Despite of many potential applications of ZnSe-based green-yellow LDs, their fast degradation is still a problem. But there is no other material including the nitrides which can obtain stimulated emission in the green spectral region. One reason of the degradation is the stability of the QW. The compressive strain of the QW attracts defects, and also Cd outdiffusion takes place into the p-doped area.
First a short review will be given of ZnSe LDs emitting in the full green spectral range. Then the advantages of QDs in those laser structures will be discussed. Finally a green monolithic VCSEL will be presented.
Laser emission in cw-mode around 560 nm from CdZnSSe QWs was reported for the first time by our group in 2001. One approach for suppressing the degradation is the introduction of strain-compensating ZnSSe barrier layers. Conventional LDs and LDs with different arranged barriers were grown by MBE. The achieved enhancement of lifetime was more than an order of magnitude in case of the p-side barrier. Further investigations revealed that a layer of high sulfur amount does not act only as strain compensating layers but also as a blocking layer for the Cd diffusion into the p-layers.
An alternative to achieve longer lifetimes is the usage of CdSe QDs in the active region. LED degradation experiments under constant current have been performed and show a lifetime of 600 h for the QD laser compared to 0.6 h for a QW laser. When processing the QD laser as a ridge waveguide structure the threshold could be reduced from 7 to 1.5 kA/cm2. This decrement is caused by a large reduction of current spreading. Compared to common ZnSe QW LDs, the discussed QD lasers show an unexpected stability against high current injection.
Furthermore, experimental data from the first monolithic ZnSe-based VCSEL will be presented. The VCSEL exhibits an emission wavelength of 514 nm by optical excitation at RT. The used DBRs can be tuned over the whole visible spectral region.