Drifts surface characterization of Zincblende nanostructured GaN
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Mat. Res. Soc. Symp. Proc. Vol. 501 ©1998 Materials Research Society
precursor Ga2 [N(CH 3)2] 6 under flowing ammonia at 600'C for four hours. The ammonia flow was maintained during the warm up and cool down phases. A greyish powder was obtained and handled under inert atmosphere. Chemical analysis revealed the material to be slightly nitrogen deficient (GaNo. 86 ). Carbon and hydrogen were also found (0.8 and 0.5% weight, respectively). FTIR experimental procedure The surface analysis was performed by diffuse reflectance infrared Fourier transform spectrometry (DRIFTS). The DRIFTS setup (supplied by Graseby-Specac) used in this study is composed of the regular diffuse reflection attachment and of a heatable vacuum chamber. This setup allows the in situ analysis of the loose GaN powder at any temperature from room temperature (rt) to 500'C under vacuum or various pressures of gases which can be controlled through a valve system. In the diffuse reflection technique, the sample is irradiated by the infrared beam while the beam diffused by each grain is collected through a spherical mirror. Therefore, the collected beam mainly consists of signals from the surface species. But, because the incident beam slightly penetrates inside the grains, signals from the bulk are also present [3]. To discriminate the bands originating from the bulk and those originating from the surface species, probe gases are adsorbed on the sample. The interaction of these gases with the surface species causes shifts or modifications of the bands from the surface groups, whereas no perturbation of the bands from the bulk can occur. To emphasize these perturbations, difference spectra are presented in the following. In such spectra, the negative bands correspond to the decreasing species during the experiment step under consideration whereas the positive bands correspond to the increasing species. The pure GaN nanopowder was kept under nitrogen until the infrared analysis was performed. Then, the powder was transferred in about 5 minutes into the environmental chamber before the latter was pumped out. The DRIFTS setup is placed in the sample chamber of a Perkin-Elmer Spectrum 2000 spectrometer equipped with a MCT cryodetector. The analyzed range varied from 4000 to 450 c1f' with a 4 cm' resolution. In this study, only the wavenumber region from 4000 to 1000 cm' where the main vibrational frequencies of the surface species absorb will be discussed. RESULTS AND DISCUSSION Bulk characterization The chemical analysis of the gallium nitride nanosized powder revealed the material to be slightly nitrogen deficient (GaN 0 86) [2]. Carbon and hydrogen were also found (0.8 and 0.5% weight, respectively). The structure of the gallium nitride powder was determined to be zinc blende by XRD analysis [2], with broadened peaks due to the small size. A low intensity peak observed in the XRD spectrum possibly originates from nitrogen vacancies. The standard infrared analysis of the GaN powder [2] pressed in potassium bromide pellets showed a main band at 550 cmf
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