Optical study of the two-dimensional electron gas and the total electronic polarization discontinuity in Sc <i>x</i> Al1− <i>x</i> N/GaN heterostructures

Abstract

Wurtzite ScxAl1−xN/GaN heterostructures are predicted to exhibit large polarization discontinuities at the interface, leading to the formation of two-dimensional electron gases with high sheet carrier concentrations. However, the magnitude of these polarization discontinuities is currently under extensive discussion. To address this, three samples with Sc contents of 4%, 13%, and 18% are investigated, providing experimental insight into the evolution of the polarization discontinuity. Broad luminescence, originating from the En=2 level in the triangular quantum well at the heterostructure interface, is observed. By extracting the energetic position of the luminescence and combining the analysis with band structure simulations, experimental values for the polarization discontinuities of the ScAlN/GaN heterostructures are determined. For the samples containing 4%, 13%, and 18% Sc, the corresponding values are 0.113±0.009, 0.071±0.014, and (0.072±0.013)Cm−2, respectively. These findings agree with theoretical expectations, as lattice-matched ScAlN on GaN at ≈13 % Sc exhibits nearly zero piezoelectric polarization, whereas at 4% and 18% Sc, the piezoelectric polarization has opposite signs, reflecting the different strain states of the epilayer. The absorption edge energies for the samples are found to be 5.95, 5.87, and 5.78 eV for 4%, 13%, and 18% Sc, respectively.