Light-induced defect creation in hydrogenated polymorphous silicon during repeated cycles of illumination and annealing

We have examined how the intensity of the ESR signal associated with dangling bonds in hydrogenated polymorphous silicon is affected by repeating light-soaking (LS) and 100°C annealing cycles. It was found that the light-induced degradation efficiency decreases with repeated LS–100°C annealing cycles. This result is accounted for in terms of the termination of dangling bonds by mobile hydrogen, i.e. the termination is enhanced by more mobile hydrogen as a result of modification of the amorphous network by the LS–annealing cycles, and consequently the net light-induced defect creation rate is reduced. This is in contrast with previous models in which the decrease in the light-induced degradation efficiency on repeated LS–annealing cycles is attributed to an increase in the amount of hydrogen being ineffective for light-induced defect creation.     

Investigation of the microstructure and optical properties of hydrogenated polymorphous silicon films prepared with pure silane

The dependences of microstructure and optical properties of hydrogenated polymorphous silicon (pm-Si:H) films on total gas pressure were studied. Instead of using high diluted silane in H₂, pure silane was used as the source gas. The films were grown by the radio-frequency plasma-enhanced chemical vapour deposition method. Fourier-transform infrared spectrometry was used to characterize the presence of Si _m H _n clusters in pm-Si:H film deposited on KBr substrate. Atomic force microscopy (AFM) analysis characterized the morphology of the pm-Si:H films and X-ray diffraction at grazing incidence angle (XRDGI) microstructure analysis also confirmed the existence of Si _m H _n nanocrystalline clusters in pm-Si:H. The thickness and optical constants of the films were measured by spectra ellipsometry as well as scanning electron microscopy. Derived using the Tauc relation, the dependence of optical bandgap, E_g , and coefficient, B, on the pressure during deposition process is discussed. The influence of inter-electrode distance on growth rate and surface smooth was analyzed using AFM.     

Thermally induced defect photoluminescence in hydrogenated amorphous silicon

The intrinsic defect photoluminescence of hydrogenated amorphous silicon (a-Si:H) films has been investigated at high intensities of optical pumping that lead to heating of the film. It has been revealed that, for short heating times, the intensity of the defect photoluminescence increases exponentially with an increase in the temperature with an activation energy of 0.85 eV, which is considerably higher than the activation energy (∼0.2 eV) determined from experiments on classical annealing. This and other experimental results on the temperature dependence of the intensity and kinetics of the defect photoluminescence have been explained in terms of the “hydrogen glass” model by thermally induced generation of intrinsic defects in amorphous silicon. The results of the calculations are in good agreement with the experimental data on the defect photoluminescence that reflects the formation and annihilation of defects for short heating times under optical excitation.     

Ordering of the structure of hydrogenated silicon films under the influence of laser radiation

A study is reported of the crystallization of amorphous hydrogenated silicon films under the influence continuous radiation from an argon laser. The structure was investigated by Raman scattering of light. The radiation power density during the annealing process was 1.5–4.5 kW/cm² and the exposure was 1/125 sec. The Raman spectra were recorded for power densities below 0.1 kW/cm². The power density threshold for the appearance of crystallites was found to be 3.0 kW/cm². The phonon localization model was used to show that the size of the crystallites produced for power densities of 3 kW/cm² was 40 Å.     

Urbach edge and the density of states in hydrogenated amorphous silicon

Photoconductivity measurements of the optical absorption edge and time-of-flight measurements of the hole drift mobility, on the same amorphous silicon film, enable us to identify the origin of the Urbach edge in amorphous silicon with the density of states in the valence band tail.     

Electronic structure and defect states of transition films from amorphous to microcrystalline silicon studied by surface photovoltage spectroscopy

In this paper, surface photovoltage spectroscopy (SPS) is used to determine the electronic structure of the hydrogenated transition Si films. All samples are prepared by using helicon wave plasma-enhanced chemical vapour deposition technique, the films exhibit a transition from the amorphous phase to the microcrystalline phase with increasing temperature. The film deposited at lower substrate temperature has the amorphous-like electronic structure with two types of dominant defect states corresponding to the occupied Si dangling bond states (D⁰/D^- and the empty Si dangling states (D⁺). At higher substrate temperature, the crystallinity of the deposited films increases, while their band gap energy decreases. Meanwhile, two types of additional defect states is incorporate into the films as compared with the amorphous counterpart, which is attributed to the interface defect states between the microcrystalline Si grains and the amorphous matrix. The relative SPS intensity of these two kinds of defect states in samples deposited above 300\du increases first and decreases afterwards, which may be interpreted as a result of the competition between hydrogen release and crystalline grain size increment with increasing substrate temperature.     

