A study of annealed GaN grown by molecular beam epitaxy using photoluminescence spectroscopy
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defects, which nucleate at dislocations. It has also been suggested by Reynolds et al [4], that the yellow band in GaN results from a transition between a shallow donor and a deep level. The deep level in the Reynolds’s model was attributed to a complex consisting of a Ga vacancy and oxygen on a nitrogen site (VGa-ON). Another commonly occurring peak in GaN is the 3.427eV peak, seen in the 4K PL spectra. It was initially attributed to oxygen forming a shallow “deep level”[5]. However, it has more recently been suggested that peaks in the PL spectra of GaN in this region 3.40eV [6-8], 3.412eV [9] and 3.42eV [10] can be related to excitons bound to stacking faults. By annealing samples of GaN in different ambients and at different temperatures, we have clarified some of the issues regarding the role that defects play, particularly in the “yellow luminescence” and the near 3.42eV luminescence regions. GaN samples grown by MBE have been used in this study and annealed at different temperatures in oxygen, oxygen plus water vapour, nitrogen, nitrogen plus water vapour and argon in a high-temperature stage. EXPERIMENTAL METHOD The layer used in this work (sample MG671) was grown by MBE in a modified Varian Mod Gen II machine. The active nitrogen was produced by radio-frequency (RF) plasma source and the Ga was produced from solid elemental sources. The sample was an unintentionally doped 1µm thick GaN layer grown on a (0001) sapphire substrate. Grown on top of the GaN was a high-mobility field effect transistor (FET) structure consisting of 3nm undoped Al0.15Ga0.85N/22nm Si-doped Al0.15Ga0.85N/15nm undoped Al0.15Ga0.85N. Pieces of the sample MG671 were annealed for 20mins using a high temperature stage at various temperatures and in various gases. The pieces of MG671 were labelled numerically and will be refered to as (number) in the remainder of the paper. The annealing details and sample numbers are shown in table 1. The photoluminescence spectra were taken using a 325nm CW HeCd laser with a maximum excitation intensity of 9mW. A 325nm band pass filter was used to attenuate lines other than the 325nm laser line. A quartz lens focused the beam onto the sample in a liquid helium cryostat. The excitation light was normal to the sample and the resulting PL was focused through a low-pass sharp cut-off filter (Oriel WG345), used to stop the laser light, onto the monochromator slit. The 0.75m Spex monochromator, with a 2400 lines/mm grating, has a resolution of 4Å/mm, which correspond to 4meV/mm in the region of 3.4eV. The light was detected by a bi-alkali Table 1. Sample numbers with annealing photomultiplier (Thorn parameters. 9924QB) with a 600V applied voltage. Standard phase Temperature (OC) 800 900 1000 1100 1200 sensitive detection techniques O2 40 41 41 44 were used to improve the O2 and H2O 63 signal to noise ratio. N2 9 46 7 N2 and H2O 61 Ar 16 18 19 Pre-annealed 39
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RESULTS
PL Intensity (arb. units)
In all of the 4K PL spectra (figure 1) there is a peak which occurs in the region from 3.467eV t
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