Polarization charge screening and indium surface segregation in (In,Ga)N/GaN single and multiple quantum wells

Energies and oscillator strengths of the optical transitions in (In,Ga)N/GaN quantum-well structures with thin cap layers are investigated theoretically. Based on a self-consistent solution of the Schrödinger-Poisson equations, the internal fields generated by spontaneous surface charges and piezoelectric interface charges are systematically discussed for different sample configurations under consideration of indium surface segregation. We vary background doping density, thickness of the cap layer, number of quantum wells, indium content, and polarity of the structure. Indium surface segregation is shown to result in a blueshift of the transition energy and in a decreased optical matrix element at the same time. Background doping influences the band-edge alignment not only via screening of the polarization charge at the material interfaces by quantum-confined carriers, but also via ionized dopants in depletion layers. This becomes particularly important in case of Ga-face-grown material with n-type doping. We find that the position of the quantum well within the sample severely affects transition energy and optical matrix element.