bluuebunny/crossref_metadata_2025 huggingface dataset:
Defect Trapping and Precipitation Processes During Annealing of Cu and Au Implanted Si
Authors: Wong-Leung J. , Nygren E. , Williams J. S. , Eaglesham D. J.
Published: April 2011

The behaviour of metallic impurities in Si is an issue of primary concern in advanced device processing. This paper addresses the annealing behaviour of Cu and Au in Si when nanocavities are present within the substrate as potential gettering and precipitation sites. In-situ, ‘hot’ Rutherford backscattering has been used to study metal accumulation to cavities at the annealing temperature. Transmission electron microscopy has been used to study the final microstructure of the Au-Si and Cu-Si system. A range of interesting phenomena have been observed, including very efficient transient gettering of both Cu and Au to cavities at the annealing temperature and precipitation of second phases at cavities. Dissolution of accumulated metal from defect bands into solution occurs for long-time annealing. This behaviour is discussed in terms of supersaturated solid solubility processes, diffusion, defect-trapping and precipitation of Cu and Au in Si.
Coherent V2O3 Precipitates in ɑ-Al2O3 Co-Implanted With Vanadium and Oxygen
Authors: Gea Laurence A. , Boatner L. A. , Rankin Janet , Budai J. D.
Published: April 2011

The oxides of vanadium VO2 and V2O3 are of fundamental and practical interest since they undergo structural phase transitions during which large variations in their optical and electronic properties are observed. In the present work, we report the formation of buried precipitates of V2O3 in sapphire by ion implantation and thermal annealing. It was found that the co-implantation of oxygen and vanadium was required in order to form nanophase V2O3 precipitates. Additionally, these precipitates, which formed only following an anneal of the co-implanted sample under reducing conditions, are coherent with the sapphire lattice. Two epitaxial relationships were observed: (0001)V2O3//(0001) ɑ-Al2O3 and (11-20)V2O3//(0001) ɑ-Al2O3. This finding is in agreement with results obtained elsewhere for thin films of V2O3 deposited on c-axis-oriented sapphire.
Structure-Property Relationship of Ion-Beam Sputtered Nd-Fe-B Magnetic Thin Films On (111) Silicon
Authors: Nazareth Anthony S. , Chopra Harsh Deep , Sood D. K. , Zmood R. B.
Published: April 2011

A focussing grid broad beam Kaufman source, using argon ions on a target of nominal composition Nd2Fei4B has been employed to sputter deposit magnetic thin films of thicknesses ranging from 800 â to 1300 â on silicon-(lll) substrates at room temperature. These films were characterised for their composition depth profile by Rutherford Backscattering Spectroscopy, while x-ray diffraction was used to study the crystallographic structure. Due to a close match between (111) Si with (220) Nd2Fej4B lattice spacings, preferred crystallographic texturing was expected, and experimental results showed a greatly enhanced (220) texture. The degradation in magnetic properties was attributed to the presence of oxygen in the films as indicated by concentration depth profiles. It is premised that another significant role of oxygen may be to relieve the misfit strain across the interface by its incorporation within the Nd2Fej4B phase.
Degradation in Euv Reflectance of Ion-Sputtered Sic Films
Authors: Schwarcz Dan , Keski-Kuha Ritva A.M.
Published: April 2011

Silicon Carbide (SiC) formed by chemical vapor deposition (CVD) has the highest reflectivity in the extreme ultraviolet (EUV) of any currently used optical material. The high temperature required for the CVD process, however, limits its suitability for coating optical components. To address this problem thin films have been sputtered onto optical surfaces from CVD βSiC targets. These films, while having reflectivity lower than that of CVD SiC, are nonetheless the best coatings available for reflectance in the spectral region below 1000À. While the initial properties are good, the EUV reflectivity degrades with time after deposition. A relative decrease of about 25% is evident in the reflectivity at 920Â after 2.5 years, and about 85% of this change occurs in the first three months. In fact, a decrease is observed in the minutes following deposition. In this study the degradation is characterized and a mechanism is proposed. Efforts underway to reduce or eliminate the degradation are discussed.
Effect of Surface Roughness on Surface Photochemistry
Authors: Myli Kari B. , Grassian Vicki H.
Published: April 2011

Photoinduced processes on surfaces is of fundamental importance in materials processing. For example, photo-assisted chemical vapor deposition has been used to deposit a number of different materials on solid substrates. The effect of substrate morphology on photoinduced processes can be an important factor in film growth. In order to clarify the role of substrate morphology, in particular surface roughness, on surface photochemistry, we have examined the wavelength dependence of the photodissociation of chlorobenzene and 3-chloropyridine adsorbed on smooth and rough silver surfaces. As discussed below, we have measured a red shift in the photodissociation threshold for these molecules adsorbed on a rough surface relative to a smooth surface. The wavelength dependence of the photodissociation cross-section near the threshold has been measured in order to discern the origin of the red shift.
Si. I-x-y. GexCy Film Formation by Pulsed Excimer Laser Crystallization of Heavily Ge and C Implanted Silicon
Authors: Fogarassy E. , Dentel D. , Grob JJ. , Prévot B. , Stoquert J.P. , Stuck R.
Published: April 2011

We investigate, for the first time, the possibility to crystallize heavily Ge and C implanted silicon substrates by excimer-laser annealing performed in the molten regime. It is demonstrated that the crystalline quality of the laser grown SiGeC alloys strongly depends on the initial dose of implanted carbon.
Germanium Partitioning and Interface Stability During Rapid Solidification of Gesi Alloys
Authors: Brunco D. P. , Thompson Michael O. , Hoglundt D. E. , Aziz M. J.
Published: April 2011

