Ma Zhong-Yuan

Electron beam evaporation deposition of cadmium sulphide and cadmium telluride thin films: Solar cell applications

Cadmium sulphide (CdS) and cadmium telluride (CdTe) thin films are deposited by electron beam evaporation. Atomic force microscopy (AFM) reveals that the root mean square (RMS) roughness values of the CdS films increase as substrate temperature increases. The optical band gap values of CdS films increase slightly with the increase in the substrate temperature, in a range of 2.42–2.48 eV. The result of Hall effect measurement suggests that the carrier concentration decreases as the substrate temperature increases, making the resistivity of the CdS films increase. CdTe films annealed at 300°C show that their lowest transmittances are due to their largest packing densities. The electrical characteristics of CdS/CdTe thin film solar cells are investigated in dark conditions and under illumination. Typical rectifying and photovoltaic properties are obtained.

Capacitance characteristics of metal-oxide-semiconductor capacitors with a single layer of embedded nickel nanoparticles for the application of nonvolatile memory

This paper reports that metal-oxide-semiconductor (MOS) capacitors with a single layer of Ni nanoparticles were successfully fabricated by using electron-beam evaporation and rapid thermal annealing for application to nonvolatile memory. Experimental scanning electron microscopy images showed that Ni nanoparticles of about 5 nm in diameter were clearly embedded in the SiO2 layer on p-type Si (100). Capacitance–voltage measurements of the MOS capacitor show large flat-band voltage shifts of 1.8 V, which indicate the presence of charge storage in the nickel nanoparticles. In addition, the charge-retention characteristics of MOS capacitors with Ni nanoparticles were investigated by using capacitance–time measurements. The results showed that there was a decay of the capacitance embedded with Ni nanoparticles for an electron charge after 104 s. But only a slight decay of the capacitance originating from hole charging was observed. The present results indicate that this technique is promising for the efficient formation or insertion of metal nanoparticles inside MOS structures.

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.

The Effect of Multiple Interface States and nc-Si Dots in a Nc-Si Floating Gate MOS Structure Measured by their G—V Characteristics

An nc-Si floating gate MOS structure is fabricated by thermal annealing of SiNx/a-Si/SiO2. There are nc-Si dots isolated by a-Si due to partial crystallization. Conductance-voltage (G—V) measurements are performed to investigate the effect of multiple interface states including Si-sub/SiO2, a-Si related (as-deposited sample) and nc-Si (annealed sample) in a charge trapping/releasing process. Double conductance peaks located in the depletion and weak inversion regions are found in our study. For the as-deposited sample, the Si-sub/SiO2 related G—V peak with weak intensity shifts to the negative as test frequency increases. The a-Si related G—V peak with strong intensity shifts slightly with the increasing frequency. For the annealed sample, little change appears in the intensity and shift of Si-sub/SiO2 related G—V peaks. The position of a-Si/nc-Si related peak is independent of frequency, and its intensity is weaker compared to that of the as-deposited sample. It is also found that as the size of nc-Si becomes larger, the a-Si/nc-Si related peak shifts to the depletion region due to the size effect of nc-Si.

Discrete Charge Storage Nonvolatile Memory Based on Si Nanocrystals with Nitridation Treatment

A nonvolatile memory device with nitrided Si nanocrystals embedded in a Boating gate was fabricated. The uniform Si nanocrystals with high density (3 × 1011 cm−2) were deposited on ultra-thin tunnel oxide layer (~ 3 nm) and followed by a nitridation treatment in ammonia to form a thin silicon nitride layer on the surface of nanocrystals. A memory window of 2.4 V was obtained and it would be larger than 1.3 V after ten years from the extrapolated retention data. The results can be explained by the nitrogen passivation of the surface traps of Si nanocrystals, which slows the charge loss rate.

