The 25th International Conference on Amorphous and Nano-crystalline Semiconductors
August 18–23, 2013 Toronto, Ontario Canada
Nano-Micro-Poly-Silicon and Multilayers: Transport and Electronic Properties I
Chair: Seiichi Miyazaki, Nagoya University
Quantifying Order at Different Length Scales in Solid Materials: Implications for Semiconductors
Department of Physics & Physical Oceanography, Memorial University of Newfoundland, St. John's, NL A1B3X7, Canada
Changes to structural periodicity can have a dramatic influence on a material's physical properties and reactivity. Nevertheless, disorder in non-glassy solids is extremely understudied. In this work, we outline a multi-faceted strategy for quantifying structural order at different length scales in solid materials, which we implement on a model system. Based on these findings, we present opportunities, as well as caveats, for applying experimental structural analyses and simulations to amorphous and nano-crystalline semiconductor materials. Our ultimate goal is to generalize this knowledge to tailor semiconducting materials for specified degrees and types of disorder in an effort to tune their electronic and optical properties.
Calcium carbonate is an electrical insulator that accommodates a wide range of structural disorder, which makes it an ideal model system for our experimental investigations. Not only does this material exhibit three different crystalline polymorphs (calcite, aragonite, vaterite), but it also accommodates extensive polyamorphism (the existence of multiple amorphous structures) that is a thriving area of research, yet still poorly understood. We show X-ray diffraction and Extended X-ray absorption fine structure data, in conjunction with Fourier transform infrared (FTIR) and Raman spectroscopic data, to demonstrate that local order in calcitic materials can be preserved in the face of many different kinds of long-range order disruptions. From these data, we pinpoint gaps that exist in experimental structural assessment methods that are appropriate for quantifying nano-crystalline and micro-crystalline structural order at length scales that lie beyond the first few spheres of an atomic-level coordination environment.
We discuss these experimental structure studies in the context of a growing body of literature that addresses the crystallization kinetics and energetics of calcium carbonate through molecular dynamics simulations. By drawing parallels with nucleation and crystallization processes in glassy materials, we identify structural features that disrupt order at different short- and intermediate-length scales in materials with poor long-range order. We focus on tracking signatures of such structural features in simulated vibrational density of states data, and we show how these can be correlated with experimentally obtained vibrational spectroscopic (FTIR and Raman) data.
Finally, we put our findings in a broader context by discussing their implications for linking structure and physical properties in semiconducting materials to be targeted for electronic and optical applications. A major challenge is that structural disorder in many semiconductors is induced kinetically, which makes the structural details very sensitive to material preparation, treatment, and usage conditions. This means that attention to structural disorder must be paid throughout the lifespan of the material. It is also important to note that the investigations described here were designed to focus on structural disorder in chemically homogeneous systems; we do not address doping effects directly. The electronic structure of poorly ordered semiconductors will be very sensitive to the specific details of the structural disorder. Furthermore, surface electronic structure can be different, and in some cases more dominant, than bulk electronic structure features.
Keywords: amorphous materials, polycrystalline materials, vibrational spectroscopy, vibrational densities of state, molecular dynamics simulations
Electronic Transport in Boron Doped Solid Phase Crystallized Poly-silicon
1. Helmholtz-Zentrum Berlin für Materialien und Energie, Institut für Silizium Photovoltaik, Kekuléstrasse 5, 12489 Berlin, Germany
2. Sony Deutschland GmbH, Hedelfinger Straße 61, 70327 Stuttgart, Germany
Polycrystalline silicon (poly-Si) is an attractive material for applications ranging from thin-film transistors to solar cells. Compared to amorphous silicon (a-Si) it exhibits a larger carrier mobility and does not suffer from long term degradation processes. Poly-Si can be fabricated on low budged substrates comprising float glasses and even plastic foil. However, the choice of substrates limits processing temperatures and hence, can impact electronic and structural properties of poly-Si.
In this paper, we present a detailed investigation on the influence of boron doping on charge-carrier transport in solid phase crystallized poly-Si. The samples were fabricated in a two-step process. First a-Si films were deposited by electron-beam evaporation on Corning glass, amorphous silicon-nitride coated Borofloat glass (SiN/Borofloat glass), and SiO2. Boron doping in the range of 4x1014 to 2x1020 cm–3 was accomplished by co-evaporation of B atoms from an effusion cell. In a second step all deposited samples were crystallized simultaneously by annealing them in a furnace at 600°C for 24 h. The resulting poly-Si specimens were composed of crystalline grains with an average size of about 1–2 μm. Information on charge transport were obtained from temperature dependent conductivity and Hall-effect measurements.
