The resistivity and mobility of carbon-doped GaAs nanowires have been studied for different doping concentrations. Surface effects have been evaluated by comparing unpassivated with passivated nanowires. We directly see the influence of the surface: the pinning of the Fermi level and the consequent existence of a depletion region lead to an increase of the mobility up to 30 cm²/Vs for doping concentrations lower than 3 × 10¹⁸ cm^–3. Electron beam induced current measurements show that the minority carrier diffusion path can be as high as 190 nm for passivated nanowires.

(© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The resistivity and mobility of carbon-doped GaAs nanowires are studied for different doping concentrations. Surface effects have been evaluated by comparing unpassivated with passivated nanowires. At low doping concentrations the highest mobility is observed for unpassivated nanowires, while at higher doping concentrations the passivated nanowires offer the best characteristics. Electron beam induced current measurements show that the minority carrier diffusion path can be as high as 190 nm for passivated nanowires.

Stacks of aluminum oxide and silicon nitride are frequently used in silicon photovoltaics. In this Letter, we demonstrate that hydrogenated aluminum nitride can be an alternative to this dual-layer stack. Deposited on 1 Ω cm p-type FZ silicon, very low effective surface recombination velocities of 8 cm/s could be reached after firing at 820 °C. This excellent passivation is traced back to a high density of fixed charges at the interface of approximately –1 × 10¹² cm^–2 and a very low interface defect density below 5 × 10¹⁰ eV^–1 cm^–2. Furthermore, spectral ellipsometry measurements reveal that these aluminum nitride layers have ideal optical properties for use as anti-reflective coatings. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The passivation of the front and rear surface of a solar cell is a key feature to reach higher efficiencies. The authors show that sputtered AlN:H can be used as a passivation layer. Excellent passivation with lifetimes exceeding 1 ms is presented. This is traced back to a very low defect density and a high density of fixed charges at the interface to silicon. Furthermore, interesting optical properties are shown for these layers which make them an ideal material for a combined passivation layer and anti-reflective coating.

The role of alloy and phonon scattering is theoretically explored in 5 nm diameter SiGe nanowires at room temperature. Low-field mobility calculations are performed by utilizing sp³d⁵ds*-spin–orbit-coupled tight binding model for electronic structure and Boltzmann transport formalism. Three different transport orientations 〈100〉, 〈110〉 and 〈111〉 are considered. Alloy scattering is found to play an important role in these Si_1–xGe_x nanowires, leading to a characteristic ‘U’ shaped mobility curve as a function of alloy composition. It is concluded that to extract any advantage of higher Ge hole mobility by alloying, Ge% > 70% is needed. Furthermore, the 〈111〉 channel orientation exhibits the highest hole mobility while 〈100〉 has the lowest hole mobility for any given alloy composition. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

SiGe is an important material for high mobility p-type transistors. In this Letter, the role of alloy and phonon scattering on the total hole mobility in SiGe nanowires is studied. At low carrier density, alloy scattering is still found to be relevant leading to an ‘U’ shaped mobility curve as a function of Ge concentration.

It is shown, that hot electrons generated in a semiconductor can transfer their excess free energy into an embedded/adjacent plasmonic metallic structure (reservoir), before it is lost irreversibly to phonons in the semiconductor. Since the plasmon–phonon (and plasmon–photon) scattering in the metallic structure could be much slower than the electron–phonon scattering in the semiconductor, free energy of the hot electrons can be this way effectively protected from phonon emission for a significant amount of time. While the cubic point-dipole crystal is proposed and studied here specifically as the plasmonic reservoir, other plasmonic structures including planar can be employed. It is also shown how the plasmon-protected energy can by recycled in a novel, 3rd generation solar cell, be employing a planar plasmonic structure that is simultaneously also an electron collector of the cell. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In solar cells, most of the absorbed photon energy is wasted to generate heat. In this Letter, it is shown how to protect this energy by storing it in plasmonic form in a nanostructure, which is simultaneously a plasmonic reservoir and an electron-collecting electrode. The stored energy increases the chemical potential of the reservoir, and thus the voltage of the cell. Such hot electron plasmon protected (HELPP) solar cell offers a dramatically improved efficiency.

