Showing papers in "Physica Status Solidi (a) in 2014"

Development of gallium oxide power devices

Abstract: Gallium oxide (Ga2O3) is a strong contender for power electronic devices. The material possesses excellent properties such as a large bandgap of 4.7–4.9 eV for a high breakdown field of 8 MV cm−1. Low cost, high volume production of large single-crystal β-Ga2O3 substrates can be realized by melt-growth methods commonly adopted in the industry. High-quality n-type Ga2O3 epitaxial thin films with controllable carrier densities were obtained by ozone molecular beam epitaxy (MBE). We fabricated Ga2O3 metal-semiconductor field-effect transistors (MESFETs) and Schottky barrier diodes (SBDs) from single-crystal Ga2O3 substrates and MBE-grown epitaxial wafers. The MESFETs delivered excellent device performance including an off-state breakdown voltage (Vbr) of over 250 V, a low leakage current of only few μA mm−1, and a high drain current on/off ratio of about four orders of magnitude. The SBDs also showed good characteristics such as near-unity ideality factors and high reverse Vbr. These results indicate that Ga2O3 can potentially meet or even exceed the performance of Si and typical widegap semiconductors such as SiC or GaN for ultrahigh-voltage power switching applications.

Enhanced strength in bulk graphene-copper composites

Abstract: Graphene nanoplatelets (GNPs) exhibit ultra-high strength and elastic modulus. Therefore, they are potential ideal reinforcements in metal–matrix composites (MMCs). In this work, we report the use of GNPs to strengthen the bulk Cu-matrix composites. GNP reinforced Cu-matrix (GNP/Cu) composites were prepared by a combination of the ball milling and hot-pressing processing, and their mechanical properties were investigated. Microstructure studies indicated that the GNPs with 0–8 vol.% contents were well dispersed in the Cu matrix by ball milling. Compared to unreinforced Cu, the GNP/Cu composites showed a remarkable increase in yield strength and Young's modulus up to 114 and 37% at 8 vol.% GNP content, respectively. The extraordinary reinforcement is attributed to the homogeneous dispersion of GNPs and grain refinement. However, the mechanical improvement of GNP/Cu composites was still below the theoretical value. The possible reasons for this deviation were discussed and the methods for further mechanical improvement of GNP/Cu composites were proposed.

Homoepitaxial growth of β-Ga2O3 layers by metal-organic vapor phase epitaxy

Abstract: Epitaxial β-Ga2O3 layers have been grown on β-Ga2O3 (100) substrates using metal-organic vapor phase epitaxy. Trimethylgallium and pure oxygen or water were used as precursors for gallium and oxygen, respectively. By using pure oxygen as oxidant, we obtained nano-crystals in form of wires or agglomerates although the growth parameters were varied in wide range. With water as an oxidant, smooth homoepitaxial β-Ga2O3 layers were obtained under suitable conditions. Based on thermodynamical considerations of the gas phase and published ab initio data on the catalytic action of the (100) surface of β-Ga2O3 we discuss the adsorption and incorporation processes that promote epitaxial layer growth. The structural properties of the β-Ga2O3 epitaxial layers were characterized by X-ray diffraction pattern and high resolution transmission electron microscopy. As-grown layers exhibited sharp peaks that were assigned to the monocline gallium oxide phase and odd reflections that could be assigned to stacking faults and twin boundaries, also confirmed by TEM. Shifts of the layer peak towards smaller 2θ values with respect to the Bragg reflection for the bulk peaks have been observed. After post growth thermal treatment in oxygen-containing atmosphere the reflections of the layers do shift back to the position of the bulk β-Ga2O3 peaks, which was attributed to significant reduction of lattice defects in the grown layers after thermal treatment.

