Ex Situ and in Situ Methods for Oxide and Carbon Removal from AlN and GaN Surfaces
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SiC(0001) or on epitaxial AIN buffer layers previously grown on 6H-SiC (0001) by OMVPE [11] and GSMBE [12], respectively. The in situ cleaning and the surface analyses of the samples subjected to ex situ and in situ cleaning were conducted in a unique ultra-high vacuum (UHV) system consisting of a 36 ft. long UHV transfer line to which were connected several surface analysis and thin film deposition units. The details of each unit and the transfer line have been described elsewhere [10]. CMOS grade acids and bases were used for the ex situ wet cleaning processes. After wet chemical cleans, samples were briefly rinsed in 18.4 MQ de-ionized water, blown dry with N2 , mounted to a molybdenum sample holder and loaded into the transfer line load lock. After evacuation for - 20 min., the samples were placed in the transfer line for immediate analysis by x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). For the ultra-violet/ozone (UV/0 3 ) oxidation studies, a specially designed box was used which incorporated a Hg lamp in a design described in ref. [13j. In situ methods consisted of exposure to a H plasma or annealing in UHV or fluxes of NH 3 , Al, or Ga. After in situ cleaning in either the GSMBE or the H plasma system, samples were immediately (< 5 rmin.) =nsferred for analysis by AES, low energy electron diffraction (LEED), and XPS. XPS analysis was performed using both the Al (hv=1486.6 eV) and Mg (hv=1253.6 eV) anodes at 20 mA and 10kV. AES spectra were obtained using a beam voltage of 3 keV and an emission c-,irrent of 1 mA. LEED was performed using rear view optics, a beam voltage of approximately 100 eV, and an emission current of 1 mA. Calibration of the XPS binding energy scale was performed by measuring the position of the Au 4f/2 and shifting the spectrums such that the peak position occurred at 83.98 eV. All sample temperatures quoted here were measured using an optical pyrometer and an emissivity of 0.5. RESULTS Ex situ and In situ cleaning of AMN Figure l(a) shows an AES spectrum of the surface of an as-received OMVPE AlN sample which is oxidized due to the strong affinity of Al for 02. Additional oxidation via UV/0 3 was investigated initially as a possible ex situ method for C removal from the nitride surfaces. Both AES and XPS were used to examine an OMVPE AIN film which had been cleaned in trichloroethylene, acetone, and methanol for 5 min. in each solvent and then exposed to UV/03 for 10 min. at room temperature. As shown in Fig. 1(b), the combination of solvent cleaning and UV/0 3 exposure reduced the surface C from the AMN surface by 50%. Longer UV/0 3 exposures (30 min. - lhr.) with or without a solvent preclean did not further decrease the surface C. However, Fig. 1(b) does show that the AMN surface was further oxidized by the UV/0 3 exposure which indicates that 03 was being generated by the Hg UV lamp. As AMN is reasonably chemically inert, oxidation of an AIN surface in a typical laboratory ambient does not proceed rapidly. Therefore, UV/0 3 exposure was used to repea
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