Photoenhanced Field-Emission Nano-Air-Channel Devices for Terahertz Generation

Photoenhanced Field-Emission Nano-Air-Channel Devices for Terahertz Generation | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Photoenhanced Field-Emission Nano-Air-Channel Devices for Terahertz Generation Feiliang Chen, Mo Li, Xiaoxu Li, Lixin Sun, Xiangyang Li, Hao Jiang, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5265138/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Nano-air-channel devices (NACDs), characterized by scattering-free ballistic electron transport in a quasi-vacuum channel, provide a new opportunity for vacuum electronics and nanoelectronics. However, enabling the NACDs for high-frequency operation is still a great challenge due to the low field-emission current and high impedance. Herein, for the first time, field-emission NACDs are demonstrated as photomixers capable of generating coherent terahertz (THz) signals from 120 GHz to 260 GHz. This achievement marks the first successful experimental demonstration of NACDs operating beyond 100 GHz. Vertical configuration NACDs with InP/InGaAs heterojunction photocathodes and 50 nm in-plane nano-air channels are designed and fabricated using a wafer-scale manufacturing process. The field-emission currents show an impressive 375-fold increase with a low-power 1550 nm CW laser irradiation. A remarkably high internal quantum efficiency of up to 1600% is achieved by exploiting the carrier multiplication effect within the heterojunction photocathodes. Furthermore, the NACDs exhibit reproducible photo-switching behavior and highly stable field-emission current even at temperatures as high as 250 ℃. These findings position NACDs as new promising candidates for photonic THz generators, opening up an exciting application prospect for NACDs in THz high-speed wireless communications, 6G networks and high-resolution radar. nano-air channel field emission Photoenhanced photomixer terahertz Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Nanotechnology has provided unprecedented opportunities for the development of novel devices with enhanced functionalities 1–3 . The emergence of nanoscale field emission devices—nano-air channel devices (NACDs), presenting a new opportunity for vacuum electronics and nanoelectronics 4–6 . These devices exploit a unique feature—a nanoscale air channel between the cathode and the anode, facilitating the ballistic transport of electrons in the air without scattering and influence from transmission media, similar to their behavior in vacuum 7–9 . This remarkable feature allows NACDs to operate at ambient conditions without a vacuum package, thus enabling miniaturization and integration that traditional vacuum electronics usually desire 10–12 . Furthermore, the presence of a nano-air channel promotes efficient field emission at low operating voltages, which significantly reduces the cathode damage—an often-cited drawback of the field emission devices, leading to a notable increase in service lifetime 5,8,13 . More importantly, the velocity of electrons in a nano-air channel can theoretically up to the speed of light (3×10 10 cm/s), which is thousands of times faster than that in semiconductors. Consequently, the NACDs combine the advantages of vacuum and semiconductor electronics, offering numerous advantages including high-speed, high-frequency, high-temperature resistance, radiation resistance, and compatibility with the integrated circuit (IC) processes. Therefore, NACDs are regarded as one of the most promising candidates for the post-Moore era 6,14 , offering potential solutions in a wide range of applications such as low-power logic circuits 15,16 , electronic circuits for harsh environments 17–19 , high-speed photodetectors 20–23 , attosecond electronics 24,25 and high-frequency electronics 7,26–28 . In particular, terahertz (THz) devices stand out as a particularly promising and disruptive domain for NACDs due to their inherent superiority of air-channel (quasi-vacuum) as an electronic transport medium beyond semiconductors. The limitations of the natural properties of semiconductors, particularly low electron mobility, are one of the main reasons for the lack of high-performance THz devices, which partly lead to the THz gap 29–31 . NACDs offer new promise for bridging the THz gap, with theoretical simulations suggesting that their cutoff frequency can exceed 4 THz 27 . A more recent study shows that the NACDs with lateral gold multi-tip field emission arrays can be used for frequency mixing in the MHz range 32 . Our recent work has further shown frequency mixers capability up to 200 MHz using vertical-air-bridge NACDs 33 . Despite these advancements, experimental implementation of NACDs working beyond gigahertz (GHz) is still lacking. The realization of high-frequency operation requires overcoming several technical hurdles. Besides the high electron transport velocity, one critical challenge is the high impedance observed in most fabricated NACDs, often reaching as high as MΩ due to their constrained field emission current (~µA) 4,7–11 , thereby impeding their operating frequency. Several initiatives have been proposed to improve the field emission current of the NACDs, such as increasing the emitter area and reducing the air channel legnth 8,12,34 . But these measures are accompanied by a rise in capacitance that limits the operating frequency in another way. To address this, introducing external excitation such as heat and light is another effective method to boost the working current without negative effect on capacitance. Among them, light is particularly promising for high-frequency NACDs because both the photoelectric emission and optical field emission are ultrafast processes 25,35 . Several previous efforts have explored photoenhanced field emission NACDs as photodetectors spanning from ultraviolet to near-infrared wavelengths 20–23 . However, their reported response speeds remain slow, and no experimental evidence has been presented regarding their RF characteristics. In this study, we report the first successful experimental demonstration of photoenhanced field emission NACDs as photomixers for THz signal generation, enabling the conversion of light into THz. A vertical configuration comprising a InP/InGaAs heterojunction photocathode, an in-plane nano-air channel, and a metal anode is proposed, which is realized using a wafer-scale manufacturing process. The in-plane nano-air channel between the photocathode and the metal anode was formed with a nanoscale sacrificial layer and the NACDs were fabricated by a wafer-scale manufacturing process, which contributes to high-efficiency field emission and electron collection, effectively improving the current and reduce the impedance. The InP/InGaAs heterojunction was designed to establish a large localized electric field inside the photocathode, leading to the carrier multiplication effect and thus achieving an exceptionally high internal quantum efficiency of up to 1600%. The NACDs were pumped with two heterodyning CW lasers to generate tunable coherent THz signals from 120 GHz to 260 GHz, providing a novel and competitive candidate for THz photomixers. This accomplishment represents a significant advance, as it is the first accomplishment in enabling NACDs for high-frequency operation above 100 GHz, opening up exciting prospects for the application of NACDs in THz applications. Results and discussion Device Structure, Principle, and Fabrication NACDs can be configured into two primary structures: lateral and vertical. In lateral devices, the field emission current flows horizontally across the wafer, while in vertical devices, it flows vertically. It is known that the field emission behavior of NACDs is extremely sensitive to the size of the nano-air channel 8 . Therefore, the lateral devices usually suffer from poor consistency due to limitations of the nanolithography technology, which are still facing serious challenges in wafer-scale manufacturing. In contrast, the nano-air channel size of the vertical device can be controlled with atomic precision through the sacrificial layer method, allowing for high consistency and high productivity through wafer-scale manufacturing process 12,34 . Consequently, the proposed NACDs featured a vertical device structure comprising an InP/In 0.53 Ga 0.47 As photocathode, an in-plane nano-air channel, and a metal anode, as shown in Fig. 1 a,b. In contrast to previously reported vertical NACDs with out-of-plane channels from cleaved edges 4,20–22 , the introduction of the in-plane nano-air channel offers advantages such as increased effective emitter area, the enhanced collection efficiency of the field emission electrons, and subsequently reduced impedance as well as improved operating frequency. Additionally, a coplanar waveguide (CPW) was integrated with the cathode and anode to guide the high-frequency output signals. The working principle of the proposed NACDs can be explained as follows. Upon application of a reverse bias voltage, specifically negative voltage to the p-InGaAs photocathode, the InGaAs undergoes depletion and forms a two-dimensional electron gas (2DEG) inversion layer at the InGaAs/Air(SiO 2 ) interface, as shown in Fig. 1 b,c. The setup, consisting of a transparent InP substrate and a highly reflective metal anode, allows for double absorption of the 1550 nm laser light within the thin InGaAs layer for backside-illuminated mode, achieving a high absorptivity with a relatively thin thickness. Under the applied electric field, the photoelectrons generated within the InGaAs move upwards to the 2DEG layer near the InGaAs/Air(SiO 2 ) interface, further increasing the 2DEG concentration and reducing the work function of the InGaAs photocathode surface. Additionally, a 50 nm air channel facilitates the establishment of a large electric field up to 1 MV/cm, at a bias voltage of 10 V, which is twice that achieved in SiO 2 , as shown in Fig. 1 d,e. Moreover, the sharp edge and rough lower surface of the metal anode induce a field enhancement effect that boosts the field emission. At a bias voltage of 10 V, the calculated band structures of the NACDs (Fig. 1 f) reveal a wide barrier of the SiO 2 of 50 nm, making it challenging for electrons to tunnel through. In contrast, a large electric field creates a narrow triangular potential barrier in the nano-air channel, allowing photoelectrons in 2DEG to pass through the barrier via the Fowler-Nordheim (F-N) tunneling rule from the surface of the InGaAs into the nano-air channel. Then the photoelectrons traverse the nano-air channel via scattering-free ballistic transport to reach the metal anode. The electron transit time through the nano-air channel is estimated to be about 75 fs (as seen in the Supplementary), which is the intrinsic advantage of NACDs for realizing high-frequency operation. Moreover, Fig. 1 e illustrates the presence of a very strong electric field of about 2.5 MV/cm near the InP/InGaAs interface, leading to a significant acceleration of photoelectrons that can surpass the threshold energy for impact ionization. As a result, the carrier multiplication