Stability of electron-hole plasma in type-I and type-II GaAs-GaAlAs single quantum wells

We calculate and compare energies of the electron-hole plasmas and ground-state excitons in type-I and type-II GaAs-GaAlAs single quantum wells with various structural conditions. Calculations are performed with a flexible and numerically stable method using realistic material parameters. We confirm that the exciton states are stable in type-I quantum wells for well widths up to 250 Å, as described in previous work [G. E. W. Bauer and T. Ando, Phys. Rev. B 34, 1386 (1986)]. We find that the electron-hole plasma becomes stable, particularly in narrow type-II quantum wells, e.g., about 2.5 meV below the ground state exciton in GaAs-AlAs quantum wells whose widths are 6 and 4 monolayers, respectively. Moreover, calculations including electric fields reveal two different conditions of the stable electron-hole plasma: one is that electrons and holes are separated and the other is that they are confined in the narrow regions. The results suggest the possibility of observing the electron-hole plasma as the lowest excited state in the GaAs-GaAlAs quantum well.