The stability of laser processed GeSi heteroepitaxial alloys on Si to partitioning driven interface instabilities is examined. Existing stability models are extended to include nonequi-librium solidification effects for nondilute alloys and are examined under typical conditions of laser induced solidification. Ge diffusion and partitioning were measured for quantitative input to the models. The Ge liquid-phase diffusivity was determined to be 2.5 x 10-4 cm2/s. The measured velocity-dependent partition coefficients k(v) were fit to the Continuous Growth Model using an equilibrium k of 0.45 and a diffusive speed of 2.7 m/s. Stability calculations based on these values and our extended stability model are presented. Although instabilities at compositions comparable to those experimentally observed to give defective films are predicted, the growth rates of these instabilities appear too slow to destabilize an interface on laser processing time scales. These results suggest that strain or other effects play an important role in the observed defective microstructures.
Modeling of Thermal, Electronic, Hydrodynamic, and Dynamic Deposition Processes for Pulsed-Laser Deposition of Thin Films
Authors: Liu C. L. , Leboeuf J. N. , Wood R. F. , Geohegan D. B. , Donate J. M. , Chen K. R. , Puretzky A. A.
Published: April 2011

Various physical processes during laser ablation of solids for pulsed-laser deposition (PLD) are studied using a variety of computational techniques. In the course of our combined theoretical and experimental effort, we have been trying to work on as many aspects of PLD processes as possible, but with special focus on the following areas: (a) the effects of collisional interactions between the particles in the plume and in the background on the evolving flow field and on thin film growth, (b) interactions between the energetic particles and the growing thin films and their effects on film quality, (c) rapid phase transformations through the liquid and vapor phases under possibly nonequilibrium thermodynamic conditions induced by laser-solid interactions, (d) breakdown of the vapor into a plasma in the early stages of ablation through both electronic and photoionization processes, (c) hydrodynamic behavior of the vapor/plasma during and after ablation. The computational techniques used include finite difference (FD) methods, particle-in-cell model, and atomistic simulations using molecular dynamics (MD) techniques.
Electron Beam Assisted Etching of Single Crystal Diamond Chips
Authors: Taniguchi Jun , Miyamoto Iwao
Published: April 2011

In order to fabricate ultra-precision diamond tools and delineate ultra-fine patterns into diamond chips without adding radiation damage, machining characteristics of diamond chips with electron beam assisted etching (EBAE) has been investigated. This processing mechanism is considered as follows: Oxygen atoms or molecules activated by electron beam bombardment on or near the chip surface react with carbon atoms of the diamond surface, resulting in formation of volatile products such as CO or C02 . An EBAE system composed of a scanning electron microscope (SEM) which has an oxygen introduction system was used to etch synthetic single crystal diamond chips. When a diamond chip was etched at an applied voltage of 10 kV and an irradiation beam current of 1.7nA, the depth of the holes increased with an increase of machining time and the diameter of the holes also increased with an increase of machining time. When a diamond chip was etched at an applied voltage of 10 kV and an irradiation beam current of 1.3nA, the depth and diameter of the etched holes merely increased with an increase of flow rate of oxygen gas ranging from 5 cc/min to 30 cc/min, then the depth decreased rapidly with an increase of oxygen gas. With this processing method, very small holes with a diameter of about 0.5 ∼ 2 /xm, and a depth of about 0.01 ∼ 0.7 fim were obtained. Line and rectagular patterns with several /xm and sub-μum depths were also fabricated.
In-situ Ellipsometry Study of Ion Bombardment Effects on Low Temperature Si Epitaxy by dc Magnetron Sputtering
Authors: Yang Y.H. , Feng G.F. , Katiyar M. , Abelson J.R.
Published: April 2011

We deposit Si films on Si(100) substrates at temperatures of 300 - 350 °C using dc magnetron sputtering, and characterize the structure by in-situ spectroscopic ellipsometry. Changes in the ion or electron bombardment, produced by biasing the sample with respect to the floating potential, are found to exert a strong effect on the kinetics of the crystalline (epitaxial) to amorphous transition for films deposited just below the apparent minimum temperature (350 °C) for sustained epitaxy. At 320°C, the best results are found at the floating potential, which is 25 V below the plasma potential and produces an ion flux equal to the depositing Si flux on the substrate. At +14 V above the floating potential, the volume fraction of crystalline Si decreases exponentially with thickness, and the characteristic decay length is a function of substrate temperature. At -14 V below the floating potential, the deposited film is amorphous with a large void content. These observations demonstrate the subtle tradeoff between enhanced surface mobility and defect creation by low energy ion bombardment.
Dynamic Behavior of Twins In BaTiO3
Authors: Katti Kalpana S. , Sarikaya Mehmet
Published: April 2011

The most prominent lattice defects in barium titanate, i.e., twins on two distinct lattice planes { 111 } and { 110 }, were characterized using dynamic transmission electron microscopy techniques in the image and diffraction modes. In-situ heating experiments from room temperature up to 130°C and then cooling down to -168°C reveal a dependence of { 110 } twins on ferroelectric transition. On the other hand, { 111 } twins are found to be completely stable through the ferroelectric transition and exist in all phases. Streaking is observed in the diffraction spots leading to an estimation of { 1111 twin wall thickness to be about 10Å while no such streaking was observed corresponding to { 1101 twins. Based on these observations and the behavior of the twin boundaries under dynamic heating cycles, an estimate of the twin wall energies indicates that { 111 } walls have three orders of magnitude higher energy than that of { 110 } walls which explains their relative thermal and mechanical stability.
Fabrication and Characterization of NiO Bicrystals
Authors: Chung Yong-Chae , Wuensch Bernhardt J.
Published: April 2011

