Evolution of microcrystalline growth pattern by ultraviolet spectroscopic ellipsometry on Si:H films prepared by Hot-Wire CVD

Ultra-violet spectroscopic ellipsometry has been applied successfully to determine the evolution of μc-Si:H film structure, including incubation layer, bulk layer and the growth zone and surface over-layer, their individual thickness and composition. In view of the availability of a significantly high atomic H density in H₂-diluted SiH₄ ensemble in Hot-Wire CVD, microcrystallization seems to be achieved easily at a low substrate temperature. Atomic H induced etching at the growing network has been identified as instrumental in controlling the microcrystallization in Si:H. Sharp elimination of the incubation layer as well as surface roughness and significant improvement in the overall crystallinity was obtained on increase in H₂ dilution to SiH₄. A H₂ dilution limited sharp transition from an amorphous dominated (a+μc)-Si mixed phase to a virtually amorphous free μc-Si phase has been identified. However, enhanced atomic H reactivity at the growth zone beyond the attainment of the amorphous free bulk and the surface layer induces porosity in the network and a gradual deviation from crystallinity.     

沉积速率对甚高频等离子体增强化学气相沉积制备微晶硅薄膜生长标度行为的影响

本文采用甚高频等离子体增强化学气相沉积技术制备了沉积速率系列不同生长阶段的微晶硅薄膜,通过椭圆偏振技术研究了生长过程中微晶硅薄膜表面粗糙度的演化.实验结果表明:沉积速率为0.08和0.24nm/s的低速沉积时,硅薄膜表面粗糙度接近,生长指数分别为β=0.21和β=0.20,对应有限扩散生长模式,此时沉积速率对硅薄膜生长影响不大,原因是低速沉积时成膜先驱物有足够时间迁移到能量低的位置;当沉积速率增加到0.66nm/s时,硅薄膜表面粗糙度明显增加,生长指数β=0.81,大于0.5,出现了异常标度行为,与低速沉积的生长模式明显不同,原因是高速沉积时成膜前驱物来不及扩散就被下一层前驱物覆盖,降低了成膜前驱物在薄膜表面的扩散,使表面粗糙度增加和生长指数β增大.β大于0.5的异常标度行为与阴影效应有关.     

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.     

Hydrogen evolution during deposition of microcrystalline silicon by chemical transport

We exposed a freshly deposited boron-doped, hydrogenated amorphous silicon (a-Si:H) layer to hydrogen plasma under conditions of chemical transport. In situ spectroscopic ellipsometry measurements revealed that atomic hydrogen impinging on the film surface behaves differently before and after crystallization. First, the plasma exposure increases hydrogen solubility in the a-Si:H network leading to the formation of a hydrogen-rich subsurface layer. Then, once the crystallization process engages, the excess hydrogen starts to leave the sample. We have attributed this unusual evolution of the excess hydrogen to the grown hydrogenated microcrystalline (μc-Si:H) layer, which gradually prevents the atomic hydrogen from the plasma reaching the μc-Si:H/a-Si:H interface. Consequently, hydrogen solubility, initially increased by the hydrogen plasma, recovers the initial value of an untreated a-Si:H material. To support the theory that the outdiffusion is a consequence and not the cause of the μc-Si:H layer growth, we solved the combined diffusion and trapping equations, which govern hydrogen diffusion into the sample, using appropriate approximations and a specific boundary condition explaining the lack of hydrogen injection during μc-Si:H layer growth.     

The study of amorphous incubation layers during the growth of microcrystalline silicon films under different deposition conditions

<正>The structural un-uniformity of μc-Si:H films prepared using a very high frequency plasma-enhanced chemical vapour deposition method has been investigated by Raman spectroscopy,spectroscopic ellipsometer and atomic force microscopy.It was found that the formation of amorphous incubation layer was caused by the back diffusion of SiH_4 and the amorphous induction of glass surface during the initial ignition process,and growth of the incubation layer can be suppressed and uniformμc-Si:H phase is generated by the application of delayed initial SiH_4 density and silane profiling methods.     

