Influence of Indium Composition on InAlAs QCLs

Influence of Indium Composition on InAlAs QCLs | 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 Research Article Influence of Indium Composition on InAlAs QCLs Smiri Badreddine, Demir Ilkay, Hizi Abir, Hélène Carrère, Altuntas Ismail, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4670192/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 In this work, we explored the impact of indium composition (x) on the structural and optical characteristics of In x Al 1-x As layers within the context of quantum cascade laser (QCL) structures grown on InP (100) substrates using the Metal Organic Vapor Phase Epitaxy (MOVPE) method. The quality of the In x Al 1-x As QCL is notably influenced by the growth with low indium composition, evident in terms of crystallinity, interface sharpness, and optical properties. The properties of the InAsP layer at the InP/ In x Al 1-x As junction are particularly sensitive to the indium composition. A drop below 0.52 in indium composition leads to a substantial lattice mismatch between the In x Al 1-x As layer and the InP substrate, typically exceeding [3 8]%. This mismatch induces defects or traps within the bandgap, significantly impacting carrier localization in this system. Our study demonstrates that cultivating In x Al 1-x As with a low indium concentration results in a strained (lattice-mismatched) In x Al 1-x As layer. This finding is significant as it can be leveraged to balance strain in high indium content InGaAs layers, particularly in the context of applications involving quantum cascade lasers. InxAl1-xAs indium composition lattice-mismatched carrier localization Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction In x Al 1−x As /InP heterostructures stand as cutting-edge materials for a diverse range of optoelectronic devices, including high electron mobility transistors (HEMTs) [ 1 ], spin field-effect transistors (TFETs) [ 2 ], infrared photodetectors [ 3 ], and terahertz (THz) and mid-infrared quantum cascade lasers (QCLs) [ 4 ]. These devices hold immense promise for high power, continuous wave operation at room temperature, and wide tunability. However, the growth of high-quality In x Al 1−x As on InP substrate with sharp interfaces presents a significant challenge compared to the more conventional AlGaAs on GaAs substrate, which is renowned for its pristine interfaces [ 5 , 6 ]. Metal Organic Vapor Phase Epitaxy (MOVPE) is a widely employed technique for growing In x Al 1−x As layers; however, it requires optimal growth conditions to ensure that Al atoms have sufficient time and kinetic energy to bind to the correct lattice site [ 7 , 8 ]. The difficulty in achieving high-quality In x Al 1−x As layers stems from the substantial bond strength difference between In-As and Al-As, leading to structural, optical, and electrical limitations [ 9 ]. In particular, the indium concentration plays a crucial role in determining the interface quality between the epilayer and the substrate [ 10 , 11 ]. Improper indium concentration can induce cluster formation and high concentrations of point defects, severely degrading the device performance [ 12 , 13 ]. Furthermore, the growth of epitaxial heterostructures inevitably introduces alloy disorder and layer thickness fluctuation due to lattice mismatch between the ultrathin heterojunction layers. This inevitably leads to the formation of interfacial defects, creating both electrical deep-level and optical localization states within the heterojunction [ 14 ]. These energy states can trigger Fermi-edge singularity (FES) phenomena, leading to abnormal luminescence and anomalous carrier transport behaviors [ 15 , 16 ]. FES can significantly impact device performance parameters, such as reduced carrier mobility [ 17 ], enhanced kink-effect in resonant tunneling [ 18 ], cotunneling during single-electron transport [ 19 ], increased electron-phonon coupling [ 20 ], and elevated electron-electron scattering in photodiodes [ 21 – 23 ]. To address these challenges, the authors have systematically grown a series of In x Al 1−x As samples, a crucial component of QCL structures, using MOVPE to investigate the effects of indium concentration on the structural and optical properties of In x Al 1−x As epilayer grown on InP substrate. The primary objective of this study is to achieve high-quality In x Al 1−x As epilayers with perfect interfaces, which will serve as injection barrier layers in high In-content InGaAs layers for QCL applications. The novelty of this work lies in the identification of optimal growth conditions for In x Al 1−x As epilayers on InP substrate, minimizing the formation of defects and preserving sharp interfaces, enabling the development of high-performance QCLs. 2. Experimental details In this study, three In x Al 1−x As epilayers (S1, S2, and S3) with different indium alloy composition (x) values were grown. In x Al 1−x As epilayers were grown on (100)-oriented, double-side polished n-doped indium phosphide (n-InP) substrates in an AIXTRON 200/4 RF-S horizontal-flow MOVPE system. Trimethylindium (TMIn, In(CH3)3) and trimethylaluminum (TMAl, Al(CH3)3) metal-organic compounds were used as In and Al precursors, respectively. Arsine (AsH3) and phosphine (PH3) hydrides were used as As and P precursors. Ultra-high-purified hydrogen (H2) and nitrogen (N2) were used as carrier gases. The growth process was first initiated by heating the substrate under PH3 environment until the epilayer growth temperature (555°C) was reached to prevent the escape of P from the InP substrate surface. An InP buffer layer (BL, ~ 250 nm thick) was grown to prevent possible unwanted structures from the substrate surface from advancing into the In x Al 1−x As epilayer and to obtain a better surface quality. The growth parameters for the buffer layer are as follows: the growth temperature is 555°C, and the TMIn and PH3 flows are 90 sccm and 300 sccm, respectively. The BL has the same growth parameters for all samples. After InP BL growth, the growth temperature was reduced to 540°C, and InAlAs epilayers were grown. In order to obtain the targeted In alloy composition (x) values in the In x Al 1−x As epilayers, TMIn and TMAl flow rates were changed while keeping AsH3 flow constant. The growth parameters of In x Al 1−x As epilayers are as follows for samples S1, S2, and S3, respectively: TMIn flows 40, 40, and 36 sccm; TMAl flows are 15, 17, and 20 sccm; AsH3 flow is 6.66 sccm for all samples. The structural properties of epitaxial layers were investigated using high-resolution X-ray diffraction (HRXRD) technique. The HRXRD measurements were performed around the InP (100) symmetry axis with 0.0004° precision using a Rigaku SmartLab diffractometer, equipped with a rotating Cu anode which provides 9 kW X-ray power (45 KV tube voltage and 200 mA tube current) and four-bounced Ge (220) monochromator. Raman scattering spectroscopy was recorded at T = 300 K using confocal micro-Raman spectrometers (Horiba-JY Xplora). Diode lasers (λ exc = 532 nm) were used for excitation. The laser beam was focused onto the sample using a 50X microscope objective down to a spot size of 2 µm2. The PL measurements were completed in the range of 10–300 K while keeping the samples in a closed-cycle helium circulation cryostat. In this case, the PL was excited by a Ti:Sa laser at λ exc = 808 nm and the spectra were recorded using a liquid nitrogen-cooled CCD. The TRPL spectra were measured at a temperature of 10 K using a closed-cycle cryostat. In this case, the PL dynamics was excited by a Ti:Sa laser at λ exc = 725 nm, the laser-pulse duration was 1 ps, pulse repetition frequency was 80 MHz, the excitation power was 1 mW, and the spectra were detected using a streak camera. 3. Results and Discussion 3.a. In Situ Reflectance Measurement Comments A semiconductor laser operating at a wavelength of 880 nm was used to monitor and control the growth steps and conditions during the growth. In situ reflectance measurement is an important measurement technique that gives valuable parameters such as growth rate, surface states, layer thickness during the growth [ 24 ]. We show in fig .1 the in-situ reflectance and growth temperature versus growth time for all samples. The In concentration of the In x Al 1−x As layer is indicated as x ln . The temperature was increased to 555°C and the InP buffer layer was grown at this temperature. There was no oscillation observed during the buffer growth because of the same refractive index of substrate and epilayer. Then, the growth temperature was reduced to 540°C and In x Al 1−x As layers with different In concentration were grown. As can be seen from the figure that oscillations were observed while growing In x Al 1−x As due to the refractive index difference between epilayer and InP buffer. The growth rate and the total film thickness were determined by using the interference patterns. The equations used to determine the film thickness and growth rate are given below: $$\varDelta d=\frac{\lambda }{2n} and r=\frac{\varDelta d}{T}$$ 1 Where Δd is an oscillation thickness, λ is the wavelength of laser light, n is the refractive index of the layer, r is the growth rate, and T is the time for a complete oscillation. High growth rate causes short periods of reflectance oscillations, while low growth rate causes long periods. The growth rate of the S1, S2, and S3 are 0.394, 0.422, and 0.444 nm/s, respectively. The phase and amplitude of the reflectance oscillations depend on the wavelength of the incident light, the optical constants of the materials, and the thickness of the growing layer. In addition, surface roughness is another important parameter that affects the reflectance curve [ 25 ]. During the three sample growth, the wavelength of the excitation light was heft constant, and the growth rate was really identical. However, a change in In content induce a change in the refraction index of the In x Al 1−x As layer. The thickness of the layers and the wavelength of the incident light are the same, but with the increase of the In fraction, the refractive index of the In x Al 1−x As layer will increase, i.e. approach the refractive index of InP. As the difference between the refractive index of the buffer layer and In x Al 1−x As layer decreases, the amplitude of the oscillations will also decrease. Therefore, the surface situation cannot be interpreted from the amplitudes of the oscillations, but the fact that the reflectance intensity did not decrease for all of the samples is an indication of the quality of the surface. 