Papers by Keyword: Amorphous Silicon

Abstract: The amorphous silicon (a-Si) grown by plasma enhanced chemical vapor deposition (PECVD) has been widely applied in advanced semiconductor devices. However, it still suffers from the bubble defects when the deposition temperature goes above 450 °C. In this work, we have investigated the influence of underlying materials on the formation of bubbles of a-Si. The a-Si was deposited on different dielectric substrates, including silicon nitrides (SiN) and silicon dioxide (SiO₂), using PECVD technique at a substrate temperature of 500 °C. A large number of bubbles of the a-Si has been observed on the thermal ALD deposited SiN underlayer, and some of them even burst. In contrast, no bubble defects were observed at the a-Si grown on PECVD SiN and PECVD SiO₂ films. Such deviation may be attributed to the quality of the underlying material, which induces the H/H₂ diffusion during the growth of a-Si and results in bubbles. A solution based on the model has been used to suppress the formation of such bubbles. An inserting layer of SiO₂ was introduced in between SiN and a-Si to improve the density of the lower layer material and the adhesion between the two materials. As a result, there is no bubble defects at the surface of a-Si observed using optical microscope. Our work reveals the mechanism of the formation of bubble defects and paves a new method to eliminate the bubbles defects and to form high-quality a-Si, which shows potential in the manufacture of semiconductor devices.

Abstract: Anisotropic periodic relief in form of ripples was formed on surface of amorphous hydrogenated silicon (a-Si:H) films by femtosecond laser pulses with the wavelength of 1.25 μm. The orientation of the surface structures relative to laser radiation polarization vector depended on the number of laser pulses N acting on the film surface. When N = 30, the structures with 0.88 μm period were formed orthogonal to the laser radiation polarization; at N = 750 the surface structures had period of 1.12 μm and direction parallel to the polarization. The conductivity of the laser-modified a-Si:H films increased by 3 to 4 orders of magnitude, up to 3.8·10^–5 (Ω∙cm)^–1, due to formation of nanocrystalline Si phase with a volume fraction from 17 to 30%. Anisotropy of the dark conductivity, as well as anisotropy of the photoconductivity spectral dependences was observed in the modified films due to depolarizing influence of periodic microscale relief and uneven distribution of nanocrystalline Si phase within such laser-induced structure.

Abstract: We demonstrated efficient crystallization of amorphous Si films induced by their direct irradiation with near-IR femtosecond laser pulses coming at sub-MHz repetition rate. Comprehensive analysis of morphology and composition of the laser-annealed film by atomic-force microscopy, Fourier-transform IR, Raman and energy dispersive X-ray spectroscopy as well as numerical modeling of optical spectra confirmed efficient crystallization of amorphous Si and high-quality of the obtained films opening pathway for applications in thin-film solar cells, transistors and displays.

Abstract: A design of a thin-film solar cell based on microcrystalline and amorphous silicon α-Si:H(n-i-p)/μс-Si:O(n-i-p)/μс-Si:H(n-i-p) was proposed. A physical model and software to calculate the functional characteristics of these solar cells were developed. The numerical simulation results show that the efficiency of the optimized thin-film solar cells may reach up to 16.3 %, open circuit voltage 1.96 V, fill factor 78 %. Improved performance of the non-crystalline solar cell is achieved by an increase in absorbance in the visible range 500 – 800 nm to 40 – 60 % and in the near-infrared range of the solar radiation 800 – 1100 nm to 70 – 75 %.

Abstract: The main aim of this work is to analyze the various heat transport mechanisms and their roles in efficiency enhancement of a thin-film solar cell due to embedded metallic nanoparticles at the rear of the cell, from both electrical and thermal aspects. The nanoparticles present deep inside the cell reflect incident radiation which then increases the optical path length for enhanced electricity generation. The increase in the optical path length also tends to induce additional but undesirable thermal heating which reduces the performance of the cells. The relationship between the improved conversion efficiency and the thermal effect is the crucial factor of maximizing the performance of thin-film solar cells and has yet to be explored. An accurate theoretical/numerical modeling is warranted in this case. Here, we present an analysis of combined light propagation and preliminary phonon transport in the cell to study solar-energy deposition and the associated thermal gradient.