The doping efficiency was deduced from Hall-effect measurements. For a doping concentration of less than 5x1017 cm–3 the hole concentration was significantly lower than the B concentration. However, at high doping concentrations all acceptors were activated indicating a doping efficiency of close to 100%. A similar dependence on the doping concentration is observed for the Hall mobility.
When comparing poly-Si on the different substrates the lowest mobilities are observed for poly-Si on Corning glass. Interestingly, the Hall-mobility is thermally activated with an activation energy of about 0.2 eV, which is independent of the acceptor concentration for all samples where [B] < 1018 cm–3. At higher B concentration the activation energy shows a pronounced decrease. On the other hand, for poly-Si on amorphous silicon-nitride coated Borofloat substrates the activation energy increases with increasing B concentration and reaches a maximum value of about 0.16 eV at a doping concentration of 1018 cm–3. At higher B concentrations the activation energy also shows a pronounced decrease. This indicates that charge transport is governed by thermionic emission over grain-boundary potential-barriers. The data are analyzed in detail to elucidate the underlying transport phenomena and are discussed in terms of an advanced transport model developed by Baccarani et al. .
 G. Baccarani, B. Ricco and G. Spadini, J. Appl. Phys. 49, 5565 (1978)
Keywords: polycrystalline silicon, charge transport, transport models, grain boundaries, boron doping
Structural and Optoelectronic Properties of Si Quantum Dots/SiC Multilayers Embedded in P-I-N Structures
National Laboratory of Solid State Microstructures and School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China
Recently, Si quantum dots (Si QDs)/SiC multilayers have attracted much attention because of their potential applications in many kinds of devices such as light emitting diodes and next generation of solar cells. Usually, Si QDs/SiC multilayers were prepared by thermally annealing amorphous Si/SiC stacked structures at high temperature. However, from the viewpoint of device application, the p-i-n structures have to be used, and high temperature annealing will cause the undesirable impurity diffusion to deteriorate the device structures.
In the present work, KrF pulsed excimer laser induced crystallization of amorphous Si (a-Si)/SiC stacked structures was used to prepare Si QDs/SiC multilayers. The formation of Si QDs with average size of 4.5 nm was confirmed by Raman spectra while the layered structures were revealed by cross-sectional transmission electron microscopy. Optical absorption of laser crystallized samples was characterized. A red shift at the absorption edge was observed with the increase of laser energy density within a proper range. Raman spectra indicate the increase in size and crystallinity with the irradiation laser energy density, while a much higher laser energy density could cause damage to the multilayered structures. The p-i-n structures containing p-a-Si/i-Si QDs/SiC multilayers/n-a-Si layers on indium-tin-oxide coated glass substrates were also obtained by laser crystallization method. The current-voltage relationship exhibits rectification characteristics, which suggests the p-i-n junction is well kept after laser annealing and the electronic characteristics were briefly discussed. Room temperature photoluminescence (PL) centered at 800 nm was observed from laser crystallized multilayered samples which cannot be detected from the as-deposited samples. The optical and luminescence behaviors imply that the light emission is originated from the recombination of photo-excited electron-hole pairs within Si QDs and/or interfacial states.
Keywords: Si quantum dots, multilayers, luminescence, electronic properties
Investigation of Metastability Effects on the Minority Carrier Transport Properties of Microcrystalline Silicon Thin Films by Using the Steady-state Photocarrier Grating (SSPG) Technique
1. Department of Physics, Faculty of Sciences, Mugla Sitki Kocman University, Kotekli Yerleşkesi, Mugla, Turkey
2. Forschungszentrum Jülich, IEK-5 Photovoltaik, 52425 Jülich, Germany
3. Institut für Physik, Carl von Ossietzky Universität Oldenburg, 26111 Oldenburg, Germany
Optoelectronic properties of microcrystalline silicon, μc-Si:H, thin films show changes after intentional or unintentional exposure to different atmospheric conditions. Investigations of the instability effects on thin films carried out for the last decade have mostly used the techniques which mainly focused on the characterization of the majority carrier transport properties. However, the operation of thin film silicon solar cells under light is controlled by both majority and minority carrier transport properties. The effect of metastability due to atmospheric gas components on the minority carrier transport properties has not been studied in detail yet. In this study, high quality hydrogenated microcrystalline silicon thin films were deposited on smooth glass substrates by using the VHF-PECVD technique under different silane gas ratios (SC=SiH4/(SiH4+H2) ) at substrate temperature of 190°C. The thickness of the samples is around 300 nm and their crystalline volume fraction changes from 0 to 75%. Silver coplanar electrodes were evaporated on the sample with 0.5 cm length and 0.5 mm separation. The samples were exposed to (i) ambient atmosphere at room temperature in the dark to cause uncontrolled metastability effect as well as to (ii) controlled gas ambient such as high purity