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.     

Diffusion of chromium in thin hydrogenated amorphous silicon films

The diffusion of a chromium bottom contact has been studied through thin 10-nm amorphous silicon film. The concentration of the diffused impurity has been analyzed by an X-ray photon spectroscopy technique and the diffusion coefficient was estimated. Diffusion annealing was carried out in vacuum (10^?6 mTorr), the temperature was kept at 400°C, and the annealing time was varied from 0 to 300 min. The authors propose that diffusion of chromium in thin hydrogenated amorphous film is limited by silicide formation at the metal-silicon interface.     

1/f noise in amorphous silicon and hydrogenated amorphous silicon thin films

Excess noise measurements have been carried out on either sputtered a-Si or sputtered a-Si:H thin films, at 300 K and in the absence of light. The dependence of excess noise amplitude on hydrogen partial pressure in the sputtering chamber during the film growth has been studied. Investigations of 1/f noise in B-doped a-Si:H thin films have also been carried out. The results of this study show that1/f noise can be correlated to both the hydrogen content in a-Si:H films and to the doping of the films. The noise is explained in terms of fluctuations in the number of gap states due to thermally activated configurational changes involving hydrogenated bonds and dangling bonds.     

Lithium doping of amorphous silicon

Interstitial doping of glow discharge a-Si with lithium leads to conductivity values comparable to phosphorus doping and offers the advantage that it is applied after the preparation of the films. Results of thermopower and conductivity measurements of an amorphous silicon film doped stepwise by Li implantation are presented. Analysis of the data indicates that the electronic transport is governed by a considerable nonlinear statistical shift of the Fermi level and an activated carrier mobility. From the position of E_F at zero temperature, determined by extrapolation, the density of localized states is inferred. Good agreement is found with published results from field effect measurements.     

非晶硅薄膜的YAG激光晶化工艺研究

应用YAG激光器在不同工艺条件(激光脉冲频率及脉宽)下对非晶硅薄膜进行了微晶化处理。采用XRD和AFM对所制薄膜的物相结构和表面形貌进行了分析,并探索了激光脉冲占空比对非晶硅薄膜晶化的影响。结果表明,非晶硅薄膜在不同激光脉冲占空比情况下的结晶变化趋势均为多晶硅衬底表层先非晶化,后与非晶硅薄膜一起结晶,而利于其结晶的最佳占空比为1/25。已晶化硅薄膜的晶粒尺寸随占空比的增加先变大后变小。     

直流磁控溅射制备非晶硅薄膜的研究

非晶硅薄膜（a-Si）是目前重要的光敏材料,在很多领域得到广泛应用。直流磁控溅射具有工艺简单．沉积温度低等优点,是制备薄膜的一种重要技术。采用直流磁控溅射工艺在玻璃基板上沉积薄膜,并对样品进行了退火处理。研究了沉积速率与溅射功率的关系。结果表明薄膜的沉积速率与溅射功率近似有线性关系。利用X射线衍射（XRD）对薄膜进行了分析鉴定,结果表明溅射的薄膜是非晶硅薄膜。利用扫描电子显微镜（SEM）对非晶硅薄膜的表面形貌进行了观察和分析,与X射线衍射测试的结果一致。所以．利用直流磁控溅射工艺能在常温下能快速制备出良好的非晶硅薄膜。     

直流磁控溅射制备非晶硅薄膜的研究

非晶硅薄膜(a-Si)是目前重要的光敏材料,在很多领域得到广泛应用.直流磁控溅射具有工艺简单,沉积温度低等优点,是制备薄膜的一种重要技术.采用直流磁控溅射工艺在玻璃基板上沉积薄膜,并对样品进行了退火处理.研究了沉积速率与溅射功率的关系.结果表明薄膜的沉积速率与溅射功率近似有线性关系.利用X射线衍射(XRD)对薄膜进行了分析鉴定,结果表明溅射的薄膜是非晶硅薄膜.利用扫描电子显微镜(SEM)对非晶硅薄膜的表面形貌进行了观察和分析,与X射线衍射测试的结果一致.所以,利用直流磁控溅射工艺能在常温下能快速制备出良好的非晶硅薄膜.     

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.     

Characterization and simulation analysis of laser-induced crystallization of amorphous silicon thin films

The effect of laser energy density on the crystallization of hydrogenated amorphous silicon (a-Si:H) thin films was studied theoretically and experimentally. The thin films were irritated with a frequency-doubled (λ=532 nm) Nd:YAG pulsed nanosecond laser. An effective finite element model was built to predict the melting threshold and the optimized laser energy density for crystallization of intrinsic amorphous silicon. Simulation analysis revealed variations in the temperature distribution with time and melting depth. The highest crystalline fraction measured by Raman spectroscopy (84.5%) agrees well with the optimized laser energy density (1000 mJ/cm²) in the transient-state simulation. The surface morphology of the thin films observed by optical microscopy is in fairly good agreement with the temperature distribution in the steady-state simulation.     

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.     

Conductivity and noise in thin films of nonhydrogenated amorphous silicon in the hopping regime

Excess noise measurements have been carried out in nonhydrogenated amorphous silicon material at room temperature. The temperature dependence of the conductivity has shown that, at room temperature, noise measurements deal with transport mechanisms dominated by hopping to nearest neighbors. At lower temperature there is variable range hopping from which the number of states near the Fermi level was determined. The ohmic behavior of structures such as molybdenum-amorphous silicon-molybdenum, and the V² dependence of the noise spectral density indicate that $\text{1}\text{f}$ noise is characteristic for this material. A simple theory of phonon assisted hopping $\text{1}\text{f}$ noise is presented. From this the fluctuations. Some preliminary data are shown for hydrogenated amorphous silicon. The noise shows a Lorentzian, indicative of generation-recombination noise.     

