Yan Qing Guo

Improvement of the electrical properties of nanocrystalline silicon films by the KrF pulsed excimer laser irradiation method

The effects of laser fluence on the microstructure and electrical properties of nanocrystalline silicon films were investigated on samples prepared by a plasma-enhanced chemical vapor deposition technique. It was found that the content of hydrogen in the films could be reduced gradually by increasing the laser fluence, accompanied by the formation of nc-Si. The evolution of hydrogen is responsible for a change in the optical band gap and dark conductivity. By controlling the laser fluence at 1.0xa0Jxa0cm−2, a dark conductivity as high as 5.9xa0×xa010−3xa0Sxa0cm−1 could be obtained. Based on measurements of the temperature-dependent conductivity, the carrier transport processes are discussed. The effusion of hydrogen and the increase of crystallinity are thought to be contributed to the high dark conductivity and low conductivity activation energy.

Nanocrystalline Silicon Films Grown at High Pressure in Very High Frequency Plasma Enhanced Chemical Vapor Deposition System

Nanocrystalline silicon films have been fabricated from SiH4 diluted with H2 in very high frequency (40.68 MHz) plasma enhanced chemical vapor deposition system at low temperatures (250oC). The influence of pressure on the structural properties of nanocrystalline silicon films has been investigated. The experimental results reveal that a very high hydrogen dilution is needed to crystallize the film grown at high pressure. If the hydrogen dilution is not high enough, the film could also be crystallized through lowering the pressure. Furthermore, the crystallinity and grain size increase with decreasing the pressure. These results could be attributed to the increase of ion bombardment energy and the higher atomic hydrogen flux toward the growing film surface at lower pressures.

Investigation of Structural, Optical an Electrical Characterization of AZO Films Used for Solar Cells

Transparent conductive Al-doped ZnO (AZO) thin films used as transparent conductive oxide (TCO) electrode layers for thin film solar cells were deposited on quartz substrate by radio frequency (RF) magnetron sputtering. After deposition, the samples were annealed at various temperatures. The structural, electrical, and optical properties of these films have been analyzed as a function of the annealing temperature. All of these samples exhibit strong (002) diffraction peaks and the average transmittance in visible range is about 85%. The crystallinity of films together with some changes of the electrical resistance has been improved after thermal annealing. The best characteristics have been obtained at 400 °C, where the lowest resistivity is 2.4×10-3 Ω cm and the optical band gap is 3.22 eV.

Evolution of Defect Photoluminescence in Annealed N-Rich a-SiNx:H Films

The N-rich a-SiNx:H films were deposited on Si substrates by very high frequency plasma enhanced chemical vapor deposition technique at a low temperature of 250 °C. Strong blue photoluminescence (PL), which originates from nitrogen dangling bond, can be observed in the as-deposited samples. The dependence of defect PL on annealing temperature was systematically investigated. The PL spectra reveal that the PL peak is almost independent of the annealing temperatures while the luminescence intensity rapidly decreases with the annealing temperature increasing from 400°C to 600°C. However, higher annealing temperatures over 700 °C results in an enhancement of luminescence intensity. Based on the relationship between PL spectra and bonding configurations of the samples, the evolution of defect PL with annealing temperatures was briefly discussed.

Abnormal Capacitance Hysteresis Phenomena in Stacked Nanocrystalline-Si Based Metal Insulator Semiconductor Memory Structure

Stack nanocrystalline-Si (nc-Si) based metal insulator semiconductor memory structure was fabricated by plasma enhanced chemical vapor deposition. The doubly stacked layers of nc-Si with the thickness of about 5 nm were fabricated by the layer-by-layer deposition technique with silane and hydrogen mixture gas. Capacitance-Voltage (C-V) measurements were used to investigate electron tunnel and storage characteristic. Abnormal capacitance hysteresis phenomena are obtained. The C-V results show that the flatband voltage increases at first, then decreases and finally increases, exhibiting a clear deep at gate voltage of 9 V. The charge transfer effect model was put forward to explain the electron storage and discharging mechanism of the stacked nc-Si based memory structure. The decreasing of flatband voltage at moderate programming bias is attributed to the transfer of electrons from the lower nc-Si layer to the upper nc-Si layer.