Abstract: Deep Level Transient Spectroscopy (DLTS) has been applied to Metal-Insulator-Semiconductor (MIS) capacitors, consisting of a p⁺ or n⁺ a-Si:H gate on an intrinsic i-a-Si:H passivation layer deposited on crystalline silicon n-or p-type substrates. It is shown that the type of gate has a pronounced impact on the obtained spectra, whereby both the kind of defects (dangling bonds at the a-Si:H/(100) c-Si interface (P_b0 defects) or in the amorphous silicon layer (D defects) and their relative importance (peak amplitude) may be varied. The highest trap densities have been found for the p⁺ a-Si:H gate capacitors on an n-type Si substrate. In addition, the spectra may exhibit unexpected negative peaks, suggesting minority carrier capture. These features are tentatively associated with interface states at the p⁺ or n⁺ a-Si:H/i-a-Si:H interface. Their absence in Al-gate capacitors is in support of this hypothesis.

Abstract: We present experiments based on neutron reflectometry in combination with ²⁹Si/^natSi isotope multilayers in order to investigate the self-diffusion in amorphous silicon. Such experiments allow the detection of diffusion processes in the amorphous state on length scales below 10 nm. First results at 650 °C show a continuous decrease of the artificial Bragg peak produced by the multilayer, corresponding to a diffusivity of (1.1 ± 0.4) x 10^-20 m²/s on a length scale of 2 - 7 nm. The diffusivity is not time-dependent for annealing times between 3 min and 1 h. Compared to recent measurements in silicon single crystals by the same method, the diffusivity is higher by a factor of about 10⁵.

Abstract: Silicon could improve soil fertility, promote crop growth, increase plant resistance to pests and harmful elements and improve yields and quality of agricultural products. Therefore, it would be theoretical significance and practical value to improve water and fertilizer conditions and apply extensively silicon fertilizers on the base of understanding silicon properties of soil in greenhouse. In this paper, the representative soils in greenhouse in Liaoning region were used to analyze the silicon change of soil. Results show that the amount of soluble silicon and amorphous silicon in protected soil were higher than those in uncovered soil, whereas the amount of active silicon in greenhouse soil was lower than that in open field. The amount of soluble silicon and amorphous silicon in 0~20cm soil layer were higher than those in 20~40cm soil layer, neither was the amount of active silicon. The amount of soluble silicon, amorphous silicon and active silicon in 0~20cm and 20~40cm soil layer increased first, then decreased with the growth of planting time.

Abstract: The new absolute PDS setup allows to measure simultaneously the absolute values of the optical transmittance T, reflectance R and absorptance A spectra in the spectral range 280 2000 nm with the typical spectral resolution 10 nm in ultraviolet and visible spectral range and 20 nm in the near infrared region. The PDS setup provides the dynamic detection range in the optical absorptance up to 4 orders of magnitude using non-toxic liquid perfluorohexane Fluorinert FC72. Here we demonstrate the usability of this setup on a series of intrinsic as well as doped a-Si:H and a-SiC:H thin films deposited on glass substrates by radio frequency (RF) plasma enhanced chemical vapor deposition (CVD) from hydrogen, silane and methane under various conditions. The increase of the Tauc gap with increasing carbon concentration in intrinsic a-SiC:H was observed. The defect-induced localized states in the energy gap were observed in doped a-Si:H as well as undoped a-SiC:H below the Urbach absorption edge.

Abstract: Contact property of aluminum and 4H-SiC wafer with crystallized amorphous-silicon (a-Si) interlayer was investigated. A phosphorus-doped a-Si layer on SiC wafer was crystallized by annealing at 1377 °C. Good ohmic contact behavior and contact resistivity of 2.1 × 10^-6 Ωcm² were obtained without silicidation annealing process. Furthermore, non-doped crystallized a-Si layer insertion layer also showed ohmic contact property. However, high contact resistivity of 8.2×10^-4 Ωcm² was obtained in the non-doped a-Si sample. X-ray photo-electron spectroscopy analysis suggests that conduction band offset is significantly reduced between crystallized a-Si and SiC wafer. Therefore, a-Si insertion layer is effective for Schottky barrier height decreasing and high doping into Si layer forms low contact resistivity between Al and SiC, indirectly.