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 [1]. 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 [2].
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.
[1] H. Wang, et al., Appl. Phys. Lett. 81 (2002) 4703.
[2] T. Tawara, et al., Appl. Phys. Lett. 83 (2003) 830.
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