Kinetic instability of AlGaN alloys during MBE growth under metal-rich conditions on m-plane GaN miscut towards the -c axis

Abstract

AlxGa1-xN layers with Al-composition above 0.6 (0.6 < x < 0.9) grown under metal-rich conditions by plasma-assisted molecular beam epitaxy on m-plane GaN miscut towards the -c axis are kinetically unstable. Even under excess Ga flux, the effective growth rate of AlGaN is drastically reduced, likely due to suppression of Ga-N dimer incorporation. The defect structure generated during these growth conditions is studied with energy dispersive x-ray spectroscopy scanning transmission electron microscopy as a function of Al flux. The AlGaN growth results in the formation of thin Al(Ga)N layers with Al-composition higher than expected and lower Al-composition AlGaN islands. The AlGaN islands have a flat top and are elongated along the c-axis (i.e., stripe-like shape). Possible mechanisms for the observed experimental results are discussed. Our data are consistent with a model in which Al-N dimers promote release of Ga-N dimers from the m-plane surface.

Intersubband Transitions in Nonpolar m‐Plane AlGaN/GaN Heterostructures

“…Figure shows the STEM and EDX elemental maps for Ga and Al for sample A. It confirms that, under the growth conditions employed in this study, the GaN and AlGaN layers are fairly planar and do not exhibit the nanostructure of layer plus flat‐top island that develops at higher Al‐fluxes . AFM scans shown in Figure b and c indicate that the top surfaces of both samples have similar roughness.…”

“…Homogeneous m‐plane Al x Ga 1−x N with an Al composition between approximately 0.6–0.8 was found to be essentially unstable under Ga‐rich growth conditions by plasma‐enhanced MBE . Specifically, we found that above a certain critical Al flux, growth of m‐plane AlGaN under excess Ga leads to formation of inhomogeneous alloys with a unique nanostructure . At high Al‐fluxes, this nanostructure consists of continuous, nearly‐pure AlN layers plus flat‐top islands of lower Al‐content bordered by m‐plane nanofacets .…”

“…We note that the c‐plane AlGaN growth rate was measured to be much closer to the GaN growth rate (8.1 nm min −1 ). Simultaneously with the drop of growth rate, the average Al‐composition (as established with HRXRD) increases super‐linearly with Al beam flux . The m‐plane Al‐composition is more than twice that of c‐plane AlGaN (56% vs. 23% for m‐ and c‐plane AlGaN, respectively).…”

“…Specifically, we found that above a certain critical Al flux, growth of m‐plane AlGaN under excess Ga leads to formation of inhomogeneous alloys with a unique nanostructure . At high Al‐fluxes, this nanostructure consists of continuous, nearly‐pure AlN layers plus flat‐top islands of lower Al‐content bordered by m‐plane nanofacets . However, this nanostructure is undesirable for ISB devices and will not be discussed any further in this paper.…”

Photoluminescence Study of Carrier Localization and Recombination in Nearly Strain‐Balanced Nonpolar InGaN/AlGaN Quantum Wells

“…[8,16,17] Furthermore, strain is anisotropic on the m-plane and generates significant alloy inhomogeneity and interface roughness in m-plane AlGaN/GaN QWs with Al compositions above 50%. [18] Therefore, to mitigate straininduced issues, this study focuses on nearly strain-balanced m-plane InGaN/AlGaN heterostructures. [19,20] Carrier localization and recombination play important roles in determining III-nitride device performance.…”