effect occurs when the photoelectrons transport through the high electric field region, thereby facilitating high responsivity and quantum efficiency. The NACDs were fabricated on a 2-inch InP wafer which contains more than 300 devices, as Fig. 1 g shows. Heterogeneous InP/In 0.53 Ga 0.47 As film was epitaxially grown on semi-insulating InP wafers, where a high-doping p-type InP layer served as the Ohmic contact layer and a low-doping p-type In 0.53 Ga 0.47 As layer was employed as the light absorption layer for telecommunication wavelengths operation. The InGaAs layer was kept at a thickness of only 200 nm to minimize the photocarrier transit time while maintaining reasonable light absorption. The diameter of the In 0.53 Ga 0.47 As mesa and the metal anode were 25 µm and 20 µm respectively, which ensured a small capacitance. A 50 nm nano-air channel was formed between the photocathode and the metal anode through wet-etching of a 50 nm SiO 2 sacrificial layer, facilitating a wafer-scale manufacturing process compatible with conventional semiconductor processes. The fabrication details are presented in the experimental section and the step-by-step fabrication process flow diagram is presented in the Supplementary Fig. S1 . As depicted in Fig. 1 h, the scanning electron microscope (SEM) image reveals a nanoscale eave structure formed by the anode edge. Figure 1 i presents the cross-sectional SEM view of the device before and after wet-etching of the SiO 2 sacrificial layer. DC Performance of the NACDs The field emission and DC photoresponse characteristics of the NACDs were measured using a probe station equipped with 1550 nm CW lasers under standard atmospheric conditions (as seen in Supplementary Fig. S2). To illustrate the advantages of the proposed in-plane channel structure, a comparison was made with the out-of-plane channel structure possessing the same geometric structures. For the out-of-plane channel structure, the SiO 2 and metal anode were edge-aligned after dry etching (as shown in Fig. 1 i). This configuration established a substantial local electric field between the edge of the anode and the InGaAs photocathode (as seen in Supplementary Fig. S3). Consequently, the electrons emitted from InGaAs reached the anode via the out-of-plane channel. Subsequent to wet etching, a part of the SiO 2 layer was removed, resulting in the formation of an in-plane channel spanning 50 nm between the InGaAs photocathode and the metal anode. The current-voltage (I-V) curves under dark conditions are shown in Fig. 2 a, revealing that the field emission current of the out-of-plane channel sample measured approximately 800 nA at 7.5 V, with a turn-on voltage of about 3 V, closely aligning with previously reported NACDs featuring out-of-plane channel 4,17 . In contrast, the in-plane channel sample exhibited a field emission current of about 40 µA at 7.5 V, representing a 50-fold increase compared to the out-of-plane sample. The results demonstrate the significant enhancement in field emission current facilitated by the in-plane channel, attributed to the increase of effective emission and collection area. The linear relation of the log(I/V 2 ) versus 1/V depicted in Fig. 2 b strongly indicate that both the out-of-plane and in-plane channel samples exhibited excellent adherence to the F-N cold field emission behavior. To further improve the field emission currents, the NACDs were operated under the illumination of a 1550 nm CW laser. Figure 2 c and 2 d demonstrates the significant influence of light on the device performance. The presence of 1550 nm light increases the electric field intensity within the nano-air channel and narrows the triangular potential barrier, both contribute to the enhancement of the field emission. At a high laser power density of 2000 W/cm 2 , the measured photoenhanced field emission current (photocurrent) show significantly increases, reaching 300 µA for the out-of-plane channel and 800 µA for the in-plane channel at 7.5 V, as shown in Fig. 2 e. The results represent an enormous enhancement in the field emission currents by 375-fold for the out-of-plane channel sample and a 26-fold for the in-plane channel sample under laser irradiation, confirming the photoenhanced field emission behavior. Furthermore, the turn-on voltages were reduced to below 0.5 V under the illumination of the laser. This phenomenon can be attributed to the narrowing of the air barrier as well as the reduction of the surface work function due to the increased 2DEG concentration. The double logarithmic coordinate I-V curves plotted in Fig. 2 f display three different slopes, corresponding to distinct emission mechanisms. At low voltages, the I-V characteristics are dominated by Schottky emission, displaying a slope of 1/2 in the double logarithm coordinate (log(I)∝V 1/2 ). As the voltage increased, the photocurrents rise rapidly with a V 2 dependence in the log-log scale plot (log(I)∝V 2 ), indicating the presence of the F-N tunneling effect and confirming the cold-field emission behavior of the photoelectrons (the corresponding log(I/V 2 ) versus 1/V plot shown in Supplementary Fig. S4 further confirms FN tunneling mechanism). At even higher voltages, the measured photocurrents exhibit a V 1.5 dependence, which can be attributed to the space-charge-limited (SCL) effect, consistent with the Child-Langmuir’s three-halves-power law as electron transport in a vacuum 36 . The photoenhanced field emission I-V characteristics were systematically investigated across various input laser power densities to evaluate the responsivity and quantum efficiency performance of the NACDs, as presented in Fig. 3 a. Notably, the observed photoresponse I-V behavior of the NACDs markedly deviates from that of conventional semiconductor photodiodes. Under reverse bias conditions (i.e. applying a negative voltage to the p-InGaAs photocathode), both field emission currents (dark currents) and photoenhanced field emission currents (photocurrents) exhibited significantly amplified values compared to those under forward bias, achieving a good rectification ratio of 10 4 . In contrast, conventional semiconductor photodiodes exhibit a forward current much larger than the dark current under the reverse bias while the photocurrent approximates the forward current 37 . This substantial distinction primarily originates from the presence of the nano-air channel and the asymmetrical semiconductor-air-metal structure. At relatively low laser power densities, the photocurrents rose rapidly at low voltages, then approached saturation at high voltages, followed by a secondary rapid rise at even higher voltages. With the increase of the laser power density, the saturation region moved towards higher voltages, and the secondary rise in current was no longer observable within the tested voltage range. For a high laser power density of 500 W/cm 2 , the current continued to increase across the tested voltage range. It is reasonable to deduce that, as long as the device doesn't break down, the saturation and the subsequent surge in current will be observable with further increases in voltage. The presence of field emission, saturation, and the subsequent surge in photocurrent was further confirmed by the F-N curves obtained at various laser power densities (as seen in Supplementary Fig. S5). Additionally, Fig. 3 b provides an alternate perspective, demonstrating that photoelectronic emission at 0 V is inefficient, resulting in a photocurrent of only a few nA and a responsivity below 10 − 4 A/W. In contrast, the photoenhanced field emission at 10 V proves highly efficient, with a photocurrent larger than 650 µA. The maximal responsivity reaches more than 4 A/W at a low laser power density of 5 W/cm 2 . Remarkably, the internal quantum efficiency reaches an impressive value of 1600% (the effective absorption rate of the 200 nm InGaAs is only 22%, as seen in Supplementary Fig. S6). As the laser power density increases, both the responsivity and quantum efficiency decline, yet even at a high laser power density of 500 W/cm 2 , the internal quantum efficiency was still as high as 170%. In comparison, the quantum efficiency of traditional vacuum phototubes is typically insufficient, with the highest attainable quantum efficiency being below 10% at near-infrared communication bands 38 . Thus, the proposed NACDs demonstrate a high quantum efficiency level in addition to miniaturization. The above phenomenon can be explained as follows. The device can be simply modelled as two adjustable series resistances, namely the resistance at the nano-air channel ( R c ) and the resistance at the InP/InGaAs heterojunction ( R h ). As shown in Fig. 3 c, the bias voltage and electric field were mainly divided by the two adjustable resistances, which dominated the I-V curves. At a low bias voltage of less than 2 V, the barrier of the nano-air channel was wide, resulting in a small field emission current and a high resistance ( R c > R h ). The R c undertook a larger voltage than R h , so the electric field at the nano-air channel increased sharply with the bias voltage (the electric field intensity doubles from 1 V to 5 V). Consequently, the field emission current experienced a rapid increase at low bias voltages, corresponding to the first rapidly rising region of the I-V curves. Meanwhile, the electric field at the InP/InGaAs heterojunction increased gradually and higher than 0.5 MV/cm when the bias voltage was higher than 1 V, which surpassed the ionization multiplication threshold electric field of InGaAs and InP. As a result, the photogenerated carrier multiplication effect was triggered, facilitating a high quantum efficiency. As the bias voltage increases continuously, the field emission current increases nonlinearly, leading to a rapid decrease of R c . Then the bias voltage and electric field would be mainly applied on the R h when R h > R c , resulting in a moderate increase in the electric field applied to the nano-air channel, thereby leading to a field emission current saturation region. Meanwhile, the electric field at the InP/InGaAs heterojunction experienced a rapid increase from 1 MV/cm at 5 V to more than 2.5 MV/cm at 10 V, greatly enhancing the photogenerated carrier multiplication effect and leading to a rapid decrease of the R h . The bias voltage and electric field would mainly applied on the R c again. As a result, the photocurrents started to increase exponentially again at a large bias voltage, leading to a significant improvement in the device’s optical responsivity and quantum efficiency. In addition, the decline of the quantum efficiency at high laser power density can be attributed to the reduction of the electric field at the InP/InGaAs heterojunction (as seen in Supplementary Fig. S7) and the space charge limiting effect within the InGaAs absorption layer caused by the high photocarrier density. THz Generation by the NACDs Beyond the large currents and high quantum efficiency, the repeatability and stability of the field emission currents are also essential for high-frequency operation and practical applications. Most of the previous NACDs with