Characterization of diamond films by Raman spectroscopy
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As the technology for diamond film preparation by plasma-assisted CVD and related procedures has advanced, Raman spectroscopy has emerged as one of the principal characterization tools for diamond materials. Cubic diamond has a single Raman-active first order phonon mode at the center of the Brillouin zone. The presence of sharp Raman lines allows cubic diamond to be recognized against a background of graphitic carbon and also to characterize the graphitic carbon. Small shifts in the band wavenumber have been related to the stress state of deposited films. The effect is most noticeable in diamond films deposited on hard substrates such as alumina or carbides. The Raman line width varies with mode of preparation of the diamond and has been related to degree of structural order. The Raman spectrum of hexagonal diamond (lonsdaleite) is distinct from that of the cubic diamond and allows it to be recognized. I. INTRODUCTION The metastable synthesis of crystalline diamond films from energized hydrocarbon gases or plasmas mixed with an excess of hydrogen has been achieved.1"4 Development of such films for application as semiconductors, hard coatings, IR-transmitting films, substrates, and other purposes requires a rather precise characterization of the material. Particularly it is important to identify the diamond structure, and to assay regions of graphitic carbon and noncrystalline and hydrogen-rich tetrahedral carbons (diamond-like carbons). Graphitic carbon and other sp2 bonded amorphous carbons are strong Raman scatterers in spite of their intense optical absorption. Likewise, diamond and related carbons have strong and easily identifiable Raman spectra. Raman spectroscopy has much to offer as a nondestructive characterization tool.5'6 With microfocus instruments individual small regions in a heterogenous film can be examined. As will be shown in this paper, Raman spectra provide a large amount of structural and phase information. It is the purpose of this paper to present Raman spectra of a wide variety of natural and synthetic diamonds and associated carbons, and to interpret these spectra in terms of the characterization of diamond films. II. EXPERIMENTAL A collection was made of natural diamonds, synthetic diamonds and diamond films, diamond-like carbons, crystalline graphites, and noncrystalline carbons of various kinds. The idea was to have a broad collection of materials so that the reproducibility of spectra from sample to sample could be assessed. Raman spectra were measured on an Instruments SA Ramanor U-1000 spectrometer. Excitation was by means of the 514.5 nm line of an argon ion laser with an output power in the range of 100 to 500 mw. The instrument is equipped with a microscope with a focal spot size in the J. Mater. Res., Vol. 4, No. 2, Mar/Apr 1989
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range of a few micrometers. Power at the sample surface was about 10% of the initial laser power. For the graphite spectra, multiple scans were collected and co-added in the instrument control compu
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