A Comparison of the Microwave Photoconductivity Decay and Open-Circuit Voltage Decay Lifetime Measurement Techniques for Lifetime-Enhanced 4H-SiC Epilayers

Abstract

This work compares the optical microwave photoconductivity decay (μPCD) and electrical open-circuit voltage decay (OCVD) techniques for measuring the ambipolar carrier lifetime in 4H-silicon carbide (4H-SiC) epitaxial layers. Lifetime measurements were carried out by fabricating P+/intrinsic/N+ (PiN) diodes on 100-μm-thick, 1 × 1014 cm−3 to 4.5 × 1014 cm−3 doped N-type 4 H-SiC epilayers, and measuring the lifetime optically using μPCD prior to metallization, then electrically using OCVD after contact deposition. Both as-grown epilayers as well as epilayers with improved lifetime (via thermal oxidation) were measured using both techniques. The observed ambipolar lifetime was improved from 1.4 μs on an unenhanced wafer to 4 μs on a wafer enhanced through the oxidation process as measured by μPCD. Little difference was observed between the μPCD and OCVD measurements on the unenhanced wafer; the ambipolar lifetime on the enhanced wafer measured by OCVD was approximately 5.5 μs, or 1.5 μs higher than the μPCD measurement. Continuous evaluation of the OCVD transient waveform was necessary due to the high lifetime in the enhanced wafer; shunt resistances included to discharge the P+/N junction capacitance were found to damp the OCVD response and yield low values for the measured lifetime. Simulation of the μPCD measurement including various surface recombination conditions yielded a good match to experimentally observed μPCD measurements for high values of the surface recombination velocity. The OCVD lifetime measurement technique is expected to yield measured lifetime values closer to the physical value due to its independence from surface conditions, provided that the experimental conditions are appropriately chosen.