Fabrication of NiO bicrystals having Σ5 (310) and Σ13 (510) coincidence-site tilt boundaries was successfully carried out by a CVT (Chemical Vapor Transport) method. The CVT method was a very advantageous way to grow ultra pure crystals since it preferentially transported NiO from the source pellet to the substrate by reaction with a HCl carrying gas. Single crystal MgO was used as a substrate for epitaxial growth of NiO as the readily available MgO crystals have only a 1% lattice mismatch with NiO. Moreover, MgO is soluble in acids while NiO is not. This permitted removal of the substrate crystal after growth to provide a free-standing NiO crystal. Using two single crystals of MgO with the desired tilt orientation as a substrate, NiO bicrystals were fabricated at growth rates greater than 100 μm/hour at 1,400K using 250 torr of HCI(g) as a carrying agent. The purity of the epitaxial NiO crystals was determined by mass spectrometry and neutron activation analysis. The grain boundary in the bicrystals is exactly perpendicular to the (100) growth surface. Highly-reflective facets along the growth direction suggest high mechanical quality. High-resolution transmission electron microscopy of the Σ13 boundary revealed structure at the atomic scale that provided no evidence for segregated phases.
Interface Reaction Enhanced Epitaxial Growth of Barium Ferrite Magnetic Thin Films
Authors: Li Jinshan , Sinclair Robert , Rosenblum Stephen S. , Hayashi Hidetaka
Published: April 2011

Using facing target sputtering, crystalline magnetoplumbite-type barium ferrite (BaFe12O19 or BaM) thin films have been prepared in-situ at a substrate temperature of 640°C without postdeposition annealing. BaM thin films grow randomly if they are directly deposited onto Si or thermally oxidized Si substrates. However, deposited onto a sputtered ZnO layer (∼230Å) on Si substrates, BaM thin films show excellent c-axis out-of-plane texture with a 0.2° c-axis dispersion angle, as indicated by X-ray diffraction (XRD) study. Cross section transmission electron microscopy (TEM) reveals that the textured films epitaxially grow on a transition layer, which is formed between BaM and ZnO. No direct epitaxial relation between BaM and ZnO was observed. This transition layer is identified by TEM and XRD as ZnFe2O4, which, from a structure point of view, reduces the lattice mismatch between BaM and ZnO, and also enhances the c-axis out-of-plane epitaxial growth.
Crystallization Kinetics of Fe-DOPED A1203
Authors: Simpson Todd W. , Mitchell Ian V. , Yu Ning , Nastasi Michael , Mcintyre Paul C.
Published: April 2011

Time resolved optical reflectivity (TRR) and Rutherford backscattering spectrometry (RBS) and ion channelling methods have been applied to determine the crystallization kinetics of Fe-doped A1203 in the temperature range of 900-1050°C. Amorphous A1203 films, approximately 250 nm thick and with Fe cation concentrations of 0, 1.85, 2.2 and 4.5%, were formed by e-beam deposition on single crystal, [0001] oriented, A1203 substrates. Annealing was performed under an oxygen ambient in a conventional tube furnace, and the optical changes which accompany crystallization were monitored, in situ, by TRR with a 633nm wavelength laser.

Crystallization is observed to proceed via solid phase epitaxy. An intermediate, epitaxial phase of -γ-Al203 is formed before the samples reach the ultimate annealing temperature. The 5% Fe-doped film transforms from γ to α-A1203 at a rate approximately 10 times that of the pure A1203 film and the 1.85% and 2.2% Fe-doped films transform at rates between these two extremes. The Fe-dopants occupy substitional lattice sites in the epilayer. Each of the four sets of specimens displays an activation energy in the range 5.0±0.2eV for the γ,α phase transition.
In Situ Hrem Observation of the Oxidation of Nickel Thin Foils
Authors: Thien-Nga Le , Hutchison J.L. , Derby B.
Published: April 2011

Growth of oxide microcrystals at the edge of nickel grains could be observed using highresolution microscopy. Three types of samples were examined: a nickel-cermet material, and pure annealed nickel, both ion-beam milled; and a standard electropolished nickel foil. Microstructural features, such as crystallite size, facets, orientation relationship and lattice parameter are described.
Heteroepitaxial Diamond Formed on Silicon Wafer Observed by High Resolution Electron Microscopy
Authors: Yang Jie , Lin Zhangda , Wang Li-Xin , Jin Sing , Zhang Ze
Published: April 2011

Diamond films with high preferential orientation (111) on silicon (100) crystalline orientation substrates had been obtained by hot-filament chemical vapor deposition (HFCVD) method. X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and high-resolution cross-sectional transmission electron microscopy (HREM) are used to characterizate the structure and morphology of the synthesised diamond films. Diamond (111) plans had been local grown epitaxially on the Si(100) substrate observed by HREM. SEM photographes show that plane diamond crystals have been obtained.
Chemical Widths at Composite Interfaces: Relationships to Structural Widths and Methods for Measurement
Authors: Carpenter R.W. , Bow J.S. , Kim M.J. , Chowdhiury K. Das , Braue W.
Published: April 2011

