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.     

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.     

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).     

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.     

Nuclear magnetic resonance investigation of H, H2 and dopants in hydrogenated amorphous silicon and related materials

Nuclear magnetic resonance has been successfully applied to the study of the microstructure of hydrogenated amorphous silicon and related materials. It has been used to determine the local bonding and structural environment of the host atoms, the hydrogen, and the dopants. First, we review some of these NMR experimental results on the hydrogen microstructure in hydrogentaed amorphous semiconductors and compare the results on plasma deposited hydrogenated amorphous silicon (a-Si:H), remote hydrogen plasma deposited a-Si:H, thermally annealed a-Si:H, doped a-Si:H, microcrystalline Si and amorphous (Si, Ge):H alloys. A common feature is that these materials exhibit a heterogeneous distribution of hydrogen bonded to the semiconductor lattice in dilute and clustered phases. In addition, the lattice contains voids of varying number and size that contain non-bonded molecular hydrogen whose quantity is altered by deposition conditions and thermal treatment. Second, we review some aspects of the local bonding structure of dopants in a-Si:H. A significant fraction of the dopants are found to be in dopant-hydrogen clusters similar to those proposed to explain hydrogen passivation in crystalline silicon. Implications of the determined local structure on the doping efficiency are discussed.     

Interface states study of intrinsic amorphous silicon for crystalline silicon surface passivation in HIT solar cell

Intrinsic hydrogenated amorphous silicon(a-Si:H) film is deposited on n-type crystalline silicon(c-Si) wafer by hotwire chemical vapor deposition(HWCVD) to analyze the amorphous/crystalline heterointerface passivation properties.The minority carrier lifetime of symmetric heterostructure is measured by using Sinton Consulting WCT-120 lifetime tester system,and a simple method of determining the interface state density(D_(it)) from lifetime measurement is proposed.The interface state density(D_(it)) measurement is also performed by using deep-level transient spectroscopy(DLTS) to prove the validity of the simple method.The microstructures and hydrogen bonding configurations of a-Si:H films with different hydrogen dilutions are investigated by using spectroscopic ellipsometry(SE) and Fourier transform infrared spectroscopy(FTIR) respectively.Lower values of interface state density(D_(it)) are obtained by using a-Si:H film with more uniform,compact microstructures and fewer bulk defects on crystalline silicon deposited by HWCVD.     

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.     

Use of optical emission spectroscopy to investigate the process of fabrication ofa-Si:H films in a combined silane-containing discharge

The results of investigations carried out with the aid of optical emission spectroscopy and concerned with the process of deposition of amorphous hydrogenated silicon (a-Si:H) films in a combined discharge are presented. Correlations between the intensities of emission lines and bands and the rate of the deposition of films were established. To control the quality of films, it is proposed to record changes in the intensity of the atomic hydrogen line (H_β) during the deposition of a-Si:H. A comparison of plasma radiation spectra for different types of discharge was made. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 4, pp. 423–426, July–August, 2000.     

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。     

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.     

Hydrogen effusion: a probe for surface desorption and diffusion

Hydrogen effusion results are discussed for hydrogenated amorphous silicon (a-Si:H) and related alloys as well as for crystalline silicon (c-Si). It is demonstrated that depending on the microstructure of the material, hydrogen effusion gives information on hydrogen diffusion or surface desorption. The results suggest for compact a-Si:H and for ion implanted c-Si a similar hydrogen diffusion process, which is a trap limited motion of atomic hydrogen. Hydrogen effusion from defect-free c-Si and from void-rich amorphous semiconductors is limited by surface desorption. Both hydrogen diffusion and desorption depend on the Fermi energy if hydrogen bonds to the host material are broken.     

