Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

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Light-emitting diodes

We demonstrate the growth of a low dislocation density, relaxed GaSb bulk layer on a (001) GaAs substrate. The strain energy generated by the 7.78% lattice mismatch is relieved by a periodic array of 90° misfit dislocations. The misfit array is localized at the GaSb∕GaAs interface and has a period of 5.6nm which is determined by transmission electron microscope images. No threading dislocations are visible. The misfits are identified as 90°, rather than 60°, using Burger’s circuit analysis, and are therefore not associated with generation of threading dislocations. A low dislocation density and planar growth mode is established after only 3 monolayers of GaSb deposition as revealed by reflection high-energy electron diffraction patterns. Calculations corroborate the materials characterization and indicate the strain energy generated by the 7.78% lattice mismatch is almost fully dissipated by the misfit array. The low dislocation density bulk GaSb material on GaAs enabled by this growth mode will lead to new devices, especially in the infrared regime, along with novel integration schemes.

Strain relief by periodic misfit arrays for low defect density GaSb on GaAs

The viability of high-power and high-energy, direct eyesafe emission from bulk erbium lasers has recently been demonstrated. In this paper, we present a review of eyesafe erbium lasers that are resonantly pumped by both fiber and diode lasers. High brightness pumping with a 1.53-/spl mu/m erbium fiber laser has yielded 60 W of continuous wave (CW) output, 10 W of repetitively Q-switched output, and as much as 16 mJ of pulse energy. Diode laser pumping has yielded 38 W of quasi-CW output and >40 mJ of Q-switched output.

Resonantly pumped eyesafe erbium lasers

The authors demonstrate and characterize type-II GaSb quantum dot (QD) formation on GaAs by either Stranski-Krastanov (SK) or interfacial misfit (IMF) growth mode. The growth mode selection is controlled by the gallium to antimony (III/V) ratio where a high III/V ratio produces IMF and a low ratio establishes the SK growth mode. The IMF growth mode produces strain-relaxed QDs, where the SK QDs remain highly strained. Both ensembles demonstrate strong room temperature photoluminescence (PL) with the SK QDs emitting at 1180nm and the IMF QDs emitting at 1375nm. Quantized energy levels along with a spectral blueshift are observed in 77K PL. Transmission electron microscope images identify the IMF array and crystallographic shape for both types of QD formation. Atomic force microscope images characterize QD geometry and density.

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III/V ratio based selectivity between strained Stranski-Krastanov and strain-free GaSb quantum dots on GaAs

We report low-noise avalanche gain from photodiodes composed of a previously uncharacterized alloy, Al0.7In0.3As0.3Sb0.7, grown on GaSb. The bandgap energy and thus the cutoff wavelength are similar to silicon; however, since the bandgap of Al0.7In0.3As0.3Sb0.7 is direct, its absorption depth is 5 to 10 times shorter than indirect-bandgap silicon, potentially enabling significantly higher operating bandwidths. In addition, unlike other III-V avalanche photodiodes that operate in the visible or near infrared, the excess noise factor is comparable to or below that of silicon, with a k-value of approximately 0.015. Furthermore, the wide array of absorber regions compatible with GaSb substrates enable cutoff wavelengths ranging from 1 μm to 12 μm.

Low-noise AlInAsSb avalanche photodiode

High-power 2.2-μm diode lasers and their arrays were designed and fabricated. Laser heterostructures were grown using solid-source molecular beam epitaxy on GaSb substrates. The device active regions contained two 1.5% compressively strained GaInAsSb quantum wells. Heavy compressive strain in the active region ensured strong carrier confinement and high differential gain. A broadened waveguide design approach was utilized to obtain an internal optical loss below 4 cm-1 and a threshold current density below 100 A/cm2. Individual high-power lasers produced 1.6 W of continuous-wave (CW) multimode power at room temperature from a single 100-μm-wide aperture. Linear laser arrays generated more than 25 W of quasi-continuous wave output power. The device power conversion efficiencies were better than 20% in peak and above 10% at maximum output power level.

High-Power 2.2- $\mu$ m Diode Lasers With Heavily Strained Active Region

Mid-IR (λ≈3–3.5 μm) light emitting diodes with quinternary AlInGaAsSb barriers and InGaAsSb strained quantum wells grown on GaSb substrates have been demonstrated. The devices produced a quasi-cw emission power of 0.7 mW at room temperature and 2.5 mW at T=80 K.

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GaSb based light emitting diodes with strained InGaAsSb type I quantum well active regions

Metamorphic heterostructures containing bulk InAs1−xSbx layers and AlInAsSb barriers were grown on GaSb substrates. The lattice mismatch (up to 2.1%) between the GaSb substrates and the InAsSb layers was accommodated by the growth of GaInSb linearly graded buffers. The 1 μm thick InAsSb0.44 layer with an absorption edge above 9 μm exhibited an in-plane residual strain of about 0.08%. InAs1−xSbx structures with x = 0.2 and x = 0.44 operated as light emitting diodes at 80 K demonstrated output powers of 90 μW and 8 μW at 5 μm and 8 μm, respectively.

Metamorphic InAsSb/AlInAsSb heterostructures for optoelectronic applications

Bulk unrelaxed InAsSb alloys with Sb compositions up to 65% were grown on compositionally graded GaInSb and AlInSb buffers on GaSb substrates by molecular beam epitaxy. The minimum energy gap for these materials at T = 77 K was estimated to be 90 meV. Benchmark material parameters were measured for barrier photodetector heterostructures with 1-μm-thick InAs0.6Sb0.4 absorbers. A minority hole lifetime of 185 ns and a diffusion length of 9 μm at T = 77 K were determined from the transient response of barrier heterostructures. The data imply a hole mobility of 103 cm2/Vs, which was confirmed with frequency response measurements. A 100-μm square mesa contact nBn heterostructure demonstrated a −3 dB frequency response bandwidth of 50 MHz.

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Development of Bulk InAsSb Alloys and Barrier Heterostructures for Long-Wave Infrared Detectors

High-power 2.3-/spl mu/m In(Al)GaAsSb-GaSb type-I double quantum-well diode laser arrays were fabricated and characterized. Linear laser arrays with 19 100-/spl mu/m-wide elements on a 1-cm-long bar generated 10 W in continuous-wave (CW) mode and 18.5 W in quasi-CW mode (30 /spl mu/s/300 Hz) at a heatsink temperature of 18/spl deg/C. Array power conversion efficiency peaked at 30 A and was about 9%. Device internal efficiency was about 50%. Individual laser differential gain with respect to current was about twice as high as in InP-based laser heterostructures, demonstrating the potential of GaSb-based material system for high-power CW room-temperature laser diode arrays.

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D. Westerfeld

Papers

High-Power 2.2- $\mu$ m Diode Lasers With Heavily Strained Active Region

GaSb based light emitting diodes with strained InGaAsSb type I quantum well active regions

Metamorphic InAsSb/AlInAsSb heterostructures for optoelectronic applications

Development of Bulk InAsSb Alloys and Barrier Heterostructures for Long-Wave Infrared Detectors

High-power 2.3-/spl mu/m GaSb-based linear laser array