Energy selected imaging with a Zeiss 912 ω-filter TEM was used to examine grain boundary solute distributions in an Si3N4/SiC(w) ceramic densified with Y2O3 + Al2O3 sintering aid. These results are compared to boundary region solute distributions in the same materials determined by field emission small probe electron energy loss spectroscopy and related methods. The intrinsic higher incident flux of the FEG small probe methods renders them the most useful for high spatial resolution local chemical width measurement. Energy selected imaging is fast and relatively simple for determining elemental distributions in boundaries at low magnifications. The methods are complementary.
Cesium/Oxide Interactions for Ultrathin Films on ∝-Al2O3(0001) and ∝-Al2O3(1102)
Authors: Zavadil Kevin R. , Ing Judith L.
Published: April 2011

The interaction of cesium at the (0001) and (1102) surfaces of sapphire has been investigated using a variety of surface analytical techniques. Reflection mass spectrometric measurements yield initial Cs adsorption probabilities of 0.9 and 0.85 for the unreconstructed (0001) and (1102) surfaces, respectively. The adsorption probability decreases dramatically for these surfaces at critical Cs coverages of 2.0×10 14 and 3.4×1014 atoms/cm2, respectively. Thermally induced reconstruction of the (0001) surface to form an oxygen deficient surface results in a decrease in the initial probability and capacity for Cs adsorption. Low energy electron diffraction (LEED) demonstrates that an intermediate, mixed domain surface yields an initial adsorption probability of 0.5 while a reconstructed surface yields a value of 0.27. Thermal desorption mass spectrometry (TDMS) shows that surface reconstruction eliminates the high binding energy states of Cs (2.7 eV/atom), consistent with the observed changes in adsorption probability. In contrast, reconstruction of the (1102) surface produces only minor changes in Cs adsorption. Xray photoelectron spectroscopy (XPS) indicates that no formal reductive/oxidative chemistry takes place at the interface. We interpret the facile adsorption and strong binding of Cs on sapphire to result from Cs interacting with coordinatively unsaturated oxygen.
Effects of Titanium Doping on Surface Properties of Alumina
Authors: Powers James D. , Kulinsky Lawrence , Kitayama Mikito , Glaeser Andreas M.
Published: April 2011

Controlled-geometry pore structures were introduced into undoped and Ti-doped sapphire using microfabrication techniques, and subsequently transferred to internal sapphire-sapphire and sapphire-polycrystalline alumina interfaces via hot pressing. The high-temperature evolution of several different types of structures was examined, in order to isolate different surface properties and evolution processes which are of interest during sintering. A study of pore channel breakup showed that Ti strongly stabilizes channels oriented in the sapphire [1100] direction, suggesting a significant alteration of the Wulff plot. In another study, the equilibration rate of isolated pores was enhanced in Ti-doped sapphire; as glassy phases are unlikely in this material, a solid-state diffusion mechanism for this enhancement is suggested. The evolution of pore channels with pre-existent perturbations was also studied, and the results of these studies are presented.
Discontinuous Dissolution of Spinel Precipitates in the Al2O3-Fe2O3 System
Authors: Lee Ho-Yong , Kang Suk-Joong L.
Published: April 2011

The discontinuous dissolution of oxide particles has been investigated in the Al2O3 -Fe2O3 system by changing oxygen partial pressure (PO2) and temperature. In PO2 change experiment, 95Al2O3-5Fe2O3 (in wt%) powder compacts were sintered at 1500°C in PO2=0.001 atm and then annealed at 1500°C in air. In temperature change experiment, 85Al2O3-15Fe2O3 (in wt%) powder compacts were sintered at 1600°C in air and then annealed at 1450°C in air. During annealing the sintered specimens, which contained corundum grains and iron aluminate spinel particles, the grain boundaries migrated concomitantly with the dissolution of spinel particles. A discontinuous dissolution of spinel precipitates thus occurred, when the grain boundary migration of alumina in a two phase mixture of corundum and spinel was induced by PO2 or temperature change. The present result would be a distinct observation of the discontinuous dissolution of precipitates in ceramic systems.
Nanoscale Control of Ferroelectric Domain Structure by AFM
Authors: Gruverman A. , Kolosov O. , Hatano J. , Takahashi K. , Tokumoto H.
Published: April 2011

An atomic force microscope is used for visualization and control of domain structure in the ferroelectric crystal guanidinium aluminum sulfate hexahydrate (GASH). Large built-in domains are delineated due to the tip-sample electrostatic interaction by the force microscope operating in friction mode. It is found that an applied voltage of a particular sign created new domains whereas a potential of opposite sign erased these domains. Domains created by an external electric field are visualised in topography mode due to piezoelectric deformation of the crystal. The dynamics of domain lateral growth has been directly observed with nanoscale resolution.
Oxygen Self-Diffusion In Mgo Grain Boundaries
Authors: Liberatore Michael , Wuensch Bernhardt J.
Published: April 2011

Oxygen grain boundary diffusion has been studied in low energy (∑5, ∑13) and high energy (asymmetric 16° tilt) MgO grain boundaries. Enhanced diffusion of oxygen was observed in all boundaries, but more so in the general, high energy boundary, by 1 - 2 orders of magnitude. This supports the theory that the better the atomic registry between the adjoining grains the lower the diffusivities in the grain boundary. Also the activation energies for bulk and grain boundary diffusion (in the ∑13 boundaries) were found to be equal to within experimental error. (≈ 3.9eV)
Observation and Characterization of Electric Fields at Grain Boundaries
Authors: Huey B. D. , Bonnell D. A. , Carroll D. L.
Published: April 2011

