Chung-Yuan Kung

Effects of rapid thermal process on structural and electrical characteristics of Y2O3 thin films by r.f.-magnetron sputtering

Abstract Y2O3 thin films have been deposited on (100) Si substrates by r.f.-magnetron sputtering and subsequently submitted to rapid thermal processing (RTP). X-ray examinations show that the sputtered Y2O3 was dominated by the (111) cubic structure. With increasing RTP temperature (>700°C), the crystallinity of films was improved, especially for the intensity of (400) diffraction peak. The as-deposited films show good dielectric properties in terms of a relative dielectric constant of 16.67 and leakage current density of 6×10−7 A cm−2 (at 1.8 MV cm−1). After the RTP treatment, both the dielectric constant and leakage current of Y2O3 were found to decrease. A typical dielectric constant decreased to 14.77 and its leakage current density lowered to 3×10−8 A cm−2 (at 1.8 MV cm−1) for the film annealed at 850°C. The observed behavior of dielectric constant may be due to the intermediate oxide formation between Y2O3 and Si. Capacitance-voltage characteristics confirm that the reduction of leakage current at high electric field comes from the improvement of interface states.

Structure and photovoltaic properties of ZnO nanowire for dye-sensitized solar cells

Aligned ZnO nanowires with different lengths (1 to approximately 4 μm) have been deposited on indium titanium oxide-coated glass substrates by using the solution phase deposition method for application as a work electrode in dye-sensitized solar cells (DSSC). From the results, the increases in length of zinc oxide (ZnO) nanowires can increase adsorption of the N3 dye through ZnO nanowires to improve the short-circuit photocurrent (Jsc) and open-circuit voltage (Voc), respectively. However, the Jsc and Voc values of DSSC with ZnO nanowires length of 4.0 μm (4.8 mA/cm2 and 0.58 V) are smaller than those of DSSC with ZnO nanowires length of 3.0 μm (5.6 mA/cm2 and 0.62 V). It could be due to the increased length of ZnO nanowires also resulted in a decrease in the transmittance of ZnO nanowires thus reducing the incident light intensity on the N3 dye. Optimum power conversion efficiency (η) of 1.49% was obtained in a DSSC with the ZnO nanowires length of 3 μm.

Enhancement of the Schottky barrier height of Au/ZnO nanocrystal by zinc vacancies using a hydrothermal seed layer

Various seed layers were prepared on a Si substrate using the sol-gel (SG) or hydrothermal (HT) method and then ZnO nanocrystal was grown on the seed layer by an HT process. Au/ZnO nanocrystal Schottky diodes (SDs) were fabricated to study the effects of various seed layers on the electrical properties of Au/ZnO SDs. The observations showed that large numbers of Zn vacancies were present near the interface of Au/ZnO with an HT seed layer. The Zn vacancy plays an acceptor-like role, which raises the barrier height of the Au/ZnO SDs to 0.79 eV with a rectifying ratio of more than 8000. Hence, a non-surface-treated Au/ZnO SD was achieved as compared to those of other reported oxygen-plasma treated surfaces. In contrast, oxygen vacancies appear near the interface of Au/ZnO with an SG seed layer. The O vacancy plays a donor-like role, which reduces the barrier height of Au/ZnO, leading to an Ohmic behavior in the I-V characteristics. Zn out-diffusion is found during Au evaporation by of x-ray photoelectron spectroscopy measurements.

Annealing effects of Au nanoparticles on the surface-plasmon enhanced p-Si/n-ZnO nanorods heterojunction photodetectors

The effects of various annealing temperatures (350–550 °C) of Au nanoparticles (NPs) on the surface-plasmon enhanced p-Si/n-ZnO nanorods (NRs) heterojunction photodetectors (HPDs) have been investigated. The photoresponse of the surface-plasmon-mediated HPDs was found to be determined by the extinction band of the Au NPs, the defects of ZnO NRs, and the Schottky-barrier height (SBH) between the Au and ZnO interface. The higher annealing temperature (550 °C) causes more defects in ZnO NRs and lowers the ultraviolet (UV) response of the fabricated p-Si/n-ZnO NRs HPDs. The higher annealing temperature also renders a rougher surface in the Au NPs, thereby leading to destructive interference and hence the narrowest extinction band. In contrast, the modest temperature (450 °C) results in fewer defects in ZnO NRs, the widest extinction band in Au NPs, and the lowest SBH at the Au/ZnO interface. Such a result enhances the UV-to-visible rejection ratio from 439.6 to 6447 as compared to the HPDs without Au NPs. A b...