Determination of the mean and the homogeneous barrier height of Cu Schottky contacts on heteroepitaxial β‐Ga2O3 thin films grown by pulsed laser deposition

Abstract: We have investigated the electrical properties of Cu Schottky contacts (SCs) on (2¯01)-oriented β-Ga2O3 thin films, which have been grown by pulsed laser deposition (PLD). The I–V characteristics of two different sample structures exhibit rectification ratios at ±2 V up to 7 orders of magnitude. The dominant current transport mechanism is thermionic emission. By fitting the I–V characteristics, we obtained the ideality factor n and the effective barrier height ΦBeff at temperatures between 50 and 320 K. Considering a Gaussian barrier height distribution, we determined a mean barrier height of 1.32 eV. The contacts are stable at high temperatures up to at least 550 K. At this temperature a homogeneous barrier height of 1.32 eV is found, consistent with the determined mean barrier height. The ideality factor for this temperature is 1.03 and barrier inhomogeneities do not influence current transport, making the contact close to ideal. Schematic band diagram of a Cu/β-Ga2O3 Schottky contact at a temperature of 550 K. The inset shows a photographic image of the sample.

Challenges for energy efficient wide band gap semiconductor power devices

Abstract: Wide band gap semiconductors, and in particular silicon carbide (4H-SiC) and gallium nitride (GaN), are very promising materials for the next generation of power electronics, to guarantee an improved energy efficiency of devices and modules. As a matter of fact, in the last decade intensive academic and industrial research efforts have resulted in the demonstration of both 4H-SiC MOSFETs and GaN HEMTs exhibiting VB2/Ron performances well beyond the silicon limits. In this paper, some of the present scientific challenges for SiC and GaN power devices technology are reviewed. In particular, the topics selected in this work will be the SiO2/SiC interface passivation processes to improve the channel mobility in 4H-SiC MOSFETs, the current trends for gate dielectrics in GaN technology and the viable routes to obtain normally-off HEMTs.

Highly sensitive magnetometer based on the off-diagonal GMI effect in Co-rich glass-coated microwire

Abstract: The design and performance of a magnetometer based on the off-diagonal GMI effect in Co-rich glass-coated microwire are presented. The sensing element of the magnetometer is a 10-mm long piece of Co–Fe–Ni–B–Si–Mo microwire with a small pick-up coil of 85 turns wounded around the microwire. The electronics with a feedback circuit is used to register an electromotive force proportional to the external magnetic field applied along the wire axis. In the absence of the feedback current the magnetometer is capable of measuring a narrow range of magnetic fields, ±3.5 μT, in the frequency range of 0–1 kHz, the level of the equivalent magnetic noise being about 10 pT Hz−1/2 at a frequency of 300 Hz. The use of the feedback circuit increases the range of the measured magnetic fields up to ±250 μT. Photo of the giant magnetoimpedance magnetometer based on the off-diagonal GMI effect in Co-rich glass-coated microwire.

On the nature and temperature dependence of the fundamental band gap of In2O3

Abstract: The onset of optical absorption in In2O3 at about 2.7 eV is investigated by transmission spectroscopy of single crystals grown from the melt. This absorption is not defect related but is due to the fundamental band gap of In2O3. The corresponding spectral dependence of the absorption coefficient is determined up to α = 2500 cm−1 at a photon energy hν = 3.05 eV at room temperature without indication of saturation. A detailed analysis of the hν dependence of α including low-temperature absorption data shows that the absorption process can be well approximated by indirect allowed transitions. It is suggested that the fundamental band gap of In2O3 is of indirect nature. The temperature dependence of the fundamental band gap is measured over a wide range from 9 to 1273 K and can be well fitted by a single-oscillator model. Compared to other semiconductors the reduction of the gap with increasing temperature is exceptionally strong in In2O3.

