Fabrication of Si nanodot single layers under ultrahigh vacuum (UHV) conditions is achieved by decomposition and self-organized growth from thermally deposited non-stoichiometric SiOx (x < 2) precursor layers provided with an ultrathin SiO2 capping layer due to phase separation upon appropriate in situ annealing. The kinetics of the thermal decomposition of the constituting Si suboxides (Sin+ , n = 0... 4) into Si nanodots and the surrounding SiO2 matrix is analyzed in situ by X-ray photoelectron spectroscopy (XPS) as a function of the annealing temperature. The maximum size and the density of the nanodots are varied by adjusting the stoichiometric coefficient x and the layer thickness. Thus, the electronic nanodot properties and the interlayer transport properties can be controlled. For initial compositions (x ranging from 0.9 to 1.4) and layer thicknesses (3 to 10 nm), phase separation was completed at 850 degrees C. The phase separation revealed by XPS is directly correlated with electrical properties as derived from atomic force microscopy measurements detecting surface potentials (KFM) and local conductivity (CS-AFM) across individual nanodots.

The self-organized growth of crystalline silicon nanodots and their structural characteristics are investigated. For the nanodot synthesis, thin amorphous silicon (a-Si) layers with different thicknesses have been deposited onto the ultrathin (2 nm) oxidized (111) surface of Si wafers by electron beam evaporation under ultrahigh vacuum conditions. The solid phase crystallization of the initial layer is induced by a subsequent in situ annealing step at 700 °C, which leads to the dewetting of the initial a-Si layer. This process results in the self-organized formation of highly crystalline Si nanodot islands. Scanning electron microscopy confirms that size, shape, and planar distribution of the nanodots depend on the thickness of the initial a-Si layer. Cross-sectional investigations reveal a single-crystalline structure of the nanodots. This characteristic is observed as long as the thickness of the initial a-Si layer remains under a certain threshold triggering coalescence. The underlying ultra-thin oxide is not structurally affected by the dewetting process. Furthermore, a method for the fabrication of close-packed stacks of nanodots is presented, in which each nanodot is covered by a 2 nm thick SiO2 shell. The chemical composition of these ensembles exhibits an abrupt Si/SiO2 interface with a low amount of suboxides. A minority charge carrier lifetime of 18 µs inside of the nanodots is determined.

The synthesis of Si/SiO2 nanostructures for the application as hetero-emitter and passivation layer in high-efficiency solar cells is explored with the long-term perspective of exploiting quantum size effects for next-generation photovoltaics. Ultrathin oxides are developed by means of oxidizing crystalline silicon using neutral oxygen atoms supplied by a RF plasma source. These oxides are characterized by an abrupt Si/SiO2 junction and good passivation of Si dangling bonds at the interface, a precondition for the implementation of Si/SiO2 nanostructures in photovoltaic devices. Another decisive issue is carrier transport across the SiO2 barrier which is demonstrated in form of a tunneling current in I-V measurements. Such tunneling oxides on Si(111) wafers are used as substrates for the subsequent deposition of Si nanodots. Nanodot synthesis is accomplished by first depositing a few nanometer thick layer of amorphous Si. During a subsequent recrystallization step at T > 600 degrees C, the surface tension between c-Si and SiO2 causes a dewetting and, thus, the formation of crystalline Si nanodots, the size of which can be controlled by the thickness of the initial amorphous Si layer. Finally the dots are cladded with a shell of tunneling oxide. A close-packing of spheres with dots stacked on top of each other is approximated by a repetition of this process. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Two different growth mechanisms are compared for the fabrication of Si/SiO2 nanostructures on crystalline silicon (c-Si) to be used as hetero-emitter in high-efficiency solar cells: (1) The decomposition of substoichiometric amorphous SiOx (a-SiOx) films with 0<x<1.3 and (2) the dewetting of thin amorphous silicon (a-Si) layers.The grown layers are investigated with regard to their structural properties, their passivation quality for c-Si wafer substrates and their electrical properties in order to evaluate their suitability as a nanodot hetero-emitter. While by layer decomposition, no passivating nanodots could be formed, the dewetting process allows fabricating nanodot passivation layers at temperatures as low as 600 degrees C. The series resistance through Ag/[Si-nanodots in SiO2]/c-Si/Al structures for dewetting is similar to nanostructured silicon rich SiOx films. Still, a nanodot hetero-emitter which exhibits both a satisfying passivation of the substrate and induces a high band bending by doping at the same time could not be fabricated yet. (C) 2011 Elsevier B.V. All rights reserved.

