# High-Power, High-Efficiency GaSb-Based Laser with Compositionally Linearly Graded AlGaAsSb Layer
## 3 Laser Performance  
The verified composition-graded layer was then applied to InGaSb/AlGaAsSb quantum-well lasers. As Figure 6 shows, the laser consists of a 1500 nm Al\({}_{0.5}\)Ga\({}_{0.5}\)As\({}_{0.04}\)Sb\({}_{0.96}\): Te bottom cladding layer, a 210 nm Al\({}_{0.5}\)Ga\({}_{0.5}\)As\({}_{0.04}\)Sb\({}_{0.96}\) to Al\({}_{0.25}\)Ga\({}_{0.75}\)As\({}_{0.02}\)Sb\({}_{0.98}\) bottom composition-graded layer, a 270 nm Al\({}_{0.25}\)Ga\({}_{0.75}\)As\({}_{0.02}\)Sb\({}_{0.98}\) bottom waveguide layer, a 40 nm active region, a 270 nm Al\({}_{0.25}\)Ga\({}_{0.75}\)As\({}_{0.02}\)Sb\({}_{0.98}\) top waveguide layer, a 210 nm Al\({}_{0.25}\)Ga\({}_{0.75}\)As\({}_{0.02}\)Sb\({}_{0.98}\) to Al\({}_{0.5}\)Ga\({}_{0.5}\)As\({}_{0.04}\)Sb\({}_{0.96}\) top composition-graded layer, a 2000 nm Al\({}_{0.5}\)Ga\({}_{0.5}\)As\({}_{0.04}\)Sb\({}_{0.96}\): Be top cladding layer, and a 250 nm GaSb: Be contact layer. The active region includes two 10 nm In\({}_{0.2}\)Ga\({}_{0.8}\)Sb quantum wells, which are separated by a 20 nm Al\({}_{0.25}\)Ga\({}_{0.75}\)As\({}_{0.02}\)Sb\({}_{0.98}\) barrier. Figure 5: AFM and HRXRD results of composition-graded layer show high-quality and lattice-matched growth of composition-graded AlGaAsSb layer. GaSb-lattice-matched AlGaAsSb transforms from a direct bandgap semiconductor to an indirect bandgap semiconductor when the Al content is higher than 0.45. Electrons injected from the Al\({}_{0.5}\)Ga\({}_{0.5}\)As\({}_{0.04}\)Sb\({}_{0.96}\) cladding layer first localize in the X valley and are then scattered into the \(\Gamma\) valley while being transported through the composition-graded AlGaAsSb layer, which is smoother than electrons crossing the abrupt Al\({}_{0.5}\)Ga\({}_{0.5}\)As\({}_{0.04}\)Sb\({}_{0.96}\)/Al\({}_{0.25}\)Ga\({}_{0.75}\)As\({}_{0.02}\)Sb\({}_{0.98}\) interface. In addition, composition-graded AlGaAsSb layers flatten the potential barrier peak of the heterostructure, lower the series resistance, and suppress the rectification effect caused by the potential barrier peak. A 200 \(\upmu\)m wide ridge pattern was prepared via the photolithography process and then etched 2 \(\upmu\)m deep in an inductively coupled plasma reactive-ion etching (ICP-RIE) system. A 250 nm thick SiO\({}_{2}\) insulation layer was deposited on the ridge waveguide by plasma-enhanced chemical vapor deposition (PECVD), through which a 190 \(\upmu\)m wide current injection window was etched by reactive-ion etching (RIE). The GaSb substrate was then thinned and polished to 120 \(\upmu\)m thickness to reduce the series resistance. AuGeNi/Au and TiPtAu were sputtered as N contact metal and P contact metal, respectively. After the deposition of AuGeNi/Au, rapid thermal processing (RTP) was conducted to form Ohmic contact. A wafer was then cleaved into bars with different cavity lengths for subsequent experiments. Single emitters were p-side-down c-mounted on a copper heatsink with In solder, while bar lasers were mounted onto a microchannel heatsink for better heat dissipation performance. The tests were all conducted in CW or QCW (with a pulse duration of 100 \(\upmu\)s at a 100 Hz repetition rate) operation. For CW output performance characterization, bars with 1 mm cavity length were 5% anti-reflection- and 95% high-reflection-coated and then cleaved into 500 \(\upmu\)m wide chips as single emitters. The CW performance at 15 \({}^{\circ}\)C of a single emitter is shown in Figure 7. The turn-on voltage is 0.65 V. Since the wavelength of our laser is 2006 nm, the theoretical lowest turn-on voltage for population inversion is the quasi-Fermi energy difference corresponding to the lasing transition, which can be expressed as Equation (4) [27]:  
\[V_{F}=\frac{ch}{\lambda e}\approx 0.62\ V \tag{4}\]  
The difference between the turn-on voltage and this theoretical lowest value is the voltage penalty induced by heterojunction barriers [29].