Influence of deposition temperature on amorphous structure of PECVD deposited a-Si:H thin films

The effect of deposition temperature on the structural and optical properties of amorphous hydrogenated silicon (a-Si:H) thin films deposited by plasma-enhanced chemical vapour deposition (PECVD) from silane diluted with hydrogen was under study. The series of thin films deposited at the deposition temperatures of 50–200°C were inspected by XRD, Raman spectroscopy and UV Vis spectrophotometry. All samples were found to be amorphous with no presence of the crystalline phase. Ordered silicon hydride regions were proved by XRD. Raman measurement analysis substantiated the results received from XRD showing that with increasing deposition temperature silicon-silicon bond-angle fluctuation decreases. The optical characterization based on transmittance spectra in the visible region presented that the refractive index exhibits upward trend with increasing deposition temperature, which can be caused by the densification of the amorphous network. We found out that the scale factor of the Tauc plot increases with the deposition temperature. This behaviour can be attributed to the increasing ordering of silicon hydride regions. The Tauc band gap energy, the iso-absorption value their difference were not particularly influenced by the deposition temperature. Improvements of the microstructure of the Si amorphous network have been deduced from the analysis.     

Effect of hydrogen on hardness of amorphous silicon

A comparative study of hardness of thin films of hydrogenated amorphous silicon (a-Si:H) and hydrogen-free amorphous silicon (a-Si) was carried out to reveal the role of hydrogen in the plastic properties of amorphous silicon. In addition, the effect of hydrogen on hardness was established by changing hydrogen concentration in the material using post-deposition processing of the samples. The hydrogen concentration in a-Si:H was decreased by thermal annealing. In a-Si hydrogen was introduced by plasma hydrogenation. The values of hardness of the as-prepared a-Si and a-Si:H films were determined by nanoindentation using depth profiling. Low-depth indentation was applied to evaluate the effect of post-hydrogenation. The results obtained show that the presence of hydrogen in the amorphous silicon network leads to the increase in hardness. The conducted experiments demonstrate that plasma hydrogenation can be used as an effective tool to increase the hardness of amorphous silicon. Hardness of a-Si:H of about 12.3–12.7 GPa is as high as of crystalline silicon, suggesting a-Si:H can be a substitute for crystalline silicon in some MEMS.     

Aluminium-induced crystallization of amorphous silicon films deposited by DC magnetron sputtering on glasses

Amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si:H) films were deposited by DC magnetron sputtering technique with argon and hydrogen plasma mixture on Al deposited by thermal evaporation on glass substrates. The a-Si/Al and a-Si:H/Al thin films were annealed at different temperatures ranging from 250 to 550 °C during 4 h in vacuum-sealed bulb. The effects of annealing temperature on optical, structural and morphological properties of as-grown as well as the vacuum-annealed a-Si/Al and a-Si:H/Al thin films are presented in this contribution. The averaged transmittance of a-Si:H/Al film increases upon increasing the annealing temperature. XRD measurements clearly evidence that crystallization is initiated at 450 °C. The number and intensity of diffraction peaks appearing in the diffraction patterns are more important in a-Si:H/Al than that in a-Si/Al layers. Results show that a-Si:H films deposited on Al/glass crystallize above 450 °C and present better crystallization than the a-Si layers. The presence of hydrogen induces an improvement of structural properties of poly-Si prepared by aluminium-induced crystallization (AIC).     

Control of epitaxial growth at a-Si:H/c-Si heterointerface by the working pressure in PECVD

The epitaxial-Si(epi-Si) growth on the crystalline Si(c-Si) wafer could be tailored by the working pressure in plasmaenhanced chemical vapor deposition(PECVD).It has been systematically confirmed that the epitaxial growth at the hydrogenated amorphous silicon(a-Si:H)/c-Si interface is suppressed at high pressure(hp) and occurs at low pressure(1p).The hp a-Si:H,as a purely amorphous layer,is incorporated in the 1p-epi-Si/c-Si interface.We find that:(i) the epitaxial growth can also occur at a-Si:H coated c-Si wafer as long as this amorphous layer is thin enough;(ii) with the increase of the inserted hp layer thickness,lp epi-Si at the interface is suppressed,and the fraction of a-Si:H in the thin films increases and that of c-Si decreases,corresponding to the increasing minority carrier lifetime of the sample.Not only the epitaxial results,but also the quality of the thin films at hp also surpasses that at lp,leading to the longer minority carrier lifetime of the hp sample than the lp one although they have the same amorphous phase.     

