Orthorhombic BaSi2 Research Articles

Comparison of Crystal and Phonon Structures for Polycrystalline BaSi<sub>2</sub> Films Grown by SPE Method on Si(111) Substrate

The search for inexpensive and efficient methods of forming thin BaSi2 films as a promising material for photovoltaic is an actual task. The co-deposition of Ba and Si atoms with alloy thickness of 100-120 nm on the silicon substrate at room temperature with following annealing (SPE method) was proposed. Ba-Si alloy compounds then were thermally annealed at different temperatures and three samples were formed: #1 at T = 600 ° C, #2 at T = 700 ° C and #3 at T = 800 ° C. Polycrystalline films with an orthorhombic BaSi2 structure were formed by XRD, UV-VIS, FIR and Raman spectroscopies data. BaSi2 grains in samples #1 and #2 have sizes 62-64 nm and 86 nm in the sample #3 from XRD data calculations by Scherrer formula. Proposed growth method resulted to strong compression of the BaSi2 unit cell volume on 1.78 – 2.70%. The strongest compression was observed after annealing at 800 °C, which was accompanied by desorption of a noticeable amount of barium and a strong decrease in the film thickness in the sample #3. The formation of nanosize Si clusters was confirmed by Raman data for samples #2 and #3, but they did not observed in the sample #3. So, the film, formed at 800 °C, is the most qualitative in terms of structure and single-phase BaSi2, but with strong decrease of initial Ba-Si alloy thickness due to Ba desorption.

Growth of BaSi2 film on Ge(100) by vacuum evaporation and its photoresponse properties

We have successfully grown a polycrystalline orthorhombic BaSi2 film on a Ge(100) substrate by an evaporation method. Deposition of an amorphous Si (a-Si) film on the Ge substrate prior to BaSi2 evaporation plays a critical role in obtaining a high-quality BaSi2 film. By controlling substrate temperature and the thickness of the a-Si film, a crack-free and single-phase polycrystalline orthorhombic BaSi2 film with a long carrier lifetime of 1.5 µs was obtained on Ge substrates. The photoresponse property of the ITO/BaSi2/Ge/Al structure was clearly observed, and photoresponsivity was found to increase with increasing substrate temperature during deposition of a-Si. Furthermore, the BaSi2 film grown on Ge showed a higher photoresponsivity than that grown on Si, indicating the potential application of evaporated BaSi2 on Ge to thin-film solar cells.

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 film’s 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.

Formation, Structure and Optical Properties of Nanocrystalline BaSi<sub>2</sub> Films on Si(111) Substrate

BaSi2 thin films were formed on Si (111) substrate by solid-phase epitaxy (SPE) (UHV deposition) using the template technology followed by vacuum annealing at temperatures of 600 °C and 750 °C. After the deposition and annealing barium silicide films were characterized by Auger electron spectroscopy, grazing incidence x-ray diffraction (GIXRD) and atomic-force microscopy (AFM). It was established that the films annealed at T = 600 °C are polycrystalline with the structure of the orthorhombic BaSi2, with grain sizes of 100-200 nm. Higher anneal temperature (T=750 °C) leads to increase of diffraction peak intensity of BaSi2 phase with grain coagulation into 300-400 nm islands. It was confirmed that nanocrystalline BaSi2 films are characterized by a direct fundamental interband transition at 1.3 eV, the second interband transition with an energy of 2.0 eV, own phonon structure with wave number peaks at 112, 119, 146 and 208 cm-1 and a high density of defect states within the band gap, which provide a noticeable subband absorption at energies of 0.8 – 1.1 eV.

Epitaxy of Orthorhombic BaSi2 with Preferential In-Plane Crystal Orientation on Si(001): Effects of Vicinal Substrate and Annealing Temperature

We attempted to grow orthorhombic BaSi2 epitaxial films having preferential in-plane crystallographic orientation on both exact and vicinal Si(001) substrates with a miscut angle of 2° toward Si[1̄10] by reactive deposition epitaxy (RDE) and subsequent molecular beam epitaxy (MBE). On the vicinal Si(001) substrate, the initial BaSi2 nuclei formed by RDE tended to grow unidirectionally with the [010] direction parallel to Si[110] when the annealing temperature of the Si substrate before the growth was increased from 830 to 1000 °C. Transmission electron microscopy showed that the grain size of the BaSi2 films grown by MBE increased up to approximately 9 µm on the vicinal Si(001) substrate when the substrate annealing temperature was 1000 °C. This is the largest grain size ever obtained for BaSi2. Even in the case of the exact Si(001) substrate, the MBE-grown BaSi2 grains preferentially grew with the [010] direction along Si[110] when the annealing temperature was 1000 °C.

Characterization of Shock-Recovered BaSi2 Powder

Shock recovery experiments at pressures of up to 22 GPa on BaSi2 powder are performed using a propellant gun. The shocked samples are characterized using X-ray diffraction analysis, Raman spectroscopy, and scanning electron microscopy (SEM). Only the orthorhombic BaSi2 phase is detected and no evidence of amorphization or phase transition is obtained. The SEM images reveal that the BaSi2 powder is consolidated at pressures below 10 GPa, whereas many cavities in addition to whiskers with diameters of several hundreds of nanometers are formed on the surface of the sample shocked at 10 GPa. These whiskers are due to the eruption of BaSi2 vapor from the cavities and the subsequent mixing of this vapor with air. The shock-induced heat may be the cause of this vaporization.