Variations in local electric fields near individual grain boundaries have been observed in TiO2 with an ac, non-contact, atomic force microscopy detection scheme. A lateral, in-plane bias was applied to the sample inducing an electric field gradient. A modified AFM then allowed accurate force and force gradient images of both topography and the electric fields to be measured. The sample-tip separation and bias dependence of the AFM probe response to electrostatic fields are used to derive the relationships that allow quantification of lateral fields. With this technique, observation and quantification of potential barriers and enhancement and depletion widths may be observed, especially as a function of boundary chemistry and orientation. Results are discussed in terms of interface potential barrier models.
Atomic Bonding at Oxide Surfaces
Authors: Henrich Victor E.
Published: April 2011

The fundamental interactions involved in the bonding of atoms and molecules to metal oxides are discussed. Surface defects play a major role in many of those interactions. Both acid/base and oxidation/reduction reactions occur at metal-oxide surfaces, with the latter dominating at point defect sites. The reaction of metals with oxide surfaces is governed largely by the relative heats of formation of the respective oxides, although surface point defects also play an important role. Preliminary studies of ceramic/ceramic interfaces indicate that interfacial interactions are much weaker than for metal/ceramic interfaces.
The Atomic-Scale Characterization of Defects on Cleaved Vanadium and Molybdenum Oxide Surfaces Using Stm
Authors: Rohrer Gregory S. , Smith Richard L.
Published: April 2011

Scanning tunneling microscopy (STM) was used to determine the structure of cleaved, single crystal surfaces of V205, V6013, Mo18052, and Mo8023. Constant current images were recorded in ultrahigh vacuum and in air. By imaging well-defined surfaces that exhibit structural and chemical similarities, and comparing the observations to the known bulk structures, it is possible to establish a reliable interpretation for the contrast in the STM images. A comparison of images from the V6013(001) and the V205(001) surfaces clearly shows that the surface V coordination polyhedra that are capped by vanadyl 0 can be distinguished from those that are not. This allows vacancies in the vanadyl 0 position to be identified on cleaved V205(001) surfaces. Mo18052(100) and Mo8023(010) provide models for two different characteristic types of surface/crystallographic shear (CS) plane intersections. The shear in Mo8023 lies in the (010) surface plane and creates dark contrast along the [001]. The CS planes in Mo18052, on the other hand, have components of shear both in and normal to the (100) surface plane and create white contrast parallel to [010]. These standards for contrast identification can be used to identify defects on inhomogeneous surfaces.
The Influence of Surface Structure on the Interaction of Water with TiO2 (100)
Authors: Henderson Michael A.
Published: April 2011

The influence of surface structure on the interaction of water with TiO2(100) (rutile) was investigated. Two unique surface structures of the (100) crystal face can be prepared by oxidation (the bulk-terminated surface) and by vacuum reduction (the {110}-microfaceted surface). Thermal desorption profiles, which measure of the strength of the adsorbate-surface interaction, indicate that submonolayer coverages of water are more stable on the bulk-terminated surface than on the {110}- microfaceted surface. Also, subtle differences exist in the way water interacts with 2-coordinate 02 sites on these two surfaces. The implications of these results are considered for the acid/base properties of microcrystalline particles.
Photoluminescence Studies of [(CdSe)1(ZnSe)2]9-ZnSeTe Multiple Quantum Wells Under High Pressure
Authors: Wang Z.P. , Liu Z.X. , Han H.X. , Zhang J.Q. , Li G.H. , Peng Z.L. , Yuan S.X.
Published: April 2011

We have performed photoluminescence (PL) measurements at liquid nitrogen temperature under high pressure up to 5.5 GPa and in the temperature range 10-300 K at atmospheric pressure on {(ZnSe)30(ZnSe0.92Te0.08)30(ZnSe)30[(CdSe)1(ZnSe)2]9}x5 multiple quantum wells. The PL peaks, EB, E1 and Ew corresponding to the band edge luminescence in ZnSe barrier layer, the transitions from the first conduction subband to the heavy-hole subband in ZnSe0.92Te0.08 layers and [(CdSe)1(ZnSe)2]9 ultra short period superlattice quantum well (SPSLQW) layers have been observed. Experimental results show that ZnSe0.92Te0.08/ZnSe forms a type-I superlattice (SL) in contrast to the type-II ZnSe/ZnTe SL. The pressure coefficients of the EB, E1 and Ew exciton peaks have been determined as 67, 63 and 56 meV/GPa, respectively. With increasing temperature (or pressure), the E1 peak-intensity drastically decreases which is attributed to the thermal effect (or the appearance of many defects in ZnSe0.92Te0.08 under higher pressure).
Ge Nanocrystals Grown on Si(111) by Molecular Beam Epitaxy with and without CaF2 Buffer Layers
Authors: Deelman Peter W. , Thundat Thomas , Schowalter Leo J.
Published: April 2011

The Stranski-Krastanov growth mode of Ge thin films on Si and the clustering behavior of Ge on calcium fluoride have been exploited to grow self-assembled nanocrystals by molecular beam epitaxy. The growth of the samples was monitored in situ with RHEED, and they were analyzed ex situ with AFM and RBS. For each system (Ge/Si and Ge/CaF2/Si), the dependence of Ge islanding on substrate temperature and on substrate misorientation was studied. When grown on Si(111) at temperatures between 500°C and 700°C, Ge clusters nucleated at step edges on vicinal wafers and nucleated homogeneously on on-axis wafers. Above 600°C, no transition to a spotty RHEED pattern, which would be expected for island growth, was observed for the vicinal samples. Ge grown at 500°C on on-axis Si(111) formed islands with a relatively narrow size distribution, typically 160nm in diameter and 10nm to 20nm in height. When grown on a CaF2 buffer layer, Ge islands nucleated homogeneously at a substrate temperature of 750°C, resulting in randomly distributed, oblate crystallites approximately 100nm to 200nm in diameter. At 650°C and 700°C, although we still observed many randomly distributed, small crystallites, most islands nucleated at step bunches and had a length scale of over 500nm.
Formation of Submicron Single Crystal Particles and Dots by Laser Ablation
Authors: Wu Hong , Vispute R. D. , Narayan J.
Published: April 2011

