# High-Power, High-Efficiency GaSb-Based Laser with Compositionally Linearly Graded AlGaAsSb Layer
## 1 Introduction  
GaSb-based lasers are the most promising candidates among mid-infrared lasers owing to their high efficiency, small volume, and low cost, which are in high demand for gas detection [1], pollution monitoring [2], aesthetic surgery [3], material processing [4], and resonant pumping of the Ho-doped host [5]. Radiation recombination in InGaAsSb/AlGaAsSb type-I quantum wells offers output radiation with wavelengths covering 2-3 \(\upmu\)m and exhibits higher efficiency and temperature stability than that in InP-based quantum wells. To achieve high output power, a broad-area ridge waveguide is the normally used structure in electrically pumped diode lasers. Benefiting from the flexibility of the energy band design and precise MBE material growth techniques, both wide wavelength coverage and room-temperature continuous-wave operation have been achieved in GaSb-based broad-area lasers. In order to achieve high-power, high-efficiency output performance, an optimized separate confinement heterostructure (SCH) design [6], a narrow waveguide layer with an optimized doping profile [7], an asymmetric waveguide layer with a carrier barrier [8], and cascade type-I quantum wells [9] have been attempted and resulted in a low threshold, low internal loss, and Watt-level high-output-power InGaAsSb/AlGaAsSb quantum-well lasers. Except for cascade type-I quantum-well lasers, the abovementioned schemes are made of composition-fixed functional layers, which limits the design flexibility of GaSb-based lasers. The composition-graded layer was first proposed in the GaAs/AlGaAs system, which is known as a graded-index separate confinement heterostructure (GRIN SCH) and has been well developed in InP and GaN systems [10; 11; 12; 13]. The composition-graded layer epitaxy technique provides more flexibility for semiconductor device designs, e.g., the modification of the built-in electric field, bandgap profile, and refractive index profile [14; 15; 16]. Further improving the performance of GaSb-based lasers requires addressing the series resistance caused by the abrupt interface between the waveguide layer and cladding layer, which remains a severe problem. On the one hand, series resistance causes power dissipation, and on the other hand, it accelerates heat accumulation, which lowers efficiency. Inserting a composition-graded layer into lasers can reduce the series resistance induced by a discontinuous bandgap and enhance the built-in electric field to raise the carrier injection efficiency [17]. For GaSb-based lasers, the composition-graded AlGaAsSb layer between the high-Al-content AlGaAsSb cladding layer and low-Al-content AlGaAsSb waveguide layer is supposed to maintain the lattice constant throughout the content regulation process. Composition-graded AlGaAsSb layer preparation is impeded by the two following facts: group V element competition makes the As incorporation behavior unclear, and the lattice constant is far more composition-dependent than GaAa/AlGaAs, whose lattice constant is almost independent of Al content. This difficulty also exists in InP-based devices with an InGaAsP composition-graded layer; conventional epitaxy processes divide the composition-graded region into thin sublayers, and each sublayer is grown with lattice-matched parameters to obtain the required composition profiles. However, such composition-graded epitaxy technology is more applicable to MOCVD, where source adjustment needs less response time, while GaSb-based device epitaxy is mainly accomplished in MBE systems. Growth interruption for changing and stabilizing the cell temperature may cause residual impurity incorporation for MBE [18]. A more competitive alternative is dominating beam equivalent pressure by precisely controlling the cell temperature and valve position to implement continuous growth. A continuously graded parabolic quantum-well laser was produced with such a method in the GaAs/AlGaAs system [19; 20]. Such a method has not been discussed in detail in GaSb-based systems.