高压射频等离子体增强化学气相沉积制备高效率硅薄膜电池的若干关键问题研究

报道了采用高压射频等离子体增强化学气相沉积(RF-PECVD) 制备高效率单结微晶硅电池和非晶硅/微晶硅叠层电池时几个关键问题的研究结果, 主要包括: 1)器件质量级本征微晶硅材料工艺窗口的确定及其结构和光电性能表征; 2)孵化层的形成机理以及减小孵化层的有效方法; 3)氢稀释调制技术对本征层晶化率分布及其对提高电池性能的作用; 4)高电导、高晶化率的微晶硅p型窗口层材料的获得, 及其对减小微晶硅电池p/i界面孵化层厚度和提高电池性能的作用等. 在解决上述问题的基础上, 采用高压RF-PECVD制备的单结微晶硅电池效率达8.16%, 非晶硅/微晶硅叠层电池效率11.61%.     

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.     

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.     

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.     

硅异质结太阳电池的物理机制和优化设计

硅异质结太阳电池是一种由非晶硅薄膜层沉积于晶硅吸收层构成的高效低成本的光伏器件,是一种具有大面积规模化生产潜力的光伏产品.异质结界面钝化品质、发射极的掺杂浓度和厚度以及透明导电层的功函数是影响硅异质结太阳电池性能的主要因素.针对这些影响因素已经有大量的研究工作在全世界范围内展开,并且有诸多研究小组提出了器件效率限制因素背后的物理机制.洞悉物理机制可为今后优化设计高性能的器件提供准则.因此及时总结硅异质结太阳电池的物理机制和优化设计非常必要.本文主要讨论了晶硅表面钝化、发射极掺杂层和透明导电层之间的功函数失配以及由此形成的肖特基势垒;讨论了屏蔽由功函数失配引起的能带弯曲所需的特征长度,即屏蔽长度;介绍了硅异质结太阳电池优化设计的数值模拟和实践;总结了硅异质结太阳电池的研究现状和发展前景.     

微晶硅薄膜的表面粗糙度及其生长机制的X射线掠角反射研究

采用等离子体增强化学气相沉积技术沉积一系列处于不同生长阶段的微晶硅薄膜.通过同步辐射X射线掠角反射技术研究微晶硅薄膜的表面粗糙度随时间等的演化，探讨微晶硅薄膜的生长动力学过程及其生长机制.研究结果表明，在衬底温度为200 ℃，电极间距为2 cm，沉积气压为6.66×1０² Pa，射频功率密度为0.22 W/cm²，氢稀释度分别为99%和98%的沉积条件下，在玻璃衬底上生长的微晶硅薄膜生长指数β分别为0.21±0.01和0.24±0.01.根据KPZ模型，微晶硅薄膜的生长机制为有限扩散生长. 关键词： X射线掠角反射 微晶硅薄膜 表面粗糙度 生长机制     

An optical study of the correlation between growth kinetics and microstructure of μc-Si grown by SiH4-H2 PECVD

Fully microcrystalline silicon, μc-Si, thin films have been deposited on corning glass by plasma enhanced chemical vapor deposition (PECVD) using SiH₄-H₂. The effects of the surface treatment and of the deposition temperature on microstructure of μc-Si films are investigated by “in situ” laser reflectance interferometry (LRI), “ex situ” spectroscopic ellipsometry (SE) and Raman spectroscopy. LRI indicated the existence of a “crystalline seeding time”, which is indicative of the crystallite nucleation, and depends on substrate treatments. Longer “crystalline seeding time” results in a lower density of crystalline nuclei, which grow laterally, yielding to complete suppression of the amorphous incubation layer and to growth of very dense, fully crystalline layer at a growth temperature as low as 120 °C.     

p种子层对单室制备微晶硅电池性能的影响

采用单室等离子体化学气相沉积技术沉积pin微晶硅电池时,硼污染降低了本征材料的晶化率并影响了p/i界面特性.针对该问题文中采用p种子层技术,即在沉积p层后采取高的H₂/SiH₄比率及适当的功率又沉积一个薄的p层,初步研究了p种子层对微晶硅i层纵向均匀性及电池性能的影响.实验结果表明：采用此方法能改善p/i界面特性,提高本征材料纵向结构的均匀性并降低硼对本征层的污染,有效地提高单结微晶硅电池的性能.最后,通过优化沉积条件,制备得到光电转换效率为881%（1 cm 关键词： 单室 甚高频等离子体增强化学气相沉积 微晶硅太阳电池 p种子层     

Research on the boron contamination at the p/i interface of microcrystalline silicon solar cells deposited in a single PECVD chamber

This paper studies boron contamination at the interface between the p and i layers of μ c-Si:H solar cells deposited in a single-chamber PECVD system. The boron depth profile in the i layer was measured by Secondary Ion Mass Spectroscopy. It is found that the mixed-phase μ c-Si:H materials with 40% crystalline volume fraction is easy to be affected by the residual boron in the reactor. The experimental results showed that a 500-nm thick μ c-Si:H covering layer or a 30-seconds of hydrogen plasma treatment can effectively reduce the boron contamination at the p/i interface. However, from viewpoint of cost reduction, the hydrogen plasma treatment is desirable for solar cell manufacture because the substrate is not moved during the hydrogen plasma treatment.     

Fractal analysis and atomic force microscopy measurements of surface roughness for Hastelloy C276 substrates and amorphous alumina buffer layers in coated conductors

In coated conductors, surface roughness of metallic substrates and buffer layers could significantly affect the texture of subsequently deposited buffer layers and the critical current density of superconductor layer. Atomic force microscopy (AFM) is usually utilized to measure surface roughness. However, the roughness values are actually relevant to scan scale. Fractal geometry could be exerted to analyze the scaling performance of surface roughness. In this study, four samples were prepared, which were electro polished Hastelloy C276 substrate, mechanically polished Hastelloy C276 substrate and the amorphous alumina buffer layers deposited on both the substrates by ion beam deposition. The surface roughness, described by root mean squared (RMS) and arithmetic average (R_a) values, was analyzed considering the scan scale of AFM measurements. The surfaces of amorphous alumina layers were found to be fractal in nature because of the scaling performance of roughness, while the surfaces of Hastelloy substrates were not. The flatten modification of AFM images was discussed. And the calculation of surface roughness in smaller parts divided from the whole AFM images was studied, compared with the results of actual AFM measurements of the same scan scales.     

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.     

Microstructure evolution and passivation quality of hydrogenated amorphous silicon oxide(a-SiOx:H) on〈100〉- and 〈111〉-orientated c-Si wafers

Hydrogenated amorphous silicon oxide(a-SiOx:H) is an attractive passivation material to suppress epitaxial growth and reduce the parasitic absorption loss in silicon heterojunction(SHJ) solar cells. In this paper, a-SiOx:H layers on different orientated c-Si substrates are fabricated. An optimal effective lifetime(τ(eff)) of 4743 μs and corresponding implied opencircuit voltage(iV(oc)) of 724 mV are obtained on〈100〉-orientated c-Si wafers. While τ(eff) of 2429 μs and iV_oc of 699 mV are achieved on 111-orientated substrate. The FTIR and XPS results indicate that the a-SiOx:H network consists of SiOx(Si-rich), Si–OH, Si–O–SiHx, SiO2 ≡ Si–Si, and O3 ≡ Si–Si. A passivation evolution mechanism is proposed to explain the different passivation results on different c-Si wafers. By modulating the a-SiOx:H layer, the planar silicon heterojunction solar cell can achieve an efficiency of 18.15%.     

Influence of Boron doping on microcrystalline silicon growth

Microcrystalline silicon (μc-Si:H) thin films with and without boron doping are deposited using the radio-frequency plasma-enhanced chemical vapour deposition method. The surface roughness evolutions of the silicon thin films are investigated using ex situ spectroscopic ellipsometry and an atomic force microscope. It is shown that the growth exponent β and the roughness exponent α are about 0.369 and 0.95 for the undoped thin film, respectively. Whereas, for the boron-doped μc-Si:H thin film, β increases to 0.534 and α decreases to 0.46 due to the shadowing effect.     

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.