Room-temperature cw Operation of GaInAsSb/AlGaAsSb Quantum Well Diode Lasers emitting beyond 2 µm

Semiconductor diode lasers emitting at wavelengths beyond 2 mu m are of great interest for applications in several fields, like molecular spectroscopy, medical surgery, and optical communication systems. Therefore, significant efforts in 2 to 3 mu m laser diodes using the III-V antimonides has been made recently. We report on the characteristics of 3 quantum well (QW) and single QW (SQW) GaInAsSb/AlGaAsSb laser structures employing a large optical cavity (LOC) design, grown by molecular beam epitaxy on n-doped (100) GaSb substrates. The active region of the 3 QW structure consists of three compressively strained 10-nm Ga(0.70)In(0.30)As(0.06)Sb(0.94) QWs separated by 20-nm-wide Al(0.28)Ga(0.72)As(0.02)Sb(0.98) barriers. The QW region is embedded between 40-nm-thick Al(0.28)Ga(0.72)As(0.02)Sb(0.98) separate confinement layers followed by 2-mu m-wide Al(0.85)Ga(0.15)As(0.07)Sb(0.93) cladding layers. The SQW laser has 480-nm-thick separate confinement layers and one QW in the active region, while the other parts of the structure are identical to the 3 QW laser. Index guided ridge-waveguide Fabry-Perot lasers were fabricated by chemically assisted ion beam etching. Cleaved laser bars were In-soldered substrate-side down onto Cu heat sinks. All devices were operated in cw mode if not stated otherwise. For both laser structures single mode at 2.25 mu m (3 QW) and 2.22 mu m (SQW) was achieved at 300 K for drive currents 10% above the threshold. For a ridge width of 64 mu m and a cavity length of 1250 mu m, threshold current densities [J[th]] of 225 A/cm2 and 156 A/cm2 were observed at 280 K for the 3 QW and SQW, respectively. A J(th) value of 144 A/cm2 for infinite cavity length was extracted for the 3 QW laser and a by a factor of three lower value, 55 A/cm2, for the SQW, as expected due to the decreasing number of QWs. The characteristic temperature for the threshold current density of the 3 QW laser was T(0) = 123 K between 200 K and 280 K. A differential quantum efficiency of 50% and a maximum total power efficiency as high as 17% was measured by the 3 QW laser at 280 K. A high internal quantum efficiency of 69% and 65%, for the 3 QW and SQW lasers. respectively, and low values for the internal loss coefficient of 7.7 cm(exp -1) (3 QW) and 5 cm(exp -1) (SQW) were determined. Maximum single facet cw output power of 240 mW was obtained at 280 K, limited by the thermal rollover, for a 64 mu m X 1000 mu m ridge-waveguide 3 QW laser with HR/AR coated mirror facets, as shown in Fig. 1. A light output power exceeding 0,5 W was achieved in pulse mode, with 5-mu s pulse width and 1% duty-cycle, for the coated laser at the same temperature. Figure 2 illustrates the gain-current curves of the 3 QW and SQW lasers at 280 K. Transparency current densities of 113 A/cm2 and 41 A/cm2 have been deduced for the 3 QW and SQW lasers, respectively. The values scale with the number of QWs. In conclusion, we have demonstrated room-temperature cw operation of large optical cavity GaInAsSb/AIGaAsSb type-I QW lasers emitting at 2.25 mu m, lasing up to at least 320 K. A maximum current efficiency of 0.16 W/A and power efficiency of 17% has been achieved.