We have investigated the formation of micron-sized single crystal dots of germanium by pulsed laser ablation. The laser ablation of a Ge target (KrF excimer laser λ=248 nm, pulse rate 10Hz, pulse duration 25x10−9 seconds, and energy 10J /cm2) results in the formation of micron and submicron liquid droplets which are ejected from the target. These droplets can be crystallized into single crystal dots on lattice-matched substrates by rapid liquid-phase recrystallization. We report the details of microstructure as a function of dot size. It is found that under these conditions, below a critical size (about 2μm), the dots are crystalline;above which dots become polycrystalline. We discuss the implications of the results in producing doped and undoped single-crystal quantum dots for device applications.
Semiconductor Nanocrystals formed in SiO2 by Ion Implantation
Authors: Zhu Jane G. , White C. W. , Budai J. D. , Withrow S. P. , Chen Y.
Published: April 2011

Nanocrystals of group IV (Si, Ge and SiGe), III-V (GaAs), and II-VI (CdSe) semiconductor materials have been fabricated inside SiO2 by ion implantation and subsequent thermal annealing. The microstructure of these nanocrystalline semiconductor materials has been studied by transmission electron microscopy (TEM). The nanocrystals form in near-spherical shape with random crystal orientations in amorphous SiO2 Extensive studies on the nanocrystal size distributions have been carried out for the Ge nanocrystals by changing the implantation doses and the annealing temperatures. Remarkable roughening of the nanocrystals occurs when the annealing temperature is raised over the melting temperature of the implanted semiconductor material. Strong red photoluminescence peaked around 1.67 eV has been achieved in samples with Si nanocrystals in SiO2.
Correlation of Size and Photoluminescence for Ge Nanocrystals in SiO2 Matrices
Authors: Yang C.M , Shcheglov K.V. , Brongersma M.L. , Polman A , Atwater H.A.
Published: April 2011

Synthesis and size-dependent photoluminescence has been performed for Ge nanocrystals in SiO2 matrices with average diameters between 2 nm and 9 nm, formed by room temperature ion implantation into SiO2 followed by precipitation during vacuum thermal anneals. Nanocrystal size distributions obtained from electron microscopy data were used in conjunction with a quantum-confined exciton recombination model[l] to generate calculated photoluminescence spectra, which were compared with experimental spectra.
Raman, Absorption, and Photoluminescence Studies of a CdSxSe1−x Semiconductor Doped Color Glass
Authors: Morgan S. H. , Pan Z. , Mu R. , Long B. H.
Published: April 2011

A CdSxSe1−x semiconductor doped sodium zinc borosilicate glass has been made and investigated by Raman, linear absorption, and photoluminescence measurements. The glass color changes from colorless through yellow and orange to red after annealing at 625 C with increasing annealing time. The average size of Cd(S,Se) quantum dots in glass has been determined by low-frequency Raman scattering in the order of several nanometers and increases with annealing time. The resonant Raman spectra from colored glasses show the longitudinal optical phonons of CdSe and CdS-like modes in alloy microcrystals, at 204 cm−1 and 284 cm−1, respectively. The stoichiometric composition of CdSxSe1−x nano-particles, x, determined by Raman scattering, is about 0.3. The photoluminescence spectra of colored glass samples show two emission bands, which are attributed to the electron-hole recombination and emission from trap centers of surface states, respectively.
Trap States in Cd(S,Se) Nanocrystals Probed by Photomodulation Spectroscopy
Authors: Stokes Kevin L. , Persans Peter D.
Published: April 2011

Intensity, temperature and frequency-dependent photoabsorption spectroscopy are used to probe trap states in CdSo.44Seo.56 nanoparticles in glass. The photoabsorption signal from 6.2 nm radius particles is found to be a combination of two effects: bleaching, due to phase space filling, and an electric-field effect. Frequency-dependent photoabsorption data show that the electric-field effect is caused by a long-lived (r=30 /µs at T=293 K) trap state and that the bleaching occurs on a faster time scale. The trap activation energy obtained from the temperature dependence of the lifetime is 0.23 ±0.06 eV. Only bleaching is observed in the smaller nanocrystallites.
The Formation of Porous Silicon Layers Formed in a Non-Aqueous Electrolyte
Authors: Rieger Melissa M. , Kohl Paul A.
Published: April 2011

The formation of porous silicon layers was examined with respect to crystal orientation and the presence of water. Unlike the etching of (111) silicon in aqueous HF solutions, no pores were formed in HF-MeCN. The etching of (111) silicon in HF-MeCN resulted in the formation of triangular pits defined by the {111} planes. A mechanism for the etching is proposed where steric hindrance of the surface terminated hydrogens induces bond strain and enhances the chemical reactivity. The mechanism also accounts for the formation of pores in (100) silicon, and the formation of a highly branched microporous structure when silicon is etched in an aqueous solution.
Effect of Rapid Thermal Oxidation on Blue and Red Luminescence Bands of Porous Silicon
Authors: Sen S. , Kontkiewicz A. J. , Kontkiewicz A. M. , Nowak G. , Siejka J. , Sakthivel P. , Ahmed K. , Mukherjee P. , Witanachchi S. , Hoff A. M. , Lagowski J.
Published: April 2011

