The growth processes and crystal structure of Ca silicides films grown by MBE at 500 °C on a Si(001) substrate

Abstract

This article investigates the growth of calcium (Ca) silicides on the Si(001)2x1 surface during molecular beam epitaxy (MBE) from two sources of Ca and Si with a variable flow ratio from three to 10 at a fixed substrate temperature of 500 °C. According to atomic force microscopy (AFM), the grown films consist of grains with a mutually perpendicular orientation and have pronounced grain boundaries with an inhomogeneous relief. The best crystalline qualities of the films, both CaSi and CaSi2, were confirmed for the minimum ratio (about 3.0) of Ca to Si deposition flows by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) data in both cross-sections and in planar sections. At the same time, according to XRD and HRTEM data, for all formed films over the CaSi(010) layer on Si(001), the growth of hR3-CaSi2 was observed with epitaxial relationships: hR3-CaSi2(001)//CaSi(010)//Si(001). An analysis of these studies showed that calcium monosilicide (CaSi) is the first silicide phase in the Ca–Si(001) system at 500 °C due to (1) the maximum modulus of its heat of formation, (2) a decrease in the Ca accommodation coefficient to silicon less than unity and the formation of a mixture close to CaSi, (3) a better matching between the parameters of the CaSi(010) plane lattice and the Si(001) plane lattice, and (4) lower CaSi (010)/Si (001) interface energy compared with the CaSi2(001)/Si(001) system. It is assumed that the formation of CaSi2 occurs in the subsurface region of the growing film at 500 °C due to increased diffusion of Ca atoms into the Si substrate and increased intergranular diffusion of Si atoms, which leads to the formation of a mixture with the CaSi2 composition. Both factors create conditions for the crystallization of CaSi2 on the surface of the CaSi layer at Т = 500 оС. The epitaxial ordering of CaSi2 on the CaSi surface and the decrease in the energy of hR3-CaSi2(001)/CaSi(010) interface is associated with small distortions of their lattices during superstructural matching, which is proved by modeling their matching.