Cham Thi Trinh

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

In order to control the electrical properties of an evaporated BaSi2 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 BaSi2. X-ray diffraction results show that a high substrate temperature is necessary for BaSi2 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 BaSi2 is found to improve, as evidenced by a decrease...

Photoresponse properties of BaSi2 film grown on Si (100) by vacuum evaporation

We have succeeded in the observation of high photoresponsivity of orthorhombic BaSi2 film grown on crystalline Si by a vacuum evaporation method, raising the prospect of its promising application in high-efficiency thin-film solar cells. Photocurrent was observed at photon energies larger than 1.28 eV, which corresponds to the band gap of evaporated BaSi2 film, indicating that the photoresponsivity originates from the BaSi2 film. The effect of the substrate temperature on the films properties was also investigated. The films grown at a substrate temperature larger than 500 °C are single-phase polycrystalline BaSi2 films, while those grown at a substrate temperature of 400 °C is a mixture of phases. We confirmed that undoped evaporated BaSi2 films are an n-type material with high carrier concentration. High carrier lifetime of 4.8 and 2.7 μs can be found for the films grown at 500 °C and 400 °C, respectively. BaSi2 film grown at a substrate temperature of 500 °C, which is crack-free and single-phase, shows the best photoresponsivity. The maximum value of photocurrent was obtained at photon energy of 1.9 eV, corresponding to an external quantum efficiency of 22% under reverse applied voltage of 2 V.

Post-annealing effects on the surface structure and carrier lifetime of evaporated BaSi2 films

To improve the surface quality for photovoltaic applications, we have investigated the effects of post-annealing on the surface structure and carrier lifetime of evaporated BaSi2 films. Structural characterizations by Raman spectroscopy and X-ray diffraction analysis show that there is an optimum post-annealing duration for fabricating a homogeneous film up to around the surface. By detailed surface analysis by X-ray photoelectron spectroscopy, the existence of a surface oxidation layer consisting of BaCO3 and barium silicate is revealed, and the thickness of the oxidation layer is found to be smallest for the optimum post-annealing duration. These surface structural changes are discussed from a thermodynamic viewpoint. Carrier lifetime is also investigated by the microwave-detected photoconductivity decay method, which shows that the structural change around the surface by post-annealing has negligible effects on carrier lifetime, possibly because the silicate layer covers the BaSi2 surface irrespective of post-annealing duration.

Preferred orientation of BaSi2 thin films fabricated by thermal evaporation

Thermal evaporation is a simple and high-speed method to grow a BaSi2 thin film, which is an emerging candidate for an absorber-layer material of thin-film solar cells. In this study, we have investigated the preferred orientation of BaSi2 films grown at substrate temperatures of 600–700 ◦C by thermal evaporation using X-ray diffraction. 2θ–ω scans show that peaks derived from (100) orientation grow steadily with increasing substrate temperature. By Xray pole figure analysis, the (100)-oriented crystals are proven to be epitaxially grown on Si(100) with two variants. The reason of epitaxial growth is discussed from the epitaxial temperature.

Minority-carrier lifetime and photoresponse properties of B-doped p-BaSi2, a potential light absorber for solar cells

600-nm-thick B-doped p-BaSi2 layers were grown on (111)-oriented n-Si substrates by molecular beam epitaxy, and the dependences of the minority carrier lifetime τ and photoresponsivity on the hole concentration p were investigated. p was varied from 1.4 × 1016 to 3.9 × 1018 cm−3. The highest τ of 2 µs was obtained for the sample with the lowest p of 1.4 × 1016 cm−3, reaching two orders of magnitude higher than that of the sample with the highest p of 3.9 × 1018 cm−3. The low-concentration-doped sample also exhibited an excellent external quantum efficiency (EQE) as large as 80% at a wavelength of approximately 800 nm at a reverse bias voltage of 0.2 V. This value is higher than any other EQEs we have ever achieved for BaSi2, showing the great potential of p-BaSi2 as a light absorber in solar cells.

Control of the electrical properties of BaSi 2 evaporated films for solar cell applications

BaSi<inf>2</inf> is a new candidate for the absorber in earth- abundant thin-film solar cells. In this study, we found that carrier density in evaporated BaSi<inf>2</inf> films can be controlled by deposition rate. First, it is shown that BaSi<inf>2</inf> films are formed at a high average deposition rate up to 2.8×10³ nm/min at 600 and 650 °C. Then, by Hall measurement, carrier (electron) density is shown to decrease with increasing deposition rate. The origin of the carrier is discussed considering the crystal quality evaluated by Raman spectroscopy. At last, using the lowest carrier-density film, rectification by n-BaSi<inf>2</inf>/p+-Si diode is demonstrated.