Electronic Properties of InGaN/GaN Vertical-cavity Lasers

In vertical-cavity surface-emitting lasers (VCSELs), the optical cavity is formed by mirrors above and below the gain region (Fig. 19.1(a)). The laser light propagates in a vertical direction and typically exhibits a circular beam shape. Internally, the photons pass the gain region in a perpendicular direction , i.e., optical gain is provided over a short propagation distance only and the amplification per photon round trip is small. Therefore, the mirrors need to be highly reflective so that photons make many round trips before they are emitted. To achieve high reflectivity, distributed Bragg reflectors (DBRs) are used with two alternating layers of high refractive index contrast. With quarter-wavelength layer thickness, the reflected waves from all DBR interfaces add up constructively, allowing for DBR reflectivities above 99% [1]. Within the active layers (quantum wells), optical gain arises from the stimulated recombination of electrons and holes, which may be generated by optical absorption of pump light or by current injection. The latter is more difficult to accomplish and it is strongly affected by the electrical DBR properties. Semiconductor DBRs allow for vertical carrier injection through the DBR directly into the active gain region (Fig. 19.1(b)) which gives good overlap of the lateral carrier and photon profile. However, suitable semiconductor materials often exhibit small index contrast and the many hetero-interfaces tend to generate a high electrical DBR resistance. An alternative choice is dielectric DBRs which typically provide a large refractive index contrast so that a few layer pairs are often sufficient for high mirror reflectivity. However, dielectric DBRs are electrically insulating and the injection current needs to be funneled into the active region from the side, typically by using ring contacts around the DBR (Fig. 19.1(c)). Some type of electrical confinement structure is required that forces the carriers to move into the small center region where the optical mode is located [2]. Last, but not least, the small lateral extension of the active region causes a potentially high thermal resistance so that good thermal