Strong Green Light Emission from Low-Temperature Grown a-SiNx:H Film after Different Oxidation Routes

Room-temperature deposited amorphous silicon nitride (a-SiNx:H) films exhibit intense green light emission after post-treated by plasma oxidation, thermal oxidation and natural oxidation, respectively. All the photolumines-cence (PL) spectra are peaked at around 500 nm, independent of oxidation method and excitation wavelength. Compared with the PL results from oxidized a-Si:H and as-deposited a-SiNx :H samples, it is indicated that not only oxygen but also nitrogen is of an important role in enhancing light emission from the oxidized a-SiNx:H. Combining the PL results with the analyses of the bonding configurations as well as chemical compositions of the films, the strong green light emission is suggested to be from radiative recombination in luminescent centres related to N–Si–O bonds.

Electronic Properties of Nanocrystalline-Si Embedded in Asymmetric Ultrathin SiO2 by In-Situ Fabrication Technique

Structures of nanocrystalline-Si (nc-Si) sandwiched between two asymmetric ultrathin SiO2 layers were fabricated. The nc-Si (dot density of 1011 cm−2) was formed by decomposition of hydrogen-diluted silane and the ultrathin SiO2 layers (about 2 nm) were prepared by plasma oxidation at a lower temperature (250 degrees C). The whole fabrication processes were completed in situ in a plasma-enhanced chemical vapour deposition system. By using the capacitance–voltage (C–V) and conductance–voltage (G–V) spectroscopy, we studied the electronic properties of the annealed samples. The experimental results show that there are distinct capacitance peaks and conductance plateau or peaks for annealed samples at room temperature, which can be explained by direct tunnelling of electrons into the nc-Si. At the same time, Coulomb blockade plays an important role in the electronic transport in the nc-Si. The effect of thermal annealing in N2 ambient on the electronic properties was studied and the results indicate that high temperature (1000 degrees C) annealing can improve the size uniformity of the nc-Si prepared by decomposition of hydrogen-diluted silane.

Two-Dimensional Cavity Resonant Modes of Si Based Bragg Reflection Ridge Waveguide

Si-based ridge-waveguides with Bragg reflectors are fabricated based on our method. Three resonant peaks could be obviously identified from the photoluminescence spectra, and field patterns of these resonant peaks, simulated by the finite difference time domain (FDTD) method, confirm that these peaks originate from cavity resonances. The resonant wavelengths and spatial angular distribution are given by the resonant models, which agree well with the experimental data. Experimentally, a simple method is proposed to testify the experimental and theoretical results. Such devices based on Bragg reflectors may have potential applications in light-emitting diodes, lasers and integrated photonic circuits.

The origin of blue photoluminescence from nc-Si/SiO2 multilayers

Intensive blue photoluminescence (PL) was observed at room temperature from the nanocrystalline-Si/SiO2 (nc-Si/SiO2) multilayers (MLs) obtained by thermal annealing of SiO/SiO2 MLs for the first time. By controlling the size of nc-Si formed in SiO sublayer from 3.5 to 1.5 nm, the PL peak blueshifts from 457 to 411 nm. Combining the analysis of TEM, Raman and absorption measurement, this paper attributes the blue PL to multiple luminescent centres at the interface of nc-Si and SiO2.

Correlation between Light Emissions from Amorphous-Si:H/SiO2 and nc-Si/SiO2 Multilayers

We investigate the properties of light emission from amorphous-Si:H/SiO2 and nc-Si/SiO2 multilayers (MLs). The size dependence of light emission is well exhibited when the a-Si:H sublayer thickness is thinner than 4 nm and the interface states are well passivated by hydrogen. For the nc-Si/SiO2 MLs, the oxygen modified interface states and nanocrystalline silicon play a predominant role in the properties of light emission. It is found that the light emission from nc-Si/SiO2 is in agreement with the model of interface state combining with quantum confinement when the size of nc-Si is smaller than 4 nm. The role of hydrogen and oxygen is discussed in detail.