Silicon nanostructures for thermoelectric devices: A review of the current state of the art

Abstract: Silicon is the most important element of modern semiconductor industry. As a thermoelectric converter material, it would be able to promote applications dramatically since device integration and scaling of the fabrication processes could be borrowed from existing technology. Though the thermoelectric performance of single crystalline silicon is only poor, silicon nanostructures have demonstrated a competitive figure of merit. To make use of these nanostructures, novel device architectures are necessary. This review provides an overview over the physical background of silicon as thermoelectric converter material as well as different nanostructures like nanowires, porous nanomeshes, and nanocrystalline bulk and their thermoelectric properties, focusing mostly on experimental data. Further, different thermoelectric converter device concepts including nanowire device concepts, pn junction thermoelectric generators and integrated spot coolers are discussed and their realization with silicon nanostructures briefly sketched. (a) Nanowires, porous nanomeshes and nanocrystalline bulk with promising thermoelectric material's figure of merit. (b) Innovative device concepts using nanowires or large area pn junctions for thermoelectric heat-to-electricity conversion.

Chemical vapor deposited graphene: From synthesis to applications

Abstract: Graphene is a material with enormous potential for numerous applications. Therefore, significant efforts are dedicated to large-scale graphene production using a chemical vapor deposition (CVD) technique. In addition, research is directed at developing methods to incorporate graphene in established production technologies and process flows. In this paper, we present a brief review of available CVD methods for graphene synthesis. We also discuss scalable methods to transfer graphene onto desired substrates. Finally, we discuss potential applications that would benefit from a fully scaled, semiconductor technology compatible production process.

Towards high efficiency segmented thermoelectric unicouples

Abstract: Segmentation of thermoelectric (TE) materials is a widely used solution to improve the efficiency of thermoelectric generators over a wide working temperature range. However, the improvement can only be obtained with appropriate material selections. In this work, we provide an overview of the theoretical efficiency of the best performing unicouples designed from segmenting the state-of-the-art TE materials. The efficiencies are evaluated using a 1D numerical model which includes all thermoelectric effects, heat conduction, Joule effects and temperature dependent material properties, but neglects contact resistance and heat losses. The calculations are performed for a fixed cold side temperature of 300 K and different hot side temperatures of 700, 900, and 1100 K. We confirm that without taking into account the compatibility of TE materials, segmentation can even decrease the total efficiency. Choosing the TE materials carefully, one is, however, rewarded by a significant improvement in the total efficiency.

Control of the conductivity of Si-doped β-Ga2O3 thin films via growth temperature and pressure

Abstract: Si-doped β-Ga2O3 thin films were grown at temperatures between 400 and 650 °C and oxygen partial pressures ranging from 3 × 10−4 mbar to 2.4 × 10−2 mbar prepared by pulsed laser deposition (PLD) on c-plane sapphire substrates. For high oxygen partial pressure the samples are composed of multiple crystalline phases; for decreasing oxygen partial pressure the crystallinity improves and single phase ()-oriented thin films are obtained for an oxygen partial pressure below at a growth temperature of 650 °C. We find a correlation between surface morphology of our thin films and their conductivity; an increasing root mean square surface roughness entails increased conductivity. Further we show that the oxygen partial pressure resulting in maximal conductivity decreases with increasing growth temperature. The results provide means to realize β-Ga2O3-based devices such as rectifiers, photodetectors or thin film transistors with optimized surface roughness, structural quality, and conductivity.

Tailored design of boron-doped diamond electrodes for various electrochemical applications with boron-doping level and sp2-bonded carbon impurities

Abstract: The effects of sp2-bonded carbon impurities on the electrochemical properties of boron-doped diamond were investigated in moderately ([B] 1021 cm−3) boron doping levels. Significant influences of sp2-bonded carbon impurities, which show glassy carbon-like electrochemical properties after anodic oxidation, were observed in heavily boron-doped diamond. This indicated that the significant effects of enhanced adsorption properties were possibly caused by surface relaxation of the strains induced by heavy boron doping and sp2-bonded carbon impurities. On the other hand, their durability was still similar to diamond electrodes rather than glassy-carbon electrodes because of the low fraction of sp2-bonded carbon impurities. Such “active” diamond electrodes are much less suitable for wastewater treatment than ordinary diamond electrodes due to a different oxygen-evolution mechanism. On the other hand, “active” BDD electrodes have a much higher efficiency for electrochemical ozone production than other BDD electrodes. The electrode properties of BDD can be designed by controlling the boron doping level and introducing the sp2-bonded carbon impurities. The guidelines proposed in this study can be used effectively to design electrodes according to their individual application, such as for use as electrochemical sensors, in wastewater treatment or electrochemical ozone production.