3.b. HR-XRD results Table 1 HR-XRD results of the In x Al 1−x As/InP QCL structures with different In alloy composition. Sample Indium Contents (%) Lattice (Å) Lattice mismatch (%) FWHM (degree) Dislocation density (10 9 cm − 2 ) S1 37.1 5.8078 -1.01% 0.093 0.173 S2 33.8 5.7946 -1.26% 0.110 0.243 S3 29.8 5.7787 -1.53% 0.142 0.408 HRXRD measurements were employed to determine the indium alloy composition (x) of the grown structures. Figure 2 exhibits the recorded 2θ/ω X-ray diffraction spectra of the three In x Al 1-x As/InP structures. It is evident from Fig. 2 that the high-intensity peaks originate from the InP substrate, while the remaining peaks emanate from the In x Al 1-x As layers. Due to the increasing Indium alloy composition within the layer, the peaks associated with the In x Al 1-x As layers are further separated from the InP substrate peaks. The peak separation angle can be utilized to ascertain the In composition within the alloy. Hence, the x In values were determined as 37.1%, 33.8%, and 29.8% for S1, S2, and S3 structures, respectively, by employing the formula provided in Reference [ 26 ] and adapting it to the In x Al 1-x As/InP structure. $$\frac{\varDelta a}{a}= -\frac{\varDelta \theta }{\text{tan}\left({}_{B}\right)}$$ 2 Where \(\varDelta \theta\) is the measured angular spacing between the epitaxial layer and substrate diffraction peaks and \({}_{B}\) is the Bragg angle for the substrate. All samples exhibited In x Al 1−x As peaks positioned to the right of the InP peak, signifying that the film was under tensile strain, leading to an expansion of the out-of-plane lattice parameter. Additionally, the FWHM values of XRD peaks for the S1, S2, and S3 structures were determined to be 0.093°, 0.110°, and 0.172°, respectively. The QCL191 structure, with the lowest FWHM value (0.093°), is the closest to the lattice-matched In x Al 1−x As/InP structure. Utilizing the FWHM values, it is possible to approximate the dislocation density (Ndis) in such epitaxial layers according to the following formula [ 27 – 28 ]: $${\text{N}}_{\text{d}\text{i}\text{s}}=2 \frac{{\left(\text{F}\text{W}\text{H}\text{M}\right)}^{2}}{{9\text{a}}_{0}^{2}}$$ 3 Where FWHM is expressed in radians and a0 represents the lattice constant of the epitaxial layer, determined according to Vegard's Law. Table I summarizes the crystal structural parameters, including the lattice parameter (a), dislocation density (Ndis), and strain (e), for each sample. Notably, the Ndis and e values of S1 and S2 are lower than those of S3. A mere 7.3% reduction in the Indium content results in a significant increase in dislocation density, approximately 135%. Additionally, thickness fringes are clearly visible in Fig. 2 for these two samples, indicating high crystalline quality and sharp interfaces between the In x Al 1−x As epilayer and the InP substrate. Based on these results, it can be concluded that the In 0.371 Al 0.629 As epitaxial layer of sample S1, with the smallest FWHM, exhibits the minimum dislocation density. 3.c. Raman results Figure 3 presents typical Raman scattering spectra at 300 K in In x Al 1−x As of various x In . The disorder-activated longitudinal acoustic (DALA) phonon of In x Al 1−x As with the middle composition range typically appears in the broad Raman band from 90 to 150 cm-1 [ 10 , 29 ]. This observation is attributed to the interactions between atomic clusters that occur during the growth of the epitaxial layer, leading to a disordered crystal arrangement at the interface. An increase in the degree of disorder (i.e., the alloy phase separating into indium-rich regions and aluminum-rich regions) contributes to an increase in dislocation density [ 10 , 29 ]. To accurately determine the peak frequencies of the various Raman modes, the Raman spectra were deconvoluted using a Lorentzian shape function. Figure 3 (b) depicts the fitting curves corresponding to the Raman data of sample S1. The five identified peaks, which are located at 190, 250, 310, 355, and 269 cm-1, correspond to LO-InAsP, LO-InAs, TO-InP, LO-InP, and LO-AlAs, respectively. The same spectral features were observed in samples S2 and S3. Notably, as the Indium content increases, the lattice parameter of the In x Al 1−x As alloy shifts between that of InAs and AlAs. This phenomenon is reflected in Fig. 3 (a), where the LO-InAs, LO-AlAs, and LO-InAsP modes exhibit a slight shift towards lower wavenumbers with increasing In content. This shift is attributed to the tensile strain induced by the lattice mismatch between InP and In x Al 1−x As [ 29 – 30 ], and it aligns well with the HR-XRD results.According to previous studies [ 27 , 31 ], the relationship between the shift of InP-like LO frequency and the residual stress \(\left(\mathcal{R}\right)\) in the In x Al 1−x As epitaxial layer is given by: $$\mathcal{R}=\frac{{2{{\omega }}_{0}^{LO}\varDelta {\omega }}^{LO}}{\left({\text{S}}_{11}+{2\text{S}}_{12}\right)\left(\text{p}+2\text{q}\right)-\left({\text{S}}_{11}-{\text{S}}_{12}\right)\left(\text{p}-\text{q}\right)}$$ 4 $${\varDelta {\omega }}^{\text{L}\text{O}}={{\omega }}^{\text{L}\text{O}}-{{\omega }}_{0}^{\text{L}\text{O}}$$ 5 $${{\omega }}_{0}^{\text{L}\text{O}}=7.096{x}^{2}-78.5x+404.1$$ 6 where, p and q are the optical phonon deformation constants, S 11 and S 12 are the elastic compliance constants, ω LO is the measured AlAs-like LO frequency in epitaxial layer, \({\omega }_{0}\) is the AlAs-like LO frequency in the ideal strain-free bulk In x Al 1-x As alloy as a function of composition x. All parameters and values of Eq. 4 are summarized in Table 2 . Table 2 The Raman results of the In x Al 1−x As epitaxial layers with different In content. Sample x In (%) \({\varvec{\omega }}_{0}^{\mathbf{L}\mathbf{O}}\left({\varvec{c}\varvec{m}}^{-1}\right)\) ω LO \(\left({\varvec{c}\varvec{m}}^{-1}\right)\) \({\varDelta \varvec{\omega }}^{\mathbf{L}\mathbf{O}} \left({\varvec{c}\varvec{m}}^{-1}\right)\) \(\mathcal{R}\) \(\left(\varvec{G}\varvec{P}\varvec{a}\right)\) S1 37.1 375.95 366.33 -9.62 1.12 S2 33.8 378.37 368.05 -10.32 1.21 S3 29.8 383.33 370.91 -12.42 1.48 Table 2 compares the residual strain of the In x Al 1−x As epitaxial layer with varying Indium content. With decreasing Indium content, the residual strain in epitaxial layers increases, and sample S1 exhibits the lowest residual strain value, indicating the highest crystalline quality. This observation corroborates the findings of our previous HR-XRD measurements. 3.d. Photoluminescence spectroscopy: Figure 4 compares the normalized PL intensity of In x Al 1−x As samples with varying In contents, measured at 10 K with a laser intensity of 0.1 W/cm 2 . A noticeable blue-shift (approximately 20 meV) in the In x Al 1−x As emission is observed as the In content decreases. The broadening of the PL peak (63 meV) with decreasing In content indicates a deterioration in optical quality, attributed to alloy phase separation (clustering effect) and stacking faults in the In x Al 1−x As layers [ 10 , 11 ]. These results suggest that clustering has a minimal impact on sample S1, and higher In content leads to improved optical quality. Two additional transitions appear at energies lower than those described in the previous section for bulk processes. The first transition at 1.37 (± 2) eV is associated with the normal interface or (e–A) transition in InP material [ 32 ]. The last transition, peaking at 1.17–1.24 eV, likely originates from a thin InAsP layer at the InP/ In x Al 1−x As interface, resulting in a higher wavelength (lower energy) luminescence band [ 10 , 32 ]. It can also be inferred that S3 is more affected by interface roughness fluctuations and compositional disorder. We conducted a detailed examination of the three samples through excitation-dependent PL measurements at 10 K in the 1.1–1.3 eV range. Figures 5 a-c present the PL spectra of the three samples with decreasing excitation power. For clarity, each spectrum is normalized based on the maximum PL intensity and shifted upward. The integrated intensity, FWHM, and PL peak energy of the three samples are extracted and depicted in Figs. 5 d-f as functions of excitation intensity. As the power intensity increases from 1 to 46 µW, the peak energy of all samples exhibits a blue shift (see Fig. 5 f), indicating a type-II band alignment and the formation of triangular wells due to band bending at the interface. This power dependence can be associated with localized states arising from alloy fluctuations and state filling effects, phenomena observed in systems displaying similar power dependencies [ 33 – 35 ]. Upon close inspection, the power dependence fit demonstrates a logarithmic relationship, consistent with systems exhibiting the spatial separation of charges expected from alloy fluctuations at this graded interface. As excitation increases from 1 to 46 µW, the spectrum gradually becomes symmetric, eventually displaying a perfect Gaussian peak. Simultaneously, the FWHM narrows from 61 to 50 meV for S3, from 21 to 27 meV for S2, and from 24 to 27 meV for S1, as shown in Fig. 5 d. Subsequently, the PL spectra become asymmetric again, with a shoulder on the higher-energy side. These features can be well explained by the filling effect of the localized exciton at low excitation and a free exciton energy state at high excitation intensity [ 11 ]. Figure 5 d also illustrates that the integrated PL intensity of the three samples increases with rising excitation power intensity. We observed that the exponent "n" approaches unity, indicating no saturation at higher excitation power. This