Grain Boundaries In Polycrystalline Silicon Research Articles

Rashba Interaction in Polysilicon Layers SemOI-Structures

The measurements of the magnetoresistance for p-type poly-Si with concentration 2.4 × 1018 cm−3 were carried out in the low temperature range 4.2–20 K and in the magnetic field up to 14 T. The results showed the presence of a negative magnetoresistance in polycrystalline silicon films in SemOI-structures. The low-temperature transport of charge carriers in p-type polycrystalline silicon films was considered within the framework of hopping conductivity and can be described by the spin–orbital interaction in the theory of weak localization. The calculated values of the coherence phase length 3–4 nm and the spin–orbit coherence length 30–50 nm at low temperatures 4.2–30 K correlate with parameters of hopping conductance and grain size, respectively, which show a contribution of Rashba spin–orbit interaction with energy ΔSO = 1.6 meV in the conductance not only inside the grains, but also its surface and between grain boundaries of polycrystalline silicon in SemOI structures.

Predoping effects of boron and phosphorous on arsenic diffusion along grain boundaries in polycrystalline silicon investigated by atom probe tomography

The effect of P or B predoping on As diffusion in polycrystalline Si was investigated by atom probe tomography. In all samples, a high concentration of As was found at grain boundaries, indicating that such boundaries are the main diffusion path. However, As grain-boundary diffusion was suppressed in the B-doped sample and enhanced in the P-doped sample. In a sample codoped with both P and B, As diffusion was somewhat enhanced, indicating competition between the effects of the two dopants. The results suggest that As grain-boundary diffusion can be controlled by varying the local concentration of P or B.

Dopant Segregation and Heat Treatment Effects on the Electrical Properties of Polycrystalline Silicon thin Films

This paper reports the effect of heat treatment on the electronic activity of grain boundaries of polycrystalline silicon. The results obtained show that for the same concentration of doping, the arsenic doped films are more resistive and have less free carriers than boron doped films. The arsenic atoms have a greater tendency to segregate at the grain boundaries than boron atoms. We also noticed that the heat treatment before implantation reduces the number of trap carriers and the quantity of doping atoms at the grain boundaries. For low doping, the concentration of the free charge carriers improves after the heat treatment by 100 % and 23 % for arsenic and boron doping respectively.

Image analysis of polycrystalline solar cells and modelling of intergranular and transgranular cracking

An innovative image analysis technique is proposed to process real solar cell pictures, identify grains and grain boundaries in polycrystalline Silicon, and finally generate finite element meshes. Using a modified intrinsic cohesive zone model approach to avoid mesh dependency, nonlinear finite element simulations show how grain boundaries and Silicon bulk properties influence the crack pattern. Numerical results demonstrate a prevalence of transgranular over intergranular cracking for similar interface fracture properties of grains and grain boundaries, in general agreement with the experimental observation.

Potential barrier at grain boundaries in polycrystalline silicon: influence of grain-boundary character and copper/iron contamination

We used Kelvin probe force microscopy to measure the potential barrier associated with grain boundaries in B-doped p-type polycrystalline silicon. The potential barrier at non-contaminated grain boundaries varied approximately from 5 to 90 meV according to grain-boundary character. The random boundaries had a higher potential barrier than the coincidence boundaries. This barrier increased with increasing twist component of the grain boundary when the misorientation angle of two adjoining grains was almost the same. Copper and iron contamination caused a significant increase in the grain-boundary potential barrier, making the potential barrier depend more strongly on misorientation, probably because of anisotropy of grain-boundary segregation.

Interfacial state and potential barrier height associated with grain boundaries in polycrystalline silicon

Importance of polycrystalline silicon has been recognized in the electronic device technology. The interfacial states in the band-gap and potential barrier associated with grain boundaries in polycrystalline silicon can exert their detrimental influence on electrical conductivity and then on device performance. However, all grain boundaries are not similarly potential sites for electrical activity because individual grain boundaries have their own character depending on the orientation relation between two adjoining grains. We apply the electron-beam-induced current technique and the Kelvin probe force microscopy to observe the carrier recombination intensity and the potential barrier height, respectively, at well-characterized grain boundaries in semiconductor-grade polycrystalline silicon. The results are compared with the previously observed ones in solar-grade silicon to examine the factors affecting electrical activity of grain boundaries.

Influence of low-energy ion-beam treatment by hydrogen on electrical activity of grain boundaries in polycrystalline silicon

The results of investigation into the effect of hydrogen plasma treatment and annealing on the electrical activity of grain boundaries (GBs) in silicon ribbons manufactured with the edge-defined film-fed growth (EFG) technique are presented. It is shown that hydrogenation with small doses of introduced hydrogen leads to decrease of the GB electrical activity, whereas further growth of hydrogenation time gives rise either to saturation or increase of electrical activity for annealed and non-annealed ribbons, respectively. The latter is explained in terms of a model taking into account the concurrence of GB defects passivation and neutralization of boron ions in the vicinity of GB during hydrogenation.

