Phase-selective growth of GaS films by chemical vapor deposition

Gallium sulfide (GaS) is a layered semiconductor that exhibits polymorphism, making the controlled synthesis of specific crystalline phases essential for tailoring properties and potential applications. In this work, we develop a phase-selective chemical vapor deposition (CVD) strategy for controlling the polymorphism and growth mode of GaS thin films using Ga₂S₃ as a single precursor and pure H₂ as the carrier gas. The effects of pressure, flux, temperature, and growth rate on the phase, crystallinity, and morphology are systematically investigated by Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). By controlling the growth conditions, the GaS films are selectively tuned from the 2H to the 3R phase, and their growth mode from island-like to a layer-by-layer. Highly crystalline and uniform 3R GaS films are obtained at 100 mbar, 50 sccm, and 550 °C, with a growth rate of approximately 10 nm min^-1. Different experimental conditions result in a more island-growth mode and worsening of structural and morphological properties. We also discuss the synergetic effects of H₂ in the improvement of the precursor reduction and the film stoichiometry allowing the deposition of 3R films with a high uniform coverage of ∼3 cm². 2H GaS films are deposited at pressure < 3 mbar and growth rate < 2.5 nm min^-1. These results establish a direct process-structure relationship for phase-selective GaS growth and provide a scalable route to centimeter-scale 3R GaS films.