Weizhi Li

Structure and 1/f noise of boron doped polymorphous silicon films

The influence of structure variation on the 1/f noise of nanometric boron doped hydrogenated polymorphous silicon (pm-Si:H) films was investigated. The films were grown by the conventional radio frequency plasma enhanced chemical vapor deposition (PECVD) method. Raman spectroscopy was used to reveal the crystalline volume fraction (X(c)) and crystal size of the pm-Si:H. The measurement of optical and structure properties was carried out with spectroscopic ellipsometry (SE) in the Tauc-Lorentz model. A Fourier transform infrared (FTIR) spectrometer was used to characterize the presence of nanostructure-sized silicon clusters in pm-Si:H film deposited on KBr substrate. The electrical properties of the films were measured using evaporated coplanar nickel as the electrode. A semiconductor system was designed to obtain the 1/f noise of pm-Si:H film as well as that of amorphous and microcrystalline silicon films. The results demonstrate that the 1/f noise of pm-Si:H is nearly as low as that of microcrystalline silicon and much lower than that of amorphous silicon. The disorder to order transition mechanism of crystallization was used to analyze the decrease of noise compared with amorphous silicon.

Preparation of room temperature terahertz detector with lithium tantalate crystal and thin film

Research on room temperature terahertz (THz) detector is essential for promoting the application of THz science and technology. Both lithium tantalate crystal (LiTaO3) and lithium tantalate thin film were used to fabricate the THz detector in this paper. Polishing process were used to reduce the thickness of LiTaO3 crystal slice by chemical mechanical polishing techniques and an improved sol-gel process was used to obtain high concentration LiTaO3 precursor solution to fabricate LiTaO3 thin film. Three dimension models of two THz detectors were set up and the temperature increasing map of two devices were simulated using finite element method. The lowest noise equivalent power value for terahertz detector using pyroelectric material reaches 6.8 × 10−9 W at 30 Hz operating frequency, which is suitable for THz imaging application.

Electron irradiation effects on the properties of heavily phosphorus-doped a-Si : H films prepared from undiluted silane

The effects of 1.0 MeV electron irradiation on the dark conductivity and amorphous network of heavily phosphorus-doped a-Si : H films have been studied. The electron irradiation leads to a strong decrease by about two orders of magnitude in the dark conductivity of heavily phosphorus-doped a-Si : H films and the degradation comes to a saturation. The evolutions of the amorphous silicon network of heavily doped a-Si : H films caused by electron irradiation were investigated by Raman spectroscopy. The observed decrease in the amorphous silicon network order in the short and intermediate range suggests that the electron irradiation induces structural defects in the films. This defect creation also tends to saturate after a long irradiation time.

A wearable and highly sensitive strain sensor based on a polyethylenimine–rGO layered nanocomposite thin film

In recent years, graphene has attracted enormous attention and has been utilized in the investigation of flexible strain sensors due to its prominent mechanical and electrical properties. In this paper, the naturally viscous material polyethylenimine (PEI) and reduced graphene oxide (rGO) were used to fabricate flexible strain sensors by facile chemical layer-by-layer self-assembly (CLS) and thermal reduction methods. The morphology, spectroscopy and thermal properties of the as-prepared sensing films were measured by SEM, AFM, FTIR and TGA. The influences of GO solution concentration and PEI–GO bilayer number on the sensing performance were studied. The optimal sensor obtained a remarkable performance with a high gauge factor (754 under 5% stretch deformation) and an ultralow limit of detection (0.1% strain). A linear relationship between the normalized response of the sensors and the stretch deformation was observed in the low strain range. The proposed sensor achieved durable properties after 500 stretching–relaxing cycles and could work and withstand a strain up to 50%. Moreover, the proposed sensor was able to detect the subtle motion of a knuckle. This research proposed a facile strategy for the large-scale fabrication of a flexible strain sensor with high sensitivity and excellent repeatability.

Preparation and Characterization of LiTaO3 Films Derived by an Improved Sol-Gel Process

In this paper, an improved sol-gel method was suggested to obtain high-concentration LiTaO3 precursor solution for simplified experimental conditions and thicker films, by mixing lithium acetate and tantalum ethoxide in a 1, 2-Propylene glycol solution. Compared to traditional methods, the process was done without weak acidic solution and absolute dry experimental condition. Results of a comparative study of LiTaO3 thin films derived by the improved sol-gel process and a traditional process using 2-methoxy ethanol as solvent were presented. Nano-crystalline LiTaO3 films with rhombohedral structures were formed in both methods after annealing at 650 ° for 5 min. The thickness of each LiTaO3 layer coated onto the substrate increased from 25 nm to 110 nm when 2-methoxy ethanol was replaced by 1, 2-Propylene glycol. LiTaO3 films with a stronger preferential orientation were obtained in 1, 2-Propylene glycol due to its higher boiling point and slower volatilization rate. On the other hand, the diffraction peak intensity of LiTaO3 thin films prepared using 1, 2-Propylene glycol was weaker than that of the films prepared using 2-methoxy ethanol due to decreased times of annealing.

