Properties of Homoepitaxially MBE-Grown GaN
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nitrogen in liquid gallium at temperatures about 1500°C and a nitrogen pressure of 1.5 GPa [8]. The high-pressure, high-temperature method of GaN synthesis yield crystals which grow in the wurtzite structure, usually as hexagonal platelets perpendicular to the c-axis. Their one side is usually very flat [9]. This surface (in crystals with lateral dimensions of 3-4 mm) was used for the GaN film depositions. Background doping in bulk samples is n-type with a carrier concentration above 1019 cm- 3 . This is presumably due to a high concentration of nitrogen vacancies. The width of the X-ray rocking curve for (0004) CuKoc reflection ranges between 30-60 arcsec. Prior to the film growth, the GaN substrate was cleaned with HF. The GaN layers were grown in a home-built MBE chamber (sample A) and in the Riber chamber (sample B), using a hollow anode source to provide activated nitrogen species [10]. The nitrogen source is designed to minimize defect formation from contamination and ion damage. The MBE chambers were cryopumped and reached base pressure in the 10-9 Torr range. For sample A, we initiated the film growth at 600 0 C and ramped the substrate temperature up to about 675 0 C within 5 minutes. This procedure allowed for a minimal decomposition of the bulk GaN substrate prior to deposition. For sample B, the deposition started from 10 min nitradation at 6950C by exposure to nitrogen plasma. The growth took place at 6750C. The films were grown in 8-hour processes, resulting in approximately 0.5 um thick layers. No buffer layer was used. The thin films were then characterized by low temperature photoluminescence (PL) and Raman scattering (T=300K). In addition, a transmission electron microscopy study were performed on Sample A. In the PL studies, which are the main subject of this paper, the samples were excited by the 325 nm line of a 50 mW HeCd laser. The luminescence signal was dispersed by a 0.85 m double monochromator and detected by an UV-sensitive photomultiplier. RESULTS AND DISCUSSION Fig. la (lb) compares the low-temperature PL (T=6K) in the sample A (sample B) consisting of the bulk GaN crystal substrate and the GaN film deposited on the bulk sample. For the sample A, the thin film shows a sharp intense peak at 3.467 eV with a full width at half maximum (FWHM) of 16 meV. This peak is assigned to a recombination of exciton bound to a shallow neutral donor (EBND). The lower energy shoulder at 3.420 eV and a peak at 3.367 are observed also. They are likely associated with more deeply bound excitons. The yellow luminescence peak at -2.3 eV which is commonly observed in GaN is almost entirely absent in the spectrum of the deposited film. The intensity ratio, R, of the band-edge luminescence to the midgap luminescence at 2.3 eV for the homoepitaxial film is 600. This compares to a ratio about 0.1 in the bulk substrates of the sample A. The PL spectra of the both bulk crystals show a weak broad peak near the band edge (FWHM of 0.1 eV-0.2 eV). The observed emission is related to band-to-band transitions. For majori
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