Photoluminescence (PL) of porous silicon after rapid thermal oxidation was studied using excitation energies of 5.0 and 6.4 eV. The emission spectra in both cases are dominated by a broad blue band centered at 430-450 nm and a much weaker red band positioned at 680-720 nm. Using atomic force microscopy (AFM), we have found a correlation between blue PL intensity and increase in feature size caused by progressive oxidation. The blue luminescence, found in porous Si only after oxidation, is identical, with respect to spectrum and fast decay, to that of thermally grown oxide on crystalline Si. Based on these findings, we conclude that the blue luminescence originates from SiO2 rather than from Si nanocrystals embedded in the oxide matrix. The red luminescence, on the contrary, has a different origin. The intensity of red band is a function of oxidation temperature and at RTP above 1050 °C, the red band is completely eliminated. The red luminescence characteristics are discussed in relation to the possible mechanism.
Size, Shape, and Crystallinity of Luminescent Structures in Oxidized Si Nanoclusters and H-Passivated Porous Si
Authors: Schuppler S. , Friedman S. L. , Marcus M. A. , Adler D.L. , Xie Y.-H. , Ross F. M. , Harris T. D. , Brown W.L. , Chabal Y. J. , Szajowski P. J. , Chaban E. E. , Brus L. E. , Citrin P. H.
Published: April 2011

Near-edge and extended x-ray absorption fine structure measurements from a wide variety of H-passivated porous Si samples and oxidized Si nanocrystals, combined with electron microscopy, ir-absorption, α-recoil, and luminescence emission data, provide a consistent structural picture of the species responsible for the luminescence observed in these systems. For luminescent porous Si samples peaking in the visible region, i. e., ≤700nm, their mass-weighted-average structures are determined here to be particles–not wires, whose short-range character is crystalline – not amorphous, and whose dimensions – typically <15 Å – are significantly smaller than previously reported or proposed. These results depend only on sample luminescence behavior, not on sample preparation details, and thus have general implications in describing the mechanism responsible for visible luminescence in porous silicon. New results are also presented which demonstrate that the observed luminescence is unrelated to either the photo-oxidized Si species in porous Si or the interfacial suboxide species in the Si nanocrystals.
Simultaneous Detection of Radiative and Non-Radiative Recombination in Porous Silicon
Authors: Grivickas Vytautas , Linnros Jan
Published: April 2011

Radiative and non-radiative recombination in porous silicon (PSi) is measured under a wide injection range by detecting of photoluminescence (PL) and free-carrier absorption (FCA) decay. We used 2.34 eV and 140 ns light pulses for carrier excitation in PSi flakes. The excited carriers were probed by focused cw IR light and the carrier concentration was calculated by using the free-carrier absorption cross section of bulk (c-Si). The results demonstrate a scaling between the total PL yield in the S-band, integrated over wavelengths, and the free carrier concentration. At lower injections the observed free carrier and PL decay follow a similar, density-independent stretched-exponential law. At injections approaching 1018 cm−3, a fast recombination component appears reducing the lifetime by a factor of ten. This component is attributed to Auger recombination of separated e-h pairs on a set of Si subclusters.
Electronic and Structural Properties of Si46: A Novel Solid of Silicon Fullerenes
Authors: Saito Susumu , Oshiyama Atsushi
Published: April 2011

We report the electronic structure of the S46 lattice having a just lnm-size unit cell and has been optimized within the density-functional theory. The Si46 lattice studied is the real geometry of Si atoms crystallized with metal atoms, and consists essentially of dodecahedral-cage Si20 fullerenes proposed by us previously. Although all the Si atoms are tetrahedrally coordinated, its electronic structure is found to be considerably different from that of the diamond Si lattice due to the pentagonal Si-Si network. Si46 is an important new Si semiconductor to be synthesized in the future. We also report the electronic structure of Na2Ba6Si46. The material is found to be metallic and the Fermi-level density of states is very high due to the hybridization between Si and Ba states, which should be of essential importance for the superconductivity observed in Na and Ba codoped Si46.
The Thermal Conductivity of Porous Silicon
Authors: Lang W. , Drost A. , Steiner P. , Sandmaier H.
Published: April 2011

The thermal conductivity of porous silicon is measured as prepared and after oxidation. The measurement method uses thermal wave propagation in the porous film. We investigate three types of porous silicon: Nanoporous p-type silicon, nanoporous n-type silicon and mesoporous p+-type silicon. The nanoporous material shows a thermal conductivity in the region of 1.2 W/mK to 1.8 W/mK as prepared and after oxidation. This value is close to silicon oxide. The mesoporous material shows a high thermal conductivity of 80 W/mK as prepared which drops to 2.7 W/mK after oxidation.
Modelling the Multiplicity of Conductance Structures in Clusters of Silicon Quantum Dots
Authors: Boeringer D. W. , Tsu R.
Published: April 2011

We have fabricated diode structures containing clusters of nanoscale silicon particles. The modelling of the observed multiple current steps from resonant tunnelling through the quantum states of these particles is presented. Whenever the applied voltage aligns the Fermi surface of the contact to a discrete state, conduction results. Since the clusters are connected in parallel, successive connections of parallel current paths can cause multiple current steps. These current steps can be drastically amplified by avalanche multiplication when the substrate is deeply depleted. The transport in this regime is quite nonlinear, and cannot be represented by a linear equivalent circuit. Apart from gaining a fundamental understanding of the measured I-V characteristics of our diodes, our results serve to promote new applications for quantum switches.
Porous Silicon Used as an Initiator in Polymerization Reactions
Authors: Heinrich Julie L. , Lee Alice , Sailor Michael J.
Published: April 2011