椭偏透射法测量氢化非晶硅薄膜厚度和光学参数

针对多角度椭偏测量透明基片上薄膜厚度和光学参数时基片背面非相干反射光的影响问题,报道了利用椭偏透射谱测量等离子增强化学气相沉积法(PECVD)制备的a-Si:H薄膜厚度和光学参数的方法,分析了基片温度T_s和辉光放电前气体温度T_g的影响.研究表明,用椭偏透射法测量的a-Si:H薄膜厚度值与扫描电镜(SEM)测得的值相当,推导得到的光学参数与其他研究者得到的结果一致.该方法可用于生长在透明基片上的其他非晶或多晶薄膜. 关键词： 椭偏测量 透射法 光学参数 氢化非晶硅薄膜     

a-Si:H/a-SiNx:H超晶格薄膜光致发光性质的研究

本文报道了a-Si:H/a-SiNx:H超晶格薄膜光致发光某些性质的研究。实验发现,这种超晶格薄膜光致发光的强度和峰值能量随交替层a-Si:H厚度,测量温度及光照时间等而变化。同时还发现,在阴、阳两极上,利用GD法沉积的样品,发光强度和峰值能量也有所不同。文中对这些实验结果作了初步解释。     

Silicon grain arrays prepared using a pattern crystallization technique of pulsed-excimer laser irradiation

Silicon grain arrays were prepared using a pattern crystallization technique of pulsed KrF excimer laser irradiation. The precursor material was hydrogenated amorphous silicon (a-Si:H) thin films deposited on single crystal Si wafers by plasma-enhanced chemical vapor deposition. It was shown that Si grains with a uniform size and a well-defined periodicity embedded in the a-Si:H matrix were obtained by this simple technique. The grain size was less than 2 μm. Relativly strong photo-luminescence with two peaks at 720 and 750 nm was observed at room temperature. We expect to reduce Si grain sizes by optimizing the growth conditions of a-Si:H thin films and controlling the temperature distribution in the film during laser irradiation. Received: 21 November 2000 / Accepted: 12 December 2000 / Published online: 9 February 2001     

镶嵌在氢化氮化硅中纳米非晶硅粒子光吸收的模拟

采用量子限制效应模型对镶嵌有纳米非晶硅粒子的氢化氮化硅薄膜的光吸收进行了理论模拟,探讨了由吸收谱分析给出该结构薄膜光学参数的方法,并通过对不同氮含量样品的讨论给出了量子限制效应和纳米硅粒子表面的结构无序对薄膜光吸收特性的影响规律。分析结果表明,随氮含量的增加,薄膜有效光学带隙增大,该结果与薄膜中纳米硅粒子平均尺寸的减小引起的量子限制效应的增强相关,而小粒度纳米硅粒子比例增加所引入的较高微观结构无序度和较多缺陷将会导致薄膜低能吸收区吸收系数增加。     

Effect of hydrogen on surface texturing and crystallization of a-Si:H thin film irradiated by excimer laser