Photoluminescence Properties of Silicon Nitride Prepared by VHF-PECVD

Since the visible photoluminescence (PL) in porous Si was observed by Canham, much attention has been paid to the light emission from silicon-based materials. In this work, luminescent amorphous silicon nitride films were prepared by very-high-frequency plasma enhanced chemical vapor deposition technique using ammonia, silane and hydrogen as source gases at a low temperature of 50 °C. It is found that the films exhibit strong visible light emissions with ranging from green to red region. Photoluminescence spectra show that the emission peaks as well as intensity strongly depends on the flow rates of ammonia. Combining with the analyses of Fourier transform infrared absorption spectra and the transmission spectra, it is suggested that the light emissions are originated from the radiative recombination in the band-tail states of amorphous silicon nitride.

Microstructures and Photoluminescence of a-Si:H/a-SiNx Multilayers Annealed at Different Temperature

Series of a-Si:H/a-SiNx multilayers were prepared by very high frequency plasma enhanced chemical vapor deposition system. As-deposited samples were thermally annealed at the various temperatures. The effects of thermal annealing on the properties of luminescence were investigated. The photoluminescence intensity of the film annealed at 600 °C is found to be higher than that of the film without annealing. However, with further increasing the annealing temperature from 600 °C to 800 °C, the photoluminescence intensity of the film rapidly decreases. Fourier transform infrared spectroscopy and Raman-scattering spectroscopy were used to study the changes of the microstructures and bonding configurations. Based on the measurements of structural and bonding configurations, the improved photoluminescence intensity is attributed to the forming of radiative defect states caused by the effusion of hydrogen in the films.

Influence of Radio Frequency Power on the Structural Properties of nc-Si Films Fabricated by VHF-PECVD

In recent years, hydrogenated nanocrystalline silicon (nc-Si:H) film has received much attention due to its potential application in various optoelectronic devices. In the present work, nanocrystalline silicon (nc-Si) films were fabricated from SiH4 diluted with H2 in very high frequency (40.68 MHz) plasma enhanced chemical vapor deposition system. The influence of radio frequency (rf) power on the structural properties of nanocrystalline silicon films has been studied. Raman spectra show that the crystallinity of the nc-Si films can be increased by promoting the rf power. But over high rf power leads to the structural deterioration of nc-Si:H film. AFM images manifest that, with the increase of deposition time, the grain size becomes larger accompanied by the decrease of the number density.

Correlation of Structural and Optical Properties of nc-Ge/a-Si Multilayers Grown by Ion Beam Sputtering

The nc-Ge/a-Si multilayer structures were fabricated by ion beam sputtering technique on silicon substrates at temperature of 400 °C. Raman scattering spectroscopy, atomic force microscopy (AFM) and room temperature photoluminescence were used to characterize the structure and optical property of the samples. It was found that the nc-Ge/a-Si multilayer sample can be obtained when the Ge sublayer is 3 nm. The room temperature photoluminescence was observed and the luminescent peak is located at 685 nm. Compared with the a-Ge/a-Si film, the intensity of PL of the nc-Ge/a-Si multilayer film becomes stronger due to the higher volume fraction of crystallized component.

Photoluminescence Characteristics of Silicon Oxynitride Films at Different Annealing Temperatures

Silicon oxynitride films were deposited in very-high-frequency (40.68 MHz) plasma enhanced chemical vapor deposition (VHF-PECVD), and subsequently annealed between 400 and 1200°C in N2 ambient. The effect of annealing temperature on the PL characteristics of the samples was investigated. The experimental results reveal that a broad PL peak around 430 nm (2.88eV) appears in all of the samples. The maximum intensity of this broad PL peak was obtained in the sample annealed at 400°C. The PL characteristics of the annealed samples were discussed.