| As one of the third generation semiconductor materials, GaN, performs many advantages, such as wide bandgap, high saturated electron drift velocity and high breakdown filed strength. Due to the excellent properties of GaN, it is a suitable material for the fabrication of various photonics and high-temperature, high-frequency, high power devices. However, it is still difficult to obtain high quality bulk GaN with large size up to till now. The electronic level GaN is commonly epitaxially grown on SiC or sapphire substrate. The epitaxial GaN layer is always high defected due to the large mismatch between the GaN layer and the substrate, degrading the device performance somewhat. Thus, the research on the high quality GaN with low defect density is one of the most important projects in this field. Furthermore, as a novel semiconductor material, many basic issues related to GaN need to be further studied in the future.A research on the AlGaN/GaN heterostructures grown on the vicinal sapphire substrates is presented in this paper. Superior structural and electrical properties of AlGaN/GaN heterostructures are obtained by using vicinal substrates, and high performance AlGaN/GaN materials are finally developed by combing the vicinal substrates and optimized growth processes. For its important role played in the fabrication of AlGaN/GaN HEMTs and various photonics, the properties of Si doped n type GaN are systematically studied in the latter part of the paper. It is expected a solid foundation could be made for the development of high performance GaN-based devices. The major works and achievements are listed as follows:1. Firstly, the properties of AlGaN/GaN heterostructures grown on vicinal sapphire substrates using low temperature AlN nucleation layers are studied in this paper. It is found that the crystallization qualities and surface morphologies of AlGaN/GaN heterostructures are greatly improved when carried on vicinal substrates. Moreover, the carrier mobility is found much higher by using vicinal substrate. The underlying physics are detailed analyzed.2. Secondly, the properties of AlGaN/GaN heterostructures grown on vicinal sapphire substrates using high temperature AlN nucleation layers are studied in this paper. In this part, two kinds of vicinal substrates are studied. It is reveled that the employ of the vicinal substrates can significantly improve the properties of AlGaN/GaN materials, whether the substrates are inclined toward a-axis or m-axis. It is also found the vicinal angular dimension is an important factor in the determination of the heterostructure properties. The physical mechanisms are analyzed and compared between the heterostructures using two kinds of AlN nucleation layers.3. The stress properties of Si doped n type GaN films are studied in this paper. The stress relaxation mechanisms are detailed analyzed. It is considered that the stress are mainly relaxed by bending dislocations in low doping samples, while the main stress relaxation units are edge dislocation in high doping samples.4. The electrical properties of Si doped n type GaN films are studied in this paper. It is found that the carrier mobility of the samples are firstly increased and then decreased with increasing the Si doping density. A reasonable explanation is proposed by analyzing the transport properties of n type GaN.5. Finally, the photoluminescence of Si doped n type GaN films are studied in this paper. It is found that the integrated intensity of the yellow luminescence in n type GaN is closely related to the edge dislocation density. It is thought that the edge dislocations can capture electrons to be negative centers, attracting plenty of positive centers under the action of coulomb force, thus the distance between donor-acceptor pairs can be greatly shorten. The relationships among the integrated intensity of the yellow luminescence in n type GaN, Si doping density and carrier mobility are also studied in this part. The underlying physical mechanisms are analyzed. |
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