Kinetic Modeling of Vertical Cation Segregation During A_xB_1-xN Epitaxy

III-nitride materials have not reached their potential for energy conversion applications largely due to the difficulty in growing thick, high-quality, single-crystal ternary alloy films. One of the most prominent examples of this is the tendency for indium gallium nitride (InGaN) to separate into In-rich and In-poor regions during growth – a phenomenon typically referred to as phase separation. This has precluded the effective growth of this material for many of the applications which have motivated InGaN research since it was first synthesized (e.g., full spectrum or tandem-with-Si solar cells and RGB LEDs). Despite a large volume of research dedicated to this topic over the past three decades, the mechanisms that drive InGaN composition variability are not fully defined. In this work, we suggest that bulk diffusion and thus spinodal decomposition in the traditional sense, is of lesser concern than surface driven processes. Consequently, we propose that InGaN “phase separation” comes primarily from a combination of at least three distinct phenomena – thermal decomposition, vertical cation segregation (VCS), and lateral cation separation (LCS). Furthermore, thermal desorption complicates the control of composition in the grown film. A new 1D model describing the vertical segregation mechanism is proposed and evaluated using parameters extracted from state-of-the-art III-nitride epitaxy. Thermal decomposition and desorption are also included in the model to better replicate realistic growth conditions.<br/>Via RHEED analysis, Moseley et al demonstrated that a critical dose of excess metal exists for epitaxy of InGaN beyond which a diffusion of indium away from the growth surface coupled with an equal gallium diffusion toward the growth surface (VCS) occurs [1]. This phenomenon can be expected to occur for any metal-rich growth condition. Our current understanding of vertical segregation suggests that it is not unique to InGaN and can be applied to any ternary III-nitride, and indeed, we have observed self-assembled super lattices in aluminum gallium nitride (AlGaN) grown by metal modulated epitaxy (MME), which does not naturally phase separate.<br/>From the observation of VCS, we have built a model for the accumulation and consumption of metal adatoms during epitaxy of III-nitrides based on our current understanding of the surface kinetics. Using a system of coupled differential equations, we can describe the group-III (cation) adatoms as they adsorb onto and desorb from the surface, grow into and decompose from the crystal, and transfer between the pseudomorphic, laterally-contracted, and droplet adlayers. MME is particularly suited to evaluate this model as MME growth of III-nitrides can be conducted at substrate temperatures where decomposition and desorption are inhibited (even for InGaN), and lateral diffusion is enhanced leading to uniform adatom mixing. However, the use of high growth rates can prevent any significant LCS. This allows us to investigate growth kinetics at experimental conditions which isolate or emphasize the kinetic mechanisms in question. Further, the cyclic nature of MME makes the VCS effects easier to observe, as they occur repeatedly (every shutter cycle) during growth.<br/>We found this model to be self-consistent and in line with our expectations for a realistic growth. We have also achieved further self-assembled super lattice structures in AlGaN by conducting growths at conditions conducive to vertical cation segregation. The simplicity of this model should make it easy to expand upon by adding other mechanisms like LCS and generalizing to other epitaxial techniques beyond MME and MBE.<br/>[1] M. Moseley, B. Gunning, J. Greenlee, J. Lowder, G. Namkoong, and W. A. Doolittle, Journal of Applied Physics <b>112</b>, 014909 (2012).