Novel blue continuous-wave current-injected light source

Abstract

Vertical-cavity surface-emitting lasers (VCSELs) contain two high-reflectivity distributed-Bragg reflectors (DBRs) as well as a short cavity characterized by a small optical-mode volume. They emit a single longitudinal mode in a circularly symmetric beam with little beam divergence. Both characteristics are superior to those achieved with edge-emitting lasers and desirable for many practical applications in, for example, high-density optical storage and laser printing. In addition, the strong field in the microcavity facilitates exploration of cavity quantum-electrodynamics effects, such as single-photon and polariton emission, controlled spontaneous emission, and low threshold (or thresholdless) lasing. Over the past decade, two developments have affected the realization of electrically pumped gallium nitride (GaN)-based VCSELs. First, high-reflectivity, high-quality DBRs composed of AlxGa1−xN and GaN exhibit a large lattice mismatch that forms cracks in the epitaxially grown DBR structure. These cracks could result in a reduction in optical reflectivity and become a leakage-current path. The second is the fabrication of novel, high-transparency, high-conductivity contacts for current injection, which was triggered by the difficulties encountered in constructing low-resistance p-type GaN layers. In 1996, GaN-based edge-emitting laser diodes were first realized at room temperature by Nakamura’s team.1 Arakawa and co-workers subsequently fabricated an In0.1Ga0.9N VCSEL and observed lasing at 77K for the first time in 1998.2 We recently fabricated a crack-free high-reflectivity DBR by incorporating AlN/GaN superlattice layers between the DBR structures to reduce V-shaped defects and act as strain buffers.3 Figure 1 shows the device structure employing hybrid mirrors. It consists of 29 pairs of AlN/GaN DBRs and a 5λ (where λ refers to the operating wavelength) active region with a Figure 1. Schematic diagram of an electrically pumped vertical-cavity surface-emitting laser. DBR: Distributed-Bragg reflector. GaN, AlN: Gallium, aluminum nitride. MQW: Multiquantum well.