Abstract
Short wavelength infrared (SWIR) and mid wavelength infrared (MWIR) photodetectors have applications in areas such as chemical sensing, gas monitoring, medical diagnostics, infrared imaging and free-space communications. Mercury Cadmium Telluride (HgCdTe) is the predominant material system used in SWIR, MWIR and long wavelength infrared (LWIR) applications. However, HgCdTe often suffers from poor material uniformity and low yield. Comparable performance can be achieved on GaSb substrates with high quality InAs/GaSb strained-layer superlattices, but significant cooling is required to achieve high detectivity performance.
Indium Phosphide (InP) based InGaAs/GaAsSb type-II quantum wells photodiodes are the promising candidates for the detection in SWIR and MWIR region, especially at room temperature. These photodiodes can take advantage of mature material and device technology of InP material system. In this dissertation, I study these InP based type-II quantum wells photodiodes theoretically and experimentally. Six-band k•p modeling was used to design these type-II quantum well structures. It is found that in order to maximize the transition wavelength and wave function overlap under strain compensated condition, the thickness of the InGaAs layer should be larger than that of the GaAsSb layer, and the GaAsSb layer should be compressively strained, while InGaAs layer should be tensile strained. Both lattice matched and conventional strain compensated type-II quantum wells photodiodes are designed and studied experimentally in this dissertation. How the performance of these type-II photodiodes changes as the detection wavelength increases is studied by comparing the performance of both photodiodes. The research has shown that the device with 100 pairs of 7nm In0.34Ga0.66As/5nm GaAs0.25Sb0.75 strain compensated type-II quantum wells absorption region has an optical response out to 3.2µm, while the device with 100 pairs of 7nm In0.53Ga0.47As/5nm GaAs0.5Sb0.5 lattice matched type-II QWs absorption region has an optical response out to 2.7µm. The strain compensated devices show detectivities of 1.4×109cm•Hz1/2•W-1 at λ=2.7m at 290K and 1.5×1010cm•Hz1/2•W-1 at 200K. For λ=3.0m, the detectivity D* is 2.0×108 cm•Hz1/2•W-1 at 290K and increases to 1.0×109 cm•Hz1/2•W-1 at 200K. They are the first 3µm results demonstrated on InP substrate, using interband transition without lattice mismatch layers.
Moreover, in order to further extend the detection wavelength, the new strain compensated InGaAs/GaAsSb QWs PIN photodiode is studied, which can lead to much longer detection wavelengths for similar GaAsSb compositions based on the modeling. This new strain compensation concept is demonstrated experimentally to have a cut-off wavelength similar to that of conventional strain compensated sample but with lower Sb composition in the GaAsSb layer. In addition, the carrier transport mechanism in the type-II quantum wells is studied in order to improve the carrier collection efficiency, and it is found that the thermionic emission is the dominant way for the photo generated electrons to get out of the quantum wells.
