In situ study of the Si-H bond in a-Si:H ultrathin films

A preliminary in situ analysis of the growth mechanisms and vibrational properties of plasma deposited amorphous silicon (a-Si:H) ultrathin films (d_f<200 Å) is presented. The hydrogen incorporation, as a function of the film thickness and the influence of the nature of the substrate on the early stage of the growth are studied by a new infrared phase modulated ellipsometer (IRPME). The Si-H bond (near 1990 cm^-1) is evidenced for the first time on films as thin as 5 Å. This new ellipsometer offers extremely high resolution thanks to a very small noise (0.02° in ψ and Δ).     

The study of a new n/p tunnel recombination junction and its application in a-Si：H/μc-Si：H tandem solar cells

This paper reports that a double N layer (a-Si:H/μc-Si:H) is used to substitute the single microcrystalline silicon n layer (n-μc-Si:H) in n/p tunnel recombination junction between subcells in a-Si:H/μc-Si:H tandem solar cells. The electrical transport and optical properties of these tunnel recombination junctions are investigated by current-voltage measurement and transmission measurement. The new n/p tunnel recombination junction shows a better ohmic contact. In addition, the n/p interface is exposed to the air to examine the effect of oxidation on the tunnel recombination junction performance. The open circuit voltage and FF of a-Si:H/μc-Si:H tandem solar cell are all improved and the current leakage of the subcells can be effectively prevented efficiently when the new n/p junction is implemented as tunnel recombination junction.     

On formation of thin SiO2/a-Si:H interface when biased oxidized semiconductor surface interacts with plasma or liquid solution

In this paper we present the results of research into a relation(s) between the bias voltage of an oxide/a-Si:H/c-Si sample during formation of very-thin and thin oxides and the resulting distribution of oxide/semiconductor interface states in the a-Si:H band gap. Two oxygen plasma sources were used for the first time in our laboratories for formation of oxide layers on a-Si:H: i) inductively coupled plasma in connection with its application at plasma anodic oxidation; ii) rf plasma as the source of positive oxygen ions for the plasma immersion ion implantation process. The oxide growth on a-Si:H during plasma anodization is also simply described theoretically. Properties of plasmatic structures are compared to ones treated by chemical oxidation that uses 68 wt% nitric acid aqueous solutions. We have confirmed that three parameters of the oxide growth process — kinetic energy of interacting particles, UV-VIS-NIR light emitted by plasma sources, and bias of the samples — determine the distribution of defect states at both the oxide/a-Si:H interface and the volume of the a-Si:H layer, respectively. Additionally, a bias of the sample applied during the oxide growth process has a similar impact on the distribution of defect states as it can be observed during the bias-annealing of similar MOS structure outside of the plasma reactor. Presented at 5-th International Conference Solid State Surfaces and Interfaces, November 19–24, 2006, Smolenice Castle, Slovakia     

Photoluminescence of very thin oxide/a-Si:H structures passivated in HCN solutions

In this contribution we present new experimental facts concerning evolution of the a-Si:H photoluminescence (PL) spectra, recorded at 6 K, induced by both formation of very thin oxide layer in the surface region of the semiconductor by nitric acid solutions and passivation of a-Si:H defect states using HCN aqueous solutions. a-Si:H layers were deposited on both n-type of crystalline Si and the Corning glass. The analysis of the set of PL spectra - their interpretation - indicates that two explanations of blue shifts of the PL band maxima are possible. The first is connected with formation of several structurally different a-Si:H-based phases inside the amorphous matrix and/or with the Street model of recombination of localized electron-hole pairs coupled with the optical phonon in Si.Additionally, as it was stated, the wet chemical oxidation of 1 μm thick a-Si:H layers deposited on the glass can induce formation of hydrogenated micro-sized a-Si grains. Passivation procedure performed in the HCN solution can transform an equivalent part of the a-Si:H semiconductor to a-Si:CN. If the multiphase model is accepted for interpretation of the PL spectra then the following main PL transitions related with different phases were observed: 1.20, 1.25, 1.38, 1.41, 1.44, and 1.48 eV.     

nc-Si/SiO2多层膜的制备及蓝光发射

在等离子体增强化学气相淀积(PECVD)系统中，采用a-Si∶H层淀积与原位等离子体氧化相结合的逐层生长的方法成功制备出a-Si∶H/SiO2多层膜 (ML)；利用限制性结晶原理通过两步退火处理使a-Si∶H层晶化获得尺寸可控的nc-Si/SiO2 ML，并观察到室温下的蓝光发射；结合Raman散射和剖面透射电子显微镜技术分析了nc-Si/SiO2 ML的结构特性；通过对晶化样品光致发光谱和紫外-可见光吸收谱的研究，探讨了蓝光发射的起源. 关键词： 纳米硅多层膜 等离子体氧化 蓝光发射 热退火     

