Showing papers in "Physica Status Solidi (a) in 2007"

Trends in electrospinning of natural nanofibers

Abstract: Nanotechnology has become in recent years a topic of great interest to scientists and engineers, and is now established as prioritized research area in many countries. The reduction of the size to the nanometer range brings an array of new possibilities in terms of material properties, in particular with respect to achievable surface to volume ratios. Electrospinning of natural fibers is a novel process for producing superfine fibers by forcing a solution through a spinneret with an electric field. An experimental study on this technique has been made in this paper and the effect of systematic parameters on electrospun nanofibers were critically analyzed. Based on this study, the key process parameters have been identified according to our experimental observations.

Theory of ferromagnetic semiconductors

Abstract: Based upon ab initio electronic structure calculations by the Korringa-Kohn-Rostoker coherent-potential approximation (KKR-CPA) method within the local-density approximation (LDA), we propose a unified physical picture of magnetism and an accurate calculation method of Curie temperature (T C ) in dilute magnetic semiconductors (DMSs) in II-VI and III-V compound semiconductors. We also propose the unified physical picture of magnetism in the DMS, where ferromagnetic Zener's double-exchange mechanism (or Zener's p-d exchange mechanism) caused by the partially occupied impurity band and anti-ferromagnetic super-exchange mechanism (or ferromagnetic super-exchange mechanism) is competing to determine the magnetic states in the DMS. We propose that the three-dimensional 3D Dairisekiphase and one-dimensional 1D Konbu-phase caused by spinodal nano-decomposition are responsible for high-T C phase in the inhomogeneous system. We propose the new methodology to go beyond LDA to describe the highly correlated electron system by taking into account the self-interaction correction (SIC) to the LDA.

Analysis of processes limiting quantum efficiency of AlGaInN LEDs at high pumping

Abstract: In this paper we argue that the quenching of external quantum efficiency (EQE) with increase of current typically observed for AlInGaN LEDs is caused by reduction of injection efficiency. It is shown as a result of numerical simulations that the current blocking AlGaN layer is inefficient at high current density due to piezoelectric field of GaN/AlGaN interface. The results of numerical simulation are in good agreement with experimental dependence of EQE on pumping. A new design of LED heterostructure is proposed, for which the EQE quenching is not expected.

Mechanical properties of crystalline calcium‐silicate‐hydrates: comparison with cementitious C‐S‐H gels

Abstract: This work explores from a theoretical viewpoint the mechanical properties of the most important hydration product present in cementitious environments, the so called C-S-H (calcium-silicate-hydrate) gel. The dependence of the bulk (K), shear (G) and Young's (E) modulus for the C-S-H crystals respect to its composition and the length of its silicate chains is analysed by lattice dynamic simulations with parameterised two-body and three-body potentials. Our simulations reveal that the mechanical properties of C-S-H crystals show a strong dependence on their composition. Nevertheless our calculated numbers systematically overestimate the experimental values for C-S-H gels. Only when the finite length of the silicate chains is taken into account this discrepancy disappears.

Pulsed laser deposition of quaternary Cu2ZnSnSe4 thin films

Abstract: We report the successful growth of quaternary Cu2ZnSnSe4 (CZTSe) thin films by pulsed laser deposition (PLD) using Nd:YAG laser. It was found that CZTSe films atomic ratios were close to target atomic ratios with a slight metal excess and selenium deficiency. Quaternary CZTSe films grew and crystallized as a stannite-type structure even at room temperature. All CZTSe films showed a p-type electrical conductivity with a high absorption coefficient of 104–105 cm–1, a bandgap of 1.5 eV and a carrier concentration of the order of 1017–1018 cm–3. These results show that pulsed laser deposition could be employed as a particularly effective deposition method for the preparation of quaternary compounds thin films. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Deagglomeration and functionalisation of detonation diamond

Abstract: In this paper we present our recent results on the production of primary particles of detonation diamond from strongly bound agglomerates using beads milling and the newly developed BASD (bead assisted sonic disintegration) method in various media. Additionally, the surface functionalisation starting from hydroxyl groups is discussed. These groups can be introduced by reductive surface homogenisation using borane. Further functionalisation includes the grafting of silanes and amino acids.