nitrogen, helium, argon and oxygen. A new reliable measurement procedure has been established and applied for the real time monitoring of dark conductivity during the controlled gas exposures as well as during the annealing process in order to cause significant metastability in the sample and to have complete annealing of the metastable state. Measurements to probe the changes after treatments were performed at 300K and in the steady-state condition. Temperature dependent dark conductivity, steady-state photoconductivity (SSPC) and steady-state photocarrier grating (SSPG)  methods have been used to detect the changes in the metastable state and after annealing. Changes in the majority carrier electron mobility-lifetime (μnτn) products were obtained from the SSPC experiments. Minority carrier hole diffusion lengths, LD, were determined from the SSPG measurements by using both linear and non-linear fits to experimentally obtained β versus grating period Λ detected in the field independent region. It was found that inert gasses (nitrogen, helium and argon) caused an increase of σD and σph within factor of 3 as well as an increase in minority carrier diffusion lengths, LD, within 20 nm. These changes are completely reversible after heat treatment at 430K. However, oxygen gas treatment at 80°C resulted in more than an order of magnitude increase in both σD and σph and an increase in LD by 50% from 213 nm of annealed state value to 320 nm. Following heat treatment almost recovers both σD and σph to their annealed state values. But LD slightly recovers to a value around 300 nm still showing a significant improvement above its annealed state value. Such increase in the LD values could be due to a decrease in the density of recombination centers for holes between valence band edge and the Fermi level. Sub-bandgap absorption measurements carried out on the same oxygen exposed and annealed states show a significant decrease in the absorption coefficient in the low energy part of spectrum, which increases the μpτp product for holes.
 D. Ritter, E. Zeldov, and K. Weiser, Appl. Phys. Lett. 49, 791 (1986)
Keywords: microcrystalline silicon, metastability, photoconductivity, steady-state photocarrier grating
We-B1.5 (invited) 9:40–10:10
45th Anniversary of Nanocrystalline Silicon: From the Past Towards the Future
Department of Chemistry, Technical University Munich, Lichtenbergstr. 4, D-85747 Garching, Germany
The first report on the deposition of nanocrystalline silicon and germanium (nc-Si, nc-Ge) by chemical transport in glow discharge hydrogen plasma in 1968 appeared at about the same time as reports of several groups about the deposition of amorphous silicon (a-Si) by plasma induced chemical vapor deposition (P CVD) from silane, followed by the pioneering paper of Spear and Le Comber about successful doping of a-Si several years later. The deposition from silane is an irreversible decomposition reaction, whereas the chemical transport is a reversible sequence of etching and decomposition reactions, where due to the control of the experimental parameters, such as temperature, plasma density and ion bombardment, the rate of the decomposition is adjusted slightly higher than the etching. In such a way it is possible to deposit single-phase nc-Si without any noticeable fraction of a-Si with an experimental detection limit of few vol. %. Kinetics and mechanistic studies in plug-flow reactors enabled to develop a relatively simple kinetic model which, when implemented into a detailed numerical modeling enabled to optimize a uniform deposition of silicon in large industrial-type deposition systems.
In order to obtain reproducible deposition of single-phase nc-Si via chemical transport, high purity hydrogen glow discharge plasma is needed with total oxygen impurities not exceeding 1–3 ppm. Therefore, as shown by Hirose et al. some time ago, it is technically much easier to use silane diluted with hydrogen to a concentration close to that corresponding to the partial chemical equilibrium in the Si + H2 heterogeneous system under the given plasma conditions. However, even in that case, it is difficult to obtain single-phase nc-Si if the oxygen impurities are not controlled at the low ppm level because the reverse, etching reaction of silicon by atomic hydrogen is absent due to temperature-dependent selectivity of the etching of silicon and silicon oxide: the maximum etch rate of Si by atomic hydrogen occurs at about 60–70°C and it strongly decreases with increasing temperature, whereas etching of SiO2 and Si surface oxidized by the oxygen impurities in the plasma requires more than 400°C where the etching of pure silicon is absent.
Oxygen present in the gas phase is preferentially incorporated into the deposited silicon during the P CVD forming relatively large ≡Si-O-Si≡ like clusters which are incommensurable with the Si-crystal lattice and disturb the short- and medium-range order thus leading to the formation of a-Si "tissue". (Amorphous Si fraction can form also in an oxygen free system when the backward, etching reaction is absent due to inappropriate choice of the deposition conditions.) An estimate shows that nc-Si can fully amorphize at oxygen impurity content of 2–3 at. %, which is incorporated into the deposited silicon already at an oxygen impurity content in the gas plasma of 0.6–0.7 at. %.
These results and the effect of oxygen impurities on the electronic properties of nc-Si will be discussed in comparison with several recent papers.
Keywords: nanocrystalline silicon, deposition conditions, etching, ion bombardment, impurities