固体纳秒激光器应用于非晶硅薄膜结晶的研究

多晶硅薄膜比非晶硅薄膜具有更高的电子迁移率,在器件中表现出更优良的性能,脉冲激光结晶非晶硅薄膜制备多晶硅薄膜的方法具有热积存小、对衬底影响小、成本低等优点。使用532 nm固体纳秒激光器进行了非晶硅薄膜激光结晶实验,为了解决直接使用高斯光束结晶时因光斑能量分布带来的结晶效果不均匀,首先基于光束整型系统将圆形的高斯光束整型成为线性平顶光束,而后研究单脉冲能量密度、脉冲个数、非晶硅薄膜厚度对结晶效果的影响。结果表明,线性平顶光束用于非晶硅薄膜结晶具有更好的均匀性,对于100 nm非晶硅薄膜,随着能量密度的增加,晶粒逐渐变大,直到表面出现热损伤,最大晶粒尺寸约为1 μm×500 nm。随着脉冲个数的增加,表面粗糙度有减小的趋势,观察到的最小粗糙度约为2.38 nm。对于20 nm超薄非晶硅薄膜,只有当能量密度位于134 mJ/cm2和167 mJ/cm2之间、脉冲个数大于或等于八个时才能观察到明显的结晶效果。     

Microstructure evolution of amorphous silicon thin films upon annealing studied by positron annihilation

Amorphous silicon (a-Si) thin films were prepared on glass substrates by plasma enhanced chemical vapor deposition (PECVD). Influence of annealing temperature on the microstructure, surface morphology, and defects evolution of the films were studied by X-ray diffraction (XRD), atomic force microscope (AFM) and positron annihilation Doppler broadening spectroscopy (DBS) based on a slow positron beam, respectively. The S parameter of the as-deposited a-Si thin film is high, indicative of amorphous state of Si film with many defects. The a-Si gradually grows into polycrystalline silicon with increasing temperature to 650 °C. For the films annealed below ~450 °C, positron diffusion lengths are rather small because most positrons are trapped in the defects of the a-Si films and annihilated there. With further rising the temperature to 600 °C, the diffusion length of positrons increases significantly due to the removal of vacancy-type defects upon annealing at a high temperature. The results indicate that the coalescence of small vacancy-type defects in a-Si thin film and the crystallization of a-Si occur around 450 °C and 650 °C, respectively.     

Effects of oxygen in Ni films on Ni-induced lateral crystallization of amorphous silicon films at various temperatures

Effects of oxygen in Ni films on the Ni-induced lateral crystallization (NILC) of amorphous silicon (a-Si) films at various temperatures have been investigated. It was found that oxygen in Ni films retarded the nucleation of polycrystalline silicon (poly-Si) from a-Si, but had little effect on the growth rate of poly-Si. This is because that needed an incubation period to be reduced to Ni metal for the subsequent mediated crystallization of a-Si.     

Topography induced preferential crystallization of thermally grown silicon dioxide films

We have investigated the crystallization of the oxide layer that grows on a deposited silicon film in a high temperature furnace. The growth of large SiO₂ crystal grains can be controlled by interfacial stress or surface topography. When the Si film is deposited on topographically patterned surfaces, the SiO₂ grains are nucleated along the edges and extremities of the relief structure. A competition in which faster growing grains terminate slower growing grains results in an average growth direction perpendicular to the edges. Single crystal grains of α-cristobalite up to hundreds of microns in length can be grown in this fashion.     