Electrical activity of grain boundaries in polycrystalline silicon – influences of grain boundary structure, chemistry and temperature

Polycrystalline silicon is extensively used in modern technology like solar-cells and thin film transistors. Towards the development of high performance devices based on polycrystalline materials, it is essential to design and control grain boundary microstructures. This article gives an overview of our recent experimental studies on electrical properties of grain boundaries in polycrystalline silicon. Particular attention is paid to the influences of the grain boundary character/structure, chemistry (hydrogenation and oxidation) and temperature on the electrical activity of individual grain boundaries. The electron beam induced current (EBIC) technique was used to observe electrical activity of grain boundaries. The observed results are discussed based on the Shockley-Read-Hall statistic.

Evaluation of trap states at front and back oxide interfaces and grain boundaries using electrical characteristic analysis and device simulation of polycrystalline silicon thin‐film transistors

AbstractThe trap states at the front and back oxide interfaces and grain boundaries of polycrystalline silicon thin‐film transistors are evaluated by using electrical characteristic analysis and device simulation. First, the method for extracting the trap densities of the front and back oxide interfaces and the grain boundaries is explained as the electrical characteristic analysis. When applied to the diagnosis of actual transistor characteristics, we see that oxygen plasma treatment or vapor heat treatment effectively decreases the trap densities at the front and back oxide interfaces, and self‐heating degradation increases the trap densities at the front and back oxide interfaces. Next, we use device simulation to examine the dependence of the transistor characteristics on the trap densities. When the simulation results were similarly applied to the diagnosis of actual transistor characteristics, we see that the oxygen plasma treatment naturally reduced the trap densities at the oxide interfaces, and the plasma‐enhanced chemical‐vapor deposition of tetraethylorthosilicate increased the trap densities at the grain boundaries. In a systematic summary of these analysis results, the effects of the trap states on the transistor characteristics, the improvement method, and the causes of degradation are discussed. © 2005 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 88(2): 1–10, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.20124

Measurements of potential barrier height of grain boundaries in polycrystalline silicon by Kelvin probe force microscopy

In recent years, the importance of polycrystalline silicon has been recognized in electronic device technology, although grain boundaries present in the material often exert a detrimental influence on the electrical properties because of the potential barriers associated with them. However, it is not true that all grain boundaries have similar properties, since they have their own character depending on the orientation relationship between two adjoining grains. We report here the first determination of the potential barrier height for well-characterized grain boundaries in polycrystalline silicon, using Kelvin probe force microscopy. The observed barrier height of the grain boundaries was found to vary in the range 10 to 100 meV depending on the grain boundary character. The most important finding is that the potential barrier height is approximately twice as high at random boundaries as at low-energy coincidence boundaries.

Reduction of grain-boundary potential barrier height in polycrystalline silicon with hot H2O-vapor annealing probed using point-contact devices

The effects of hot H2O-vapor annealing were investigated on local carrier transport properties over a few grain boundaries in polycrystalline silicon. It shows that hot H2O-vapor annealing effectively reduces grain-boundary dangling bonds and the potential barrier height. In addition, it narrows the distribution of the barrier height value significantly. These effects are thought to originate from oxidation in the vicinity of the film surface, and from hydrogenation in the deeper region. Our results suggest that H2O annealing can improve the carrier transport properties by opening up shorter percolation paths and by increasing the effective carrier mobility and density.

Characterization of Tunnel Barriers in Polycrystalline Silicon Point-Contact Single-Electron Transistors

Potential barrier properties of grain boundaries in polycrystalline silicon have been studied using nanometer-scale point-contact transistors. The devices, with channel length and width ranging from 30 nm to 50 nm, were fabricated in a 50-nm-thick film. We found that grain boundaries oxidized at 1000°C acted as tunneling barriers, resulting in the appearance of single-electron charging effects. However, potential barrier heights estimated for oxidized grain boundaries are still low, less than 90 meV, and barrier thickness is as large as ∼4 nm. The grain boundary oxides appear to be silicon sub-oxides which are formed by oxygen diffusion into grain boundaries.

Nanocrystalline silicon from hot-wire deposition — a photovoltaic material?

Nanocrystalline silicon (nc-Si:H) attracts a great deal of attention due to the hope for more efficient and stable solar cells, as well as better thin-film transistors and optical sensors. In this study, we report on improvements in the structural and electronic qualities of intrinsic nc-Si:H grown from hot-wire chemical vapour deposition. For examining a wide range of deposition parameters, we use a design-of-experiments approach. In contrast to our previous films obtained from tungsten and tantalum filaments, a novel type of filament greatly enhances preferential growth in the 〈110〉 direction over a wide range of deposition conditions. General considerations on the orientation and electronic activity of grain boundaries in polycrystalline silicon explain why the electronic quality of this 〈110〉-oriented film is remarkably higher than the one previously grown, mixed-phase nanocrystalline silicon. Mobility–lifetime products of films from our novel filaments are two orders of magnitude higher than those of samples from Ta wires. In photoluminescence spectra, no band tail contributions occur, and the amorphous and defect peaks are greatly reduced. Moreover, the transverse optical Raman signal is red-shifted, and thereby indicates a reduction in mechanical strain in our novel nanocrystalline silicon films.

