A Study of Pt/AlN/6H-SiC MIS Structures for Device Applications
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Table I. Deposition conditions and thickness of the AlN films grown via PSMBE. Deposition Acceleration Gas Flow, r.f. power, Thickness, Temperature, Voltage, sccm W nm °C V Film 1 500 -15 20N2/40Ar 250 53 Film 2 640 -15 20N2/40Ar 250 56 Film 3 800 -15 20N2/40Ar 250 79 Film 4 500 -15 20N2/40Ar 200 120 0.055 ohm-cm. Each 6H-SiC piece was ultrasonically degreased for 20 minutes in acetone and methanol, in succession. Prior to deposition, the substrates were heated inside the PSMBE chamber at 800°C for one hour. The deposition conditions for all AlN films discussed here are presented in Table I. The films were characterized in situ by Reflection High Energy Electron Diffraction (RHEED) using a KSA 300 system by k-Space Associates, Inc. Atomic Force Microscopy (AFM) was performed on a Digital Instruments Nanoscope III in contact mode. Spectroscopic ellipsometry characterization in the range 0.73-6.5 eV was performed on a standard deepultraviolet range ellipsometer (DUV-VASE™), while in the range 4.25-8.75 eV a prototype of a vacuum-ultraviolet range ellipsometer (VUV-VASE™) was used. Both ellipsometers were equipped with an AutoRetarder™ for maximum measurement precision [7]. The SiC substrate optical constants were fixed at the values given by Zollner et al. [8]. No significant anisotropy effects are expected from the substrate since the optical axis is normal to the sample surface and the substrates were single-side polished. Each AlN film was described using the GBPS dispersion relationship as developed by Herzinger-Johs [8, 9]. A Bruggeman Effective Medium Approximation was used to model the surface roughness of each film. In addition, Film 3 and Film 4 required vertical index grading to improve the results fit. For electrical measurements, Pt electrodes were deposited using a d.c. magnetron sputtering system. Circular electrodes with a diameter of 150 µm were deposited on the surface of the films through a hard mask and blanket contacts were deposited on the back of the 6H-SiC substrate. I-V measurements were performed on a HP4140B pico-ammeter with a d.c. voltage source. C-V characterization was carried through on a HP4192A LF impedance analyzer at 1 MHz. A heated chuck was used to reach temperatures up to 250°C. RESULTS AND DISCUSSION RHEED and x-ray diffraction analysis revealed that all films are single crystalline and epitaxial to the 6H-SiC substrates. Figure 1 shows RHEED patterns and AFM images from the surface of the thin AlN films deposited as a function of deposition temperature. The RHEED patterns consist of oblong spots, which indicate the presence of island-like features on the surface of the films. The RHEED pattern from the film grown at 800°C consists of wider, more round spots, which is an indication for increased surface roughness. AFM revealed that the root mean square (RMS) roughness is higher for the 800°C film and the surface features for this film are substantially larger compared to the films grown at lower deposition temperature. This indicates that the film deposited at 800°C most probably
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