激光限制结晶技术制备nc-Si/SiO2多层膜

在等离子体增强化学气相淀积系统中，采用aSi:H层淀积和原位等离子体氧化相结合的逐层生长技术制备了aSi:H/SiO_2多层膜.在激光诱导限制结晶原理基础上，使用KrF准分子脉冲激光为辐照源，对aSi:H/SiO_2多层膜进行辐照，使纳米级厚度的aSi:H子层晶化.Raman散射谱和电子衍射谱的结果表明，经过激光辐照后纳米Si颗粒在原始的aSi:H子层内形成，晶粒尺寸可以根据aSi:H层的厚度精确控制.还研究了样品的光致发光（PL）特性以及激光辐照能量密度对PL性质的影响. 关键词： 脉冲激光 多层膜 限制结晶     

Effect of charged deep states in hydrogenated amorphous silicon on the behavior of iron oxides nanoparticles deposited on its surface

Langmuir-Blodgett technique has been used for the deposition of ordered two-dimensional arrays of iron oxides (Fe₃O₄/Fe₂O₃) nanoparticles onto the photovoltaic hydrogenated amorphous silicon (a-Si:H) thin film. Electric field at the a-Si:H/iron oxides nanoparticles interface was directly in the electrochemical cell modified by light soaking and bias voltage (negative or positive) pretreatment resulting in the change of the dominant type of charged deep states in the a-Si:H layer. Induced reversible changes in the nanoparticle redox behavior have been observed. We suggest two possible explanations of the data obtained, both of them are needed to describe measured electrochemical signals. The first one consists in the electrocatalytical effect caused by the defect states (negatively or positively charged) in the a-Si:H layer. The second one consists in the possibility to manipulate the nanoparticle cores in the prepared structure immersed in aqueous solution via the laser irradiation under specific bias voltage. In this case, the nanoparticle cores are assumed to be covered with surface clusters of heterovalent complexes created onto the surface regions with prevailing ferrous or ferric valency. Immersed in the high viscosity surrounding composed of the wet organic nanoparticle envelope these cores are able to perform a field-assisted pivotal motion. The local electric field induced by the deep states in the a-Si:H layer stabilizes their “orientation ordering” in an energetically favourable position.     

用正电子湮没技术研究a-Si:H/a-SiNx:H多层膜中的界面缺陷

本文报道应用正电子湮没技术(PAT)对a-Si:H/a-SiN_x:H(x≈0.5)多层膜系列样品所进行的研究。发现,由于a-Si:H和a-SiN_x:H在结构方面的失配,导致在a-Si:H/a-SiN_x:H多层膜中的界面区,产生大量缺陷。在a-Si:H子层中,紧靠界面的是应变层,厚度约为8?;在应变层之后是过渡层,厚度约为50?。在过渡层中存在大量缺陷,这就是所谓界面缺陷。 关键词：     

后退火增强氢化非晶硅钝化效果的研究

在本征氢化非晶硅(a-Si:H(i))/晶体硅(c-Si)/a-Si:H(i)异质结构上溅射ITO时, 发现后退火可大幅增加ITO/a-Si:H(i)/c-Si/a-Si:H(i)的少子寿命(从1.7 ms到4 ms). 这一增强效应可能的三个原因是: ITO/a-Si:H(i)界面场效应作用、退火形成的表面反应层影响以及退火对a-Si:H(i)材料本身的优化, 但本文研究结果表明少子寿命增强效应与ITO和表面反应层无关; 对不同沉积温度制备的a-Si:H(i)/c-Si/a-Si:H(i)异质结后退火的研究表明: 较低的沉积温度(<175 ℃)后退火增强效应显著, 而较高的沉积温度(>200 ℃)后退火增强效应不明显, 可以确定“低温长高温后退火”是获得高质量钝化效果的一种有效方式; 采用傅里叶红外吸收谱(FTIR)研究不同沉积温度退火前后a-Si:H(i)材料本身的化学键构造, 发现退火后异质结少子寿命大幅提升是由于a-Si:H(i)材料本身的结构优化造成的, 其深层次的本质是通过材料的生长温度和退火温度的优化匹配来控制包括H含量、H键合情况以及Si原子无序性程度等微观因素主导作用的一种竞争性平衡, 对这一平衡点的最佳控制是少子寿命大幅提升的本质原因.     