Influence of Y-doped induced defects on the optical and magnetic properties of ZnO nanorod arrays prepared by low-temperature hydrothermal process

One-dimensional pure zinc oxide (ZnO) and Y-doped ZnO nanorod arrays have been successfully fabricated on the silicon substrate for comparison by a simple hydrothermal process at the low temperature of 90°C. The Y-doped nanorods exhibit the same c-axis-oriented wurtzite hexagonal structure as pure ZnO nanorods. Based on the results of photoluminescence, an enhancement of defect-induced green-yellow visible emission is observed for the Y-doped ZnO nanorods. The decrease of E2(H) mode intensity and increase of E1(LO) mode intensity examined by the Raman spectrum also indicate the increase of defects for the Y-doped ZnO nanorods. As compared to pure ZnO nanorods, Y-doped ZnO nanorods show a remarked increase of saturation magnetization. The combination of visible photoluminescence and ferromagnetism measurement results indicates the increase of oxygen defects due to the Y doping which plays a crucial role in the optical and magnetic performances of the ZnO nanorods.

Using the Surface Plasmon Resonance of Au Nanoparticles to Enhance Ultraviolet Response of ZnO Nanorods-Based Schottky-Barrier Photodetectors

Surface plasmon resonance mediated by Gold (Au) nanoparticles (NPs) was employed to enhance the ultraviolet (UV) response of ZnO nanorod (NR)-based Schottky-barrier photodetectors (SB-PDs). The defect-level emissions of ZnO NRs induce surface plasmon resonance in Au NPs and enhance the electromagnetic field near Au NPs, which excites a great deal of electrons from Au NPs crossing over the barrier height of Au NP/ZnO interface. This causes the band-to-band emission of ZnO (384 nm) is increased by a magnitude of three, and the deep-level emissions (450-700 nm) are drastically decreased as compared to ZnO NRs without coverings of Au NPs. Also, it drastically enhances the UV (340 nm)-to-visible (560 nm) rejection ratio from 115 to 443. The effect of surface plasmon resonance is verified by that the responsivity of SB-PD with the covering of Au NPs exhibits a greater increasing with applied reverse-bias voltage than that without Au NPs.

A research on the persistent photoconductivity behavior of GaN thin films deposited by r.f. magnetron sputtering

Abstract The persistent photoconductivity (PPC) behavior has been characterized in sputtered GaN thin films using the room illumination with a wavelength of 254 nm under a 5-V bias. It was found that the N 2 partial pressure in the sputtering atmosphere has an evident effect on the PPC behavior. The obtained data show that the nitrogen vacancy is the candidate for PPC effect in the sputtered GaN film. At lower N 2 partial pressures, the nitrogen vacancy can be induced and resulted in GaN films with more donor levels as compared with those of samples deposited at higher N 2 contents. An energy band model that can account for the experimental observation of PPC behavior is proposed.

Silicon-Based Solar Cell Fabricated by Metal-Induced Lateral Crystallization of Amorphous Silicon Film

Silicon-based polycrystalline solar cells are first fabricated by metal-induced lateral crystallization in which n-type polycrystalline-silicon (poly-Si) films, processed using nickel-induced amorphous silicon, are grown on p-silicon substrate at 550 °C by furnace annealing. The fabricated n-type poly-Si/p-substrate solar cell exhibits a conversion efficiency of 10.4% and an open-circuit voltage of 0.54 V without any passivation or antireflection coating layers.

Structural Dependence of Photoluminescence and Room-Temperature Ferromagnetism in Lightly Cu-Doped ZnO Nanorods

Well-defined 1-D ZnO and ZnO:Cu semiconductor nanostructures have been fabricated by a low temperature solution process. Cu (0.073 nm) is chosen as a dopant due to the similar ionic radius with Zn (0.074 nm). The radius of ZnO:Cu nanorods observed by FE-SEM is lager than that of pure ZnO which indicates the growth rate of the nanorods can be obviously enhanced by the light doping of Cu. The XRD patterns of both compositions with single diffraction peak (002) show the same wurtzite hexagonal structure. Photoluminescence spectra show a red-shift of the UV emission peak position and a decrease of the luminescence intensity in green-yellow region. From the room temperature hysteresis loop, ferromagnetism is observed and the saturation magnetization decreases with the increase of the Cu concentration for ZnO:Cu nanorods.

Improve the properties of p-i-n α-Si:H thin-film solar cells using the diluted hydrochloric acid-etched GZO thin films

Gallium-doped zinc oxide (GZO) thin films were deposited on glass, and the process parameters are RF power of 50W and working pressure of 5mTorr, and the substrate temperature was changed from roomtemperature to 300°C. At first, the thickness was around 300 nm by controlling the deposition time. The effects of substrate temperature on the crystallinity, lattice constant (c), carrier mobility, carrier concentration, resistivity, and optical transmission rate of the GZO thin films were studied. The 200°C-deposited GZO thin films had the best crystallinity, the larger carrier concentration and carrier mobility, and the lowest resistivity. For that, the thickness of the GZO thin films was extended to around 1000 nm. Hydrochloric (HCl) acid solutions with different concentrations (0.1%, 0.2%, and 0.5%) were used to etch the surfaces of the GZO thin films, which were then used as the substrate electrodes to fabricate the p-i-n α-Si:H thin-film solar cells. The haze ratio of the GZO thin films increased with increasing HCl concentration, and that would effectively enhance light trapping inside the absorber material of solar cells and then improve the efficiency of the fabricated thin-film solar cells.