We fabricated approximately 200-nm-thick BaSi2 films on Si(111) substrates at 600 °C. The formation of BaSi2 was demonstrated by X-ray diffraction and Raman spectroscopy. A reduction in the electron concentration (n = 2 × 1016 cm−3) by 3 orders of magnitude compared to that previously reported (n = 7 × 1019 cm−3) and resultant photoresponsivity enhancement by more than two orders of magnitude were achieved. The photoresponsivity increased with the bias voltage Vbias applied between the top and bottom electrodes, and reached approximately 0.19 A/W at 2.0 eV, room temperature, and |Vbias| = 0.5 V.
We formed carbon (C)-doped BaSi2 films by radio-frequency (RF) sputtering of BaSi2 and SiC targets simultaneously, and measured their optical properties. In the Raman spectra of BaSi2 films, peaks corresponding to vibrational modes of Si tetrahedra in the lattice of BaSi2 appear.On the other hand, in C-doped BaSi2 films, new peaks at around 260, 310, and 630 cm -1 other than those of BaSi2 films were observed. As the RF power of the SiC target (PSiC) increased, these intensities increased. The absorption edge of C-doped BaSi2 films was shifted to higher energies from 1.19 eV to 1.30 eV with increasing PSiC. We achieved the highest photoresponsivity of 1 A/W ever achieved for undoped BaSi2 films at a bias voltage of 0.1 V applied between the top and bottom electrodes. The marked enhancement of photoresponsivity was interpreted to originate from the increased carrier lifetime in C-doped BaSi2 films.
We investigate the influence of deposition pressure in the range 0.25−1.0 Pa on the photoresponsivity of 200-nm-thick BaSi2 films grown by sputtering at 600 °C. BaSi2 films formed at 0.8 Pa exhibit a high photoresponsivity. The deposited Ba-to-Si atomic ratio depends significantly on the sputtering pressure. That's why the pressure influences the photoresponsivity. BaSi2 films grown by a two-step growth technique show much higher photoresponsivity almost equivalent to those grown by molecular beam epitaxy. The photoresponsivity reaches 0.75 A/W at 2.0 eV at a bias voltage of 0.5 V applied between the top and bottom electrode.
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.
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