Significant reduction of major deep levels in n-type 4H-SiC(0001) epilayers by means of thermal oxidation is demonstrated. By thermal oxidation of epilayers at 1150 -1300 C, the concentration of the Z 1=2 and EH 6=7 centers has been reduced from ð0:3 { 2Þ Â 10 13 cm À3 to below the detection limit (1  10 11 cm À3 ). The depth-profile analysis of the Z 1=2 center has revealed that the Z 1=2 center is eliminated to a depth of about 50 m from the surface after thermal oxidation at 1300 C for 5 h. The carrier lifetime in an n-type 4H-SiC epilayer measured by differential microwave photoconductance decay has been significantly improved from 0.73 s (as-grown) to 1.62 s (after oxidation: 1300 C, 5 h  2). The reduction mechanism of the Z 1=2 and EH 6=7 centers is discussed. #

By thermal oxidation of 4H-SiC at 1150 -1300 C, the Z 1=2 and EH 6=7 concentrations can be reduced to below 1 Â 10 11 cm À3 . By the oxidation, however, a high concentration of HK0 center (E V þ 0:78 eV) is generated. Additional annealing in Ar at 1550 C results in elimination of the HK0 center. Thus, all the major deep levels can be eliminated by the two-step thermal treatment. Based on the depth profiles of deep levels, a model for the defect generation and elimination is proposed. The carrier lifetime in 4H-SiC epilayers has been improved from 0.64 (as-grown) to 4.52 s by this method.

After remarkable reduction in the Z1/2 center in n-type 4H–SiC epilayers, the measured carrier lifetimes can be severely affected by other recombination paths. Impacts of carrier recombination at the surface as well as in the substrate are investigated in detail by using numerical simulation based on a diffusion equation. The simulation reveals that a very thick (>100 μm) epilayer is required for accurate measurement of carrier lifetimes if the bulk lifetime in the epilayer is longer than several microsecond, due to the extremely short lifetimes in the substrate. The fast decay often observed at the initial stage of decay curves can be explained by fast recombination at the surface and in the substrate. In experiments, the carrier lifetime is improved from 0.69 to 9.5 μs by reducing the Z1/2 center via two-step thermal treatment (thermal oxidation and Ar annealing) for a 148-μm-thick n-type epilayer. This lifetime must be still, to large extent, affected by the recombination at the surface and in the substrate, and the real bulk lifetime may be much longer. The carrier recombination paths and their impacts on the decay curves are discussed.