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。     

低应力非晶硅薄膜的制备

本文研究了三种降低非晶硅材料应力的手段:优化工艺参数.通过N2退火,将非晶硅的压应力变为张应力.通过额外的掺杂硼来改变非晶硅的应力.最初加入硼掺杂后应力会发生突变,但随着同步掺杂的浓度增加,最终的应力变化会趋向缓和.通过综合利用以上三个手段,最终的实际结果达到了预定的低应力目标.     

The effects of extended heat treatment on Ni induced lateralcrystallization of amorphous silicon thin films

The effects of extended heat treatment on the rate of metal induced lateral crystallization (MILC) of amorphous silicon (a-Si) were investigated. Orientation image microscopy and transmission electron microscopy were employed to reveal the crystallinity of the thin film and to measure the MILC length. It was found that for circular Ni disc patterns, the radial dimensions of the resulting MILC rings increased with the radii of the Ni discs. The longest MILC lengths were obtained from straight-edged Ni patterns, which effectively had infinite radii of curvature. The MILC rate decreased upon extended heat treatment. One reason is the continuously changing state of the a-Si during the treatment. An additional reason could be the diminishing supply of Ni from the Ni covered area. The contribution of both to the reduction of the MILC rate is discussed     

Conductivity of thin nanocrystalline silicon films

It is shown in this paper that thin (200–250 Å) hydrogenated nanocrystalline silicon films have low longitudinal conductivity, comparable to that of undoped amorphous silicon, and high transverse conductivity. These films can be used as doping layers in barrier structures with low surface current leakage. It was found that film conductivity decreases by 8–10 orders of magnitude along the layer as the layer thickness is reduced from 1500 to 200 Å. The observed dependence of the conductivity on thickness can be explained (in terms of percolation theory) by destruction of a percolation cluster made up of nanocrystallites as the layer thickness is decreased.     

Hydrogenated amorphous silicon thin film transistor fabricated onplasma treated silicon nitride

We have demonstrated that the performance of the inverted staggered, hydrogenated amorphous silicon thin film transistor (a-Si:H TFT) is improved by a He, H₂, NH₃ or N₂ plasma treatment for a short time on the surface of silicon nitride (SiN _x) before a-Si:H deposition. With increasing plasma exposure time, the field-effect mobility increase at first and then decrease, but the threshold voltage changes little. The a-Si:H TFT with a 6-min N₂ plasma treatment on SiN_x exhibited a field effect mobility of 1.37 cm²/Vs, a threshold voltage of 4.2 V and a subthreshold slope of 0.34 V/dec. It is found that surface roughness of SiN_x is decreased and N concentration in the SiN _x at the surface region decreases using the plasma treatment     

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。     

Properties of thin LPCVD silicon oxynitride films

Thin, uniform silicon oxynitride films with films thicknesses of ≤ 10 nm were successfully deposited by low pressure chemical vapor deposition (LPCVD). The reactant gases were SiH₂Cl₂, N₂O, and NH₃. The compositional uniformity of these films as a function of depth was good. The structure of these oxynitride films was found to be dominated by the mixed matrix of Si, N, and 0, rather than a physical mixture of SiO₂ and Si₃N₄ clusters. N-H bonding was observed and the total amount of hydrogen in the as-deposited film was on the order of 5 x l0²⁰/cm³. No H-OH or Si-OH bonds were detected. Excellent dielectric breakdown distributions were found for oxynitride films with equivalent oxide film thicknesses as low as 7.5 nm. The conduction of Si-N-0 films depended on film composition. A small capacitor-voltage (C-V) window (< 0.1 V) was observed for the Si-N-O/Si structures. The midgap surface state density was on the order of 5 x 10¹⁰/cm² /eV. Either trapping of holes or the generation of positive states were found after high field stressing of the oxynitride films.     

Copper gate hydrogenated amorphous silicon TFT with thin bufferlayers

We demonstrated a Cu gate hydrogenated amorphous silicon thin-film transistor (TFT) with buffer layers. We introduced an AlN/Cu/Al₂ O₃ multilayer for a gate of an a-Si:H TFT. The Al₂ O₃ improves the adhesion to glass substrate and AlN protect the Cu diffusion to the TFT and plasma damage to Cu during plasma enhanced chemical vapor deposition of silicon-nitride. An a-Si:H TFT with a Cu gate exhibited a field effect mobility of 1.18 cm² V/s, a gate voltage swing of 0.87 V/dec., and a threshold voltage of 3.5 V     

Optoelectronic study in porous silicon thin films

A photoconductivity (PC) study in as deposited porous silicon (PS) thin films is presented in this work. PS thin films were produced by the electrochemical anodizing method at different anodizing times. The films surfaces were characterized by SEM and porosity was determined by gravimetric methods. Photoluminescence and PC measurements were taken at room temperature. The maximum of the photoluminescence spectra are located around 650 nm, whereas those of PC are placed around 400 nm. The maximum of the photoluminescence signal shifts toward short wavelengths as the quantum dimension of the material skeleton diminishes, while any spectral displacement of the photocurrent signal as the porosity of the material increases is not observed. The spectral position of the PC signal does not change because it is strongly affected by the large quantity of defects present in the sample surface which diminishes the mean free path of the carriers to reach the electrodes. In all the samples photocurrent is small around 10^?1 μA and the intensity of the signal goes down as the porosity increases. Two mechanisms exist that compete with one another, the carrier generation and recombination through light emission centers which diminish the photocurrent.