Comparison of the optical gain of wurtzite GaN/AlGaN quantum well lasers grown on the (0001)- and (101̄0) oriented substrates

Abstract

We calculate the optical gain of wurtzite (WZ) GaN/AlGaN QWs grown on the (0001) and (101̄0) oriented substrates, taking into account the valence subband structure modification due to the crystal orientation effect and the pseudomorphic strain which is anisotropic in the QW plane. We show that, for the (101̄0) GaN/AlGaN quantum well (QW), the two topmost valence subbands are more widely separated than the HH1 and LH1 subbands in the (0001) GaN/AlGaN QW. The in-plane energy dispersion of the (10̄10) QW also becomes anisotropic, giving rise to a reduced band-edge density-of-states in comparison with the (0001) QW. Moreover, states constituting the topmost valence subband at the Γ point favor TE mode gain. A combination of the reduced band-edge density-of-states and the existence of the preferred symmetry at the valence band maximum contributes to an improvement of the TE optical gain. A comparison of the QWs of both orientations shows that the (101̄0) QW is capable of achieving lower transparency current densities. Therefore, the (101̄0) orientation is promising in improving the threshold performance of WZ GaN-based QW lasers.