Gain and loss processes in GaAlAs-GaAs heterostructure lasers

Heterostructure lasers have been made by growing an epitaxial p⁺layer of GaAlAs onto an n+-GaAs substrate. The p-GaAs active region, formed by diffusion of zinc into the substrate during the growth process, is about 1.6 μ thick. Threshold current densities of these lasers lie between 8000 and 12 000 A.cm^-2at 300°K, depending on the cavity length. External incremental quantum efficiencies are about 40 percent and change very little with cavity length. It is found that the gain coefficient is a superlinear function of current and the loss coefficient appears to vary with cavity length as a result of the variation in photon energy, which occurs with different threshold currents. The threshold currents of the heterostructure lasers are a factor of 4 lower than previous homojunction lasers. The conclusion is drawn that the improvement is largely a result of the carrier confinement that occurs at the heterojunction. Some improvement can be related to a reduction of optical losses but it is a consequence of the superlinear gain-current relationship that even a loss coefficient of zero would be insufficient to explain the low-threshold currents.