Surface charge, surface state density and interface recombination behavior on polished float zone (FZ) solar cell substrates were investigated after various wet-chemical pre-cleaning procedures and deposition of amorphous silicon (a-Si:H) or silicon nitride (a-SiNx: H). Applying surface photo voltage (SPV), microwave detected photo conductance decay (mu W-PCD) and transient microwave conduction (TRMC) measurements, electronic interface properties were monitored repeatedly during the preparation processes. As shown for an inverted a-Si:H/c-Si hetero-junction structure, with front side passivation by a-SiNx:H and a p-type a-Si:H emitter on the rear side, the effect of optimised wet-chemical pre-treatment can be preserved during the subsequent soft plasma enhanced chemical vapour deposition of a-Si:H or a-SiNx:H. This leads to hetero-interfaces with low interface recombination velocities. These results were compared to previously reported findings, obtained on textured Czochralski (CZ) single crystalline substrates. a-SiNx:H is known to result in a field effect passivation. Nevertheless a strong influence of wet-chemical treatments on surface charge and recombination losses was observed on both flat and textured a-SiNx:H/c-Si interfaces. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A theoretical model is developed to account for the kinetics of the decrease of the density of recombination active centres at the interface of SiO2 layers with Si(111) substrates as a result of treatment by thermalised hydrogen plasma. The recombination active centres at the SiO2/Si interface are ascribed to be the Pb centres. The model developed takes into account the kinetics of the incorporation of hydrogen atoms into the SiO2 layer from the plasma source and their interaction with both passivated and unpassivated Pb centres. A special attention is given to ultrathin (∼2nm) SiO2 layers. A simplification of the general model to this important case allows one to obtain analytical solution for the density of unpassivated recombination active Pb centres as a function of the flow of hydrogen atoms from plasma, the initial density of Pb centres, and the treatment time. The model applicability is verified comparing the results of calculations with the experimentally measured values of the relative density of recombination active centres at the ultrathin SiO2/Si(111) substrate interface upon passivation by atomic hydrogen from thermalised plasma source. A conformity of the model predictions to the experimental results is demonstrated.

A complete in situ process from preparation and hydrogen passivation to interface gap state analysis by near-UV photoelectron spectroscopy (NUV-PES) without breaking ultrahigh vacuum (UHV) conditions is applied to ultrathin oxide layers on Si(1 1 1), (1 1 0), and (1 0 0). RF plasma oxidation with thermalized neutral oxygen atoms allows the growth of homogeneous ultrathin SiO2 layers (< 2 nm) and the preparation of compositionally and structurally abrupt Si/SiO2 interfaces with minimal amounts of suboxides ranging from 2% on Si(1 0 0) to 4% on Si(1 1 0). The oxide growth is independent of the crystallographic orientation. Appropriate plasma treatment with nearly thermalized hydrogen atoms (Ekin < 1 eV) leads to significant passivation of dangling bonds at the ultrathin-SiO2/Si interfaces and is most efficient on Si(I 0 0). In contrast, energetic hydrogen plasma treatment of these interfaces with kinetic energies exceeding 120 eV, which is conventionally applied for polycrystalline Si thin-film solar cells, imparts large amounts of energy and deteriorates the electrical properties as is reflected in interface degradation and increased densities of defect states. (C) 2008 Elsevier B.V. All rights reserved.

