Real-time growth study of plasma assisted atomic layer epitaxy of InN films by synchrotron x-ray methods

Abstract

The temporal evolution of high quality indium nitride (InN) growth by plasma-assisted atomic layer epitaxy (ALEp) on a-plane sapphire at 200 and 248 °C was probed by synchrotron x-ray methods. The growth was carried out in a thin film growth facility installed at beamline X21 of the National Synchrotron Light Source at Brookhaven National Laboratory and at beamline G3 of the Cornell High Energy Synchrotron Source, Cornell University. Measurements of grazing incidence small angle x-ray scattering (GISAXS) during the initial cycles of growth revealed a broadening and scattering near the diffuse specular rod and the development of scattering intensities due to half unit cell thick nucleation islands in the Yoneda wing with correlation length scale of 7.1 and 8.2 nm, at growth temperatures (Tg) of 200 and 248 °C, respectively. At about 1.1 nm (two unit cells) of growth thickness nucleation islands coarsen, grow, and the intensity of correlated scattering peak increased at the correlation length scale of 8.0 and 8.7 nm for Tg = 200 and 248 °C, respectively. The correlated peaks at both growth temperatures can be fitted with a single peak Lorentzian function, which support single mode growth. Post-growth in situ x-ray reflectivity measurements indicate a growth rate of ∼0.36 Å/cycle consistent with the growth rate previously reported for self-limited InN growth in a commercial ALEp reactor. Consistent with the in situ GISAXS study, ex situ atomic force microscopy power spectral density measurements also indicate single mode growth. Electrical characterization of the resulting film revealed an electron mobility of 50 cm2/V s for a 5.6 nm thick InN film on a-plane sapphire, which is higher than the previously reported mobility of much thicker InN films grown at higher temperature by molecular beam epitaxy directly on sapphire. These early results indicated that in situ synchrotron x-ray study of the epitaxial growth kinetics of InN films is a very powerful method to understand nucleation and growth mechanisms of ALEp to enable improvement in material quality and broaden its application.

Atomic Layer Deposition as the Enabler for the Metastable Semiconductor InN and Its Alloys

“…Illustrations of a typical scattering geometry and scattering pattern are shown in Figure a and b, in which the scattering intensity is described as a function of in-plane and out-of-plane momentum transfer, q y and q z , respectively. GISAXS studies of ALD processes have been reported for a broad range of material systems, including various metals, − oxides, , and nitrides. − …”

“…Researchers at the U.S. Naval Research Laboratory have reported several in situ GISAXS studies of InN plasma ALD growth kinetics. − The most significant results are summarized and discussed here. The experiments were performed in the G3 hutch of the Cornell High Energy Synchrotron Source using a custom plasma ALD system with an ICP source, X-ray-transparent beryllium windows, and air-cooled dry vacuum pump.…”

“…The X-ray incidence and exit angles are 0.8°and 0.2°, respectively. Reprinted with permission from Nepal et al 58 Copyright 2017 American Vacuum Society. −n fit of GISAXS linecuts at high in-plane scattering angle q y , which is related to island sidewall sharpness.…”

Coherent X-ray Spectroscopy Elucidates Nanoscale Dynamics of Plasma-Enhanced Thin-Film Growth

“…Within the context of the 3D growth morphology, the mounded configuration is very stable after this point. Nepal et al 34 suggested that around 2-unit cell thickness, corresponding to 4 layers, the substrate surface is more than 50% covered by the film and the mound structure stabilizes. These experiments provide direct evidence for the latter part of that supposition.…”

“…An individual PE-ALD cycle is explained schematically in Figure . Under the experimental conditions, it takes 8–10 PE-ALD cycles to form a monolayer of wurtzite InN, with a monolayer being half a unit cell height. , …”