Room Temperature Laser Action in Laterally Overgrown GaN Pyramids on (111) Silicon
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photolithography and reactive ion etching. The openings were arranged in a hexagonal pattern with a 20 gim spacing and the average diameter of the openings ranged from 2 to 5 jin depending on the size of pyramids to be grown. GaN growth was then performed in the MOCVD reactor with the ammonia flow set at 1.8 slm and the triethylgallium (TEG) flow varying 1.9 to 5.3 jtmol/min over the course of 3 hours at a temperature of 1050 °C. The smaller TEG flow for the initial growth was used to avoid nucleation on the mask. The growth conditions are described elsewhere9 in more detail. The result of the selective lateral overgrowth was a two-dimensional array of GaN pyramids. A scanning microscope image (SEM) image of one of the samples is shown in Fig. 1. The base diameter of the pyramids was estimated to be about 5 and 15 Am for the two different arrays of pyramids, which is considerably larger than corresponding 2 and 5 jiLm openings in the mask, indicating substantial lateral growth of the pyramids. Transmission electron microscope pictures revealed a drastic reduction in defect densities. The samples were mounted on a translation stage that allowed 3-D positioning of the sample with -1 micron resolution. The third harmonic of an injection-seeded Nd:YAG laser was used as the pumping source. The pulse width of the laser was varied from 5 to 25 ns by changing the Q-switch delay. The laser beam was focused to a diameter of 4 jLm through a microscope objective. The laser light intensity was attenuated continuously using a variable neutral density filter. This study was performed in a surface emitting geometry where emission from the sample was collected through the same microscope objective in the direction normal to the sample surface.
FIG. 1. SEM image of GaN pyramids with a 15 prm wide hexagonal base, grown on a (111) Si substrate by selective lateral overgrowth. The details of the experimental configuration are described elsewhere.I0 This allowed us to pump, image, and spectrally analyze emission from separate pyramids.
RESULTS AND DISCUSSION The RT emission spectra at different pump densities (-20-ns-pulses) near the lasing threshold for the 15-jim-wide pyramids are shown in Fig. 2. At excitation densities below the lasing threshold, only a spontaneous emission peak with a full width at half maximum (FWHM) of approximately 14 nm is present. The energy position as well as the spectral width of the peak are very similar to that observed from high-quality single-crystal GaN epilayers.' As the pump density is increased to values slightly above the lasing threshold, a very narrow peak with a FWHM of less than 0.3 nm appears on the low energy side of the spontaneous emission peak. Note that the FWHM of the stimulated emission peak in high-quality GaN epilayers is typically 2 nm at RT. The intensity of the peak shown in Fig. 2 increases superlinearly with excitation power. Both drastic narrowing of the spectra and the superlinear increase of intensity suggest that we observe single-mode laser action in the pyramids. The las
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