The effects of crystallographic orientation and strain on the properties of excitonic emission from wurtzite InGaN/GaN quantum wells

We have examined in detail crystal orientation effects on the properties of excitonic emission from wurtzite InxGa1-xN/GaN quantum wells (QWs) with piezoelectric polarization using exciton binding and transition energy calculations based on a single-band effective-mass theory. We show numerical results for the bandgaps, effective heavy-hole masses, piezoelectric polarizations and fields, exciton wavefunctions, exciton binding and transition energies and radiative lifetimes of excitonic emission as a function of the QW crystallographic growth planes. Band-edge and effective-mass parameters for a continuum of GaN crystallographic orientations, on which InGaN/GaN QWs are grown, were obtained from In-composition- and strain-dependent k·p calculation for wurtzite InxGa1-xN, using the 6x6 k·p Hamiltonian in appropriate {hkil} representations. We have performed calculations for a continuum of technologically relevant QW growth planes {hh-bar0l} oriented at various angles θ relative to the (0001) c-plane. The excitonic ground- and first-excited-state energies and wavefunctions were calculated using an effective potential method. A strong reduction of average in-plane heavy-hole effective mass and normal to the plane piezoelectric polarization and field is observed as θ varies from θ = 0 deg. (i.e. the c-axis direction) to θ = 49.5 deg., where the piezoelectric polarization and electric field reverse their orientation with respect to the plane of the QW. The decrease of the electric field in the InGaN/GaN QW growth direction leads to an increased exciton transition energy and oscillator strength, which results in the increase of the exciton binding energy and decrease of the excitonic radiative lifetime. For angles θ>49.5 deg. only small variations on the order of ∼10% in the exciton binding and transition energies and excitonic radiative lifetime are observed for narrow In0.12Ga0.88N/GaN QWs that have widths less than ∼ 3.5 nm. The average in-plane heavy-hole effective mass reaches its minimum for θ = 90 deg., i.e. m-plane {11-bar00} growth. These results indicate that InGaN/GaN QW structures grown on non-(0001)-oriented planes in a wide variety of angles 49.5 deg. ≤θ≤90 deg. can be used for optimized operation of optoelectronic devices