Formation of high-photoresponsivity BaSi₂ films on glass substrate by radio-frequency sputtering for solar cell applications

Abstract

The formation of high-photoresponsivity semiconducting films composed of earth-abundant elements on a SiO2 substrate is of particular importance for large-scale deployment of solar cells. We investigated the deposition of high-photoresponsivity BaSi2 films by radio-frequency sputtering on indium-tin-oxide (ITO), Ti, or TiN electrodes formed on a SiO2 substrate. Raman spectroscopy and x-ray diffraction measurements revealed the formation of randomly oriented polycrystalline BaSi2 films only on TiN/SiO2 substrates at 570 °C–650 °C. In contrast, impurity phases such as Ba oxides and TiSi2 were included when ITO and Ti layers were used, respectively. The photoresponsivity of the BaSi2 films on TiN electrodes reached 1.1 A W−1 at a wavelength of 790 nm under a bias voltage of 0.5 V applied between the front ITO and bottom TiN electrodes. This value is equivalent to the highest photoresponsivity ever achieved for BaSi2 epitaxial films on Si(111) substrates by molecular beam epitaxy.

Recent Progress Toward Realization of High‐Efficiency BaSi2 Solar Cells: Thin‐Film Deposition Techniques and Passivation of Defects

“…We recently found that metallic TiN is a good choice for this purpose. [29] TiN is a low-ρ metal (ρ ¼ 10 À5 -10 À4 Ω cm) [76] and possesses high chemical and thermal stabilities. [77,78] Figure 5a,b shows the optical images and Raman spectra of films grown by sputtering at T S ¼ 600 C on ITO(270 nm)/SiO 2 and TiN(250 nm)/SiO 2 substrates, respectively.…”

“…[77,78] Figure 5a,b shows the optical images and Raman spectra of films grown by sputtering at T S ¼ 600 C on ITO(270 nm)/SiO 2 and TiN(250 nm)/SiO 2 substrates, respectively. [29] The top surface was capped with a 3 nm-thick a-Si layer. All of the layers were formed by sputtering.…”

“…[39] Reproduced with permission. [29] Copyright 2021, IOP Publishing. [21] Copyright 2017, Royal Society of Chemistry.…”

“…[15,21] In this review, we focus on the recent progress of BaSi 2 to bridge the gap between the present status and that described in previous review articles. [14,15] Great progress has especially been made in thinfilm deposition techniques [22][23][24][25][26][27][28][29][30][31] and understanding the surface structure of BaSi 2 epitaxial films, [32] the reactions at the air/BaSi 2 and BaSi 2 /Si interfaces during post annealing, [33][34][35][36][37] and understanding the importance of satisfying the stoichiometry. [38,39] By positron annihilation spectroscopy, [39] it has been revealed that the electron concentration increases to %10 18 cm À3 with increasing vacancy-type defects [38] when the Ba/Si atomic ratio is far from the stoichiometric ratio.…”

“…In addition, novel device structures have been proposed, [40][41][42][43][44][45][46][47][48][49][50] and passivation of defects in BaSi 2 has been demonstrated by atomic-hydrogen (H) [51][52][53][54][55][56] and carbon (C) doping. [27,30,57] For deposition techniques, vacuum evaporation of BaSi 2 granules [21][22][23][24][25] and sputtering of BaSi 2 targets [26][27][28][29][30] or cosputtering of Si and Ba targets [31] are suitable to form BaSi 2 films for large-scale deployment on inexpensive substrates like SiO 2 . Large minority-carrier lifetimes (>4 μs) have been obtained for polycrystalline BaSi 2 films by these methods, [27,58] and they are comparable with those of BaSi 2 epitaxial films obtained by molecular beam epitaxy (MBE), [52] showing the superiority of these deposition techniques.…”