Study On Dependence Of Electrical, Optical Properties On Point Defects Point Defects In GaN Epitaxial Film

As one of the third generation semiconductor materials, GaN has many advantages, such as wide bandgap, high saturated electron drift velocity and high breakdown filed strength. Due to these properties GaN is a suitable material for the fabrication of various photonics and high-temperature, high-frequency, high power devices. The electronic level GaN is commonly epitaxial grown on heterogeneous substrates by metal organic chemical vapor deposition(MOCVD). The epitaxial GaN layer has high density defects which include dislocations and point defects, due to the large lattice and thermal expansion coefficient mismatch between the GaN layer and the substrate, degrading the device performance somewhat. Thus, the research on the different types of defects putting different impact on materials and devices is one of the most important projects in this field, and so is how to acquire high quality GaN with low defect density. Furthermore, as a novel semiconductor material, GaN still have many related basic issues that need to be further studied in the future.A study on the relationship between the point defects and the electrical and optical properties in GaN epitaxial film are presented in this paper. Almost all the experimental samples are grown by MOCVD equipments which are designed by Xidian University. The characterization of point defects and physical mechanism in GaN film are studied by theoretical analysis and experiments in this paper. The major works and achievements are listed as follows:1. Firstly, leakage current in GaN epitaxial film of AlGaN/GaN HEMT structure is studied from the electrical properties aspect. First, we have determined the leakage current because of the point defects in GaN that provide the background carriers. Eliminate a major source of the background carriers in n-type GaN epitaxial layer is nitrogen vacancy (VN), interstitial nitrogen (Ni), interstitial gallium (Gai), substituted nitrogen (NGa) all of which are considered as donors by theoretical analysis. Then, we draw the conclusion that oxygen impurities (O) which come from the sapphire substrates decomposing is the main source of the background carriers by secondary ion mass spectroscopy (SIMS).2. Secondly, we draw the conclusion that buried charge layer near the substrate in GaN epitaxial layer with low concentration of carbon (C) and oxygen (O) is the leading cause of GaN buffer layer leakage, using the method of many sets of comparative experiments and the possible factors excluded. At the same time, we find out that the density of screw dislocation and the quality of nucleation layer are the major cause of impurity O diffusion upward from the substrate. In addition, we induce X-ray photoelectron spectroscopy (XPS) analysis to characterize the binding energy of carbon, and reveal the phenomenon that a small amount of carbon can compensate the background carries in GaN epitaxial layer.3. The reason of yellow luminescence (YL) is discussed in this part. We find the fact that the concentration of carbon in GaN epitaxial layer has consistency with the intensity of YL. Then we select 7 typical samples with different structures and carbon concentration. From comparing the 7 samples, we conclude that C-involved defects are originally responsible for the YL. Afterward, we detail the mechanism of the role of C-involved defects. When the CN0 after inspired by laser radiates to more stable-state CN^-1, GaN material emits YL.4. An indirect method for the characterization of gallium vacancy concentration has been proposed. From analyzing V/III ratio growth data, detecting the concentration of n-type carrier through SIMS, and discovering the Ga/N ratio through XPS, we compositely estimate the relative concentration of gallium vacancy (VGa).5. Furthermore, we find out the reasons why different samples have different C concentration. The MO-source is the major origin of C in MOCVD while none-C source is in HVPE growth pattern. There is more C concentration in non-polar a-plane GaN than polar c-plane GaN because of the crystal orientation. The protection effect of the Ga atom to the N atom determines the C concentration, and there is lacking this type of protection in a-plane GaN. Fe-doped GaN has lower C concentration because Fe atom can play a better role than Ga.6. Finally, the type of nucleation layer has been compared. We point out that the growth temperature and the thickness are the main factor to influence the concentration of C and O (The thicker, the lower). The growth mode of high temperature (HT) AlN nucleation layer is crystallization and nucleation simultaneously while of low temperature (LT) nucleation layer is crystallization after nucleation, so HT-AlN can grow thicker, and GaN epitaxial layer with HT-AlN nucleation layer has the best crystal quality with lower density of dislocations and lower concentration of C and O.