Mechanisms of Optical Gain in Cubic GaN and InGaN
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ERIMENTAL
Downloaded from https://www.cambridge.org/core. Auckland University of Technology, on 11 Aug 2020 at 09:44:53, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S109257830000226X
Cubic GaN films with a phase purity better than 99.9% were grown on semi-insulating GaAs (001) substrates by RF-plasma assisted molecular beam epitaxy (MBE) at a substrate temperature of 720°C. Undoped epitaxial layers were grown under carefully controlled stoichiometric growth conditions[13]. Details of the growth procedure were reported in Ref.13. The optical properties of the c-GaN layers investigated under low and high excitation intensities are reported in Ref. 10. By cleaving we obtained c-GaN samples with cavity lengths of 550, 450 and 250 µm along the common (001) direction and performed high-excitation and gain measurements. The InGaN/GaN/GaAs (001) heterostructures were grown by plasma assisted MBE. The GaN buffer layers with a typical thickness in the range from 100-200nm were grown at T=720° C under carefully controlled stoichiometric conditions, exploiting Reflection High Energy Diffraction (RHEED) measurements of the surface reconstruction as an in-situ control of the composition of the layer surface during growth [14]. The InGaN layers had a thickness between 200-300 nm and were deposited at lower temperatures (T=610-680° C). During the InGaN epitaxy we used a Ga flux which was reduced by about 20% compared to that of the GaN layer deposition. The In flux was adjusted to establish a metal rich surface taking into account the extremely small and strongly temperature dependent sticking coefficient of In. To obtain the high excitation density necessary for our investigations we used a dye laser pumped by an excimer laser, providing pulses with a duration of 15 ns at a rate of 30 Hz and a total energy of up to 20 µJ at 340 nm. The sample was mounted in a bath cryostat at 1.8 K. Gain measurements were performed using the variable-stripe-length method [15] . The excitation spot was focused onto a l x 50 µm 2 stripe, where l denotes the excitation length. The photoluminescence spectra were recorded from the top of the sample with a continuous-wave (cw) helium-cadmium laser. RESULTS Optical Amplification in cleaved c-GaN samples Figure 1 shows the spectra of the edge emission from a cleaved sample with a cavity length of 450 µm at different excitation densities on a linear scale at 2 K. Above the threshold excitation of 1 MW/cm2 a peak at 3.26 eV appears exhibiting a strong increase of the edge emission with excitation intensity and a strong polarization dependence. The emitted light is strongly TE polarized, as expected for an edge emitting cleaved facet. For higher densities up to 5 MW/cm2 a slight shift to lower energies of the peak position is observed, indicating the increased carrier density in the sample. In Fig. 1a (inset of Fig. 1) the results of intensity dependent edge emission measurements for the 250, 450 and 550 µm c-GaN samples are summari
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