suggests that this PL transition is not attributed to any impurity or defects but is an intrinsic recombination (band-to-band). Previous studies have demonstrated that the three transitions are indirect with a type II transition [ 10 ]. Figures 6 a-c present the results of temperature-dependent photoluminescence (PL) spectra for the three samples, revealing distinct behaviors for each. Firstly, in Fig. 6 e, the FWHM initially narrows as the temperature increases from 10 to 50 K for samples S1 and S2, and from 10 to 80 K for sample S3. This behavior is attributed to localized excitons initially occupying a broad distribution of inhomogeneous fluctuations, which are then thermally excited into the narrower distribution of the free exciton state. Subsequently, the FWHM continuously widens with further temperature increases. The kinetic energy of the free exciton increases with temperature, leading to a larger FWHM. Secondly, as depicted in Fig. 6 f, the PL peak energy exhibits an "S-shape" behavior (red/blue/red-shift) with increasing temperature, particularly pronounced at low indium concentration. The S-shape behavior at low temperature is attributed to the strong localization of excitons at an energy level within the material band gap. However, the maximum redshift is 10 meV for S3 and only 5 meV for the other two samples. At around 60 K, the excitons thermalize and relax to the local minima (alloy disorder), resulting in the first red shift. As the temperature rises from 60 K to 100 K, localized carriers gain enough thermal energy to transfer to higher energy levels in the band tails until reaching the maximum of the band continuum, causing a blue shift in the PL peak energy. Finally, when the lattice temperature surpasses 100 K, the PL peaks of all samples shift to lower energies, with S1 and S2 redshifting faster than S3. In this phase, carriers are thermally activated and prevented from localization, indicating free carrier recombination. Similar anomalous behavior of PL peak energy with temperature increase has been observed in other QW InGaAs/InAlAs systems at lower temperatures, attributed to the presence of band tails in the density of states (DOS) due to potential fluctuations caused by the interface in the quantum well. We used the Arrhenius relation to fit the experimental data of the integrated PL intensity to understand the mechanism of thermal quenching of carriers in this system [ 37 – 38 ]: $$\varvec{I}\left(\varvec{T}\right)=\frac{{\varvec{I}}_{0}}{1+\sum _{\varvec{i}}{\varvec{C}}_{\varvec{i}}\varvec{e}\varvec{x}\varvec{p}\varvec{e}\varvec{x}\varvec{p}(-\frac{{\varvec{E}}_{\varvec{a}\varvec{i}}}{{\varvec{K}}_{\varvec{B}}\varvec{T}}) }$$ 7 Where T is the measured temperature, I 0 is a variable parameter intensity, I(T) is the integrated PL emission intensity, C i are constants related to the densities of non-radiative recombination centers, E ai are the activation energies corresponding to the non-radiative recombination centers and K B is Boltzmann’s constant. The Arrhenius fits, depicted in Fig. 6 d along with the experimental data, reveal a noteworthy trend in activation energies. As illustrated, there is a clear correlation between the decrease in indium content and the rise in thermal activation energy [ 11 ]. This observation can be rationalized by the pronounced defect trapping effect in S3, demanding a higher activation energy for the release of charge carriers from their localized states. The distinct thermal quenching mechanisms exhibited by these three QWs are evident in the variability of their activation energies. 3.e. Time-resolved PL (TRPL) measurement TRPL measurements were conducted to gain insights into the impact of indium concentration on carrier dynamics in the In x Al 1−x As QW and the inverted interface transition (type II transition). Figure 7 displays the PL lifetime curves for both the fundamental transition energy of the QW and the type II transition energy. To extract the PL lifetime from these curves, background subtraction from the detected signal and a single exponential model were employed. \(I\left(t\right)={I}_{0}\text{*}\text{e}\text{x}\text{p}(-\frac{t}{\tau })\) [ 3 ] where \({I}_{0}\) and τ are amplitude and decay time-constant, respectively. The lifetimes in the In x Al 1−x As QW (direct transition) are determined to be 0.37 ns, 0.53 ns, and 0.71 ns, while in the type II transition (indirect transition), they are measured to be 1.01 ns, 2.94 ns, and 6.47 ns for the S3, S2, and S1 samples, respectively. It is evident that in both transitions, the carrier lifetime increases with the rise in indium concentrations, indicating a significant difference in the material quality of these three samples. Two factors contribute to this increase in lifetime with higher indium concentration: the localization of excitons and the interface defect density. Exciton localization is known to increase radiation lifetime [ 39 ], while defect density at the interface decreases the radiation lifetime [ 3 ]. In this case, the effect of interface defect density appears to be predominant over the effect of exciton localization on the carrier lifetime. Notably, the sample S1, with the best crystalline quality, exhibits a high PL lifetime of 0.71 ns for the In x Al 1−x As epilayer and 6.47 ns for the type II transition. This sample, characterized by a high indium content close to the lattice mismatch between the In x Al 1−x As layer and the InP substrate (x In =0.52), demonstrates superior optical quality, lower peak FWHM, higher crystalline quality, better interface quality, fewer hetero-interface scattering effects, reduced composition fluctuation, and fewer defects in the QCLs active region. Conclusions The investigation focused on the phase separation phenomenon in the In x Al 1−x As epilayer, an integral component of QCL structures grown via MOCVD on InP substrate at a temperature of 540 ℃. The results clearly demonstrated an increase in dislocation density and residual strain values with a decrease in In content due to alloy phase separation. A systematic examination of PL at various temperatures revealed an inverted S-shaped behavior, indicative of the competition between localized and delocalized states at the interface. This effect was more prominent in samples with lower In content. The findings highlighted that the best crystallinity and optical quality, featuring abrupt interfaces, were achieved with an indium concentration of 0.371 and a low growth rate of approximately 0.394 nm/s. This conclusion was supported and confirmed by in-situ reflectance, HRXRD, Raman spectroscopy, PL, and TRPL measurements. In future work, we plan to conduct a quantitative analysis of exciton localization. Additionally, we aim to explore variations in growth conditions to leverage our structure as an active region in high In-content InGaAs layers for applications in Quantum Cascade Lasers. Declarations Acknowledgments The authors acknowledge the usage of the Nanophotonics Research and Application Center at Sivas Cumhuriyet University (CUNAM) facilities. This work is supported by both The Scientific and Technological Research Council of Turkey (TUBITAK, project number 22AG074), and Sivas Cumhuriyet University Scientific Research Projects (CUBAP, project number MRK-2024-004). Conflict of interest The authors have no conflicts to disclose Availability of data and materials AIP Publishing believes that all datasets underlying the conclusions of the paper should be available to readers. We encourage authors to deposit their detasets in publicity available repositories (where available and appropriate) or present them in the main manuscript. Ethical Approval, Our institution does not require ethical approval for reporting individual cases or case series. Statement of informed consent Written informed consent was obtained from the patient(s) for their anonymized information to be published in this article. Consent to participate Not applicable Consent for publication Not applicable Competing interests Non-financial associations that may be relevant to the submitted manuscript. Funding Not applicable References H. Jo, Do-Young Yun, Ji-Min Baek, Jung-Hee Lee, Tae-Woo Kim, Dae-Hyun Kim, Takuya Tsutsumi, Hiroki Sugiyama and Hideaki Matsuzaki, Lg = 25 nm InGaAs/InAlAs high-electron mobility transistors with both fT and fmax in excess of 700 GHz, Appl. Phys. Express 12 054006 (2019). 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Smiri Badreddine, Ben Arabia Marwa, Ilkay Demir, Saidi Faouzi, Othmen Zied, Dkhil Brahim, Ismail Altuntas, Sezai Elagoz, Fredj Hassen, Maaref Hassen., Optical and Structural properties of high-x InxGa1-xAs epitaxial layers on (100) InP for SWIR detectors. Materials Science & Engineering B 262 (2020) 114769. S. Z. Chang, T. C. Chang, J. L. Shen, S. C. Lee, Y. F. Chen, Material and Electrical Properties of Highly Mismatched InxGa1-xAs on GaAs by Molecular-beam Epitaxy, J. Appl. Phys. 74 (1993) 6912. Badreddine Smiri, Saidi Faouzi, Adnen Mlayah, Hassen Maaref, Comparative optical studies of InAlAs/InP quantum wells grown by MOCVD on (311)A and (311)B InP planes.. J. of Materials Science: Materials in Electronics, (2020), 31, 10750–10759. A. Sayari, N. Yahyaoui, M. Oueslati, H. Maaref, K. Zellama, Raman study of V/III flux ratio effect in InP/InAlAs/InP heterostructures grown by MOCVD. J. Raman Spectrosc. 