Observation of Native Ga Vacancies in GaN by Positron Annihilation
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transition leading to the yellow luminescence is observed both in GaN bulk crystals and layers, but even the positions of the electronic levels participating this optical process are under discussion [1; 2; 3; 4]. In this work we use positron annihilation spectroscopy to obtain structural information on native point defects in GaN bulk crystals as well as in epitaxial layers [5]. Positrons get trapped at neutral and negative vacancies because of the missing positive charge of the ion cores. The reduced valence and core electron densities at vacancies increase the positron lifetime and narrows the positron-electron momentum distribution. EXPERIMENT The bulk crystals were grown at the nitrogen pressure of 1.5 GPa and tem erature of 1500 'C [6]. The free electron concentration in the undoped material is 5x'1019 cm- at 300 K. The Mg-doped bulk crystals are semi-insulating. The undoped epitaxial layers were grown on sapphire substrates in an atmospheric-pressure MOCVD equipment with the gas system and the quartz reactor especially designed for the growth of nitride compounds. Ammonia and trimethylgallium were used as source gases and hydrogen as the carrier gas. After thermal annealing at 1000 'C in H2 and nitridization of the surface in NH 3, a low temperature GaN buffer layer was deposited. Then a GaN layer with a thickness of 2 gtm was grown at temperature in the range of 1000 - 1050 °C. 757 Mat. Res. Soc. Symp. Proc. Vol. 482 © 1998 Materials Research Society
The GaN layers were characterized by Hall effect and photoluminescence measurements at 300 K. All layers were n-type with n - 1017 - 1018 cm- 3 . The intensity of the yellow luminescence was determined by exciting with the 325 nm line of a He-Cd laser. In order to probe approximately the same region below the surface of the epilayer as in the positron experiments, the luminescence was excited from the substrate side of the sample. The emitted radiation was analyzed by a 0.5-m monochromator equipped with a photomultiplier. In order to compare the yellow luminescence of different samples its intensity was averaged over the surface of a particular sample and the same optical alignment was used to collect the light emitted by each sample. The GaN bulk crystals were studied by sandwiching two identical sample pieces with a 30 4tCi 22 Na positron source. Both positron lifetime and Doppler broadened shape of the 511 keV annihilation radiation were recorded using conventional instrumentation [7]. The lifetime spectrum n(t) is the sum of exponential decay components n(t) = I Iiexp(-t/tci). The positron in the state i (e. g. delocalized state in the lattice or localized state at a vacancy) annihilates with the lifetime ri and the intensity Ii. The increase of the average lifetime Tav = I Iii above TB obtained in the defect-free lattice is a clear sign of vacancy defects in the sample. The GaN layers were investigated with Doppler broadening experiments using a positron beam. The energy of the beam was chosen so that all positrons annihilate in the GaN epilayer. The sha
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