ZnO thin films were deposited by atomic layer deposition (ALD) at various temperatures and the resulting electrical and chemical properties were examined. The fraction of O-H bonds in ZnO films decreased from 0.39 to 0.24 with increasing processing temperatures. The O/Zn ratio decreased from 0.90 at 70 • C to 0.78 at 130 • C. The carrier concentration and resistivity changed sharply with decreasing temperature. The ZnO thin film transistors (TFTs) were fabricated at processing temperatures of 70 to 130 • C and the electrical properties of the TFT were as follows: the field-effect mobility ranged from 8.82 × 10 −3 to 6.11 × 10 −3 cm 2 V −1 s −1 , the on/off current ratio ranged from 1.28 × 10 6 to 2.43 × 10 6 , the threshold voltage ranged from −12.5 to 14.7 V and the subthreshold swing ranged from 1.21 to 24.1 V/decade. The electrical characteristics of the ZnO TFT were enhanced as the processing temperature decreased.

Abstract

In this study, ZnO thin films were deposited by atomic layer deposition ͑ALD͒ at various process temperatures. The purpose of this paper was to investigate the controllability of the preferred orientations of ZnO thin films by varying the process temperature and to determine the effect of the preferred orientations on the electrical properties of the films. The process temperature was varied from 70 to 250°C at several increments while the other ALD process parameters were fixed. The deposition rates and uniformities, crystal structures, and electrical properties of these films were evaluated at the various process temperatures. At process temperatures of 70 and 90°C, ZnO thin films showed strong ͑002͒ preferred orientations with cylindrical, fine, columnar crystal structures, almost a 1:1 stoichiometric chemical ratio of Zn to O, and n-type carrier concentrations in the range of 10 16 cm −3 with resistivities of 0.1-1 ⍀ cm. ZnO thin films deposited at temperatures higher than 110°C had wedge-shaped crystal structures, high oxygen deficiencies, and higher n-type carrier concentrations up to 10 20 cm −3 than the films deposited at lower temperatures.

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Various characteristics of dimethyl ether (DME) as an alternative fuel for compression ignition engines were experimentally investigated including its spray characteristics, combustion performance, and emission reduction in a common-rail diesel engine. The spray behavior of DME was analyzed in terms of injection rate, spray development, and spray tip penetration, which were measured by an injection rate meter and a high-pressure spray chamber equipped with a spray visualization system. In addition, the engine performance and indicated mean effective pressure (IMEP), as well as exhaust emissions, including oxides of nitrogen (NO x ), soot, hydrocarbons, and carbon monoxide were measured at various injection and operating cycle parameters. The combustion characteristics of DME fuel were compared with those of conventional diesel fuel in a diesel engine. Experimental results show that DME has an injection delay 0.03 ms shorter and a maximum injection rate 21% higher than those of diesel fuel at a constant injection pressure of 50 MPa and an injection mass of 8 mg/cycle. At a fixed energizing duration and injection pressure, a greater mass of DME was injected than that of diesel fuel. The DME-operated engine produced almost negligible soot emissions but also considerably higher NO x emissions than the engine operated with diesel fuel at a fixed IMEP.