| AlGaN-based ultraviolet light-emitting diodes (UV-LEDs) have a wide range of potentital applications in the ultraviolet field such as chemical detection, water purification, disinfection, environmental protection, ultraviolet communication, and white light illumination. Research and development of AlGaN-based UV-LEDs materials and devices have attracted considerable attention in recent years. However, there exists a strong polarization field in the traditional c-plane AlGaN LEDs which is produced by spontaneous and strain-induced piezoelectric polarizations in the c-axis oriented hexagonal AlGaN. This polarization field is undesirable for light-emitting devices since the associated electric fields separate the electrons and holes at the opposite interfaces of the quantum well and thus reduce the overlap of their wave functions, which leads to a reduction of the recombination efficiency in light-emitting devices. This phenomenon is known as the quantum confined stark effect. It is possible to eliminate this effect by growing on non-polar planes, such as the α-plane or the m-plane. In this case there are no polarization fields perpendicular to the layer interfaces, because the polar c-axis lies within the growth plane. However, films grown along these non-polar directions suffer from rough surface as well as a high density of dislocations and stacking faults. It is very difficult to obtain non-polar materials with high crystal quality and smooth surface. In this dissertation, the a-AlGaN films grown on r-plane sapphire substrates have been investigated via metalorganic chemical vapor deposition (MOCVD). The main works and results are as follows:(1) The effects of substrate nitridation process on a-GaN films are investigated in the the first part of this dissertation. The role of nitridation process in the film growth process is discussed by comparing the crystal quality and surface properties of a-GaN grown on r-plane sapphire substrates with nitridation and without nitridation.(2) The different effects of the low temperature (LT) AIN nucleation layer (NL) and high temperature (HT) AIN nucleation layer on the surface morphology and crystal quality of α-Al0.1Ga0.9N films are studied. Results show that the crystal quality and surface properties of a-Al0.1Ga0.9N films grown on HT-A1N NL demonstrate better performance. The thickness of HT-A1N NL is optimized and the crystal quality and surface properties of a-Al0.1Ga0.9N films exhibit best performance when the thickness of HT-A1N NL is40nm.(3) We found that a reasonable increase in growth temperature can improve the crystal quality of the α-Al0.1Ga0.9N films. But when temperature is too high the Al0.1Ga0.9N films will be decomposed and surface pits arise. The most suitable growth temperature for α-Al0.1Ga0.9N film is between1010℃to1030℃.(4) Increase in the thickness of α-Al0.1Ga0.9N films is not effective to improve the quality of crystal along the c direction in (1120) plane, while the crystal quality of α-Al0.1Ga0.9N films along the m direction in (1120) plane can be improved to a certain extent. At the same time, the anisotropy extent of thin films can also be improved with increasing thickness.(5) A smooth surface of α-Al0.1Ga0.9N film is obtained by decreasing the V/III ratio in-three steps along the growth direction and the RMS roughness of sample within the area of5×5μm2is1.29nm.(6) The crystal quality of α-Al0.1Ga0.9N films can be effectively improved by inserting AlN/Al0.1Ga0.9N superlattices. The best crystal quality of α-Al0.1Ga0.9N film is obtained when the superlattice period number is20. The XRC results show that the FWHMs of (1120) plane along c direction and m direction are673arcsec and1101arcsec, respectively.(7) The effect of SiH4flow rate on the n-type carrier concentration is studied. The electron concentration of n-Al0.1Ga0.9N can reach up to2.48×1018cm-3. The effect of CP2Mg flow rate on the p-type carrier concentration is also studied. The hole concentration of p-Al0.1Ga0.9N can reach up to4.11×1017cm-3. The doping of n-type and p-type leads to the deterioration of the crystal quality and surface properties of the Al0.1Ga0.9N-films.(8) GaN/Al0.1Ga0.9N MQWs with diferent well thicknesses are successfully achieved on α-Al0.1Ga0.9N template. The results show that the PL emission intensity of MQWs is significantly enhanced with the increase of thickness of the well layer in a certain range.
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