Abstract
We demonstrate the selective, pulsed-laser deposition of hexagonal GaS and monoclinic Ga2S3 films on sapphire substrates from a single Ga2S3 target in high-vacuum conditions. Growth at substrate temperatures below 550 °C causes GaS film formation, which indicates non-stoichiometric transfer from the target to the film. Surprisingly, stoichiometric transfer occurs at substrate temperatures above 650 °C with monoclinic Ga2S3 as the preferred, higher S-content phase. Through a series of growth and annealing experiments, we show that GaS nucleation under S-deficient conditions leads to the preferred growth of this layered, hexagonal phase below 550 °C. Furthermore, GaS films annealed above 650 °C under high vacuum are transformed to Ga2S3, reflecting the greater stability of the monoclinic phase. By first growing Ga2S3 at a higher temperature and subsequently growing GaS at a lower temperature, we can fabricate GaS/Ga2S3 heterostructures in a single growth process.
Highlights
Gallium sulfide is a group III–VI semiconductor with multiple stable phases, including the hexagonal GaS phase and monoclinic Ga2S3 phase
Thin GaS and Ga2S3 films have been grown using a variety of methods, including chemical vapor deposition (CVD), chemical vapor transport (CVT), and chemical bath deposition (CBD)
The narrowing of the S and Ga signals in the Rutherford backscattering spectrometry (RBS) spectra as the substrate temperature increases shows that the films become correspondingly thinner despite otherwise similar growth conditions
Summary
Gallium sulfide is a group III–VI semiconductor with multiple stable phases, including the (layered) hexagonal GaS phase and monoclinic Ga2S3 phase. The bandgap of GaS is ∼2.6 eV.4,5 Another form of gallium sulfide, the monoclinic Ga2S3 phase, consists of covalent bonds in three dimensions (i.e., not layered) and has a reported bandgap of ∼3.0 eV.. Thin GaS and Ga2S3 films have been grown using a variety of methods, including chemical vapor deposition (CVD), chemical vapor transport (CVT), and chemical bath deposition (CBD).. Thin GaS and Ga2S3 films have been grown using a variety of methods, including chemical vapor deposition (CVD), chemical vapor transport (CVT), and chemical bath deposition (CBD).9–15 Studies using these methods have demonstrated that depending on growth conditions, GaS, Ga2S3, or a combination of both phases is synthesized. One report of gallium sulfide growth was achieved by physical vapor transport (PVT) using GaS powder as the source material. PLD is a physical vapor process that enables “digital” deposition of target materials using a pulsed laser to supply energy for source material evaporation. For example, PLD has been used to grow few-layer nanosheets of layered GaSe.
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