planar nano-tips usually suffered from the current degradation and tip destruction 8,32 . In contrast, our vertical NACDs exhibited excellent robustness, as shown in Fig. 4 a-c. The photoenhanced field emission currents showed a slight increase (less than 2.5%) with repeated testings, which could be attributed to the heat-assisted electron emission resulting from the heat accumulation effect due to continuous laser irradiation. Furthermore, periodic on-off laser tests revealed that both the field-emission currents and photoenhanced field emission currents feature excellent repeatability. The maximum photoenhanced field emission current can reach up to 1.56 mA at a bias voltage of 10 V and laser power density of 2000 W/cm 2 . Additionally, the long-term stability of the photoenhanced field emission currents in air was also examined, which shows no degradation over time but a slight increase in the initial phase and eventually stabilized due to the thermal equilibrium. The slight fluctuation of the currents was caused by the fiber vibration and the laser power fluctuation. The high stability of the photoenhanced field emission is due to the low voltage operation, which can significantly reduce the ion bombardment damage and increase the lifetime. The high-temperature stability of the field emission currents was further investigated as shown in Fig. 4 d. At low voltages, the temperature dependence confirms the Schottky emission behavior. In contrast, at high voltage, the I-V curves exhibited reduced temperature dependence, consistent with F-N cold field emission behavior. It is worth noting that the current of our NACD at 8 V showed a slight fluctuation of about 10% from 25 ℃ to 200 ℃ (as seen in Supplementary Fig. S8) while the traditional semiconductor InGaAs photodetectors suffer from poor temperature stability. For example, the dark current of the InGaAs APD deteriorated by two orders of magnitude from 200 K to 340 K 39 . These results strongly underscore the superiority of our photo-enhanced NACDs for high-temperature operation. Above 200 ℃, thermally excited electrons could easily pass through the thin air potential barrier, making the thermally promoted field emission dominated the I-V curves. The improved field emission currents, along with the high stability, make the proposed NACDs well-suited for high-frequency operation. Subsequently, the NACDs were employed to generate THz signals using a laser heterodyne measurement set-up, as illustrated in Fig. 4 e. Two tunable C-band CW lasers were used to produce a heterodyning pump beam, which was directed onto the NACDs by a single-mode fiber. An RF probe with a bias tee and a frequency analyzer were used to measure the output signals from the NACDs. By varying the wavelength difference between the two lasers, at the output end of the NACDs, THz signals ranging from 120 GHz to 260 GHz were obtained, as shown in Fig. 5b. The photomixing THz signals exhibited very stable and flat output within the test range, showing properties of ultra-wide bandwidth. The measured output power exhibits a maximum of approximately − 45 dBm at 200 GHz, which is mainly limited by the small field-emission photocurrent at the current stage. Although the power is still below the state-of-the-art terahertz photomixers such as uni-travelling-carrier PD (UTC-PD), it is worth noting that our NACDs exhibit 10 dB higher radiated output power than previously reported plasmonic internal-photoemission detectors 40 , which are suitable for on-chip, inter-chip, and near field terahertz wireless communication 41 . The output of the NACDs at higher frequencies is limited by the working frequency range of our RF probe. The linewidth of the generated THz signals is as narrow as 5 kHz, which is primarily determined by that of the lasers and can be further improved to obtain highly coherence and ultralow phase noise THz sources 42 . To the best of our knowledge, this is not only the first successful demonstration of NACDs as photomixers for THz generation, but also the first experimental realization of NACDs operating above 100 GHz, which opens up exciting prospects for the practical application of NACDs in high-frequency fields. The frequency response of the NACDs is mainly determined by two factors: the transit time and resistor-capacitor (RC) time constant. The total transit time of the NACDs comprises two components: the drift-diffusion time τ InGaAs within the InGaAs material and the ballistic transport time τ Air through the nano-air channel. The τ InGaAs is estimated to be about 1 ps for a 200 nm thick InGaAs layer (assuming that the average speed of the carriers in InGaAs is 2×10 7 cm/s), while the τ Air is less than 75 fs. Thus, the transit time is mainly limited by the InGaAs photocathode. On the other hand, the RC time constant is primarily determined by the intrinsic capacitance, parasitic capacitance, and series resistance. In our NACDs, the RC time constant is the main limiting factor rather than the transit time. The field emission current can be further improved by energy-band engineering and work function engineering to reduce the series resistance. Consequently, the NACDs have the potential to generate even higher frequency and power output in the future, thus playing a critical role in high-speed wireless communication and high-resolution radar at THz bands 43–45 . Moreover, benefited from the IC-compatible manufacturing process, our NACDs can be further integrated with lasers, optical delay lines, electrooptical modulators, and arrayed antennas, then used for compact and efficient chip-scale THz transmitter and beam steering 46,47 . The performances of our NACDs are compared with the various state-of-the-art NACDs reported in previous studies, as summarized in Table 1 . The field emission currents of typical NACDs with planar tips are usually of the order of milliamperes due to their small emission area 48,49 . In contrast, vertical configuration NACDs with long channel perimeter could obtain large emission currents up to 10 mA, but the device area was as large as several square millimeters, resulting in a large capacitance and low response speed with tens of nanoseconds 34 . Our NACDs with a reasonable anode diameter of 20 µm, strike a balance between size and performance. They exhibited a small capacitance approximately one hundred fF. Meanwhile, the photoenhanced field emission currents of our NACDs exceed 1.5 mA, which is one of the highest values among the state-of-the-art NACDs. Furthermore, our NACDs also achieves a record quantum efficiency among various photoenhanced NACDs. Table 1 Performance comparison for state-of-the-art NACDs. Device structures Emission current Quantum efficiency Working frequency Years Vertical Si/SiO 2 /Al with 20 nm channel 4 0.8 µA at 2 V - - 2012 Lateral Si tips with 150 nm channel 7 40 µA at 2 V - - 2012 Lateral Si nanowire with 50 nm channel 6 3 µA at 5 V - - 2017 Lateral W tips with 11 nm channel 13 0.15 µA at 2 V - - 2018 Vertical SiC/SiO 2 /Si with 200 nm channel 17 12 µA at 20 V - - 2019 Lateral GaN tips with 30 nm channel 8 0.45 µA at 1 V - - 2021 Lateral Au tip-to-edge with sub-10 nm channel 9 1 µA at 2 V - - 2021 Vertical Si/SiO 2 /Au with 80 nm channel 12 160 µA at 2 V - - 2021 Vertical GaN/SiO 2 /Au with 50 nm channel 34 11 mA at 10 V - - 2023 Lateral Au comb with 30 nm channel 32 0.7 µA at 8 V - 85 MHz 2023 Lateral Ga 2 O 3 nanowires with 50 nm channel 49 2.7 µA at 15 V - - 2024 Vertical air-bridge with 50 nm channel 33 0.5 µA at 1 V - 200 MHz 2024 Photoenhanced vertical Si/SiO 2 /graphene with 23 nm channel 20 0.95 mA at 5 V 384% at 325 nm - 2015 Photoenhanced vertical Si/SiO 2 /graphene with 20 nm channel 21 0.6 mA at 5 V 350% at 633 nm - 2016 Photoenhanced vertical Au/Air/Au resonant surface with 200 nm channel 35 0.65 mA at 5 V 5% at 785 nm - 2016 Photoenhanced vertical Si/SiO 2 /Au nanoantennas with 30 nm channel 22 0.7 µA at 2 V 4% at 800 nm - 2020 Photoenhanced TiN planar tips array with 30 nm channel 49 7 µA at 5 V 0.1% at 1.2 µm - 2021 Photoenhanced vertical Si tips/Air/CsPbBr 3 with 100 nm channel 50 2 nA at 15 V 400% at 532 nm - 2022 Photoenhanced vertical InP/InGaAs/SiO 2 /CrAu with 50 nm channel (This work) 1.56 mA at 10 V 1600% at 1.55 µm 120 GHz-260 GHz - Conclusions In this work, we proposed a new concept of utilizing photoenhanced field-emission NACDs to work as photomixers, which brings forward a fresh perspective for the high-frequency applications of nano-air-channel devices. A vertical configuration device with in-plane nano-air channels of 50 nm was introduced to achieve efficient field emission, resulting in a 50-fold increase in current compared to traditional vertical NACDs. A scalable and high-yield fabrication approach compatible with conventional semiconductor processing techniques was developed for wafer-scale production of the proposed NACDs, which unlocks the potential for seamless integration with optoelectronic integrated circuits. The working mechanism and photosensitive field emission behavior of the proposed NACDs were systematically analyzed, providing valuable insights for device design. By leveraging the photoenhanced field emission effect, ultra-low turn-on voltage down to ~ 0.5 V and high field emission current up to 1.56 mA was achieved. Furthermore, long-term stability and high-temperature reliability investigations demonstrate the superiority of cold field emission NACDs. Notably, the distinguishing feature of the nano-air channel lies in its ability to generate substantial local electric fields within both the nano-air channel and the InP/InGaAs heterojunction photocathode at notably low bias voltages. This facilitates efficient field emission of electrons and multiplication of photogenerated carriers, culminating in an exceptional responsivity reaching up to 4 A/W and an internal quantum efficiency as high as 1600% at 1550 nm. As a result, the high-efficiency photoenhanced field emission provides an effective approach to reduce the impedance of NACDs, facilitating high-speed and high-frequency operation. With these advancements, we have demonstrated the generation of coherent THz signals from 120 GHz to 260 GHz, which is the first successful experimental demonstration of NACDs operating beyond 100 GHz known to date. The proposed NACDs provide a highly promising solution for photonic THz generators, thereby facilitating and advancing the development of THz-wireless and fiber transparent seamless integration communication links, 6G networks and high-resolution radar. Methods Device Simulation. The electric field distribution, energy band structures, optical reflectivity and absorption were simulated using a commercial TCAD tool (Silvaco). Structural parameters and doping density used in the simulation were consistent with the actual fabricated device. Material parameters such as dielectric constant and work function were based on the default values provided by the Silvaco internal library. The Fowler–Nordheim (F–N) tunneling model was utilized in the simulation. Device Fabrication. A 200 nm high-doping (5e18 cm -3 ) p-type InP Ohmic contact layer followed by a 200 nm low-doping (1e16 cm -3 ) p-type In 0.53 Ga 0.47 As film was grown on 2 inches semi-insulating InP substrates by metal-organic chemical vapor deposition (MOCVD) method. The mesa and electrode patterns were formed by UV photolithography with a resolution of about 1 µm. The In 0.53 Ga 0.47 As and InP mesa were fabricated by ICP etching and wet etching techniques. A 50 nm SiO 2 film was deposited on the InGaAs by atomic layer deposition (ALD) and a 200 nm CrAu anode was deposited on the SiO 2 by electron beam evaporation (EBE). Then, the electrode and CPW were constructed by ion beam etching (IBE). Wet etching was employed to remove part of the SiO 2 between the InGaAs photocathode and the CrAu anode to form a nano-air channel by using a BOE solution. The diameter of the active area is 20 µm and the depth of the nano-air channel is about 200 nm. Finally, the backside thinning and polishing were carried out to thin the substrate down to 300 µm. Measurements Setup. The morphology of the NACDs was characterized by an SEM system (ZEISS Gemini 300). The optical responses and photomixing of the NACDs were measured by an RF probe station equipped with a single-mode optical fiber and a THz Probes (GGB Model 220), a SourceMeter SMU instrument (Keithley 2450), two tunable lasers (Pure Photonics PPCL550), and a frequency analyzer (Ceyear 4051H). The heterodyning laser spectra were measured by a high-resolution optical spectrum analyzer (Yokogawa AQ6370D-22). Declarations Acknowledgements The authors would like to acknowledge the financial support from the National Natural Science Foundation of China (No. 62474032, 61704162, 61704163) and the Fundamental Research Funds for the Central Universities (Grant No. ZYGX2021J028). The authors would also like to thank Dr. Qian Li from MTRC for her help in device simulation. 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Nagatsuma T, Ducournau G, Renaud C C. Advances in terahertz communications accelerated by photonics. Nat. Photonics , 10 , 371-379 (2016). Kittlaus E A, Eliyahu D, Ganji S, et al. A low-noise photonic heterodyne synthesizer and its application to millimeter-wave radar. Nat . Commun . 12 , 4397 (2021). Nellen S, Lauck S, Peytavit E, et al. Coherent wireless link at 300 GHz with 160 Gbit/s enabled by a photonic transmitter. J. Lightwave Technol. 40 , 4178-4185 (2022). Sengupta K, Nagatsuma T, Mittleman D M. Terahertz integrated electronic and hybrid electronic–photonic systems. Nat. Electron. 1 , 622–635 (2018). Che M, Kondo K, Kanaya H, et al. Arrayed photomixers for THz beam-combining and beam-steering. J. Lightwave Technol. 40 , 6657-6665 (2022). Tang M, Ma C, Liu L, et al. β-Ga 2 O 3 Air-Channel Field-Emission Nanodiode with Ultrahigh Current Density and Low Turn-On Voltage. Nano Lett. 24 , 1769-1775 (2024). Nardi A, Turchetti M, Britton W A, et al. Nanoscale refractory doped titanium nitride field emitters. Nanotechnology 32 , 315208 (2021). Zeng X, Li S, Liu Z, et al. High Responsivity Vacuum Nano-Photodiode Using Single-Crystal CsPbBr 3 Micro-Sheet. Nanomaterials 12 , 4205, (2022). Additional Declarations There is no conflict of interest Supplementary Files Supplementaryinformation.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5265138","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":376158457,"identity":"08312712-eae2-4871-b86d-ea4880bb4945","order_by":0,"name":"Feiliang Chen","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAyklEQVRIiWNgGAWjYBAC9nYGNiBlA+MzE9bCcxisJQ2mmngth0nRwsz87MHHHecT+/nPH/zAUGGd2MB+9gABLWzmhjPP3E6cOSOZWYLhTHpiA09eAl4t9sw8bNK8bbcTN9xgZmNgbDuc2CDBY0DAFqCWv23nEvefPwzU8o9YLYxtBxI3MCQDtTQQpYXNTLK3Ldl4xo1kY4mEY+nGbTw5BLSwNz+T+NlmJ9vff/Dhhw811rL97Gfwa0EFCUDMRoL6UTAKRsEoGAU4AAC34zxYvQrlwAAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0001-6069-8699","institution":"University of Electronic Science and Technology of China","correspondingAuthor":true,"prefix":"","firstName":"Feiliang","middleName":"","lastName":"Chen","suffix":""},{"id":376158458,"identity":"6c21a166-a3a6-4a02-9c17-9b7ad79b0a31","order_by":1,"name":"Mo Li","email":"","orcid":"","institution":"University of Electronic Science and Technology of 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05:15:07","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5265138/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5265138/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":70034088,"identity":"92eac383-8724-41ee-b202-149f63c91529","added_by":"auto","created_at":"2024-11-27 17:02:26","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":376822,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStructure, working principle, and photo of the NACDs.\u003c/strong\u003e \u003cstrong\u003ea,\u003c/strong\u003e The 3D schematic diagram of the proposed NACDs. \u003cstrong\u003eb,\u003c/strong\u003e Profile structure and working process schematic diagram of the NACDs. \u003cstrong\u003ec,\u003c/strong\u003e Energy band diagram and photon-generated electron emission principle of the NACDs. \u003cstrong\u003ed,\u003c/strong\u003e Calculated electric field distribution of the NACDs at a bias voltage of 10 V. \u003cstrong\u003ee,\u003c/strong\u003e Comparison of the electric field intensity in the nano-air channel and the SiO\u003csub\u003e2\u003c/sub\u003e. \u003cstrong\u003ef,\u003c/strong\u003e Calculated band structures along the SiO\u003csub\u003e2\u003c/sub\u003e and the nano-air channel at a bias voltage of 10 V. \u003cstrong\u003eg,\u003c/strong\u003e Images of the fabricated NACDs on a 2-inch InP wafer. \u003cstrong\u003eh,\u003c/strong\u003e SEM image of the fabricated device (insert is a close-up of the nano-air channel from a 45° side view). \u003cstrong\u003ei,\u003c/strong\u003e A cross-sectional SEM view of the device before and after wet-etching.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-5265138/v1/28e917dabd47c3b59b3259a5.png"},{"id":70035213,"identity":"9470cdba-f5c9-4d83-92f8-1a897a553ef1","added_by":"auto","created_at":"2024-11-27 17:10:26","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":343211,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePhotoenhanced field emission characterization for fabricated NACDs.\u003c/strong\u003e \u003cstrong\u003ea,\u003c/strong\u003e Comparison of the field emission currents of the out-of-plane and in-plane channel devices under dark conditions (the inserts are the corresponding structure diagram). \u003cstrong\u003eb,\u003c/strong\u003e The linearity of the FN curves indicates electron transport following the tunneling mechanism for both the devices. \u003cstrong\u003ec,d, \u003c/strong\u003eComparison of the electric fields (\u003cstrong\u003ec\u003c/strong\u003e) and band structures (\u003cstrong\u003ed\u003c/strong\u003e) under dark and light conditions at a bias voltage of 10 V. \u003cstrong\u003ee,\u003c/strong\u003e Measured photoenhanced field emission currents of before and after wet-etching samples (the inserts are the corresponding SEM images). \u003cstrong\u003ef,\u003c/strong\u003e The double logarithm coordinate photoenhanced I-V characteristics together with different emission principles.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-5265138/v1/8ec35ec11e3114f3b61aa550.png"},{"id":70034092,"identity":"23019519-891d-4444-8d05-adf96b7d4b62","added_by":"auto","created_at":"2024-11-27 17:02:26","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":280291,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eOptical response characterization of the NACDs at different laser power densities.\u003c/strong\u003e \u003cstrong\u003ea,\u003c/strong\u003e Measured I-V curves of NACDs at different laser power densities. \u003cstrong\u003eb,\u003c/strong\u003eComparison of the currents and responsivity for a bias voltage of 0 V and 10 V at different laser power densities. \u003cstrong\u003ec,\u003c/strong\u003eCalculated electric field of the NACDs at different bias voltages. \u003cstrong\u003ed,\u003c/strong\u003e Responsivity and internal quantum efficiency of the NACDs at different laser power densities.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-5265138/v1/b27f7395e5ea6c43ebfe02fd.png"},{"id":70034090,"identity":"957b7e71-8eb4-43fb-b6c3-aa8e7127a437","added_by":"auto","created_at":"2024-11-27 17:02:26","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":331695,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eRepeatability, stability and THz signals generation by the NACDs.\u003c/strong\u003e \u003cstrong\u003ea,\u003c/strong\u003e Repeated test results of the photoenhanced field emission current. \u003cstrong\u003eb,\u003c/strong\u003e Laser on-off tests of the photoenhanced field emission current at a bias voltage of 10 V and laser power density of 2000 W/cm\u003csup\u003e2\u003c/sup\u003e. \u003cstrong\u003ec,\u003c/strong\u003e Long-term stability of the photoenhanced currents. \u003cstrong\u003ed,\u003c/strong\u003e Temperature dependence of the NACDs. \u003cstrong\u003ee\u003c/strong\u003e, Measured THz signals from 120 GHz to 260 GHz generated by the NACDs.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-5265138/v1/81c8440231a10363ef1f0cef.png"},{"id":70430256,"identity":"ee5a44dc-7d74-457b-b007-6a56a6c661b7","added_by":"auto","created_at":"2024-12-03 06:06:46","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2042111,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5265138/v1/4a533089-1259-4255-81fb-c68fc1791de3.pdf"},{"id":70034089,"identity":"b964dc54-e04d-4db4-bcf9-478de836ac77","added_by":"auto","created_at":"2024-11-27 17:02:26","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1565238,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Supplementaryinformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-5265138/v1/676f9e749f38ef8a18a155ca.docx"}],"financialInterests":"There is no conflict of interest","formattedTitle":"Photoenhanced Field-Emission Nano-Air-Channel Devices for Terahertz Generation","fulltext":[{"header":"Introduction","content":"\u003cp\u003eNanotechnology has provided unprecedented opportunities for the development of novel devices with enhanced functionalities\u003csup\u003e1\u0026ndash;3\u003c/sup\u003e. The emergence of nanoscale field emission devices\u0026mdash;nano-air channel devices (NACDs), presenting a new opportunity for vacuum electronics and nanoelectronics\u003csup\u003e4\u0026ndash;6\u003c/sup\u003e. These devices exploit a unique feature\u0026mdash;a nanoscale air channel between the cathode and the anode, facilitating the ballistic transport of electrons in the air without scattering and influence from transmission media, similar to their behavior in vacuum\u003csup\u003e7\u0026ndash;9\u003c/sup\u003e. This remarkable feature allows NACDs to operate at ambient conditions without a vacuum package, thus enabling miniaturization and integration that traditional vacuum electronics usually desire\u003csup\u003e10\u0026ndash;12\u003c/sup\u003e. Furthermore, the presence of a nano-air channel promotes efficient field emission at low operating voltages, which significantly reduces the cathode damage\u0026mdash;an often-cited drawback of the field emission devices, leading to a notable