R. L. Gunshor

We have employed spectrally resolved, subpicosecond transient four-wave-mixing techniques to identify and to study the four-particle “antibound states” associated with excitonic molecules in a binary ZnSe single quantum well. Previously, the existence of robust bound biexcitons have been observed in this wide band-gap semiconductor heterostructure, including a density regime where optical gain is dominated by the molecular states. We employ selective circular polarization excitation to suppress the bound (ground state) exciton-pair state contribution and find a spectrally distinct and isolated feature emerging approximately 3.5 meV higher in photon energy than the n=1 heavy-hole exciton transition. The new resonance is distinct only in the negative time delay regime, and is interpreted as a manifestation of coherent four-particle correlated states within the biexciton continuum.

Spectroscopy on a single ZnSe quantum well characterized by a sharp exciton resonance (ΔE=0.6 meV) shows how a gas of interacting excitonic molecules induces blue-green laser emission in an optically pumped heterostructure at cryogenic temperatures. Spectrally resolved four-wave-mixing experiments yield a molecular binding energy of 5 meV. However, the lasing occurs in an electron-hole density regime, where strong many-exciton interaction effects are evident. © 1996 The American Physical Society.

Picosecond laser pulses were used to simulate current injection and generate dark defects in ZnSSe/ZnCdSe quantum well structures. The formation of the defects was monitored in situ with a photoluminescence microscope setup, with spatial resolution of about 1 μm. Dark defects are observed to form in the ZnCdSe quantum well, analogous to electrical injection in separate confinement heterostructure diode lasers. Transmission electron microscopy of the optically generated dislocation network gives further insight to the origin of the defect microstructure.

The degradation mechanism of ZnMgSSe/ZnSSe/ZnCdSe separate confinement heterostructure laser diodes is studied under continuous wave and pulsed operation at room temperature. The degradation of the active quantum well is caused by formation of optically inactive areas which nucleate at paired stacking faults. These ‘‘dark defects’’ are identified as areas of dislocation network and point defect segregation.

The microstructure of a degraded II-VI blue-green laser diode based on the ZnCdSe/ZnSSe/ ZnMgSSe pseudomorphic separate confinement heterostructure has been examined by transmission electron microscopy. Triangular nonluminescent dark defects observed in the laser stripe region by electroluminescence microscopy have been identified to be dislocation networks developed at the quantum-well region. The dislocation networks have been observed to be nucleated at threading dislocations originating from pairs of V-shaped stacking faults which are nucleated at or near the II-VI/GaAs interface and extending into the n-ZnMgSSe lower cladding layer.