Chung Wei Liu

Characterization of the quality of ZnO thin films using reflective second harmonic generation

A polar mirror symmetrical contribution originated from the arrangement of grain boundaries existing in the ZnO film is detected by reflective second harmonic generation pattern. The ordering of ZnO grain boundary is dependent on the kinetic energy of deposited atoms and affects the quality of ZnO films. The net direction of the grain boundary in ZnO film trends toward the [1¯10] direction of Si(111) to reach the minimum grain energy for better quality ZnO film. The polar structure of the mirrorlike boundaries under the optically macroscopic viewpoint presents a correlation with film quality.

Probing Surface Structure Quality of ZnO Nanorods by Second Harmonic Generation

The surface quality of ZnO nanorod after postannealing was inspected by second harmonic generation (SHG) with the assistance of photoluminescence (PL) spectra, and the X-ray photoelectron (XPS) spectroscopy. The SHG is sensitive to the quality of the surface structure, which involves the elimination of surface defects and restructuring. Oxygen (O)-deficient defects and surface defects were generated during the growth of ZnO nanorods. The PL and XPS analyses indicate that the surface defects on ZnO nanorods were reduced at annealing temperatures above 600 °C. The bulk crystal structure was repaired for higher activation temperatures (> 700°C) concluded by X-ray diffraction. Results from SHG revealed the relationship between surface restructuring of ZnO nanorods and annealing temperature.

Noise Properties of Low-Temperature-Grown Co-Doped ZnO Nanorods as Ultraviolet Photodetectors

The growth of vertically aligned cobalt-doped ZnO (Co-ZnO) nanorods on a glass substrate using a low-temperature hydrothermal method is reported. A Co-ZnO nanorod metal-semiconductor-metal ultraviolet photodetector (PD) was also fabricated. The ratio of UV-to-visible rejection of the fabricated PD was approximately 11700 when biased at 1 V with a sharp cutoff at 380 nm. With an incident light wavelength of 380 nm and an applied bias of 1 V, the measured responsivity of the PD was found to be 19.8 A/W. Furthermore, the dark noise equivalent power (NEP) and photo NEP of the fabricated Co-ZnO nanorod MSM PD were 1.3 × 10^-13 and 1.8 × 10^-11 W at the corresponding dark detectivities (D*) and photo D* of 1.1 × 10¹⁴ and 7.3 × 10¹¹ cm · Hz ^0.5 · W ^-1, respectively.

Enhancement in the structure quality of ZnO nanorods by diluted Co dopants: Analyses via optical second harmonic generation

We report a systematic study about the effect of cobalt concentration in the growth solution over the crystallization, growth, and optical properties of hydrothermally synthesized Zn1−xCoxO [0 ≤ x ≤ 0.40, x is the weight (wt.) % of Co in the growth solution] nanorods. Dilute Co concentration of 1 wt. % in the growth solution enhances the bulk crystal quality of ZnO nanorods, and high wt. % leads to distortion in the ZnO lattice that depresses the crystallization, growth as well as the surface structure quality of ZnO. Although, Co concentration in the growth solution varies from 1 to 40 wt. %, the real doping concentration is limited to 0.28 at. % that is due to the low growth temperature of 80 °C. The enhancement in the crystal quality of ZnO nanorods at dilute Co concentration in the solution is due to the strain relaxation that is significantly higher for ZnO nanorods prepared without, and with high wt. % of Co in the growth solution. Second harmonic generation is used to investigate the net dipole dis...

Noise Properties of Fe-ZnO Nanorod Ultraviolet Photodetectors

This letter reports the development of Fe-doped ZnO (FZO) nanorods via a low-temperature hydrothermal method and the fabrication of FZO nanorod metal-semiconductor-metal (MSM) photodetector (PD). The average length and diameter of the FZO nanorods were 4.5 μm and ~ 100 nm, respectively. The ultraviolet-to-visible rejection ratio of the sample was ~ 23 when biased at 1 V, and the fabricated PD was visible-blind with a sharp cut-off at 375 nm. Furthermore, the noise equivalent power and detectivity of the fabricated FZO nanorod MSM PD were 6.27×10^-10 W and 2.12×10¹⁰ cm·Hz^0.5·W^-1, respectively.

