Study On Growth Of GaN Films With Low Dislocation Density

With the development of society and the progress of science and technology, semiconductor material plays an extremely important role in modern technological revolution. As a representative of the third generation of semiconductor materials, gallium nitride (GaN) and other nitrides materials are hot topics in field of photoelectron materials and high temperature, high-power device. GaN-based semiconductor materials have prominent advantages, such as large band gap, electron drift velocity, good thermal conductivity and high breakdown filed strength. Due to these excellent properties, GaN films possess the unique advantage in the production of high-power, high frequency and high temperature electronic devices and optoelectronic devices. However, due to the large lattice mismatch and thermal mismatch between the GaN layer and the substrate, the epitaxial GaN film often receives a high dislocation density, which greatly degrades the GaN-based device performance. Therefore, researching for low defect GaN materials is always the key topic, which also is the main research direction in GaN field.This paper systematically studied the surface morphology, crystallization quality and strain of GaN films grown on the vicinal sapphire substrates.We effectively improved the quality of GaN by using vicinal sapphire with different vicinal angle and direction, and made a reasonable explanation to the related experimental phenomena. Moreover, the studies on dislocation annihilating mechanism in GaN were presented too, laying a solid foundation to make vicinal substrate applied to the growth of high quality GaN. In addition, we used a variety of growth technology including vicinal substrate homogeneous growth, different substrate surface treatment and TiN mask layer to improve the quality of GaN films, and the relevant mechanism was in-depth studied too. Our studies provide new ideas for low defect GaN heteroepitaxy. The major works and achievements are listed as follows:1. Firstly , we comparatively studied the crystallization quality, surface morphology and internal stress of GaN grown on vicinal substrate inclined toward m-axis and a-axis, and made a reasonable explanation to the related experimental phenomena. It is reveled that the vicinal substrate can induct effectivly the annihilation of dislocation in GaN films. It is also found that the vicinal substrate inclined toward m-axis is better for mergence of micro steps and release of stress, therefore, the GaN films on vicinal substrate inclined toward m-axis process higher crystallization quality.2. In the small angle range, the vicinal angular dimension is an important factor in the determination of crystallization quality, surface morphology and stress of GaN films.With the increasing of vicinal angular, the crystallization qualities are greatly improved, the width of surface micro steps are increased, and the internal stress is reduced too. This shows that substrate with larger vicinal angle is better for the mergence of micro steps, and then promote the bending of dislocations. In addition, there are some big steps on the morphology of GaN films on vicinal substrate, and the direction of big steps is different for different substrate. The underlying physics are detailed analyzed.3. The physical mechanisms of annihilation in GaN films on vicinal substrate are analyzed by TEM. It is found that dislocations appear together and annihilate each other in local area. It is also reveled that dislocations tend to annihilate concentratly in different thickness areas.4. The properties of GaN grown on vicinal substrate and nomal substrate by HVPE are studied in this paper. It is found that the GaN film carried on vicinal substrate processes much higher crystallization quality then the one on nomal substrate.There are two reasons for the phenomenon. Firstly, the vicinal substrate can induct effectivly the annihilation of dislocation in GaN films, thus dislocations can not extend to the top films; secondly, there are areas where dislocations annihilate concentratly in GaN films on vicinal substrate. Therefore, the advantage of vicinal substrate is reflected more fully for GaN thick film grown by HVPE.5. Finally, the effects of surface treatment to the secondary grown GaN films are studied in this paper. It is found that the GaN films on the substrate treated by HF acid have the higher quality and better photoluminescence. The relevant physical mechanism was discussed in the paper.6. The dislocation density of GaN films are reduced effectively using TiN as a mask layer. It is found that Ti metal deposited on surface of GaN films turns into TiN by nitriding, forming natural mask area and window area, which effectively inhibit the extension of dislocation. The underlying growth mechanisms are analyzed.