40, 1023–1027 (2009). L. Zhao, Z. Guo, M. Zhang, S.Yang, L. Zhao, Surface-Interface Analysis of InxGa1‐xAs/InP Heterostructure in Positive and Negative Mismatch System, Surf. Interface Anal. 51 (2019) 498–505. J. Hellara, K. Borgi, H. Maaref, V. Souliere, Y. Monteil, Mater. Sci. Eng. C 21 (2002) 231e236. Poças LC, Lopes EM, Duarte JL, Dias IFL, Lourenço SA, Laureto E et al (2005) The effect of potential fluctuations on the optical properties of InGaAs/ InAlAs superlattices. J Appl Phys 97:103518 Yang QK, Chen JX, Li AZ (2000) Growth and characterization of high-quality GaInAs/AlInAs triple wells. J Cryst Growth 209:8–14 Pocas LC, Duarte JL, Lopes EM, Harmand JC (2006) The effect of potential fluctuations on the optical properties of InGaAs/InGaAlAs single and coupled double quantum wells. J Appl Phys 100:053519. Ying Wang, Xinzhi Sheng, Qinglin Guo, Xiaoli Li, Shufang Wang, Guangsheng Fu, Yuriy I. Mazur, Yurii Maidaniuk, Morgan E. Ware, Gregory J. Salamo, Baolai Liang and Diana L. Huffaker, Photoluminescence Study of the Interface Fluctuation Effect for InGaAs/InAlAs/InP Single Quantum Well with Different Thickness, Wang et al. Nanoscale Research Letters (2017) 12:229. Yang QK, Chen JX, Li AZ (1998) Photoluminescence study of InGaAs/InAlAs single and multiple quantum wells. J Cryst Growth 194:31–36. Gu Y, Zhang YG, Li AZ, Wang K, Li C, Li YY (2009) Structural and photoluminescence properties for highly strain-compensated InGaAs/InAlAs superlattice. Chin Phys Lett 26:077808. B. Deveaud, F. Cl´erot, N. Roy, K. Satzke, B. Sermage, D. Katzer, Enhanced radiative recombination of free excitons in GaAs quantum wells, Phys. Rev. Lett. 67 (17) (1991) 2355–2358. Additional Declarations No competing interests reported. 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-4670192","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":330705718,"identity":"6e58f3d5-4e38-496b-a26a-12d3d695976a","order_by":0,"name":"Smiri Badreddine","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA+0lEQVRIiWNgGAWjYBACAzBiY+YB8xIYJORA9IEHpGgxBmtJIEILXCCxAaIXNzBnP7ztw4cyaxlz9vanGx7usUifH3b4IdAWOzndBuxaLHvSimfOOJfOY9lzxuxGwjOJ3I230wyAWpKNzQ7gcNiBHGNm3rbDPAY3cthuJBwAapmdANJyIHEbLi3n3xgz/wVpuf/8GUhLuuHs9A/4tdwA2sIItoXBDKQlQV46B78tljOeFTP2AP1icCYHrMVwg3ROwYEEA9x+MedP3szwo8za3uD48Wc3fxyok5efnb75w4cKOzlcWrAFCJgkVjkIyDeQonoUjIJRMApGAgAAmdRloD+AIdsAAAAASUVORK5CYII=","orcid":"","institution":"Université de Toulouse, INSA-CNRS-UPS, LPCNO","correspondingAuthor":true,"prefix":"","firstName":"Smiri","middleName":"","lastName":"Badreddine","suffix":""},{"id":330705719,"identity":"c7f0a571-9252-4580-9e59-6c0769a1a430","order_by":1,"name":"Demir Ilkay","email":"","orcid":"","institution":"Sivas Cumhuriyet University","correspondingAuthor":false,"prefix":"","firstName":"Demir","middleName":"","lastName":"Ilkay","suffix":""},{"id":330705721,"identity":"f7bba226-4a50-4fba-bb9a-fe582ec6f07c","order_by":2,"name":"Hizi Abir","email":"","orcid":"","institution":"Université de Toulouse, INSA-CNRS-UPS, LPCNO","correspondingAuthor":false,"prefix":"","firstName":"Hizi","middleName":"","lastName":"Abir","suffix":""},{"id":330705722,"identity":"67812ca7-b61b-4aee-a6ff-9d5642e28a85","order_by":3,"name":"Hélène Carrère","email":"","orcid":"","institution":"Université de Toulouse, INSA-CNRS-UPS, LPCNO","correspondingAuthor":false,"prefix":"","firstName":"Hélène","middleName":"","lastName":"Carrère","suffix":""},{"id":330705724,"identity":"fab1449b-0897-4fdc-ba15-adbee02b3b33","order_by":4,"name":"Altuntas Ismail","email":"","orcid":"","institution":"Sivas Cumhuriyet University","correspondingAuthor":false,"prefix":"","firstName":"Altuntas","middleName":"","lastName":"Ismail","suffix":""},{"id":330705725,"identity":"742e7421-19ed-4f1b-b763-3e911e50b0b3","order_by":5,"name":"Adnen Mlayah","email":"","orcid":"","institution":"CNRS-Université de Toulouse","correspondingAuthor":false,"prefix":"","firstName":"Adnen","middleName":"","lastName":"Mlayah","suffix":""},{"id":330705726,"identity":"889aae7d-d292-4ee7-8f2d-78c352c4e15f","order_by":6,"name":"Maaref Hassen","email":"","orcid":"","institution":"Université de Monastir","correspondingAuthor":false,"prefix":"","firstName":"Maaref","middleName":"","lastName":"Hassen","suffix":""},{"id":330705729,"identity":"e145260b-60ea-44f2-b3e6-3dda63ec2db3","order_by":7,"name":"Marie Xavier","email":"","orcid":"","institution":"Université de Toulouse, INSA-CNRS-UPS, LPCNO","correspondingAuthor":false,"prefix":"","firstName":"Marie","middleName":"","lastName":"Xavier","suffix":""}],"badges":[],"createdAt":"2024-07-01 20:53:22","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4670192/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4670192/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":61067078,"identity":"d963770b-d560-4e8a-aca0-94769c0c4c71","added_by":"auto","created_at":"2024-07-25 07:51:14","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":370121,"visible":true,"origin":"","legend":"\u003cp\u003eIn situ optical reflectance measurements (left axis) of In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs and temperature (right axis) QCL for different Indium concentration during growth.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-4670192/v1/3fb62874d5359e5d09c64e7a.png"},{"id":61067079,"identity":"f48be093-0d7c-4630-902d-1725edd62b69","added_by":"auto","created_at":"2024-07-25 07:51:14","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":151665,"visible":true,"origin":"","legend":"\u003cp\u003e2θ/ω HR-XRD spectra of the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1-x\u003c/sub\u003eAs layers having different In alloy compositions grown on InP by MOVPE.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-4670192/v1/6023c030936bd5fa88faddeb.png"},{"id":61067805,"identity":"2f1c976c-4348-46a4-9174-c5cc1c6b8a7b","added_by":"auto","created_at":"2024-07-25 07:59:14","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":989005,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Raman spectra recorded at room temperature with incident energy E\u003csub\u003ei \u003c/sub\u003e= 2.33 eV for the all samples. (b) Line shape fitting of Raman spectrum obtained from sample S1.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-4670192/v1/2ecde9b265830e907a9efd4f.png"},{"id":61067804,"identity":"ea80f191-6ebb-4e31-a816-51af410358df","added_by":"auto","created_at":"2024-07-25 07:59:14","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":213518,"visible":true,"origin":"","legend":"\u003cp\u003ePL spectra recorded at 10 K with incident energy Ei = 1.53 eV for the all samples.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-4670192/v1/915c7c878dbd89dec1c5e363.png"},{"id":61067803,"identity":"eb4f159e-6325-4b5b-bb99-b2ab65d4903e","added_by":"auto","created_at":"2024-07-25 07:59:14","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":191376,"visible":true,"origin":"","legend":"\u003cp\u003ePL measured as a function of the excitation intensity. \u003cstrong\u003ea)\u003c/strong\u003e PL spectra for the S1. \u003cstrong\u003eb)\u003c/strong\u003e PL spectra for the S2. \u003cstrong\u003ec)\u003c/strong\u003e PL spectra for the S3. \u003cstrong\u003ed)\u003c/strong\u003e Integrated PL intensity of the type II transition as a function of the excitation intensity. \u003cstrong\u003ee)\u003c/strong\u003e FWHM of PL spectra as a function of excitation intensity et \u003cstrong\u003ef)\u003c/strong\u003e PL peak energy as a function of the excitation intensity.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-4670192/v1/21ef84b20f5c65f577763513.png"},{"id":61067082,"identity":"11f04b5d-3806-4ce1-9608-0a15c2c90694","added_by":"auto","created_at":"2024-07-25 07:51:14","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":293466,"visible":true,"origin":"","legend":"\u003cp\u003ePL\u003cstrong\u003e \u003c/strong\u003emeasured as a function of temperature \u003cstrong\u003ea)\u003c/strong\u003e PL spectra for the S1, \u003cstrong\u003eb)\u003c/strong\u003e PL spectra for the S2 and c\u003cstrong\u003e)\u003c/strong\u003e PL spectra for the S3. \u003cstrong\u003ed)\u003c/strong\u003e integrated PL intensity, \u003cstrong\u003ee)\u003c/strong\u003e The PL FWHM, \u003cstrong\u003ef)\u003c/strong\u003e PL peak energy shift vary as a function of temperature for the type II transition measured at the laser excitation intensity of I\u003csub\u003e0\u003c/sub\u003e = 0.1 W/cm\u003csup\u003e2\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-4670192/v1/1d8ea6c332f620a3a03377cb.png"},{"id":61067083,"identity":"bef97636-b54b-4d6c-8aff-403a80a2b280","added_by":"auto","created_at":"2024-07-25 07:51:14","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":337556,"visible":true,"origin":"","legend":"\u003cp\u003eNormalized TRPL decay curves for (a) In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1-x\u003c/sub\u003eAs layer and (b) type II transition for x= 0.371 to 0.298 at 10K with excitation power 1 mW. Exponential data (symbols) and single exponential (solid lines).\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-4670192/v1/5c4b07de3bbb165b10cffce9.png"},{"id":74794422,"identity":"041f8efb-c6d1-4e7d-b1e3-ea1002c293af","added_by":"auto","created_at":"2025-01-27 01:01:25","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2732722,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4670192/v1/b6fa44de-a23b-41b9-8221-8540a4da37a7.