Microstructure-based lasing in GaN/AlGaN separate confinement heterostructures
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studied a 4.2-µm-thick GaN epilayer grown on (0001) 6H-SiC. The samples were mounted on a copper heat sink attached to a wide temperature range cryostat. Conventional photoluminescence (PL) spectra were measured in the back-scattering geometry using a frequency-doubled Ar+ laser (244 nm) as the excitation source. In order to study the lasing phenomena, a tunable dye laser pumped by a frequency-doubled, injection-seeded Nd:YAG laser was used as the primary optical pumping source. The excitation beam was focused to a line on the sample using a cylindrical lens. The emission from the edge of the sample was coupled into a 1-m spectrometer with a side-mounted optical multi-channel analyzer and photomultiplier tube. Special precautions were taken to avoid fluctuations in sample position due to the thermal expansion of the mounting system. This allowed us to spatially "pin" the sample and obtain lasing modes from a single microcavity over the entire temperature range studied. DISCUSSION In order to evaluate the effects of cracks on the optical properties, we cleaved our samples into submillimeter-wide bars (note that when GaN is grown on SiC, it can be easily cleaved along the ( 11 2 0 ) direction [3]). Before cleaving, the samples did not exhibit any noticeable defects on the surface. After the cleaving process, however, cracks were observed along all three cleave planes associated with a hexagonal crystal structure, with the majority running parallel to the length of the bar, as shown in Figure 1.
30 µ m Figure 1. A picture of the sample surface of a GaN/AlGaN separate confinement heterostructure after cleaving. Before cleaving, the sample exhibited no noticeable defects on the surface. After the cleaving process, however, cracks can be seen running parallel to the length of the bar.
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When the sample was excited above the lasing threshold, high-finesse cavity modes were observed. Typical emission spectra at pump densities above and below the lasing threshold are depicted in Figure 2. A series of equally spaced and strongly polarized (TE:TM ≥ 300: 1) lasing modes with full width at half maximum of ~3 Å appears on the low energy side of the GaN-active-region peak. We note, however, that when the spontaneous emission was collected from the sample edge instead of the surface, the lasing modes appeared on the higher energy side of the spontaneous emission peak [4]. This phenomenon is due to strong re-absorption that introduces a shift of several nanometers to the spontaneous emission peak. Lasing in our samples occurs in the wavelength range of 360 to 364 nm at room temperature, which is much deeper in the UV than for InGaN/GaN-based structures. We were able to correlate the spacing between the modes in Figure 2 to the distance between the cracks depicted in Figure 1. Assuming that the unity round-trip condition is satisfied and there is no loss due to absorption in the GaN layer, the threshold gain can be estimated from [5]:
GaN/AlGaN SCH RT Intensity (arb. units)
lasing
spontaneous emission
3.35
3
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