钙钛矿/硅叠层太阳电池中平面a-Si:H/c-Si异质结底电池的钝化优化及性能提高

最近,旋涂法制备的钙钛矿/平面硅异质结高效叠层太阳电池引起人们广泛关注,主要原因是相比于绒面硅衬底制备的钙钛矿/硅叠层太阳电池,其制备工艺简单、制备成本低且效率高.对于平面a-Si:H/c-Si异质结电池, a-Si:H/c-Si界面的良好钝化是获得高转换效率的关键,进而决定了钙钛矿/硅异质结叠层太阳电池的性能.本文主要从硅片表面处理、a-Si:H钝化层和P型发射极等方面展开研究,通过对硅片表面的氢氟酸(HF)浸泡时间和氢等离子体预处理气体流量、a-Si:H钝化层沉积参数、钝化层与P型发射极(I/P)界面富氢等离子体处理的综合调控,获得了相应的优化工艺参数.对比研究了p-a-Si:H和p-nc-Si:H两种缓冲层材料对I/P界面的影响,其中高电导、宽带隙的p-nc-Si:H缓冲层既能够降低I/P界面的缺陷态,又可以增强P型发射层的暗电导率,提高了前表面场效应钝化效果.通过上述优化,制备出最佳的P-type emitter layer/aSi:H(i)/c-Si/a-Si:H(i)/N-type layer (inip)结构样品的少子寿命与implied-Voc分别达到2855μs和709 mV,表现出良好的钝化效果.应用于平面a-Si:H/c-Si异质结太阳电池,转换效率达到18.76%,其中开路电压达到681.5 mV,相对于未优化的电池提升了34.3 mV.将上述平面a-Si:H/c-Si异质结太阳电池作为底电池,对应的钙钛矿/硅异质结叠层太阳电池的开路电压达到1780 mV,转换效率达到21.24%,证明了上述工艺优化能够有效地改善叠层太阳电池中的硅异质结底电池的钝化及电池性能.     

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.     

A thin-film transistor with a very thin a-Si:H layer and its application for an LC panel

One of the disadvantages of applying an a-Si:H thin-film transistor (TFT) to an active matrix-addressed liquid crystal (LC) panel is that a TFT with an a-Si:H has a very large photo-leakage current because of the high photo-conductivity of an a-Si:H itself.We have tried decreasing the photo-leakage current by varying the thickness of an a-Si:H layer (L) in TFTs and investigated the characteristics of TFTs, mainly drain voltage versus drain current containing photo-leakage current (I _ph).As a result, it is shown that lnI _ph is proportional to InL, and its gradient is 1.5–2.0. We assume that the thinner an a-Si:H layer is, the more effective the recombination of carriers at the interface states is forI _ph.We have applied TFT with a very thin a-Si:H layer (30nm) to a full-color active matrix-addressed LC panel for a moving picture display and realized a display of good quality under illuminated condition of 5×10⁴lx without a shading layer in it.     

On interface properties of ultra-thin and very-thin oxide/a-Si:H structures prepared by oxygen based plasmas and chemical oxidation

Amorphous hydrogenated silicon (a-Si:H) belongs still to most promising types of semiconductors for its utilization in fabrication of TFTs and thin film solar cell technology due to corresponding cheap a-Si:H-based device production in comparison with, e.g. crystalline silicon (c-Si) technologies. The contribution deals with both two important modes of preparation of very-thin and ultra-thin silicon dioxide films in the surface region of a-Si:H semiconductor (oxygen plasma sources and liquid chemical methods) and electrical, optical and structural properties of produced oxide/semiconductor structures, respectively. Dominant aim is focused on investigation of oxide/semiconductor interface properties and their comparison and evaluation from view of utilization of used technological modes in the nanotechnological industry. Following three basic types of oxygen plasma sources were used for the first time in our laboratories for treatments of surfaces of a-Si:H substrates: (i) inductively coupled plasma in connection with its applying at plasma anodic oxidation; (ii) rf plasma as the source of positive oxygen ions for plasma immersion ion implantation process; (iii) dielectric barrier discharge ignited at high pressures.The liquid chemical manner of formation SiO₂/a-Si:H structures uses 68 wt% nitric acid aqueous solutions (i.e., azeotropic mixture with water). Their application in crystalline Si technologies has been presented with excellent results in the formation of ultra-thin SiO₂/c-Si structures [H. Kobayashi, M. Asuha, H.I. Takahashi, J. Appl. Phys. 94 (2003) 7328].Passivation of surface and interface states by liquid cyanide treatment is additional original technique applied after (or before) formation of almost all formed thin film/a-Si:H structures. Passivation process should be used if high-quality electronical parameters of devices can be reached.     

