Growth Window, Morphology, and Chemical-State Evolution of Tensile-Strained GaP Nanostructures on GaAs(001) by Droplet Epitaxy

Abstract

Tensile-strained GaP nanostructures (anti-quantum dots, anti-QDs) were systematically fabricated on GaAs(001) by droplet epitaxy and mapped as a function of P₂ flux (5.0 × 10⁻⁸ to 2.4 × 10⁻⁶ Torr) and manipulator temperature during P₂ exposure (T_P = 200−300 °C). Atomic force microscopy and scanning electron microscopy reveal that the areal density remains nearly unchanged (∼39−55 μm⁻²), closely matching the initial Ga droplet density, indicating that nucleation is predetermined at the droplet-formation step and is insensitive to subsequent phosphorization conditions. In contrast, pronounced morphology transitionsdome → 4-fold → 2-fold → ringemerge from the interplay of temperature-activated Ga surface diffusion, flux-dependent phosphorization and structural transformation kinetics, and GaAs(001) symmetry (⟨110⟩ dimer-row−induced diffusion anisotropy). Height−width statistics demonstrate a template-limited crystallization regime at T_P ≥ 250 °C, where the GaP height does not exceed the initial droplet height, while lateral dimensions may surpass the droplet footprint via spill-over−assisted overgrowth, evidencing a Ga-reservoir role of droplets at elevated temperatures. X-ray photoelectron spectroscopy of P 2p core levels further shows that morphology and detectable Ga−P bond formation are not one-to-one correlated: the integrated P−Ga peak area generally increases with temperature but exhibits a non-monotonic dependence on P₂ flux, indicating an optimal phosphorization window rather than a simple monotonic increase with phosphorus supply. Finally, cross-sectional HR-TEM confirms that the anti-QD morphology is preserved after 200 nm GaAs capping, revealing strain-related internal features while maintaining the buried structural integrity after overgrowth. These results establish a unified morphology−chemistry map for tensile-strained GaP/GaAs droplet epitaxy and identify surface kinetics and volatile group-V chemistry as tunable knobs to engineer anti-QD architectures beyond conventional temperature-only crystallization frameworks.