Structural and electrical characterizations of crack-free BaSi2 thin films fabricated by thermal evaporation

Abstract

The BaSi 2 semiconductor is a promising candidate for an earth-abundant solar cell absorber. In this study, we have realized a crack-free BaSi 2 film by a simple thermal evaporation technique on a CaF 2 substrate at a growth temperature of 500 • C for electrical characterization. A Si layer preliminarily formed on the substrate by sputtering is a key to obtain stoichiometric BaSi 2 film. Detailed structural characterization of the evaporated films with different Si layer thicknesses by X-ray diffraction, scanning and transmission electron microscopy demonstrates that a crack-free 370-nm-thick BaSi 2 film is formed by consuming the Si layer. It is observed that the 90-nm-thick bottom part is microcrystalline and contains Ar atoms, which come from Si deposition atmosphere. The surface of the BaSi 2 layer is found to be covered by an amorphous Si layer due to Si-rich vapor at the last stage of evaporation. Electrical properties of the BaSi 2 film is revealed by Hall measurement and the electron density and mobility are found to be 6×10 20 cm −3 and 0.04 cm 2 /V·s, respectively. Owing to a better crystalline quality contributed by the preliminarily-deposited Si layer, minority-carrier lifetime of the evaporated film (0.6 µs) is twenty times longer than previous films on glass substrates.

Field Modeling the Impact of Cracks on the Electroconductivity of Thin-Film Textronic Structures

“…In order to find the φ i potential, the elliptical coordinates η, ψ defined by the relations (8) were used:…”

“…The resistance of thin layers is also affected by the anisotropic properties of the substrate [7]. The impact of defects on the electrical properties of films has been discussed by [8][9][10][11][12][13][14][15]. Physical vacuum deposition is used primarily when a high deposition index is required for a high-quality thin layer.…”

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

“…Si atoms to compensate excess Ba can originate from deposited Si thin films on various substrates. Hara et al have shown that pre-deposited amorphous Si (a-Si) is the key to obtaining stoichiometric BaSi2 thin films, and they succeeded in obtaining crack-free BaSi2 thin films on CaF2 substrates [39], which have a similar linear expansion coefficient to BaSi2 [40]. Other substrates, such as stainless steel, titanium [41], and Ge [42], have been used successfully as substrates for BaSi2 thin films.…”

“…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].…”

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

“…We are therefore developing a thermal evaporation technique for BaSi 2 . So far, we have realized BaSi 2 films on various substrates such as silicon, 20 glass, 5 and CaF 2 , 21 and have revealed the BaSi 2 formation mechanism. 5,20,22,23 The composition of the vapor produced from BaSi 2 by thermal evaporation is Ba-rich in the initial stage and it changes to Si-rich as evaporation proceeds.…”