Characterisation of polar (0001) and non-polar (11-20) ultraviolet nitride semiconductors

Abstract

UV and deep-UV emitters based on AlGaN/AlN heterostructures are very inefficient due to the high lattice mismatch of these films with sapphire substrates, leading to high dislocation densities. This thesis describes the characterisation of the nanostructures of a range of UV structures, including c-plane (polar) AlGaN epilayers grown on AlN template, and nonpolar GaN/AlGaN MQWs grown on a-plane GaN template. The results are based primarily on transmission electron microscopy (TEM), cathodoluminescence in the scanning electron microscope (SEM-CL), high-resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM) measurements. The structural and optical properties of various types of defect were examined in the c-plane AlGaN epilayers. Strain analysis based on in-situ wafer curvature measurements was employed to describe the strain relief mechanisms for different AlGaN compositions and to correlate the strain to each type of defect observed in the epilayers. This is followed by the investigation of AlN template growth optimisation, based on the TMA pre-dose on sapphire method to enhance the quality and the surface morphology of the template further. The initial growth conditions were shown to be critical for the final AlN film morphology. A higher TMA pre-dose has been shown to enable a better Al coverage leading to a fully coalesced AlN film at 1 μm thickness. An atomically smooth surface of the template was achieved over a large 10 x 10 μm AFM scale. Finally, the investigation of UV emitters based on nonpolar crystal orientations is presented. The SiNx interlayer was able to reduce the threading dislocation density but was also found to generate voids with longer SiNx growth time. The relationship between voids, threading dislocations, inversion domain boundaries and their associated V-defects and the variation in MQW growth rate has been discussed in detail.