Heteroepitaxy of Sb₂Se₃ on GaAs(001) via Molecular Beam Epitaxy

The quasi-1D antimony selenide (Sb₂Se₃) material is an emerging candidate for phase change programmable photonics. Beyond large refractive index contrast, Sb₂Se₃’s amorphous-crystalline phase transition is rare in that it maintains near-zero extinction coefficients in the near and mid-infrared spectral ranges [1]. To leverage the pronounced anisotropy of the crystalline form, on-chip Sb₂Se₃ phase change material technology would benefit from investigation of film interactions with functional single crystal substrates. The lengthy chained structure imparts birefringence, axial crystal growth morphology, as well as undesired polymer-like crystallization and quenching behavior [2]. Here, we aim to realize structurally oriented Sb₂Se₃thin films towards tuning these qualities.<br/><br/>Bottom-up synthesis control of layered Sb₂Se₃ has motivated the use of ultra-high vacuum growth techniques. Without controlled surface preparation and slower growth rates, Sb₂Se₃ has a propensity to form sharded and prismatic polycrystalline structures [3]. In this work, we demonstrate a synthesis route towards textured-epitaxial and amorphous Sb₂Se₃ films (&lt; 232 nm) directly on GaAs(001) substrates via molecular beam epitaxy (MBE).<br/><br/>X-ray diffraction indicates that for growth temperatures of 230 – 265 °C, Sb₂Se₃ is prone to mixed-orientation nucleation and growth out-of-plane, although it remains oriented along the covalent axis in-plane. We hypothesize this behavior is due to an abundance of low energy van der Waals terminations in the system. More importantly, we identify a narrow epitaxial growth window at a lower range of 180 – 200 °C. Transmission electron microscopy shows an unusual growth progression in this regime, where initially disordered growth transitions into (010)-oriented Sb₂Se₃ with increasing film thickness. Atomic force microscopy reveals the nm-level smoothness and signature ribbon-like surface morphology in these films.<br/><br/>Furthermore, we demonstrate that MBE-synthesized films can span the structural spectrum from epitaxial to amorphous character. Below 150 °C, MBE conditions can produce glassy Sb₂Se₃ films, opening opportunities for solid phase epitaxy studies. We will present initial results on laser-irradiated amorphous Sb₂Se₃. With room for further tuning of film-substrate interaction and stoichiometry, these initial synthesis results offer a preliminary model system grown on a conventional III-V substrate, representing new ways to probe and integrate the photonic functionality of thin film Sb₂Se₃.<br/> <br/>[1] K. Aryana et al., <i>Opt. Mat. Express</i> <b>13</b>(11), 3277–3286 (2023).<br/>[2] A.A. Sokol et al., in <i>Growth of Crystals, </i>ed. by E. I. Givargizov, (Springer, 1986), pp. 322 – 331.<br/>[3] R. Kondrotas et al., <i>Sol. Energy Mater. </i><b>199</b>, 16-23 (2019).