Effects of deposition rate on the structure and electron density of evaporated BaSi2 films

Abstract

In order to control the electrical properties of an evaporated BaSi 2 film, which is an emerging candidate for the absorber-layer material of earth-abundant thin-film solar cells, we have investigated the effects of deposition rate on the produced phases, microstructure, and carrier density of the thin films grown by thermal evaporation of BaSi 2. X-ray diffraction results show that a high substrate temperature is necessary for BaSi 2 formation at a high deposition rate, which is discussed from viewpoints of vapor composition and diffusion time. Microstructural characteristics such as grain size of 30-120 nm, oxide particle arrays present around the interface, and partial oxidation at a low substrate temperature are revealed by cross-sectional transmission electron microscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy combined with an energy-dispersive X-ray spectroscopy. With increasing deposition rate, the crystalline quality of BaSi 2 is found to improve, as evidenced by a decrease in full-width at half maximum of a [Si 4 ] 4À vibration band in Raman spectra. At the same time, electron density, which is determined by Hall measurement, decreases with deposition rate. The variation of electron density is discussed on the basis of microstructural characteristics and BaSi 2 formation mechanism. The most probable reason is concluded to be composition deviation from stoichiometry.

Exploring the potential of semiconducting BaSi2for thin-film solar cell applications

“…Intrinsically doped BaSi2 shows n-type conductivity regardless of thin-film growth method. An electron concentration n varies from n ~ 10 16 cm -3 for MBE-grown BaSi2 films [16,50] to n >10 18 cm -3 for sputtered or vacuum-evaporated BaSi2 films [48,51,52]. A first-principles DFT supercell approach revealed that this n-type conductivity arises from Si vacancies [53].…”

Detection of local vibrational modes induced by intrinsic defects in undoped BaSi2 light absorber layers using Raman spectroscopy

“…First, we confirm the reproducibility of this calculation. Table I shows 34,36,37 and by calculation. 35 Terai et al also pointed out such a feature of modes in the measurement of the angle dependencies of polarized Raman spectra of a-axis-oriented undoped BaSi 2 epitaxial films on Si(001).…”

“…The Raman lines originate from Si tetrahedra with T h symmetry in the lattice of BaSi 2 . In the past, they were assigned based on Si tetrahedra with T d symmetry; 34,36,37 however, we found very recently that T h symmetry is more suitable on the basis of experiments by polarized Raman spectroscopy and by infrared absorption spectroscopy. Besides, the Si-Si distances within each Si tetrahedron are different as in 0.234, 0.237, 0.237, 0.241, 0.248, and 0.248 nm, 57 meaning that T d symmetry is not valid.…”

“…32,33 However, there has been no such discussion about defects in BaSi 2 although there have been several reports on Raman spectra of BaSi 2 from both experimental and theoretical viewpoints so far. [34][35][36][37] In this work, we first estimate residual stresses in the a-axis-oriented BaSi 2 epitaxial films grown on a Si(111) substrate based on the lattice constants deduced from x-ray diffraction measurements. This is to qualitatively understand the shift of the Raman peak position and discuss their origins.…”

“…34-37. They are based on the results of powdered BaSi 2 ,34 phonon density of states (DOS),35 BaSi 2 film on Si(001) by vacuum evaporation,36 and BaSi 2 film on Si(001) by MBE 37. Schematics of Raman active A g modes calculated at the C point in a BaSi 2 perfect crystal at (a) 499.8 cm À1 and (b) 502.5 cm À1 .TABLE III.…”