Room temperature MBE deposition of Bi2Te3 and Sb2Te3 thin films with low charge carrier densities

Sb2Te3 and Bi2Te3 thin films were grown at room temperature on SiO2 substrates using MBE and were subsequently annealed at 250°C. The films were stoichiometric, polycrystalline, textured, and yielded strikingly low charge carrier densities of about 2.7 × 10(exp 19) cm-3. The in-plane transport properties were measured at room temperature, the thermopower was 130 µVK(exp -1) for Sb2Te3 and -153 µVK(-1) for Bi2Te3 thin films. The small charge carrier densities are explained by a reduced antisite defect density due to the low temperatures to which the thin films were exposed during annealing

Sb2Te3 and Bi2Te3 Thin Films Grown by Molecular Beam Epitaxy at Room Temperature

Nano-alloyed p-type Sb 2Te 3 and n-type Bi 2Te 3 thin films were grown on SiO 2/Si and BaF 2 substrates by molecular beam epitaxy (MBE) in two steps: (i) Repeated deposition of five-layer stacks with sequence Te-X-Te-X-Te (X = Sb or Bi) with elemental layer thicknesses of 0.2 nm on substrates at room temperature, (ii) annealing at 250 °C for two hours at which phase formation of Sb 2Te 3 or Bi 2Te 3 occurred. The room temperature MBE deposition method reduces surface roughness, allows the use of non lattice-matched substrates, and yields a more accurate and easier control of the Te content compared to Bi 2Te 3 thin films, which were epitaxially grown on BaF 2 substrates at 290 °C. X-ray diffraction revealed that the thin films were single phase, poly-crystalline, and textured. The films showed grain sizes of 500 nm for Sb 2Te 3 and 250 nm for Bi 2Te 3, analyzed by transmission electron microscopy (TEM). The in-plane transport properties (thermopower S, electrical conductivity , charge carrier density n, charge carrier mobility , power factor S 2) were measured at room temperature. The nano-alloyed Sb 2Te 3 thin film revealed a remarkably high power factor of 29 W cm -1 K -2 similar to epitaxially grown Bi 2Te 3 thin films and Sb 2Te 3 single crystalline bulk materials. This large power factor can be attributed to a high charge carrier mobility of 402 cm 2 V -1 s -1 similar to high-ZT Bi 2Te 3/Sb 2Te 3 superlattices. However, for the nano-alloyed Bi 2Te 3 thin film a low power factor of 8 W cm -1 K -2 and a low charge carrier mobility of 80 cm 2 V -1 s -1 were found. Detailed microstructure and phase analyses were carried out by energy-filtered TEM in cross-sections. Quantitative chemical analysis by energy-dispersive x-ray spectroscopy (EDS) was also applied. In Bi 2Te 3 thin films, few nanometer thick Bi-rich blocking layers at grain boundaries and Te fluctuations by 1.3 at.% within the grains were observed. The small charge carrier densities are explained by a reduced antisite defect density due to the low temperatures to which the thin films were exposed during annealing

Structural and compositional characterization of Bi1−x_{1−x}Sbx_x nanowire arrays grown by pulsed deposition to improve growth uniformity

Arrays of Bi1-xSbx nanowire with various compositions (0 <= x <= 1) are grown in etched ion-track membranes by pulsed electrochemical deposition. Nanowires of diameter from 130 nm down to 18 nm are characterized by means of X-ray diffraction and high-resolution electron microscopy combined with electron diffraction and energy dispersive X-ray analysis. Compared to potentiostatic deposition, the pulsed synthesis method leads to a more uniform growth and higher filling rate of the wires across the entire template. By tuning the deposition parameters, we demonstrate excellent control over the wire composition and crystallographic orientation. The deposition process presented facilitates the development of future nanowire-based thermoelectric sensors, which are expected to exhibit a higher sensitivity and a faster response compared to thin film sensors. (C) 2015 Elsevier B.V. All rights reserved