Effects of oxygen content on the microstructures and optical properties of thermochromic vanadium oxide thin films

Reactive direct current magnetron sputtering and in situ thermal oxidation were used to prepare vanadium oxide (VO X ) thin films with different oxygen contents. X-ray diffraction, Fourier transform infrared spectroscopy and a field emission scanning electron microscope were employed to characterize the films. The optical properties of the VO X films at room temperature and 90 °C were investigated by applying an spectroscopic ellipsometer with a three-layer model of BEMA/Brendel–Bormann oscillator/substrate. It was demonstrated that the vanadium–oxygen bonds were strengthened, the film thickness and roughness decreased, while the grain size increased with increasing oxygen content. The increase in oxygen content had the effect of decreasing the near-infrared reflectance and free-electron concentration of the film at 90°C due to the decrease in the amount of VO2.

High-quality silicon nitride films prepared by low-frequency plasma-enhanced chemical vapor deposition

With their excellent mechanical, thermal, and optical properties, silicon nitride (SiNx) films are widely used as both supporting and insulating materials for MEMS structures. Practical applications of SiNx films rely on their film quality, which will affect the performance and stability of the related devices. In general, SiNx films are deposited by low pressure chemical vapor deposition (LPCVD) or high-frequency (13.56 MHz) plasma-enhanced chemical vapor deposition (HF-PECVD). However, up to now, less reports about the deposition of SiNx films by low-frequency plasma-enhanced chemical vapor deposition (LF-PECVD) have been made. This paper reports the preparation of SiNx thin films by PECVD with a low frequency (380 kHz). The process parameters were carefully optimized for the growth of SiNx films. And the thicknesses and refractive indices of SiNx films were characterized by spectroscopic ellipsometry with small mean square error (MSE<2.5) using Tauc-Lorentz fitting model. It was revealed that the thickness uniformity, deposition rate, and wet etching rate of the as-prepared SiNx films strongly depend on the key process parameters, including RF frequency, power, gas flow ratio, deposition temperature and pressure. Our results also indicated that the refractive index of SiNx film can be rationally tuned to be 1.874 ~ 2.145 by LF-PECVD. Moreover, the wet etching rate of SiNx film in a diluted HF solution can be controlled to be 7.4 to 65.9 nm/min, and the deposition rate ranges from 23.5 to 260.8 nm/min. We also experimentally confirmed that the SiNx thin films deposited with low frequency (380 kHz) exhibit better thickness uniformity, higher deposition rate and lower wet etching rate, compared with those deposited with high frequency (13.56 MHz). Particularly, 150-mm-diameter SiNx thin films with high thickness uniformity (thickness nonuniformity <1.0%) were successfully produced in this work. With their tunable physical properties, the LF-PECVD SiNx thin films exhibit great potential in microelectronics and optoelectronics applications. Moreover, the SiNx films prepared by LF-PECVD are compared with those produced by HF-PECVD and DF-PECVD.

Effects of gas temperature on optical and transport properties of a-Si:H films deposited by PECVD

Effects of silane temperature (T g) before glow-discharge on the optical and transport properties of hydrogenated amorphous silicon (a-Si:H) thin films were investigated. The optical measurements show that the refractive index increases with increasing T g. The transport characterizations show that when T g increases, the dark conductivity increases. However, the temperature coefficient of resistance decreases. In addition, after holding at 130°C for 20 h, the resistance variation, ΔR/R, of the films deposited at T g = room temperature (10.8%) is much larger than those deposited at silane temperatures of 80°C (3%) and 160°C (2%). This can be attributed to different rates of defect creation in a-Si:H films caused by various T g.

Phosphorescent molecularly doped light-emitting diodes with blended polymer host and wide emission spectra.

Stable green light emission and high efficiency organic devices with three polymer layers were fabricated using bis[2-(4′-tert-butylphenyl)-1-phenyl-1H-benzoimidazole-N,C2′] iridium(III) (acetylacetonate) doped in blended host materials. The 1 wt% doping concentration showed maximum luminance of 7841 cd/cm2 at 25.6 V and maximum current efficiency of 9.95 cd/A at 17.2 V. The electroluminescence spectra of devices indicated two main peaks at 522 nm and 554 nm coming from phosphor dye and a full width at half maximum (FWHM) of 116 nm. The characteristics of using blended host, doping iridium complex, emission spectrum, and power efficiency of organic devices were investigated.

Design and fabrication of micro-bolometer array with double layers structure

In order to increase the fill factor of small size micro-bolometer, double layer micro-bolometer was designed in this paper with bottom sensitive/ top absorber structure. The deformation and residual stress characters of single layer and double layer structure models were simulated and optimized, and with the optimized results, double layer structure micro-bolometer was fabricated with multifarious semiconductor recipes. The surface image and deformation information of the fabricated micro-bolometer was tested. By using double layer structure, the area of membrane increases by a factor of 1.99 and 3.6 for the “L” shape leg structure and long “S” shape structure, respectively.