Enhancing the superconducting transition temperature of BaSi2 by structural tuning

We present a joint experimental and theoretical study of the superconducting phase of the layered binary silicide BaSi2. Compared with the layered AlB2 structure of graphite or diboride-like superconductors, in the hexagonal structure of binary silicides the sp3 arrangement of silicon atoms leads to corrugated sheets. Through a high-pressure synthesis procedure we are able to modify the buckling of these sheets, obtaining the enhancement of the superconducting transition temperature from 4 K to 8.7 K when the silicon planes flatten out. By performing ab-initio calculations based on density functional theory we explain how the electronic and phononic properties of the system are strongly affected by changes in the buckling. This mechanism is likely present in other intercalated layered superconductors, opening the way to the tuning of superconductivity through the control of internal structural parameters.Comment: Submitte

Fabrication of SrGe2 thin films on Ge (100), (110), and (111) substrates

Semiconductor strontium digermanide (SrGe2) has a large absorption coefficient in the near-infrared light region and is expected to be useful for multijunction solar cells. This study firstly demonstrates the formation of SrGe2 thin films via a reactive deposition epitaxy on Ge substrates. The growth morphology of SrGe2 dramatically changed depending on the growth temperature (300−700 °C) and the crystal orientation of the Ge substrate. We succeeded in obtaining single-oriented SrGe2 using a Ge (110) substrate at 500 °C. Development on Si or glass substrates will lead to the application of SrGe2 to high-efficiency thin-film solar cells

Transport properties of n- and p-type polycrystalline BaSi2

Electron and hole mobilities versus temperature in semiconducting barium disilicide (BaSi2) have been systematically studied both experimentally and theoretically. The experiments were performed with undoped 250 nm-thick BaSi2 polycrystalline films grown by molecular beam epitaxy. The grain size of films ranged from 0.2 to 5 μm with the electron concentration of 5.0 × 1015 cm−3. To investigate the hole mobility, B-doped p-BaSi2 films with various dopant concentrations were fabricated and studied. The experimental temperature dependence of the electron mobility in the range of 160–300 K was found to have a maximum of 1230 cm2/V∙s at 218 K, while at room temperature (RT) it dropped down to 816 cm2/V∙s. We demonstrate that the temperature dependence of the electron mobility cannot be adequately reproduced by involving standard scattering mechanisms. A modified approach accounting for the grained nature of the films has been proposed for the correct description of the mobility behavior. The highest hole mobility in p-BaSi2 films reaching ~ 80 or 200 cm2/V∙s (for the films grown on (111) or (001) Si substrates, respectively) at RT is about an order or four times of magnitude smaller than that in n-BaSi2 films. Such a great difference we ascribe to the specific features of electron-phonon and hole-phonon coupling in semiconducting BaSi2

Simple way of finding Ba to Si deposition rate ratios for high photoresponsivity in BaSi2 films by Raman spectroscopy

Since the photoresponsivity of BaSi2 is sensitive to a Ba-to-Si deposition rate ratio (R Ba/R Si), there is a need to determine the optimum value of R Ba/R Si. We grew 0.5 μm thick BaSi2 films with R Ba/R Si varied from 1.1–3.6 at 580 °C and 0.4–4.7 at 650 °C. The photoresponsivity reached a maximum at R Ba/R Si = 2.2 and 1.2, respectively. Raman spectroscopy revealed that the crystalline quality of BaSi2 became better with decreasing R Ba/R Si. However, as R Ba/R Si decreased further beyond these values, excess Si precipitated, showing that the optimum value of R Ba/R Si should be as small as possible without causing Si precipitates to form

Preparation and Characterization of Clathrates in the Systems Ba – Ge, Ba – Ni – Ge, and Ba – Ni – Si: Preparation and Characterization of Clathrates in the Systems Ba – Ge, Ba – Ni – Ge, and Ba – Ni – Si

