Time Evolution of the Screening of Piezoelectric Fields in InGaN Quantum Wells

We have measured the time response of the emission spectra of In _0.07Ga_0.93N quantum wells with widths of 2, 3, and 4nm in GaN following pulsed optical excitation. We observe a blue shift of the emission peak during the excitation and a subsequent red shift as the carriers recombine in the 3- and 4-nm wells, and a negligible shift for the 2-nm well. Using a comprehensive theory we are able to fit both the time evolution of the peak emission energy and the integrated emission intensity. The shift of the emission peak (by about 17 meV) arises from the balancing of the change in screening of the internal piezoelectric field as the carrier density changes and bandgap renormalization. We have projected the calculations to quantify the degree of screening at typical threshold carrier densities. At transparency we estimate carrier densities of 4.3times10¹⁶ m^-2 and 4.8times10¹⁶ m^-2 for the 4- and 3-nm wells, respectively, which reduce the internal piezoelectric field in the well to 0.97times10⁸ (4 nm) and 1.03times10 ⁸ (3 nm) Vmiddotm^-1 compared with the unscreened value of about 1.23times10⁸ Vmiddotm^-1. Thus, a substantial field remains in these wells under laser conditions. We find that this partially screened field is beneficial in reducing the threshold current compared with that of a square well for modal gains up to about 150 cm^-1