Phonon drag thermopower in an AlGaN/GaN heterostructure

The theoretical thermopower of a two-dimensional electron gas (2DEG) is studied in a clean AlGaN/GaN heterostructure of zincblende (ZB) and wurtzite (WZ) symmetry over the temperature range 0.1-10K. The phonon drag thermopower (S g ) due to 2DEG coupling with 3D longitudinal and transverse acoustic phonons is calculated for high-density electrons. It is found that, in both ZB and WZ symmetries, the S g due to piezoelectrically coupled transverse acoustic phonons is dominant over the S g due to longitudinal acoustic phonons via piezoelectric and deformation potential coupling over the entire temperature range. This dominance is attributed to the pronounced piezoelectricity of nitrides and the smaller velocity of transverse acoustic phonons. It is shown that the total S g ∼ T n with n = 3.8 at temperatures T<2.0K and is close to n = 4 in the Bloch-Gruneisen limit. Diffusion thermopower (S d ) is also presented and is found to be significant for T below 0.4K. The calculations are compared with the results for GaAs/GaAlAs heterostructures.