Title: Fabrication and Characterization of Optoelectronics Devices Based on III-V Materials for Infrared Applications by Molecular Beam Epitaxy
Description: Optoelectronic devices based on III-V materials operating in infrared wavelength range have been attracting intensive research effort due to their applications in optical communication, remote sensing, spectroscopy, and environmental monitoring. The novel semiconductor lasers and photodetectors structures and materials investigated in this thesis cover the spectral range from 1.3µm to 12µm. This spectral region includes near-infrared (NIR), mid-infrared (MIR) and long wavelength infrared. This thesis demonstrated infrared optoelectronic devices, based on III-V compound semiconductors grown by Molecular Beam Epitaxy (MBE,) utilizing various combinations of novel III-V materials, device structures and substrate orientations. This thesis will be presented in two parts; the first part focuses on two types of photodetectors; type-II InAs/GaSb superlattice IR detector and AlGaAsSb/InGaAsSb mid-infrared heterojunction p-i-n photodetector. The second part of this thesis focuses on the three types of quantum well (QW) lasers; phosphor-free1.3µm InAlGaAs strain-compensated multiple-quantum-well (SCMQW) lasers on InP (100), InGaAsNSb/GaAs quantum wells (QWs) grown on GaAs (411)A substrates and mid-infrared InGaAsSb lasers with digitally grown tensile-strained AlGaAsSb barriers.

Type-II InAs/GaSb superlattice IR detectors with various spectral ranges were grown by MBE. Two superlattice structures with 15 monolayers (ML) of InAs/12ML GaSb and 17ML InAs/7ML GaSb are discussed. Based on X-ray diffraction (XRD) measurements both InAs/GaSb superlattices exhibit excellent material qualities with the full width at half maximum (FWHM) of the 0th-order peak about 20arcsec, which is among the narrowest ever reported. The 50% cutoff wavelengths at 80K of the two photodiodes with 15ML InAs/12ML GaSb and 17ML InAs/7ML GaSb superlattices are measured to be 10.2 µm and 6.6 µm, respectively. Mid-infrared heterojunction p-i-n photodetector, AlGaAsSb/InGaAsSb lattice-matched to GaSb grown by solid source molecular beam epitaxy using As and Sb valved crackers greatly facilitated the lattice-matching of the quaternary InGaAsSb absorbing layer to the GaSb substrates, as characterized by X-ray diffraction. The resulting device exhibited low dark current and a breakdown voltage of 32V at room temperature.

A record Johnson-noise-limited detectivity of 9.0 × [10]^10 cm Hz^(1/2)/W was achieved at 290K. The 50% cutoff wavelength of the device was 2.57 µm. Thus, our result has clearly demonstrated the potential of very high-performance lattice-matched InGaAsSb p-i-n photodetectors for mid-infrared wavelengths. For phosphor-free1.3 µm InAlGaAs multiple-quantum-well (MQW) lasers, the substrate temperature has been found to be a critical growth parameter for lattice-matched InAl(Ga)As layers in the laser structures. As shown by X-ray diffraction measurements, in the temperature range of 485-520° C, spontaneously ordered superlattices (SLs) with periods around 7-10 nm were formed in the bulk InAl(Ga)As layers. Based on photoluminescence (PL) measurements, a large band gap reduction of 300 meV and a broadened PL peak were observed for the In_0.52 Al_0.48 As layers with SL, as compared to those without SL. The undesirable, spontaneously-ordered SL can be avoided by using MBE growth temperatures higher than 530 °C.

This results in a high laser performance. Threshold-current density as low as 690 A/cm² and T_0 as high as 80 K were achieved for InAlGaAs laser bars emitting at 1310 nm. InGaAsNSb/GaAs QWs on GaAs (411)A exhibited remarkably enhanced photoluminescence efficiency compared with the same structures on conventional GaAs (100) substrates. It was further observed that the optimum growth temperature for (411)A was 30 °C higher than that for (100). To explain this phenomenon, a model based on the self-assembling of local rough surface domains into a unique global smooth surface at the lowest energy state of the system is proposed. Lastly, the digital-growth approach for tensile-strained AlGaAsSb barriers improved the reliability and controllability of MBE growth for the MQW active region in the mid-infrared InGaAsSb quantum well lasers. The optical and structural qualities of InGaAsSb MQW were improved significantly, as compared to those with random-alloy barriers due to the removal of growth interruption at the barrier/well interfaces in digital growth.

As a result, high-performance devices were achieved in the InGaAsSb lasers with digital AlGaAsSb barriers. A low threshold current density of 163 A/cm² at room temperature was achieved for 1000-µm-long lasers emitting at 2.38 µm. An external differential quantum efficiency as high as 61% was achieved for the 880-µm-long lasers, the highest ever reported for any lasers in this wavelength range.