Recent progress of high efficiency white LEDs

Abstract: We fabricated three types of white light emitting diodes (LEDs). The first is the white LED, which has a high general color rendering index (R 0 ) of 97 and CRI-No. 9 of 96. The CRI-No. 9 denotes the color reproduction in the red region. These values are higher than those of a tri-phosphor fluorescent lamp (R a = 85 and CRI-No. 9 = 8). The second is the high efficiency white LED fabricated from the small-size high efficiency blue LED chip. The output power (P 0 ), the external quantum efficiency (η ex ) and wall-plug efficiency (WPE) of the small-size blue LED were 35.0 mW, 63.3% and 56.3%, respectively, at a forward-bias current of 20 mA. The luminous flux (Φ), luminous efficiency (η L ) and WPE of the second white LED are 8.6 lm, 138 Im/W and 41.7%, respectively. The luminous efficiency is 1.5 times greater than that of a tri-phosphor fluorescent lamp (90 lm/W). The third is the high power white LED fabricated from the larger-size blue LED chip. Po, η ex and W.P.E. are 458 mW, 47.2% and 39.7%, respectively, at 350 mA. Φ, η L and WPE of the third white LED are 106 lm, 91.7 Im/W and 27.7% at 350 mA, respectively. Moreover, Φ of 247 lm and 402 lm at 1 A and 2 A are obtained, respectively. Φ at 2 A is equivalent to the total flux of a 30 W incandescent lamp.

Hydrogen blistering of silicon: Progress in fundamental understanding

Abstract: When silicon is implanted with a sufficient concentration of H ions, at low to moderate temperature, and subsequently annealed at high temperature, dome-shaped gas-filled blisters and/or craters of exploded blisters appear on the surface. Under particular conditions, blistering can be produced by plasma hydrogenation as well. The phenomenon is another facet of hydrogen behaviour in silicon, a question with both fundamental and applied implications. Blistering is at the origin of the "ion-cutting" process for the fabrication of silicon-on-insulator and other heterostructures; this process is particularly useful whenever atomically sharp interfaces between layers are required. The novelty and vast potential of this process has spurred since the mid-1990's a burst of experimental activity on blistering. The purposes of those works were either to improve or extend the ion-cut process, or to clarify its underlying mechanisms. In "mechanisms", the plural is used to convey the fact that it is a multi-step phenomenon. Because of this complexity, the theoretical work, in comparison, is far less abundant. Hydrogen blistering of silicon is qualitatively understood in broad terms: H being insoluble in Si, it tends to segregate into cavities which grow and coalesce at high temperature, and the H 2 pressure in the cavities finally deforms the surface. In fact, our understanding of the microscopic mechanisms has progressed much beyond that level thanks to the sophisticated work that has been carried out using techniques such as transmission electron microscopy, Ruther-ford backscattering in the channelling mode, infrared spectroscopy of local vibrational modes, stress and strain measurements, and others. The effects of n- or p-doping, He ion coimplantation, and isotope substitution have also greatly helped in discriminating between different hypotheses. After a review of the most relevant experimental facts, the blistering mechanisms that have been proposed in the literature will be discussed and their conformity with the data assessed. Finally an attempt will be made to identify the key questions and suggest a few avenues for future work.

Nanocrystalline MgB4O7:Dy for high dose measurement of gamma radiation

Abstract: Magnesium borate activated by dysprosium (MgB4O7:Dy) is a low-Zeff, tissue-equivalent material that is commonly used for medical dosimetry of ionizing radiations such as gamma and X-rays using the thermoluminescence (TL) technique. Nanocrystals of the same material are produced and their TL characteristics are studied. It is found that the nanocrystalline MgB4O7:Dy with a dopant concentration of 1000 ppm is the most sensitive amongst varying dopant concentrations, with its sensitivity equal to 0.025 times that of the standard phosphor CaSO4:Dy. The glow curve has two peaks at 154 °C and 221 °C. The nanophosphor has very poor sensitivity for low doses up to 10 Gy but beyond this dose the phosphor exhibits a linear response up to 5000 Gy. On increasing the dose further the response first becomes supralinear and then sublinear, finally resulting into saturation. Considering also its low fading particularly under post-irradiation annealing and excellent reusability features, this nanophosphor may be used for high dose (10–5000 Gy) measurements of ionizing radiations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The transfer doping of graphite and graphene

