Carrier lifetime measurement in n− 4H-SiC epilayers

The effects of measurement technique and measurement conditions (e.g., injection level, temperature) on measured carrier lifetimes in $n^{-}$ 4H-SiC epilayers are investigated both experimentally and through detailed carrier dynamics simulations to better understand differences between reported lifetimes. Three common, optically based techniques are compared: time resolved photoluminescence, transient free carrier absorption, and microwave photoconductivity decay. From the details of these measurement techniques it is shown from both theory and experiment that for the limits of high or low injection, these techniques can reflect very different lifetimes. The effect of measurement conditions on the carrier lifetime was approached by simulating the carrier dynamics assuming a dominant $Z_1/Z_2$ defect in order to calculate the evolution of the lifetimes and the carrier and defect charge state concentrations for arbitrary injection level or temperature, as a closed-form solution to this problem does not exist. The simulated behavior was found to be in reasonable agreement with experiment and the resulting values for the electron and hole capture cross sections for the $0/+$ transition of $Z_1/Z_2$ were found to be ${\sigma }_{n2}\approx \left(2-4\right)\times {10}^{-15}{\text{cm}}^2$ and ${\sigma }_{p2}\approx \left(1-2\right)\times {10}^{-14}{\text{cm}}^2$ , respectively. The simulations provide insight into the dominant processes controlling the lifetime and identify four distinct stages of decay. A simple expression for the ratio of high- to low-injection lifetimes is presented which compares well with experiment. The temperature dependence of the lifetime is found to be relatively weak below 500 K and thermally activated immediately above this temperature due to electron emission from the $Z^0$ state. Electron emission from $Z^{-}$ and hole emission become important only at higher temperatures. Simulations with both $Z_1/Z_2$ and ${\text{EH}}_6/{\text{EH}}_7$ defects suggest the latter does not contribute significantly to the lifetime in as-grown epilayers, due primarily to a small capture cross section for holes.