氢化硅薄膜光吸收近似特性研究

利用等离子体化学气相沉积系统在直流电压源和射频源的双重激励下,以康宁7059玻璃为衬底制备了氢化硅薄膜．过测定氢化硅薄膜Raman光谱,对薄膜微结构进行了表征;建立氢化硅薄膜的光吸收模型,计算出薄膜的光吸收系数和光学带隙,和实验结果基本一致,说明该模型符合实验结果;并利用该模型计算的光吸收系数和光学带隙,结合AMPS软件对设计的太阳电池结构进行了模拟,给出的I-V特性曲线变化趋势与实验结果基本符合,同时对实验结果与模拟结果存在差异的原因进行了分析,并给出合理解释． 关键词： 氢化硅薄膜 光吸收系数 光吸收模型     

Density of states and charge distribution in hydrogenated amorphous silicon

The local density of states and the charge distribution in a cluster model of hydrogenated amorphous silicon is calculated using the recursion method. The results indicate an average charge transfer of 0.3 electrons from silicon to hydrogen. In addition Si-H dipoles carry small negative charges, which are neutralized by a positively charged bulk of silicon atoms. There are also static spatial charge fluctuations, which are due to the topological disorder.     

非晶微晶过渡区域硅薄膜的微区喇曼散射研究

通过改变氢气对硅烷气体的稀释程度,并保持其他的沉积参量不变,用等离子体增强化学气相沉积(PECVD)方法成功地制备出处于非晶微晶相变过渡区域的硅薄膜样品.测量了样品的室温光电导和暗电导,样品的光电性能优越,在50mW·cm-2的白光照射下,光电导和暗电导的比值达到106.在室温下用微区喇曼谱研究了薄膜的微结构特性,用高斯函数对喇曼谱进行了拟合分析.结果表明,在我们的样品制备条件下,当H2和SiH4的流量比R较小时,样品表现出典型的非晶硅薄膜的结构特性;随流量比R的增大,薄膜表现出两相结构,其中的微晶成分随 关键词： 非晶硅 薄膜 喇曼散射 微结构     

Study on stability of hydrogenated amorphous silicon films

Hydrogenated amorphous silicon （a-Si：H） films with high and same order of magnitude photosensitivity （-10^5） but different stability were prepared by using microwave electron cyclotron resonance chemical vapour deposition system under the different deposition conditions. It was proposed that there was no direct correlation between the photosensitivity and the hydrogen content （CH） as well as H-Si bonding configurations, but for the stability, they were the critical factors. The experimental results indicated that higher substrate temperature, hydrogen dilution ratio and lower deposition rate played an important role in improving the microstructure of a-Si：H films. We used hydrogen elimination model to explain our experimental results.     

Photo-chemical vapor deposition of hydrogenated amorphous silicon films

A photo-chemical vapor deposition, using ultraviolet light excitation and a mercury photo-sensitization, was investigated for depositing hydrogenated amorphous silicon films from SiH₄. The photoelectric and structural properties were examined to characterize the deposited films. Those properties were depended strongly on substrate temperature, and the films which were deposited at a substrate temperature more than 200°C contained dominant SiH configurations. A relatively large single crystalline grain size of about 0.5 m was observed in a 1.0 m thick film, which was obtained at a substrate temperature as low as 200°C. Phosphorus impurity doping into the films and Pt-Schottky diode fabrication were also attempted.     

Dispersive processes of light-induced defect creation in hydrogenated amorphous silicon

The growing curve of light-induced dangling bonds under illumination has been observed for various intensities of illumination in a-Si:H. It is fitted to a stretched exponential function and then two parameters β and τ involved in the function are estimated as a function of saturated dangling bond density . The experimental values of β, τ, and are compared with those calculated based on our model of light-induced defect creation in a-Si:H.     

HW-MWECR-CVD法制备氢化微晶硅薄膜及其微结构研究

用热丝辅助微波电子回旋共振化学气相沉积方法制备出高晶化体积分数的氢化微晶硅(μc-Si:H)薄膜.拉曼散射和X射线衍射技术对样品的微观结构测量分析表明，当反应气体中SiH₄浓度在3.6%—50%之间大范围变化时，μc-Si:H薄膜均具有高的晶化体积分数.进一步的分析表明，在SiH₄浓度较大时制备的薄膜，其结构以非晶-微晶的过渡相为主.薄膜易于晶化或生长为过渡相的主要原因是微波电子回旋共振使SiH₄气体高度分解，等离子体高度电离. 关键词： 微波电子回旋共振化学气相沉积 氢化微晶硅薄膜 拉曼散射 X射线衍射