Fabrication and tensile properties of graphene/copper composites prepared by electroless plating for structrual applications

Abstract: Despite a growing interest in graphene nanoplatelets (GNPs)-based metal matrix composites, developing high-performance GNP/metal composites with full exploration of graphene properties remains a great challenge. Here, we propose an effective method to prepare GNP/Cu composites by an electroless plating process and followed by a spark plasma sintering technique. The precoating of Cu nanoparticles on GNPs effectively inhibited the GNPs aggregations and made the dispersion of GNPs homogeneous in the composites. Transmission electron microscopy studies revealed the formation of oxygen mediated bonding between the Cu matrix and the GNPs arising from the residue functional groups on GNPs. Tensile tests demonstrated 107 and 21% increases in tensile strength and Young's modulus, respectively, in 1.3 wt.% GNP/Cu composites. The measured modulus was in good agreement with the theoretical estimation from the Halpin–Tsai model.

Atomic layer deposition of functional films for Li‐ion microbatteries

Abstract: The lithium ion battery concept is a promising energy storage system, both for larger automotive systems and smaller mobile devices. The smallest of these, the microbatteries, are commonly based on the all-solid state concept consisting of thin layers of electroactive materials separated by a solid state electrolyte. The fact that solid state electrolytes are required puts rather severe constraints on the materials in terms of electronic and ionic conductivity, as well as lack of pinholes otherwise leading to self-discharge. The atomic layer deposition (ALD) technology is especially suitable for realization of such microbatteries for the Li-ion technology. ALD has an inherent nature to deposit conformal and pinhole free layers on complex geometrical shapes, an architecture most commonly adopted for microbattery designs. The current paper gives an overview of ALD-type deposition processes of functional battery materials, including cathodes, electrolytes, and anodes with the aim of developing all-solid-state batteries. Deposition of Li-containing materials by the ALD technique appears challenging and the status of current efforts is discussed.

Template‐assisted Co–Ni alloys and multisegmented nanowires with tuned magnetic anisotropy

Abstract: Homogeneous Co85Ni15 and Co54Ni46 alloys, together multisegmented Co85Ni15/Co54Ni46 magnetic nanowire arrays have been synthesized by means of potentiostatic and pulsed potentiostatic electrodeposition techniques, respectively, into the pores of hard-anodic alumina templates. Morphological, compositional and structural characterization indicate that the composition of Co–Ni alloy segments and nanowires can be properly adjusted by varying the electrochemical deposition potential, leading to different crystalline structures (fcc or hcp) depending on the Co–Ni alloy composition. The magnetic properties of homogeneous and multisegmented Co–Ni alloy nanowire arrays, obtained from hysteresis loops and switching field distribution calculations, are correlated with their crystalline structure and effective magnetic anisotropy, to shed light on the magnetization processes that determine the magnetic behaviour of the nanowire arrays. Part (a) shows a schematic drawing of freestanding multisegmented Co85Ni15/Co54Ni46 nanowires. Part (b) displays the magnetic hysteresis loops of multisegmented nanowire arrays measured along the parallel and perpendicular directions with respect to the nanowires long axis.

