Phase separation in semiconductor alloys, such as InGaN, is a critical phenomenon that influences material properties and device performance. This process occurs when a homogeneous alloy decomposes into regions with distinct compositions due to thermodynamic instabilities. Unlike homogeneous phase changes, phase separation involves the formation of compositionally inhomogeneous domains, driven by factors like chemical immiscibility, coherency strain, and kinetic limitations during growth.

The thermodynamic basis for phase separation is often described using spinodal decomposition theory. A spinodal curve defines the region within a phase diagram where the alloy is unstable to infinitesimal composition fluctuations. Inside this region, the second derivative of the free energy with respect to composition is negative, leading to spontaneous phase separation. For InGaN, the miscibility gap is significant due to the large lattice mismatch between InN and GaN, which creates a strong tendency for phase separation. The spinodal temperature for InGaN is estimated to be above typical growth temperatures, meaning that metastable alloys are often obtained under standard growth conditions. The spinodal curve can be approximated for InGaN using models that incorporate enthalpy of mixing and elastic strain energy, revealing a broad immiscibility region for indium compositions between approximately 10% and 90%.

Coherency strain plays a pivotal role in phase separation. When InGaN alloys form, the lattice mismatch between InN and GaN induces significant strain. The elastic energy associated with maintaining coherency between phases can suppress or modify phase separation. If the strain energy exceeds a critical threshold, the system may undergo strain relaxation through dislocation formation, but coherent phase separation can still occur if the domains remain below a critical size. The interplay between chemical driving forces and coherency strain leads to modulated structures with periodic composition variations. For InGaN, this often manifests as indium-rich clusters embedded in a Ga-rich matrix, with domain sizes ranging from a few nanometers to tens of nanometers, depending on growth conditions.

Experimental evidence for phase separation in InGaN comes primarily from advanced microscopy techniques, particularly scanning transmission electron microscopy (STEM). High-angle annular dark-field (HAADF) STEM provides atomic-number contrast, allowing direct visualization of indium-rich and gallium-rich regions. Energy-dispersive X-ray spectroscopy (EDS) in STEM further quantifies local composition variations. Studies have shown that InGaN layers grown by metalorganic chemical vapor deposition (MOCVD) exhibit nanoscale compositional fluctuations, with indium-rich clusters forming even at relatively low indium concentrations. These clusters are often associated with local strain fields, visible in geometric phase analysis (GPA) of high-resolution STEM images.

The kinetics of phase separation in InGaN are influenced by growth parameters such as temperature, V/III ratio, and growth rate. Lower growth temperatures favor phase separation due to reduced adatom mobility, while higher temperatures may promote homogenization. However, the metastable nature of InGaN means that even at elevated temperatures, phase separation can persist if the system is trapped in a local energy minimum. Post-growth annealing can further alter the phase-separated morphology, either enhancing or reducing compositional fluctuations depending on the annealing conditions.

Phase separation has profound implications for the optical and electronic properties of InGaN. Indium-rich regions act as localization centers for carriers, affecting recombination dynamics in light-emitting devices. In GaN-based LEDs, phase separation can lead to inhomogeneous broadening of emission spectra and variations in quantum efficiency. However, controlled phase separation has also been exploited to improve device performance, such as in white LEDs where broad emission spectra are desirable.

Theoretical models of phase separation in InGaN often combine thermodynamic calculations with kinetic Monte Carlo simulations to predict domain sizes and distributions. These models must account for strain effects, surface diffusion, and interfacial energies to accurately describe experimental observations. Comparisons between simulated and experimental STEM data have validated the role of coherency strain in stabilizing nanoscale phase separation.

In summary, phase separation in semiconductor alloys like InGaN is a complex interplay of thermodynamics, strain, and kinetics. Spinodal decomposition theory provides a framework for understanding the instability driving phase separation, while coherency strain modifies the resulting morphology. STEM-based techniques offer direct evidence of compositional fluctuations, confirming the nanoscale heterogeneity inherent to these materials. The controlled manipulation of phase separation remains an important avenue for optimizing semiconductor devices, particularly in optoelectronic applications where compositionally inhomogeneous structures can be leveraged for enhanced performance.