Valence subband structure and optical gain of GaAs-AlGaAs (111) quantum wells
The authors use the envelope function method to calculate the hole confinement energies and valence subband dispersion energies of (111) GaAs-AlGaAs quantum wells of varying widths. The heavy-hole mass is significantly larger along (111) than along the conventional (001) growth direction. This increases the number of heavy-hole confined states for a given well width. Away from the zone centre, the subband dispersion shows less mixing between heavy- and light-hole bands than for (001) growth and, in thin wells, the highest subband has a low in-plane effective mass over a greater energy range ( approximately 25 meV for a 50 AA well). It has recently been shown that (111) GaAs-AlGaAs quantum well lasers can have a lower threshold current density than equivalent (001) lasers. They perform laser gain calculations which show that this reduced threshold current density can be explained by the enhanced light-hole behaviour at the valence band maximum of (111) quantum wells. No theoretical evidence is found to support an increased in-plane heavy-hole mass or enhanced optical matrix element, as have been deduced from recent experiments on (111) quantum well structures.