The Effect of Thermal Annealing of Au Contacts on 6h-Sic and 4h-Sic
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fabrication and annealing procedures were the same for both 4H- and 6H-SiC samples. The adopted cleaning procedure for SiC was as following: diced SiC substrates were first washed with distilled water and acetone, then subsequently immersed and/or washed in NH4 OH:H 2 0 2 :H20 (1:1:2) at 85 'C for 5 minutes, HCI:H 20 2 :H20 (1:1:2) at 85 'C for 5 min, HF:H 20 (20% solution) at room temperature (RT) for 5 min, HNO 3 (50%) at 90 'C for 5 min, and HF:H 20 (10% solution) for 5 min. Between each step above, the sample was rinsed by deionized water. After chemical cleaning, the sample was kept immersed in methanol to prevent contamination before being loaded into a sputtering chamber. The Au ultra-thin films were deposited onto the SiC substrates by employing an rf sputtering system (from Kurt J. Lesker Company). The chemically cleaned SiC substrates were put in the sputtering chamber and baked at 350 'C (the maximum baking temperature available for the system) for 30 minutes at a base pressure of lxI0"6 torr. The sputtering parameters were as following: 5 inches' sample-to-source distance, 35 W rf power, 12 mtorr Ar pressure, 30 and 150 second sputtering times for the thinner and thicker films, respectively, and at about 40 'C of operating temperature. The purity of the gold source was 99.95%. The resulting thickness of the films was about 30 A and 150 A based on XPS and/or AFM measurements. The as-deposited samples with the thinner Au film were first studied with XPS/AES at room temperature after a brief exposure to air during transportation and then imaged using AFM. For samples with the thicker Au film, only surface morphology was investigated. A Kratos XSAM 800 spectrometer that contains both XPS and AES was employed for sample analysis. The spectrometer with a resolution of 0.1 eV uses Mg K,, (ho = 1253.6 eV) x-rays to create photo-emissions. The operating vacuum was 108 torr. SPM systems (NanoScope E & III from Digital Instruments, Inc.) were operated in contact and/or tapping modes for the sample surface imaging. Annealed samples were obtained by sequentially putting all the Au/SiC samples into a tube furnace. The samples were annealed at 200 'C, 300 'C or 400 'C for 30 minutes each and quenched in air at RT after annealing. The annealing procedure was an analogue to the operation of a real SiC based device that is operated and/or maintained at both high and room temperatures. AFM analyses of the annealed samples were conducted after XPS/AES studies. RESULTS AND DISCUSSIONS A: Morphological Analyses Surface morphology of the Au/SiC systems was obtained using AFM as displayed in Figs. 1-3. Fig. 1 shows the surfaces of the thinner (30 A) Au films on 4H- and 6H-SiC for asdeposited, 300 'C annealed and 400 'C annealed samples, respectively. The image sizes are all 1 tmx 1 ×mx 10nm. Surface morphologies of the thicker (150 A) Au films on 4H- and 6H-SiC are revealed in fig. 2 for the 200 'C, 300 'C and 400 'C annealed samples, respectively. The image sizes are all I gmx 1 tm with a z range of 10 nm. The exception is the
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