Abstract: It has been commonly thought that diamond is unique in its class in that it can undergo p-type transfer doping when covered by a thin layer of electrolyte. In this study, density-functional theory has been used to show that the work functions of graphite and graphene are low enough for transfer doping to occur between a graphite or graphene substrate and an aqueous adsorbate layer. The possible electrochemical reactions are the same as those favoured in the case of diamond, e.g. O 2 +2H 2 O+4e - → 40H - , where the electrons have been provided by the graphite or graphene substrate. Notably, the extraordinary nature of carrier transport in (single-layer) graphene renders this non-disruptive doping method an exciting possibility.

Radiation hardness of diamond and silicon sensors compared

Abstract: The radiation hardness of silicon charged particle sensors is compared with single crystal and polycrystalline diamond sensors, both experimentally and theoretically. It is shown that for Si- and C-sensors, the NIEL hypothesis, which states that the signal loss is proportion al to the Non-Ionizing Energy Loss, is a good approximation to the present data. At incident proton and neutron energies well above 0.1 GeV the radiation damage is dominated by the inelastic cross section, while at non-relativistic energies the elastic cross section prevails. The smaller inelastic nucleon-Carbon cross section and the light nuclear fragments imply that at high energies diamond is an order of magnitude more radiation hard than silicon, while at energies below 0.1 GeV the difference becomes significantly smaller.

Valence and coordination of chromium ions in ZnO-Sb2O3-B2O3 glass system by means of spectroscopic and dielectric relaxation studies

Abstract: ZnO–Sb2O3–B2O3 glasses containing different concentrations of chromium ions (introduced as CrO2–4) ranging from 0 to 1.0 mol% were prepared. A number of studies viz., differential thermal analysis, infrared, optical absorption, Raman and ESR spectra and dielectric properties (constant e ′, loss tan δ, a.c. conductivity σac over a wide range of frequency and temperature) of these glasses have been carried out. The results have been analyzed in the light of different oxidation states of chromium ions. The analysis indicates that when the concentration of chromium ions is ∼0.2 mol%, these ions mostly exist in Cr6+ state, occupy network forming positions with CrO2–4 structural units and increase the rigidity of the glass network. When the concentration of the chromium ions is increased beyond 0.2 mol%, these ions seem to be reduced from Cr6+ state to Cr5+ and Cr3+ states. A gradual transformation of chromium ions from tetrahedral positions to octahedral positions could also be established from these results. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Lithium-ion insertion in anodic TiO2 nanotubes resulting in high electrochromic contrast

Abstract: The present work investigates lithium insertion into highly ordered nanotubular layers of TiO 2 and its electrochromic effects. The nanotubes were formed by anodization of titanium in 1 M (H 3 PO 4 ) + 1 M (NaOH) + 0.5wt% HF electrolyte at 20 V. This leads to nanotubular layers with a thickness of 1 ± 0.1 μm, individual tube diameter of 100 ± 10 nm and a tube wall thickness of 10 ± 2 nm. The cathodic behavior of tubes in the "as formed" amorphous phase and annealed to anatase were studied in a solution of 1 M LiCl0 4 in acetonitrile. Cyclic voltammograms and potential step experiments combined with reflectance measurements show for anatase nanotubes a well defined uptake potential for Li + and a very strong and reversible electrochromic effect, while on the other investigated TiO 2 structures the effects were much less pronounced.

Raman studies of ZnO:Co thin films

Abstract: In this paper we investigate cobalt doped ZnO thin films prepared via different sol-gel methods. XRD shows only the diffraction peaks from wurtzite ZnO without secondary phase, as widely reported in literature, for the Co contents of our samples of 3 to 12%. Raman spectroscopy reveals the existence of anti-ferromagnetic cobalt oxides like CoO and Co 3 O 4 already in intermediately doped samples. Furthermore, the Raman shifts of the doped samples exhibit a shift to higher energy with increasing Co doping content. Unfortunately, Raman spectroscopy is not sensitive to metallic Co and, therefore, cannot be employed to explore the presence of Co clusters. However, we demonstrate that XRD is not a suitable method to rule out the existence of magnetic secondary phases in ZnO doped with transition metals.

