Carrier lifetimes are one of the most crucial parameters that govern the performance of high voltage/high power devices. The lack of understanding of the factors that determine the carrier lifetimes in silicon carbide is currently a major impediment in the development of high voltage/high power technology based on this material. The objective of this dissertation was to identify and subsequently, characterize various recombination channels present in silicon carbide. Of special importance was identification of lifetime limiting defects in the high quality epitaxial layers grown by state-of-the-art chemical vapor deposition technique for high voltage application.
The effect of growth conditions (C/Si ratio, growth temperature, seed polarity, epilayer thickness, and background doping) on the concentrations of various defects were investigated with the aim of manipulating carrier lifetimes by controlling different growth parameters. Based on the qualitative correlations between various point defects and carrier lifetimes in more than thirty epitaxial layers, three defects (Z-defect, EH6/7 center, and P1 center) were identified as potential lifetime limiting defects.
The P1 center was shown to act as efficient recombination channel whenever present in concentrations greater than 1013 cm-3. Such concentrations were observed in layers grown on the C-face and at low C/Si ratio (less than 1.5). The measurement of recombination rates of electrons and holes via the Z-defect and the EH6/7 center (as a function of temperature) were performed by analyzing the carrier dynamics in specially designed p-n diodes. At 300 K, the capture cross section of the two states of the Z-defect were σn1∼6x10-15 cm2 (electron capture at the donor state), σp1∼2x1014 cm2 (hole capture at the donor state), σn2∼1x10 16 cm2 (electron capture at the acceptor state), and σ p2∼1e-13 cm2 (hole capture at the acceptor state). The electron capture cross section for the EH6/7 centers was measured as ∼2.3x10-15 cm2 and an upper value for the hole capture cross section was estimated as ∼10-19 cm2. The recombination rate of carriers was six orders of magnitude higher through the Z-defects as compared to the EH6/7 centers, thus, the former acted as lifetime limiting defect in the high quality epitaxial layers that were investigated in this research.
The minority carrier lifetime in high quality epitaxial layers can be predicted with sufficient accuracy using a simple relationship between the lifetimes, τMCL, and concentration of the Z-defect, N T (cm-3), expressed as τMCL∼10 6/NT. The minority carrier lifetimes in the as-grown epitaxial layers (investigated in this research) were in 0.1–2 μs range corresponding to the Z-defect concentrations of 1011-10 12 cm-3. The temperature and injection rate dependence of carrier lifetimes were also simulated based on the Z-defect recombination parameters, and were in good agreement with the experimental data.
The rate of carrier recombination via extended defects (threading edge and screw dislocations) was also measured for 4H-SiC epitaxial layers. The dislocation recombination velocity and effective dislocation core radius which characterize the rate of recombination through the dislocations were determined to be 105 cm/s and 2.5x10-5 cm, respectively. The carrier lifetimes were shown to be affected locally in the regions where the dislocation density was higher than a certain threshold density. The threshold dislocation density on the studied epilayer was 106 cm -2.