The Growth and Characterization of Germanium-Carbon Alloy Thin Films
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THE GROWTH AND CHARACTERIZATION OF GERMANIUM-CARBON ALLOY THIN FILMS Haojie Yuan and R. Stanley Williams Department of Chemistry and Biochemistry and Solid State Science Center, University of California, at Los Angeles, Los Angeles, CA 90024 ABSTRACT Thin films of pure germanium-carbon alloys (GexCl-x with x - 0.0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) have been grown on Si(100) and A12 0 3 (0001) substrates by pulsed laser ablation in a high vacuum chamber. The films were analyzed by x-ray 0-20 diffraction (XRD), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), conductivity measurements and optical absorption spectroscopy. The analyses of these new materials showed that films of all compositions were amorphous, free of contamination and uniform in composition. By changing the film composition, the optical band gap of these semiconducting films was varied from 0.00eV to 0.85eV for x = 0.0 to 1.0 respectively. According to the AES results, the carbon atoms in the Ge-C alloy thin film samples has a bonding configuration that is a mixture of sp 2 and sp 3 hybridizations. I. INTRODUCTION A new class of semiconducting materials would result from alloys of group IV elements with each other. Although much research work has been done with the Si-C and Si-Ge systems, the Ge-C system has received less attention. No stable compound containing only Ge and C has been synthesized to date [1]. Compounds of Ge and C are of interest because of their potential for forming a new system of semiconducting material with unique properties. It has been suggested that the Ge-C alloy thin films could be an important material for photothermal [2] and photovoltaic applications [3]. Although GeC alloy thin films have been grown by chemical vapor deposition [2], sputtering [3] and glow discharge [4] methods, it is only possible to produce amorphous hydrogenated Ge-C alloys. In search of pure metastable alloys of Ge and C, we attempted to grow GexCi-x (where x is equal to 0.0, 0.2, 0.4, 0.5, 0.8, and 1.0) alloy thin films on Si(100) and sapphire(0001) substrates by laser ablation. Films were characterized by x-ray 0-20 diffraction experiments (XRD), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), conductivity measurements, and optical absorption spectroscopy. I. EXPERIMENTS The films were deposited in a high vacuum chamber. The chamber base pressure was less than 5x10-7 Torr. Powder mixtures of Ge and C with different compositions were ground and pressed into pellets to be used as targets. The ablating pulses were supplied by an excimer laser, using ArF with a wavelength of 193nm. The laser power used was 160mJ/pulse at pulse repetition rate 10Hz. The laser beam was focused to a spot of roughly 1mm diameter onto the 2.5mm diameter target, which was rotated at a speed of Mat. Res. Soc. Symp. Proc. Vol. 270. @1992 Materials Research Society
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25rpm to avoid surface texturing. Films were deposited both on Si(100) and on sapphire(0001) substrates. The target to substrate distance was 3cm. The substrat
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