All-epitaxial VCSELs with tunnel QW-QD InGaAs-InAs gain medium

Abstract

Quantum dot (QD) size distribution and limitations in carrier capture and thermalization rates are still limiting the maximum saturation gain in QD-based laser diodes and the utilization of QD-medium in all-epitaxial vertical cavity surface emitting lasers (VCSELs). To overcome these problems structures of tunnel coupled pairs consisting of InGaAs quantum wells grown on top of self-assembled InAs QDs (QW-on-QDs) were employed as a gain medium for VCSELs. Photoluminescence, transmission electron microscopy and electroluminescence were used to study the properties of the multiple-layer QW-on-QDs active medium. QW-on-QDs tunnel structures with 3 - 5 nm tunnel barrier thicknesses and with different ground state (GS) relative separations were grown with varying InGaAs QW while the QD growth process parameters were kept constant. We have developed a tunnel QW-on-QDs structure with a QD PL line red-shifted by 32 meV relative to QW GS line. The narrow linewidth (22 meV) of this QD transition likely indicates an efficient LOphonon assisted tunneling of carriers from QW into QD ensemble states. Optimized tunnel (with 3 nm barrier thickness) QW-on-QDs structures were evaluated in VCSELs. All-epitaxial VCSELs with triple-pair tunnel QW-on-QDs as active medium demonstrated continuous wave mode lasing. These QD-based VCSELs with n-doped AlGaAs/GaAs mirrors and tunnel n-p junction exhibited 1.8 mA (Jth ~ 800 A/cm2) minimum threshold current at QD GS emission wavelength, 1135 nm, with 0.7mW optical power and 12% slope efficiency.