Porous silicon is investigated as an initiator for radical polymerization reactions. Surface radicals and photogenerated e−/h+ pairs in porous silicon can be used to initiate polymerization reactions. Porous silicon wafers were placed in ethanolic solutions of methyl methacrylate, N-vinylcarbazole, acrylonitrile, or styrene and either left in the dark or excited by light. Methyl methacrylate polymerized in the absence of light, presumably via silicon-based radical initiator sites at the porous silicon surface. The N-vinylcarbazole polymerized to a small extent in the dark, and to a much greater extent during white light irradiation of the porous silicon surface. Photopolymerization is thought to occur via excited state hole transfer from the porous silicon to the N-vinylcarbazole monomer. Very little to no polymerization of acrylonitrile or styrene could be detected either in the dark or under illumination.
Electronic Properties of Silicon - M Binary Clusters (M = C & Na)
Authors: Nakajima A. , Nakao K. , Gomei M. , Kishi R. , Iwata S. , Kaya K.
Published: April 2011

Electronic properties of silicon-carbon and silicon-sodium binary clusters, produced by laser vaporization, were investigated by photoelectron spectroscopic or photoionization spectroscopic method. The photoelectron spectra of the C1Sim-1- clusters are similar to those of pure Sim- clusters in the peak positions and their envelopes, which is attributed to the similar electronic structure of Si and C atoms, leading to a similar geometry. In contrast, the similarity in the photoelectron spectra is not observed between Cn- and Cn-1Si1 clusters, which is attributed to a change in their geometry; from chain to ring.

The ionization energies (Ei) of the SinNam clusters (l≤n≤15) were determined from the threshold energy of their ionization efficiency curves. The clear parallelism between the ionization energy of SinNa and the electron affinity (EA) of Sin is found; there are three local minima at n=4, 7 and 10. This implies the facts that (1) the structure of the SinNa clusters keeps the frame of the corresponding Sin cluster unchanged and that (2) the parentage of singly occupied molecular orbital (SOMO) of SinNa is the LUMO of Sin. Furthermore, the EAs of SinNa (4≤n≤7) were determined from the threshold energy in the photoelectron spectra of SinNa". When the EAs of SinNa are compared with those of Sin, the EAs decrease at n=4-6, but the EA increases at n=7. The results of ab initio calculation show that the Na atom is bound by two Si atoms (bridge site) at n=4-6, whereas it is bound by one Si atom (apex site) at n=7.
Blue and Green Electroluminescence from Porous Materials
Authors: Mimura H. , Matsumoto T. , Kanemitsu Y.
Published: April 2011

In applying porous Si (PS) to color display technology, it is important to fabricate light emitting devices with three primary colors. However, there have been few reports on blue and green electroluminescence (EL), and its mechanism (even the relationship between PL and EL spectra) is unclear. To obtain blue and green EL and to investigate its mechanism, we have formed PS anodized under UV illumination (UV-PS) with green photoluminescence (PL) and porous SiC with blue PL. Consequently, green and blue light emitting devices were successfully fabricated by using these materials. The observed spectra are from 350 to 750 nm with a peak of, 520 nm for ITO / UV-PS junctions and from 300 to 600 nm with a peak of 470 nm for ITO / porous SiC junctions. The EL mechanism is also discussed by reference to experimental results of comparing PL and EL spectra and of investigating the dependence of EL intensity on current.
The Influence of Local Ambient Atmosphere on the Electroluminescent Stability of Porous Silicon Diodes
Authors: Zhang Libing , Coffer Jeffery L. , Gnade Bruce E. , Xu DaXue , Pinizzotto Russell F.
Published: April 2011

In this work, the influence of surrounding ambient atmosphere on the stability of electroluminescent (EL) porous Si (PS) diodes fabricated from anodic oxidation of epitaxially grown p-type layers on n-type Si substrates is investigated. These structures are characterized using photoluminescence (PL), electroluminescence (EL), and infrared (IR) spectrosopies, as well as scanning electron microscopy (SEM). Such structures yield orange emission with maxima near 620 nm upon the application of moderate applied voltages (3-7 V). In strong oxidizing environments, EL intensity degrades completely within 30 minutes; in contrast, the integrated intensity remains essentially unchanged in the same timeframe in the presence of a vigorous flow of inert gases such as nitrogen and argon. Infrared spectroscopic studies strongly suggest that electroluminescence degradation is related to porous silicon surface oxidation.
Stabilization of electroluminescence and photoluminescence of porous n-silicon by chemical oxidation in H2O2
Authors: Kozlowski F. , Wagenseil W. , Steiner P. , Lang W.
Published: April 2011

We have oxidized porous (n-)silicon samples in solutions of H202 and have found that PL can be stabilized at a high level. The PL intensity of as prepared samples (PL quantum efficiencies in the range of 5 %) degrades to about 1/3 of the initial value in some ten minutes (the exact value depends on the experimental conditions). After having treated the samples for about 30 - 45 minutes in hydrogen peroxide it can be observed that bright PL remains stable for hours. These results confirm similar experiments performed with porous silicon made from p-substrates1. While red electroluminescing samples have shown long time stability for about 100 hours2, samples with blue-green electroluminescence have a lifetime of about 20 - 40 minutes. Oxidizing electroluminescent samples as described above results in a stabilization of electroluminescence for more than 7 hours.