Hydrogenated amorphous silicon (a-Si:H) thin films have been considered for use in solar cell applications because of their significantly reduced cost compared to crystalline bulk silicon. However, their overall efficiency and stability are lower than that of their bulk crystalline counterpart. Limited work has been performed on simultaneously solving the efficiency and stability issues of a-Si:H. Previous work has shown that surface texturing and crystallization on a-Si:H thin film can be achieved through a single-step laser processing, which can potentially alleviate the disadvantages of a-Si:H in solar cell applications. In this study, hydrogenated and dehydrogenated amorphous silicon thin films deposited on glass substrates were irradiated by KrF excimer laser pulses and the effect of hydrogen on surface morphologies and microstructures is discussed. Sharp spikes are focused only on hydrogenated films, and the large-grained and fine-grained regions caused by two crystallization processes are also induced by presence of hydrogen. Enhanced light absorptance is observed due to light trapping based on surface geometry changes of a-Si:H films, while the formation of a mixture of nanocrystalline silicon and original amorphous silicon after crystallization suggests that the overall material stability can potentially improve. The relationship between crystallinity, fluence and number of pulses is also investigated. Furthermore, a step-by-step crystallization process is introduced to prevent the hydrogen from diffusing out in order to reduce the defect density, and the relationship between residue hydrogen concentration, fluence and step width is discussed. Finally, the combined effects show that the single-step process of surface texturing and step-by-step crystallization induced by excimer laser processing are promising for a-Si:H thin-film solar cell applications.     

Femtosecond-laser-induced-crystallization and simultaneous formation of light trapping microstructures in thin a-Si:H films

We report the observation of crystallization and simultaneous formation of surface microstructures in hydrogenated amorphous silicon (a-Si:H) thin films as one step laser processing. Light trapping microstructures of around 300 nm in height were formed on a-Si:H films of thickness in the range of 1.5 μm to 2 μm deposited on soda lime glass after exposure to femtosecond laser pulses. Scanning electron microscope (SEM) images show the formation of spikes that are around 1 μm part and their heights could be controlled by the laser fluences. Atomic force microscope (AFM) images were taken to study the roughness created on the surface. The mean roughness of the textured surface increases with laser fluence at smaller power densities, and for power densities beyond 0.5 J/cm² the film removal deteriorates the texturing. X-ray diffraction results indicate the formation of a nano-crystalline structure with (111) and (311) crystal orientation after the laser treatment. The observed black color and enhanced optical absorption in the near infrared region in laser treated films may be due to a combined effect of light trapping in the micro-structured silicon surface because of multiple total internal reflections, phase change in the film, possible defect sites induced after laser treatment and formation of SiO_x. Demonstration of light trapping microstructures in thin a-Si:H films and simultaneous crystallization could provide new opportunities for optoelectronic devices. PACS 42.55.Px; 42.62.Cf; 81.05.Ge     

Electric-field-enhanced metal-induced crystallization of hydrogenated amorphous silicon at room temperature

A novel simple method of crystallization of hydrogenated amorphous silicon (a-Si:H) thin films is described. Namely, we studied a metal-induced crystallization enhanced by a dc electric field in sandwich p⁺–i–n⁺structures. The samples were fabricated from wide-bandgap a-Si:H with high hydrogen content (13–51 at. % H). Macroscopic islands of a-Si:H (up to ∼1 mm in diameter) in the region between upper (CrNi) and lower (ITO) contacts crystallize instantaneously when a sufficiently high dc electric field (≳10⁵ V cm^-1) is applied. The crystallization sets in at room temperature and ambient atmosphere and is spatially selective. A proposed microscopic mechanism of such an easy macroscopic crystallization consists in easy diffusion of Ni and/or Ni silicides (representing nucleation sites) through a dense network of voids in hydrogen-rich a-Si:H. Received: 30 November 2000 / Accepted: 3 May 2001 / Published online: 27 June 2001     

Microstructure of hydrogenated silicon thin films prepared from silane diluted with hydrogen

We report results obtained from FTIR and TEM measurements carried out on silicon thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) from silane diluted with hydrogen. The hydrogen content, the microstructure factor, the mass density and the volume per Si-H vibrating dipoles were determined as a function of the hydrogen dilution. Hydrogen dilution of silane results in an inhomogeneous growth during which the material evolves from amorphous hydrogenated silicon (a-Si:H) to microcrystalline hydrogenated silicon (μc-Si:H). With increasing dilution the transition from amorphous to microcrystalline phase appears faster and the average mass density of the films decreases. The μc-Si:H films are mixed-phase void-rich materials with changing triphasic volume fractions of crystalline and amorphous phases and voids. Different bonding configurations of vibrating Si-H dipoles were observed in the a-Si:H and μc-Si:H. The bonding of hydrogen to silicon in the void- and vacancy-dominated mechanisms of network formation is discussed.     

