Low-Voltage-Threshold Microlasers

Abstract

We have reduced the voltage required for threshold in vertical cavity surface emitting lasers (VCSEL) to 1.7 V [l], the lowest yet reported for a CW-operating VCSEL [2,3]. Room-temperature current threshold was 3 mA pulsed, 4 mA CW. This advance in VCSEL technology leads to manageable heat dissipation for high packing densities. It was achieved in a structure which can be further optimized for high wallplug efficiency and high powers. Furthermore the thickness of the molecular beam epitaxially (MBE) grown portion of the structure was reduced by about 1.5 μm compared to conventional VCSELs, resulting in decreased MBE costs, significantly shallower processing depths and easier integration of VCSELs with transistors or other electronics. The (resistance x area) products of our VCSELs are nearly as low as those reported for high-power edge-emitting lasers. MBE was used to grown-doped Al_(0.15)Ga_(0.85)As/GaAs bottom mirror layers, the active region containing 3 GaAs quantum wells, and a 1-μm-thick p-doped top contact layer. 12 pairs of alternating SiO_2/Si_3N_4 layers formed a high-reflectivity mirror which completed the laser cavity. The reactive sputter-deposited mirrors produce reflectivities of 98.3% for 9.5 pairs [3]. Individual laser elements were defined by ion milling of mesas through the p-n junction, followed by planarization with SiO_2 to define the current path. Then, Au-Zn p-contacts were deposited around the mesa tops and alloyed for current injection. A final ion-milling step was used to isolate individual contacts. In this way microlasers with diameters ranging from 7.5-25 μm were fabricated and measured.