Plasma‐assisted molecular beam epitaxy of Sn‐doped In2O3: Sn incorporation, structural changes, doping limits, and compensation

Abstract: A comprehensive study of Sn doping in In2O3 during plasma-assisted molecular beam epitaxy (PA-MBE) is given, covering growth aspects and application-relevant aspects such as structural and transport properties. Single crystalline, (001) oriented indium oxide (In2O3) thin films were grown on Y-stabilized ZrO2(001) and systematically doped with 1018 cm−3 to 6 × 1021 cm−3 tin (Sn) by PA-MBE. The Sn incorporation was proportional to the Sn flux up to a Sn concentration of ≈1020 cm−3 indicating well-controllable doping in this regime. Toward higher Sn concentrations the Sn incorporation was increasingly impeded, which could be somewhat mitigated by increasing the oxygen-to-indium flux ratio. The surface faceting of undoped In2O3(001) during growth under oxygen rich conditions was prevented by doping to Sn concentrations 4×1020 cm−3. Up to Sn concentrations of 1.4 × 1021 cm−3 no detrimental effects on the film crystal quality were observed by X-ray diffraction, but concentrations cm−3 resulted in structural deterioration with the formation of secondary crystalline phases. The electron concentration increased and resistivity decreased with increasing Sn concentration. The electron concentration was limited to ≈2 × 1021 cm−3 despite higher Sn concentrations and a minimum resistivity of 9 × 10−5 Ω cm was reached at a Sn concentration of ≈1021 cm−3. The highest electron concentrations and lowest resistivities were realized by a post-growth vacuum annealing to remove compensating acceptors. Guidelines to obtain low resistivity, high-quality indium tin oxide (ITO) films are given. Textured reference films grown on r-plane sapphire, Al2O3(10–12), showed very similar behavior in terms of incorporation, doping limit, and compensation, which indicates that our results are qualitatively not limited to single crystalline films.

Growth, characterization, and properties of bulk SnO2 single crystals

Abstract: SnO2 is a semiconductor with a wide optical bandgap (3.5 eV), which makes it an attractive transparent semiconducting oxide (TSO) for electronic and opto-electronic applications. At elevated temperatures it is, however, much more unstable than other TSOs (such as ZnO, Ga2O3, or In2O3). This leads to a rapid decomposition even under very high oxygen pressures. Our experiments showed that stoichiometric SnO2 does not melt up to 2100 °C, in contradiction to earlier published data. Bulk SnO2 single crystals, that could provide substrates for epitaxial growth, have not been reported so far. Hereby we report on truly bulk SnO2 single crystals of 1 inch diameter grown by physical vapor transport (PVT). The most volatile species during SnO2 decomposition is, in addition to oxygen, SnO, which is stable in the gas phase at high temperature and reacts again with oxygen at lower temperatures to form SnO2. We identified a relatively narrow temperature window, temperature gradients and a ratio of SnO/O2 for providing the best conditions for SnO2 single crystal growth. X-ray powder diffraction (XRD) proved the single SnO2 phase. Moreover, by selecting a suitable SnO/O2 ratio it was possible to obtain either n-type conductivity with electron concentrations up to 2 × 1018 cm−3 and electron mobilities up to 200 cm2 V−1 s−1, or insulating behavior. The crystals exhibited an optical absorption edge located at 330–355 nm, depending on the crystal orientation, and a good transparency over visible and near infrared (NIR) spectra.

Improvement of dislocation density in thick CVD single crystal diamond films by coupling H2/O2 plasma etching and chemo‐mechanical or ICP treatment of HPHT substrates

Abstract: H2/O2 plasma treatments offer advantages over other etching processes of diamond as a technique to prepare the substrate's surface prior to homoepitaxial CVD diamond growth particularly in the case of thick films. It allows removing surface defects induced by polishing, thus leading to an improved morphology and limiting the stress within the grown crystal. Nevertheless, this treatment induces surface roughness leading to dislocation formation when CVD growth is initiated. In this paper we combined H2/O2 plasma etching with smoother surface treatment such as RIE-ICP etching or Chemo-Mechanical Polishing and we showed a significant reduction in dislocation density of the thick CVD epitaxial layers.

