Terahertz spectroscopy of an electron-hole bilayer system in AlN/GaN/AlN quantum wells

Abstract

We report studies on the nanoscale transport dynamics of carriers in strained AlN/GaN/AlN quantum wells: an electron-hole bilayer charge system with a large difference in transport properties between charge layers. From electronic band diagram analysis, the presence of spatially separated two-dimensional electron and hole charge layers is predicted at opposite interfaces. Since these charge layers exhibit distinct spectral signatures at terahertz frequencies, a combination of terahertz and far-infrared spectroscopy enables us to extract (a) individual contributions to the total conductivity and (b) effective scattering rates for charge-carriers in each layer. Furthermore, by comparing direct-current and THz-extracted conductivity levels, we are able to determine the extent to which structural defects affect charge transport. Our results evidence that (i) a non-unity Hall-factor and (ii) the considerable contribution of holes to the overall conductivity lead to a lower apparent mobility in Hall-effect measurements. Overall, our work demonstrates that terahertz spectroscopy is a suitable technique for studying bilayer charge systems with large differences in transport properties between layers such as quantum wells in III-nitride semiconductors.