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Influence of Indium Composition on InAlAs QCLs","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eIn\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1\u0026minus;x\u003c/sub\u003eAs /InP heterostructures stand as cutting-edge materials for a diverse range of optoelectronic devices, including high electron mobility transistors (HEMTs) [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e], spin field-effect transistors (TFETs) [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], infrared photodetectors [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], and terahertz (THz) and mid-infrared quantum cascade lasers (QCLs) [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. These devices hold immense promise for high power, continuous wave operation at room temperature, and wide tunability. However, the growth of high-quality In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1\u0026minus;x\u003c/sub\u003eAs on InP substrate with sharp interfaces presents a significant challenge compared to the more conventional AlGaAs on GaAs substrate, which is renowned for its pristine interfaces [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eMetal Organic Vapor Phase Epitaxy (MOVPE) is a widely employed technique for growing In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1\u0026minus;x\u003c/sub\u003eAs layers; however, it requires optimal growth conditions to ensure that Al atoms have sufficient time and kinetic energy to bind to the correct lattice site [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. The difficulty in achieving high-quality In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1\u0026minus;x\u003c/sub\u003eAs layers stems from the substantial bond strength difference between In-As and Al-As, leading to structural, optical, and electrical limitations [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. In particular, the indium concentration plays a crucial role in determining the interface quality between the epilayer and the substrate [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Improper indium concentration can induce cluster formation and high concentrations of point defects, severely degrading the device performance [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFurthermore, the growth of epitaxial heterostructures inevitably introduces alloy disorder and layer thickness fluctuation due to lattice mismatch between the ultrathin heterojunction layers. This inevitably leads to the formation of interfacial defects, creating both electrical deep-level and optical localization states within the heterojunction [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. These energy states can trigger Fermi-edge singularity (FES) phenomena, leading to abnormal luminescence and anomalous carrier transport behaviors [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. FES can significantly impact device performance parameters, such as reduced carrier mobility [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], enhanced kink-effect in resonant tunneling [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], cotunneling during single-electron transport [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e], increased electron-phonon coupling [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], and elevated electron-electron scattering in photodiodes [\u003cspan additionalcitationids=\"CR22\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTo address these challenges, the authors have systematically grown a series of In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1\u0026minus;x\u003c/sub\u003eAs samples, a crucial component of QCL structures, using MOVPE to investigate the effects of indium concentration on the structural and optical properties of In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1\u0026minus;x\u003c/sub\u003eAs epilayer grown on InP substrate. The primary objective of this study is to achieve high-quality In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1\u0026minus;x\u003c/sub\u003eAs epilayers with perfect interfaces, which will serve as injection barrier layers in high In-content InGaAs layers for QCL applications. The novelty of this work lies in the identification of optimal growth conditions for In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1\u0026minus;x\u003c/sub\u003eAs epilayers on InP substrate, minimizing the formation of defects and preserving sharp interfaces, enabling the development of high-performance QCLs.\u003c/p\u003e"},{"header":"2. Experimental details","content":"\u003cp\u003eIn this study, three In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1\u0026minus;x\u003c/sub\u003eAs epilayers (S1, S2, and S3) with different indium alloy composition (x) values were grown. In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1\u0026minus;x\u003c/sub\u003eAs epilayers were grown on (100)-oriented, double-side polished n-doped indium phosphide (n-InP) substrates in an AIXTRON 200/4 RF-S horizontal-flow MOVPE system. Trimethylindium (TMIn, In(CH3)3) and trimethylaluminum (TMAl, Al(CH3)3) metal-organic compounds were used as In and Al precursors, respectively. Arsine (AsH3) and phosphine (PH3) hydrides were used as As and P precursors. Ultra-high-purified hydrogen (H2) and nitrogen (N2) were used as carrier gases. The growth process was first initiated by heating the substrate under PH3 environment until the epilayer growth temperature (555\u0026deg;C) was reached to prevent the escape of P from the InP substrate surface. An InP buffer layer (BL, ~\u0026thinsp;250 nm thick) was grown to prevent possible unwanted structures from the substrate surface from advancing into the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1\u0026minus;x\u003c/sub\u003eAs epilayer and to obtain a better surface quality. The growth parameters for the buffer layer are as follows: the growth temperature is 555\u0026deg;C, and the TMIn and PH3 flows are 90 sccm and 300 sccm, respectively. The BL has the same growth parameters for all samples. After InP BL growth, the growth temperature was reduced to 540\u0026deg;C, and InAlAs epilayers were grown. In order to obtain the targeted In alloy composition (x) values in the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1\u0026minus;x\u003c/sub\u003eAs epilayers, TMIn and TMAl flow rates were changed while keeping AsH3 flow constant. The growth parameters of In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1\u0026minus;x\u003c/sub\u003eAs epilayers are as follows for samples S1, S2, and S3, respectively: TMIn flows 40, 40, and 36 sccm; TMAl flows are 15, 17, and 20 sccm; AsH3 flow is 6.66 sccm for all samples.\u003c/p\u003e \u003cp\u003eThe structural properties of epitaxial layers were investigated using high-resolution X-ray diffraction (HRXRD) technique. The HRXRD measurements were performed around the InP (100) symmetry axis with 0.0004\u0026deg; precision using a Rigaku SmartLab diffractometer, equipped with a rotating Cu anode which provides 9 kW X-ray power (45 KV tube voltage and 200 mA tube current) and four-bounced Ge (220) monochromator.\u003c/p\u003e \u003cp\u003eRaman scattering spectroscopy was recorded at T\u0026thinsp;=\u0026thinsp;300 K using confocal micro-Raman spectrometers (Horiba-JY Xplora). Diode lasers (λ\u003csub\u003eexc\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;532 nm) were used for excitation. The laser beam was focused onto the sample using a 50X microscope objective down to a spot size of 2 \u0026micro;m2.\u003c/p\u003e \u003cp\u003eThe PL measurements were completed in the range of 10\u0026ndash;300 K while keeping the samples in a closed-cycle helium circulation cryostat. In this case, the PL was excited by a Ti:Sa laser at λ\u003csub\u003eexc\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;808 nm and the spectra were recorded using a liquid nitrogen-cooled CCD.\u003c/p\u003e \u003cp\u003eThe TRPL spectra were measured at a temperature of 10 K using a closed-cycle cryostat. In this case, the PL dynamics was excited by a Ti:Sa laser at λ\u003csub\u003eexc\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;725 nm, the laser-pulse duration was 1 ps, pulse repetition frequency was 80 MHz, the excitation power was 1 mW, and the spectra were detected using a streak camera.\u003c/p\u003e"},{"header":"3. Results and Discussion","content":"\u003ch3\u003e3.a. In Situ Reflectance Measurement Comments\u003c/h3\u003e\n\u003cp\u003e \u003c/p\u003e \u003cp\u003eA semiconductor laser operating at a wavelength of 880 nm was used to monitor and control the growth steps and conditions during the growth. In situ reflectance measurement is an important measurement technique that gives valuable parameters such as growth rate, surface states, layer thickness during the growth [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eWe show in fig .1 the in-situ reflectance and growth temperature versus growth time for all samples. The In concentration of the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs layer is indicated as x\u003csub\u003eln\u003c/sub\u003e. The temperature was increased to 555°C and the InP buffer layer was grown at this temperature. There was no oscillation observed during the buffer growth because of the same refractive index of substrate and epilayer.\u003c/p\u003e \u003cp\u003eThen, the growth temperature was reduced to 540°C and In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs layers with different In concentration were grown. As can be seen from the figure that oscillations were observed while growing In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs due to the refractive index difference between epilayer and InP buffer. The growth rate and the total film thickness were determined by using the interference patterns. The equations used to determine the film thickness and growth rate are given below:\u003c/p\u003e\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\varDelta d=\\frac{\\lambda }{2n} and r=\\frac{\\varDelta d}{T}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003cp\u003e\u003c/p\u003e \u003cp\u003eWhere Δd is an oscillation thickness, λ is the wavelength of laser light, n is the refractive index of the layer, r is the growth rate, and T is the time for a complete oscillation.\u003c/p\u003e \u003cp\u003eHigh growth rate causes short periods of reflectance oscillations, while low growth rate causes long periods. The growth rate of the S1, S2, and S3 are 0.394, 0.422, and 0.444 nm/s, respectively. The phase and amplitude of the reflectance oscillations depend on the wavelength of the incident light, the optical constants of the materials, and the thickness of the growing layer. In addition, surface roughness is another important parameter that affects the reflectance curve [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e].\u003c/p\u003e\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eDuring the three sample growth, the wavelength of the excitation light was heft constant, and the growth rate was really identical. However, a change in In content induce a change in the refraction index of the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs layer.