Electrical and optical properties of TiO2 thin films prepared by plasma-enhanced atomic layer deposition

Abstract: We report on the electrical and optical characterisation of the high-permittivity (high-κ) TiO2 thin films grown by plasma enhanced atomic layer deposition on Si (100) and glass substrates, respectively. TiO2 films were incorporated in metal-oxide semiconductor (MOS) capacitor structures with an Al metal gate electrode. The as-deposited films were amorphous; however upon annealing in the temperature range 500–900 °C, crystalline TiO2 in the anatase phase was formed. This was further confirmed by performing Raman measurements where the characteristic features corresponding to the anatase phase were observed. Transmittance and absorption spectra of the as-deposited and annealed films were performed by UV–Vis measurements showing more than 70% of transmittance. The formation of stoichiometric TiO2 was revealed by X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectroscopy (RBS) analysis performed on annealed samples (500–900 °C). The dielectric constants were calculated from capacitance–voltage (C–V) curves of the MOS structure on the as-deposited film and annealed films revealing a significant improvement of the dielectric constants from 10 to 75 at AC frequencies of 100 kHz for the 700 °C annealed TiO2 thin films. The increase in the dielectric constant for annealed films could be attributed to the transformation of film structure from amorphous to polycrystalline (anatase). However, the transformation of amorphous to crystalline phase, leads to an increase in the leakage current which was also found best fitted with Schottky emission mechanism at moderated electric fields.

Reactively magnetron sputtered Bi2O3 thin films: Analysis of structure, optoelectronic, interface, and photovoltaic properties

Abstract: Bi2O3 thin films deposited by RF magnetron sputtering have been studied in situ by using photoelectron spectroscopy. UV/VIS transmission spectroscopy and XRD measurements were carried out to determine the optical and structural properties of the films. Thin film solar cells were built up with ITO|Bi2O3|Au layer structures. These devices were characterized by current–voltage and capacitance–frequency measurements. Open-circuit voltages up to 680 mV and short-circuit current densities of about 0.3 mA cm−2 were observed. In addition, relative permittivities of approximately 45 have been measured. In order to determine the energy band alignment of Bi2O3 with different contact materials, interface experiments were carried out. With stepwise depositions of the contact material combined with in situ observation of the Fermi level shift via X-ray photoelectron spectroscopy, it is possible to measure the energy barrier height between a semiconductor and a metallic contact. The work functions of the different materials were determined by UV photoelectron spectroscopy.

Passive charge state control of nitrogen-vacancy centres in diamond using phosphorous and boron doping

Abstract: The control and stabilisation of the charge state of nitrogen-vacancy centres in diamond is an important issue for the achievement of reliable processing of spin-based quantum information. The effect of phosphorous and boron doping of diamond on the charge state of nitrogen-vacancy (NV) centres is shown here. Ensembles of NV centres are produced at a depth of 60 nm in ultrapure diamond by implantation of nitrogen ions. Overlapping with the NV ensembles, donor and acceptor doped regions of different doping levels are prepared by ion implantation of phosphorus and boron followed by annealing in vacuum at 1500 °C. We show how the charge state of NV centres is controlled by the presence of phosphorous or boron atoms in their neighbourhood. For the lowest doping level, spectral measurements on the ensemble of NV centres reveal a higher amount of NV0 in the case of boron and a higher amount of NV− in the case of phosphorus, as compared with undoped regions. This behaviour is strengthened when the doping level is increased. Interestingly, the charge state control of native silicon-vacancy centres is also evidenced. Finally, we discuss the role of the surface termination of diamond on the average charge state of the NV ensemble (still dominant even at a depth of 60 nm) and confirm that the surface 2D-hole-gas (H-termination) can be compensated by nitrogen itself.