激光波长对非晶硅薄膜晶化效果的影响

为研究波长对连续激光晶化非晶硅(a-Si) 薄膜过程的影响,利用连续Ar＋-Kr＋激光对a-Si薄膜晶 化,在5ms固定照射时间下,改变激光波长,采用拉曼光谱测试技术和场发射扫描电子显微 镜(SEM)研究在不同 激光功率密度下薄膜晶化后的特性。结果表明,a-Si薄膜的晶化阈值随着波长的 增大而增大,当波长为 458nm时薄膜晶化阈值为13.2kW/cm²,波长 为647nm时,晶化阈值为19.2kW/cm²;在激光功率密度范 围为0～27.1kW/cm²内,薄膜的最大晶化率受波长的影响相对较小 ,但总体也随着波长的增大而呈增大 趋势,当波长为647nm时,在激光功率密度26.5kW/cm²处,晶化率达到最大值75.85%。     

Steady state and transient photoconductivity in amorphous thin films of Se0.8Te0.2

The present paper reports on steady state and transient photoconductivity measurements in amorphous thin films of Se_0.8Te_0.2. The paper also reports the effect of crystallization on the photoconductive behaviour. The photosensitivity(I _ph /I _d ) reduces by a factor of 5 and the decay of photocurrent becomes much slower in the films annealed above the crystallization temperature (state B) as compared to the films annealed below the crystallization temperature (state A). A detailed analysis of the photoconductive decay in the state B shows that the recombination within localized states may be the predominant recombination mechanism in this state.     

BF3-doped amorphous silicon thin films

BF₃ has been used as a p-type dopant in thin films of hydrogenated amorphous silicon (a-Si:H). The films were deposited in a capacitively coupled radio frequency (13.56 MHz) glow discharge system in which silane (SiH₄) was the principal gaseous component. The boron content in the films was measured by secondary ion mass spectroscopy (SIMS) using a calibrated boron-implanted silicon standard. The optical and transport properties of these films were measured and compared with films of similar boron film content deposited using B₂H₆ BF₃-doped films showed less sub-band gap absorption (as determined by photothermal deflection spectroscopy), no band gap narrowing, and higher conductivity prefactors σ_o for low doping levels compared to B₂H₆-doped films. The possible utility of lightly BF₃-doped a-Si:H as the i-layer in a p/i/n photovoltaic device is suggested.     

多晶硅薄膜的新型激光晶体制备方法

采用两步激光晶化方法制备了多晶硅薄膜,其晶粒尺寸为１．１μｍ,比用传统单步晶化制备的薄膜晶粒尺寸大,表明该方法法对扩大晶粒尺寸很有效。拉曼光谱分析表明０．３０Ｊ／ｃｍ＾２晶化的薄膜结晶程度已很高。     

多晶硅薄膜的新型激光晶化制备方法

采用两步激光晶化方法制备了多晶硅薄膜 ,其晶粒尺寸为 1.1μm,比用传统单步晶化制备的薄膜晶粒尺寸大 ,表明该方法对扩大晶粒尺寸很有效。拉曼光谱分析表明 0 .30 J/ cm2晶化的薄膜结晶程度已很高     

Development of highly conducting n-type micro-crystalline silicon oxide thin film and its application in high efficiency amorphous silicon solar cell

Wide band gap and highly conducting n-type nano-crystalline silicon film can have multiple roles in thin film solar cell. We prepared phosphorus doped micro-crystalline silicon oxide films (n-μc-SiO:H) of varying crystalline volume fraction (X_c) and applied some of the selected films in device fabrication, so that it plays the roles of n-layer and back reflector in p-i-n type solar cells. It is generally understood that a higher hydrogen dilution is needed to prepare micro-crystalline silicon, but in case of the n-μc-SiO:H an optimized hydrogen dilution was found suitable for higher X_c. Observed X_c of these films mostly decreased with increased plasma power (for pressure<2.0 Torr), increased gas pressure, flow rate of oxygen source gas and flow rates of PH₃>0.08 sccm. In order to determine deposition conditions for optimized opto-electronic and structural characteristics of the n-μc-SiO:H film, the gas flow rates, plasma power, deposition pressure and substrate temperature were varied. In these films, the X_c, dark conductivity (σ_d) and activation energy (E_a) remained within the range of 0–50%, 3.5×10⁻¹⁰ S/cm to 9.1 S/cm and 0.71 eV to 0.02 eV, respectively. Low power (30 W) and optimized flow rates of H₂ (500 sccm), CO₂ (5 sccm), PH₃ (0.08 sccm) showed the best properties of the n-μc-SiO:H layers and an improved performance of a solar cell. The photovoltaic parameters of one of the cells were as follows, open circuit voltage (V_oc), short circuit current density (J_sc), fill-factor (FF), and photovoltaic conversion efficiency (η) were 950 mV, 15 mA/cm², 64.5% and 9.2% respectively.     