氢稀释对FTS沉积a-Si:H薄膜结构特性的影响

采用对靶磁控反应溅射技术,以氢气作为反应气体在不同的氢稀释比条件下制备了氢化非晶硅薄膜.利用台阶仪、傅里叶红外透射光谱、Raman谱和紫外-可见光透射谱测量研究了不同氢稀释比对氢化非晶硅薄膜生长速率和结构特性的影响.分析结果发现,利用对靶磁控溅射技术能够实现低温快速沉积高质量氢化非晶硅薄膜的制备.随着氢稀释比不断增加,薄膜沉积速率呈现先减小后增大的趋势.傅里叶红外透射光谱表明,氢化非晶硅薄膜中氢含量先增大后变小.而Raman谱和紫外-可见光透射谱分析发现,氢稀释比的增加使氢化非晶硅薄膜有序度和光学带隙均先增大后减小.可见,此技术通过改变氢稀释比R能够实现氢化非晶硅薄膜结构的有效控制.     

Nanoindentation of hydrogenated amorphous silicon

Nanoindentation was carried out on thin films of hydrogenated amorphous silicon (a-Si:H) prepared by plasma-enhanced chemical vapor deposition. The composite values of elastic (Young's) modulus, E _c, and hardness, H _c, of the film/substrate system were evaluated from the load–displacement curves using the Oliver–Pharr approach. The film-only parameters were obtained employing the extrapolation of the depth profiles of E _c and H _c. Scanning probe microscopy was employed to image the nanoindenter impressions and to estimate the effect of film roughness and material pile-up on the testing results. It was established that the elastic modulus of thin a-Si:H films is in the range 117–131 GPa, which is lower than for crystalline silicon. In contrast, the values of hardness are in the range 12.2–12.7 GPa, which is comparable to crystalline silicon and higher than for hydrogen-free amorphous silicon. It is suggested that the plastic deformation of a-Si:H proceeds through plastic flow and it is the presence of hydrogen in the amorphous matrix that leads to a higher hardness.     

Thermal conductivity of hydrogenated amorphous silicon

The thermal conductivity of amorphous silicon thin films is measured in one dimension steady state condition. The experimental method is based on heating the sample front surface and monitoring the temperatures at its two sides. The experiments were carried out in conditions ensuring one-direction heat flow from top to bottom throughout the sample thickness. Sputtered a-Si:H films prepared with different conditions are used in order to investigate the dependence of thermal conductivity on material properties (i.e. hydrogen content and microstructure). The results show that, firstly, amorphous silicon is a very bad thermal conductor material. Secondly, the disorder in the film network plays an important role in thermal conduction. The highly disordered film exhibits the lowest thermal conductivity.     

基于反射和透射光谱的氢化非晶硅薄膜厚度及光学常量计算

采用紫外-可见透射光谱仪测量了对靶磁控溅射沉积法制备的氢化非晶硅（a-Si:H）薄膜的透射光谱和反射光谱.利用T/(1-R)方法来确定薄膜的吸收系数,进而得到薄膜的消光系数|通过拟合薄膜透射光谱干涉极大值和极小值的包络线来确定薄膜折射率和厚度的初始值,并利用干涉极值公式进一步优化薄膜的厚度值和折射率|利用柯西公式对得到的薄膜折射率进行拟合,给出了a-Si:H薄膜的色散关系曲线.为了验证该方法确定的薄膜厚度和光学常量的可靠性,将理论计算得到的透射光谱与实验数据进行了比较,结果显示两条曲线基本重合,可见这是确定a-Si:H薄膜厚度及光学常量的一种有效方法.     

