Different pressure behavior of GaN/AlGaN quantum structures grown along polar and nonpolar crystallographic directions

Abstract

High quality GaN/AlGaN multiquantum well (QW) structures were grown by ammonia molecular beam epitaxy along the (0001) polar and (112¯0) nonpolar directions. Each sample contains three QWs with thicknesses of 2, 3, and 4 nm as well as 10 nm Al0.30Ga0.70N barriers. The measured photoluminescence (PL) spectrum consists of three peaks originating from the radiative recombination of excitons in individual QWs. In the nonpolar sample, the energy positions (EPL) of the observed peaks are separated because of the quantum confinement effect, whereas in the polar sample an additional redshift is induced by the quantum confined Stark effect. The dependence of EPL on QW width was used to estimate the built-in electric field magnitude in the latter sample to be about 2 MV/cm. Hydrostatic pressure studies of the PL in both samples gave qualitatively different results. In the polar sample, the pressure shift of EPL, dEPL/dp decreases significantly with QW width. The important finding is derived from the observation of a QW width independent dEPL/dp in the nonpolar sample. It shows that for GaN/Al0.30Ga0.70N, the quantum confinement remains practically independent of the applied hydrostatic pressure. This result reveals that in the polar sample, the variation in dEPL/dp with the QW width is due to the pressure-induced increase in the built-in electric field Fint. Thus, a more quantitative analysis of the latter effect becomes justified. We found that the Fint increases with pressure with a rate of about 80 kV(cm GPa)−1.