Electrically active defects in 4H silicon carbide

Abstract: Development of future technology needs to widen the application areas of semiconductor devices. The increased requirements are beyond the limits of the most common semiconductors today like silicon or gallium arsenide. Use of new materials could resolve many performance issues and also offer increased reliability and reduced cost of the devices. Silicon carbide (SiC) is a wide bandgap semiconductor with properties suitable for high-power, high-temperature and high frequency applications. However, many material related issues have to be solved before SiC could compete in the semiconductor market. Understanding and controlling intrinsic defects and residual impurities is one of the remaining issues limiting progress and applications of this material. Intrinsic defects is a very important issue for all semiconducting materials, since they are unintentionally introduced during different device processing steps or are present directly after the growth, where the amount and nature of the defects is a very sensitive function of the growth parameters. These defects can act as effective trapping and recombination centers and therefore have negative influence on the carrier lifetime. Detailed knowledge about the defect dynamics is needed to be able to control and remove them. The concentration of the intrinsic defects is usually very low in SiC material grown in horizontal hot-wall CVD reactor due to the low growth rate and temperature. Therefore great attention must be paid to reduction of residual impurities, since even a small amount can degrade the electronic properties of the material.This thesis contributes to the increased understanding of some major intrinsic defects and residual boron acceptor in 4H-SiC. It is divided into introduction and seven papers. In paper I, an energy-level scheme in the bandgap for the D1 defect is established. Using Minority Carrier Transient Spectroscopy (MCTS) we show that the D1 defect luminescence is correlated to a hole trap at about 0.35 eV above the valence band, proving that the D1 defect is a pseudodonor.In paper II, studies of intrinsic and impurity related defects in SiC epilayers grown using chemical vapor deposition (CVD) with high growth rate in a vertical hot-wall reactor are presented. Using capacitance transient techniques, the concentration of electrically active defects as a function of growth parameters are investigated.Defects, created by low energy electron irradiation in the range of 80 300 keV are investigated in papers III and IV. We report face-dependent introduction rates of point defects in 4H-SiC in paper III. Clear difference for the defect introduction rates is obtained for irradiations performed from silicon and carbon faces. Energy dependence of the ratio of introduction rates from different faces has been investigated. In paper IV, irradiation energy dependence of the defect introduction rates and annealing dynamics are presented. Defects, caused by carbon displacements are identified, and their possible microstructure is discussed.Recombination enhanced defect annealing is reported in paper V. The signal from the HS2 trap, which usually needs 900°C to anneal out, disappears after introducing nonequilibrium density of minority carriers and indicates it being an effective recombination center. Other defects, labeled as EHi and EH3, also anneal out by recombination enhanced reaction, but at a lower rate.One of the most common trace impurities in SiC is boron. Electrical properties of residual boron acceptor in high quality epitaxial layers have been examined by MCTS in paper VI. Comparison with the optical decay measurements shows that boron is related to the observed lateral variations of the minority carrier lifetime in low doped 4H-SiC epilayers. Boron related photoluminescence and capacitance transient spectroscopy peaks are investigated around SIMS craters in paper VII. Enhancement of boron and hydrogen related PL is observed in the vicinity of the crater, whereas the concentration of electrically active boron as measured by MCTS was found to decrease considerably. Possible models of boron luminescence center are discussed.