Lead carbonate chloride, Pb₂CO₃Cl₂, known as mineral phosgenite, is introduced as a novel SRS-active carbonate crystal with tetragonal symmetry. Under picosecond one-micron laser pumping Raman-induced χ⁽³⁾-nonlinear generation in the near-IR is observed. All recorded high-order Stokes and anti-Stokes sidebands are identified and attributed to two SRS-promoting vibration modes with ω_SRS1 ≈ 1062 cm^–1 and ω_SRS2 ≈ 86 cm^–1.

Example of a natural crystal of phosgenite, Pb₂CO₃Cl₂, from the locality of Monte Poni, Sardinia. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Laser frequency conversion via stimulated Raman scattering (SRS) in carbonate crystals is usually dominated by the totally symmetric A_1g vibration mode of the [CO₃] groups. In contrast to other known SRS-active carbonates (such as calcite or aragonite), phosgenite is a representative of carbonates with non-coplanar [CO₃] groups in the crystal structure. The possibility of efficient SRS without contribution of the A_1g mode in a carbonate crystal is observed for the first time in phosgenite.

Steady-state and time-resolved photoluminescence of silicon nanoparticles dispersed in low-polar liquids at above room temperature is studied. The roles of low-polar liquids as well as mechanisms responsible for their temperature-dependent photoluminescence are discussed. The thermal sensitivity of the photoluminescence is estimated and application of the nanoparticles as nanothermometers is proposed.

(© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this Letter, luminescent silicon nanoparticles are investigated for the first time in view of their new application as nanothermometers for a contactless temperature determination. Thermal sensitivity of different photoluminescence parameters at above room temperature is estimated. Photoluminescence lifetime is found to be the most appropriate parameter for luminescent nanothermometry. The thermal sensitivity of lifetime for silicon nanoparticles is comparable or higher than for different II-VI semiconductor quantum dots.

Si nanodots of high density and hexagonal short-range order are observed upon normal-incidence bombardment of hot, crystalline Si with Bi₃⁺ ions having a kinetic energy of a few tens of keV. The heights of nanodots are comparable to their widths of ∼20 nm. The implanted Bi accumulates in tiny Bi nanocrystals in a thin Si top layer which is amorphous due to implantation damage. Light and heavy ions up to Xe cause smoothing of surfaces, but Bi₃⁺ ions considered here have a much higher mass. Atomistic simulations prove that each Bi₃⁺impact deposits an extremely high energy density resulting in a several nanometer large melt pool, which resolidifies within a few hundreds of picoseconds. Experiments confirm that dot patterns form only if the deposited energy density exceeds the threshold for melting. Comparing monatomic and polyatomic Bi ion irradiation, Bi–Si phase separation and preferential ion erosion are ruled out as driving forces of pattern formation. A model based on capillary forces in the melt pool explains the pattern formation consistently.

High-density Si nanodots are formed by polyatomic Bi ion irradiation of hot Si surfaces. Each impact causes local transient melt pools smaller than the dots. Hexagonally ordered patterns evolve by self-organization driven by repeated ion-induced melting of tiny volumes. Homogeneously distributed Bi nanocrystals are found in the a-Si film. These nanocrystals are related to particularities of the Si–Bi phase diagram. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High-density Si nanodots are formed by polyatomic Bi ion irradiation of hot Si surfaces. Each impact causes local transient melt pools smaller than the dots. Hexagonally ordered patterns evolve by self-organization driven by repeated ion-induced melting of tiny volumes. Homogeneously distributed Bi nanocrystals are found in the a-Si film. These nanocrystals are related to particularities of the Si-Bi phase diagram.

We present an L-shaped nanoprobe for scanning electrochemical microscopy–atomic force microscopy (SECM–AFM) capable of imaging the surface topography and the electrochemical activity of nanostructures of interest. Owing to the geometry of the protrusive peak in the L-shaped probe, the distance between the probe electrode and the substrate is maintained precisely at ∼100 nm during surface scanning. The reduction in electrode-to-substrate distance significantly improves the positive feedback current on top of the electrochemically active nanomaterials. The L-shaped nanoprobe successfully acquired simultaneous a topographical image and an electrochemical current image of individual carbon nanotubes (CNTs) in a two-dimensional (2D) CNT network.

