Sn – Induced decomposition of SiGeSn alloys grown on Si by molecular-beam epitaxy A.B. Talochkin a,b,⇑ , V.A. Timofeev a , A.K. Gutakovskii a,b , V.I. Mashanov a a A.V. Rzhanov Institute of Semiconductor Physics, Lavrentyev Avenue 13, Novosibirsk 630090, Russia b Novosibirsk State University, Novosibirsk 630090, Russia article info Article history: Received 17 June 2017 Received in revised form 31 August 2017 Accepted 5 September 2017 Available online 8 September 2017 Communicated by K.H. Ploog Keywords: A1. Solid solutions A1. Nanostructures A3. Molecular beam epitaxy B1. Germanium silicon alloys B2. Semiconducting silicon compounds abstract Structural features of Si 1xy Ge x Sn y alloy layers grown on Si by molecular-beam epitaxy are studied. These layers with the thickness of 2.0 nm, the nominal Ge composition of x 0  0.3, and the Sn-content of y  2–6 at.% have been grown at low temperatures (100–150 °C). We have used high-resolution trans- mission electron microscopy to analyze atomic structure of grown layers and Raman spectroscopy to evaluate the real Ge-content x from the observed optical phonon frequencies. It is found that the x value coincides with the nominal one at low Sn-content (2–3 at.%), and when it is increased (y  5 at.%), the decomposition of alloys into two fractions occurs. One of them is enriched by Ge with x up to 0.6 and the other fraction is Si-enriched. It is shown that the observed decomposition is Sn-induced and related to increase in Ge adatoms mobility in the growth process. This mechanism is similar to that theoretically predicted by Venezuela and Tersoff (Phys. Rev. 58, 10871 (1998)) for the case of high growth temperature. Ó 2017 Elsevier B.V. All rights reserved. 1. Introduction The incorporation of Sn atoms into the Ge lattice allows a direct-bandgap semiconductor to be created in order to use its light emission capability in optoelectronic devises based on Si- compatible technology [1–5]. The required Sn-content is 6–11 at. % [6–8]. However, Sn has low equilibrium solubility (<1 at.%) in Ge and a tendency for surface segregation. These obstacles can be obviated by using such techniques as low-temperature chemical vapor deposition (CVD) [3,9,10] and molecular-beam epitaxy (MBE) [11–13]. For the latter, the operating temperature range is 100–200 °C, which is far from equilibrium one (400–500 °C) and related to change in the growth mechanism due to the consider- able reduction of the adatom surface diffusion length [14]. In this case, interplay between the growth components becomes an essential factor determining the appearance of unusual structural features of GeSn layers. For example, these layers grown on Si con- sist of pure Ge islands surrounded by GeSn alloy with the high Sn- content [15]. In addition, the strong enhancement of light absorp- tion in GeSn layers grown on Ge was observed in Ref. [16]. It is induced by sizeable compositional disorder. So, the low- temperature growth causes structural distortions affecting layer properties [16,17]. Triple SiGeSn alloy can also become a direct-bandgap material [18–20]. The third component (Si) provides an additional degree of freedom by allowing for tuning electron states and strain asso- ciated with the lattice mismatch between Ge, Si and Sn. Therefore, this alloy is a prospective candidate with more opportunities and higher thermal stability in comparison with a binary one [9,7]. Recently, Nikiforov et al. [21] suggested to grow ultrathin pseudo- morphic SiGeSn layers directly on Si using low-temperature MBE. Their evident advantage against thick relaxed layers is that they are free from dislocations, and also it is possible to control of strain by means of changing an alloy composition. Despite the fact that according to theoretical results [18–20] the direct-bandgap state in this structure is hard to achieve due to compressive strain in the growth plane, anomalously high photoresponse in the lower energy range than the Si bandgap (0.6–0.8 eV) was observed in the grown multilayer SiGeSn/Si samples [21]. This indicates that the given structure is attractive for creation of high efficiency pho- todetectors. At the same time, whether structural features of SiGeSn layers related to low growth temperature, similar to those of GeSn alloys [16–19], are formed remains an open question. Because of their influence on layer properties, an examination of these features seems to be a pressing task. http://dx.doi.org/10.1016/j.jcrysgro.2017.09.005 0022-0248/Ó 2017 Elsevier B.V. All rights reserved. ⇑ Corresponding author at: Institute of Semiconductor Physics, Siberian Branch of the Russian Academy of Sciences, Lavrentjeva 13, 630090 Novosibirsk, Russia. E-mail address: tal@isp.nsc.ru (A.B. Talochkin). Journal of Crystal Growth 478 (2017) 205–211 Contents lists available at ScienceDirect Journal of Crystal Growth journal homepage: www.elsevier.com/locate/crys