Advances in Type-I GaSb Based Lasers Gregory BELENKY  , Leon SHTERENGAS, Dmitry DONETSKY, Mikhail KISIN, and Gela KIPSHIDZE Department of Electrical and Computer Engineering, SUNY at Stony Brook, NY 11794-2350, U.S.A. (Received December 13, 2007; accepted June 13, 2008; published online October 17, 2008) We show that high level of compressive strain in the optically active quantum wells is a key condition for efficient continuous- wave room-temperature operation of the type-I GaSb-based diode lasers. Lasers with two highly strained InGaAsSb quantum wells and AlGaAsSb barriers demonstrate an output CW power of 1050 mW at 2.4 mm and 85 mW at 3.1 mm. [DOI: 10.1143/JJAP.47.8236] KEYWORDS: high-power, mid-infrared, GaSb, type-I, diode lasers Mid-infrared light emitters capable of room temperature continuous-wave (CW) operation are in demand for variety of applications ranging from medical diagnostics to missile countermeasures. Room temperature operated GaSb-based CW lasers and laser arrays operating in the spectral range from 2.3 mm to over 3 mm have been reported. 1–7) It has been noted 3) that in GaSb-based type-I quantum well (QW) diode lasers the reduction of the hole confinement barriers in active QWs is responsible for the decrease of the laser output power with increasing wavelength. In this work we demonstrate that highly strained InGaAsSb QWs incorporated in AlGaAsSb waveguide allow implementation of high-power 2.4 mm diode lasers emitting 1050 mW CW at room temperature with maximum power conversion efficiency (PCE) of 17.5%. Long wave- length 3.1 mm emitting lasers produce 85 mW of CW output optical power. The laser heterostructures were grown using a Veeco GEN-930 solid source molecular beam epitaxy system on Te-doped GaSb substrates. The cladding layers were 1.5 mm wide Al 0:9 Ga 0:1 As 0:07 Sb 0:93 doped with Te (n-side) and Be (p-side). For 3.1 mm emitting devices the n-cladding thick- ness was increased up to 2.5 mm to eliminate modal leakage into high refractive index GaSb substrate. Graded bandgap heavily doped transition layers were introduced between the substrate and n-cladding and between the p-cladding and p-cap to assist carrier injection. We used a nominally undoped Al 0:25 Ga 0:75 As 0:02 Sb 0:98 waveguide for 2.4 mm lasers and Al 0:35 Ga 0:65 As 0:03 Sb 0:97 waveguide for 3.1 mm lasers. For short wavelength devices, the active region contained two 12-nm-wide compressively strained (1.6%) In 0:37 Ga 0:63 As 0:1 Sb 0:9 QWs centered in the 800 nm wave- guide and separated 20 nm apart. Another structure with lower strained (1.2%) QWs was manufactured using higher QW arsenic concentration of 16%. 12-nm-wide 100-nm- spaced 1.8% compressively strained InGaAsSb QWs with nominal indium composition of about 50% were used to fabricate lasers with  ¼ 3:1 mm. The total width of the 3.1 mm emitting device waveguide was about 1 mm. The wafers were processed into 100-mm-wide oxide confined gain-guided lasers. Devices with anti-reflection (AR) 3% (neutral, NR, 30% for 3.1 mm lasers) and high- reflection (HR) 95% coating were In-soldered epi-side down onto Au-coated copper blocks and characterized. Figure 1 shows the CW light-current and power-conver- sion characteristics for 1-mm-long and 2-mm-long AR/HR coated lasers. The CW output power above 850 mW was achieved for the 1-mm-long lasers at 4 A while the 2-mm- long device shows 1.050 W at 6.2 A. The lasers power- conversion efficiency was peaked at 17.5% for the 1-mm- long lasers and was above 10% over the whole range of operation for the 2-mm-long devices. The CW output power levels and power conversion efficiencies obtained for these heavily strained 2.4 mm emitting lasers are superior to previously reported 2.3 – 2.5 mm emitting devices 2,4) (650 mW at 2.35 mm for 1-mm-long lasers with peak PCE  11% and 1W CW at 7A at 2.5 mm for 2-mm-long lasers with peak PCE  12%). The improved power- conversion performance correlates with the high device external efficiency and low threshold current density. Figure 2 shows the peak modal gain as a function of under-threshold current as obtained from Hakki–Paoli measurements for 1-mm-long devices. The rate of increase of the modal gain with current exceeds 200 cm 1 /A. The corresponding data for lasers with lower strain in active region 4) are presented also in Fig. 2. The enhanced QW compressive strain leads to improved device differential gain and nearly halves the laser threshold current density. The experimental results obtained for 2.4 mm lasers can be explained by the improvement of the hole confinement barriers in the heavily strained quantum wells. In contrast to strain-induced density of states (DOS) balancing, which is efficient in A 3 B 5 materials for strain values only up to 1%, 8) the enhancement of the hole confinement due to the 0 5 10 15 20 0 1 2 3 4 5 6 7 0.0 0.2 0.4 0.6 0.8 1.0. 1.2 2mm 1mm 2mm 1mm CW, T=18°C epi-down Power (W) Current (A) Power Conversion Efficiency (%) 2.3 2.4 2.5 2A Wavelength (um) Fig. 1. CW current dependences of the output power and power- conversion efficiencies of 2.4 mm emitting lasers with cavity lengths of 1 and 2 mm. The insert shows the laser spectrum of the 1-mm-long device at 2 A.  E-mail address: garik@ece.sunysb.edu Japanese Journal of Applied Physics Vol. 47, No. 10, 2008, pp. 8236–8238 #2008 The Japan Society of Applied Physics 8236 Communication