Schematic diagram of an L-shaped nanoprobe for SECM–AFM: the nanoprobe has a triangular frame nanoelectrode and a protrusive insulating peak at the end of the AFM tip.

(© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A new type of SECM-AFM probe with an L-shaped tip and nanoelectrode is reported. The L-shaped tip geometry enables the probe to maintain a short electrode-to-substrate distance and to collect dual information of topography and electrochemical reactivity. The L-shaped probe successfully visualizes the topographical and electrochemical characteristics of a carbon nanotube network.

The characteristics of Al₂O₃ film grown by atomic-layer deposition as blocking layer with and without fluorine plasma treatment were investigated based on a capacitor structure of Al/Al₂O₃/TaON/SiO₂/Si. The physical structure was studied by transmission electron microscopy, and the chemical composition of the blocking layer was analyzed by X-ray photoelectron spectroscopy and secondary ion mass spectroscopy. Moreover, the surface roughness of the blocking layer was investigated by atomic force microscopy. Compared with a capacitor with Al₂O₃ blocking layer, the one with fluorinated Al₂O₃ displayed higher programming/erasing speeds, better endurance property and better charge retention characteristic because the fluorination could reduce excess oxygen and traps in the blocking layer, thus forming a larger barrier height at the interface between the charge-trapping layer and the blocking layer. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this Letter, the authors describe the use of a fluorinated Al₂O₃ film as blocking layer for flash memory devices. The fluorine plasma treatment can assist the excess oxygen in the Al₂O₃ film to diffuse out during its annealing and suppress the traps in the bulk, thus forming a larger barrier height at the interface between the charge-trapping layer and the blocking layer, and leading to a much better memory performance.

Ordered arrays of vertically aligned self-catalyzed GaAs nanowires have been grown by gas source molecular beam epitaxy (GS-MBE) on silicon substrates using nano-patterned oxide templates. Several growth processes of different duration were performed under identical conditions and with identical sample preparation. To determine the influence of pattern parameters, the samples were prepared with 20 patterned areas, each with progressively increasing hole diameters and pitch. Measurements of the average lengths and diameters of the vertically oriented nanowire areas were then used to calculate the overall axial and radial growth rates. These experiments confirm that significant accompanying radial growth occurs. Furthermore, the rate of radial growth increases with increasing pattern pitch. We propose that gallium-rich conditions may increase the size of the liquid droplet, resulting in an inverse tapered morphology which promotes step-flow radial growth via secondary adsorption on the nanowire sidewalls. The pitch dependence of the radial growth rate may therefore be due to shadowing or competition for the flux of material desorbing from the oxide surface between the nanowires. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Periodic arrays of self-catalyzed GaAs nanowires are grown by gas-source MBE on nano-patterned oxidized Si substrates. The average nanowire length and diameter increase significantly with both time and increasing inter-wire spacing as revealed by growth of different duration under identical conditions. A mechanism for the radial and axial growth is proposed in this Letter, which accounts for the observed dependence on the pattern parameters.

Partially filled polycrystalline p-type skutterudites of nominal compositions Yb_x Co₃FeSb₁₂ were synthesized and their thermoelectric properties characterized. The compositions and filling fractions were confirmed with a combination of Rietveld refinement and elemental analysis. The thermoelectric properties were evaluated from 300 K to 810 K. The Seebeck coefficient and resistivity increase while the thermal conductivity decreases with increasing Yb content. A maximum ZT value of 0.85 was obtained at 810 K. This work is part of a continuing effort to enhance the thermoelectric properties of p-type skutterudites, as this class of materials continues to be of interest for thermoelectrics applications. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Yb-filled Fe-substituted p-type CoSb₃ skutterudites, Yb_xCo₃FeSb₁₂, are structurally and physically characterized. Both filling fraction and Fe-to-Co ratio affect the transport properties. High ZT is obtained by a combination of ∼ 20% Yb filling and a slightly higher Fe content.

