Xiang-na Liu

The structure and properties of nanosize crystalline silicon films

Nanosize crystalline silicon films are fabricated by using highly hydrogen‐diluted silane as the reactive gas and activated with rf+dc double‐power sources, in a conventional plasma‐enhanced chemical‐vapor‐deposition system. The structure of the deposited films as studied by means of high‐resolution electron microscopy, Raman scattering spectra, x‐ray‐diffraction pattern, IR transmission spectra, and ultraviolet ray analysis. The results show that there are many novel structural features and new physical properties for these nanosize crystalline silicon films. In particular, it is found that the optical‐absorption coefficient α is higher than that of a‐Si:H and μc‐Si:H films, the room‐temperature conductivity σd has the value of 10−3–10−1 Ω−1u2009cm−1, and the hydrogen content CH in nc‐Si:H films is higher than 30 at.u2009%. The nc‐Si:H films have their peculiar features which are different from both a‐Si:H and μc‐Si:H films.

Study of photoluminescence in nanocrystalline silicon/amorphous silicon multilayers

We report in this letter the observation of visible photoluminescence (PL) at room temperature from hydrogenated nanocrystalline silicon (nc‐Si:H)/amorphous silicon (a‐Si:H) multilayers (MLs) prepared in a plasma enhanced chemical vapor deposition system without any postprocessing. The PL peak wavelength can be controlled, blueshifting from 750 to 708 nm, through reducing the width of the nc‐Si:H sublayers from 4.0 to 2.1 nm. Quantum size effect in nc‐Si:H sublayers of the ML is responsible for the emission above the band gap of bulk crystal Si.

Intense violet-blue photoluminescence in as-deposited amorphous Si:H:O films

Three photoluminescence (PL) bands at 340–370, 400–430, and 740 nm were observed at room temperature in a-Si:H:O films fabricated by plasma enhanced chemical vapor deposition without any postprocessing. The violet-blue emission is very strong and stable, and its intensity is closely related to the oxygen content in the films, which can be controlled by the applied dc biases on the sample substrates during deposition. The first two PL peaks are ascribed to Si–O related species, and the last one to the quantum size effect of the nanocrystallites embedded in the a-Si:H:O matrix.

VISIBLE ELECTROLUMINESCENCE FROM NANOCRYSTALLITES OF SILICON FILMS PREPARED BY PLASMA ENHANCED CHEMICAL VAPOR DEPOSITION

We have observed visible electroluminescence (EL) from silicon nanocrystallites which are embedded in a‐Si:H films prepared in a plasma enhanced chemical vapor deposition system. The EL spectra are in the range of 500–850 nm with two peaks located at about 630–680 and 730 nm, respectively. We found that the intensity of EL peaks is related closely to the conductivity of the deposited films. The carrier conduction path is discussed in terms of the material structural characteristics, and a tentative explanation of the light emission mechanism is proposed.

Photoluminescence of nanocrystallites embedded in hydrogenated amorphous silicon films

We have fabricated light‐emitting nanocrystallites embedded in an a‐Si:H matrix using a conventional plasma‐enhanced chemical‐vapor‐deposition system. It was found that the photoluminescence properties are directly related to the deposition parameters. The quantum size effect model is proposed to explain the photoluminescence. Two structural prerequisites are proposed for this kind of films to exhibit effective light emission: One is an upper limit for mean crystallite size of about 3.4 nm, the other is an upper limit for crystallinity of about 30%.

Magnetic properties enhancement of Nd2Fe14B/α–Fe nanocomposites with a combined addition of Cu and Ti

The effect of combined Cu and Ti additions on the magnetic properties and phase transformation behavior of Nd8Fe86B6 nanocomposites prepared by melt spinning have been investigated. For pure ternary Nd8Fe86B6 ribbons, the α–Fe phase precipitates directly from amorphous phase and its grains grow quickly during crystallization. But for Nd8Fe86B6 ribbons with a combined addition of Cu and Ti, the crystallization temperature is increased by about 30u200a°C and a metastable hexagonal phase with a TbCu7-type structure is formed prior to the α–Fe and Nd2Fe14B phase. When the annealing temperature is increased, a metastable 1:7 phase is decomposed into the α–Fe and Nd2Fe14B phase. At the same time, the growth of α–Fe grains is suppressed and the microstructure becomes more uniform. When the content of Cu reaches 1.0 at.u200a% and that of Ti reaches 0.5 at.u200a%, the optimal magnetic properties iHc=4.8u2009kOe, σr=110u2009emu/g, (BH)max=15u2009MGOe were obtained after annealing at the temperature of 650u200a°C for 8 min. So, a combined addi...

Electron spin resonance in doped nanocrystalline silicon films

Abstract We report studies of electron spin resonance (ESR) and its related defect states in doped nanocrystalline silicon films (nc-Si:H). The samples used, which were prepared by the plasma enhanced CVD method, contain two phases, i.e. nanocrystallites embedded in an amorphous matrix. For phosphorus doped nc-Si:H samples, the measured ESR g-values are 1.9990–1.9991, the line width ΔHpp 40–42xa0G, and the ESR density Nss is of order of 1017xa0cm−3. For boron doped nc-Si:H samples, the measured ESR g-values are 2.0076–2.0078, the ΔHpp is about 18xa0G, and the Nss is of order of 1016xa0cm−3. Considering the micro-structural and conducting characteristics of these kinds of films, we discuss and give explanations to the ESR sources, their ΔHpp and Nss as well. We intend to ascribe the ESR signal in phosphorus doped nc-Si:H to the unpaired electrons in the high density defect states located in the interfaces of nanocrystallites/amorphous matrix, and that in boron doped ones to the unpaired electrons in the valence band-tail states in the a-Si:H tissue of their amorphous matrix.

Temperature dependence of visible photoluminescence from PECVD nanocrystallites embedded in amorphous silicon films

Abstract The temperature dependence of visible range photoluminescence (PL) properties of nanocrystallites embedded in silicon films deposited at the substrate temperature T s = 50–150°C by PECVD method was studied. It was found that there are many differences between them and that of the near-infrared range PL in ordinary a -Si : H films. Combining with the results of photo-absorption studies of these nanocrystalline samples, we discussed the mechanism of their visible range PL. We suggest that the light excitation occurs in both the nanocrystallites core and their surface regions; however, the radiative recombination can only occur between the electrons and holes in the localized band tail states of the crystallite surface regions, which are more disordered than ordinary a -Si : H with their defect states extending more deeply into the band gap.

Intense violet photoluminescence at room temperature in as-deposited a-Si:H:O films

Abstract Three photoluminescence (PL) bands at ∼355, ∼415, and 740 nm were observed at room temperature in a-Si:H:O films fabricated by plasma-enhanced chemical vapor deposition without post-processing. The violet emission is intense and stable, and its intensity is closely related to the oxygen content in the films, which can be controlled by the applied dc bias on the sample substrates during deposition. The first two PL peaks are ascribed to oxygen-related color centers, and the last one to the quantum size effect of the crystallites embedded in a-Si:H:O matrix.