Thermally evaporated tellurium possesses an intriguing crystallization behavior, where an amorphous to crystalline phase transition happens at near‐ambient temperature. However, a comprehensive understanding and delicate control of the crystallization process for the evaporated Te films is lacking. Here, the kinetics and dynamics of the crystallization of thermally evaporated Te films is visualized and modeled. Low‐temperature processing of highly crystalline tellurium films with large grain size and preferred out‐of‐plane orientation ((100) plane parallel to the surface) is demonstrated by controlling the crystallization process. Tellurium single crystals with a lateral dimension of up to 6 µm are realized on various substrates including glass and plastic. Field‐effect transistors based on 5 °C crystallized Te single grains (6‐nm‐thick) exhibit an average effective hole mobility of ≈100 cm2 V−1 s−1, and on/off current ratio of ≈3 × 104.
Tellurium, as an elemental van der Waals semiconductor, has intriguing anisotropic physical properties owing to its inherent 1D crystal structure. To exploit the anisotropic and thickness‐dependent behavior, it is important to realize orientated growth of ultrathin tellurium. Here, van der Waals epitaxial growth of Te on the surface of 2D transition metal dichalcogenides is systematically investigated. Orientated growth of Te with a thickness down to 5 nm is realized on three‐fold symmetric substrates (WSe2, WS2, MoSe2, and MoS2), where the atomic chains of Te are aligned with the armchair directions of substrates. 1D/2D moiré superlattices are observed for the Te/WSe2 heterostructure. This method is extended to the growth of SeTe alloys, providing flexibility for band engineering. Finally, growth of textured Te film is demonstrated on the lower‐symmetry surface of WTe2.
There is a considerable interest in lowering the operating temperature of solid oxide fuel cells. In this respect, (La,Sr)CoO 3 -(La,Sr) 2 CoO 4 dual phase oxides have attracted much attention as cathode materials due to their enhanced oxygen reduction reaction kinetics. The main drawback in lanthanum strontium cobaltite cathodes is Sr-segregation at operating temperatures which causes a sudden degradation in performance. In the current study, this segregation is verified by a specially designed experiment where a (La 0.8 Sr 0.2 )CoO 3 -(La 0.5 Sr 0.5 ) 2 CoO 4 bilayer is deposited and annealed for an extended period of time. Thin film cathodes are then deposited via co-sputtering of (La 0.8 Sr 0.2 )CoO 3 and (La 0.5 Sr 0.5 ) 2 CoO 4 yielding non-crystalline structures with acceptable area specific resistance values at temperatures as low as 575°C. The stability of these cathodes is investigated over an extensive range of compositions (La 0.8 Sr 0.2 )CoO 3 : (La 0.5 Sr 0.5 ) 2 CoO 4 = 0.10:0.90 -0.90:0.10. Prolonged annealing of cathodes at temperature of the same initial area specific resistance shows an exceptionally stable cathode performance as measured by electrochemical impedance spectroscopy responses. It is therefore concluded that co-sputtered (La 0.8 Sr 0.2 )CoO 3 -(La 0.5 Sr 0.5 ) 2 CoO 4 dual phase cathodes with their amorphous/nanocrystalline structures, especially at mid-compositions, provide an extremely stable microstructure with a strong resistance to Sr segregation.
The study of Cd1-xZnxTe (Cadmium Zinc Telluride) bulk-crystal growth and surface processing technology at the Middle East Technical University (METU) began in 2012. The initial R&D efforts were started with the growing of CdZnTe ingots up to a size of 15 mm in diameter in a three-zone vertical Bridgman furnace located in a limited laboratory area of 15 m 2 . Following promising development in terms of single crystal yield and the crystal growth process, a new vertical gradient freeze (VGF) multi-zone furnace setup was designed and developed to accommodate the production of 60 mm diameter CdZnTe ingots. The entire furnace setup is located in a newly founded 90 m 2 laboratory named the METU Crystal Growth Laboratory (METU-CGL) in 2013. The laboratory is fully dedicated to the CdZnTe material growth and surface processing technology. Currently, METU-CGL is capable of producing 60 mm diameter CdZnTe ingots with one large grain and a few small grains. CdZnTe material is continuously grown in order to serve as either a substrate material (Cd0.96Zn0.04Te) for infrared detectors or an active material (Cd0.90Zn0.10Te) for X-ray/Gamma-ray detectors. As a typical yield, 2-3 oriented wafers per radial slice are retrieved from the grown ingots. The target wafer dimensions are 20 mm x 20 mm; however, larger or smaller crystals can be obtained based on the application of interest. The crystalline quality of the produced crystals is way below 50 arcsec of FWHM (Full width at half maximum) values from the DCRC (Double crystal rocking curves) measurements and the EPD (Etch-pit density) values are typically mid-10 4 /cm 2 . Infrared (IR) transmission of the home-grown CdZnTe crystals is exceeding 60% and stays constant within 2-20 µm wavelength interval showing that the crystals have low density of inclusions and precipitates. Not only limited to CdZnTe bulk growth technology, the METU-CGL is also capable of slicing and surface processing technologies including optimized lapping, rough mechanical polishing, and performing final chemo-mechanical polishing steps with extreme care regarding surface roughness and subsurface damage. Achievable surface roughness values of produced wafers are well below 0.5 nm (Rrms). Various state-of-theart characterization techniques including HRTEM (High-resolution transmission electron microscopy) and APT (Atom probe tomography) were conducted to study nanoscale defects in CdZnTe as a material property. This paper reviews many aspects of CdZnTe bulk-growth, surface finishing, and characterization technologies at METU-CGL as well as the laboratory infrastructure itself.
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