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

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 LO-phonon 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 similar to 800 A/cm2) minimum threshold current at QD GS emission wavelength, 1135 nm, with 0.7 mW optical power and 12% slope efficiency.