A low temperature growth of Ca silicides on Si(100) and Si(111) substrates: Formation, structure, optical properties and energy band structure parameters

Abstract

The low-temperature formation of Ca silicides on silicon substrates with (100) and (111) orientations and on the oxidized silicon surface during deposition using solid phase epitaxy (SPE) and molecular beam epitaxy (MBE) was studied by the in situ electron spectroscopy. The temperature ranges for the formation of Ca2Si (273–346 °C) and CaSi (554–663 °C) on the Si(100) 2 × 1 surface were established during isochronous annealing of a thin Ca layer (10 nm). It was shown that an amorphous Ca2Si layer was formed by a SPE process at 330 °C from a Ca–Si mixture on the Si(100)2 × 1 surface, but under the surface the silicon and CaSi nanocrystals (NCs) were formed at the interface with the substrate. According to the optical spectroscopy data, films grown by the SPE method are semiconducting with the first direct interband transition at 1.16 eV, but the intraband absorption below 1 eV was determined by the carrier transitions in the CaSi NCs and transitions to defect levels. Raman active and far IR active peaks in amorphous Ca2Si films were firstly determined. When the MBE method (T = 330 °C) with an additional annealing at 330 °C was used, a Ca2Si layer was formed on the film surface, but the main contribution is made by faceted CaSi crystals (100 × 25 nm2) according to AFM, X-ray and electron diffraction. In the film consisting of the Ca2Si and CaSi phases, there are Raman peaks from both phases, which is associated with their separation according to the positions of the Raman shifts. A decrease in the temperature of the MBE process to 190 °C on the Si(111)7 × 7 surface with additional annealing at T = 300 °C leads to the formation of amorphous calcium silicide with a high density of embedded CaSi NCs (3–7 nm) with a stressed structure. When these CaSi NCs films were studied by the Raman spectroscopy method, 8 Raman phonons in the range of 105–422 cm−1 were detected. It was established that films containing CaSi in the crystalline phase are a semimetal with a constant absorption coefficient ((1–2)∙104 cm−1) at photon energies (0.5–1.1 eV) and a quasi-band gap with an energy from 1.26 eV to 1.36 eV depending on the size of CaSi NCs. MBE growth from a mixture of Ca–Si on an oxidized Si(100) surface led to the formation of shapeless CaSi and Ca2Si grains (weak contribution) with sizes from 50 to 200 nm, creating a structural-continuous film.