The main focus of this work is the preparation, chemical and structural characterization along with the investigation of physical properties of intermetallic clathrates. Starting from the history of clathrate research, classification of clathrate types, their structural properties and possible application areas are evaluated in chapter 2. The methodologies of sample preparation and materials characterization as well as quantum chemical calculations are discussed in chapter 3. The complete characterization of Ba8Ge433 ( is a Schottky-symbol standing for vacancies),12-14 which is a parent compound for the variety of ternary variants, is the subject of chapter 4. Ba8Ge433 is a high temperature phase,12 which was prepared for the first time as single phase bulk material in this work.15, 16 In this way, the intrinsic transport properties could be investigated without influence of grain boundary and impurity effects. The transport behavior is analyzed at low and high temperatures and referred to the former results. In addition, crystal structure and vacancy ordering in terms of the reaction conditions are discussed. Chemical bonding in Ba8Ge433 is investigated by topological analysis of the electron localizability indicator and the electron density. Chapter 5 deals with the preparation, phase analysis, crystal structure and physical properties of BaGe5, which constitutes a new clathrate type oP60.17, 18 So far, two clathrate types were known in the binary system Ba – Ge, namely the clathrate cP124 Ba6Ge25,19-21 and the clathrate-I Ba8Ge433. Originally, BaGe5 was detected by optical and scanning electron microscopy within the grains of Ba8Ge433.12 Once the preparation of phase-pure Ba8Ge433 was achieved, it became possible to make detailed investigations of its decomposition along with the formation of BaGe5. A detailed theoretical and experimental analysis on the relation between crystal structure and physical properties of BaGe5 is presented. In chapter 6, a thorough structural characterization and the physical properties of clathrates in the system Ba – Ni – Ge is presented based on the subtle relation between the crystal structure containing vacancies and the thermoelectric properties. During the investigations in this system, a large single crystal was grown by Nguyen et al. 22, 23 from the melt with the composition Ba8Ni3.5Ge42.10.4. A systematic reinvestigation of the phase relations in this system was performed and the influence of different Ni content to the crystal structure and physical properties is evaluated. The Si-based ternary clathrate with composition Ba8–δNixySi46–x–y is the subject of chapter 7. The phase relations and the homogeneity range are established. The crystal structure taking into account vacancies in the framework is discussed. Physical properties of bulk pieces are analyzed and the results are related to the sample composition. In addition, first-principles electronic structure calculations are carried out to assess variations in the electronic band structure, phase stability and chemical bonding.24 Chapter 8 reports on the intermetallic compound Ba3Si4,25, 26 which was encountered during the investigations on the Ba – Ni – Si phase diagram. The discussion covers issues related to preparation, crystal structure, phase diagram analysis, electrical and magnetic properties, NMR measurements, quantum mechanical calculations and oxidation to nanoporous silicon with gaseous HCl. Besides my contributions to the NoE CMA, I studied under the Priority Program 1178 of Deutsche Forschungsgemeinschaft “Experimental electron density as the key for understanding chemical interactions” with the project of “Charge distribution changes by external electric fields: investigations of bond selective redistributions of valence electron densities”. Chapter 9 deals with the preparation of chalcopyrites ZnSiP2 and CuAlS2 for experimental charge density analysis. Both phases show semiconducting properties and have non-centrosymmetric structures with high space group symmetry as needed to investigate the structural changes induced by external electric field. In this chapter, I describe the preparation and the crystal structure analyses of ZnSiP2 and CuAlS2 including issues related to the data collection as well as the results of NMR investigation

Boron-doped p-BaSi 2/n-Si solar cells formed on textured n-Si(001) with a pyramid structure consisting of {111} facets

BaSi2 films were fabricated on textured Si(0 0 1) substrates that consisted of {1 1 1} facets using molecular beam epitaxy. The light-trapping effect of these films and their performance when incorporated into solar cells were measured. X-ray diffraction and reflectivity measurements showed that the BaSi2 films were grown epitaxially on the textured Si(0 0 1) substrate and confirmed the light-trapping effect. The critical thickness over which BaSi2 relaxes increased from approximately 50 to 100 nm when comparing the BaSi2 films on a flat Si(1 1 1) substrate and the textured substrate, respectively. p-BaSi2/n-Si solar cells were fabricated with varying BaSi2 layer thickness and with hole concentrations in the range between 2.0 × 1018 and 4.6 × 1018 cm−3. These cells exhibited a maximum energy conversion efficiency of 4.62% with an open-circuit voltage of 0.30 V and a short-circuit current density of 27.6 mA/cm2 when the p-BaSi2 layer was 75 nm-thick. These results indicated that the use of BaSi2 films on textured Si(0 0 1) substrates in solar cells shows great promise