Characterization of Carbon Nitride Films Produced by Pulsed Laser Deposition
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EXPERIMENTAL The PLD vacuum chamber is described elsewhere[ 13]. It is capable of attaining a base pressure of 1 x 10-6 Pa and is equipped with a broad-beam Kaufman-type ion source. The pyrolytic graphite targets (Union Carbide) were ablated with a KrF (248 nm) laser capable of generating 450-mJ pulses of 17-ns duration. The deposition time was 20 minutes at a laser repetition rate of 20 Hz. Samples grown at elevated temperatures were deposited at a 40-Hz repetition rate. The laser light was focused into the vacuum chamber using a spherical lens with a 35-cm focal length. The laser beam illuminates the rotating target at a 450 angle from the target normal. The beam forms a rectangular spot on the target with an area of 0.01 cm 2 , giving an energy density of 45 J/cm 2 . To improve the thickness uniformity, the sample substrate was rotated, the center of the ablated plume struck the substrate off the center line, and the sample substrate was 18 cm from the target. Using these conditions, we obtain an ablation deposition with ±10% thickness uniformity over 10-cm diameter wafers at an ablation deposition rate of 0.2 A/pulse (no ion beam) for a total sample thickness of 1250 ± 250A. Before deposition, the uncoated silicon (100) substrates (n-doped with P to 0.02 Q)cm) were cleaned to remove the surface oxide layer by a wet dip procedure in a HF/NH 4F solution[ 16]. The "room temperature" rotating Si substrates were not actively heated during deposition, although some residual heating (< 75 'C) occurred due to the power dissipated by the normal operation of the ion gun and condensation of energetic species from the ablation plume. Sample depositions at elevated temperature were performed by resistively heating the Si substrate. The temperature was measured by a bare thermocouple pressed directly to the back of the wafer. The 3-cm Kaufman-type ion source (mounted 10 cm from the substrate) was aimed (coincident with the laser plume) at a 250 angle from the substrate surface normal and slightly off axis from the center of the rotating Si substrate. The current density at the substrate was measured with a biased (-30 V) retractable ball of 1 cm 2 diameter. For this study, the experimentally varied parameters were the ion feed gas composition (0 - 100% flow of N2 in argon at a constant total flow of 4 sccm), the ion energy (0 - 1000 eV), and the ion current density (0 - 260 gA/cm 2). During deposition the background gas pressure in the chamber was -2.7 x 10-2 Pa. To measure the effect of the ion beam on the atomic bonding, the films were characterized by Raman spectroscopy (514 nm light at 50 mW of power) and FTIR spectroscopy. Graphite has two Raman active modes at 1350 cm- 1 and 1581 cm- 1 . The Raman band at 1580 - 1590 cm-1 (the "graphite" or "G" peak) is a fundamental Brilloun-zone-center mode of graphite. The Raman band at 1350 - 1360 cm- 1 (the "disorder" or "D" peak) is believed to be a Brilloun-zone-edge phonon mode that intensifies for graphite crystal domains less than 100 nm in size. "Glassy" carbon is a nanocry
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