Huang Xin-Fan

Enhancement of Band Edge Emission from ZnS/Zn(OH)2 Quantum Dots

ZnS and Zn(OH)2 capped ZnS semiconductor quantum dots (QDs) have been synthesized by the colloidal chemical method using inorganic reagents. Transmission electron microscopy and electron diffraction results have shown that the monodispersed ZnS QDs have a diameter of 1-5 nm and the wurtzite structure. The polarities of the precursors and surfactant solvents have shown strong effects on the properties of the photoluminescence for ZnS QDs. For ZnS QDs capped with a wider band gap Zn(OH)2 shell, the surface trap states were passivated and hence the band-edge emissions have been enhanced.

Blue Light Emission from Hydrogenated Amorphous Silicon Carbide Prepared by Xylene Source in Plasma-Enhanced Chemical Vapour Deposition System

We chose xylene C8H10 as carbon source instead of conventional methane CH4 to fabricate a-Si1-xCx: H films using plasma-enhanced chemical vapour deposition method. The optical band gap of this material could be widened to 3.1 eV. And the infrared absorption spectra of our films manifest the existence of aromatic ring in them. Strong blue light emission with a peak wavelength of 490 nm has been observed at room temperature.

Improved Luminescence Properties and Thermal Stability of ZnS Quantum Dots by Organic and Inorganic Passivation

ZnS quantum dots (QDs) synthesized in water and ethanol solutions were coated with polystyrene (PS) and SiO2 shells, respectively. The band edge emission was enhanced by nearly five times after PS coating and by about thirteen times after SiO2 coating, because the surface trap states were removed. From the photoluminescence properties of ZnS QDs coated with PS and SiO2 shells we have detected the improvement of thermal stability. This is due to the fact that the surface passivation can prevent the further growth of the ZnS QDs and the diffusion of oxygen on the surface of ZnS QDs during thermal oxidation.

Hydrogen Passivation Effect on Enhanced Luminescence from Nanocrystalline Si/SiO2 Multilayers

Nanocrystalline Si/SiO2 multilayers are prepared by thermally annealing amorphous Si/SiO2 stacked structures. The photoluminescence intensity is obviously enhanced after hydrogen passivation at various temperatures. It is suggested that the hydrogen trapping and detrapping processes at different temperatures strongly influence the passivation effect. Direct experimental evidence is given by electron spin resonance spectra that hydrogen effectively reduces the nonradiative defect states existing in the Si nanocrystas/SiO2 system which enhances the radiative recombination probability. The luminescence characteristic shows its stability after hydrogen passivation even after aging eight months.

TRANSPORT PROPERTIES OF µc-Si:H FILMS PREPARED BY VERY HIGH HYDROGEN-DILUTED SILANE PLASMA *

Highly hydrogen-diluted silane plasma is used to fabricate microcrystalline silicon films in a plasma-enhanced chemical vapour deposition system. X-ray diffraction and micro-Raman scattering spectroscopy are utilized to characterize their microstructure properties. Dark conductivity and drift mobility are measured by the travelling wave method. With the decreasing gas flow ratio of silane-to-hydrogen from 2% to 0.2%, the crystalline volume fraction and the drift mobility increase at room temperature. Meanwhile, the dark conductivity increases initially and then decreases. The relationship between the microstructures and transport properties is discussed.

Self-focusing matched filter produced by computer-generated hologram

A self-focusing matched filter is proposed in this paper. The filter function is constructed by adding a quadratic phase factor to the conjugate Fourier spectrum of an object to be recognized. The filter is recorded by using computer-generated holographic techniques. Because of the property of self-focusing, the optical path of the recognition system becomes much simpler and is convenient to adjust. The experiments on the pattern recognition are performed. It is shown that this filter is suitable for recognizing the objects with different scales.

Strong Electron Field Emission from Nano-CdS Modified Porous Silicon

A nano-CdS modified porous silicon (nano-CdS/PS) field emitter is fabricated by chemical method at room temperature. The electron field emission characteristics show that the turn-on field for nano-CdS/PS is about 4.0 V/m and the emission current reaches about 20 μA/cm2 at 5.0 V/μm. This emission current is 20 times larger than that of the PS substrate without nano-CdS modification. The strong field emission properties make the nano-CdS/PS field emitter a good candidate for applications in the field of electronic and optoelectronic devices.

Origin of Electron and Hole Charging Current Peaks in Nanocrystal-Si Quantum Dot Floating Gate MOS Structure

The nanocrystal-Si quantum dot (nc-Si QD) floating gate MOS structure is fabricated by using plasma-enhanced chemical vapour deposition (PECVD) and furnace oxidation technology. The capacitance hysteresis in capacitance-voltage (C – V) measurements confirm the charging effect of nc-Si QDs. Asymmetric charging current peaks both for electrons and holes have been observed in current-voltage (I – V) measurements at room temperature for the first time. The characteristic and the origin of these current peaks in this nc-Si QD MOS structure is investigated systematically. Moreover, the charge density (10−7 C/cm2) calculated from the charging current peaks in the I – V measurements at different sweep rates shows that each quantum dot is charged by one carrier. The difference of charging threshold voltages between the electrons and holes charging peaks, ΔVG, can be explained by the quantum confinement effect of the nc-Si dots in size of about 3.5 nm.

Resonant Tunnelling and Storage of Electrons in Si Nanocrystals within a-SiNx/nc-Si/a-SiNx Structures

The a-SiNx/nanocrystalline silicon (nc-Si)/a-SiNx sandwiched structures with asymmetric double-barrier are fabricated in a plasma enhanced chemical vapour deposition (PECVD) system on p-type Si substrates. The nc-Si layer in thickness 5nm is fabricated from a hydrogen-diluted silane gas by the layer-by-layer deposition technique. The thicknesses of tunnel and control SiNx layers are 3nm and 20nm, respectively. Frequency-dependent capacitance spectroscopy is used to study the electron tunnelling and the storage in the sandwiched structures. Distinct frequency-dependent capacitance peaks due to electrons tunnelling into the nc-Si dots and capacitance-voltage (C - V) hysteresis characteristic due to electrons storage in the nc-Si dots are observed with the same sample. Moreover, conductance peaks have also been observed at the same voltage region by conductance-voltage (G - V) measurements. The experimental results demonstrate that electrons can be loaded onto nc-Si dots via resonant tunnelling and can be stored in our a-SiNx/nc-Si/a-SiNx structures.

Preparation of a Single Layer of Luminescent Nanocrystalline Si Structures by Laser Irradiation Method

KrF excimer laser annealing on ultrathin hydrogenated amorphous Si films with various initial Si thicknesses is carried out to obtain a single layer of nanocrystalline Si structures. It is found that Si nanograins can be obtained with the area density as high as 1011 cm−2 under the irradiation with suitable laser fluence. Raman and planar transmission electron microscopy are used to characterize the formation process of Si nanocrystals from amorphous phase. Moreover, a strong photoluminescence is observed at room temperature from well-relaxed nanocrystalline Si structures.