Excimer-laser-assisted RF glow-discharge deposition of amorphous and microcrystalline silicon thin films

We combine the deposition of Hydrogenated amorphous Silicon (a-Si:H) by rf glow discharge with XeCl-excimer laser irradiation of the growing surface in order to obtain different kinds of silicon films in the same deposition system. In-situ UV-visible ellipsometry allows us to measure the optical properties of the films as the laser fluence is increased from 0 up to 180 mJ/cm² in separate depositions. For fixed glow-discharge conditions and a substrate temperature of 250° C we observe dramatic changes in the film structure as the laser fluence is increased. With respect to a reference a-Si:H film (no laser irradiation) we observe at low laser fluences (15–60 mJ/cm²) that the film remains amorphous but demonstrates enchanced surface roughness and bulk porosity. At intermediate fluences (80–165 m/Jcm²), we obtain an amorphous film with an enhanced density with respect to the reference film. Finally, at high fluences (165–180 mJ/cm²), we obtain microcrystalline films. The in-situ ellipsometry measurements are complemented by ex-situ measurements of the dark conductivity, X-ray diffraction, and Elastic Recoil Detection Analysis (ERDA). Simulation of the temperature profiles for different film thicknesses and for three laser fluences indicates that crystallization occurs if the surface temperature reaches the melting point of a-Si:H ( 1420 K). The effects of laser treatment on the film properties are discussed by taking into account the photonic and thermal effects of laser irradiation.Presented at LASERION 93, Munich, June 21–23, 1993     

沉积压力对氢化非晶硅薄膜特性的影响

采用等离子增强化学气相沉积(PECVD)系统,以乙硅烷和氢气为气源,普通钠钙玻璃为衬底制备了氢化非晶硅(a-Si∶H)薄膜,研究了沉积压力对非晶硅薄膜的沉积速率、光学带隙以及结构因子的影响。采用台阶仪、紫外可见分光光度计、傅里叶变换红外光谱仪和扫描电子显微镜等手段分别表征了a-Si∶H薄膜的沉积速率,光学带隙、结构因子和表面形貌。结果表明： 随着沉积压力的增加,沉积速率呈现先上升后下降的趋势,光学带隙不断下降。当沉积压力小于210 Pa时,以SiH键存在的H原子较多,而以SiH₂或SiH₃等形式存在的H较少; 当沉积压力大于210 Pa时,以SiH₂,(SiH₂)_n或SiH₃等形式存在的H较多。通过结构因子的计算,发现沉积压力在110～210 Pa的范围内沉积的薄膜质量较好。     

Effects of gas temperature on optical and transport properties of a-Si:H films deposited by PECVD

Effects of silane temperature (T _g) before glow-discharge on the optical and transport properties of hydrogenated amorphous silicon (a-Si:H) thin films were investigated. The optical measurements show that the refractive index increases with increasing T _g. The transport characterizations show that when T _g increases, the dark conductivity increases. However, the temperature coefficient of resistance decreases. In addition, after holding at 130°C for 20 h, the resistance variation, ΔR/R, of the films deposited at T _g = room temperature (10.8%) is much larger than those deposited at silane temperatures of 80°C (3%) and 160°C (2%). This can be attributed to different rates of defect creation in a-Si:H films caused by various T _g.     

EPR investigation of a-Si:H aerosol particles formed under silane thermal decomposition

The dangling bond defects were investigated in a-Si:H particles formed under silane thermal decomposition in flow reactor. EPR together with hydrogen evolution method were used. The experimental results allowed us to conclude that there are two kinds of dangling bond defects in a-Si:H aerosol particles. The defects of the first kind are localized on the surface of interconnected microvoids and microchannels (surface dangling bonds) and those of the second kind are embedded in amorphous silicon network (volume dangling bonds). The thermal equilibration of dangling bonds and temperature dependence of equilibrium dangling bond concentration were investigated. It was found that at temperatures > 400 K the dangling bond concentrationNApplied Magnetic Resonance _s reversibly depends on sample temperature. The volume dangling bond concentration increases with temperature increasing (the effective activation energy of dangling bond formationU > 0), and the surface dangling bond concentration decreases with temperature increasing (U < 0). It has been found that EPR line is considerably asymmetric for samples with high hydrogen content and for low hydrogen content the EPR line is weakly asymmetric. A conclusion was drawn that the asymmetry degree depends on amorphous silicon lattice distortions. This conclusion has been confirmed by EPR spectra simulations.