Stability of H-terminated BDD electrodes: an insight into the influence of the surface preparation

Abstract: EIS, CV and XPS experiments were carried out to address the evolution of H-terminated BDD electrochemical properties under intensive use and air aging. A drastic drop of apparent electron transfer rate k' 0 was thus recorded using Fe(CN 6 ) 3-/4- as the redox mediator by identifying components of the Randles equivalent circuit. Excellent agreement was observed between theoretical and experimental curves. The feasibility of recovering and stabilizing high reactivity and reversible behaviour by applying a suitable electrochemical post treatment was then demonstrated. Such a treatment was developed empirically and seemed to significantly improve the electrode performances. On the basis of our results, we claim that the electrochemical evolutions observed are strongly linked to chemical termination modifications and partial inactivation of the electrode surface. Further research on the mechanisms that govern electron mediation at the diamond surface would be of high interest for better control of diamond electrode stability and reactivity towards their applications for bio-electronic devices.

Synchrotron-radiation computed laminography for high-resolution three-dimensional imaging of flat devices

Abstract: Synchrotron-radiation computed laminography (SRCL) is developed as a method for high-resolution three-dimensional (3D) imaging of regions of interest (ROIs) in all kinds of laterally extended devices. One of the application targets is the 3D X-ray inspection of microsystems. In comparison to computed tomography (CT), the method is based on the inclination of the tomographic axis with respect to the incident X-ray beam by a defined angle. With the microsystem aligned roughly perpendicular to the rotation axis, the integral X-ray transmission on the two-dimensional (2D) detector does not change exceedingly during the scan. In consequence, the integrity of laterally extended devices can be preserved, what distinguishes SRCL from CT where ROIs have to be destructively extracted (e.g. by cutting out a sample) before being imaged. The potential of the method for three-dimensional imaging of microsystem devices will be demonstrated by examples of flip-chip bonded and wire-bonded devices.

Normally-off GaN-MISFET with well-controlled threshold voltage

Abstract: A new device structure with four epitaxial layers and a recessed gate is proposed for normally-off operation in GaN-FETs. Employment of this structure makes it possible to control accurately the threshold voltage without accurate control of recess etching depth. The high breakdown voltage and the low on-resistance of the fabricated devices are also reported.

Electronic activity of SiC precipitates in multicrystalline solar silicon

Abstract: In the upper part of block-cast multicrystalline silicon one often finds silicon carbide and silicon nitride precipitates and inclusions. These contaminants can cause severe ohmic shunts in solar cells and thus decrease the efficiency of the solar cells very strongly. It is well known that the silicon carbide precipitates cause the ohmic shunts. However, the electrical properties of the silicon carbide was unknown so far. To study the electrical properties of these silicon carbide particles we isolated them from the silicon bulk material and performed different electrical measurements on them. The measurements show that the silicon carbide precipitates are highly conductive. An investigation of the heterojunction silicon – silicon carbide was also performed and a simulation of this heterojunction leads to a new model of the ohmic shunt mechanism. It is concluded that the shunt current flows inside of the filaments.

Strain and wafer curvature of 3C-SiC films on silicon : influence of the growth conditions

Abstract: We study the influence of the growth conditions on the residual strain and related optical and structural properties in the case of 3C-SiC films grown on (001) silicon substrates. We show that two possible mechanisms compete to manage the final sample bow: one is by controlling the composition of the gaseous phase (C/Si ratio) the other one by adjusting the growth temperature and duration (creep effect). In both cases, we compare the low temperature photoluminescence spectra of samples grown under tensile or compressive final stress. We show that better results can be obtained when using the creep effect.

Synthesis and magnetic properties of nano Ba‐hexaferrite/NiZn ferrite composites

Abstract: Ba-hexaferrite and Ni 0.5 Zn 0.5 ferrite nano composites have been successfully fabricated by a self-propagating combustion method. Transmission electron microscopy and X-ray diffractometer analysis showed that two ferrite phases were homogeneously distributed and the measured grain size of them around 20nm. The saturation magnetizations of the composites were revealed to be increased compared with the theoretical values. These increments of the saturation magnetization values could be explained by the exchange spring interaction between soft and hard magnetic phases.