SiO2/Si/SiO2 single quantum wells (QWs) were prepared under ultrahigh vacuum conditions in order to study their structural, chemical and photoelectrical properties with respect to a possible application in photovoltaic devices. Amorphous silicon (a-Si) layers (thickness <10nm) were deposited onto quartz glass (SiO2) substrates and subsequently oxidized with neutral atomic oxygen at moderate temperatures of 600°C. Under these conditions, the formation of suboxides is mostly suppressed and abrupt Si/SiO2 interfaces are obtained. Crystallization of a-Si QWs requires temperatures as high as 1000°C resulting in a nanocrystalline structure with a small amorphous fraction. The spectral dependence of the internal quantum efficiency of photoconductivity correlates well with the nanocrystalline structure and yields mobility lifetime products of <10−7cm2V−1. This rather low value points towards a strong influence of Si/SiO2 interface states on the carrier mobility and the carrier lifetime in Si QWs. Electronic passivation of interface states by subsequent hydrogen treatment in forming gas enhances the internal quantum efficiency by nearly one order of magnitude.

An essential prerequisite for the successful application of Si/SiO(2) nanostructures in photovoltaics is the realization of well-defined and abrupt interfaces with low densities of interface gap states. Here, a complete in situ process from preparation and hydrogen passivation to interface gap state analysis by near-UV photoelectron spectroscopy without breaking ultrahigh vacuum (UHV) conditions is introduced. It is demonstrated that by RF plasma oxidation of Si(111) substrates with thermalized neutral oxygen atoms, ultrathin SiO(2) layers can be realized with compositionally and structurally abrupt Si/SiO(2) interfaces and a minimal amount of intermediate oxidation states bridging the transition from Si to SiO(2). Plasma oxidized samples have significantly lower interface gap states than samples oxidized by thermal oxidation at 850 °C. Interface gap state densities were further reduced by in situ hydrogen plasma passivation with nearly thermalized H atoms. The resulting reduction of interface recombination velocity and the increase of effective majority and minority carrier lifetimes are revealed by constant photocurrent measurements and quasi-steady-state photoconductance, respectively.

A main loss factor in conventional solar cells is t he mismatch of solar spectrum and bandgap energy of the absorber material. Thus, photons with energi es above the bandgap generate hot carriers which lose their excess energy via thermalization. A conc ept in 3rd generation photovoltaics that aims to circumvent thermalization losses is to realize appr o riate Si based nanound quantum structures, such as stacked Si/SiO 2 multi quantum wells, that provide bandgap tunabili ty and thus adaptation to the solar spectra via utilization of quantum size e ff cts [1]. However, due to increased interface-tovolume ratios at reduced dimensions, charge carrier ecombination at the SiO 2/Si interfaces is facilitated, which lowers the solar cell efficiency . To overcome this drawback, experimental methods are required, that are capable of preparing well-de fined Si and SiO2 layers and of minimizing interface losses. Moreover, adequate analytical too ls have to be developed to reveal information about the chemical, structural, and photoelectrical properties at atomic level. These are the main issues that are addressed in thi s paper. The focus is on the study of a model system consisting of an ultrathin SiO 2 layer upon a Si(111) wafer representing one of the interfaces as a constituting building block of such a Si/SiO 2 quantum well device. For highest possible purity and control, complete preparation as well as full i nterface characterization was performed under ultrahigh vacuum (UHV) conditions. Under such condi tions plasma oxidation using neutral, thermalized oxygen atoms is found to be superior to conventional thermal oxidation. The transmission electron microscopy image (TEM) in Fig . 1a shows such a homogeneous SiO 2 layer on Si(111) with uniform thickness of about 2 nm and pr oves the successful formation of an abrupt interface structure. Analysis of the chemical shift by x-ray photoelectron spectroscopy (XPS) revealed dominant contributions of the oxidation st ates Si 0+ and Si , corresponding to Si and SiO 2, respectively, and extremely low amounts of the inte rmediate oxidation states Si , Si, and Si 3+ (Fig. 1b) [2]. Thus, high quality ultrathin SiO 2/Si interfaces with compositionally abrupt transiti ons were obtained by UHV plasma oxidation.

Si/SiO2 single quantum wells and quantum dot layers were prepared under ultrahigh vacuum conditions and studied with respect to possible photovoltaic applications. The detection of a photocurrent in such structures is demonstrated. Its spectral dependence correlates with the respective structural properties. Internal quantum efficiencies of photoconductivity and, thus, carrier mobilities and lifetimes, are strongly affected by Si/SiO2 interface states and were enhanced upon hydrogen treatment due to passivation of interface gap states.