On the Discontinuity of Polycrystalline Silicon Thin Films Realized by Aluminum-Induced Crystallization of PECVD-Deposited Amorphous Si

Crystallization of glass/Aluminum (50, 100, 200 nm) /hydrogenated amorphous silicon (a-Si:H) (50, 100, 200 nm) samples by Aluminum-induced crystallization (AIC) is investigated in this article. After annealing and wet etching, we found that the continuity of the polycrystalline silicon (poly-Si) thin films was strongly dependent on the double layer thicknesses. Increasing the a-Si:H/Al layer thickness ratio would improve the film microcosmic continuity. However, too thick Si layer might cause convex or peeling off during annealing. Scanning electron microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX) are introduced to analyze the process of the peeling off. When the thickness ratio of a-Si:H/Al layer is around 1 to 1.5 and a-Si:H layer is less than 200 nm, the poly-Si film has a good continuity. Hall measurements are introduced to determine the electrical properties. Raman spectroscopy and X-ray diffraction (XRD) results show that the poly-Si film is completely crystallized and has a preferential (111) orientation.     

Wet-chemical passivation of atomically flat and structured silicon substrates for solar cell application

Special sequences of wet-chemical oxidation and etching steps were optimised with respect to the etching behaviour of differently oriented silicon to prepare very smooth silicon interfaces with excellent electronic properties on mono- and poly-crystalline substrates. Surface photovoltage (SPV) and photoluminescence (PL) measurements, atomic force microscopy (AFM) and scanning electron microscopy (SEM) investigations were utilised to develop wet-chemical smoothing procedures for atomically flat and structured surfaces, respectively. Hydrogen-termination as well as passivation by wet-chemical oxides were used to inhibit surface contamination and native oxidation during the technological processing. Compared to conventional pre-treatments, significantly lower micro-roughness and densities of surface states were achieved on mono-crystalline Si(100), on evenly distributed atomic steps, such as on vicinal Si(111), on silicon wafers with randomly distributed upside pyramids, and on poly-crystalline EFG (Edge-defined Film-fed-Growth) silicon substrates.The recombination loss at a-Si:H/c-Si interfaces prepared on c-Si substrates with randomly distributed upside pyramids was markedly reduced by an optimised wet-chemical smoothing procedure, as determined by PL measurements. For amorphous-crystalline hetero-junction solar cells (ZnO/a-Si:H(n)/c-Si(p)/Al) with textured c-Si substrates the smoothening procedure results in a significant increase of short circuit current I_sc, fill factor and efficiency η. The scatter in the cell parameters for measurements on different cells is much narrower, as compared to conventional pre-treatments, indicating more well-defined and reproducible surface conditions prior to a-Si:H emitter deposition and/or a higher stability of the c-Si surface against variations in the a-Si:H deposition conditions.     

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.     

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.     

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。     

激光干涉结晶技术制备二维有序分布纳米硅阵列

利用结合移相光栅掩模 (PSGM) 的激光结晶技术在超薄a-SiN_x/a-Si:H/ a-SiN _x三明治结构样品中制备出二维有序分布的纳米硅阵列.原始样品是用等离子体 增强化学气相淀积法生长.a-Si:H层厚为10nm，a-SiN_x 为50nm，衬底材料为SiO ₂/Si或 熔凝石英.原子力显微镜、剖面透射电子显微镜、高分辨透射电子显微镜对样品表面形貌和 微结构的观测结果表明，采用该方法可以在原始淀积的a-Si:H层中得到位置可控的晶化区域 ：每个晶化区域直径约250nm，具有同PSGM一致的2μm周期；晶化区域内形成的纳米硅 颗粒尺寸接近原始淀积的a-Si:H层厚，且晶粒的择优取向为<111>. 关键词： 纳米硅 激光结晶 定域晶化 移相光栅     

原位等离子体逐层氧化a-Si:H/SiO2多层膜的光致发光研究

采用在等离子体增强化学汽相沉积系统中沉积a-Si:H和原位等离子体逐层氧化的方法制备a-Si:H/SiO2多层膜.改变a-Si:H层的厚度，首次在室温下观察到来自a-Si:H/SiO2多层膜较强的蓝色光致发光和从465到435nm的蓝移.x射线能谱证明，SiO2层是化学配比的SiO2；C-V特性表明，a-Si:H/SiO2界面得到了很好的钝化；透射电子显微镜表明，样品形成了界面陡峭的多层结构.结合光吸收谱和光致发光谱的研究，对其发光机理进行了讨论.用一维量子限制模型对光致发光峰随着a-Si:H层厚度的减小 关键词： a-Si:H/SiO2多层膜 光致发光