Room temperature II–VI lasers with 2.5 mA threshold

Abstract

Buried-ridge II–VI laser diodes operating at 511 nm at room temperature were fabricated from separate-confinement heterostructures. The layers consisted of a pseudomorphic CdZnSe quantum well and lattice-matched ZnSSe light-guiding and MgZnSSe cladding layers. Stripe gain-guide lasers fabricated from similar wafers exhibited threshold current densities as low as 630 A/cm 2 and threshold voltages less than 9 V. The buried-ridge devices operated in single lateral and transverse modes under pulsed excitation at room temperature. Threshold currents as low as 2.5 mA and 50% duty cycles at room temperature were demonstrated with these structures. A conventional model for the threshold currents was used in an initial attempt to fit the thresholds as functions of cavity length and temperature (100–325 K); excitonic mechanisms were not included. The electron lifetime in the light-guiding layer was the only adjustable parameter in the model. Good agreement between measurement and theory was obtained with the electron lifetime taken as 42 ps; this value also agrees well with independent lifetime measurements. All reports of II–VI diode lasers to date indicate very short device lifetimes at room temperature. Electroluminescence topography and TEM studies were carried out on both LEDs and stripe lasers in order to gain insight to the failure mechanisms and to study their evolution. Two types of dark line defects (along the 〈100〉 and 〈110〉 directions) and patches with extremely large densities (10 10 cm -2) of a dislocation network consisting of elongated dislocation segments were observed in degraded devices. This work represents the first detailed study of II–VI device failure and indicates that the rapid failure is due to the formation of dark defects at the high current densities required for lasing.