Compositionally and structurally abrupt Si/SiO2 interfaces were prepared under ultrahigh vacuum conditions by RF plasma oxidation of Si(111) substrates with thermalized neutral oxygen atoms. The chemical, structural and electronic properties of the interface were analyzed and discussed with respect to a possible application in photovoltaic Si/SiO2 quantum well structures. The benefits of using neutral atomic oxygen were explored and turned out to be manifold: ultrathin SiO2 layers (thickness: 1–2 nm) can be precisely grown, the formation of suboxides is mostly suppressed and abrupt Si/SiO2 interfaces are obtained even at moderate substrate temperatures of 300 to 600 °C. Due to the perfect Si/SiO2 interfaces, the SiO2 layers allow thermal post-oxidation treatment up to 1000 °C without significant change in oxide thickness or stoichiometry. This is an essential prerequisite for the envisaged realization of Si/SiO2 superlattices with high crystallinity and low strain. It was shown that a post-oxidation annealing step lowers the strain and disorder at the interface resulting in lower intrinsic density of interface states. A further decrease of the density of interface states was achieved by hydrogen passivation as a result of saturation of dangling bonds.

Time-dependent structural relaxations in a melt-mixed 38/62 vol% poly(ethylene oxide)/atactic poly(methyl methacrylate) blend were studied using several techniques: differential scanning calorimetry, pressure-volume-temperature analysis, positron annihilation lifetime spectroscopy, dynamic mechanical analysis, and solid-state nuclear magnetic resonance. The internal volume (free volume hole size) and the external volume (specific volume) of the blend are found to decrease with aging time. The time scale of the volume changes is the same, suggesting that internal and external volumes can be calculated from each other. Increasing mobility of poly(ethylene oxide), composition fluctuations, and shifting glass transition temperatures are observed upon aging. Phase separation in terms of spinodal decomposition below an upper critical solution temperature occurs within minutes and results in two amorphous phases of different composition. Subsequent crystallization then causes further structural changes.

This review should demonstrate that polyelectrolyte solutions are currently advancing to a fascinating topic in polymer science. The increasing effort in analytical theory, simulations and experiments is about to result in a deeper, fundamental understanding of the interacting macroions and of the conformational properties of flexible polyions. As these “simple” problems have still not been completely solved, new challenges have already emerged, such as the effect of polyion chain architecture (polyelectrolyte stars, combs, rings, μ-gels), the effect of bivalent or multivalent metal ions bound to the polyelectrolyte chain (intramolecular cross-linking, polyion coil collapse), and the complex field of surfactant-polyion and polyanion-polycation complexes. In comparison with the growing technical importance of such complex ionic systems, the fundamental academic research in this area has hardly started yet.

Die Makromolekulare Chemie, Rapid CommunicationsVolume 14, Issue 7 p. 433-438 Article Size-exclusion chromatography of cationic and anionic polyelectrolytes in aqueous media Andreas Kühn, Andreas Kühn Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, GermanySearch for more papers by this authorStephan Förster, Stephan Förster Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, GermanySearch for more papers by this authorRainer Lösch, Rainer Lösch Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, GermanySearch for more papers by this authorMarianne Rommelfanger, Marianne Rommelfanger Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, GermanySearch for more papers by this authorChristine Rosenauer, Christine Rosenauer Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, GermanySearch for more papers by this authorManfred Schmidt, Corresponding Author Manfred Schmidt Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, GermanyMax-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, GermanySearch for more papers by this author Andreas Kühn, Andreas Kühn Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, GermanySearch for more papers by this authorStephan Förster, Stephan Förster Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, GermanySearch for more papers by this authorRainer Lösch, Rainer Lösch Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, GermanySearch for more papers by this authorMarianne Rommelfanger, Marianne Rommelfanger Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, GermanySearch for more papers by this authorChristine Rosenauer, Christine Rosenauer Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, GermanySearch for more papers by this authorManfred Schmidt, Corresponding Author Manfred Schmidt Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, GermanyMax-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, GermanySearch for more papers by this author First published: July 1993 https://doi.org/10.1002/marc.1993.030140711Citations: 7AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume14, Issue7July 1993Pages 433-438 RelatedInformation