Temperature-dependent ferroelectric dynamic hysteresis properties of modified Pb(Mg_1/3Nb_2/3)O₃–Pb(Zr,Ti)O₃ (PMN–PZT) ceramics have been investigated in a wide range from 298 to 433 K. Interestingly, it was found that back-switching polarization P_bc increased while the hysteresis area 〈A 〉, coercive field E_c, saturation polarization P_s, and remnant polarization P_r all decreased linearly with temperature below 398 K (close to the macro–micro domain transition temperature T_nr). A set of simple linear temperature scaling relations was established, which is different from the power-law scaling relations for soft and hard PZT ceramics. At further increased temperature, double-loop features appeared and the ferroelectric properties varied nonlinearly due to the reversible macro–micro domain transition in the T > T_nr region. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The reverse current of solar cells with acceptor C₆₀ shows a linear current–voltage relation under illumination. This current is due to photoconductivity in the pure layers, generating a photoshunt and internal quantum efficiencies larger than unity for negative bias. The photoshunt represents a loss path reducing the fill factor.

Heavily n-type doped and several nanometres thick In_0.485Ga_0.515P layers are necessary for various devices. We studied the delta-doping of this ternary with tellurium; the layers were grown by metalorganic vapour phase epitaxy (MOVPE), and diethyltelluride was used as the precursor. A maximum Hall sheet concentration of 2.75 × 10¹³ cm^–2 was achieved in our samples grown at 560 °C. The Te profiles were analyzed with secondary ion mass spectrometry (SIMS), and a very narrow spectrum with a full width at half maximum of 7.5 nm was measured. This value indicates that the memory effect, referred to in the literature, was practically eliminated with appropriate growth conditions. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This Letter reports on tellurium delta-doped InGaP layers prepared by metalorganic vapour phase epitaxy (MOVPE) and demonstrates the elimination of the memory effect, which is a serious problem in Te-doped InGaP. The authors suppose that the elimination is a consequence of the surface morphology formed on exactly (001)-oriented substrate at low growth temperature and at a higher V/III ratio. An extremely narrow dopant distribution was measured with secondary ion mass spectrometry (SIMS) in the prepared samples.

We observe that the reverse current under illumination in solar cells containing C₆₀ and ZnPc is dominated by a photoshunt. This shunt, not present in the dark, causes a linear current–voltage relation under illumination showing no saturation. Although observable in bulk heterojunctions, this effect is more pronounced in the presence of a pristine C₆₀ layer. An internal quantum efficiency larger than unity under an applied negative voltage and in the spectral range where C₆₀ absorbs identifies charges which are injected in addition to those photogenerated. The photoshunt is also present in the power-generating region and represents a loss mechanism limiting the fill factor in particular for flat heterojunction devices. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The reverse current of solar cells with acceptor C₆₀ shows a linear current–voltage relation under illumination. This current is due to photoconductivity in the pure layers, generating a photoshunt and internal quantum efficiencies larger than unity for negative bias. The photoshunt represents a loss path reducing the fill factor.

The possibility of multiferroicity arising from charge ordering in LuFe₂O₄ and structurally related rare earth ferrites is reviewed. Recent experimental work on macroscopic indications of ferroelectricity and microscopic determination of coupled spin and charge order indicates that this scenario does not hold. Understanding the origin of the experimentally observed charge and spin order will require further theoretical work. Other aspects of recent research in these materials, such as geometrical frustration effects, possible electric-field-induced transitions, or orbital order are also briefly treated.

Bilayer subunit in R Fe₂O₄ compounds with proposed charge order rendering it polar.

(© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetoelectric multiferroics have a large application potential, for example in information technology, with the main challenge being the identification of suitable materials and mechanisms of ferroelectricity. “Ferroelectricity originating from charge ordering” is a particularly intriguing mechanism for obtaining multiferroics, but experimentally indicated examples are exceedingly rare. In this article, recent research on rare earth ferrites, the prototypical “proof of principle” example materials, is reviewed.