increase in service lifetime\u003csup\u003e5,8,13\u003c/sup\u003e. More importantly, the velocity of electrons in a nano-air channel can theoretically up to the speed of light (3\u0026times;10\u003csup\u003e10\u003c/sup\u003e cm/s), which is thousands of times faster than that in semiconductors. Consequently, the NACDs combine the advantages of vacuum and semiconductor electronics, offering numerous advantages including high-speed, high-frequency, high-temperature resistance, radiation resistance, and compatibility with the integrated circuit (IC) processes. Therefore, NACDs are regarded as one of the most promising candidates for the post-Moore era\u003csup\u003e6,14\u003c/sup\u003e, offering potential solutions in a wide range of applications such as low-power logic circuits\u003csup\u003e15,16\u003c/sup\u003e, electronic circuits for harsh environments\u003csup\u003e17\u0026ndash;19\u003c/sup\u003e, high-speed photodetectors\u003csup\u003e20\u0026ndash;23\u003c/sup\u003e, attosecond electronics\u003csup\u003e24,25\u003c/sup\u003e and high-frequency electronics\u003csup\u003e7,26\u0026ndash;28\u003c/sup\u003e. In particular, terahertz (THz) devices stand out as a particularly promising and disruptive domain for NACDs due to their inherent superiority of air-channel (quasi-vacuum) as an electronic transport medium beyond semiconductors. The limitations of the natural properties of semiconductors, particularly low electron mobility, are one of the main reasons for the lack of high-performance THz devices, which partly lead to the THz gap\u003csup\u003e29\u0026ndash;31\u003c/sup\u003e. NACDs offer new promise for bridging the THz gap, with theoretical simulations suggesting that their cutoff frequency can exceed 4 THz\u003csup\u003e27\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eA more recent study shows that the NACDs with lateral gold multi-tip field emission arrays can be used for frequency mixing in the MHz range\u003csup\u003e32\u003c/sup\u003e. Our recent work has further shown frequency mixers capability up to 200 MHz using vertical-air-bridge NACDs\u003csup\u003e33\u003c/sup\u003e. Despite these advancements, experimental implementation of NACDs working beyond gigahertz (GHz) is still lacking. The realization of high-frequency operation requires overcoming several technical hurdles. Besides the high electron transport velocity, one critical challenge is the high impedance observed in most fabricated NACDs, often reaching as high as MΩ due to their constrained field emission current (~\u0026micro;A)\u003csup\u003e4,7\u0026ndash;11\u003c/sup\u003e, thereby impeding their operating frequency. Several initiatives have been proposed to improve the field emission current of the NACDs, such as increasing the emitter area and reducing the air channel legnth\u003csup\u003e8,12,34\u003c/sup\u003e. But these measures are accompanied by a rise in capacitance that limits the operating frequency in another way. To address this, introducing external excitation such as heat and light is another effective method to boost the working current without negative effect on capacitance. Among them, light is particularly promising for high-frequency NACDs because both the photoelectric emission and optical field emission are ultrafast processes\u003csup\u003e25,35\u003c/sup\u003e. Several previous efforts have explored photoenhanced field emission NACDs as photodetectors spanning from ultraviolet to near-infrared wavelengths\u003csup\u003e20\u0026ndash;23\u003c/sup\u003e. However, their reported response speeds remain slow, and no experimental evidence has been presented regarding their RF characteristics.\u003c/p\u003e \u003cp\u003eIn this study, we report the first successful experimental demonstration of photoenhanced field emission NACDs as photomixers for THz signal generation, enabling the conversion of light into THz. A vertical configuration comprising a InP/InGaAs heterojunction photocathode, an in-plane nano-air channel, and a metal anode is proposed, which is realized using a wafer-scale manufacturing process. The in-plane nano-air channel between the photocathode and the metal anode was formed with a nanoscale sacrificial layer and the NACDs were fabricated by a wafer-scale manufacturing process, which contributes to high-efficiency field emission and electron collection, effectively improving the current and reduce the impedance. The InP/InGaAs heterojunction was designed to establish a large localized electric field inside the photocathode, leading to the carrier multiplication effect and thus achieving an exceptionally high internal quantum efficiency of up to 1600%. The NACDs were pumped with two heterodyning CW lasers to generate tunable coherent THz signals from 120 GHz to 260 GHz, providing a novel and competitive candidate for THz photomixers. This accomplishment represents a significant advance, as it is the first accomplishment in enabling NACDs for high-frequency operation above 100 GHz, opening up exciting prospects for the application of NACDs in THz applications.\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e\u003cb\u003eDevice Structure, Principle, and Fabrication\u003c/b\u003e\u003c/h2\u003e \u003cp\u003eNACDs can be configured into two primary structures: lateral and vertical. In lateral devices, the field emission current flows horizontally across the wafer, while in vertical devices, it flows vertically. It is known that the field emission behavior of NACDs is extremely sensitive to the size of the nano-air channel\u003csup\u003e8\u003c/sup\u003e. Therefore, the lateral devices usually suffer from poor consistency due to limitations of the nanolithography technology, which are still facing serious challenges in wafer-scale manufacturing. In contrast, the nano-air channel size of the vertical device can be controlled with atomic precision through the sacrificial layer method, allowing for high consistency and high productivity through wafer-scale manufacturing process\u003csup\u003e12,34\u003c/sup\u003e. Consequently, the proposed NACDs featured a vertical device structure comprising an InP/In\u003csub\u003e0.53\u003c/sub\u003eGa\u003csub\u003e0.47\u003c/sub\u003eAs photocathode, an in-plane nano-air channel, and a metal anode, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea,b. In contrast to previously reported vertical NACDs with out-of-plane channels from cleaved edges\u003csup\u003e4,20\u0026ndash;22\u003c/sup\u003e, the introduction of the in-plane nano-air channel offers advantages such as increased effective emitter area, the enhanced collection efficiency of the field emission electrons, and subsequently reduced impedance as well as improved operating frequency. Additionally, a coplanar waveguide (CPW) was integrated with the cathode and anode to guide the high-frequency output signals.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe working principle of the proposed NACDs can be explained as follows. Upon application of a reverse bias voltage, specifically negative voltage to the p-InGaAs photocathode, the InGaAs undergoes depletion and forms a two-dimensional electron gas (2DEG) inversion layer at the InGaAs/Air(SiO\u003csub\u003e2\u003c/sub\u003e) interface, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb,c. The setup, consisting of a transparent InP substrate and a highly reflective metal anode, allows for double absorption of the 1550 nm laser light within the thin InGaAs layer for backside-illuminated mode, achieving a high absorptivity with a relatively thin thickness. Under the applied electric field, the photoelectrons generated within the InGaAs move upwards to the 2DEG layer near the InGaAs/Air(SiO\u003csub\u003e2\u003c/sub\u003e) interface, further increasing the 2DEG concentration and reducing the work function of the InGaAs photocathode surface. Additionally, a 50 nm air channel facilitates the establishment of a large electric field up to 1 MV/cm, at a bias voltage of 10 V, which is twice that achieved in SiO\u003csub\u003e2\u003c/sub\u003e, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed,e. Moreover, the sharp edge and rough lower surface of the metal anode induce a field enhancement effect that boosts the field emission. At a bias voltage of 10 V, the calculated band structures of the NACDs (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef) reveal a wide barrier of the SiO\u003csub\u003e2\u003c/sub\u003e of 50 nm, making it challenging for electrons to tunnel through. In contrast, a large electric field creates a narrow triangular potential barrier in the nano-air channel, allowing photoelectrons in 2DEG to pass through the barrier via the Fowler-Nordheim (F-N) tunneling rule from the surface of the InGaAs into the nano-air channel. Then the photoelectrons traverse the nano-air channel via scattering-free ballistic transport to reach the metal anode. The electron transit time through the nano-air channel is estimated to be about 75 fs (as seen in the Supplementary), which is the intrinsic advantage of NACDs for realizing high-frequency operation. Moreover, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee illustrates the presence of a very strong electric field of about 2.5 MV/cm near the InP/InGaAs interface, leading to a significant acceleration of photoelectrons that can surpass the threshold energy for impact ionization. As a result, the carrier multiplication effect occurs when the photoelectrons transport through the high electric field region, thereby facilitating high responsivity and quantum efficiency.\u003c/p\u003e \u003cp\u003eThe NACDs were fabricated on a 2-inch InP wafer which contains more than 300 devices, as Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eg shows. Heterogeneous InP/In\u003csub\u003e0.53\u003c/sub\u003eGa\u003csub\u003e0.47\u003c/sub\u003eAs film was epitaxially grown on semi-insulating InP wafers, where a high-doping p-type InP layer served as the Ohmic contact layer and a low-doping p-type In\u003csub\u003e0.53\u003c/sub\u003eGa\u003csub\u003e0.47\u003c/sub\u003eAs layer was employed as the light absorption layer for telecommunication wavelengths operation. The InGaAs layer was kept at a thickness of only 200 nm to minimize the photocarrier transit time while maintaining reasonable light absorption. The diameter of the In\u003csub\u003e0.53\u003c/sub\u003eGa\u003csub\u003e0.47\u003c/sub\u003eAs mesa and the metal anode were 25 \u0026micro;m and 20 \u0026micro;m respectively, which ensured a small capacitance. A 50 nm nano-air channel was formed between the photocathode and the metal anode through wet-etching of a 50 nm SiO\u003csub\u003e2\u003c/sub\u003e sacrificial layer, facilitating a wafer-scale manufacturing process compatible with conventional semiconductor processes. The fabrication details are presented in the experimental section and the step-by-step fabrication process flow diagram is presented in the Supplementary Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eh, the scanning electron microscope (SEM) image reveals a nanoscale eave structure formed by the anode edge. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ei presents the cross-sectional SEM view of the device before and after wet-etching of the SiO\u003csub\u003e2\u003c/sub\u003e sacrificial layer.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eDC Performance of the NACDs\u003c/h3\u003e\n\u003cp\u003eThe field emission and DC photoresponse characteristics of the NACDs were measured using a probe station equipped with 1550 nm CW lasers under standard atmospheric conditions (as seen in Supplementary Fig. S2). To illustrate the advantages of the proposed in-plane channel structure, a comparison was made with the out-of-plane channel structure possessing the same geometric structures. For the out-of-plane channel structure, the SiO\u003csub\u003e2\u003c/sub\u003e and metal anode were edge-aligned after dry etching (as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ei). This configuration established a substantial local electric field between the edge of the anode and the InGaAs photocathode (as seen in Supplementary Fig. S3). Consequently, the electrons emitted from InGaAs reached the anode via the out-of-plane channel. Subsequent to wet etching, a part of the SiO\u003csub\u003e2\u003c/sub\u003e layer was removed, resulting in the formation of an in-plane channel spanning 50 nm between the InGaAs photocathode and the metal anode. The current-voltage (I-V) curves under dark conditions are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea, revealing that the field emission current of the out-of-plane channel sample measured approximately 800 nA at 7.5 V, with a turn-on voltage of about 3 V, closely aligning with previously reported NACDs featuring out-of-plane channel\u003csup\u003e4,17\u003c/sup\u003e. In contrast, the in-plane channel sample exhibited a field emission current of about 40 \u0026micro;A at 7.5 V, representing a 50-fold increase compared to the out-of-plane sample. The results demonstrate the significant enhancement in field emission current facilitated by the in-plane channel, attributed to the increase of effective emission and collection area. The linear relation of the log(I/V\u003csup\u003e2\u003c/sup\u003e) versus 1/V depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb strongly indicate that both the out-of-plane and in-plane channel samples exhibited excellent adherence to the F-N cold field emission behavior.\u003c/p\u003e \u003cp\u003eTo further improve the field emission currents, the NACDs were operated under the illumination of a 1550 nm CW laser. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed demonstrates the significant influence of light on the device performance. The presence of 1550 nm light increases the electric field intensity within the nano-air channel and narrows the triangular potential barrier, both contribute to the enhancement of the field emission. At a high laser power density of 2000 W/cm\u003csup\u003e2\u003c/sup\u003e, the measured photoenhanced field emission current (photocurrent) show significantly increases, reaching 300 \u0026micro;A for the out-of-plane channel and 800 \u0026micro;A for the in-plane channel at 7.5 V, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee. The results represent an enormous enhancement in the field emission currents by 375-fold for the out-of-plane channel sample and a 26-fold for the in-plane channel sample under laser irradiation, confirming the photoenhanced field emission behavior. Furthermore, the turn-on voltages were reduced to below 0.5 V under the illumination of the laser. This phenomenon can be attributed to the narrowing of the air barrier as well as the reduction of the surface work function due to the increased 2DEG concentration. The double logarithmic coordinate I-V curves plotted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef display three different slopes, corresponding to distinct emission mechanisms. At low voltages, the I-V characteristics are dominated by Schottky emission, displaying a slope of 1/2 in the double logarithm coordinate (log(I)\u0026prop;V\u003csup\u003e1/2\u003c/sup\u003e). As the voltage increased, the photocurrents rise rapidly with a V\u003csup\u003e2\u003c/sup\u003e dependence in the log-log scale plot (log(I)\u0026prop;V\u003csup\u003e2\u003c/sup\u003e), indicating the presence of the F-N tunneling effect and confirming the cold-field emission behavior of the photoelectrons (the corresponding log(I/V\u003csup\u003e2\u003c/sup\u003e) versus 1/V plot shown in Supplementary Fig. S4 further confirms FN tunneling mechanism). At even higher voltages, the measured photocurrents exhibit a V\u003csup\u003e1.5\u003c/sup\u003e dependence, which can be attributed to the space-charge-limited (SCL) effect, consistent with the Child-Langmuir\u0026rsquo;s three-halves-power law as electron transport in a vacuum\u003csup\u003e36\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe photoenhanced field emission I-V characteristics were systematically investigated across various input laser power densities to evaluate the responsivity and quantum efficiency performance of the NACDs, as presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea. Notably, the observed photoresponse I-V behavior of the NACDs markedly deviates from that of conventional semiconductor photodiodes. Under reverse bias conditions (i.e. applying a negative voltage to the p-InGaAs photocathode), both field emission currents (dark currents) and photoenhanced field emission currents (photocurrents) exhibited significantly amplified values compared to those under forward bias, achieving a good rectification ratio of 10\u003csup\u003e4\u003c/sup\u003e. In contrast, conventional semiconductor photodiodes exhibit a forward current much larger than the dark current under the reverse bias while the photocurrent approximates the forward current\u003csup\u003e37\u003c/sup\u003e. This substantial distinction primarily originates from the presence of the nano-air channel and the asymmetrical semiconductor-air-metal structure. At relatively low laser power densities, the photocurrents rose rapidly at low voltages, then approached saturation at high voltages, followed by a secondary rapid rise at even higher voltages. With the increase of the laser power density, the saturation region moved towards higher voltages, and the secondary rise in current was no longer observable within the tested voltage range. For a high laser power density of 500 W/cm\u003csup\u003e2\u003c/sup\u003e, the current continued to increase across the tested voltage range. It is reasonable to deduce that, as long as the device doesn't break down, the saturation and the subsequent surge in current will be observable with further increases in voltage. The presence of field emission, saturation, and the subsequent surge in photocurrent was further confirmed by the F-N curves obtained at various laser power densities (as seen in Supplementary Fig. S5). Additionally, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb provides an alternate perspective, demonstrating that photoelectronic emission at 0 V is inefficient, resulting in a photocurrent of only a few nA and a responsivity below 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e A/W. In contrast, the photoenhanced field emission at 10 V proves highly efficient, with a photocurrent larger than 650 \u0026micro;A. The maximal responsivity reaches more than 4 A/W at a low laser power density of 5 W/cm\u003csup\u003e2\u003c/sup\u003e. Remarkably, the internal quantum efficiency reaches an impressive value of 1600% (the effective absorption rate of the 200 nm InGaAs is only 22%, as seen in Supplementary Fig. S6). As the laser power density increases, both the responsivity and quantum efficiency decline, yet even at a high laser power density of 500 W/cm\u003csup\u003e2\u003c/sup\u003e, the internal quantum efficiency was still as high as 170%. In comparison, the quantum efficiency of traditional vacuum phototubes is typically insufficient, with the highest attainable quantum efficiency being below 10% at near-infrared communication bands\u003csup\u003e38\u003c/sup\u003e. Thus, the proposed NACDs demonstrate a high quantum efficiency level in addition to miniaturization.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe above phenomenon can be explained as follows. The device can be simply modelled as two adjustable series resistances, namely the resistance at the nano-air channel (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e) and the resistance at the InP/InGaAs heterojunction (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec, the bias voltage and electric field were mainly divided by the two adjustable resistances, which dominated the I-V curves. At a low bias voltage of less than 2 V, the barrier of the nano-air channel was wide, resulting in a small field emission current and a high resistance (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e\u0026gt;\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e). The \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e undertook a larger voltage than \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e, so the electric field at the nano-air channel increased sharply with the bias voltage (the electric field intensity doubles from 1 V to 5 V). Consequently, the field emission current experienced a rapid increase at low bias voltages, corresponding to the first rapidly rising region of the I-V curves. Meanwhile, the electric field at the InP/InGaAs heterojunction increased gradually and higher than 0.5 MV/cm when the bias voltage was higher than 1 V, which surpassed the ionization multiplication threshold electric field of InGaAs and InP. As a result, the photogenerated carrier multiplication effect was triggered, facilitating a high quantum efficiency. As the bias voltage increases continuously, the field emission current increases nonlinearly, leading to a rapid decrease of \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e. Then the bias voltage and electric field would be mainly applied on the \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e when \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e\u0026gt;\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e, resulting in a moderate increase in the electric field applied to the nano-air channel, thereby leading to a field emission current saturation region. Meanwhile, the electric field at the InP/InGaAs heterojunction experienced a rapid increase from 1 MV/cm at 5 V to more than 2.5 MV/cm at 10 V, greatly enhancing the photogenerated carrier multiplication effect and leading to a rapid decrease of the \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e. The bias voltage and electric field would mainly applied on the \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e again. As a result, the photocurrents started to increase exponentially again at a large bias voltage, leading to a significant improvement in the device\u0026rsquo;s optical responsivity and quantum efficiency. In addition, the decline of the quantum efficiency at high laser power density can be attributed to the reduction of the electric field at the InP/InGaAs heterojunction (as seen in Supplementary Fig. S7) and the space charge limiting effect within the InGaAs absorption layer caused by the high photocarrier density.\u003c/p\u003e\n\u003ch3\u003eTHz Generation by the NACDs\u003c/h3\u003e\n\u003cp\u003eBeyond the large currents and high quantum efficiency, the repeatability and stability of the field emission currents are also essential for high-frequency operation and practical applications. Most of the previous NACDs with planar nano-tips usually suffered from the current degradation and tip destruction\u003csup\u003e8,32\u003c/sup\u003e. In contrast, our vertical NACDs exhibited excellent robustness, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea-c. The photoenhanced field emission currents showed a slight increase (less than 2.5%) with repeated testings, which could be attributed to the heat-assisted electron emission resulting from the heat accumulation effect due to continuous laser irradiation. Furthermore, periodic on-off laser tests revealed that both the field-emission currents and photoenhanced field emission currents feature excellent repeatability. The maximum photoenhanced field emission current can reach up to 1.56 mA at a bias voltage of 10 V and laser power density of 2000 W/cm\u003csup\u003e2\u003c/sup\u003e. Additionally, the long-term stability of the photoenhanced field emission currents in air was also examined, which shows no degradation over time but a slight increase in the initial phase and eventually stabilized due to the thermal equilibrium. The slight fluctuation of the currents was caused by the fiber vibration and the laser power fluctuation. The high stability of the photoenhanced field emission is due to the low voltage operation, which can significantly reduce the ion bombardment damage and increase the lifetime. The high-temperature stability of the field emission currents was further investigated as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed. At low voltages, the temperature dependence confirms the Schottky emission behavior. In contrast, at high voltage, the I-V curves exhibited reduced temperature dependence, consistent with F-N cold field emission behavior. It is worth noting that the current of our NACD at 8 V showed a slight fluctuation of about 10% from 25 ℃ to 200 ℃ (as seen in Supplementary Fig. S8) while the traditional semiconductor InGaAs photodetectors suffer from poor temperature stability. For example, the dark current of the InGaAs APD deteriorated by two orders of magnitude from 200 K to 340 K\u003csup\u003e39\u003c/sup\u003e. These results strongly underscore the superiority of our photo-enhanced NACDs for high-temperature operation. Above 200 ℃, thermally excited electrons could easily pass through the thin air potential barrier, making the thermally promoted field emission dominated the I-V curves.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe improved field emission currents, along with the high stability, make the proposed NACDs well-suited for high-frequency operation. Subsequently, the NACDs were employed to generate THz signals using a laser heterodyne measurement set-up, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee. Two tunable C-band CW lasers were used to produce a heterodyning pump beam, which was directed onto the NACDs by a single-mode fiber. An RF probe with a bias tee and a frequency analyzer were used to measure the output signals from the NACDs. By varying the wavelength difference between the two lasers, at the output end of the NACDs, THz signals ranging from 120 GHz to 260 GHz were obtained, as shown in Fig.\u0026nbsp;5b. The photomixing THz signals exhibited very stable and flat output within the test range, showing properties of ultra-wide bandwidth. The measured output power exhibits a maximum of approximately \u0026minus;\u0026thinsp;45 dBm at 200 GHz, which is mainly limited by the small field-emission photocurrent at the current stage. Although the power is still below the state-of-the-art terahertz photomixers such as uni-travelling-carrier PD (UTC-PD), it is worth noting that our NACDs exhibit 10 dB higher radiated output power than previously reported plasmonic internal-photoemission detectors\u003csup\u003e40\u003c/sup\u003e, which are suitable for on-chip, inter-chip, and near field terahertz wireless communication\u003csup\u003e41\u003c/sup\u003e. The output of the NACDs at higher frequencies is limited by the working frequency range of our RF probe. The linewidth of the generated THz signals is as narrow as 5 kHz, which is primarily determined by that of the lasers and can be further improved to obtain highly coherence and ultralow phase noise THz sources\u003csup\u003e42\u003c/sup\u003e. To the best of our knowledge, this is not only the first successful demonstration of NACDs as photomixers for THz generation, but also the first experimental realization of NACDs operating above 100 GHz, which opens up exciting prospects for the practical application of NACDs in high-frequency fields.\u003c/p\u003e \u003cp\u003eThe frequency response of the NACDs is mainly determined by two factors: the transit time and resistor-capacitor (RC) time constant. The total transit time of the NACDs comprises two components: the drift-diffusion time τ\u003csub\u003eInGaAs\u003c/sub\u003e within the InGaAs material and the ballistic transport time τ\u003csub\u003eAir\u003c/sub\u003e through the nano-air channel. The τ\u003csub\u003eInGaAs\u003c/sub\u003e is estimated to be about 1 ps for a 200 nm thick InGaAs layer (assuming that the average speed of the carriers in InGaAs is 2\u0026times;10\u003csup\u003e7\u003c/sup\u003e cm/s), while the τ\u003csub\u003eAir\u003c/sub\u003e is less than 75 fs. Thus, the transit time is mainly limited by the InGaAs photocathode. On the other hand, the RC time constant is primarily determined by the intrinsic capacitance, parasitic capacitance, and series resistance. In our NACDs, the RC time constant is the main limiting factor rather than the transit time. The field emission current can be further improved by energy-band engineering and work function engineering to reduce the series resistance. Consequently, the NACDs have the potential to generate even higher frequency and power output in the future, thus playing a critical role in high-speed wireless communication and high-resolution radar at THz bands\u003csup\u003e43\u0026ndash;45\u003c/sup\u003e. Moreover, benefited from the IC-compatible manufacturing process, our NACDs can be further integrated with lasers, optical delay lines, electrooptical modulators, and arrayed antennas, then used for compact and efficient chip-scale THz transmitter and beam steering\u003csup\u003e46,47\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe performances of our NACDs are compared with the various state-of-the-art NACDs reported in previous studies, as summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The field emission currents of typical NACDs with planar tips are usually of the order of milliamperes due to their small emission area\u003csup\u003e48,49\u003c/sup\u003e. In contrast, vertical configuration NACDs with long channel perimeter could obtain large emission currents up to 10 mA, but the device area was as large as several square millimeters, resulting in a large capacitance and low response speed with tens of nanoseconds\u003csup\u003e34\u003c/sup\u003e. Our NACDs with a reasonable anode diameter of 20 \u0026micro;m, strike a balance between size and performance. They exhibited a small capacitance approximately one hundred fF. Meanwhile, the photoenhanced field emission currents of our NACDs exceed 1.5 mA, which is one of the highest values among the state-of-the-art NACDs. Furthermore, our NACDs also achieves a record quantum efficiency among various photoenhanced NACDs.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePerformance comparison for state-of-the-art NACDs.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDevice structures\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eEmission current\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eQuantum efficiency\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eWorking frequency\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eYears\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVertical Si/SiO\u003csub\u003e2\u003c/sub\u003e/Al with 20 nm channel\u003csup\u003e4\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.8 \u0026micro;A at 2 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2012\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLateral Si tips with 150 nm channel\u003csup\u003e7\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e40 \u0026micro;A at 2 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2012\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLateral Si nanowire with 50 nm channel\u003csup\u003e6\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3 \u0026micro;A at 5 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2017\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLateral W tips with 11 nm channel\u003csup\u003e13\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.15 \u0026micro;A at 2 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2018\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVertical SiC/SiO\u003csub\u003e2\u003c/sub\u003e/Si with 200 nm channel\u003csup\u003e17\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e12 \u0026micro;A at 20 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2019\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLateral GaN tips with 30 nm channel\u003csup\u003e8\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.45 \u0026micro;A at 1 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2021\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLateral Au tip-to-edge with sub-10 nm channel\u003csup\u003e9\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1 \u0026micro;A at 2 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2021\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVertical Si/SiO\u003csub\u003e2\u003c/sub\u003e/Au with 80 nm channel\u003csup\u003e12\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e160 \u0026micro;A at 2 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2021\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVertical GaN/SiO\u003csub\u003e2\u003c/sub\u003e/Au with 50 nm channel\u003csup\u003e34\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e11 mA at 10 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2023\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLateral Au comb with 30 nm channel\u003csup\u003e32\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.7 \u0026micro;A at 8 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e85 MHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2023\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLateral Ga\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e nanowires with 50 nm channel\u003csup\u003e49\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.7 \u0026micro;A at 15 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2024\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVertical air-bridge with 50 nm