Impact of annealing on spiro‐OMeTAD and corresponding solid‐state dye sensitized solar cells

Abstract: Annealing is a route that has proven to be successful at enhancing the hole mobility of some hole transporting materials. Under this consideration, we systematically investigated the effect of annealing treatment on the spiro-OMeTAD and its corresponding solar cells by annealing the dye-sensitized TiO2 electrodes at 80 and 140 °C. Upon annealing, the crystallization and oxidation of spiro-OMeTAD enhanced, thereby favoring higher hole transfer and transport, ultimately leading to higher short-circuit current (Jsc). However, the annealed devices showed lower open-circuit voltage, which was due to the downshifting of TiO2 Fermi level as revealed by the transient photovoltage measurement. Results obtained in this study indicate the bis(trifluoromethylsulfonyl)imide (Li-TFSI) is responsible for the spiro-OMeTAD oxidization and TiO2 Fermi level reduction during the annealing. Further to be annealed at 140 °C, the 4-tert-butyl pyridine (tBP) evaporated to further down-shift the TiO2 Fermi level and dramatically reduce the Jsc, hence, the corresponded solar cell revealed the lowest efficiency. Typically, the standard temperature for thermal stress testing of solar cells in commercial applications is 85 °C. Therefore, it is highly imperative to have a better understanding of this phenomenon in order to achieve long-term stability of solid-state dye sensitized solar cells.

Phase transitions in lipid vesicles detected by a complementary set of methods: heat‐transfer measurements, adiabatic scanning calorimetry, and dissipation‐mode quartz crystal microbalance

Abstract: We report on the use of the heat transfer method, a novel surface-sensitive technique based on heat transfer through solid–liquid interfaces, to detect phase transitions of model lipid membranes. We selected the lipid DPPC because of its rich phase behavior in an experimentally accessible temperature range. The vesicles were adsorbed on nanocrystalline diamond films, known as a versatile platform material for biosensing with outstanding heat-conduction properties. Complementary Peltier-element-based adiabatic scanning calorimetry (pASC) and quartz crystal microbalance with dissipation monitoring (QCM-D) measurements were carried out to monitor the phase transitions in multilamellar and small unilamellar vesicles, respectively. The heat-transfer measurements revealed reversible jumps upon heating and cooling in the thermal resistance in the vicinity of the expected transition temperature and they agree qualitatively with molecular simulations of the thermal conductivity across a lipid bilayer. The results show the capability of the heat transfer method to detect the main phase transition in DPPC, opening new perspectives for the study of more complex lipid systems and different solid platforms. This work confirms QCM-D as a useful tool for the assessment of the structural changes upon the phase conversion and shows the capability of pASC to provide high-resolution thermodynamic information on biophysical systems. Temperature profile of the heat transfer resistance Rth during the main phase transition of a DPPC supported vesicle layer adsorbed on a hydrogen-terminated nanocrystalline diamond substrate. The arrows indicate the sense of the run: heating (red solid line) and cooling (blue solid line).

Temperature dependence of silicon nitride deposited by remote plasma atomic layer deposition

Abstract: We investigated the characteristics of silicon nitride (SiNx) thin films deposited by remote plasma atomic layer deposition (RPALD) using trisilyamine (TSA) and ammonia (NH3) plasma at low temperatures. Although the process window of SiNx thin films is 150–350 °C, considering the refractive index (RI), SiNx thin films deposited at 250–350 °C were focused on for analyses. All of the SiNx films were nearly stoichiometric, regardless of the deposition temperature. As the deposition temperature increased, the RI increased, while the hydrogen content decreased. The defect density also changed at higher deposition temperatures; as the deposition temperature increased, all of the trap densities increased because of the low-hydrogen content in the SiNx thin films. The characteristics of the SiNx thin film deposited by RPALD could be controlled to adjust the defect density for charge trap flash memory applications by changing the deposition temperature.

