Growth and characterization of aluminum gallium nitride containing thin films for UV device applications

The main focus of this thesis is the development of good quality n-type conducting high Al mole fraction AlGaN thin films for UV applications via metalorganic chemical vapor deposition. To achieve this, an extensive buffer layer optimization had to be done, which evolved from high Al composition AlGaN buffers to AlN buffer layer, finally reaching a milestone in the demonstration of lateral epitaxial overgrowth of AlN films. The electrical properties of AlxGa1-xN films are then discussed in relation to the chemical concentrations of Silicon and residual impurities in the films. Of importance was attaining highly conductive Al0.65Ga0.35 N films fabricated using the indium-silicon codoping technique. The In-Si codoped Al0.65Ga0.35N layers exhibited an n-type carrier density as high as 2.5 x 1019 cm-3 with an electron mobility of 22 cm 2/Vs. In contrast, for samples grown without Indium the n -type carrier concentration dropped 2 orders of magnitude. Evaluation of the structural properties of Al0.49Ga0.51N films revealed that increased Si doping promoted the relaxation of the compressively strained layers. The relaxation is achieved by the inclination of pure edge threading dislocation lines with respect to the layer surface normal. The relaxation is not assisted by dislocation glide but rather is caused by the "effective climb" of edge dislocations. The effective dislocation climb may result from the film growth and it is not necessarily related to bulk diffusion processes. The contribution of the dislocation inclination to strain relaxation has been formulated and the energy release due to the dislocation inclination in mismatched stressed layers has been determined. Finally, AlGaN based ultraviolet solar blind photodiodes operating at a peak wavelength of 274nm with external quantum efficiencies of up to 46% at zero bias were grown. The photodiodes operate via backside illumination, allowing the use of Mg doped GaN instead of a high Al mole fraction p-AlGaN layer. The effects of the individual layer thicknesses, the Al composition difference between the n-type and intrinsic layer, the use of an AlGaN grade at the i-n interface, and the inclusion of an electron blocking layer at the p-i interface on the device performance were investigated.