Atomic Layer Annealing with Radio-Frequency Substrate Bias for Control of Grain Morphology in Gallium Nitride Thin Films

A method of performing atomic layer annealing (ALA) with radio-frequency (RF) substrate bias on insulating and amorphous substrates is demonstrated for GaN deposition at 275°C. GaN is typically deposited by metal–organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) at >600°C, resulting in strain upon cooling; this makes low-temperature process alternatives desirable. Tris(dimethylamido) gallium (III) and hydrazine served as precursors whereas Ar and Kr were used for ion bombardment. Optimization of substrate bias potential is demonstrated by grazing incidence x-ray diffraction (GI-XRD) and x-ray reflectivity (XRR). Reference films were deposited by thermal ALD and non-substrate-biased ALA processes. X-ray photoelectron spectroscopy (XPS) surface and depth-profiling studies show that applied RF bias decreases film oxygen and carbon content relative to the reference films; these films also show crystallites broadening with increasing film thickness by TEM in contrast to the reference films. In summary, ALA with RF substrate bias is demonstrated as an effective method to deposit GaN thin films at a low deposition temperature on insulators. Atomic layer annealing (ALA) presents the ability to enhance desirable properties of thin films deposited by atomic layer deposition processes, such as crystallinity, density, purity, and resistivity. ALA could also enable the deposition of materials typically grown by high-temperature processes at lower temperatures required for device integration. In this article, RF substrate-biased ALA is demonstrated as an effective means to deposit minimally contaminated and polycrystalline GaN films on insulating and amorphous substrates at 275°C. RF bias is required for deposition on insulating or patterned substrates to avoid charge damage. This method could be useful to deposit thin films of other group III-nitrides at low temperatures, such as AlN, InGaN, and AlGaN. Such materials are of interest to serve as heat spreaders, piezoelectric films, back-end thin-film transistors, and optoelectronic devices. These materials are typically deposited by metal–organic chemical vapor deposition at temperatures exceeding 800°C. Accordingly, lower-temperature deposition processes could enable their more widespread implementation in accelerating future technologies such as heterogenous integration, advanced packaging, and microLED displays.