Noise in boron doped amorphous/microcrystallization silicon films

Hydrogenated silicon (Si:H) film was grown by radio frequency plasma enhanced chemical vapor deposition (PECVD) method. The transition between hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (μc-Si:H) was characterized by X-ray diffraction analysis. A semiconductor system was used to measure low frequency noise (1/f noise) and random telegraph switching noise of Si:H films. The results show that the 1/f noise of μc-Si:H is 4 orders of magnitude lower than that of a-Si:H and no RTS noise was found in both films. It also shows that using μc-Si:H instead of a-Si:H film as a sensing layer will enable the development of high performance uncooled microbolometer.     

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.     

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.     

Optical absorption in PECVD deposited thin hydrogenated silicon in light of ordering effects

We report results obtained from measurements of optical transmittance spectra carried out on a series of silicon thin films deposited by plasma-enhanced chemical vapour deposition (PECVD) from silane diluted with hydrogen. 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). Spectral refractive indices and absorption coefficients were determined from transmittance spectra. The spectral absorption coefficients were used to determine the Tauc optical band gap energy, the B factor of the Tauc plots, E ₀₄ (energy at which the absorption coefficient is equal to 10⁴ cm⁻¹), and the Urbach energy as a function of the hydrogen dilution. The results were correlated with microstructure, namely volume fractions of the amorphous and crystalline phase with voids, and with the grain size.      

The influence of boron concentrations on structural properties in disorder silicon films

In this work we present a detailed structural of a series of B-doped hydrogenated microcrystalline silicon (μc-Si:H) films deposited by plasma-enhanced chemical vapor deposition (PECVD) and B-doped polycrystalline silicon (poly-Si) films produced by step-by-step laser crystallization process from amorphous silicon. The influence of doping on the structural properties and structural changes during the sequential crystallization processes were monitored by Raman spectroscopy. Unlike μc-Si:H films, that consist of a two-phase mixture of amorphous and ordered Si, partially crystallized sample shows a stratified structure with polycrystalline silicon layer at the top of an amorphous layer. With increasing doping concentration the LO-TO phonon line in poly-Si shift to smaller wave numbers and broadens asymmetrically. The results are discussed in terms of resonant interaction between optical phonons and direct intraband transitions known as a Fano resonance. In μc-Si:H films, on the other hand, the Fano effect is not observed. The increase of doping in μc-Si:H films suppressed the crystalline volume fraction, which leads to an amorphization in the film structure. The structural variation in both μc-Si:H and poly-Si films leads to a change in hydrogen bonding configuration.     

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.     

椭圆偏振光谱表征单晶硅衬底上生长的非晶硅和外延硅薄膜

本文采用射频等离子体增强化学气相沉积(rf-PECVD)技术在单晶硅衬底上沉积了两个系列的硅薄膜. 通过对样品进行固定角度椭圆偏振测试, 结果表明第一个系列硅薄膜为非晶硅, 形成了突变的a-Si:H/c-Si异质结构, 此结构在HIT电池中有利于形成好的界面特性, 对于非晶硅薄膜采用通常的Tauc-Lorentz摇摆模型(Genosc)拟合结果很好; 第二个系列硅薄膜为外延硅, 对于外延硅薄膜, 随着膜厚增加晶化率降低, 当外延硅薄膜厚度为46 nm时开始非晶硅生长. 对于外延硅通常采用EMA模型(即将硅薄膜体层看成由非晶硅和c-Si构成的混合层)拟合结果较好, 当硅薄膜中出现非晶硅生长时, 将体层分成混合层和非晶硅两层, 采用三层模型拟合结果很好. 本文证实了椭偏光谱分析采用不同的模型可对单晶硅衬底上不同结构的硅薄膜进行有效表征.     

Influence of substrate temperature on growth of a-Si:H films by reactive facing target sputtering deposition

Hydrogenated amorphous silicon(a-Si:H) films were deposited by reactive facing target sputtering(FTS) technique with a mixture of Ar and H2 reaction gas.Fourier transform infrared(FTIR) absorption,Raman scattering and ultraviolet-visible optical absorption are used to investigate the microstructure and optical properties of the deposited films.The decrease of the concentration of bonded hydrogen,especially that of(Si-H2)n with increasing substrate temperature(Ts),was observed in FTIR spectra,suggesting the ...