\u003c/p\u003e\u003cp\u003eThe thickness of the layers and the wavelength of the incident light are the same, but with the increase of the In fraction, the refractive index of the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs layer will increase, i.e. approach the refractive index of InP. As the difference between the refractive index of the buffer layer and In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs layer decreases, the amplitude of the oscillations will also decrease. Therefore, the surface situation cannot be interpreted from the amplitudes of the oscillations, but the fact that the reflectance intensity did not decrease for all of the samples is an indication of the quality of the surface.\u003c/p\u003e\u003c/div\u003e\u003cp\u003e\u003c/p\u003e\n\u003ch3\u003e3.b. HR-XRD results\u003c/h3\u003e\n\u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e\u003cdiv class=\"gridtable\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\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\u003eHR-XRD results of the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs/InP QCL structures with different In alloy composition.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003c/colgroup\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eIndium Contents\u003c/p\u003e \u003cp\u003e(%)\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLattice\u003c/p\u003e \u003cp\u003e(Å)\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eLattice mismatch (%)\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eFWHM\u003c/p\u003e \u003cp\u003e(degree)\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eDislocation density (10\u003csup\u003e9\u003c/sup\u003e cm\u003csup\u003e− 2\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e37.1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.8078\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e-1.01%\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.093\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.173\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS2\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e33.8\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.7946\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e-1.26%\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.110\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.243\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS3\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e29.8\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.7787\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e-1.53%\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.142\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.408\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/table\u003e\u003c/div\u003e \u003cp\u003e\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eHRXRD measurements were employed to determine the indium alloy composition (x) of the grown structures. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e exhibits the recorded 2θ/ω X-ray diffraction spectra of the three In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1-x\u003c/sub\u003eAs/InP structures. It is evident from Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e that the high-intensity peaks originate from the InP substrate, while the remaining peaks emanate from the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1-x\u003c/sub\u003eAs layers. Due to the increasing Indium alloy composition within the layer, the peaks associated with the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1-x\u003c/sub\u003eAs layers are further separated from the InP substrate peaks. The peak separation angle can be utilized to ascertain the In composition within the alloy. Hence, the x\u003csub\u003eIn\u003c/sub\u003e values were determined as 37.1%, 33.8%, and 29.8% for S1, S2, and S3 structures, respectively, by employing the formula provided in Reference [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] and adapting it to the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1-x\u003c/sub\u003eAs/InP structure.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Equ2\" class=\"Equation\"\u003e \u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\frac{\\varDelta a}{a}= -\\frac{\\varDelta \\theta }{\\text{tan}\\left({}_{B}\\right)}$$\u003c/div\u003e \u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e \u003cp\u003e\u003c/p\u003e \u003cp\u003eWhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\varDelta \\theta\\)\u003c/span\u003e\u003c/span\u003e is the measured angular spacing between the epitaxial layer and substrate diffraction peaks and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({}_{B}\\)\u003c/span\u003e\u003c/span\u003e is the Bragg angle for the substrate. All samples exhibited In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs peaks positioned to the right of the InP peak, signifying that the film was under tensile strain, leading to an expansion of the out-of-plane lattice parameter. Additionally, the FWHM values of XRD peaks for the S1, S2, and S3 structures were determined to be 0.093°, 0.110°, and 0.172°, respectively. The QCL191 structure, with the lowest FWHM value (0.093°), is the closest to the lattice-matched In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs/InP structure. Utilizing the FWHM values, it is possible to approximate the dislocation density (Ndis) in such epitaxial layers according to the following formula [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e–\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]:\u003c/p\u003e\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$${\\text{N}}_{\\text{d}\\text{i}\\text{s}}=2 \\frac{{\\left(\\text{F}\\text{W}\\text{H}\\text{M}\\right)}^{2}}{{9\\text{a}}_{0}^{2}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003cp\u003e\u003c/p\u003e \u003cp\u003eWhere FWHM is expressed in radians and a0 represents the lattice constant of the epitaxial layer, determined according to Vegard's Law. Table I summarizes the crystal structural parameters, including the lattice parameter (a), dislocation density (Ndis), and strain (e), for each sample. Notably, the Ndis and e values of S1 and S2 are lower than those of S3. A mere 7.3% reduction in the Indium content results in a significant increase in dislocation density, approximately 135%. Additionally, thickness fringes are clearly visible in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e for these two samples, indicating high crystalline quality and sharp interfaces between the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs epilayer and the InP substrate. Based on these results, it can be concluded that the In\u003csub\u003e0.371\u003c/sub\u003eAl\u003csub\u003e0.629\u003c/sub\u003eAs epitaxial layer of sample S1, with the smallest FWHM, exhibits the minimum dislocation density.\u003c/p\u003e\n\u003ch3\u003e3.c. Raman results\u003c/h3\u003e\n\u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e presents typical Raman scattering spectra at 300 K in In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs of various x\u003csub\u003eIn\u003c/sub\u003e. The disorder-activated longitudinal acoustic (DALA) phonon of In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs with the middle composition range typically appears in the broad Raman band from 90 to 150 cm-1 [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. This observation is attributed to the interactions between atomic clusters that occur during the growth of the epitaxial layer, leading to a disordered crystal arrangement at the interface. An increase in the degree of disorder (i.e., the alloy phase separating into indium-rich regions and aluminum-rich regions) contributes to an increase in dislocation density [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTo accurately determine the peak frequencies of the various Raman modes, the Raman spectra were deconvoluted using a Lorentzian shape function. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b) depicts the fitting curves corresponding to the Raman data of sample S1. The five identified peaks, which are located at 190, 250, 310, 355, and 269 cm-1, correspond to LO-InAsP, LO-InAs, TO-InP, LO-InP, and LO-AlAs, respectively. The same spectral features were observed in samples S2 and S3. Notably, as the Indium content increases, the lattice parameter of the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs alloy shifts between that of InAs and AlAs. This phenomenon is reflected in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a), where the LO-InAs, LO-AlAs, and LO-InAsP modes exhibit a slight shift towards lower wavenumbers with increasing In content. This shift is attributed to the tensile strain induced by the lattice mismatch between InP and In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e–\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e], and it aligns well with the HR-XRD results.According to previous studies [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e], the relationship between the shift of InP-like LO frequency and the residual stress \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\left(\\mathcal{R}\\right)\\)\u003c/span\u003e\u003c/span\u003e in the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs epitaxial layer is given by:\u003c/p\u003e\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\mathcal{R}=\\frac{{2{{\\omega }}_{0}^{LO}\\varDelta {\\omega }}^{LO}}{\\left({\\text{S}}_{11}+{2\\text{S}}_{12}\\right)\\left(\\text{p}+2\\text{q}\\right)-\\left({\\text{S}}_{11}-{\\text{S}}_{12}\\right)\\left(\\text{p}-\\text{q}\\right)}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$${\\varDelta {\\omega }}^{\\text{L}\\text{O}}={{\\omega }}^{\\text{L}\\text{O}}-{{\\omega }}_{0}^{\\text{L}\\text{O}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ6\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ6\" name=\"EquationSource\"\u003e\n$${{\\omega }}_{0}^{\\text{L}\\text{O}}=7.096{x}^{2}-78.5x+404.1$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e6\u003c/div\u003e\u003c/div\u003e\u003cp\u003e\u003c/p\u003e \u003cp\u003ewhere, p and q are the optical phonon deformation constants, S\u003csub\u003e11\u003c/sub\u003e and S\u003csub\u003e12\u003c/sub\u003e are the elastic compliance constants, ω\u003csup\u003eLO\u003c/sup\u003e is the measured AlAs-like LO frequency in epitaxial layer, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\omega }_{0}\\)\u003c/span\u003e\u003c/span\u003e is the AlAs-like LO frequency in the ideal strain-free bulk In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1-x\u003c/sub\u003eAs alloy as a function of composition x. All parameters and values of Eq.