GaN-based surface-emitting lasers

The debut of semiconductor lasers came in a simple format of Fabry Pérot (F P) cavity with two cleaved facets as parallel mirrors. The light is generated in the active region (typically made of multiple quantum wells (MQWs) through the radiative recombination of electron and hole pairs provided by the n- and p-cladding layers, respectively. The n- and p-cladding layers are connected to the external current source to form a p-n diode. These layers are epitaxially grown on the semiconductor substrate by epitaxial equipment, for example, molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD). In order to form an F-P cavity, the substrates have to be cleaved to reveal two end facets so that the light is able to travel back and forth parallel to the junction plane to complete a round-trip oscillation, shown as in Figure 18.1a. After the population inversion and laser threshold are met, the coherent laser light is out-coupling through the edge of the cavity. Therefore, this type of semiconductor laser is termed as the edge-emitting laser (EEL) and has been adopted and commercialized in many optoelectronic systems. However, the laser action of EELs is observed only after the substrates are cleaved. On-wafer testing would be very challenging, and the protection of cleaved facets is also critical. If the laser light emits out of the wafer surface, the cleaved facets are no longer needed, and two-dimensional laser arrays can be implemented. As shown in Figure 18.1b, the emitting direction of laser light is perpendicular to the junction plane. This type of semiconductor laser is called the surface-emitting laser (SEL).