The intrinsic zinc oxide (ZnO) thin films with controllable crystallographic orientation have been synthesized on Si(100) substrates using plasma-enhanced chemical vapor deposition (PECVD) system without any buffer layer. Based on X-ray diffraction (XRD) results, the evolution of crystallographic orientation of ZnO thin films from polar c-plane (0002), polar c-plane and nonpolar m-plane (101̅0) coexist to nonpolar m-plane and a-plane (112̅0) coexist was achieved by a simple factor of controlling synthesized temperature. The plane-view morphological images exhibited that the surface texture and grain shape of ZnO thin films could have evolved from hexagonal to stripelike grains when the ZnO crystallographic orientation changed from perpendicular to parallel to the substrate. The characterization analysis indicated that the zinc precursor [diethylzinc (DEZn), Zn­(C2H5)2] played a key role on the crystallographic orientation evolution of ZnO thin films during the early stage of the growth process because DEZn not only can serve as Zn precursor but also can be employed as passivating agent to influence the crystal growth under different synthesized temperatures. Room-temperature Hall effect measurement showed that intrinsic ZnO thin film with stripelike grains possessed the lowest value of resistivity ∼7.11 × 102 Ω cm, which had an estimated carrier concentration and mobility of about 5.73 × 1014 cm–3 and 15.34 cm2/V s, respectively. The water contact angle (CA) measurement was also provided to determine the surface wettability and surface free energy of ZnO thin films, indicating that CA could be adjusted via different crystallographic orientation of ZnO thin film.

This paper describes a fabrication and characterization of ultraviolet (UV) photodetectors based on Ohmic contacts using Pt electrode onto the epitaxial ZnO (0002) thin film. Plasma enhanced chemical vapor deposition (PECVD) system was employed to deposit ZnO (0002) thin films onto silicon substrates, and radio-frequency (RF) magnetron sputtering was used to deposit Pt top electrode onto the ZnO thin films. The as-deposited Pt/ZnO nanobilayer samples were then annealed at450∘Cin two different ambients (argon and nitrogen) to obtain optimal Ohmic contacts. The crystal structure, surface morphology, optical properties, and wettability of ZnO thin films were analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), photoluminescence (PL), UV-Vis-NIR spectrophotometer, and contact angle meter, respectively. Moreover, the photoconductivity of the Pt/ZnO nanobilayers was also investigated for UV photodetector application. The above results showed that the optimum ZnO sample was synthesized with gas flow rate ratio of 1 : 3 diethylzinc [DEZn, Zn(C2H5)2] to carbon dioxide (CO2) and then combined with Pt electrode annealed at450∘Cin argon ambient, exhibiting good crystallinity as well as UV photo responsibility.

A plasma-enhanced chemical vapor deposition system was used to fabricate ultraviolet (UV) photodetectors based on polar and nonpolar zinc oxide (ZnO) thin films combined with interdigitated platinum top electrodes. The performance of photodetectors was demonstrated by current–voltage characteristics and time-dependent photoresponse measurements. Both polar and nonpolar detectors showed a prominent photocurrent gain under UV light illumination, compared with dark conditions. However, the response and recovery times for the nonpolar detectors were significantly faster compared to the polar detectors. These variations in response and recovery times can be explained by the dipole effect between the electrode and sensing thin film, which is due to the adsorption and desorption of gas molecules on polar and nonpolar ZnO thin film surfaces.

In this study, zinc oxide (ZnO) thin films with high c-axis (0002) preferential orientation have been successfully and effectively synthesized onto silicon (Si) substrates via different synthesized temperatures by using plasma enhanced chemical vapor deposition (PECVD) system. The effects of different synthesized temperatures on the crystal structure, surface morphologies and optical properties have been investigated. The X-ray diffraction (XRD) patterns indicated that the intensity of (0002) diffraction peak became stronger with increasing synthesized temperature until 400 (o)C. The diffraction intensity of (0002) peak gradually became weaker accompanying with appearance of (10-10) diffraction peak as the synthesized temperature up to excess of 400 (o)C. The RT photoluminescence (PL) spectra exhibited a strong near-band-edge (NBE) emission observed at around 375 nm and a negligible deep-level (DL) emission located at around 575 nm under high c-axis ZnO thin films. Field emission scanning electron microscopy (FE-SEM) images revealed the homogeneous surface and with small grain size distribution. The ZnO thin films have also been synthesized onto glass substrates under the same parameters for measuring the transmittance. For the purpose of ultraviolet (UV) photodetector application, the interdigitated platinum (Pt) thin film (thickness ~100 nm) fabricated via conventional optical lithography process and radio frequency (RF) magnetron sputtering. In order to reach Ohmic contact, the device was annealed in argon circumstances at 450 (o)C by rapid thermal annealing (RTA) system for 10 min. After the systematic measurements, the current-voltage (I-V) curve of photo and dark current and time-dependent photocurrent response results exhibited a good responsivity and reliability, indicating that the high c-axis ZnO thin film is a suitable sensing layer for UV photodetector application.

Non-polar coexisting m-plane (10 10) and a-plane (11 20) zinc oxide (ZnO) thin films have been synthesized onto commercial silicon (100) substrates by using plasma enhanced chemical vapor deposition (PECVD) system at different working pressures. The effects of the working pressure on crystal orientation, microstructure, surface morphology, and optical properties of the ZnO thin films were investigated. From the X-ray diffraction patterns, the non-polar ZnO thin films were successfully synthesized at the working pressures of 6 and 9 Torr, respectively. The nonpolar ZnO thin films showed stripes-like surface morphology and with smooth surface roughness (>3.53 nm) was performed by field emission scanning electron microscope (FE-SEM) and atomic force microscope (AFM), respectively. All the ZnO films show a remarkable near-band-edge (NBE) emission peak located at ultraviolet (UV) band accompanying a negligible deep-level (DL) emission at visible region detected by photoluminescence (PL) spectra at room temperature. From the above systematic measurement analysis, indicating the better crystallinity and optical character of ZnO thin film was improved with reducing the working pressure. The wettability of non-polar ZnO thin films was also explored in this presented article.