MgZnO-based ultraviolet avalanche photodetectors (APDs) have been fabricated from Au/MgO/Mg_0.44Zn_0.56O/MgO/Au Schottky structures. The carrier avalanche multiplication is realized via an impact ionization process occurring in the MgO layer under relatively large electric field. The APDs exhibit an avalanche gain of 587 at 31 V bias, and the response speed of the APDs is in the order of microseconds.

(© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In virtue of carrier-multiplication occurring via an impact ionization process in Schottky structures, MgZnO-based avalanche photodetectors have been obtained. The results reported in this Letter may promise high-performance MgZnO UV avalanche photodetectors by bypassing the challenging issue of p-type doping of wide bandgap semiconductors.

Using first-principles calculations, we investigate the fully oxidized silicene and germanene with stoichiometric ratio Si:O/Ge:O = 1:1. For both compounds, the zigzag ether-like conformation (z-sSiO/z-sGeO) is found to be the most energetically favorable structure. These z-sSiO and z-sGeO nanosheets have prominent elastic characteristics, which even exhibit an unconventional auxetic behavior with negative Poisson ratios. After oxidation, the semi-metallic nanosheets are transformed into semiconductors with narrow direct band gaps. Due to the anisotropic mechanical and electronic properties, the z-sSiO and z-sGeO possess an axially high intrinsic charge mobility up to the order of 10⁴ cm²/Vs, which is comparable to that of graphene nanoribbons. Our studies demonstrate that the silicene and germanene oxides have peculiar mechanical and electronic properties, which endow these nanostructures for potential applications in nanoelectronics and devices. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this Letter, the authors report the intriguing mechanical and electronic behaviors in stoichiometric silicene and germanene oxides. The zigzag ether-like conformations are found to be dynamically stable and have an unusual axially auxetic feature with negative Poisson ratios. Oxidation transforms the semi-metallic sheets into direct-band-gap semiconductors, which possess high intrinsic charge mobilities even comparable to graphene.

This article reviews recent developments in nanowire-based photovoltaics (PV) with an emphasis on III–V semiconductors including growth mechanisms, device fabrication and performance results. We first review the available nanowire growth methods followed by control of nanowire growth direction and crystal structure. Important device issues are reviewed, including optical absorption, carrier collection, strain accommodation, design for high efficiency, tunnel junctions, Ohmic contact formation, passivation and doping. Performance data of III–V nanowire cells and the primary challenges in nanowire PV are summarized. Many of the issues discussed here are also applicable to other nanowire devices such as photodetectors. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

To overcome the limitations of current III–V multi-junction solar photovoltaic (PV) cells, III–V nanowire-based PV is being aggressively pursued by numerous groups. This article provides a comprehensive review of nanowire-based PV covering all aspects of design for high power conversion efficiency including nanowire growth methods, growth direction, crystal structure, optical absorption, carrier collection, strain accommodation, tunnel junctions, Ohmic contact formation, passivation and doping.

The properties of the core–shell p–n junction in GaAs nanowires have been investigated via the self-consistent solution of the Poisson–Schrödinger equations in cylindrical coordinates and the effective mass approximation for doping levels between 10¹⁷–10¹⁹ cm^–3 and radii ≤100 nm at 300 K. Only electrons or holes are confined in the n- or p-type core for equal core–shell thicknesses and doping levels but the shell is depleted even when a flat-band condition exists at the surface for equal core–shell volumes. In contrast, a δ-doped n–i–n–i–p–i junction has a balanced charge distribution and flat-band potential in the core and shell which is critical for the realization of high performance nanowire devices. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The aim of this work is to describe the properties of the core–shell p–n junction in semiconductor nanowires and explain the difficulty in obtaining a flat band condition at the surface and simultaneous confinement of electrons and holes in the n- and p-type segments, which is necessary for optimum device performance and may be overcome by δ-doping which allows one to tailor the charge distribution and potential profile.

