# High-Power, High-Efficiency GaSb-Based Laser with Compositionally Linearly Graded AlGaAsSb Layer
## 2 Composition-Graded AlGaAsSb Layer Epitaxy
where \(\theta_{s}\) is the diffraction angle of the substrate, and \(\theta_{e}\) is the diffraction angle of the epitaxial layer. Figure 2 shows how the As BEP influences As incorporation when the Al and Ga growth rates are both 0.5 ML/s. When the As BEP varies from 1.40 \(\times\) 10\({}^{-7}\) hPa to 1.67 \(\times\) 10\({}^{-7}\) hPa, the lattice mismatch is maintained within \(\pm\)50 ppm. A quasi-linear dependence of As content on the As BEP is observed, and the slope is about 0.0045/1 \(-\) 10\({}^{8}\) hPa, which reveals a constant As stacking coefficient within the researched As BEP range. The experiment was conducted under Sb overpressure (1.33 \(\times\) 10\({}^{-6}\) hPa), where Sb BEP has little influence on the As content and group III composition does not enhance As incorporation [25]. Therefore, it is reasonable to assume a constant As stacking coefficient throughout the composition-graded AlGaAsSb layer epitaxy process. Valued cracker cells overcome the flux instability by providing a large reservoir of group V element vapor and controlling the flux with a needle valve. Real-time As flux monitoring and feedback control have been demonstrated by taking advantage of the rapid and repeatable flux response to the valve position [26]. In group V element composition-varying conditions, a step-like flux sequence is usually adopted to obtain the designed composition profile [27]. In this work, As BEP quasi-linear variation was accomplished by ramping the As cracker valve position to prevent background impurity incorporation induced by growth interruption. The As BEP is monitored by an ion gauge installed in the growth chamber. Figure 3 illustrates the As BEP profile obtained by ramping up the As valve position. The As BEP's linearly ascending segment lasts for \(\sim\)830 s, corresponding to the valve position ramp-up procedure. The valve position is fixed after the As BEP reaches the setpoint, and the As BEP is maintained for subsequent high-Al-content AlGaAsSb epitaxy. According to Figure 2, an 800 ppm lattice mismatch is the outcome of a 2.67 \(\times\) 10\({}^{-8}\) hPa As BEP deviation. Since an As BEP deviation within 2.67 \(\times\) 10\({}^{-8}\) hPa is acceptable for lattice-matched epitaxy, the demonstrated As BEP profile can be considered a linear modulation procedure, which is functional for composition-graded AlGaAsSb layer epitaxy. Figure 2: Influence of As BEP on As content and lattice mismatch (dots: measured; lines: fitted). A quasi-linear dependence of As BEP on As incorporation is observed, indicating a constant stacking coefficient under Sb overpressure and fixed growth temperature. Group III element diffusion cells show different operating characteristics from group V valved cracker cells: the group III element flux is directly produced by metal evaporation, which causes an exponential flux-temperature dependence in the steady-state case, and flux transients occur when the cell operating condition changes. In addition, cell temperature ramping is not as rapid as valve position ramping. However, [PERSON] demonstrated linear Ga growth rate modulation with a rate of change of 12.4 (A/h)/s by linearly ramping the Ga cell temperature [28], indicating that the linear modification of group III BEP is feasible by adopting a sufficiently slow ramping rate. A similar approach was taken in our experiment, and Figure 4 shows the flux obtained in the Al temperature time-varying growth. The Al cell temperature ramp-up procedure lasts for \(\sim\)830 s, and the cell temperatures of the starting and finishing points are calibrated in advance from the RHEED oscillation curve for the specified Al growth rate, namely, 0.16 ML/s for Al\({}_{0.25}\)Ga\({}_{0.75}\)As\({}_{0.02}\)Sb\({}_{0.98}\) and 0.5 ML/s for Al\({}_{0.5}\)Ga\({}_{0.5}\)As\({}_{0.04}\)Sb\({}_{0.96}\). Linear Al flux modulation followed by flux fluctuation as the cell temperature reaches the setpoint is observed.