Citation: Stanionyt ˙ e, S.; Malinauskas, T.; Niaura, G.; Skapas, M.; Devenson, J.; Krotkus, A. The Crystalline Structure of Thin Bismuth Layers Grown on Silicon (111) Substrates. Materials 2022, 15, 4847. https:// doi.org/10.3390/ma15144847 Received: 10 June 2022 Accepted: 8 July 2022 Published: 12 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article The Crystalline Structure of Thin Bismuth Layers Grown on Silicon (111) Substrates Sandra Stanionyt ˙ e 1, * , Tadas Malinauskas 2 , Gediminas Niaura 1 , Martynas Skapas 1 , Jan Devenson 1 and Ar ¯ unas Krotkus 1 1 Center for Physical Sciences and Technology, Saul˙ etekio av. 3, LT-10257 Vilnius, Lithuania; gediminas.niaura@ftmc.lt (G.N.); martynas.skapas@ftmc.lt (M.S.); jan.devenson@ftmc.lt (J.D.); arunas.krotkus@ftmc.lt (A.K.) 2 Institute of Photonics and Nanotechnology, Vilnius University, Sauletekio av. 3, LT-10257 Vilnius, Lithuania; tadas.malinauskas@ff.vu.lt * Correspondence: sandra.stanionyte@ftmc.lt Abstract: Bismuth films with thicknesses between 6 and ∼30 nm were grown on Si (111) substrate by molecular beam epitaxy (MBE). Two main phases of bismuth — α-Bi and β-Bi — were identified from high-resolution X-ray diffraction (XRD) measurements. The crystal structure dependencies on the layer thicknesses of these films were analyzed. β-Bi layers were epitaxial and homogenous in lateral regions that are greater than 200 nm despite the layer thickness. Further, an increase in in-plane 2θ values showed the biaxial compressive strain. For comparison, α-Bi layers are misoriented in six in-plane directions and have β-Bi inserts in thicker layers. That leads to smaller (about 60 nm) lateral crystallites which are compressively strained in all three directions. Raman measurement confirmed the XRD results. The blue-sift of Raman signals compared with bulk Bi crystals occurs due to the phonon confinement effect, which is larger in the thinnest α-Bi layers due to higher compression. Keywords: bismuth thin film; molecular beam epitaxy; high-resolution X-ray diffraction 1. Introduction Bismuth (Bi) is a semimetal with unique physical properties. Its electron energy dispersion is very anisotropic, the effective masses of the carriers are small, and their free-flight distances are large. When the Bi layer is thinned to approximately 30 nm, it is converted from a semimetal to a semiconductor [1]. Bi nanowires can also become semiconducting when their diameter is below 60 nm [2]. Interest in thin Bi layers has grown in particular recently, as it has become clear that a few atomic layers of the thick structures of this material can become topological insulators [3,4]. This variety of bismuth phases has even led to it being seen as the most important electronic material of the future [5]. In addition, nanometre-thin bismuth layers are being investigated for many different applications, such as sensors [6], thermoelectricity [7], contacts for Na-ion batteries [8], femtosecond optical switches [9], and so on. Various technologies were used to obtain high-quality Bi layers: thermal evaporation [10], electrodeposition [11], magnetron sputtering [6], pulsed laser deposition [12], and molecular beam epitaxy (MBE) [13]. Epitaxial Bi layers were grown by MBE on a variety of substrates, such as graphene [14], highly oriented pyrolytic graphite [15], NaCl [6], InAs [16], SiC [17], and silicon [13]. Compatibility with existing silicon technology, the ability to grow full wafer-sized homogeneous layers of several nanometers thickness that can be transferred to other secondary substrates, has made Si (111) substrates [18] the most popular for growing Bi layers. However, even on the (111)-oriented silicon substrates, the Bi layers do not always grow in the same way. Depending on the technological conditions, the growth starting from individual islands of the Stranski–Krastanov type or continuous layer-by-layer growth is possible. In the second case, we obtain a homogeneous, hexagonal symmetry layer Materials 2022, 15, 4847. https://doi.org/10.3390/ma15144847 https://www.mdpi.com/journal/materials