Growth issues for blue-green laser diodes

Abstract

Laser device lifetime is linked to electrical dissipation, hence vertical transport. One significant issue is the resistivity of ZnSe and alloys of ZnSe. The results of our recent studies into the origins of both doping level limitations in ZnSe, and the relatively high acceptor activation energies found in wide bandgap alloys of ZnSe, will be reviewed. The experimental evidence, including the presence or absence of persistent photoconductivity in wide bandgap alloys of p-ZnSe and p-ZnSe:N, points to the presence of deep levels associated with lattice relaxation as the origin of the increased acceptor activation energy. Key issues of growth include the evaluation of sticking coefficient for the alloy constituents, and the maintenance of constant substrate temperature to avoid variations in alloy fraction. The appearance of composition modulation in ZnSe alloys will also be discussed. Understanding the degradation of II–VI blue-green laser diodes has become important after the demonstrations of room-temperature continuous-wave operation from (Zn,Mg)(S,Se)/Zn(S,Se)/(Zn,Cd)Se separate confinement heterostructure (SCH) laser diodes. The observed degradation seems associated with the existence of stacking faults originated from the II–VI/GaAs heterovalent interfaces. Segments of these extended structural defects which penetrate the active region become sources for the generation of macroscopically dark patches under lasing conditions. Lessons from III–V semiconductor laser device research are consistent with the observation that the presence of non-radiative point defects and built-in strain contribute to the propagation and growth of dislocations. Efforts to reduce the density of stacking faults continue, while the temperature dependence of photoluminescence points the way to a reduction in as-grown point defects.