Recent Progress in the Growth of Mid-ir Emitters by Metalorganic Chemical Vapor Deposition
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Ideally, a laser with an N-stage active region could produce N photons for each carrier injected from the external power supply. Thereby, multi-staging of the laser active region may increase gain, lower threshold current, and finally increase the operating temperature of gain-limited, midinfrared lasers. The success of the unipolar, quantum cascade laser demonstrates the benefit of multi-stage gain regions.7 However, the nonradiative (optical phonon) lifetimes of the unipolar devices are orders of magnitude shorter than the Auger-limited lifetimes for interband devices, and with multi-staging, mid-infrared interband Sb-based lasers may have lower threshold currents than unipolar quantum cascade lasers. Gain regions with multiple electron-hole recombination stages have been proposed for Sbbased lasers.I'6 These devices utilize a semimetal layer, formed by an InAs (n) / GaAsSb (p) heterojunction, as an internal electron-hole source between staFes. Recently, cascaded lasers with type II InAs/GaInSb active regions have been demonstrated. The type II lasers were grown by molecular beam epitaxy, and characteristic of multi-stage lasers, large differential quantum efficiencies (> 1) are now reported.9 In this work we demonstrate 10-stage lasers composed of InAsSb quantum wells with type I band offsets. A band diagram of our multi-stage active region, under forward bias, is shown in Figure 1. Electron-hole recombination occurs in compressively strained InAsSb quantum wells separated by tensile strained InAsP barriers. The type I, InAsSb/InAsP quantum wells have a large light-heavy hole splitting required for suppression of Auger recombination. 2 Electron-hole pairs for each stage are generated at a semi-metal, GaAsSb (p)/ InAs (n) heterojunction. An AlAsSb layer prevents electrons from escaping; nominal hole confinement is provided by the InAsSb quantum well valence band offset relative to the InAsP barrier layer in this initial device. Ideally, electronhole generation replenishes the carriers which recombine in each stage, and for each carrier injected 19 Mat. Res. Soc. Symp. Proc. Vol. 484 01998 Materials Research Society
AlAsSb dectron block
(500 A)
Compressed InAsSb active regio
p InAs(n) /GaAsSb(p) semimretl Figure 1 - Band diagram of a multi-stage laser active region with compressed, type I InAsSb quantum wells separated by InAsP barriers, electron-hole generation by an InAs(n)/GaAsSb(p) semimetal heterojunction, and an A1AsSb electron block from the external circuit, the multi-stage active region can emit several photons resulting in an overall quantum efficiency greater than unity. In practice, charge can accumulate in the active region, shift the Fermi-level, and turn-off electron generation at the semi-metal. Previously, we have found that a AlGaAsSb graded layer between the AlAsSb and GaAsSb layers reduces hole trapping, thus increasing laser duty cycles and lowering turn-on voltages. Unlike previous cascaded lasers, our device was grown by metal-organic chemical vapor deposition (MOCVD). Our devices are a
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