Chemical and Structural Analysis of Nitridated Sapphire
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Despite a large lattice mismatch with GaN (- 14%), sapphire is the most used substrate so far. It was reported that a high quality epitaxial growth was obtained with the initial growth of a buffer layer at low temperature [3]. Furthermore, nitridation of the substrate before the buffer layer growth was reported to improve morphological and optical properties of GaN films [4-6]. It is clear from these reports, that the quality of thick GaN films depends on the initial surface treatments. The exact mechanism of the initial nitridation of sapphire is, however, not well understood. Uchida et al. [5] observed an amorphous layer of AINxO,.x after substrate was exposure to NH 3 at 1020 'C. On the other hand, Moustakas et al. [6] reported that a relaxed layer of AIN was formed at 850 'C after substrate exposure to a nitrogen plasma from electron cyclotron resonance (ECR). Based on RHEED studies, Grandjean et al. [7] also reported the formation of a relaxed AIN layer after the nitridation of sapphire by NH3(g). In this paper, we report the first direct observation of the formation of both AIN layer and amorphous AINxO,-_ after the substrate was exposed to a nitrogen plasma. EXPERIMENT Nitridation was performed in a rebuilt Riber 1000 MBE system with a constricted glow discharge nitrogen plasma source. The details of the plasma source design is reported elsewhere [8]. Basal plane sapphire substrates were obtained from Union Carbide. They were degreased by 45 Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society
boiling in acetone and ethyl alcohol for 5minutes each, and blow-dried with nitrogen before they were placed in the MBE chamber. Once they were placed in the load-lock, the substrates were degassed for 30 minutes at 500 'C. The susbtrates were then transferred to the main chamber and heated up to 700 'C for thermal desorption of residue surface contaminants. Nitridation was performed by exposing the substrates to activated nitrogen at a flux of 35 sccm for 5 to 60 minutes. The XPS of the substrates were collected using a PHI 5300 ESCA system with a Mg target. The nitridated sapphire samples were exposed to air during the transfer from the growth vacuum chamber to the XPS vacuum chamber which was maintained at 10. to 10' torrs. High resolution spectra were taken in the binding energy regions of Al2p, N I s, O 1s and C I s spectral peaks. A 6 1 tm-thick AIN grown on sapphire was used as a standard. Because the substrates were highly insulating, electrostatic charging resulted in positive binding energy shifts. Adventitious carbon Is peak (284.8 eV) from the surface hydrocarbon was used as a reference peak to correct for binding energy shifts for all spectra. An AutoProbe M5 AFM system manufactured by Park Scientific Instruments was used to investigate the surface morphology. Areas of 21am by 2[tm were scanned in contact mode. To be consistent, images from the several areas near the center of the substrates were analyzed. The structure of the nitridated sapphire and the formation of thin AIN layers were
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