Working Around HTS Thickness Limitations–towards 1000+A/cm–Class Coated Conductors

AbstractIncreasing the HTS film thickness would be the straightforward route to enhance the current transport capacity of coated conductors. Usually, however, growth defects lead to a strong deterioration of the critical current density with thickness. The unique growth mode of HTS films on graded and tilted MgO buffer layers grown by inclined substrate deposition (ISD) paves the way to overcome this limit. This contribution presents micro-structural and performance data of coated conductors with monolithic REBCO-films up to 7.5μm thickness exhibiting perfect crystallinity and critical currents in excess of 1000 A/cm-width

Origin of microscopically coupled ferromagnetic Cu-ions in a distorted system of Cu-doped ZnO and their synchrotron-based electronic structures

Spintronics-based studies have produced significant attention in the last decade while claiming the observation of room temperature ferromagnetism (RTFM). Nevertheless, there is a lack of consensus on a mechanism responsible for this phenomenon. In this study, we focus on Cu-doped ZnO (ZCO) to understand the microscopic origin of RTFM and the role of different oxidation states of Cu in RTFM. We have performed different spectroscopic techniques using synchrotron facilities. The values of spin-moment obtained from x-ray magnetic circular dichroism sum-rule truly exhibit a ferromagnetic interaction in the nanocrystalline powder of ZCO with ∼0.58 μB for 5% of Cu concentration in the total fluorescence yield mode. Such an enhanced magnetization is attributed to the presence of Cu2+, which is mainly localized in the bulk region. Cu in ZCO is mostly dominated by the presence of Cu2+. This is clearly reflected by the profiles of x-ray photoemission spectroscopy. Consequently, the weakly magnetized total electron yield mode is attributed to a state of magnetic frustration as the majority of Cu3+ is found on the surface. Some of these Cu3+ when come in the vicinity of Cu2+ ions result in a highly correlated state of double exchange mechanism, which is the microscopic origin of RTFM in ZCO. The coupling between Cu2+-Cu3+ is mediated via oxygen vacancies (VO), the presence of which is confirmed through the features of electron energy loss spectroscopy over different edges. The confirmation of VO is also supported by the deconvolution of E2high-phonon in the Raman spectra. Moreover, the defects in the local electronic structures of ZCO are demonstrated by the deconvoluted spectra of Cu L3 x-ray absorption spectroscopy. The images obtained from high-resolution transmission electron microscopy confirm the incorporation of Cu into the wurtzite crystal of ZnO. A clear enhancement in magnetization upon an increase in carriers of Cu in ZCO indicates carrier-induced ferromagnetism. Cu2+ and VO are the two attributes of RTFM in ZCO

Sb2Te3 and Bi2Te3 Thin Films Grown by Room-Temperature MBE

Sb2Te3 and Bi2Te3 thin films were grown on SiO2 and BaF2 substrates at room temperature using molecular beam epitaxy. Metallic layers with thicknesses of 0.2 nm were alternately deposited at room temperature and the films were subsequently annealed at 250A degrees C for 2 h. x-Ray diffraction and energy-filtered transmission electron microscopy (TEM) combined with high-accuracy energy-dispersive x-ray spectrometry revealed stoichiometric films, grain sizes of less than 500 nm, and a texture. High-quality in-plane thermoelectric properties were obtained for Sb2Te3 films at room temperature, i.e., low charge carrier density (2.6 x 10(19) cm(-3)), large thermopower (130 V K-1), large charge carrier mobility (402 cm(2) V-1 s(-1)), and resulting large power factor (29 W cm(-1) K-2). Bi2Te3 films also showed low charge carrier density (2.7 x 10(19) cm(-3)), moderate thermopower (-153 V K-1), but very low charge carrier mobility (80 cm(2) V-1 s(-1)), yielding low power factor (8 W cm(-1) K-2). The low mobilities were attributed to Bi-rich grain boundary phases identified by analytical energy-filtered TEM