氢等离子体辅助铝诱导晶化非晶硅的研究

本文中提出了一种通过氢等离子体改进和加速铝诱导晶化的工艺方法。通过拉曼散射谱、SIMS测试晶化多晶硅,结果说明了氢等离子体的作用缩短了铝诱导晶化的时间。这项技术使退火时间由10小时缩短到4小时同时使霍尔迁移率从22.1 cm2/V增加到42.5 cm2/V。另外也对氢等离子体辅助铝诱导晶化的可能机理做了讨论。     

Effects of oxygen on crystallization of amorphous silicon films and polysilicon TFT characteristics

Oxygen ions were implanted into the amorphous silicon film deposited at 540°C in order to study the effects of oxygen on the solid phase crystallization of silicon films. The resulting films were investigated using transmission electron microscopy, x-ray diffraction (XRD), and also by measuring the electrical characteristics of polycrystalline silicon thin film transistors (TFTs) fabricated in the crystallized films. The development of {111} texture as a function of annealing time is similar to films implanted with Si, with higher oxygen samples showing more texture. Transmission electron microscopy shows that the grain size of completely crystallized films varies little with oxygen concentration. The electrical performances of TFTs are found to degrade with increasing oxygen dose. The trap state density increases from 5.6 × 10¹²/cm² to 9.5 × 10¹²/cm² with increasing oxygen dose. It is concluded that for a high performance TFT, oxygen incorporation in the Si film should be kept to 10¹⁹/cm³ or less.     

Study on thermal solid-phase crystallization of amorphous ZnO thin films stacked on vanadium-doped ZnO films

Thermal solid-phase crystallization (SPC) of an amorphous ZnO film stacked on a vanadium-doped ZnO (VZO) film was investigated. ZnO films were deposited on 30-nm-thick amorphous VZO films on c-face sapphire substrates at room temperature by RF magnetron sputtering. Stacked film was subsequently calcined at 800 °C in a nitrogen atmosphere. ZnO film grew in an amorphous state up to about 150-nm thick on the amorphous VZO film, but self-orientation occurred in a thicker layer. Any secondary phase such as Zn₂VO₄ was not formed in the case of total film thickness (t_total) ≥100 nm. V concentration decreased by thermal diffusion of V through the ZnO layer from the VZO film, and thereby the formation of secondary phase was effectively avoided. The amorphous ZnO layer was crystallized from highly-aligned initial thin layer of VZO film when t_total ≤200 nm and crystal orientation of the stacked film was superior to single VZO film. However, the c-axis orientation was deteriorated drastically at t_total ≈400 nm due to SPC affected by the tilted regions existed in the as-deposited ZnO film. Therefore, it is suggested that careful selection of ZnO film thickness is necessary to obtain the high-quality ZnO films in this method.     

The effects of hydrogen on aluminum-induced crystallization of sputtered hydrogenated amorphous silicon

The effects of hydrogen on aluminum-induced crystallization (AIC) of sputtered hydrogenated amorphous silicon (a-Si:H) were investigated by controlling the hydrogen content of a-SiH films. Nonhydrogenated (a-Si) and hydrogenated (a-Si:H) samples were deposited by sputtering and plasma-enhanced chemical vapor deposition (PECVD). All aluminum films were deposited by sputtering. Hydrogen was introduced into the sputter-deposited a-Si films during the deposition. After deposition, the samples were annealed at temperatures from 200°C to 400°C for different periods of time. X-ray diffraction (XRD) patterns were used to confirm the presence and degree of crystallization in the a-Si:H films. For nonhydrogenated films, crystallization initiates at a temperature of 350°C. The crystallization of sputter-deposited a-Si:H initiates at 225°C when 14% hydrogen is present in the film. As the hydrogen content is decreased, the crystallization temperature increases. On the other hand, the crystallization initiation temperature for PECVD a-Si:H containing 11at.%H is 200°C. Further study revealed that the crystallization initiation temperature is a function, not only of the total atomic percent hydrogen in the film, but also a function of the way in which the hydrogen is bonded in the film. Models are developed for crystallization initiation temperature dependence on hydrogen concentration in a-Si:H thin films.     

