Understanding the effect of nitrogen plasma exposure on plasma assisted atomic layer epitaxy of InN monitored by real time grazing incidence small angle x-ray scattering

Abstract

The authors present an in situ study of the effect of nitrogen plasma pulse time on the temporal evolution of the surface morphology of InN growth on a-plane sapphire at 250 °C by plasma assisted atomic layer epitaxy (ALEp). The growth surface evolution was monitored in real-time using grazing incidence small angle x-ray scattering (GISAXS) measurements at an x-ray incidence angle of 0.8°. Nitrogen plasma pulse time (tp) was varied between 15 and 30 s in 5-s steps, and for all tp, the near specular scattering broadens and correlated peaks develop and evolve along the Yoneda Wing (YW). For tp ≥ 20 s, a YW with one correlated length scale evolves. At the end of the growth, the longest correlated length scale is 16.54 nm for tp = 25 s. Porod analysis of GISAXS intensity at high qy for tp = 25 s shows the formation of mounded shapes at the early stage of nucleation that transitioned to cylinders after about 3 unit cells of InN growth. Additionally, at tp = 25 s, the growth rate is highest with root mean square surface roughness and carbon impurity levels at or below atomic force microscopy and x-ray photoelectron spectroscopy sensitivity limits, respectively. At tp < 25 s, the growth surface may be undersaturated and at tp > 30 s, it appears that trimethylindium precursor molecules start to decompose, resulting in higher carbon content in the film. Thus, the nature of GISAXS correlated length scale directly correlates with the material quality. Additional ex situ characterizations reveal an electron mobility of 6–31 cm2/V s for a 3–5 nm thick InN film on a-plane sapphire, which is similar to the reported value of 30 cm2/V s for a 1300 nm thick InN film grown by molecular beam epitaxy directly on sapphire. Thus, the combination of in situ synchrotron x-ray analysis and ex situ characterization is a powerful approach to develop understanding of the growth mechanisms of ALEp of III-N materials in order to improve the quality by reducing impurities and broaden material applications.