II–VI blue-green light emitters

Abstract

Mg in ZnCd-chalcogenides opens up a wide range of II–VI materials having both a wide energy gap and a large lattice constant. ZnMgSSe, fully lattice-matched to GaAs (001) substrates, with an energy gap tunable from 2.8 up to ∼ 4.5 eV, has made possible the photopumped operation of MBE or MOCVD-grown Zn(Cd)Se/ZnSSe/ZnMgSSe separate-confinement heterostructure samples and continuous-wave operation of MBE-grown green and blue laser diodes at room temperature. Valence-band engineering by using a superlattice based on the amphoteric native defect model is proposed for removing the doping limit in alloy semiconductors. The doping concentration could be increased by one order of magnitude for (ZnSe)m(ZnMgSSe)n superlattice with m = 4–6 and n = 10–15. The device characteristics of the MBE-grown II–VI wide-gap light emitters are becoming as good as those of the established III–V materials-based LDs except for the device lifetime. We have reported structural studies of degraded ZnMgSSe-based LEDs. Pre-existing stacking faults turn into non-radiative regions with a high density of dislocation dipoles and dislocation loops, spreading out in the 〈100〉 directions during current injection. The dislocation dipoles themselves are aligned along both 〈110〉 directions lying in the {111} plane, with Burgers vectors of the type (a2)〈011〉 inclined at an angle of 45° against the (001) junction plane. Analysis of the degradation process shows that no catastrophically fast degradation occurs when II–VI LDs degrade. We believe that we can establish the reliability of the ZnMgSSe-based LDs through prevention of the start of rapid degradation by eliminating the pre-existing defects, and by slowing down the gradual degradation through reduction of point defects.