X-Ray Diffraction Study of Clusters in a-tC Films
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Amorphous carbon films grown using plasma deposition and pulsed laser deposition (PLD) techniques are made of nanocrystalline aggregates or clusters with sizes in the range of tens to thousands of angstroms. The clusters' size may depend on several film parameters, such as deposition energies, pressure and film thickness, and can affect both the electrical as well as the mechanical properties of the films [1]. In this paper, we present the initial results of an x-ray diffraction experiment on at-C films prepared by PLD in a high-vacuum environment. We have characterized the films as a function of laser energy density and film thickness. We use small angle x-ray scattering to determine the presence and size distribution of clusters. High angle diffraction yields information on bond sizes present in the films. EXPERIMENTAL The at-C films used in our study are grown by PLD, using 248nm pulsed laser radiation. A pyrolitic graphite target serves as the carbon source. The films are deposited on a rotating p-type Si (100) wafer. Details of the deposition process have been presented elsewhere [1]. The laser energy density varies between 5 and 45 J/cm 2 , and the deposition rate is approximately IA/sec. Approximately 1/3 - 1/2 of the silicon wafer in each sample used in this experiment is left exposed. This enables the measurement and subtraction of a background scattering signal. 857 Mat. Res. Soc. Symp. Proc. Vol. 358 @1995 Materials Research Society
Small angle x-ray scattering (SAXS) is a non-destructive way of characterizing the structure and density of colloidal size particles [3]. The method has been used in the analysis of metal and semiconducting clusters, as well as superlattices and quantum well structures [4-10]. The SAXS measurements average the properties of a large number of clusters since the area illuminated by the x-rays is large. Varying film depths may be probed using different angles of incidence for the x-ray beam. Data obtained using SAXS can be used to calculate the radial distribution function p(r) which provides information on the different clusters present in the sample [3, 10]. Both the SAXS and high angle experiments are performed on a 12 kW rotating anode source, operating between 3 and 7 kW, with a flat LiF (100) monochromator and a Braun metal wire position sensitive detector (PSD), using the KaX doublet radiation from a Cu anode. The inplane resolution determined by the LiF monochromator, is Aqx = 0.012A-1, and Aqz < 0.00001A-1 FWHM. The vertical resolution, determined by slits is Aqy = 0.015A•1 FWHM. The PSD is calibrated to measure 0.0120 per channel. Another series of experiments are performed using synchrotron radiation, using 1.3776A radiation. The in-plane resolution, determined by a Ge(1 11) monochromator, is Aqx = 0.008A-1. The vertical resolution was determined by slits to
be Aqy = 0.015A•1. RESULTS AND DISCUSSION Several samples prepared using different laser energy densities and of different thicknesses were studied using SAXS. We show in Figure 1 the results of SAXS measuremen
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