Nanotube film metallicity and its effect on the performance of carbon nanotube–silicon solar cells

Abstract: Research into silicon solar cells that use a thin film of single walled carbon nanotubes as the front electrode is an important area of increasing research activity. This paper provides the first ever direct performance comparison between devices fabricated with either semiconducting, metallic or mixed nanotubes to probe the effect of the semiconducting/metallic nature, or 'metallicity', of the nanotube film on solar cell performance and properties. HiPCO nanotube material sorted using the gel chromatography technique is highly purified in either metallic or semiconducting nanotube species. The solar cells fabricated with the metallic nanotubes greatly outperform their semiconducting or mixed counterparts. The operating mechanisms underlying this observation and its implications in regards to current understanding are discussed in light of recent literature. Dramatic increases in performance as well as substantial changes in the effect of metallicity due to subsequent hole doping of the sorted nanotube films are also demonstrated.[GRAPHICS].Using highly pure semiconducting and metallic carbon nanotubes, film metallicity is shown to be a vital factor in the performance of carbon nanotube-silicon solar cells. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Stability of graphene–silicon heterostructure solar cells

Abstract: The stability of undoped graphene–silicon heterostructure solar cells was investigated. Single-layer graphene was grown by chemical vapor deposition on copper foil. Prior to the transfer of graphene to the silicon wafer, the flat Si(111) surface was passivated with hydrogen or methyl groups (CH3). The conversion efficiency, η, of the H terminated Si device was negligible small (0.1%), whereas that of the CH3 passivated Si was 2 and 4.2% at 100 mW (AM 1.5) and 20 mW of light intensity, respectively. After 28 days in ambient atmosphere η decreased only slightly to 1.5 and 3.7%. This small change of η is due to the high stability of the CH3 passivated graphene–Si(111) interface. The methylated Si surface shows a high degree of chemical stability especially during the graphene transfer process.

Top-down fabrication and characterization of axial and radial III-nitride nanowire LEDs

Abstract: Complex axial and radial type III-nitride InGaN/GaN nanowire LEDs are realized using a recently developed top–down fabrication approach which enables high quality GaN-based nanowires with independently controlled height, pitch, and diameter. In this paper, we report on the fabrication, structural characterization, and luminescence of these two different structures and discuss their relative merits, weaknesses, and prospects in the context of the field. Axial (left) and radial (right) nanowire LEDs fabricated by a two-step top–down method.

Influence of power factor enhancement on the thermoelectric figure of merit in Mg2Si0.4Sn0.6 based materials

Abstract: The influence of power factor enhancement on the superior thermoelectric (TE) performance of Mg2(Si,Sn) is studied based on a comparison with Mg2Si. Doped compounds with compositions of Mg2(Si0.4Sn0.6) are synthesized and the TE properties measured between room temperature and 773 K. Carrier concentration and hall mobilities at room temperature are measured on selected samples. Enhancement of the density-of-states effective mass and mobilities comparable to Mg2Si indicate band convergence in the solid solutions to be the cause of the power factor enhancement. Microstructural analysis shows the presence of secondary Si rich phases while the matrix corresponds to Mg2(Si0.3Sn0.7). A doped compound with this composition is synthesized and exhibits superior ZT values (compared to the Mg2(Si0.4Sn0.6) specimens) with a ZTmax ∼1.3 at 773 K.

Photoemission spectra and effective masses of n‐ and p‐type oxide semiconductors from first principles: ZnO, CdO, SnO2, MnO, and NiO

Abstract: While there is a persistent interest in oxides, e.g., for semiconductor technology or optoelectronics, it seems to be difficult to achieve n-type and p-type doping for one and the same material. At the same time, it is important to understand the electronic structure for both types of doping individually. In this work, we use modern electronic-structure calculations to compute the density of states as well as effective electron and hole masses for n-type (ZnO, CdO, SnO2) and p-type (MnO, NiO) oxide materials. We establish our ab initio electronic structures by comparison to photoemission experiments at various incident photon energies. Taking into account the photoionization cross-sections, we are able to analyze the contributions of different atomic states and to verify the results by comparison to measured data. Based on these electronic structures, we calculate free-electron and free-hole masses as well as their dependence on the concentration of free carriers in the system. For SnO2, we compare with experimental results from another article (see M. Feneberg et al., Phys. Status Solidi A, DOI 10.1002/pssa.201330147 (2013) ) in this special issue. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim