Wafer scale III-nitride deep-ultraviolet vertical-cavity surface-emitting lasers featuring nanometer-class control of cavity length

Wafer scale III-nitride deep-ultraviolet vertical-cavity surface-emitting lasers featuring nanometer-class control of cavity length | 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 Wafer scale III-nitride deep-ultraviolet vertical-cavity surface-emitting lasers featuring nanometer-class control of cavity length Fujun Xu, Chen Ji, Jiaming Wang, Lisheng Zhang, Jing Lang, Ziyao Zhang, and 11 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7024020/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 III-nitride AlGaN-based deep-ultraviolet vertical-cavity surface-emitting lasers (DUV-VCSELs) have shown a great application potential in optical atomic clock, maskless photolithography, etc. Nevertheless, the detuning issue owing to the uncontrolled cavity length, i.e. the difference between the resonance wavelength and gain peak, severely impairs the device performance. Herein, a DUV-VCSEL strategy featuring the uniform nanometer-class control of the cavity length in a 4 inch wafer is proposed in the DUV framework based on GaN templates, which ensures the wafer-scale removal of the sapphire substrates by laser lift-off, and then provides space for the subsequent deposition of dielectric distributed Bragg reflector (DBR). It is more significant that the strategy brings about a GaN/AlGaN sharp interface with an Al composition difference up to 80%, whereby self-terminated etching with an ultrahigh selectivity of 100:1 is achieved. The cavity length can hence be accurately determined by epitaxy itself instead of fabrication process, so as to minimize the detuning. As such, 285.6 nm optically pumped DUV-VCSELs with double dielectric DBRs are fabricated, exhibiting a record low threshold of 0.38 MW/cm 2 as well as a narrow linewidth of 0.11 nm. What’s more, the lasing wavelength varies within 1.9 nm across the 4 inch wafer, indicating a cavity length variation of only 0.81%. This work establishes a promising strategy for III-nitride DUV-VCSELs, which will definitely speed up the development of devices featuring high performance and scalability. Physical sciences/Materials science/Materials for optics/Lasers, LEDs and light sources/Semiconductor lasers Physical sciences/Optics and photonics/Lasers, LEDs and light sources/Semiconductor lasers Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Vertical-cavity surface-emitting lasers (VCSELs) have attracted much attention owing to their advantages of circular far field distribution, single longitudinal mode emission, and two-dimensional integration capability, leading to a rapidly growing billion-dollar market in data communication, sensing, and display 1-3 . From III-arsenides, III-phosphides to III-nitrides, the lasing wavelength blue-shifts from infrared/red 4-6 to blue/ultraviolet (UV) 7-9 , which further expands the application scenarios to high-resolution 3D nanoprinting, maskless photolithography, and miniature atomic clocks 10-12 . Nevertheless, a serious challenge for III-nitride VCSELs is the realization of distributed Bragg reflectors (DBRs) with a high reflectivity. Unlike the mature AlGaAs/GaAs DBRs, the lower refractive index difference and larger lattice mismatch between III-nitrides make it quite hard to achieve a crack-free epitaxial DBR (e.g. AlN/GaN DBR) with a high reflectivity as well as a wide stopband 13 . The situation is even worse when it enters the UV band, since AlGaN with high Al composition is essential in AlGaN/Al(Ga)N DBRs to avoid the light absorption, that further reducing the refractive index difference. An alternative and effective solution is the adoption of dielectric DBRs, e.g. TiO 2 /SiO 2 14 , ZrO 2 /SiO 2 15 , Ta 2 O 5 /SiO 2 16 and HfO 2 /SiO 2 17 . In order to furthest minimize the mirror losses and thus decrease the threshold, VCSELs with double dielectric DBRs is proposed, which, however, brings about new challenges: (i) how to remove the substrate without cracks and subsequently obtain a smooth exposed surface for the deposition of the second DBR 18-20 ; (ii) how to precisely control the cavity length after the substrate removal, so as to reduce the detuning between the resonance wavelength and gain peak 21 . In terms of the substrate removal, a series of approaches have been proposed, including laser lift-off (LLO) 9,18,20 , chemical/electrochemical lift-off 8,22,23 and etch thinning 19 . Wherein, LLO is the most promising candidate by comprehensively considering the productivity, yield and cost, hence widely adopted in GaN-based visible optoelectronics 24-26 . It is, however, fairly hard to apply LLO in AlGaN-based DUV devices grown on AlN templates. The key issue is that the precipitated Al from AlN decomposition is rigid, resulting in serious cracking of the lifted-off epilayers 27 . A ground-breaking DUV optoelectronic framework featuring structural stacking on GaN templates has been demonstrated in our previous work, where the 4 inch sapphire substrate can be entirely removed by LLO without cracks, and then the wafer-scale fabrication of 280 nm vertical injection DUV light-emitting diodes is realized 28 . Definitely, the framework should also be applicable to the fabrication of DUV-VCSELs. It is worth noting that the substrate removal by LLO brings about a rough exposed surface, which will cause severe optical scattering losses in the cavity and then make lasing difficult 29 . Chemical mechanical polishing (CMP) is hence widely employed to smoothen the surface as well as control the cavity length 9,30,31 , although it is not really satisfactory. On the one hand, the lifted-off epilayer generally curls owing to the residual stress, consequently, the uniformity of the cavity length by CMP is quite poor in a large-size wafer 9 . On the other hand, the thickness control of CMP cannot meet the precision requirement of cavity, wherein the uncontrolled cavity length will result in the detuning between the resonance wavelength and gain peak, and then lead to an evident increase in the threshold 21 . This issue has long plagued III-nitride VCSELs, from InGaN-based visible to AlGaN-based UV ones. Besides CMP, an approach of photo-assisted chemical etching is proposed to smooth the exposed surface, by which a measure of reduction in the roughness is achieved 32 . The high chemical reactivity of N-polar surface in aqueous acid/alkali, however, plays a negative role and restricts the effect of this approach. In this work, a DUV-VCSEL strategy featuring the nanometer-class control of the cavity length is proposed in the DUV optoelectronic framework based on GaN templates, which ensures the LLO removal of the sapphire substrates for the deposition of dielectric DBR, meanwhile maintains a high radiative recombination efficiency for lasing. After the sapphire removal, a self-terminated dry etching technology is developed, whereby the cavity length can be accurately determined by epitaxy instead of fabrication process, so as to minimize the detuning between the resonance wavelength and gain peak. Moreover, the etching brings about a smooth surface to reduce the optical scattering losses at the cavity/DBR interface. As such, a record low threshold 0.38 MW/cm 2 is realized in 285.6 nm optically pumped DUV-VCSELs with double dielectric DBRs, and a cavity length variation of only 0.81% within a total length of 1.08 μm is demonstrated across the 4 inch wafer. Epitaxy and optical properties of the cavity The epitaxy and fabrication of the DUV-VCSELs are outlined in Fig. 1. Our proposed DUV framework is adopted for the sake of removing the sapphire substrate, and a 9λ cavity for DUV lasing (~280 nm) is grown on GaN templates. The cavity consists of the decoupling structure 28 (a 90 nm-thick Al 0.8 Ga 0.2 N pre-crack layer and an 870 nm-thick Al 0.65 Ga 0.35 N healing layer), 8-pair Al 0.5 Ga 0.5 N/Al 0.37 Ga 0.63 N multiple quantum wells (MQWs) as well as a 45 nm-thick Al 0.5 Ga 0.5 N capping layer in sequence. Wherein the position of MQWs is thoroughly designed. On the one hand, both the pre-crack and healing layers in the decoupling structure are thick enough to fully release the lattice-mismatch-induced strain between the GaN template and MQWs, and subsequently rebuild a flat surface for MQWs; meanwhile on the other hand, the MQWs are placed on an optical antinode of the standing wave according to the theoretical calculations as shown below, which can help to reduce the threshold 1 . Figure 1a presents the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of MQWs, where the thicknesses of the barriers and wells are determined to be 9 and 1.9 nm, respectively. The radiative recombination efficiency (RRE), as one of the decisive factors for lasing, is further evaluated via the temperature-dependent photoluminescence (PL) measurements from 10 to 300 K (Fig. 1b). Assuming the non-radiative recombination centers frozen at cryogenic temperature (10 K), the MQWs exhibit a room-temperature RRE of 71.5%, almost the same as our previous report 28 . Moreover, the optical polarization of MQWs is characterized by polarization-dependent PL, where the transverse-electric (TE) and transverse-magnetic (TM) polarized light is separately collected as shown in Fig. 1c. It is found that the TE light hugely dominates, and the degree of polarization (DOP) is estimated as 60.6%, assuring the feasibility of the surface-emitting lasing. The fabrication of DUV-VCSELs starts with the bottom HfO 2 /SiO 2 DBR deposited on the as-grown surface, followed by the wafer bonding and LLO of the sapphire substrate. The subsequent removal of the exposed GaN template is the key in precisely controlling the cavity length. An inductively coupled plasma (ICP) etching process with ultrahigh selectivity is then developed, by which the etching front self-terminates at the interface between GaN and Al 0.8 Ga 0.2 N (the pre-crack layer). In other words, the cavity length can be determined by epitaxy instead of etching, hence significantly enhancing the precision and uniformity of the cavity length. Meanwhile, the self-terminated etching brings about a smooth surface as shown below, which lays a good foundation for the deposition of the top HfO 2 /SiO 2 DBR. The reflection spectra of the top and bottom DBRs are recorded in Fig. 1e and f, respectively, and the corresponding transmission spectra can be found in Supplementary Fig. S1. Both DBRs show a high reflectivity exceeding 96% within the wavelength range of 280–300 nm, consistent with the theoretical expectations. Self-terminated etching of N-polar GaN over Al 0.8 Ga 0.2 N It should be noted the exposed GaN surface after the LLO removal of sapphire is N-polar, exhibiting greater chemical reactivity in comparison with the metal-polar one 33 . As a consequence, it is more difficult to precisely control the cavity length by etching; worse still, the realization of a smooth surface for the top DBR seems to be an illusion. Fortunately, an AlGaN layer with extremely high Al composition of 80% (the pre-crack layer) is adjacent to the GaN template (Supplementary Fig. S2), making it possible to realize a highly selective etching to address these issues. Herein, ICP etching is performed in both chlorine- and fluorine-based chemistry, and the etching rates of GaN and Al 0.8 Ga 0.2 N (Fig. 2a) are evaluated by atomic force microscopy (AFM), where photoresist is employed as the mask of the unetched region. It is demonstrated that in comparison with Cl 2 /BCl 3 mixtures, the etching remarkably decelerates in SF 6 /BCl 3 ones, since the fluorides of Ga and Al have much higher boiling/sublimation points than the chlorides 34 . More importantly, the formation of nonvolatile AlF x almost terminates the Al 0.8 Ga 0.2 N etching with a quite slow rate of only 1 nm/min, leading to an ultrahigh selectivity of 100:1 for N-polar GaN relative to Al 0.8 Ga 0.2 N. Figure 2b presents the AFM image of the etched Al 0.8 Ga 0.2 N for 4 mins in SF 6 /BCl 3 mixtures, where the etched and masked regions can be easily distinguished by the boundary of the photoresist residue. To determine the etching rate, a line scan of the relative height is performed along the white arrow across the boundary. The etched depth is then determined to be ~4 nm in Fig. 2c; as such, the Al 0.8 Ga 0.2 N etching rate in SF 6 /BCl 3 mixtures is 1 nm/min. Such high etching selectivity greatly benefits the precise control and repeatability of the cavity length during the fabrication of DUV-VCSELs. Moreover, it can bring about a smooth surface for the subsequent DBR. The surface morphology before/during etching is characterized by scanning electron microscopy (SEM) and AFM, as shown in Fig. 2d-i. A typical rough surface is present in the lifted-off epilayer in Fig. 2d, attributed to the spot-to-spot irradiation in the LLO process; meanwhile in a scan area of 3×3 μm 2 by AFM (Fig. 2g), dense and whisker-like microcolumns are observed as reported 35 , leading to a root-mean-square (RMS) roughness of 12.8 nm. This morphology is almost inherited during GaN etching (Fig. 2e and h), since there is no etching selectivity inside the GaN template. Until the etching front reaches the GaN/Al 0.8 Ga 0.2 N interface, exposed Al 0.8 Ga 0.2 N in the valley self-terminates the process, while residual GaN is still rapidly etched. As a consequence, the surface gradually flattens out, and a roughness of 0.7 nm is eventually achieved (Fig. 2i). It is worth noting that small protrusions can be observed in Fig. 2f, which are regularly distributed along some straight lines. These lines are actually corresponding to the pre-cracks, and the protrusions are AlGaN grown in the pre-cracks during healing (Supplementary Fig. S3). In consideration of the quite small surface coverage of pre-cracks, it is convinced that these protrusions have less effects on the subsequent deposition of the top DBR. Prior to the deposition of the top DBR, the impacts of LLO on the optical properties of MQWs should be revealed, since the laser reaching MQWs during LLO may result in the formation of additional defects for non-radiative recombination 36,37 . Herein the fluence of the KrF excimer laser employed in the LLO process is decreased as much as possible, so as to reduce the potential damage. Time-resolved PL measurements at room temperature are further carried out to investigate the lifetime of carriers in MQWs 38 , which can quantify the variation of the non-radiative recombination lifetime (i.e. defects) between as-grown and processed MQWs. According to the images recorded by a streak camera (Fig. 3a and b), a redshift of the peak wavelength is observed from 283 nm (as-grown) to over 285 nm (processed), attributed to the variation of residual strain in the epilayer owing to the processes of bonding and LLO (details in Supplementary Fig. S4 and S5) 39 . Notably, the PL decay profiles at the peak wavelengths are shown in Fig. 3c, where the carrier lifetime is extracted to be ~660 ps for both samples. This indicates that the LLO and ICP processes in this study do not introduce additional point defects into MQWs, assuring the cavity quality in DUV-VCSELs. Performance of the DVU-VCSELs The 4 inch wafer-scale fabrication of DUV-VCSELs is further accomplished along with the deposition of the top DBR. According to the cross-sectional HAADF-STEM measurement shown in Fig. 4a, the cavity length is determined to be 1.08 μm, almost equivalent to the sum of the epitaxial thickness from the Al 0.8 Ga 0.2 N pre-crack layer to the Al 0.5 Ga 0.5 N capping one. That demonstrates the nanometer-class control of the cavity length through the combination of LLO and self-terminated ICP etching. Also it is found that both the top and bottom DBRs present flat interfaces, attributed to the aforementioned smooth etched and as-grown surface, respectively. The longitudinal optical field inside the DUV-VCSEL structure is then calculated by the transfer matrix method, as shown in Fig. 4b. On the one hand, most of the QWs align with the optical antinode as designed to obtain a great gain; meanwhile, both interfaces between cavity and top/bottom DBR are placed at the optical node, beneficial for the reduction of the optical scattering loss 29 . The calculation of the cavity reflection spectrum is further carried out (Fig. 4c), where multi-mode emission is observed due to the relatively long cavity length of 1.08 μm. Considering that the spontaneous emission peak locates at over 285 nm (Fig. 3b), a stimulated emission one at ~285 nm can be expected. Eventually, DUV-VCSELs are optically pumped at room temperature by a 266 nm laser with 5 ns pulse duration and 20 Hz repeat frequency (details in Supplementary Fig. S7). As the pump power increases, a stimulated emission peak with a linewidth of 0.11 nm is highlighted at 285.6 nm in Fig. 4d, bringing about a Q factor of 2596. The dependence of the integrated intensity of the stimulated peak on the pump power density is further depicted (Fig. 4e), where a clear kink (the threshold power density, P th ) around 0.38 MW/cm 2 is observed. The record low threshold in DUV-VCSELs is undoubtedly attributed to the precise control of the cavity length 8,9,21,23,40 ; as a consequence, the resonance wavelength is perfectly matched with the gain peak (Supplementary Fig. S8). Besides, more dies across the 4 inch wafer are characterized to investigate the uniformity of lasing wavelength/cavity length, as shown in Fig. 4f. Specifically, 4 random dies are picked up in each quadrant, and the lasing wavelength shifts in the range of 284.8–286.7 nm in the four quadrants (typical stimulated emission spectra in Supplementary Fig. S9), corresponding to a cavity length variation of 8.7 nm, ~0.81% of the total length (Supplementary Fig. S10). In summary, a DUV-VCSEL strategy featuring the precise and uniform nanometer-class control of the cavity length in the 4 inch wafer is demonstrated in the DUV framework based on GaN templates, which ensures the LLO removal of the sapphire substrates, and then provides space for the subsequent deposition of dielectric DBR. It is more significant that the strategy brings about a GaN/AlGaN sharp interface with an Al composition difference up to 80%, whereby self-terminated etching with an ultrahigh selectivity of 100:1 is achieved for the exposed N-polar GaN template relative to the Al 0.8 Ga 0.2 N pre-crack layer after the sapphire removal. The cavity length, as a consequence, can be accurately determined by epitaxy instead of fabrication process, which helps to minimize the detuning between the resonance wavelength and gain peak. Meanwhile, the etching brings about a smooth surface with the RMS roughness of 0.7 nm in an area of 3×3 μm 2 , efficiently reducing the optical scattering losses at the interface between cavity and the top DBR. As such, a record low threshold of 0.38 MW/cm 2 as well as a narrow linewidth of 0.11 nm is realized in the 285.6 nm optically pumped DUV-VCSELs with double dielectric DBRs. What’s more, the lasing wavelength varies within 1.9 nm across the 4 inch wafer, corresponding to a cavity length variation of 8.7 nm, ~0.81% of the total length of 1.08 μm. This work establishes a promising strategy for III-nitride DUV-VCSELs, which will greatly promote the development of this field and bring about devices featuring high performance and scalability. Methods MOCVD Growth of DUV-VCSELs. The DUV-VCSEL structures in this study were grown on 4 inch double-polished sapphire substrates, by an Aixtron 1×4 in. close-coupled showerhead MOCVD system, and repeated by an AMEC Prismo HiT3 (4×4 in.) MOCVD system. A 4 μm-thick GaN template was first grown by the two-step method, followed by a 90 nm-thick Al 0.8 Ga 0.2 N pre-crack layer and an 870 nm-thick Al 0.65 Ga 0.35 N healing layer grown at 1075°C and 1095°C, respectively. Then, 8-pair 9 nm/1.9 nm Al 0.5 Ga 0.5 N/Al 0.37 Ga 0.63 N MQWs as well as a 45 nm-thick Al 0.5 Ga 0.5 N capping layer were grown in sequence. Fabrication of DUV-VCSELs. 8.5-pair 35.5 nm/40.8 nm alternating HfO 2 /SiO 2 were deposited on the as-grown epitaxial surface as the bottom DBR. Then, the wafer was bonded to a Si submount, followed by the laser lift-off process by employing a 248 nm KrF excimer laser. After the removal of sapphire, the exposed N-polar GaN surface was cleaned in the HCl solution for 1 min to remove the Ga metal from GaN decomposition. The residual GaN layer was then etched down to the Al 0.8 Ga 0.2 N pre-crack layer by ICP. Eventually, 7.5-pair HfO 2 /SiO 2 were deposited on the N-polar surface as the top DBR. The SEM image of DUV-VCSELs is shown in Supplementary Fig. S11. Characterization. TEM (Thermo Scientific Themis Z STEM operated at 200 kV) was employed to reveal the control of cavity length in this study, where the TEM specimen was prepared by focused ion beam (FIB, Thermo Scientific Helios G4 HX Dual Beam). The surface morphology of wafers was evaluated by AFM (Bruker Dimension Icon) and SEM (FEI Nova NanoSEM 430). Reflection and transmission spectra were recorded by Agilent Cary 7000 UV-Vis-NIR spectrophotometry. Temperature-dependent PL was characterized by a homemade system at Peking University, where a 213 nm laser (Xiton Photonics Impress 213) was employed as the excitation source. Temperature-dependent measurements were performed by employing a closed-cycle helium cryostat (JANIS SVT-400) attached to the temperature controller (Scientific Instruments 9700). Time-resolved as well as polarization-dependent PL was characterized by a homemade system at Peking University, where a 266 nm laser (Coherent Chameleon Ultra II) was employed as the excitation source, and a streak camera (Hamamatsu C10910-03) and a spectrometer (Horiba iHR550) were used for time-resolved and polarization-dependent detection, respectively. The optical pump of DUV-VCSELs was performed by a homemade system at Xiamen University, and a 266 nm laser with 5 ns pulse duration and 20 Hz repeat frequency was employed as the pump source. Declarations Competing Interests The authors declare no competing interests. Author Contributions C.J. and J.W. conceived the experiments. C.J., J.W., C.Z.J. and J.Z. grew the samples and performed relevant measurements. F.X., L.Z., X.Y., N.T., X.W., W.G. and B.S. gave support in the measurements and analyses. C.J., J.W. and J.L. performed device fabrication under X.K. and Z.Q. supervision. J.W. wrote the manuscript with the assistance of F.X., W.G. and B.S. All authors discussed the results and commented on the manuscript. Acknowledgments The authors thank Prof. Baoping Zhang and Prof. Yang Mei from Xiamen University for the optically pumping measurements. This work was supported by the National Key Research and Development Program of China (2023YFB3609700 to F.X.), the National Natural Science Foundation of China (62234001 and 61927806 to B.S.; 62135013 to F.X.; 62374007 and 62422401 to J.W.; 62204005 to J.L.), and the Basic Research Program of Jiangsu (BK20243027 to F.X.). Data Availability Data are available from the corresponding authors upon request. Source data are provided with this paper. References Michalzik, R. VCSELs: fundamentals, technology and applications of vertical-cavity surface-emitting lasers. Springer-Verlag Berlin Heidelberg 2013. Koyama, F. Recent advances of VCSEL photonics. J. Lightwave Technol. 24 , 4502–4513 (2006). Liu, A., Wolf, P., Lott, J. 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Electrochemical etching of AlGaN for the realization of thin-film devices. Appl. Phys. Lett. 115 , 182103 (2019). Zheng, Z., Wang, Y., Hoo, J., Guo, S., Mei, Y., Long, H., Ying, L., Zheng, Z. & Zhang, B. High-quality AlGaN epitaxial structures and realization of UVC vertical-cavity surface-emitting lasers. Sci. China Mater. 66 , 1978–1988 (2023). Additional Declarations There is NO Competing Interest. Supplementary Files SupplementaryInformationNCOMMS2547463.docx Supplementary Information 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. 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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-7024020","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":479594940,"identity":"3df9a938-7dce-4ec0-b4a1-cf8522ea763c","order_by":0,"name":"Fujun Xu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAvUlEQVRIiWNgGAWjYBAC9oYDDAwfGBJAbAPitPAcOMDAOINELQwMzDykaWE8YyZtuyMtsYG9eZsEQ80dIrQwHEuTzj2Tk9jAc6xMguHYM8Ja7BkOH5PObatIbJDIMZNgbDhMjC0H26QtQVrk3xCtBWgLYxvQYRI8RGs5lmzZeybNuI0nrdgi4RgxWiTOGN74uSNZtp/98MYbH2qI0MIgcYCBgbGBgYENxEkgQgMDA38DRMsoGAWjYBSMApwAALdbOHuE/W3zAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-0751-6566","institution":"Peking University","correspondingAuthor":true,"prefix":"","firstName":"Fujun","middleName":"","lastName":"Xu","suffix":""},{"id":479594941,"identity":"4698a491-eb0d-47ec-8cf8-5cfbb525c654","order_by":1,"name":"Chen Ji","email":"","orcid":"","institution":"Peking University","correspondingAuthor":false,"prefix":"","firstName":"Chen","middleName":"","lastName":"Ji","suffix":""},{"id":479594942,"identity":"9a591f8a-14be-49e5-80ac-dc27e297af23","order_by":2,"name":"Jiaming 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University","correspondingAuthor":false,"prefix":"","firstName":"Xinqiang","middleName":"","lastName":"Wang","suffix":""},{"id":479594955,"identity":"f362920b-527e-4505-9d0d-fec3c01833d5","order_by":15,"name":"Weikun Ge","email":"","orcid":"","institution":"Peking University","correspondingAuthor":false,"prefix":"","firstName":"Weikun","middleName":"","lastName":"Ge","suffix":""},{"id":479594956,"identity":"991f3a5f-f025-4184-9ae5-0765e28e437d","order_by":16,"name":"Bo Shen","email":"","orcid":"https://orcid.org/0000-0003-2786-8400","institution":"Peking University","correspondingAuthor":false,"prefix":"","firstName":"Bo","middleName":"","lastName":"Shen","suffix":""}],"badges":[],"createdAt":"2025-07-02 01:10:21","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7024020/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7024020/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":85925115,"identity":"570f9767-3da9-4354-9d91-1833d25e010c","added_by":"auto","created_at":"2025-07-03 08:34:13","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":5749381,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEpitaxy and fabrication of DUV-VCSELs. a,\u003c/strong\u003e Cross-sectional HAADF-STEM image of MQWs in the cavity. \u003cstrong\u003eb,\u003c/strong\u003e Temperature-dependent PL spectra of MQWs in Panel a. \u003cstrong\u003ec,\u003c/strong\u003ePolarization-dependent PL spectra of MQWs in Panel a. \u003cstrong\u003ed,\u003c/strong\u003e Schematic illustration of the fabrication of DUV-VCSELs with double dielectric DBRs. \u003cstrong\u003ee,f,\u003c/strong\u003e Reflection spectra of the top and bottom DBRs, respectively.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7024020/v1/d6a546c4c0b3cd39f21b1b78.png"},{"id":85925118,"identity":"ef3db118-0efd-4dac-8d87-ada64f998066","added_by":"auto","created_at":"2025-07-03 08:34:13","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":5899768,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSelf-terminated etching of N-polar GaN over Al\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e0.8\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eGa\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e0.2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eN. a,\u003c/strong\u003e Etching rates of N-polar GaN and Al\u003csub\u003e0.8\u003c/sub\u003eGa\u003csub\u003e0.2\u003c/sub\u003eN in the Cl\u003csub\u003e2\u003c/sub\u003e/BCl\u003csub\u003e3\u003c/sub\u003e and SF\u003csub\u003e6\u003c/sub\u003e/BCl\u003csub\u003e3\u003c/sub\u003e mixtures. \u003cstrong\u003eb,\u003c/strong\u003e The AFM image of the etched Al\u003csub\u003e0.8\u003c/sub\u003eGa\u003csub\u003e0.2\u003c/sub\u003eN for 4 mins in SF\u003csub\u003e6\u003c/sub\u003e/BCl\u003csub\u003e3\u003c/sub\u003e mixtures, where the right half is masked by photoresist during etching. \u003cstrong\u003ec,\u003c/strong\u003e The line scan of relative height along the white arrow in Panel b. \u003cstrong\u003ed,e,f,\u003c/strong\u003e Surface morphology of the exposed GaN surface (after LLO), etched GaN and etched Al\u003csub\u003e0.8\u003c/sub\u003eGa\u003csub\u003e0.2\u003c/sub\u003eN characterized by SEM, respectively. \u003cstrong\u003eg,h,i,\u003c/strong\u003e Surface morphology by AFM corresponding to Panels d, e, and f, respectively.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7024020/v1/fd2008a00ef44a6cb6e3d79f.png"},{"id":85926642,"identity":"8e6b9e56-81e8-49ae-800c-74bb50d5303a","added_by":"auto","created_at":"2025-07-03 08:42:13","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":2300554,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eImpact of LLO and etching on the optical properties of MQWs. a,b,\u003c/strong\u003eTime-resolved PL recorded at room temperature for MQWs before and after LLO/ICP, respectively. \u003cstrong\u003ec,\u003c/strong\u003e PL decay profiles at the peak wavelengths in Panels a and b.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7024020/v1/169be203562ec855aa4ae84f.png"},{"id":85926641,"identity":"c140a0a3-1dce-4443-a7fe-445b135b087b","added_by":"auto","created_at":"2025-07-03 08:42:13","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":2651347,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePerformance of DUV-VCSELs. a,\u003c/strong\u003e Cross-sectional HAADF-STEM image of the device. \u003cstrong\u003eb,\u003c/strong\u003e Calculation result of the optical field inside the cavity. \u003cstrong\u003ec,\u003c/strong\u003eCalculated cavity reflection spectrum. \u003cstrong\u003ed,\u003c/strong\u003ePL emission spectra at room temperature under different optical pump power densities. \u003cstrong\u003ee,\u003c/strong\u003e Dependence of the integrated intensity of stimulated peak on the pump power density. \u003cstrong\u003ef,\u003c/strong\u003e Lasing wavelengths of 4 random dies in each quadrant of the 4 inch wafer.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7024020/v1/5b01529846ab92efea287806.png"},{"id":87606847,"identity":"1cb7321c-8e5c-40f9-b9d4-bd24bdf92751","added_by":"auto","created_at":"2025-07-25 18:38:28","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":17320366,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7024020/v1/8abc6b62-1e2c-4a6f-ae75-a43a92d42155.pdf"},{"id":85925123,"identity":"96f744eb-25ed-4f12-8ebc-c6dcabb5adab","added_by":"auto","created_at":"2025-07-03 08:34:13","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":10280470,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"SupplementaryInformationNCOMMS2547463.docx","url":"https://assets-eu.researchsquare.com/files/rs-7024020/v1/96232875c2fa5aa0a6ffeb96.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Wafer scale III-nitride deep-ultraviolet vertical-cavity surface-emitting lasers featuring nanometer-class control of cavity length","fulltext":[{"header":"Introduction","content":"\u003cp\u003eVertical-cavity surface-emitting lasers (VCSELs) have attracted much attention owing to their advantages of circular far field distribution, single longitudinal mode emission, and two-dimensional integration capability, leading to a rapidly growing billion-dollar market in data communication, sensing, and display\u003csup\u003e1-3\u003c/sup\u003e. From III-arsenides, III-phosphides to III-nitrides, the lasing wavelength blue-shifts from infrared/red\u003csup\u003e4-6\u003c/sup\u003e to blue/ultraviolet (UV)\u003csup\u003e7-9\u003c/sup\u003e, which further expands the application scenarios to high-resolution 3D nanoprinting, maskless photolithography, and miniature atomic clocks\u003csup\u003e10-12\u003c/sup\u003e. Nevertheless, a serious challenge for III-nitride VCSELs is the realization of distributed Bragg reflectors (DBRs) with a high reflectivity. Unlike the mature AlGaAs/GaAs DBRs, the lower refractive index difference and larger lattice mismatch between III-nitrides make it quite hard to achieve a crack-free epitaxial DBR (e.g. AlN/GaN DBR) with a high reflectivity as well as a wide stopband\u003csup\u003e13\u003c/sup\u003e. The situation is even worse when it enters the UV band, since AlGaN with high Al composition is essential in AlGaN/Al(Ga)N DBRs to avoid the light absorption, that further reducing the refractive index difference. An alternative and effective solution is the adoption of dielectric DBRs, e.g. TiO\u003csub\u003e2\u003c/sub\u003e/SiO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e14\u003c/sup\u003e, ZrO\u003csub\u003e2\u003c/sub\u003e/SiO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e15\u003c/sup\u003e, Ta\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/SiO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e16\u003c/sup\u003e and HfO\u003csub\u003e2\u003c/sub\u003e/SiO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e17\u003c/sup\u003e. In order to furthest minimize the mirror losses and thus decrease the threshold, VCSELs with double dielectric DBRs is proposed, which, however, brings about new challenges: (i) how to remove the substrate without cracks and subsequently obtain a smooth exposed surface for the deposition of the second DBR\u003csup\u003e18-20\u003c/sup\u003e; (ii) how to precisely control the cavity length after the substrate removal, so as to reduce the detuning between the resonance wavelength and gain peak\u003csup\u003e21\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eIn terms of the substrate removal, a series of approaches have been proposed, including laser lift-off (LLO)\u003csup\u003e9,18,20\u003c/sup\u003e, chemical/electrochemical lift-off\u003csup\u003e8,22,23\u003c/sup\u003e and etch thinning\u003csup\u003e19\u003c/sup\u003e. Wherein, LLO is the most promising candidate by comprehensively considering the productivity, yield and cost, hence widely adopted in GaN-based visible optoelectronics\u003csup\u003e24-26\u003c/sup\u003e. It is, however, fairly hard to apply LLO in AlGaN-based DUV devices grown on AlN templates. The key issue is that the precipitated Al from AlN decomposition is rigid, resulting in serious cracking of the lifted-off epilayers\u003csup\u003e27\u003c/sup\u003e. A ground-breaking DUV optoelectronic framework featuring structural stacking on GaN templates has been demonstrated in our previous work, where the 4 inch sapphire substrate can be entirely removed by LLO without cracks, and then the wafer-scale fabrication of 280 nm vertical injection DUV light-emitting diodes is realized\u003csup\u003e28\u003c/sup\u003e. Definitely, the framework should also be applicable to the fabrication of DUV-VCSELs.\u003c/p\u003e\n\u003cp\u003eIt is worth noting that the substrate removal by LLO brings about a rough exposed surface, which will cause severe optical scattering losses in the cavity and then make lasing difficult\u003csup\u003e29\u003c/sup\u003e. Chemical mechanical polishing (CMP) is hence widely employed to smoothen the surface as well as control the cavity length\u003csup\u003e9,30,31\u003c/sup\u003e, although it is not really satisfactory. On the one hand, the lifted-off epilayer generally curls owing to the residual stress, consequently, the uniformity of the cavity length by CMP is quite poor in a large-size wafer\u003csup\u003e9\u003c/sup\u003e. On the other hand, the thickness control of CMP cannot meet the precision requirement of cavity, wherein the uncontrolled cavity length will result in the detuning between the resonance wavelength and gain peak, and then lead to an evident increase in the threshold\u003csup\u003e21\u003c/sup\u003e. This issue has long plagued III-nitride VCSELs, from InGaN-based visible to AlGaN-based UV ones. Besides CMP, an approach of photo-assisted chemical etching is proposed to smooth the exposed surface, by which a measure of reduction in the roughness is achieved\u003csup\u003e32\u003c/sup\u003e. The high chemical reactivity of N-polar surface in aqueous acid/alkali, however, plays a negative role and restricts the effect of this approach.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn this work, a DUV-VCSEL strategy featuring the nanometer-class control of the cavity length is proposed in the DUV optoelectronic framework based on GaN templates, which ensures the LLO removal of the sapphire substrates for the deposition of dielectric DBR, meanwhile maintains a high radiative recombination efficiency for lasing. After the sapphire removal, a self-terminated dry etching technology is developed, whereby the cavity length can be accurately determined by epitaxy instead of fabrication process, so as to minimize the detuning between the resonance wavelength and gain peak. Moreover, the etching brings about a smooth surface to reduce the optical scattering losses at the cavity/DBR interface. As such, a record low threshold 0.38 MW/cm\u003csup\u003e2\u003c/sup\u003e is realized in 285.6 nm optically pumped DUV-VCSELs with double dielectric DBRs, and a cavity length variation of only 0.81% within a total length of 1.08 \u0026mu;m is demonstrated across the 4 inch wafer.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEpitaxy and\u003c/strong\u003e \u003cstrong\u003eoptical properties of the cavity\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe epitaxy and fabrication of the DUV-VCSELs are outlined in Fig. 1. Our proposed DUV framework is adopted for the sake of removing the sapphire substrate, and a 9\u0026lambda; cavity for DUV lasing (~280 nm) is grown on GaN templates. The cavity consists of the decoupling structure\u003csup\u003e28\u003c/sup\u003e (a 90 nm-thick Al\u003csub\u003e0.8\u003c/sub\u003eGa\u003csub\u003e0.2\u003c/sub\u003eN pre-crack layer and an 870 nm-thick Al\u003csub\u003e0.65\u003c/sub\u003eGa\u003csub\u003e0.35\u003c/sub\u003eN healing layer), 8-pair Al\u003csub\u003e0.5\u003c/sub\u003eGa\u003csub\u003e0.5\u003c/sub\u003eN/Al\u003csub\u003e0.37\u003c/sub\u003eGa\u003csub\u003e0.63\u003c/sub\u003eN multiple quantum wells (MQWs) as well as a 45 nm-thick Al\u003csub\u003e0.5\u003c/sub\u003eGa\u003csub\u003e0.5\u003c/sub\u003eN capping layer in sequence. Wherein the position of MQWs is thoroughly designed. On the one hand, both the pre-crack and healing layers in the decoupling structure are thick enough to fully release the lattice-mismatch-induced strain between the GaN template and MQWs, and subsequently rebuild a flat surface for MQWs; meanwhile on the other hand, the MQWs are placed on an optical antinode of the standing wave according to the theoretical calculations as shown below, which can help to reduce the threshold\u003csup\u003e1\u003c/sup\u003e. Figure 1a presents the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of MQWs, where the thicknesses of the barriers and wells are determined to be 9 and 1.9 nm, respectively. The radiative recombination efficiency (RRE), as one of the decisive factors for lasing, is further evaluated via the temperature-dependent photoluminescence (PL) measurements from 10 to 300 K (Fig. 1b). Assuming the non-radiative recombination centers frozen at cryogenic temperature (10 K), the MQWs exhibit a room-temperature RRE of 71.5%, almost the same as our previous report\u003csup\u003e28\u003c/sup\u003e. Moreover, the optical polarization of MQWs is characterized by polarization-dependent PL, where the transverse-electric (TE) and transverse-magnetic (TM) polarized light is separately collected as shown in Fig. 1c. It is found that the TE light hugely dominates, and the degree of polarization (DOP) is estimated as 60.6%, assuring the feasibility of the surface-emitting lasing.\u003c/p\u003e\n\u003cp\u003eThe fabrication of DUV-VCSELs starts with the bottom HfO\u003csub\u003e2\u003c/sub\u003e/SiO\u003csub\u003e2\u003c/sub\u003e DBR deposited on the as-grown surface, followed by the wafer bonding and LLO of the sapphire substrate. The subsequent removal of the exposed GaN template is the key in precisely controlling the cavity length. An inductively coupled plasma (ICP) etching process with ultrahigh selectivity is then developed, by which the etching front self-terminates at the interface between GaN and Al\u003csub\u003e0.8\u003c/sub\u003eGa\u003csub\u003e0.2\u003c/sub\u003eN (the pre-crack layer). In other words, the cavity length can be determined by epitaxy instead of etching, hence significantly enhancing the\u0026nbsp;precision and uniformity of the cavity length. Meanwhile, the\u0026nbsp;self-terminated etching brings about a smooth surface as shown below, which lays a good foundation for the deposition of the top\u0026nbsp;HfO\u003csub\u003e2\u003c/sub\u003e/SiO\u003csub\u003e2\u003c/sub\u003e DBR. The reflection spectra of the top and bottom DBRs are recorded in Fig. 1e and f, respectively, and the corresponding transmission spectra can be found in Supplementary Fig. S1. Both DBRs show a high reflectivity exceeding 96% within the wavelength range of 280\u0026ndash;300 nm, consistent with the theoretical expectations.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSelf-terminated etching of N-polar GaN over Al\u003csub\u003e0.8\u003c/sub\u003eGa\u003csub\u003e0.2\u003c/sub\u003eN\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIt should be noted the exposed GaN surface after the LLO removal of sapphire is N-polar, exhibiting greater chemical reactivity in comparison with the metal-polar one\u003csup\u003e33\u003c/sup\u003e. As a consequence, it is more difficult to precisely control the cavity length by etching; worse still, the realization of a smooth surface for the top DBR seems to be an illusion. Fortunately, an AlGaN layer with extremely high Al composition of 80% (the pre-crack layer) is adjacent to the GaN template (Supplementary Fig. S2), making it possible to realize a highly selective etching to address these issues.\u003c/p\u003e\n\u003cp\u003eHerein, ICP etching is performed in both chlorine- and fluorine-based chemistry, and the etching rates of GaN and Al\u003csub\u003e0.8\u003c/sub\u003eGa\u003csub\u003e0.2\u003c/sub\u003eN (Fig. 2a) are evaluated by atomic force microscopy (AFM), where photoresist is employed as the mask of the unetched region. It is demonstrated that in comparison with Cl\u003csub\u003e2\u003c/sub\u003e/BCl\u003csub\u003e3\u003c/sub\u003e mixtures, the etching remarkably decelerates in SF\u003csub\u003e6\u003c/sub\u003e/BCl\u003csub\u003e3\u003c/sub\u003e ones, since the fluorides of Ga and Al have much higher boiling/sublimation points than the chlorides\u003csup\u003e34\u003c/sup\u003e. More importantly, the formation of nonvolatile AlF\u003csub\u003ex\u003c/sub\u003e almost terminates the Al\u003csub\u003e0.8\u003c/sub\u003eGa\u003csub\u003e0.2\u003c/sub\u003eN etching with a quite slow rate of only 1 nm/min, leading to an ultrahigh selectivity of 100:1 for N-polar GaN relative to Al\u003csub\u003e0.8\u003c/sub\u003eGa\u003csub\u003e0.2\u003c/sub\u003eN. Figure 2b presents the AFM image of the etched Al\u003csub\u003e0.8\u003c/sub\u003eGa\u003csub\u003e0.2\u003c/sub\u003eN for 4 mins in SF\u003csub\u003e6\u003c/sub\u003e/BCl\u003csub\u003e3\u003c/sub\u003e mixtures, where the etched and masked regions can be easily distinguished by the boundary of the photoresist residue. To determine the etching rate, a line scan of the relative height is performed along the white arrow across the boundary. The etched depth is then determined to be ~4 nm in Fig. 2c; as such, the Al\u003csub\u003e0.8\u003c/sub\u003eGa\u003csub\u003e0.2\u003c/sub\u003eN etching rate in SF\u003csub\u003e6\u003c/sub\u003e/BCl\u003csub\u003e3\u0026nbsp;\u003c/sub\u003emixtures is 1 nm/min.\u003c/p\u003e\n\u003cp\u003eSuch high etching selectivity greatly benefits the precise control and repeatability of the cavity length during the fabrication of DUV-VCSELs. Moreover, it can bring about a smooth surface for the subsequent DBR. The surface morphology before/during etching is characterized by scanning electron microscopy (SEM) and AFM, as shown in Fig. 2d-i. A typical rough surface is present in the lifted-off epilayer in Fig. 2d, attributed to the spot-to-spot irradiation in the LLO process; meanwhile in a scan area of 3\u0026times;3 \u0026mu;m\u003csup\u003e2\u003c/sup\u003e by AFM (Fig. 2g), dense and whisker-like microcolumns are observed as reported\u003csup\u003e35\u003c/sup\u003e, leading to a root-mean-square (RMS) roughness of 12.8 nm. This morphology is almost inherited during GaN etching (Fig. 2e and h), since there is no etching selectivity inside the GaN template. Until the etching front reaches the GaN/Al\u003csub\u003e0.8\u003c/sub\u003eGa\u003csub\u003e0.2\u003c/sub\u003eN interface, exposed Al\u003csub\u003e0.8\u003c/sub\u003eGa\u003csub\u003e0.2\u003c/sub\u003eN in the valley self-terminates the process, while residual GaN is still rapidly etched. As a consequence, the surface gradually flattens out, and a roughness of 0.7 nm is eventually achieved (Fig. 2i). It is worth noting that small protrusions can be observed in Fig. 2f, which are regularly distributed along some straight lines. These lines are actually corresponding to the pre-cracks, and the protrusions are AlGaN grown in the pre-cracks during healing (Supplementary Fig. S3). In consideration of the quite small surface coverage of pre-cracks, it is convinced that these protrusions have less effects on the subsequent deposition of the top DBR.\u003c/p\u003e\n\u003cp\u003ePrior to the deposition of the top DBR, the impacts of LLO on the optical properties of MQWs should be revealed, since the laser reaching MQWs during LLO may result in the formation of additional defects for non-radiative recombination\u003csup\u003e36,37\u003c/sup\u003e. Herein the fluence of the KrF excimer laser employed in the LLO process is decreased as much as possible, so as to reduce the potential damage. Time-resolved PL measurements at room temperature are further carried out to investigate the lifetime of carriers in MQWs\u003csup\u003e38\u003c/sup\u003e, which can quantify the variation of the non-radiative recombination lifetime (i.e. defects) between as-grown and processed MQWs. According to the images recorded by a streak camera (Fig. 3a and b), a redshift of the peak wavelength is observed from 283 nm (as-grown) to over 285 nm (processed), attributed to the variation of residual strain in the epilayer owing to the processes of bonding and LLO (details in Supplementary Fig. S4 and S5)\u003csup\u003e39\u003c/sup\u003e. Notably, the PL decay profiles at the peak wavelengths are shown in Fig. 3c, where the carrier lifetime is extracted to be ~660 ps for both samples. This indicates that the LLO and ICP processes in this study do not introduce additional point defects into MQWs, assuring the cavity quality in DUV-VCSELs.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePerformance of the DVU-VCSELs\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe 4 inch wafer-scale fabrication of DUV-VCSELs is further accomplished along with the deposition of the top DBR. According to the cross-sectional HAADF-STEM measurement shown in Fig. 4a, the cavity length is determined to be 1.08 \u0026mu;m, almost equivalent to the sum of the epitaxial thickness from the Al\u003csub\u003e0.8\u003c/sub\u003eGa\u003csub\u003e0.2\u003c/sub\u003eN pre-crack layer to the Al\u003csub\u003e0.5\u003c/sub\u003eGa\u003csub\u003e0.5\u003c/sub\u003eN capping one. That demonstrates the nanometer-class control of the cavity length through the combination of LLO and self-terminated ICP etching. Also it is found that both the top and bottom DBRs present flat interfaces, attributed to the aforementioned smooth etched and as-grown surface, respectively. The longitudinal optical field inside the DUV-VCSEL structure is then calculated by the transfer matrix method, as shown in Fig. 4b. On the one hand, most of the QWs align with the optical antinode as designed to obtain a great gain; meanwhile, both interfaces between cavity and top/bottom DBR are placed at the optical node, beneficial for the reduction of the optical scattering loss\u003csup\u003e29\u003c/sup\u003e. The calculation of the cavity reflection spectrum is further carried out (Fig. 4c), where multi-mode emission is observed due to the relatively long cavity length of 1.08 \u0026mu;m. Considering that the spontaneous emission peak locates at over 285 nm (Fig. 3b), a stimulated emission one at ~285 nm can be expected.\u003c/p\u003e\n\u003cp\u003eEventually, DUV-VCSELs are optically pumped at room temperature by a 266 nm laser with 5 ns pulse duration and 20 Hz repeat frequency (details in Supplementary Fig. S7). As the pump power increases, a stimulated emission peak with a linewidth of 0.11 nm is highlighted at 285.6 nm in Fig. 4d, bringing about a \u003cem\u003eQ\u003c/em\u003e factor of 2596. The dependence of the integrated intensity of the stimulated peak on the pump power density is further depicted (Fig. 4e), where a clear kink (the threshold power density, P\u003csub\u003eth\u003c/sub\u003e) around 0.38 MW/cm\u003csup\u003e2\u003c/sup\u003e is observed. The record low threshold in DUV-VCSELs is undoubtedly attributed to the precise control of the cavity length\u003csup\u003e8,9,21,23,40\u003c/sup\u003e; as a consequence, the resonance wavelength is perfectly matched with the gain peak (Supplementary Fig. S8). Besides, more dies across the 4 inch wafer are characterized to investigate the uniformity of lasing wavelength/cavity length, as shown in Fig. 4f. Specifically, 4 random dies are picked up in each quadrant, and the lasing wavelength shifts in the range of 284.8\u0026ndash;286.7 nm in the four quadrants (typical stimulated emission spectra in Supplementary Fig. S9), corresponding to a cavity length variation of 8.7 nm, ~0.81% of the total length (Supplementary Fig. S10).\u003c/p\u003e\n\u003cp\u003eIn summary, a DUV-VCSEL strategy featuring the precise and uniform nanometer-class control of the cavity length in the 4 inch wafer is demonstrated in the DUV framework based on GaN templates, which ensures the LLO removal of the sapphire substrates, and then provides space for the subsequent deposition of dielectric DBR. It is more significant that the strategy brings about a GaN/AlGaN sharp interface with an Al composition difference up to 80%, whereby self-terminated etching with an ultrahigh selectivity of 100:1 is achieved for the exposed N-polar GaN template relative to the Al\u003csub\u003e0.8\u003c/sub\u003eGa\u003csub\u003e0.2\u003c/sub\u003eN pre-crack layer after the sapphire removal. The cavity length, as a consequence, can be accurately determined by epitaxy instead of fabrication process, which helps to minimize the detuning between the resonance wavelength and gain peak. Meanwhile, the etching brings about a smooth surface with the RMS roughness of 0.7 nm in an area of 3\u0026times;3 \u0026mu;m\u003csup\u003e2\u003c/sup\u003e, efficiently reducing the optical scattering losses at the interface between cavity and the top DBR. As such, a record low threshold of 0.38 MW/cm\u003csup\u003e2\u003c/sup\u003e as well as a narrow linewidth of 0.11 nm is realized in the 285.6 nm optically pumped DUV-VCSELs with double dielectric DBRs. What\u0026rsquo;s more, the lasing wavelength varies within 1.9 nm across the 4 inch wafer, corresponding to a cavity length variation of 8.7 nm, ~0.81% of the total length of 1.08 \u0026mu;m. This work establishes a promising strategy for III-nitride DUV-VCSELs, which will greatly promote the development of this field and bring about devices featuring high performance and scalability.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eMOCVD Growth of DUV-VCSELs.\u0026nbsp;\u003c/strong\u003eThe DUV-VCSEL structures in this study were grown on 4 inch double-polished sapphire substrates, by an Aixtron 1\u0026times;4 in. close-coupled showerhead MOCVD system, and repeated by an AMEC Prismo HiT3 (4\u0026times;4 in.) MOCVD system.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eA 4 \u0026mu;m-thick GaN template was first grown by the two-step method, followed by a 90 nm-thick Al\u003csub\u003e0.8\u003c/sub\u003eGa\u003csub\u003e0.2\u003c/sub\u003eN pre-crack layer and an 870 nm-thick Al\u003csub\u003e0.65\u003c/sub\u003eGa\u003csub\u003e0.35\u003c/sub\u003eN healing layer grown at\u0026nbsp;1075\u0026deg;C and 1095\u0026deg;C, respectively. Then, 8-pair 9 nm/1.9 nm Al\u003csub\u003e0.5\u003c/sub\u003eGa\u003csub\u003e0.5\u003c/sub\u003eN/Al\u003csub\u003e0.37\u003c/sub\u003eGa\u003csub\u003e0.63\u003c/sub\u003eN MQWs as well as a 45 nm-thick Al\u003csub\u003e0.5\u003c/sub\u003eGa\u003csub\u003e0.5\u003c/sub\u003eN capping layer were grown\u0026nbsp;in sequence.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFabrication of DUV-VCSELs.\u003c/strong\u003e 8.5-pair 35.5 nm/40.8 nm alternating HfO\u003csub\u003e2\u003c/sub\u003e/SiO\u003csub\u003e2\u003c/sub\u003e were deposited on the as-grown epitaxial surface as the bottom DBR. Then, the wafer was bonded to a Si submount, followed by the laser lift-off process by employing a 248 nm KrF excimer laser. After the removal of sapphire, the exposed N-polar GaN surface was cleaned in the HCl solution for 1 min to remove the Ga metal from GaN decomposition. The residual GaN layer was then etched down to the Al\u003csub\u003e0.8\u003c/sub\u003eGa\u003csub\u003e0.2\u003c/sub\u003eN pre-crack layer by ICP. Eventually, 7.5-pair HfO\u003csub\u003e2\u003c/sub\u003e/SiO\u003csub\u003e2\u003c/sub\u003e were deposited on the N-polar surface as the top DBR. The SEM image of DUV-VCSELs is shown in Supplementary Fig. S11.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCharacterization.\u0026nbsp;\u003c/strong\u003eTEM (Thermo Scientific Themis Z STEM operated at 200 kV) was employed to reveal the control of cavity length in this study, where the TEM specimen was prepared by focused ion beam (FIB, Thermo Scientific Helios G4 HX Dual Beam). The surface morphology of wafers was evaluated by AFM (Bruker Dimension Icon) and SEM (FEI Nova NanoSEM 430). Reflection and transmission spectra were recorded by Agilent Cary 7000 UV-Vis-NIR spectrophotometry. Temperature-dependent PL was characterized by a homemade system at Peking University, where a 213 nm laser (Xiton Photonics Impress 213) was employed as the excitation source. Temperature-dependent measurements were performed by employing a closed-cycle helium cryostat (JANIS SVT-400) attached to the temperature controller (Scientific Instruments 9700). Time-resolved as well as polarization-dependent PL was characterized by a homemade system at Peking University, where a 266 nm laser (Coherent Chameleon Ultra II) was employed as the excitation source, and a streak camera (Hamamatsu C10910-03) and a spectrometer (Horiba iHR550) were used for time-resolved and polarization-dependent detection, respectively. The optical pump of DUV-VCSELs was performed by a homemade system at Xiamen University, and a 266 nm laser with 5 ns pulse duration and 20 Hz repeat frequency was employed as the pump source.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting Interests\u003c/h2\u003e \u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contributions\u003c/h2\u003e \u003cp\u003eC.J. and J.W. conceived the experiments. C.J., J.W., C.Z.J. and J.Z. grew the samples and performed relevant measurements. F.X., L.Z., X.Y., N.T., X.W., W.G. and B.S. gave support in the measurements and analyses. C.J., J.W. and J.L. performed device fabrication under X.K. and Z.Q. supervision. J.W. wrote the manuscript with the assistance of F.X., W.G. and B.S. All authors discussed the results and commented on the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eThe authors thank Prof. Baoping Zhang and Prof. Yang Mei from Xiamen University for the optically pumping measurements. This work was supported by the National Key Research and Development Program of China (2023YFB3609700 to F.X.), the National Natural Science Foundation of China (62234001 and 61927806 to B.S.; 62135013 to F.X.; 62374007 and 62422401 to J.W.; 62204005 to J.L.), and the Basic Research Program of Jiangsu (BK20243027 to F.X.).\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e \u003cp\u003eData are available from the corresponding authors upon request. Source data are provided with this paper.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eMichalzik, R. VCSELs: fundamentals, technology and applications of vertical-cavity surface-emitting lasers. 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China Mater.\u003c/em\u003e\u003cstrong\u003e66\u003c/strong\u003e, 1978\u0026ndash;1988 (2023).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"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":"","lastPublishedDoi":"10.21203/rs.3.rs-7024020/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7024020/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIII-nitride AlGaN-based deep-ultraviolet vertical-cavity surface-emitting lasers (DUV-VCSELs) have shown a great application potential in optical atomic clock, maskless photolithography, etc. Nevertheless, the detuning issue owing to the uncontrolled cavity length, i.e. the difference between the resonance wavelength and gain peak, severely impairs the device performance. Herein, a DUV-VCSEL strategy featuring the uniform nanometer-class control of the cavity length in a 4 inch wafer is proposed in the DUV framework based on GaN templates, which ensures the wafer-scale removal of the sapphire substrates by laser lift-off, and then provides space for the subsequent deposition of dielectric distributed Bragg reflector (DBR). It is more significant that the strategy brings about a GaN/AlGaN sharp interface with an Al composition difference up to 80%, whereby self-terminated etching with an ultrahigh selectivity of 100:1 is achieved. The cavity length can hence be accurately determined by epitaxy itself instead of fabrication process, so as to minimize the detuning. As such, 285.6 nm optically pumped DUV-VCSELs with double dielectric DBRs are fabricated, exhibiting a record low threshold of 0.38 MW/cm\u003csup\u003e2\u003c/sup\u003e as well as a narrow linewidth of 0.11 nm. What’s more, the lasing wavelength varies within 1.9 nm across the 4 inch wafer, indicating a cavity length variation of only 0.81%. This work establishes a promising strategy for III-nitride DUV-VCSELs, which will definitely speed up the development of devices featuring high performance and scalability.\u003c/p\u003e","manuscriptTitle":"Wafer scale III-nitride deep-ultraviolet vertical-cavity surface-emitting lasers featuring nanometer-class control of cavity length","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-03 08:34:09","doi":"10.21203/rs.3.rs-7024020/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":"3fded0d9-d0a2-4e49-901a-be5586d27b9d","owner":[],"postedDate":"July 3rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":50927015,"name":"Physical sciences/Materials science/Materials for optics/Lasers, LEDs and light sources/Semiconductor lasers"},{"id":50927016,"name":"Physical sciences/Optics and photonics/Lasers, LEDs and light sources/Semiconductor lasers"}],"tags":[],"updatedAt":"2025-07-25T18:30:16+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-03 08:34:09","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7024020","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7024020","identity":"rs-7024020","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}