Spontaneous and stimulated recombination in p+-n-n+(AlGa)As-GaAs heterojunction laser diodes

A detailed study has been made of the lasing properties of p⁺-n-n⁺double heterojunction lasers where the n-type active region was lightly doped n type ( cm^-3) and only weakly compensated. The structure of the diodes allowed a study of the spontaneous emission (below and above lasing threshold) through the nonabsorbing higher band-gap (AlGa)As surface of the diode. Only a single major spontaneous band is seen which peaks near the band-gap energy of the n-type active region. (However, when viewed only through the diode edge, a second spurious lower energy "ghost peak" is seen due to selective internal absorption of the spontaneous radiation in the GaAs substrate.) With increasing current density, the spontaneous emission band broadens with a slight upward shift in its peak energy. The lasing peak eventually emerges from the low-energy tail of the band at an energy where relatively little spontaneous radiation is observed at low current densities and where the absorption coefficient is relatively small. Observations made by cathodoluminescence of GaAs with similar doping shows an analogous behavior, with the energy separation between the spontaneous and lasing peaks increasing with increasing temperature between 77 and 300 K. The observed behavior is consistent with the hypothesis that the lasing transitions involve conduction and valence band tail states due to the screening of the crystal field (and carrier interaction) by the high density of injected carriers in the active region of the laser.