| Summary: | The terahertz (THz) range in the electromagnetic radiation spectrum lies between the high-frequency edge of the microwave band and the long-wavelength edge of the far-infrared band. THz technology is important for both scientific and commercial applications so the production of coherent, high power sources operating at room temperature is of great interest. Quantum cascade lasers (QCLs) are unipolar emitters operating from THz to infrared range. Molecular beam epitaxy (MBE) is the main technique for the growth of THz QCLs, offering precise control of layer thickness and composition under ultra-high vacuum conditions. The output power, maximum operating temperature, threshold and dynamic range are essential parameters in THz QCL performances. Since high power in THz QCLs is a desirable performance in particular to areas such as imaging and remote sensing. The threshold and dynamic ranges are influenced by the injector doping levels determining the losses. Moreover, the interface roughness affects the electrical and optical properties of semiconductor devices, which can be improved by misorientation of (100) GaAs substrates. This thesis reviews the development of THz QCLs, investigates MBE growth, fabrication and electrical characterisation process, and the enhancement of THz QCL performances. Chapter 1 summarizes the development and application of THz technology, different THz sources, as well as the development of THz QCLs. Chapter 2 outlines the MBE system, and investigates the calibration methods, the growth process, and growth reproducibility. Chapter 3 explains the fabrication techniques, electrical characterisation of THz QCLs, and the factors that influence the device performance. Chapter 4 investigates the enhancement in THz QCL power performance; whilst Chapter 5 probes the injector doping effects and background impurity level influences on BTC and hybrid THz QCL performance. In Chapter 6, the effects and possible significance of misorientation on THz QCLs are discussed. Chapter 7 concludes the thesis and suggests future directions.
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