Grain Boundaries In Silicon Research Articles

Scattering of phonons from a high-energy grain boundary in silicon: Dependence on angle of incidence

We use molecular-dynamics simulation to elucidate phonon scattering from the high-energy $\ensuremath{\Sigma}29$ twist grain boundary in silicon. In particular, we have computed the dependence of energy transmission through the grain boundary on the wavelength and angle of incidence. Transmission through the grain boundary is found to be predominantly a function of the incident phonon frequency. In agreement with previous results, modes with wave vectors perpendicular to the grain-boundary plane exhibit relatively large energy-transmission coefficients. However, as the wavelength decreases and frequency increases, the energy transmission through the interface tends to sharply decrease. To develop a comprehensive picture of elastic phonon scattering, we have studied longitudinal-acoustic, transverse-acoustic, and some longitudinal-optical modes. By considering a simple theory that relates the energy-transmission coefficients to the Kapitza conductance, we are able to make a quantitative prediction based on detailed transmission probabilities. Predictions obtained using this model are relevant for comparison to both the classical (i.e., high-temperature) and quantum (i.e., low-temperature) regimes. We discuss the temperature dependence of the Kapitza conductance and suggest avenues of inquiry including experimental verification.

Atomic Structure of Faceted Σ3 CSL Grain Boundary in Silicon: HRTEM and <I>Ab-initio</I> Calculation

{112} Σ3 CSL grain boundary in silicon was investigated by high-resolution transmission electron microscopy (HRTEM) and ab-initio calculation. A {112} Σ3 CSL boundary consisted of two segments which differed in atomic structure. The segment near the connected corner to {111} Σ3 CSL boundary showed symmetric structure and the other long segment, being distant region from the corner, showed asymmetric structure. It was shown that the asymmetric structure is more stable than the symmetric one. In the symmetric segment a 5-fold coordinated atom presented, which elevated the structure energy of the boundary and produced a new state in the band gap.

Modification of Local Structure and Its Influence on Electrical Activity of Near (310) Σ5 Grain Boundary in Bulk Silicon

We report on the modification of the local structures of (310) Σ5 grain boundary in bulk silicon as the growth of crystal and its influence on electrical activity. The grain boundary was formed via floating-zone growth method utilizing bicrystal seed. The misalignment of the seed resulted in formation of the grain boundary with small deviations of crystal orientation from the Σ5 singular coincidence orientation. The deviations consist of tilt and twist components and they were found to monotonically decrease with respect to the distance from the seed crystal accompanied by the decrease in density of dislocations on the grain boundary. The change in the grain boundary structure allows a systematic study on the correlation between the structure and the electrical activity at the GB. The density of dislocations was found to control the electrical activity and the (310) Σ5 coincidence grain boundary without any dislocations is expected to show a very low electrical activity.

Grooving of grain boundaries in multicrystalline silicon: Effect on solar cell performance

In this work, we investigate the effect of grooving of grain boundaries (GB) in multicrystalline silicon using chemical etching in HF/HNO3 solutions. The grain boundaries were grooved in order to reduce the area of these highly recombining regions. Using optimized conditions, grooved GBs enable deep phosphorus diffusion and deep metallic contacts. As a result, the internal quantum efficiency (IQE), and the I–V characteristics under the dark and AM1.5 illumination were improved. It was also observed a reduction of the GB recombination velocity, which was deduced from light-beam-induced-current (LBIC) measurements. Such grooving in multicrystalline silicon enables passivation of GB-related defects. These results are discussed and compared to solar cells based on untreated multicrystalline silicon wafers.

Order and structural units in simulations of twist grain boundaries in silicon at absolute zero

In contrast to previous results of simulations of the ground-state structures of high-angle twist grain boundaries (GB's) in silicon at absolute zero, in which amorphous intergranular films were found, recent experimental results imply structural order in some high-angle twist boundaries. With a novel protocol for simulating twist GB's, which allows the number of atoms at the boundary to vary, we have found new, lower energy, ordered structures. We have also found structural units common to many of these boundaries, which is further evidence that these boundary structures are not arbitrary or disordered. We give a detailed exposition of the results for five boundaries and conclude that there is structural order present in all of them.

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 Grain Boundary Properties on Microcracking of AlN in Sliding Test

Grain boundary microcracking in sliding test using a silicon pin was evaluated on several kinds of AlN to investigate the behavior of damage generation and the influence of material properties on it. The grain boundary fracture toughness (KIC gb ) was estimated from a percentage of the intergranular fracture as a grain boundary property for each specimen. KIC gb was greatly influensed by an amount and kinds of additives. After the sliding test, silicon debris accumulation and grain boundary microcracking of AlN were observed at the wear traces on the surface of AlN. The density of grain boundary microcracks increased linearly with increasing the contact load. Empirical relationship indicated that its density depended on KIC gb and the AlN grain size, and decreased mainly with increasing KIC gb. The density of grain boundary microcracks greatly degreased at the KIC gbvalue above 1.9 (MPa·m1/2).