Design of Ag nanograting for broadband absorption enhancement in amorphous silicon thin film solar cells

Light trapping is one of the key issues to improve the light absorption and increase the efficiency of thin film solar cells. The effects of the triangular Ag nanograting on the absorption of amorphous silicon solar cells were investigated by a numerical simulation based on the finite element method. The light absorption under different angle and area of the grating has been calculated. Furthermore, the light absorption with different incident angle has been calculated. The optimization results show that the absorption of the solar cell with triangular Ag nanograting structure and anti-reflection film is enhanced up to 96% under AM1.5 illumination in the 300–800 nm wavelength range compared with the reference cell. The physical mechanisms of absorption enhancement in different wavelength range have been discussed. Furthermore, the solar cell with the Ag nanograting is much less sensitive to the angle of incident light. These results are promising for the design of amorphous silicon thin film solar cells with enhanced performance.     

Laser pattern-write crystallization of amorphous SiC alloys

The aim of this work is to investigate the cw-laser crystallization of amorphous a-Si_1−xC_x alloys as a function of laser power and alloy composition. As the microRaman analysis reveals, many cases occur: in a silicon rich alloy (x ∼0.3) we can obtain two crystalline phases, i.e. polycrystalline Si or polycrystalline C, depending on the laser energy density irradiated on the film. The presence of polycrystalline SiC is observed only in quasi-stoichiometric alloy (x ∼ 0.48) in the cubic β-SiC phase. The experiment has been performed with a laser pattern-writing system that permits simultaneous control of annealing energy and focused spot size. PC control allows several patterns to be traced on the same film.     

Explosive crystallization of rf-sputtered amorphous CdTe films

Explosive crystallization of about 2-μm thick amorphous CdTe films prepared by rf-sputtering in an Ar-N₂ ambient has been observed. The transformation can be initiated with or without external thermal or mechanical impulses depending on the oxygen content of the films. The crystallization is accompanied by a light flash. X-ray diffraction, Auger analysis, and resistivity measurements were made on these films to arrive at a qualitative model to explain the observations.     

Optical characterization of hydrogenated amorphous silicon thin films deposited at high rate

The aim of this paper is to provide a better understanding of hydrogenated amorphous silicon thin films (a-Si:H) in relation to their optical properties: refractive index, optical gap, absorption coefficient, thickness and surface roughness. The transmission spectrum of the films, deposited with various rf discharge power densities by an optimized plasma enhanced chemical vapor deposition (PECVD) method, at a high rate (>10 Å/sec), was measured over a range in wavelength from 500 to 1100 nm. An approximate model is utilized to describe the surface roughness. In this model, the surface roughness is modeled as a mixed layer of 50 percent of a-Si:H and 50 percent of air and the optical constant of the rough layer is derived using the Bruggemann effective medium approximation (EMA). The gradient iteration method of numerical analysis is used to solve the nonlinear equations in the study. Our results show that the potential underestimation of refractive index and resulting overestimation of film thickness can be overcome by considering the reflection of the rough surface. The method is carried out on the transmission data and the influence of rf discharge power density on the properties of the film is discussed in detail.     

用催化CVD法研制优质a—Si薄膜

用新的催化ＣＶＤ法在约３５０℃的低温条件下研制出了ａ－Ｓｉ薄膜。本文是利用催化剂，使ＳｉＨ和Ｈ２混合气体在一定温度下反应而裂解，在衬底上沉积ａ－Ｓｉ薄膜。所制出的ａ－Ｓｉ膜膜具有良好的光电特性，光电灵敏度超过１０＾６，电子自旋密度约２．５×１０＾６ｃｍ＾－３。