GaN-Based Vertical Cavity Surface Emitting Lasers
Takehiko Tawara, Hideki Gotoh, Tetsuya Akasaka, Naoki Kobayashi, and Tadashi Saitoh
Physical Science Laboratory
Optical devices based on III-nitride quantum wells (QWs) have been actively studied. However, the refractive index contrasts between III-nitride and air are relatively small compared with those in GaAs-based semiconductors and the reflectivity is about 18%. This leads to an increase in the driving current needed for optical devices . With lasers, one way to reduce the lasing threshold is to use the vertical cavity surface emitting laser (VCSEL) structure. If the cavity size is of the order of the emission wavelength, the spontaneous emission rate can be controlled and the lasing threshold can be reduced.
VCSEL structures usually consist of a cavity with active layers and distributed Bragg reflectors (DBRs), which are formed by monolithic growth with the same kind of semiconductor. When a high reflectivity DBR is constituted from GaN-based semiconductors, however, the large lattice mismatch between each layer of the DBR generates surface roughening and cracking of the cavity layer.
We prepared a GaN-based cavity layer including InGaN QWs on a SiC substrate and oxide-based DBRs separately. The VCSEL structure was formed by using a wafer bonding technique after removing the SiC substrate by selective dry etching. We realized high quality VCSEL structures without cracking or surface roughening (Fig. 1). The spectral linewidth of the cavity mode at the InGaN emission wavelength of 400 nm was less than 1 nm, and this indicates that we realized a microcavity with low optical loss. Moreover, we measured the luminescence intensity from the fabricated VCSELs for different optical excitation powers at room temperature (Fig. 2). A clear nonlinear characteristic was observed, and this shows that the VCSEL is lasing. Furthermore, the spontaneous emission factor (the coupling efficiency of the spontaneous emissions to the lasing mode) was about 0.01, and this is 1000 times more efficient than in a typical edge-emitting laser .
In such a structure, there is an advantage in that the reflectivity and the stopband width of the DBR can be chosen freely while maintaining the crystal quality of the cavity layers. Moreover, the realization of strong exciton-photon coupling at room temperature will be possible by using such high quality microcavities, because the exciton oscillator strength and binding energy of III-nitride semiconductors are large compared with GaAs-based semiconductors.
 H. Wang, et al., Appl. Phys. Lett. 81 (2002) 4703.
 T. Tawara, et al., Appl. Phys. Lett. 83 (2003) 830.
Fig. 1. Cross-sectional image of the VCSEL.
Fig. 2. Lasing characteristics.
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