Liquid-Target Pulsed Laser Deposition and Its Application to Grow Diamond and Diamond-Like Carbon Films
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Instead of using a solid material as a target, we use a liquid target as suggested first by Sankur et al. [4]. In this way, the focused laser beam can always ablate a smooth surface and, hence, target deterioration can be completely prevented without target rotation. As for the splashing of large particulates, the problem can be solved by keeping the target liquid in a viscous state, which most materials possess at temperatures slightly higher than their melting point. If a naturally-occurring liquid source material is unavailable, or whose melting point is below or above the operation temperature, an additional thermally insulated cooling (for some gaseous sources) or heating (for solid sources with a low or moderate high melting point) can be used to obtain the necessary liquid target. In fact, a molten Ge target used by Sankur, et al. has produced much smoother thin films than those produced in a solid target PLD process [4]. In comparison to plasma chemical vapor deposition and hot-filament chemical vapor deposition, the two major diamond growth techniques, the liquid target pulsed laser deposition (LTPLD) offers an extremely high power delivery (> 108 W) in each ultra-short pulse duration. Instead of a purely thermal effect, the photolytic interaction between laser and target material with specific bond-breaking can significantly lower the deposition temperature. The LTPLD is particularly suitable for the growth of diamond and diamond-like carbon films since carbon-containing organic liquids are widely available. EXPERIMENTAL DESCRIPTION The details of our LTPLD system have been described elsewhere [5]. Basically, the system consists of an
rotation motor,,
exciler Liqrid
liquid ArF excimer laser (Lambda Physik, target-.. model: LPX210icc, wavelength: 193 viwingo nm, maximum pulse energy: 400 mJ, window.,/ repetition rate: 1 - 100 Hz) and a small, homebuilt upright vacuum deposition wate chamber (Fig. 1). The ArF excimer laser beam is focused by a quartz lens and guided through a quartz window
Heating wires
lens.
0
/H0
substrate
gravity
vacuum pump
onto the liquid target which is held by Fig. 1 Schematic diagram of the liquid-target a water-cooled stainless steel cup. The pulsed laser deposition vacuum chamber ablated carbon-containing plume is deposited on a heated substrate which is silver-pasted onto the substrate holder surrounded by a stainless steel heat-screening foil. Reactive gases can be added into the deposition chamber from a needle-valve controlled feedthrough, and a mechanical vacuum pump is used to maintain the necessary vacuum condition that is required for the deposition. The wall of the vacuum chamber is water-cooled and the deposition process can be viewed through a glass window. The 284
experimental conditions for the results reported here are as follows: A pulsed laser with a 220 2 mJ pulse energy and a 10 Hz repetition rate was focused onto an area of 2 x 5 mm on the surface of the liquid target which was located about 4 cm away from the substrate. The substrate temperature w
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