The Study On The Intrinsic Defects And Annealing Charateristics Of Unintentionally Doped 4H-SiC

High-quality Semi-insulating silicon carbide (SiC) crystals can be used for many applications related to Integrated Circuits (IC), high power MESFETs, PIN devices, and so on. One example is the requirements for high-quality semi-insulating substrates for high-power, high-frequency devices based on SiC and GaN. In addition, high-quality semi-insulating SiC crystals can also be applied in device isolation or junction termination which is very important for improving the performances of devices. It is well known there are some difficulties to measure and characterize the high resistance materials. Another key factor in semi-insulating SiC is the characters of native defects in high-quality materials. It is reported that the commercial semi-insulating 4H- and 6H-SiC have been prepared by physical vapor transport (PVT) and high temperature chemical vapor deposition (HTCVD), meanwhile the kinds of intrinsic defects, geometrical configurations and annealing characters have been studied, in which there are many differences among the experimental results.At present, there are obviously progresses about the growth of 4H-SiC homoepitaxial layers, in which the concentration of impurity can be decreased greatly. But there are many difficulties on systematically testing the intrinsic defects in unintentionally doped 4H-SiC, meanwhile the results obtained from measurements are different for different samples so that the origin of native deep energy level is not quit clear. In this case, there are still different opinions for the native deep energy level in unintentionally doped 4H-SiC. In this dissertation, the electronic structures of 4H-SiC crystal are investigated and the influences of some intrinsic defects on the electronic structures are computed, from which the formative energy of each native defects and their concentration can be deduced at room temperature. On the other hand, the intrinsic defects in unintentionally doped 4H-SiC prepared by ourselves are comprehensively studied. The main studies and contributions of this dissertation are as follows.1. With the simulator CASTEP, the electronic structure of perfect 4H-SiC and 4H-SiC with intrinsic defects such as VC,VSi,VC-VSi,VC-C and VSi-Si have been calculated. The results indicate that 4H-SiC is an indirect wide band gap semiconductor. The bottom of conduction band and the top of valence band are located in the M and G point in the Brillouin zone respectively. The energy band (Eg) of 4H-SiC is 2.36eV, less than that of experimental value, which is caused by the calculation approach of General Gradient Approximate (GGA). C vacancy (VC) produces two donor defect energy levels in the band gap, located in Ec-～0.6eV and Ev+～1.0eV respectively. Si vacancy (VSi) produces an acceptor level located in Ev+～0.5eV, meanwhile each of the VC-VSi, VC-C and VSi-Si defects introduce one defect energy level, located in Ec-1.2eV, Ec-1.6eV and Ec-1.1eV respectively. In addition, the formative energy of different intrinsic defects is calculated in this paper. The result shows that the formative energy of different intrinsic defects is ranked, from small to large as VSi→VC-C→VSi-Si→VC→VC-VSi. Based on this the concentration of intrinsic defects is computed and the concentrations of VC-VSi, VC-C and VC are larger than others. The computation results are basically accordance with that of experimental conclusions in high quality semi- insulating 4H-SiC.2. The Electron Paramagnetic Resonance (ESR) characters of intrinsic defects in unintentionally doped 4H-SiC under dark are investigated. Firstly, the ESR spectrums express an obvious anisotropy in unintentionally doped 4H-SiC. The asymmetric chart and wider ESR line width should be attributed to the higher testing temperature, non-homogeneous distribution of the defect concentration and the unsymmetrical crystal structure in 4H-SiC. The ESR results show that the ESR characters of intrinsic defects correspond with that of VC and its complex compounds based on the ESR theory, which is accordance with simulating results.On the other hand, with exposed in Xenon (Xe) light and testing temperature kept at 120K, the results show that there are two other apexes presented in the ESR spectra, which are likely the intrinsic defects VSi and VCCSi respectively by computing their g vector. Meanwhile the ESR results show that the intrinsic defects in illuminated samples are the same as that of as-grown samples. While some variations happened on the ESR spectrums of illuminated samples, such as the ESR peak and line width, which show the concentration and distribution of intrinsic defects in illuminated samples are changed. On the other hand, with the illumination time increase, the two other apexes are fade-away and the ESR intensity is changed periodically with the variation range decreased gradually. The ESR intensity gets to the maximum when the illumination time is 2.5min, which results from the intrinsic defects have a strong interactions and reciprocal transformation in the Xe illumination.3. The energy level of intrinsic defects is analyzed by photoluminescence (PL) technique at 10K. The broadband green and yellow luminescence has been observed with the PL peak located in 570nm. The broadband green and yellow luminescence may be composed of two Gauss-type spectra and the centers of defects energy level located 0.8eV to 1.3eV below the conduction band. The results strongly imply that this originates from the radiative transitions between carbon vacancies (VC) and complex- compounds-related VC, which is accordance with the ESR results.4. The effects of annealing treatment on the intrinsic defects in the unintentionally doped 4H-SiC are studied. It is obvious that annealing treatments make the intrinsic defects concentration decrease. The ESR and PL results show that native defects consist of carbon vacancy (VC) and complex-compounds-related VC after annealing series performed. The ESR and PL intensity of intrinsic defects increases and reaches its maximum at 1573K, then decreases with elevating anneal temperature when annealing time is less than 30min, which means there are the process for intrinsic defects stabilized and a strong interaction among the intrinsic defects during the annealing treatments. While the trends both in ESR and PL results are different absolutely with that of annealing time less than 30min when annealing time is 60min. The difference between annealing time less than 30min and 60min is caused by not only the process of stabilizing intrinsic defects and a strong interaction among the intrinsic defects but also the silicon atom escaping. With annealing time 60min, the Si atoms escaping is not ignored which causes the ratio of C/Si increasing and introduces a lot of vacancy of Si (VSi). Both annealing temperature and time have important effects on the concentration of intrinsic defects.The luminescent models have been established during the annealing treatment. There are three processes to affect the concentration of intrinsic during the annealing treatment, and they are the metastable transforming to stable defects, the interaction among native defects and Si atoms escaping respectively, which make the PL wave shifts from green to yellow. There are two reasons lead to the PL spectrum shifts from"green"to"yellow". One is the defect production with different size and located in deeper energy level, coming from strong interaction among the intrinsic defects and the other is lots of VSi and complex-compounds-related VSi in the samples because of the Si atoms escaping during the annealing treatment.