channel\u003csup\u003e33\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.5 \u0026micro;A at 1 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e200 MHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2024\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePhotoenhanced vertical Si/SiO\u003csub\u003e2\u003c/sub\u003e/graphene with 23 nm channel\u003csup\u003e20\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.95 mA at 5 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e384% at 325 nm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2015\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePhotoenhanced vertical Si/SiO\u003csub\u003e2\u003c/sub\u003e/graphene with 20 nm channel\u003csup\u003e21\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.6 mA at 5 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e350% at 633 nm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2016\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePhotoenhanced vertical Au/Air/Au resonant surface with 200 nm channel\u003csup\u003e35\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.65 mA at 5 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5% at 785 nm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2016\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePhotoenhanced vertical Si/SiO\u003csub\u003e2\u003c/sub\u003e/Au nanoantennas with 30 nm channel\u003csup\u003e22\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.7 \u0026micro;A at 2 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4% at 800 nm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2020\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePhotoenhanced TiN planar tips array with 30 nm channel\u003csup\u003e49\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7 \u0026micro;A at 5 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.1% at 1.2 \u0026micro;m\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2021\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePhotoenhanced vertical Si tips/Air/CsPbBr\u003csub\u003e3\u003c/sub\u003e with 100 nm channel\u003csup\u003e50\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2 nA at 15 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e400% at 532 nm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e-\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2022\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003ePhotoenhanced vertical InP/InGaAs/SiO\u003c/b\u003e\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e\u003cb\u003e/CrAu with 50 nm channel (This work)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e1.56 mA at 10 V\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e1600% at 1.55 \u0026micro;m\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e120 GHz-260 GHz\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn this work, we proposed a new concept of utilizing photoenhanced field-emission NACDs to work as photomixers, which brings forward a fresh perspective for the high-frequency applications of nano-air-channel devices. A vertical configuration device with in-plane nano-air channels of 50 nm was introduced to achieve efficient field emission, resulting in a 50-fold increase in current compared to traditional vertical NACDs. A scalable and high-yield fabrication approach compatible with conventional semiconductor processing techniques was developed for wafer-scale production of the proposed NACDs, which unlocks the potential for seamless integration with optoelectronic integrated circuits. The working mechanism and photosensitive field emission behavior of the proposed NACDs were systematically analyzed, providing valuable insights for device design. By leveraging the photoenhanced field emission effect, ultra-low turn-on voltage down to ~\u0026thinsp;0.5 V and high field emission current up to 1.56 mA was achieved. Furthermore, long-term stability and high-temperature reliability investigations demonstrate the superiority of cold field emission NACDs. Notably, the distinguishing feature of the nano-air channel lies in its ability to generate substantial local electric fields within both the nano-air channel and the InP/InGaAs heterojunction photocathode at notably low bias voltages. This facilitates efficient field emission of electrons and multiplication of photogenerated carriers, culminating in an exceptional responsivity reaching up to 4 A/W and an internal quantum efficiency as high as 1600% at 1550 nm. As a result, the high-efficiency photoenhanced field emission provides an effective approach to reduce the impedance of NACDs, facilitating high-speed and high-frequency operation. With these advancements, we have demonstrated the generation of coherent THz signals from 120 GHz to 260 GHz, which is the first successful experimental demonstration of NACDs operating beyond 100 GHz known to date. The proposed NACDs provide a highly promising solution for photonic THz generators, thereby facilitating and advancing the development of THz-wireless and fiber transparent seamless integration communication links, 6G networks and high-resolution radar.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e \u003cb\u003eDevice Simulation.\u003c/b\u003e The electric field distribution, energy band structures, optical reflectivity and absorption were simulated using a commercial TCAD tool (Silvaco). Structural parameters and doping density used in the simulation were consistent with the actual fabricated device. Material parameters such as dielectric constant and work function were based on the default values provided by the Silvaco internal library. The Fowler\u0026ndash;Nordheim (F\u0026ndash;N) tunneling model was utilized in the simulation.\u003c/p\u003e \u003cp\u003e \u003cb\u003eDevice Fabrication.\u003c/b\u003e A 200 nm high-doping (5e18 cm\u003csup\u003e-3\u003c/sup\u003e) p-type InP Ohmic contact layer followed by a 200 nm low-doping (1e16 cm\u003csup\u003e-3\u003c/sup\u003e) p-type In\u003csub\u003e0.53\u003c/sub\u003eGa\u003csub\u003e0.47\u003c/sub\u003eAs film was grown on 2 inches semi-insulating InP substrates by metal-organic chemical vapor deposition (MOCVD) method. The mesa and electrode patterns were formed by UV photolithography with a resolution of about 1 \u0026micro;m. The In\u003csub\u003e0.53\u003c/sub\u003eGa\u003csub\u003e0.47\u003c/sub\u003eAs and InP mesa were fabricated by ICP etching and wet etching techniques. A 50 nm SiO\u003csub\u003e2\u003c/sub\u003e film was deposited on the InGaAs by atomic layer deposition (ALD) and a 200 nm CrAu anode was deposited on the SiO\u003csub\u003e2\u003c/sub\u003e by electron beam evaporation (EBE). Then, the electrode and CPW were constructed by ion beam etching (IBE). Wet etching was employed to remove part of the SiO\u003csub\u003e2\u003c/sub\u003e between the InGaAs photocathode and the CrAu anode to form a nano-air channel by using a BOE solution. The diameter of the active area is 20 \u0026micro;m and the depth of the nano-air channel is about 200 nm. Finally, the backside thinning and polishing were carried out to thin the substrate down to 300 \u0026micro;m.\u003c/p\u003e \u003cp\u003e \u003cb\u003eMeasurements Setup.\u003c/b\u003e The morphology of the NACDs was characterized by an SEM system (ZEISS Gemini 300). The optical responses and photomixing of the NACDs were measured by an RF probe station equipped with a single-mode optical fiber and a THz Probes (GGB Model 220), a SourceMeter SMU instrument (Keithley 2450), two tunable lasers (Pure Photonics PPCL550), and a frequency analyzer (Ceyear 4051H). The heterodyning laser spectra were measured by a high-resolution optical spectrum analyzer (Yokogawa AQ6370D-22).\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors would like to acknowledge the financial support from the National Natural Science Foundation of China (No. 62474032, 61704162, 61704163) and the Fundamental Research Funds for the Central Universities \u0026nbsp;(Grant No. ZYGX2021J028). The authors would also like to thank Dr. Qian Li from MTRC for her help in device simulation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupplementary information\u003c/strong\u003e is available for this paper at http://XXX.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eHenry Huang X M, Zorman C A, Mehregany M, et al. Nanodevice motion at microwave frequencies. \u003cem\u003eNature\u003c/em\u003e\u003cstrong\u003e421\u003c/strong\u003e, 496-496 (2003).\u003c/li\u003e\n\u003cli\u003eAtabaki A H, Moazeni S, Pavanello F, et al. 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High Responsivity Vacuum Nano-Photodiode Using Single-Crystal CsPbBr\u003csub\u003e3\u003c/sub\u003e Micro-Sheet. \u003cem\u003eNanomaterials\u003c/em\u003e\u003cstrong\u003e12\u003c/strong\u003e, 4205, (2022).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"nano-air channel, field emission, Photoenhanced, photomixer, terahertz","lastPublishedDoi":"10.21203/rs.3.rs-5265138/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5265138/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eNano-air-channel devices (NACDs), characterized by scattering-free ballistic electron transport in a quasi-vacuum channel, provide a new opportunity for vacuum electronics and nanoelectronics. However, enabling the NACDs for high-frequency operation is still a great challenge due to the low field-emission current and high impedance. Herein, for the first time, field-emission NACDs are demonstrated as photomixers capable of generating coherent terahertz (THz) signals from 120 GHz to 260 GHz. This achievement marks the first successful experimental demonstration of NACDs operating beyond 100 GHz. Vertical configuration NACDs with InP/InGaAs heterojunction photocathodes and 50 nm in-plane nano-air channels are designed and fabricated using a wafer-scale manufacturing process. The field-emission currents show an impressive 375-fold increase with a low-power 1550 nm CW laser irradiation. A remarkably high internal quantum efficiency of up to 1600% is achieved by exploiting the carrier multiplication effect within the heterojunction photocathodes. Furthermore, the NACDs exhibit reproducible photo-switching behavior and highly stable field-emission current even at temperatures as high as 250 ℃. These findings position NACDs as new promising candidates for photonic THz generators, opening up an exciting application prospect for NACDs in THz high-speed wireless communications, 6G networks and high-resolution radar.\u003c/p\u003e","manuscriptTitle":"Photoenhanced Field-Emission Nano-Air-Channel Devices for Terahertz Generation","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-11-27 17:02:22","doi":"10.21203/rs.3.rs-5265138/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a557e2b9-f753-4172-85fe-34bbda9e5b82","owner":[],"postedDate":"November 27th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-12-03T05:50:39+00:00","versionOfRecord":[],"versionCreatedAt":"2024-11-27 17:02:22","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5265138","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5265138","identity":"rs-5265138","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}