Structural and optical properties of Cu3SnS4 sprayed thin films

Abstract: This paper reports the successful deposition of Cu3SnS4 thin films using a spray pyrolysis technique. These films were deposited on glass substrates using a solution containing CuCl2 · 2H2O, SnCl2 · 2H2O and SC(NH2)2 as precursors with appropriate chemical compositions of elements ([Sn] = 10–2 M and [Cu]/[Sn] = 2 × 2.2) at a substrate temperature of 360 °C. X-ray diffraction analysis show that unannealed and annealed Cu3SnS4 thin films under sulphur atmosphere crystallise in the tetragonal structure and the crystallites exhibit preferential (112) orientation of the grains. Optical measurements show that Cu3SnS4 sprayed thin film annealed at 500 °C has a direct band gap of about 1.22 eV. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Gate-lag and drain-lag effects in (GaN)/InAlN/GaN and InAlN/AlN/GaN HEMTs

Abstract: Gate and drain-lag effects are studied in (GaN)/InAlN/GaN and InAlN/AlN/GaN HEMTs grown on sapphire. Electron trapping on the surface states between the gate and the drain forming the net negative charge up-to similar to 2 x 10(13) cm(-2) is found to be responsible for the gate-lag effect in the (GaN)/InAlN/GaN HEMTs. If the polarization charge at the device surface is decreased by GaN capping, then density of the trapped charge is not changed, however the electron de-trapping process becomes faster. The drain-lag effect is caused by electron injection and trapping in the source-gate area reaching similar to 1 x 10(13) cm(-2) of the trapped charge in the steady state. In the studied voltage range the InAlN/AlN/GaN HEMT is shown to be gate-lag-free suggesting that this parasitic transient can be avoided if thin AlN is used in the epi-layer growth sequence. It is assumed that this breakthrough quality relates to the decreased or even reverted electric field in the MAIN layer if AlN is inserted. Surface states need not to be generated in this case. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Spatial resolution in Bragg-magnified X-ray images as determined by Fourier analysis

Abstract: The spatial resolution of an imaging system is most often experimentally determined by using model objects and idealized imaging conditions. This traditional approach does not properly take into account the influence of noise and the dependence of contrast on the object. The latter influence is essential especially in the case of magnified X-ray images acquired by two fold asymmetric Bragg diffraction. For this phase sensitive technique the application of a criterion for spatial resolution based upon Fourier analysis is shown to provide reliable resolution values in a simple way, overcoming the limitations mentioned above. For the case of Bragg-magnified imaging of an ant leg with 166 fold magnification (effective pixel size 0.094 μm), the physical resolution achieved in the image is found to be 0.4 μm.

Surface Plasmon Enhanced Bright Light Emission from InGaN/GaN

Abstract: Surface plasmon (SP) coupling technique was used to enhance blue and green light emissions from InGaN/GaN quantum wells (QWs). Large enhancement of photoluminescence (PL) of both blue and green emissions was observed with silver coated samples, whereas the enhancements were not so effective for the gold coated samples. We could obtain well enhanced green emission by tuning the matching condition of QW-SP coupling with nano-grating structures at gold layers. This method should be useful to design even more efficient structures and to fabricate super bright light emitting devices.

Using a porous silicon photonic crystal for bacterial cell-based biosensing

Abstract: This work demonstrates a method for non-invasively monitoring the growth and Φ6 virus infection of Pseudomonas syringae bacteria using a 1-D porous silicon photonic crystal and a white-light source coupled to a CCD spectrometer. Bacteria growth on porous Si leads to increased scattering efficiency that can be measured as a change in intensity of light reflected from a 1-D porous Si photonic crystal. A linear relationship between bacteria concentration and intensity of light at the photonic resonance is observed, and detection limits are similar to those obtained from optical density measurements. Upon infection with virus, decreases in scattered light intensity are observed at times that correlate with cell lysis caused by viral replication and subsequent bursting of bacteria cells. It is shown that measurements can be performed in real-time and in an incubator where cells remain in their ideal environment throughout the experiment. This method is well-suited for cell-based biosensing, because bacteria cells can be monitored remotely without the need for sampling cells for plating or optical density measurements.