We have demonstrated an effective method of enhancing the power efficiency of double–emissive solution-processed blue phosphorescent organic light-emitting diode (PHOLED) by controlling the charge transport in the heterojunction and emissive layer. The first emissive layer consists of poly(vinylcarbazole) (PVK) and bis(4,6 difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic) mixed with 4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA) or 1,3-bis[(4-tert- butylphenyl)-1,3,4 oxidiazolyl] phenylene (OXD-7). The second layer consists of an alcohol-soluble 2,7-bis(diphenylphosphoryl)-9,9′-spirobi[fluorene] (SPPO13) and FIrpic blend. The incorporation of OXD-7 into PVK blurs the interface between the emissive layers and widens the recombination zone while blending TCTA into PVK reduces the hole- injection barrier from PEDOT:PSS to PVK. By adding TCTA or OXD-7 into the first emissive layer, we have achieved a power efficiency of 10 lm/W and 11 lm/W, respectively, at 1000 cd/m².

(© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this Letter, the authors enhance the power efficiency of a double emissive solution-processed blue PHOLED by blending commercially available OLED material, i.e. TCTA or OXD-7, into the first emissive layer. The addition of TCTA reduces the hole injection barrier at the anode whereas the incorporation of OXD-7 widens the recombination zone.

Potential-induced degradation (PID) of photovoltaic solar modules gained a lot of interest during the last three years. Many PID-affected modules based on crystalline silicon show intense shunting of the solar cells. In the past, character and origin of these PID-shunts have not been clear. In the contribution by Naumann et al. (see pp. 315–318) monocrystalline and multicrystalline silicon solar cells have been prepared with PID of the shunting type. In plan view, SEM using electron beam induced current and secondary ion mass spectrometry measurements reveal local PID-shunts correlated with sodium accumulations. Detailed TEM investigations at cross sections of PID-shunts reveal stacking faults in silicon decorated with sodium. It is concluded that the sodium decorated stacking faults provide paths with high conductivity across the p–n junction, thus leading to the observed shunting.

The realization of memory devices on various unconventional substrates has been made possible by use of the transfer printing technique. The memory devices have been fabricated onto a range of unconventional substrates including paper, an insect (cicada), glass, polyethersulphone, polyimide, double-sided tape, Al foil, fabric, a mask, a leather wallet, a name card, a banknote, a latex glove, and a squeezed plastic bottle. In their Letter on pp. 326–331 Choi et al. show that the constraints imposed by process compatibility between the substrates and the device materials are completely eliminated by the use of the transfer printing technique. It is confirmed that the electrical characteristics of the RRAM devices do not degrade during the transfer process. Stable resistive switching properties, reliable endurance levels, and good retention characteristics are demonstrated. The mechanical stability is also analysed and an encapsulation protection layer on top of the memory devices is suggested for long-term reliability. The possibility of the realization of integrated electronic systems onto various substrates will enable the versatile use of these electronics anywhere and anytime in many different environments.

Mono- and multicrystalline solar cells have been stressed by potential-induced degradation (PID). Cell pieces with PID-shunts are imaged by SEM using the EBIC technique in plan view as well as after FIB cross-section preparation. A linear shaped signature is found in plan-view EBIC images at every potential-induced shunt position on both mono- and multicrystalline solar cells. Cross-sectional SEM and TEM images reveal stacking faults in a {111} plane. Combined TEM/EDX measurements show that the stacking faults are strongly decorated with sodium. Thus, the electric conductivity of stacking faults is assumed to arise under the influence of sodium ion movement through a high electric field across the SiN_x anti-reflective layer, resulting in PID. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this Letter, shunts caused by potential-induced degradation (PID) of crystalline solar cells are investigated in detail. Electron beam induced current measurements at low acceleration voltage reveal that PID shunts are always correlated with stacking faults in a {111} plane. Secondary ion mass spectroscopy mapping and cross-sectional high-resolution transmission electron microscopy (TEM) with EDX analyses prove that these shunting stacking faults are decorated with sodium. Accordingly, a model for PID-shunting is introduced.

Emitter formation for industrial crystalline silicon (c-Si) solar cells is demonstrated by the deposition of phosphorous-doped silicate glasses (PSG) on p-type monocrystalline silicon wafers via in-line atmospheric pressure chemical vapor deposition (APCVD) and subsequent thermal diffusion. Processed wafers with and without the PSG layers have been analysed by SIMS measurements to investigate the depth profiles of the resultant phosphorous emitters. Subsequently, complete solar cells were fabricated using the phosphorous emitters formed by doped silicate glasses to determine the impact of this high-throughput doping method on cell performance. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Using phosphosilicate glass thin films deposited by APCVD, the authors report on improved control of phosphorus surface concentration for c-Si solar cells formed by in-line diffusion. They demonstrate doping from APCVD films in a high-throughput, dynamic deposition system, offering an alternative to in-line emitter formation via H₃PO₄ doping, a technology that suffers from high phosphorus surface concentration.

For the first time we present a free energy loss analysis (FELA) of heterojunction silicon solar cells (HSSC) to study the influence of the intrinsic buffer layer thickness (t_buffer) on the solar cell efficiency (η). The main advantage of the FELA is that the impact of various loss mechanisms can be directly expressed in absolute percentage of η. Furthermore, it is possible to extract the magnitude of every loss for each region of the solar cell. All quantities required to perform the FELA are obtained by the simulation software AFORS-HET. The FELA yields an optimum efficiency of 21.24% for t_buffer ≈ 5 nm. The efficiency drop for t_buffer £ 5 nm is ascribed to a lower maximum usable generated power Φ_G(22.84% @ 2 nm, 23.98% @ 5 nm). Lower efficiencies for t_buffer ³ 5 nm are attributed to the increased transport loss of holes in the intrinsic buffer layer (0.05% @ 2 nm, 0.65% 8 nm). The η values yielded by the FELA are in agreement with the ones calculated by AFORS-HET, demonstrating the applicability of the FELA to the HSSC concept. Therewith, we demonstrate that the FELA can be employed to obtain a deeper understanding of the HSSC concept. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A free energy loss analysis (FELA) of heterojunction silicon solar cells is presented in order to obtain a deeper understanding of this solar cell concept. In particular, this Letter focuses on the influence of the intrinsic buffer layer thickness on the solar cell efficiency.

Electronic devices on unconventional substrates are attractive beyond the conventional solid-state rigid electronic system or even flexible electronics. Here, by use of the transfer printing technique, we report the memory devices onto a range of unconventional substrates, including paper, an insect (cicada), glass, polyethersulphone, polyimide, double-sided tape, Al foil, fabric, a mask, a leather wallet, a name card, a banknote, a latex glove, and a squeezed plastic bottle. The device can be possibly realized on any substrate. The memory structure used in this work is non-volatile resistive random access memory (RRAM). Constraints imposed by process compatibility between the substrates and device materials are completely eliminated by the use of the transfer printing techniques. It is confirmed that the electrical characteristics of the RRAM devices do not degrade during the transfer process. Stable resistive switching properties, reliable endurance levels, and good retention characteristics are demonstrated. The mechanical stability is also analysed and an encapsulation protection layer on top of the memory devices is suggested for long-term reliability. The possibility of the realization of integrated electronic systems onto various substrates will enable the versatile use of these electronics anywhere and anytime in many different environments.

(© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This Letter reports the realization of memory devices on various nonconventional substrates by use of the transfer printing technique. The electrical characteristics of the RRAM devices do not degrade during the transfer process. The possibility of the realization of integrated electronic systems onto various substrates will enable the versatile use of these electronics anywhere and anytime in many different environments.

Magnetization switching by a spin-polarized current in perpendicular anisotropy devices with magnetic nanocontact (NC) is investigated using a micromagnetic formalism. The critical switching current (i_cr) and switching time (τ₀) can be reduced when a soft layer is exchange coupled to the NC. The study reveals that devices with fewer NCs have smaller i_cr compared to those with a large number. Furthermore, τ₀ for nanoconstricted devices is almost constant with anisotropy field (H_k), in contrast to devices without NCs that show an exponential increase with H_k. This suggests that nanoconstricted devices could be used to improve thermal stability, while reducing i_cr and τ₀. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A composite free layer with continuous and nano-constricted parts is proposed to reduce both switching current and time of magnetization. This is the result of an increase of spin torque efficiency. Furthermore, the switching time becomes less dependent on anisotropy field of the free layer. For spin torque magnetic memory application, nano-constricted devices can be scalable with high frequency response and better thermal stability.