Structural evolution of implanted vicinal Si(111) during annealing via analysis of the dipole contribution

The changes in the amplitude and orientation of the dipole at the surface layer of phosphorous (P) implanted vicinal Si(111) show the evolution of its restructuring. The different electronegativities between Si and participated P atoms changed the dipolar configuration of Si surface. The dipolar configuration of implanted vicinal Si(111) would be variant for the different annealing conditions at which silicon recrystallization and P activation occur. Reflective second harmonic generation (RSHG) is a sensitive technique for studying the symmetrical dipole structure on the surface. We offer a model to distinguish dipolar configurations of the top surface from the one of the implanted bulk Si(111) and explain their relative phase variation in the results of RSHG experiment.

The Formation of p-Type ZnO Films by Thermal Diffusion from the Low Energy, High Dose Phosphorus-Implanted Si Substrate

The p-type ZnO film is obtained out of thermal diffusion of phosphorus (P) atoms from the low energy, high dose implanted Si substrate through rapid thermal annealing (RTA). Many nonactivated P atoms exist on the surface of the shallow-implanted Si substrate and easily out-diffuse into the ZnO films at lower RTA temperatures. The concentration of the p-type ZnO reached 1.13 × 10 19 cm -3 . The p-ZnO/n-Si heterojunction was fabricated using the one-step RTA process. This method offers various choices of dopants with low energy, high dose implanting into silicon without considering the restriction of the substrate.

Crystal Orientation Dynamics of Collective Zn dots before Preferential Nucleation

The island nucleation in the context of heterogeneous thin film growth is often complicated by the growth kinetics involved in the subsequent thermodynamics. We show how the evolution of sputtered Zn island nucleation on Si(111) by magnetron sputtering in a large area can be completely understood as a model system by combining reflective second harmonic generation (RSHG), a 2D pole figure with synchrotron X-ray diffraction. Zn dots are then oxidized on the surfaces when exposed to the atmosphere as Zn/ZnO dots. Derived from the RSHG patterns of Zn dots at different growth times, the Zn dots grow following a unique transition from kinetic to thermodynamic control. Under kinetic-favoring growth, tiny Zn dots prefer arranging themselves with a tilted c-axis to the Si(111) substrate toward any of the sixfold in-plane Si<110> directions. Upon growth, the Zn dots subsequently evolve themselves to a metastable state with a smaller tilting angle toward selective <110> directions. As the Zn dots grow over a critical size, they become most thermodynamically stable with the c-axis vertical to the Si(111) substrate. For a system with large lattice mismatch, small volume dots take kinetic pathways with insignificant deviations in energy barriers.

Diluted Magnetic Nanosemiconductor: Fe-Doped ZnO Vertically Aligned Nanorod Arrays Grown by Hydrothermal Synthesis

Room-temperature ferromagnetism (RTFM) was revealed in Fe-doped ZnO vertically aligned nanorod arrays grown by hydrothermal synthesis method. The evolution of RTFM properties and ZnO nanorod quality were studied by different growth temperature. At the growth temperature of 80 °C, the vertically aligned ZnO nanorods were well-formed, and Fe readily substituted for Zn in the ZnO nanorod arrays. The weak room temperature ferromagnetism of the Fe-doped ZnO nanorod arrays was determined by magnetization measurements. The morphology and quality of the Fe-doped ZnO nanorods were examined by many structure and composition analysis tools. The Fe atoms were found to be readily incorporated into the ZnO lattice without any precipitation or segregation of the secondary phase in the vertically aligned ZnO nanorod arrays.

Inspecting the effects of post-annealing on ZnO nanorods by optical second harmonic generation

We report on the effects of post-annealing on ZnO nanorods array using optical second harmonic generation (SHG). The high-quality and nearly vertically aligned ZnO nanorod arrays were deposited on ZnO-seeded glass substrates by aqueous solution method. The SHG result indicated the transformation of oxygen deficiency on ZnO nanorods surface after post-annealing treatment process. The chemical composition of the nanorods was investigated by X-ray photoelectron spectroscopy. The corresponding structural properties of the nanorod arrays were investigated by scanning electron microscopy and X-ray diffraction.