Extended-state mobility in hydrogenated amorphous silicon

We use the results of time-of-flight experiments in conjunction with recent conclusions about the behavior of the density of localized states below the conduction-band mobility edge to calculate the mobility of electrons moving in extended states in a-Si:H. We find that the extended-state mobility is considerably larger than previous estimates, which were based on the assumption that the exponential behavior responsible for dispersive transport extends all the way to the mobility edge. Using a recent estimate for the density of localized states, we find that the extended state mobility in a-Si:H is about 500 cm²/V-s, a value consistent with the results deduced from high-level injection experiments on p-i-n structures.     

The relationship between hydrogen and paramagnetic defects in thin film silicon irradiated with 2 MeV electrons

After irradiation of hydrogenated amorphous and microcrystalline silicon (a-Si:H and μc-Si:H) with 2 MeV electrons at 100 K, we observe satellite-like components close to the dominating electron spin resonance (ESR) signal of these materials. The satellites overlap with the commonly observed dangling bond resonance and are proposed to originate from a hyperfine interaction with the nuclear magnetic moment of hydrogen atoms in a-Si:H and μc-Si:H. Our present study is focused on the verification of this hypothesis. Equivalent hydrogenated and deuterated a-/μc-Si:H/D materials have been investigated with ESR before and after 2 MeV electron bombardment. From the difference between ESR spectra of hydrogenated and deuterated samples we identify the doublet structure in the ESR spectra as a hyperfine pattern of hydrogen-related paramagnetic centers. The observations of H-related paramagnetic centers in a-/μc-Si:H are of particular interest in view of metastability models of a-Si:H, which include H-related complexes as precursors for the stabilization of the metastable Si dangling bonds. The nature of the observed center is discussed in the light of known H-related complexes in crystalline Si and suggested H-related dangling bonds in a-Si:H.     

A chemical-bond approach to doping,compensation and photo-induced degradation in amorphous silicon

We propose that during deposition of a-Si:H films a chemical equilibrium is established that relates the density of dangling-bond defects near mid-gap to the densities of electrons and holes in the conduction and valence band states. We develop the appropriate chemical reaction formalism and show that our model allows doping, compensation and photo-induced degradation to be treated within a single and unifying approach.     

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.     

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.     

Fast diffusion of H and creation of dangling bonds in hydrogenated amorphous silicon studied by in situ ESR

The interaction of atomic hydrogen with a-Si:H films was studied by means of in situ ESR during H plasma treatment. H diffuses into the a-Si:H film and creates additional Si dangling bonds ( approximately 10(13) cm (-2)). We observed a high diffusion coefficient (>10(-10) cm (2) s (-1)) at the very initial stage of H treatment (<1 s). The resulting additional dangling bonds are spatially distributed ( approximately 100 nm) into the bulk film. The characteristic depth of dangling bond (db) distribution decreases with increasing H treatment temperature. The activated rate constants of db creation and annihilation reactions determine the distribution of additional dangling bonds at different treatment temperatures.     

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     

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.     

a-Si:H中本征缺陷引起的赝隙态

本文取原子集团模型Si₈H₁₈,Si₁₇及从连续无规网络中挖取的集团模型Si₂₉和Si₄₇,用CNDO LCAO-MO方法计算其电子结构,探讨了a-Si:H中由弱键、弯键和带电组态等本征缺陷引起的赝隙态分布。结果表明,当弱键拉伸时,两个弱键态移动并收缩至带隙中央;过剩电荷使弱键能级移至价带顶或导带底附近;弯键态主要出现在价带顶附近。当弯键曲率较大时,弯键态上移至带隙中央以下的区域。结构拓扑无序导致 关键词：     

Post-transit-time analysis of time-of-flight photocurrents

It is shown how the density of localized gap states in amorphous semiconductors can be deduced from time-of-flight photocurrents on the basis of the trap-limited band transport model, post-transit currents giving information on deep-state densities; and how strong deep-trapping of the photo-induced excess carriers invalidates the analysis. Results from a-Si:H, p-type a-Si:H, a-Si,S:H and a-Si,C:H samples are used to illustrate these points.     

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.     

Investigation of the Occupation Function of Localized States in Amorphous Semiconductors Under Infrared-Light Excitation

The occupation function of localized states in a-Si:H under i.r. light excitation is directly calculated from the observed infrared-stimulated photocurrent spectra. The calculated occupation function in dual-beam and one-beam excitation cases are compared. The overshot phenomenon in the dual-beam experiment is explained as the result of the difference of the initial condition between two cases when i.r. light is turned on. In the dual-beam excitation case, at the beginning, in the upper part of the band gap, there are many trapped electrons to be excited into the conduction band to cause the photoconductivity overshot.     

