Tuning electrical and thermal conductivities of the two-dimensional electron gas in AlN/GaN heterostructures by piezoelectricity.

We investigate the electrical and thermal conductivities of the two-dimensional electron gas confined in the quantum well formed at the heterojunction between a thin GaN layer and an AlN layer strained by an AlxGa1-xN capping layer in the temperature range from 10 to 360 K. The experimental protocol developed to deduce from calorimetric and Hall-effect measurements at a variable temperature the critical characteristics and transport properties of the confined two-dimensional electron gas is presented. It is found that, in the measured temperature range (10-360 K), the electrical conductivity of the two-dimensional electron gas is temperature-independent, due to the predominance of scattering processes by interface defects. However, the thermal conductivity shows a linear temperature dependence, mirroring the specific heat of free electrons. The temperature-independent relaxation time associated with the overall electron scattering means that the values obtained for electrical and thermal conductivities are in excellent agreement with those stipulated by the Weidemann-Franz law. It is also found that for weak strain fields in the AlN layer, both the electrical and thermal conductivities of the two-dimensional interfacial electrons increase exponentially with strain. The importance of two-dimensional electron gas in AlN/GaN quantum wells lies in the fact that the strong piezoelectricity of AlN allows the transport properties of the two-dimensional electron gas to be tuned or modulated by a weak electric field even with the high density of lattice mismatch induced defects at the AlN-GaN interface .