# High-Power, High-Efficiency GaSb-Based Laser with Compositionally Linearly Graded AlGaAsSb Layer
## 2 Composition-Graded AlGaAsSb Layer Epitaxy  
In this work, composition-graded AlGaAsSb layers were inserted between an Al\({}_{0.25}\)Ga\({}_{0.75}\)As\({}_{0.02}\)Sb\({}_{0.98}\) waveguide layer and an Al\({}_{0.5}\)Ga\({}_{0.5}\)As\({}_{0.04}\)Sb\({}_{0.96}\) cladding layer. There is a bandgap difference of -0.3 eV between Al\({}_{0.25}\)Ga\({}_{0.75}\)As\({}_{0.02}\)Sb\({}_{0.98}\) and Al\({}_{0.5}\)Ga\({}_{0.5}\)As\({}_{0.04}\)Sb\({}_{0.96}\). The composition-graded AlGaAsSb layer is supposed to be beneficial to the electrical characteristics and efficiency of lasers. According to [PERSON]'s law [21], the AlGaAsSb lattice constant is expressed as:  
\[a_{AlxGa(1-x)AsySb(1-y)}=xya_{AlAs}+x(1-y)a_{AlSb}+(1-x)ya_{GaAs}+(1-x)(1-y)a_{ GaSb} \tag{1}\]  
where \(a_{AlAs}\) is 5.6611 A, \(a_{AlSb}\) is 6.1355 A, \(a_{GaAs}\) is 5.6533 A, and \(a_{GaSb}\) is 6.0959 A. Linear interpolation Equation (1) is visualized in Figure 1. A 1% Al content deviation from the GaSb-lattice-matched condition will cause a 65 ppm lattice mismatch, and the same deviation for As content will lead to a 750 ppm lattice mismatch. A strict lattice match is unnecessary for thick-layer epitaxy. Maintaining a lattice mismatch within 1000 ppm is sufficient to effectively prevent dislocation generation. The line segment connecting Al\({}_{0.25}\)Ga\({}_{0.75}\)As\({}_{0.02}\)Sb\({}_{0.98}\) and Al\({}_{0.5}\)Ga\({}_{0.5}\)As\({}_{0.04}\)Sb\({}_{0.96}\) represents the ideal composition modulation process throughout composition-graded AlGaAsSb layer epitaxy, and the actual composition modulation process with a deviation within the marked acceptable region can also result in high-quality AlGaAsSb epitaxy. As Figure 1 illuminates, the Al content fluctuation is acceptable on the condition that the deviation from the designed value is less than 16%, and the As content fluctuation within 1.3% is acceptable. Therefore, precise As content control is much more essential for GaSb-lattice-matched AlGaAsSb quaternary epitaxy. While group III elements with unit sticking coefficients are capable of being directly compositionally modified by adjusting the growth rate, the control of group V element incorporation is more complicated. Materials grown by MBE are usually in group V over-pressure conditions, which means that group V elements are in excess of the available surface group V vacancies, causing their sticking coefficients to be less than one unit. The consequent group V competition for incorporation in mixed-anion semiconductor alloys causes difficulty in group V content control [22]. Considering that group V element incorporation is sensitive to group V flux and growth temperature, we kept the growth temperature, group III flux, and Sb flux constant and vary the As cell valve position to determine the GaSb-lattice-matched AlGaAsSb epitaxy condition in the following experiment. A linear relationship between the As content of AlGaAsSb and the As beam equivalent pressure (BEP) in low-As-content conditions has been reported [23]. An experiment was conducted to determine the As BEP dependence of As content in our equipment. The substrate temperature was calibrated by the \(2\times 5\to 1\times 3\) reconstruction transition of the GaSb surface, which is set as Tc, according to RHEED observations, and the growth temperature is 130 \({}^{\circ}\)C above Tc for high-quality AlGaAsSb epitaxy. The lattice mismatch was experimentally deduced from the HRXRD diffraction peak position difference of the epitaxial layer and substrate, according to Equation (2) [24]. \[\frac{\Delta a}{a}=(\theta_{s}-\theta_{e})\cot\theta_{s} \tag{2}\]  
Figure 1: Dependence of lattice mismatch for Al and As contents. AlGaAsSb composition is assumed to vary along the line between Al\({}_{0.25}\)Ga\({}_{0.75}\)As\({}_{0.02}\)Sb\({}_{0.98}\) and Al\({}_{0.5}\)Ga\({}_{0.5}\)As\({}_{0.04}\)Sb\({}_{0.96}\). Fluctuation within the dotted line is acceptable for GaSb-lattice-matching epitaxy.