\u0026nbsp;\u003cspan refid=\"Equ4\" class=\"InternalRef\"\u003e4\u003c/span\u003e are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\u003cdiv class=\"gridtable\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eThe Raman results of the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs epitaxial layers with different In content.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003c/colgroup\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ex\u003csub\u003eIn\u003c/sub\u003e (%)\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\varvec{\\omega }}_{0}^{\\mathbf{L}\\mathbf{O}}\\left({\\varvec{c}\\varvec{m}}^{-1}\\right)\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eω\u003csup\u003eLO\u003c/sup\u003e \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\left({\\varvec{c}\\varvec{m}}^{-1}\\right)\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\varDelta \\varvec{\\omega }}^{\\mathbf{L}\\mathbf{O}} \\left({\\varvec{c}\\varvec{m}}^{-1}\\right)\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\mathcal{R}\\)\u003c/span\u003e\u003c/span\u003e \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\left(\\varvec{G}\\varvec{P}\\varvec{a}\\right)\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e37.1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e375.95\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e366.33\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-9.62\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1.12\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS2\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e33.8\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e378.37\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e368.05\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-10.32\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1.21\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS3\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e29.8\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e383.33\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e370.91\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-12.42\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1.48\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/table\u003e\u003c/div\u003e \u003cp\u003e\u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e compares the residual strain of the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs epitaxial layer with varying Indium content. With decreasing Indium content, the residual strain in epitaxial layers increases, and sample S1 exhibits the lowest residual strain value, indicating the highest crystalline quality. This observation corroborates the findings of our previous HR-XRD measurements.\u003c/p\u003e\n\u003ch3\u003e3.d. Photoluminescence spectroscopy:\u003c/h3\u003e\n\u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e compares the normalized PL intensity of In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs samples with varying In contents, measured at 10 K with a laser intensity of 0.1 W/cm\u003csup\u003e2\u003c/sup\u003e. A noticeable blue-shift (approximately 20 meV) in the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs emission is observed as the In content decreases. The broadening of the PL peak (63 meV) with decreasing In content indicates a deterioration in optical quality, attributed to alloy phase separation (clustering effect) and stacking faults in the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs layers [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. These results suggest that clustering has a minimal impact on sample S1, and higher In content leads to improved optical quality.\u003c/p\u003e \u003cp\u003eTwo additional transitions appear at energies lower than those described in the previous section for bulk processes. The first transition at 1.37 (± 2) eV is associated with the normal interface or (e–A) transition in InP material [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. The last transition, peaking at 1.17–1.24 eV, likely originates from a thin InAsP layer at the InP/ In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs interface, resulting in a higher wavelength (lower energy) luminescence band [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. It can also be inferred that S3 is more affected by interface roughness fluctuations and compositional disorder.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWe conducted a detailed examination of the three samples through excitation-dependent PL measurements at 10 K in the 1.1–1.3 eV range. Figures\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea-c present the PL spectra of the three samples with decreasing excitation power. For clarity, each spectrum is normalized based on the maximum PL intensity and shifted upward. The integrated intensity, FWHM, and PL peak energy of the three samples are extracted and depicted in Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed-f as functions of excitation intensity.\u003c/p\u003e \u003cp\u003eAs the power intensity increases from 1 to 46 µW, the peak energy of all samples exhibits a blue shift (see Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef), indicating a type-II band alignment and the formation of triangular wells due to band bending at the interface. This power dependence can be associated with localized states arising from alloy fluctuations and state filling effects, phenomena observed in systems displaying similar power dependencies [\u003cspan additionalcitationids=\"CR34\" citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e–\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. Upon close inspection, the power dependence fit demonstrates a logarithmic relationship, consistent with systems exhibiting the spatial separation of charges expected from alloy fluctuations at this graded interface. As excitation increases from 1 to 46 µW, the spectrum gradually becomes symmetric, eventually displaying a perfect Gaussian peak. Simultaneously, the FWHM narrows from 61 to 50 meV for S3, from 21 to 27 meV for S2, and from 24 to 27 meV for S1, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed. Subsequently, the PL spectra become asymmetric again, with a shoulder on the higher-energy side. These features can be well explained by the filling effect of the localized exciton at low excitation and a free exciton energy state at high excitation intensity [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed also illustrates that the integrated PL intensity of the three samples increases with rising excitation power intensity. We observed that the exponent \"n\" approaches unity, indicating no saturation at higher excitation power. This suggests that this PL transition is not attributed to any impurity or defects but is an intrinsic recombination (band-to-band). Previous studies have demonstrated that the three transitions are indirect with a type II transition [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigures \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea-c present the results of temperature-dependent photoluminescence (PL) spectra for the three samples, revealing distinct behaviors for each. Firstly, in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee, the FWHM initially narrows as the temperature increases from 10 to 50 K for samples S1 and S2, and from 10 to 80 K for sample S3. This behavior is attributed to localized excitons initially occupying a broad distribution of inhomogeneous fluctuations, which are then thermally excited into the narrower distribution of the free exciton state. Subsequently, the FWHM continuously widens with further temperature increases. The kinetic energy of the free exciton increases with temperature, leading to a larger FWHM.\u003c/p\u003e \u003cp\u003eSecondly, as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef, the PL peak energy exhibits an \"S-shape\" behavior (red/blue/red-shift) with increasing temperature, particularly pronounced at low indium concentration. The S-shape behavior at low temperature is attributed to the strong localization of excitons at an energy level within the material band gap. However, the maximum redshift is 10 meV for S3 and only 5 meV for the other two samples. At around 60 K, the excitons thermalize and relax to the local minima (alloy disorder), resulting in the first red shift. As the temperature rises from 60 K to 100 K, localized carriers gain enough thermal energy to transfer to higher energy levels in the band tails until reaching the maximum of the band continuum, causing a blue shift in the PL peak energy. Finally, when the lattice temperature surpasses 100 K, the PL peaks of all samples shift to lower energies, with S1 and S2 redshifting faster than S3. In this phase, carriers are thermally activated and prevented from localization, indicating free carrier recombination. Similar anomalous behavior of PL peak energy with temperature increase has been observed in other QW InGaAs/InAlAs systems at lower temperatures, attributed to the presence of band tails in the density of states (DOS) due to potential fluctuations caused by the interface in the quantum well.