| Summary: | III-V semiconductor alloys possess interesting electronic properties that can be engineered and optimised for highly-efficient light absorption and emission in optoelectronic devices. Although GaAs is by far the most mature and cost-effective III-V semiconductor material, its use as a starting material for optoelectronic devices is limited by the feasibility of growing high-quality single crystals with different lattice parameters on GaAs substrates. In this thesis, p-i-n diodes comprised of two types of GaAs-based alloys with bandgap energies in the near and short-wave infrared (SWIR) range for photo-detection applications, are investigated experimentally. All of the samples are grown by molecular beam epitaxy (MBE) technique. The main focus of this thesis is on the first type of alloy, GaInNAs, in which the addition of a small amount of substitutional N atoms into GaAs results in an anomalously large energy band gap reduction, while the addition of In compensates the tensile strain introduced by the small N atoms to achieve lattice matching to GaAs. The second alloy, GaInAsSb, is conventionally grown on GaSb substrate, but epitaxial growth on GaAs is possible via a special growth mode, which enables a very thin GaSb epilayer to be grown directly on GaAs with minimum or absence of threading dislocations. As both of the material systems investigated in this work are known for various growth-related problems, attempts were made, whenever possible, to infer the electronic properties of the devices irrespective of the growth process. Growth-related defects in the dilute nitride GaInNAs alloy system are known to affect optoelectronic device performance with issues such as short minority-carrier diffusion lengths and high background doping densities. Despite these difficulties, interest into the dilute nitride alloys has continued to grow for over two decades, as evident by a recent breakthrough in high-efficiency, multi-junction solar cells utilising 1 eV GaInNAs absorber layers. Hence the thesis has to first deal with the effects of growth-related defects on the optoelectronic performance of GaInNAs devices. Investigation of the photovoltaic characteristics of GaInNAs diodes are useful in revealing issues affecting the efficiency and detectivity of photodetectors and solar cells, such as those associated with dark currents, quantum efficiency, minority carrier diffusion lengths, and lifetime. In addition, the effect of Sb, which is commonly used as a surfactant during the growth process of Ga(In)NAs, is investigated. The unique electronic band-structure properties in GaInNAs semiconductors are investigated through experimental measurements of the avalanche breakdown voltage, multiplication and excess noise in GaInNAs photodiodes with a range of N compositions below 4%. The results reveal evidence that both the electron and hole ionisation coefficients in GaInNAs with 3.8% N and 10% In are suppressed by up to two orders of magnitude at low fields, compared with GaAs. At high fields, analysis of the avalanche breakdown behavior suggests that electrons impact ionise at energies above the upper subband, E+(k). Consequently, the electron and hole ionisation coefficients are not so significantly disparate as previously thought, which has important implications for avalanche photodiode applications. Finally, the detectivity performance of GaInAsSb SWIR p-i-n photodiodes grown on GaAs substrate via a special growth mode is investigated. The study reveals that very smooth epilayer surface morphology and robust, high-detectivity GaInAsSb photodiodes can be realised on GaAs despite the presence of threading dislocations. These photodiodes yield an estimated maximum detectivity of mid 1010 cm Hz1/2 W-1 at room temperature, with a zero-bias differential-resistance area product of 260 O cmand a peak responsivity of 0.8 A/W at 2 µm. Although not having the best crystal quality at present, the performance of the GaAs-based GaInAsSb photodetectors is comparable to, if not marginally better, than similar GaSb-based devices reported to date in the literature. This further substantiates the potential of extending the detection wavelength of GaAs-based, high-operating-temperature photodetectors to the short and mid-wave infrared range.
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