GaN CVD Reactions: Hydrogen and Ammonia Decomposition and the Desorption of Gallium
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Abstract Isotopic labeling experiments have revealed correlations between hydrogen reactions, Ga desorption, and ammonia decomposition in GaN CVD. Low energy electron diffraction (LEED) and temperature programmed desorption (TPD) were used to demonstrate that hydrogen atoms are available on the surface for reaction after exposing GaN(0001) to deuterium at elevated temperatures. Hydrogen reactions also lowered the temperature for Ga desorption significantly. Ammonia did not decompose on the surface before hydrogen exposure. However, after hydrogen reactions altered the surface, N15H3 did undergo both reversible and irreversible decomposition. This also resulted in the desorption of N2 of mixed isotopes below the onset of GaN sublimation. This suggests that the driving force of the high nitrogen-nitrogen bond strength (226 kcal/mol) can lead to the removal of nitrogen from the substrate when the surface is nitrogen rich. Overall, these findings indicate that hydrogen can influence GaN CVD significantly, being a common factor in the reactivity of the surface, the desorption of Ga, and the decomposition of ammonia. Keywords: nitrides, GaN, GaN(0001), MOCVD, hydrogen, ammonia, Ga, surfaces
Introduction For a model of GaN CVD processes to be applicable over a wide range of conditions, the chemistry model must utilize accurate rate constants. However, this first requires identification of the reactions and the reaction products formed on the deposition surface and in the gas phase. This study has revealed relationships between reactions of the hydrogen carrier gas, Ga desorption, and ammonia decomposition on GaN. Shown in equation 1, this was achieved using (1)
Ga(CH3)3 g + N15H3 g + D2 g + GaN14
Æ GaN
+ reaction products
isotopic labeling to distinguish nitrogen in ammonia from that in the substrate and the carrier hydrogen from that originating in ammonia.
Experimental D2 reactions were performed in a small cold-wall batch reactor connected directly to a UHV (ultra-high vacuum) surface analysis chamber via isolation seals. N15H3 reactions were conducted in UHV and consisted of 1x10-7 Torr exposures lasting in duration from 30 to 300s. The UHV chamber was equipped with x-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and mass spectroscopy for temperature programmed desorption (TPD). The mass spectrometer was enclosed in a liquid nitrogen cooled shroud with an opening of only 1.5mm approximately 5mm in line of sight of the GaN(0001) surface. The GaN(0001) surface used in this study had a hexagonal LEED pattern with sharp spots [1] and a high contrast to background (figure 1). Although the symmetry and orientation of this excellent diffraction pattern remained unchanged over the entire surface (1cm2), XRD and TEM have been used to show that this substrate was polycrystalline, as found for other GaN films.
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