Low Temperature Growth of Oriented Gallium Nitride using Pulsed Laser Deposition
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of many materials
[2,3]
especially multicomponent
oxides, such as high temperature superconductors and ferroelectrics. In PLD, a pulsed laser beam is focused into a vacuum chamber onto a target of the material to be grown, producing a dense and energetic plume of material that is ejected into the gas phase. The plume has both kinetic energy (1- >100 eV) [4-6] and potential energy in the form of electronic excitation [7-9]. A reactive background gas is often present at low pressures (< 1 torr) that aids in the deposition process. The ablated jet of material interacts chemically and physically with the reactive ambient prior to condensation on a substrate located a short distance away from the target. Film growth is straightforward because of stoichiometric transfer of material from the target. Furthermore, the energetic properties of the ablated vapor in PLD can lead to oriented film growth at low temperatures even on poorly matched substrates [10]. 343 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
The goal of this research was to examine the suitability of the PLD deposition process for the growth of oriented gallium nitride. Since low temperature processing is desirable for electronic device fabrication, it was also our intent to explore a variety of processing variables to optimize epitaxial GaN film growth at relatively low substrate temperatures. EXPERIMENTAL A schematic of the experimental set-up used for gallium nitride film growth is shown in Figure 1. Since this is a novel approach to GaN film Substrate & Heater fabrication, a detailed description of the deposition process is as follows. The ablation laser was a KrF Laser 'Mesh excimer (248 nm, 30nsec FWHM) that was focused inside a stainless steel vacuum deposition chamber Target [ onto the target (-0.1 cm 2 focal area). Typical pulse energies were I 0.13-0.20 J/pulse, yielding a fluence range of 1.3-2.0 J/cm 2 . Bias The target-substrate distance was -6 cm and the base vacuum Figure 1. A diagram of the PLD set-up. pressure was -5x10- 8 torr using a The laser impinges on the target (-79'), producing a vapor plume that deposits cryopump. The ablation targets on a heated substrate. A plasma was were either a GaN pellet or pure Ga induced between the target and substrate by applying a voltage to an metal. The GaN target was pressed from GaN powder (99.0% purity) at aluminum mesh with a bias supply. 20,000 psi for 5 minutes.
S Il
The target was held
horizontally
in the deposition
chamber
to
accommodate the gallium metal target (Tm-29°C), which normally liquefied upon both handling and laser processing. The metal boule had a nominal diameter of 2.5 cm. The vapor pressure of Ga metal was on the order of 10-7 torr at temperatures of -500 'C, as estimated from critical vapor pressure data for Ga [11]. The GaN target was rotated (6 rpm) during ablation while the Ga metal target was held stationary. A motivation for trying the Ga metal target was the possibility of greatly reducing the size and density of ejected asperities from a
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