Residual strain effects on the two-dimensional electron gas concentration of AlGaN/GaN heterostructures

Ga-face AlGaN/GaN heterostructures with different sheet carrier concentrati ons have been studied by photoluminescence and Raman spectroscopy. Compared to bulk GaN, an energy shift of the excitonic emission lines towards highe r energies was observed, indicating the presence of residual compressive st rain in the GaN layer. This strain was confirmed by the shift of the E-2 Ra man line, from which biaxial compressive stresses ranging between 0.34 and 1.7 GPa were deduced. The spontaneous and piezoelectric polarizations for e ach layer of the heterostructures have been also calculated. The analysis o f these quantities clarified the influence of the residual stress on the sh eet electron concentration (n(s)). Possible causes for the discrepancies be tween the calculated and experimentally determined sheet carrier densities are briefly discussed. (C) 2001 American Institute of Physics.