Effects of annealing temperature on the structure and surface feature of BaSi2 films grown on Si(111) substrates

The optimum growth temperature is determined for the epitaxial growth of semiconducting orthorhombic BaSi2 films on Si(111) substrates by magnetron sputtering (MS) and annealing in vacuum. The structure and morphological feature of the films were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Crystal planes of BaSi2 show preferred orientation. The optimum annealing temperature is 800 °C, 12 hours is a feasible annealing time.

Effects of Heat Treatment on Growth of BaSi<sub>2</sub> Film on Si(111) Substrates

Semiconducting orthorhombic BaSi2 films were synthesized on Si(111) substrates using magnetron sputtering (MS) and subsequent annealing by interdiffusion between the deposited Ba film and Si(111) substrate. The structural and morphological features of the result films are analysed. The growth mechanism and the evolution of the silicides are discussed. The effects of annealing temperature and annealing time on the growth of the BaSi2 film are studied.

First-principles calculations on the electronic structure and optical properties of BaSi2

The electronic structure, densities of states and optical properties of the stable orthorhombic BaSi2 have been calculated using the first-principle density function theory and pseudopotential method. The results show that BaSi2 is an indirect semiconductor with the band gap of 1.086 eV, the valence bands of BaSi2 are mainly composed of Si 3p, 3s and Ba 5d, and the conduction bands are mainly composed of Ba 6s, 5d as well as Si 3p. The static dielectric function ɛ 1(0) is 11.17, the reflectivity n 0 is 3.35, and the biggest peak of the absorption coefficient is 2.15×105 cm−1.

Epitaxial Growth of Semiconducting BaSi2 Thin Films on Si(111) Substrates by Reactive Deposition Epitaxy

The optimum growth temperature is determined for the epitaxial growth of semiconducting orthorhombic BaSi2 films on Si(111) substrates by reactive deposition epitaxy (RDE). X-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED) analyses confirmed that smooth, [100]-oriented epitaxial BaSi2 films could be obtained by RDE at a substrate temperature of 600–650°C when the Ba deposition rate was 10 nm/min.

Investigation of the energy band structure of orthorhombic BaSi2 by optical and electrical measurements and theoretical calculations

Optical and electrical properties of polycrystalline orthorhombic BaSi2 prepared by arc melting in Ar atmosphere were investigated. The optical absorption spectra measured at room temperature showed that indirect and direct absorption edges were 1.15 and 1.25 eV, respectively. The activation energy estimated from temperature dependence of the resistivity was 1.10 eV. These results agreed well with a calculated band structure of the orthorhombic BaSi2 by first principles calculation using density functional theory.

ＢａＳｉ２の高温高圧下における相転移とクエンチされた準安定相の電気的性質

This paper reviews structural phase transitions of BaSi2 at high pressures and high temperatures, and electrical resistivity measurements of two quenched high-pressure, high-temperature phases and a stable phase at ambient conditions. In situ x-ray diffraction measurements reveal that BaSi2 has three high pressure, high temperature phases: trigonal, cubic and an additional phase, at pressures up to 7 GPa and temperatures up to 1300 K. All of the high pressure, high temperature phases can be quenched at ambient conditions. The electrical resistivity measurements of trigonal, cubic and orthorhombic BaSi2, a stable phase at ambient conditions, at 1 atm show that the electrical resistivity strongly depends on their crystal structure: orthorhombic and cubic BaSi2 are n-type semiconductors, and trigonal BaSi2 is a hole metal that shows superconductivity with an onset temperature of 6.8 K.

Electrical Resistivity of Three Polymorphs of Basi2 and P-T Phase Diagram

AbstractThis paper reports a pressure-temperature phase diagram for BaSi2 and evaluates the electrical resistivity of orthorhombic, cubic and trigonal BaSi2. In-situ X-ray diffraction measurements revealed the transition sequence of cubic and trigonal BaSi2 from orthorhombic BaSi2 at high pressures and high temperatures. The electrical resistivity measurements of three polymorphs show that the electrical properties depend on the crystal structure: orthorhombic BaSi2is an n-type semiconductor as previously reported; cubic BaSi2is an n-type semiconductor; trigonal BaSi2is a hole metal that shows superconductivity with an onset temperature of 6.8K.

Kristallstruktur von Sr0,5Ba0,5Si2 bei hohen Drücken / Crystal Structure of Sr0,5Ba0,5Si2 at High Pressures

Abstract A cubic polymorph (SrSi2 type of structure) is obtained if orthorhombic (BaSi2 type of structure) Sro.5Bao.5Si2 is treated in a belt type apparatus (40 kbar, 1000 °C): P4332 (or P4i32), Z = 4, a = 662.2 pm, xSi = 0.419.

Notizen: Kristallstruktur von Bariumdigermanid bei hohen Drücken / Crystal Structure of Bariumdigermanide at High Pressures

After treatment of orthorhombic (BaSi2 type of structure) bariumdigermanide at 40 kbar and 1000 °C in a belt type apparatus and quenching to ambient conditions a tetragonal polymorph (a-ThSi2 type of structure) is obtained. Its crystal data are: I41/amd, a = 0.4755 nm, c = 1.473 nm, zGe = 0.415.