Transparent Probe Test Structure for Electrical and Physical Characterization of Defects in Thin Films

This paper presents a new test structure, the transparent probe, which allows transmission electron microscopy analysis techniques on small regions of a thin film which have been physically isolated and tested electrically. In this way, physical structure may be correlated with associated electrical properties with a resolution not previously available by other techniques. An example described here is taken from a study on grain boundaries in polycrystalline silicon, but the test structure may be reasonably expected to be applicable to any thin film material which may be similarly patterned and etched. The technique can provide quantitative structural and electrical information on electrically active defects as small as a few tenths of a micrometer wide. © 2000 The Electrochemical Society. All rights reserved.

Effect of Stress and Impurities on Preferential Amorphization on Grain Boundaries in Polycrystalline Silicon

AbstractClarifying the local amorphization on the grain boundaries, the in-situ observation during ion-irradiation was carried out for poly-crystalline Si film. The critical dose of amorphous formation increased exponentially with increasing temperature, where the local amorphization was developed at middle temperature. The critical dose was affected by the doped impurity and the grain size. The preferential amorphization on and near grain boundaries had two processes; first stage with rapid growth rate and second stage with almost constant growth rate. The importance of stress was demonstrated from the acceleration due to the stress on the first stage of amorphization.

Kinetics of arsenic segregation at grain boundaries in polycrystalline silicon

By comparison between chemically and electrically active As concentrations in polycrystalline silicon thin films with a well-stabilized grain size, the equilibrium segregation coefficient relating the grain boundary and the Si grain is experimentally obtained in the 750 - C temperature range. Moreover, by using isothermal short anneals, it has been possible to study the kinetics of the segregation phenomena. It is found that the time constant corresponding to the achievement of the equilibrium state strongly depends on the annealing temperature. The kinetics coefficients controlling the rates of exchange between the grains and the grain boundaries have been extracted on the basis of a simple kinetics model. The technological relevance of these results for the doping control of the polycrystalline silicon layers is pointed out.

A high-resolution scanning Kelvin probe microscope for contact potential measurements on the 100 nm scale

The article describes the principles and current performance of a scanning Kelvin probe microscope for contact potential measurements with a lateral resolution on the 100 nm scale and a sensitivity in the millivolt range. Preliminary results are presented regarding the variation of the surface potential across charged grain boundaries in polycrystalline silicon.

Energetics of bond-centered hydrogen in strained Si-Si bonds.

The energetics of hydrogen incorporation into strained Si-Si bonds are examined using first-principles density-functional-pseudopotential calculations. It is found that an increase in the bond length of the initial Si-Si bond leads to a lower formation energy for the Si-H-Si configuration. A simple linear relationship, describing the change in formation energy as a function of the Si-Si bond length, is presented, which applies to bond-stretching as well as bond-bending distortions. The results are discussed in the context of hydrogen interaction with grain boundaries in polycrystalline silicon.

Energy distribution of trapping states at grain boundaries in polycrystalline silicon

A method to obtain the energy distribution of traps at grain boundaries in polycrystalline silicon films from the activation energy of resistivity measured as a function of doping concentration is proposed. The energy distribution of trapping states in the energy gap for plasma-hydrogenated and nonhydrogenated polycrystalline silicon films doped with boron are determined: nonhydrogenated films have a Gaussian distribution with a peak at the midgap, and after plasma hydrogenation the peak at the midgap has diminished. The validity of the monoenergetic trap model applied to the conduction in nonhydrogenated and plasma-hydrogenated polycrystalline silicon films is discussed.

Mechanism of Enhanced Hydrogen Diffusion in Solar Cell Silicon

ABSTRACTExperimental results that support our previously reported observations of enhanced diffusivity of hydrogen in some solar cell silicon are presented. The diffusivities of hydrogen, implanted at low energies, were measured for several commercial silicon substrates. It is shown that the diffusivity of hydrogen in some solar cell substrates is about two orders of magnitude higher than that in Float Zone silicon in a temperature range of 100° - 300°C. This value of diffusivity is also close to that observed along some grain boundaries in polycrystalline silicon. It is determined that hydrogen in-diffusion is accompanied by an out-diffusion of boron, and that the hydrogen diffusivity is retarded by high concentrations of oxygen. A mechanism for enhanced hydrogen diffusion is proposed which invokes formation of a highly mobile vacancy-hydrogen complex.