Metal precipitation at grain boundaries in silicon: Dependence on grain boundary character and dislocation decoration

Synchrotron-based analytical microprobe techniques, electron backscatter diffraction, and defect etching are combined to determine the dependence of metal silicide precipitate formation on grain boundary character and microstructure in multicrystalline silicon (mc-Si). Metal silicide precipitate decoration is observed to increase with decreasing atomic coincidence within the grain boundary plane (increasing Σ values). A few low-Σ boundaries contain anomalously high metal precipitate concentrations, concomitant with heavy dislocation decoration. These results provide direct experimental evidence that the degree of interaction between metals and structural defects in mc-Si can vary as a function of microstructure, with implications for mc-Si device performance and processing.

Modelling of grain boundary effects in nanocrystalline/multicrystalline silicon heterojunction solar cells

Heterojunction solar cells formed by nanocrystalline silicon films on fine-grained multicrystalline silicon substrates are simulated in the presence of grain boundaries. The effects of grain boundaries on the dark and illuminated current–voltage (I–V) characteristics and spectral response (SR) of heterojunction (HJ) solar cells are assessed using 1D and 2D device simulations. The grain boundary in fine-grained multicrystalline silicon is modelled in two ways: as a defective surface with continuous defect distribution throughout the bandgap, and as a hypothetical sheet with a certain recombination velocity for electrons and holes. The SR and I–V characteristics of HJs are exploited to characterize grain boundary effects on the photovoltaic properties of the solar cells and photodetectors. Simulation results show noticeable differences on the dark I–V and SR of on- and off-grain boundary HJs. Grain boundary effects become important when fine-grained multicrystalline substrates are used. Measurement results of tiny test structures fabricated on the grain boundary show consistently inferior dark I–V and SR characteristics compared to those fabricated away from the grain and allow us to quantify the recombination at the grain boundary.

Are the Structures of Twist Grain Boundaries in Silicon Ordered at 0 K?

Contrary to previous simulation results on the existence of amorphous intergranular films at high-angle twist grain boundaries (GBs) in elemental solids such as silicon, recent experimental results imply structural order in some high-angle boundaries. With a novel protocol for simulating twist GBs, which allows the number of atoms at the boundary to vary, we have found new low-energy ordered structures. We give a detailed exposition of the results for the simplest boundary. The validity of our results is confirmed by first-principles calculations.

Dedication: phys. stat. sol. (a) 202/15

The papers in this issue are dedicated to Professor Horst Paul Strunk on the occasion of his 65th birthday and his retirement from active teaching. This volume honours a scientist who has made a lasting impact on the field in electron microscopic characterisation of growth and relaxation phenomena in epitaxial growth of semiconductors. Born in The Hague, The Netherlands, on 13 June 1940, he studied physics in Stuttgart where he received his degree in Physics in 1968. He joined the group of Prof. Seeger at the Max-Planck-Institut für Metallforschung and defended his Ph.D. on defects in NaCl at Stuttgart University in 1973. He spent one year at Cornell University as a visiting Professor before joining Technische Universität Hamburg–Harburg in 1983. There he created the “Zentralbereich Elektronenmikroskopie” and was a professor for materials analytics from 1983 till 1989. In 1989 he changed to the University of Erlangen–Nürnberg, where he established the “Verbundlabor für hochauflösende Elektronenmikroskopie” and directed the “Lehrstuhl Mikrocharakterisierung” at the Institut für Werkstoffwissenschaften of the same university. He spent two research periods at the Universities of Rennes in France and Campinas in Brazil. Together with his colleague Prof. Jürgen Werner he created the series of conferences on polycrystalline semiconductors POLYSE which he has been supervising together with Jürgen Werner since 1990. The research activities of Horst P. Strunk are focused on microstructure of materials and their relation to macroscopic physical properties. Main topics are dislocations, their formation and interaction mechanisms, strain relaxation as well as fundamental mechanisms of epitaxial growth. The spectrum of materials covers a wide range starting from metals over ionic crystals, e.g. NaCl to elemental and compound semiconductors. From the beginning, the main tool of study has been the transmission electron microscope. However, Horst P. Strunk recognised that a thorough understanding of materials problems would require the combined use of structural characterisation, advanced spectroscopy and modelling. Therefore he complemented electron microscopic approaches by optical methods e.g. Raman spectroscopy and cathodoluminescence. Modelling of strain states by finite elements and of defect structures by ab-initio calculations became an important topic especially in the last years. It is characteristic for the scientific approach of Horst Strunk that methodological developments were not an end in itself but linked to problems in solid state physics and materials sciences. Among the scientific works of Strunk, a few examples should be highlighted which mark important stages in his scientific career. Pioneering work has been done on the influence of dislocations in homoepitaxial growth of Si and GaAs in collaboration with Elisabeth Bauser in Stuttgart. Strunk correlated growth spirals on the surface to dislocations that caused these step sources. Studying the dislocation structure of heteroepitaxial Ge/GaAs layers, Strunk discovered that a new dislocation multiplication source works which, later known as Hagen–Strunk source, had a strong impact on understanding of relaxation processes by dislocations in heteroepitaxial semiconductor systems. Work on electrical and structural properties of grain boundaries in silicon was performed together with Jürgen Werner. This was the starting point of a long lasting research on photovoltaic materials that accompanies Strunk till today. Fundamental studies on heteroepitaxial growth were performed in the system SiGe grown from solution. In this context, finite elements were established for the first time in the study of nanostructured materials. In the last years correlated studies on structural and optical properties on III-nitride heterostructures were done by cathodoluminescence in the transmission electron microscope. The impact of Horst P. Strunk's work is evident from the fact that his lab became part of collaborative international projects based on the unique facilities at the “Verbundlabor für Hochauflösende Elektronenmikroskopie” and the profound knowledge in the field of crystal growth and solid state physics present in his group. The articles in this issue contain original research results contributed by his friends, collaborators and former students. They are a testimony of the lasting impact of Horst P. Strunk's work and they express the authors' gratefulness for benefiting from his work. This volume gives us a unique opportunity to say “thank you” to Horst P. Strunk and to wish him a new period in his life that should continue to be scientifically as fruitful as up to now but less affected by the burden of administrative work than during the last years.

