Materialia 11 (2020) 100666 Contents lists available at ScienceDirect Materialia journal homepage: www.elsevier.com/locate/mtla Full Length Article Vacancy-mediated diffusion of atoms at Ge/Si interfaces: An atomistic perspective Sweta Kumari, Amlan Dutta ∗ Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, West Bengal - 721302, India a r t i c l e i n f o Keywords: Semiconductors Diffusion Interfaces Simulation and modelling a b s t r a c t The diffusive transport of atoms at the interface between germanium and silicon is a phenomenon of key impor- tance in understanding the electronic, thermal, and structural behavior of many hetero-epitaxial semiconductor devices and superlattices. Here we explore this phenomenon from the perspective of vacancy-diffusion at the Ge/Si interfaces. By employing the atomistic nudged elastic band method, we obtain the diffusion enthalpies, transition pathways, and activation barriers for the vacancy-mediated diffusion of interfacial atoms. In particu- lar, the {100}, {111}-glide, and {111}-shuffle pseudomorphic interfaces have been examined, and the tendency of a vacancy to cross the interface has been evaluated. Such calculations facilitate the identification of paths and interfaces with minimum and maximum resistance against the vacancy-mediated migration of interfacial atoms. Our computations reveal that the {111}-glide interface exhibits the least tendency of interface-crossing diffusion of vacancies, whereas the {111}-shuffle interface promotes such diffusion. In addition, the atomistic calculations also reveal a unique stable state of Ge monovacancy at the Ge/Si {111}-shuffle interface. 1. Introduction The heteroepitaxy of germanium layer on silicon is being perceived as a promising route to the fabrication of enhanced optoelectronic de- vices [1,2]. The quest for understanding the electronic and growth be- haviors of such heterosystems is not confined to conventional micro- electronics, but motivate studies on nanostructures like quantum dots as well [3–6]. It is well known that the electronic properties and per- formances of heteroepitaxial semiconductors depend on the structure and behavior of interfaces [7–11]. In this context, the diffusive trans- port of atoms at the interface dictates its integrity and quality. For in- stance, Huangfu et al. [12] have demonstrated the diffusion of Ge into Si across the {100} Ge/Si-heteroepitaxial interface. Therefore, the pre- diction of properties, reliability, and durability of heteroepitaxy-based devices requires a detailed qualitative and quantitative comprehension of the elementary atomistic events involved in interfacial diffusion. In the present study, we investigate the lattice diffusion of vacan- cies at the coherent pseudomorphic Ge/Si interfaces. Such interfaces appear in ultrathin epitaxial layers, where the interfacial strain is ac- commodated even in the absence of the misfit dislocations [13]. More- over, much thicker coherent interfaces are also possible when the Si substrate is suitably patterned [2,12,14]. Apart from thin layers, the pseudomorphic interfaces have also been studied in semiconductor core- shell nanowires [15,16] and Si-Ge superlattice [17–20]. Using atom- ∗ Corresponding author. E-mail address: amlan.dutta@metal.iitkgp.ac.in (A. Dutta). istic simulations, we estimate the minimum-energy-paths (MEPs) of the diffusive migration of vacancies, both across and away from the inter- faces in Ge/Si heterostructures. These computations involve the {100}, {111}-glide, and {111}-shuffle interfaces with pseudomorphic struc- tures, where the migration of monovacancies both across and away from the interfaces have been studied. Furthermore, this study examines and compares the diffusive vacancy migrations from Ge to Si, and from Si to Ge separately. As the calculations yield the energy barriers along the diffusion pathways in addition to the enthalpies of diffusion, it is pos- sible to assess the feasibility of these diffusion processes at reasonable temperatures of interest. Thus, the structures with easy interdiffusion paths have been identified, and the possible role of vacancy-mediated mass transport in the structure and composition of the interface has been analyzed. 2. Theoretical calculations The Ge/Si coherent interface is simulated by employing the Tersoff interatomic potential [21], which has been widely used for simulating a variety of Si-Ge systems including both alloys and interfaces [22–25]. Both {100} and {111} interfaces have been simulated, and in the latter case, the interface can either be the glide or shuffle type. For creating the simulated samples, the initial structure is con- structed as a diamond-cubic lattice with the lattice parameter as the https://doi.org/10.1016/j.mtla.2020.100666 Received 9 January 2020; Accepted 17 March 2020 Available online 15 May 2020 2589-1529/© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.