Time-Resolved Electroabsorption Measurement of Electron Velocity in InGaN Heterostructures due to Internal Electric Fields

Carrier transport was measured in c-plane, $p$-down, $n$-GaN/$i$-In$_{\mathrm{1-x}}$Ga$_{\mathrm{x}}$N/$p$-GaN solar cell heterostructures using a time-resolved electroabsorption pump-probe technique with sub-picosecond resolution. Large built-in electric fields are present in the InGaN region associated with the termination of large polarization at hetero-interfaces. The change in transmission of a probe beam (tuned for maximum sensitivity to changes in the band edge) due to the transport of photogenerated carriers under the built-in field is monitored to determine the electron transit time and average electron velocity. Time-domain THz measurements indicate the direction of electron transport is dominated by drift towards the $n$-GaN. Samples with a 200-nm In$_{0.13}$Ga$_{0.87}$N layer show a change in signal rise time with carrier density. At the lowest injection level, an $\sim$ 1.5-ps rise time is observed, which corresponds to an average electron velocity of 6.7x10$^{6}$~cm/s for an average distance of travel of 100~nm in an internal field of $\sim$ 135~kV/cm. This velocity is significantly smaller than in GaN with a similar field, which may be indicative of transport through compositional inhomogeneities.