Suppression of phase separation in InGaN due to elastic strain

Abstract

The effect of elastic strain in epitaxial InGaN layers coherently grown on GaN wafers on spinodal decomposition of the ternary compound is examined. The effect results in considerable suppression of phase separation in the strained InGaN layers. To predict correctly the position of the miscibility gap in the T-x diagram it is important to take into account the compositional dependence of the elastic constants of the ternary compound. The contribution of the elastic strain to the Gibbs free energy of InGaN is calculated assuming uniform compression of the epitaxial layer with respect to the underlying GaN wafer. The interaction of binary constituents in the solid phase is accounted for on the base of regular solution model. The enthalpy of mixing is estimated using the Valence Force Field approximation. The strain effect becomes stronger with increasing In content in the InGaN. As a result the miscibility gap shifts remarkably into the area of higher InN concentration and becomes of asymmetrical shape. Various growth surface orientations and the type of crystalline structure (wurtzite or sphalerite) provide different effects of the elastic strain on phase separation in ternary compounds.

Research challenges to ultra‐efficient inorganic solid‐state lighting

“…Early theoretical calculations predicted that compositional inhomogeneities in InGaN are a natural consequence of microscopic-strain-induced immiscibility of InN and GaN materials [29], and hence increase in severity as the In content increases from zero towards that necessary for deep green emission (and presumably continuing to increase in severity as the 50:50 alloy is approached). However, the applicability of these conclusions to thin, fully strained InGaN QWs grown on GaN has been challenged [37].…”

Solid-State Lighting

“…18 In addition, more sophisticated thermodynamic calculations, which took into account the fact that the InGaN quantum wells were strained, suggested that the strain should suppress the decomposition and that InGaN should be a homogeneous random alloy up to an indium content of at least 40%. 19 Hence, the bright light emission from InGaN cannot be due to the formation of indium-rich clusters. So the puzzle remained: Why do InGaN quantum wells emit brilliant light when the dislocation density is so high?…”

Does In form In-rich clusters in InGaN quantum wells?

“…However Karpov [32] has calculated the phase diagram for an InGaN layer epitaxially matched to a GaN layer, which puts the InGaN into biaxial compression. The effect of the strain is to stabilise the InGaN, and no decomposition is predicted for normal growth conditions.…”