Optical, electrical and magnetic properties of transparent, n-type conductive Zn0.90−xV0.10Alx O thin films elaborated from aerogel nanoparticles

Abstract: Vanadium-aluminium co-doped ZnO thin films have been grown by rf-magnetron sputtering of Zn0.90-xV0.10AlxO (x = 0.01 to 0.03) aerogel nanoparticles. Even though the films were deposited at room temperature they were of good crystal quality and showed c-axis texturing normal to the film surface. The films are highly transparent in the visible with a transmittance higher than 90%. The highest conductivity of 5.10(3) Omega(-1) cm(-1) is obtained for 1% Al doping. The incorporation of V, a notoriously difficult dopant in ZnO, is confirmed by the observation of different absorption bands near 800 nm which we attribute to intracenter transitions of Zn substituted V2+. The magnetic properties of the aerogel particles have been studied by electron paramagnetic resonance (EPR) spectroscopy. No evidence for a ferromagnetic phase is found contrary to theoretical predictions. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Loading and release of a model protein from porous silicon powders

Abstract: Porous silicon (p-Si) has been investigated as a novel delivery system for protein therapeutics. The loading of a model hydrophilic protein, Papain into anodised and stain etched p-Si powders has been investigated using X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (FTIR) and correlations made with the release kinetics. Variation in specific Papain (Amide I/II) and p-Si (Si–Hx) functional group absorbances with Papain loading level was characterised using FTIR, while the surface chemical distribution was assessed using XPS. A combination of burst release and sustained release of Papain was observed from p-Si powders; this was dependent on p-Si type and the Papain loading level. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Betavoltaic and photovoltaic energy conversion in three-dimensional macroporous silicon diodes

Abstract: A three-dimensional p-n diode structure is presented for the generation of energy via photovoltaic and betavoltaic modes of operation. Macroporous Silicon (MPS) has a large degree of internal surface area and its vertically oriented pores, which extend deep into the bulk of the Si substrate, allow for the creation of three-dimensional structures. In this device the MPS will not only serve as a means for creating 3D diode structures, it will also serve as a host matrix for a tritium isotope which emits energetic beta particles. By varying electrochemical etching conditions and using a prepatterning technique, 1.1 μm diameter pores with a spacing of 2.5 μm were achieved. The p-n junction was created using a rapid thermal process (RTP) which relies on the diffusion from an n-type solid source into the MPS. To ensure the quality of the diode structure, devices were tested using a light source which resulted in a photovoltaic response. Finally, betavoltaic operation was demonstrated by exposing devices to a tritium gas source. The average energy conversion efficiency of the first generation 3D diode was one order of magnitude higher than that of a similar planar device.

Photovoltaic effects in InGaN structures with p–n junctions

Abstract: InGaN photovoltaic structures with p-n junctions have been fabricated by metal organic chemical vapour deposition. Using double-crystal X-ray diffraction measurements, it was found that the room temperature band gaps of p-InGaN and n-InGaN films were 2.7 and 2.8 eV, respectively. Values of 3.4 x 10(-2) mA cm(-2) short-circuit current, 0.43 V open-circuit voltage and 0.57 fill factor have been achieved under ultraviolet illumination (360 nm), which were related to p-n junction connected back-to-back with a Schottky barrier and many defects of the p-InGaN film. 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Nanocomposite of SWNTs and SnO2 fabricated by soldering process for ammonia gas sensor application

Abstract: We introduce a simple method to fabricate an ammonia gas sensor with a nanocomposite of single-walled carbon nanotubes (SWNTs) and SnO 2 . The nanocomposite showed a semiconducting property. With exposure to ammonia gas, the electrical resistance of the nanocomposite sensor increases rapidly. It was found that the sensor can detect the concentration of NH 3 down to the 10 ppm level at room temperature. The sensor exhibited a fast response time of less than 100 s and good sensing response and recovery.