\u003c/p\u003e \u003cp\u003eWe used the Arrhenius relation to fit the experimental data of the integrated PL intensity to understand the mechanism of thermal quenching of carriers in this system [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e–\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]:\u003c/p\u003e\u003cdiv id=\"Equ7\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ7\" name=\"EquationSource\"\u003e\n$$\\varvec{I}\\left(\\varvec{T}\\right)=\\frac{{\\varvec{I}}_{0}}{1+\\sum _{\\varvec{i}}{\\varvec{C}}_{\\varvec{i}}\\varvec{e}\\varvec{x}\\varvec{p}\\varvec{e}\\varvec{x}\\varvec{p}(-\\frac{{\\varvec{E}}_{\\varvec{a}\\varvec{i}}}{{\\varvec{K}}_{\\varvec{B}}\\varvec{T}}) }$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e7\u003c/div\u003e\u003c/div\u003e\u003cp\u003e\u003c/p\u003e \u003cp\u003eWhere T is the measured temperature, I\u003csub\u003e0\u003c/sub\u003e is a variable parameter intensity, I(T) is the integrated PL emission intensity, C\u003csub\u003ei\u003c/sub\u003e are constants related to the densities of non-radiative recombination centers, E\u003csub\u003eai\u003c/sub\u003e are the activation energies corresponding to the non-radiative recombination centers and K\u003csub\u003eB\u003c/sub\u003e is Boltzmann’s constant.\u003c/p\u003e \u003cp\u003eThe Arrhenius fits, depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed along with the experimental data, reveal a noteworthy trend in activation energies. As illustrated, there is a clear correlation between the decrease in indium content and the rise in thermal activation energy [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. This observation can be rationalized by the pronounced defect trapping effect in S3, demanding a higher activation energy for the release of charge carriers from their localized states. The distinct thermal quenching mechanisms exhibited by these three QWs are evident in the variability of their activation energies.\u003c/p\u003e\n\u003ch3\u003e3.e. Time-resolved PL (TRPL) measurement\u003c/h3\u003e\n\u003cp\u003e \u003c/p\u003e \u003cp\u003eTRPL measurements were conducted to gain insights into the impact of indium concentration on carrier dynamics in the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs QW and the inverted interface transition (type II transition). Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e displays the PL lifetime curves for both the fundamental transition energy of the QW and the type II transition energy. To extract the PL lifetime from these curves, background subtraction from the detected signal and a single exponential model were employed.\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(I\\left(t\\right)={I}_{0}\\text{*}\\text{e}\\text{x}\\text{p}(-\\frac{t}{\\tau })\\)\u003c/span\u003e\u003c/span\u003e [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] where \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({I}_{0}\\)\u003c/span\u003e\u003c/span\u003e and τ are amplitude and decay time-constant, respectively. The lifetimes in the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs QW (direct transition) are determined to be 0.37 ns, 0.53 ns, and 0.71 ns, while in the type II transition (indirect transition), they are measured to be 1.01 ns, 2.94 ns, and 6.47 ns for the S3, S2, and S1 samples, respectively. It is evident that in both transitions, the carrier lifetime increases with the rise in indium concentrations, indicating a significant difference in the material quality of these three samples. Two factors contribute to this increase in lifetime with higher indium concentration: the localization of excitons and the interface defect density. Exciton localization is known to increase radiation lifetime [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e], while defect density at the interface decreases the radiation lifetime [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. In this case, the effect of interface defect density appears to be predominant over the effect of exciton localization on the carrier lifetime. Notably, the sample S1, with the best crystalline quality, exhibits a high PL lifetime of 0.71 ns for the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs epilayer and 6.47 ns for the type II transition. This sample, characterized by a high indium content close to the lattice mismatch between the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs layer and the InP substrate (x\u003csub\u003eIn\u003c/sub\u003e=0.52), demonstrates superior optical quality, lower peak FWHM, higher crystalline quality, better interface quality, fewer hetero-interface scattering effects, reduced composition fluctuation, and fewer defects in the QCLs active region.\u003c/p\u003e "},{"header":"Conclusions","content":"\u003cp\u003eThe investigation focused on the phase separation phenomenon in the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1−x\u003c/sub\u003eAs epilayer, an integral component of QCL structures grown via MOCVD on InP substrate at a temperature of 540 ℃. The results clearly demonstrated an increase in dislocation density and residual strain values with a decrease in In content due to alloy phase separation. A systematic examination of PL at various temperatures revealed an inverted S-shaped behavior, indicative of the competition between localized and delocalized states at the interface. This effect was more prominent in samples with lower In content. The findings highlighted that the best crystallinity and optical quality, featuring abrupt interfaces, were achieved with an indium concentration of 0.371 and a low growth rate of approximately 0.394 nm/s. This conclusion was supported and confirmed by in-situ reflectance, HRXRD, Raman spectroscopy, PL, and TRPL measurements.\u003c/p\u003e\u003cp\u003eIn future work, we plan to conduct a quantitative analysis of exciton localization. Additionally, we aim to explore variations in growth conditions to leverage our structure as an active region in high In-content InGaAs layers for applications in Quantum Cascade Lasers.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors acknowledge the usage of the Nanophotonics Research and Application Center at Sivas Cumhuriyet University (CUNAM) facilities. This work is supported by both The Scientific and Technological Research Council of Turkey (TUBITAK, project number 22AG074), and Sivas Cumhuriyet University Scientific Research Projects (CUBAP, project number MRK-2024-004).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors have no conflicts to disclose\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAIP Publishing believes that all datasets underlying the conclusions of the paper should be available to readers. We encourage authors to deposit their detasets in publicity available repositories (where available and appropriate) or present them in the main manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval,\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eOur institution does not require ethical approval for reporting individual cases or case series.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eStatement of informed consent\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWritten informed consent was obtained from the patient(s) for their anonymized information to be published in this article.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNon-financial associations that may be relevant to the submitted manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eH. 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J Cryst Growth 194:31\u0026ndash;36.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGu Y, Zhang YG, Li AZ, Wang K, Li C, Li YY (2009) Structural and photoluminescence properties for highly strain-compensated InGaAs/InAlAs superlattice. Chin Phys Lett 26:077808.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eB. Deveaud, F. Cl\u0026acute;erot, N. Roy, K. Satzke, B. Sermage, D. Katzer, Enhanced radiative recombination of free excitons in GaAs quantum wells, Phys. Rev. Lett. 67 (17) (1991) 2355\u0026ndash;2358.\u003c/span\u003e\u003c/li\u003e\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":"InxAl1-xAs, indium composition, lattice-mismatched, carrier localization","lastPublishedDoi":"10.21203/rs.3.rs-4670192/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4670192/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn this work, we explored the impact of indium composition (x) on the structural and optical characteristics of In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1-x\u003c/sub\u003eAs layers within the context of quantum cascade laser (QCL) structures grown on InP (100) substrates using the Metal Organic Vapor Phase Epitaxy (MOVPE) method. The quality of the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1-x\u003c/sub\u003eAs QCL is notably influenced by the growth with low indium composition, evident in terms of crystallinity, interface sharpness, and optical properties. The properties of the InAsP layer at the InP/ In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1-x\u003c/sub\u003eAs junction are particularly sensitive to the indium composition. A drop below 0.52 in indium composition leads to a substantial lattice mismatch between the In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1-x\u003c/sub\u003eAs layer and the InP substrate, typically exceeding [3 8]%. This mismatch induces defects or traps within the bandgap, significantly impacting carrier localization in this system. Our study demonstrates that cultivating In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1-x\u003c/sub\u003eAs with a low indium concentration results in a strained (lattice-mismatched) In\u003csub\u003ex\u003c/sub\u003eAl\u003csub\u003e1-x\u003c/sub\u003eAs layer. This finding is significant as it can be leveraged to balance strain in high indium content InGaAs layers, particularly in the context of applications involving quantum cascade lasers.\u003c/p\u003e","manuscriptTitle":"Influence of Indium Composition on InAlAs QCLs","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-25 07:51:09","doi":"10.21203/rs.3.rs-4670192/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":"85081786-acac-4e59-8b02-da036d2e483e","owner":[],"postedDate":"July 25th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-06-09T20:23:12+00:00","versionOfRecord":[],"versionCreatedAt":"2024-07-25 07:51:09","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4670192","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4670192","identity":"rs-4670192","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}