Characterization of grain growth, nature and role of grain boundaries in microcrystalline silicon—review of typical features

A boundary between amorphous and microcrystalline growth of silicon thin films was explored for the study of grain growth by changing parameters of plasma enhanced deposition (substrate temperature, silane dilution or deposition duration and thus the film thickness). Resulting series of a-Si:H/μc-Si:H samples were characterized by values of dark conductivity and corresponding activation energy and prefactor, diffusion length, hydrogen content and morphology. The abrupt change of conductivity at the boundary is accompanied by a peak in surface roughness and diffusion length, which were connected to the formation of a percolating network of microcrystalline large grains. Results are discussed using the idea of barriers for electronic transport forming at the large grain boundaries. Comparison of transport properties with the hydrogen content pointed out the fundamental role of hydrogen present at the grain boundaries.

Electron-beam-induced current study of hydrogen passivation on grain boundaries in multicrystalline silicon: Influence of GB character and impurity contamination

The impacts of grain boundary (GB) character and impurity contamination level on the hydrogen passivation of GBs in multicrystalline silicon (mc-Si) were studied by means of an electron-beam-induced current (EBIC) technique. In mc-Si with a low contamination of Fe, the 300 K EBIC contrast of all kinds of GBs in the H-passivated state was weak and similar to that in the as-grown state. The 100 K EBIC contrast of Σ ( Σ = 3 , 9, and 27) GBs decreased about 75–80%, whereas that of random and small-angle GBs decreased about 35–40%. Due to the different impurity gettering ability of different GBs, the variation in 100 K EBIC contrast has suggested that the effect of H-passivation depends on both the GB character and impurity contamination level. In the mc-Si with heavy contamination of Fe, at both 300 and 100 K, the EBIC contrast of both Σ ( Σ = 3 ) and random GBs decreased but the ratio was <40%, suggesting that the H-passivation is mainly affected by the impurity contamination level.

Electron-beam-induced current study of small-angle grain boundaries in multicrystalline silicon

Abstract Recombination activity of small-angle grain boundaries (SA GBs) in multicrystalline silicon (mc-Si) was studied by means of electron-beam-induced current (EBIC) technique. In the as-grown mc-Si, the EBIC contrasts of special Σ and random GBs were weak at both 300 and 100 K, whereas those of SA GBs were weak (

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.

Simulation of carbon containing complexes at silicon–silicon grain boundaries in cluster approximation

The transformation of the “core” for the tilt Σ5 θ=37° [001]/(130) grain boundary (GB) in polycrystalline silicon due to incorporation of carbon atoms is studied with a method of molecular orbital–linear combinations of atomic orbital (MO LCAO) in PM3 approximation. Insertion of carbon atoms into 5- or 7-fold interstitial positions at GB “core” formed donor-like complexes which are composed of combined Si 2C, Si 3C or Si 4C configurations depending on number x of incorporated C-atoms. Energy levels of such complexes are shifted to the bottom of the conductive band from E C—0.536 eV to E C—0.043 eV with increasing x from 1 to 4.

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

Electronic and structural properties of grain boundaries in electron-irradiated edge-defined film-fed growth silicon

Edge-defined film-fed growth (EFG) is an economical method of producing multicrystalline silicon ribbon for solar cells. Such silicon is heavily doped with carbon. After electron irradiation, the dominant defect found in this material is the G-centre, which is associated with the CsCi defect. In this paper, the techniques of scanning cathodoluminescence and electron backscattered diffraction pattern analysis are used to correlate the luminescence from the G-centre with the grain boundary structure in electron-irradiated EFG silicon. A localized enhancement of G-centre luminescence is found near twin boundaries at temperatures above 20 K, whereas no such enhancement is found near low-angle grain boundaries at temperatures up to 80 K or at twin boundaries below 20 K. This behaviour may be caused by thermal ionization of excitons from traps at the twin boundaries, and their subsequent capture at G-centres.

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.