Ca silicide films—promising materials for silicon optoelectronics

Abstract

Single-phase films of semiconductor and semimetallic calcium silicides (Ca2Si, CaSi, and CaSi2), as well as films with a significant contribution of Ca5Si3 and Ca14Si19 silicides, were grown on single-crystal silicon and sapphire substrates. The analysis of the crystal structure of the grown films was carried out and the criterion of their matching with silicon and sapphire substrates was determined. Some lattice matching models were proposed, and the subsequent deformations of the silicide lattices were estimated. Film’s optical functions, including the optical transparency, were calculated from the optical spectroscopy data and an extended comparison was performed with the results of ab initio calculations. The real limits of the optical transparency for the films on sapphire substrates were established. The maximum transparency limit (3.9 eV) was observed for the CaSi film. Based on an analysis of the photoelectric properties of Ca2Si/Si diodes on n- and p-type silicon substrates, a perspective of their applications in silicon optoelectronics was discussed.

Reactive Spark Plasma Sintering and Thermoelectric Properties of Zintl Semiconducting Ca14Si19 Compound

“…Orthorhombic Ca 2 Si is semiconductive with a direct energy bandgap of about 0.88 eV, recently reported from optical spectra of a Ca 2 Si thin film on sapphire substrate [32], which is slightly lower than the value of 1.02 eV found with GW calculations, a post-DFT method that corrects the energy bandgap obtained from the DFT method [40]. Growth of thin films has been performed for applications such as light-emitting diodes in the near-infrared range [32]. Thermoelectric and electrical transport experiments on orthorhombic Ca 2 Si [43,44] showed that it is p-type and that the electrical conductivity (σ) is too weak, even after doping with Na, and that the energy activation found is much smaller than the energy bandgap found from the optical experiments [32].…”

“…Among all the other calcium silicide compounds, density functional theory (DFT) calculations show that only Ca 2 Si, Ca 3 Si 4 , and Ca 14 Si 19 are semiconducting compounds [28,[39][40][41][42]. Orthorhombic Ca 2 Si is semiconductive with a direct energy bandgap of about 0.88 eV, recently reported from optical spectra of a Ca 2 Si thin film on sapphire substrate [32], which is slightly lower than the value of 1.02 eV found with GW calculations, a post-DFT method that corrects the energy bandgap obtained from the DFT method [40]. Growth of thin films has been performed for applications such as light-emitting diodes in the near-infrared range [32].…”

“…As far as we know, the calcium-silicide family consists of six members in the equilibrium phase diagram: orthorhombic CaSi [Cmcm, No 63] -30]. In addition to the stable R6 rhombohedral CaSi 2 phase with 6 layers stacking, there is a metastable R3 rhombohedral CaSi 2 phase [R-3m, No 166] with 3 layers stacking that can be obtained from the sintering of R6 CaSi 2 under H 2 atmosphere or from thin film epitaxial growth [31,32]. The R6 rhombohedral CaSi 2 phase undergoes two structural phase transitions under high pressure and below 700 K, to a trigonal [P-3m1, No 164] phase between 8 and 16 GPa and to a hexagonal phase [P6/mmm, No 191] above 16 GPa [33,34].…”

“…Growth of thin films has been performed for applications such as light-emitting diodes in the near-infrared range [32]. Thermoelectric and electrical transport experiments on orthorhombic Ca 2 Si [43,44] showed that it is p-type and that the electrical conductivity (σ) is too weak, even after doping with Na, and that the energy activation found is much smaller than the energy bandgap found from the optical experiments [32]. Concerning Ca 5 Si 3 , there are several contradicting results.…”

“…According to the phonon glass electron crystal concept [11][12][13][14], their structural complexity should facilitate low k l values and enhance ZT appreciably. The drawback of the structural complexity is, however, a difficult synthesis process that comprises long-time annealing for several weeks for bulk materials [39], and only very recently thin films containing significant amounts of Ca 14 Si 19 in addition to CaSi 2 have been obtained [32]. As a consequence, while the elements Ca and Si are abundant and cheap, their casting into semiconducting thermoelectric compounds has been tedious, long-lasting, and expensive.…”