Novel processing methods for gallium nitride nanocrystalline powders and thin films

Chemical precursors based on oxygen (gallium isopropoxide, GIP) and nitrogen (gallium dimethyl amide, GDA) were used to prepare nanocrystalline gallium nitride (GaN) powders and films by pyrolysis and crystallization in an ammonia atmosphere. Growth of nanocrystalline wurtzite GaN grains embedded within an amorphous matrix occurred by boundary motion at lower temperatures followed by evaporation-condensation at higher temperatures. Low temperature pyrolysis of GIP in an ammonia atmosphere yielded gallium oxynitride nanocrystals with a structure similar to wurtzite GaN, and where grain growth occurs by evaporation-condensation. Higher processing temperatures transformed the nanocrystalline powders to wurtzite GaN possibly by a volatile species, Ga₂O that acted as an intermediate towards the formation of GaN. Amorphous GaN films were produced by spin-coating solutions of GDA and GIP on single crystal Al₂O ₃ substrates and subsequently pyrolyzed and crystallized in ammonia. At low temperatures, epitaxial grains nucleated at the substrate interface and grew through the polycrystalline overlayer at higher temperatures primarily by evaporation-condensation. Films exhibited luminescence at 380nm and a broad peak centered near 530nm usually associated with nitrogen vacancies.; GaN films were produced by a vapor-solid reaction between reactive species of ammonia and the LiGaO₂ substrate material at elevated temperatures (700°C–1000°C). Two types of films (A and B) were produced with the formula Li_xGa_(2−x)O_2xN_2(1−x) , x ≈ 0.35 (A) and x = 0 (B) depending on the flow rate of ammonia over the substrate. It is postulated that an intermediate species (e.g., Ga ₂O) was liberated from the LiGaO₂ and reacted further with ammonia to form GaN.