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。     

Hydrogen diffusion and electronic metastability in amorphous silicon

Hydrogen diffusion and its role in the many electronic metastability phenomena in hydrogenated amorphous silicon (a-Si:H) is reviewed. A-Si:H contains about 10 at% hydrogen, most of which is bonded to silicon. The hydrogen diffuses at relatively low temperatures by releasing hydrogen from the Si-H bonds into interstitial sites. The reactions of hydrogen with the silicon dangling bonds and the weak bonds provide a hydrogen-mediated mechanism for electron-structural interactions, which are manifested as electronic metastability. The annealing of light-induced defects, the equilibration of defects and dopants, the stretched exponential relaxation kinetics, and the atomic structure formed during growth, are all attributed to hydrogen diffusion.     

Photoinduced absorption spectrum of a-Si:H between 0.65 and 1.6 eV

Photoinduced absorption in a-Si:H is measured between 0.65 eV and 1.6 eV using a guided wave technique. Spectra are interpreted in terms of holes transitions from trap states in the valence band tail to the valence band.     

Relationship between optical and structural properties of hydrogenated amorphous silicon

A series of experimental studies has been made on the relationship between optical and structural properties of hydrogenated amorphous silicon (a-Si:H) prepared under various conditions. It has been clarified by analysing the results that the shape of the energy spectrum near the band edge and the distribution of the valence-band tail states depend primarily on the structural disorder of the Si network in a-Si:H. On the other hand, the total content and the bonding mode of bonded hydrogen have little effects on these electronic properties of a-Si:H. It has also been found that the distribution of the valenceband tail states might be related to other unidentified factor(s) besides the structural disorder. The present results have been compared with those of the previous experimental and theoretical studies.     

Influence of heterogeneities on the electronic properties of hydrogenated amorphous silicon

An attempt is made to highlight the importance of inhomogeneities in hydrogenated amorphous silicon (a-Si:H), in controlling its electronic properties. We note that hydrogen increases the gap in a-Si:H and that hydrogen is distributed inhomogeneously in it. This gives rise to long-range potential fluctuations, which are mostly uncorrelated and usually ignored. These and other such considerations have not only enabled us to gain new insights into the behaviour of a-Si:H in general, but have also allowed us to resolve several unsolved puzzles. Among these are questions like why undoped a-Si:H is n-type, why the creation of dangling bonds upon light soaking (LS) so inefficient, why a-Si:H degrades more upon LS when it is doped, why the reciprocity fails for light-induced degradation, why presence of nanocrystalline silicon improves stability and so on. We provide evidence to support some of our ideas and make suggestions for verifying the others.     

载流子选择性接触:高效硅太阳电池的选择

太阳电池可看成由光子吸收层和接触层两个基本单元组成,接触层是高复合活性金属界面和光子吸收层之间的区域.为了进一步提高硅太阳电池的转换效率,关键是降低光子吸收层和接触之间的复合损失.近年来,载流子选择性接触引起了光伏界的研究兴趣,其被认为是接近硅太阳电池效率理论极限的最后的障碍之一.本文分析了三种类型的载流子选择性接触:在光子吸收层与金属界面之间引入薄的重掺杂层,即所谓的发射极或背面场;利用两种材料之间的导带或价带对齐;利用高功函数的金属氧化物与晶硅接触从而在晶硅中感应能带弯曲.基于一维太阳电池模拟软件wx AMPS,模拟了扩散同质结硅太阳电池[结构为(p~+)c-Si/(n)c-Si/(n~+)c-Si]、非晶硅薄膜硅异质结太阳电池[结构为(p~+)a-Si/(i)a-Si/(n)c-Si/(i)a-Si/(n~+)a-Si]和氧化物薄膜硅异质结太阳电池[结构为(n)MoO_x/(n)c-Si/(n)TiO_x]暗态下的能带结构和载流子浓度的空间分布,其中c-Si为晶硅;a-Si为非晶硅;(i),(n)和(p)分别表示本征、n型掺杂和p型掺杂.模拟结果表明:载流子选择性接触的核心是在接触处晶硅表面附近形成载流子浓度空间分布的不对称进而使得电导率的不对称,形成了对电子的高阻和空穴的低阻或者对空穴的高阻和电子的低阻,从而让空穴轻松通过同时阻挡电子,或者让电子轻松通过同时阻挡空穴,形成空穴选择性接触或者电子选择性接触.     

B掺杂a-SiC:H/本征a-Si:H异质结中的B扩散

本文用 ¹¹B(p,a)⁸ Be(E_{r=163keV)核反应分析方法,研究不同生长温度和退火温度对a-SiC:H(B)/a-Si:H异质结构中B原子浓度剖面分布的影响,由B原子浓度剖面分布的变化,分别估算了B在a-Si:H生长和退火过程中的扩散系数,结合电导率随膜厚度变化的测量,并依据最新提出的热平衡缺陷观点,对B的扩散过程作了分析。 关键词：}