We have fabricated multi-peak and chromaticity-stable top-emitting white organic light-emitting diodes (TEWOLEDs) using single blue emitter. Besides the intrinsic emission of blue emitter, the additional emission can be well realized by simply adjusting the thickness of hole transporting layer (HTL), thus modifying the optical cavity length to obtain different resonant wavelengths. The detailed variation process for multi-peak spectra with the increase of HTL thickness is studied, which provides a guidance for the design of microcavity TEWOLEDs.

(© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Multi-peak and chromaticity-stable top-emitting white organic light-emitting diodes (TEWOLEDs) using single blue emitter are demonstrated. The detailed variation process for multi-peak spectra with the increase of the hole transporting layer thickness is studied, which provides guidance for the design of microcavity TEWOLEDs.

We demonstrate the fabrication of a solid state heterojunction photovoltaic device with solution-processed graphene oxide (GO) and n-Si. Partially reduced GO with a high optical gap (2.8 eV) was spin-coated on the n-Si substrate and a heterojunction device was fabricated with the structure of Au/pr-GO/n-Si. In the fabricated device, incident light was transmitted through the thin GO film to reach the junction interface, generating photoexciton, and thereby a photovoltaic action was observed. By means of a built-in electric potential at the GO/n-Si junction, photoexcited electrons and holes can be separated, transported and collected at the electrodes.

Solid state heterojunction device fabricated with solution-processed graphene oxide (high optical gap ∼2.8 eV) and n-Si.

(© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Kalita et al. demonstrate the fabrication of a solid state heterojunction device with solution-processed graphene oxide (GO, high optical band gap ∼2.8 eV) and n-type silicon. In the fabricated device, a built-in electric potential was created at the junction, by which photo-excited electrons and holes were transported and collected to obtain photovoltaic action. The simple fabrication technique of the GO/Si heterojunction can be exploited in various applications replacing high-cost fabrication techniques.

We observed crossed transitions and anti-Stokes emissions in single quantum-dot-like objects embedded in the active layer of InGaN/GaN quantum disks by two-photon absorption techniques. We proposed a phenomenological model based on the interplay between Auger effect and crossed transitions to explain the origin of anti-Stokes emissions and the preferential excitation of 0D objects at the expense of their surroundings. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The experimental results of the authors reveal that crossed-transitions are the key point to understand phenomena related to two-photon absorption in quantum dot systems. The combined effects of Auger recombination and crossed transitions explain the preferential excitation of 0D objects at the expense of their surroundings and the origin of anti-Stokes recombination that was observed under two-photon absorption spectroscopy.

Microwave noise technique is applied to study in-plane electronic properties of epitaxial graphene grown on sapphire by chemical vapor deposition and subjected to high electric field applied in the plane. The noise spectrum is measured in the field direction at room temperature. While a 1/f^1.25-type dependence is observed in the 200 MHz–2.5 GHz band, a shot noise contribution is resolved at 10 GHz. The shot noise is possibly associated with hole jumps across the potential barriers located in the graphene layer. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The authors report on noise spectrum and dc current measured parallel to the electric field applied in the epitaxial graphene plane grown on sapphire. A 1/f^1.25 type noise is found in the 200 MHz-2.5 GHz frequency range. The shot noise contribution is resolved at 10 GHz. The low spectral density of the shot noise is possibly caused by strongly anticorrelated hole jumps between potential barriers in the graphene layer.

By a combination of chemical etching, atomic layer deposition (ALD) and galvanic deposition it became possible to produce Ni nanowires (NWs) with ultra-high aspect ratio (∼1000), embedded in a semi-insulating, piezoelectric, single-crystalline InP matrix. This Letter discusses the structural and magnetic properties of these Ni NWs. They are crystalline with a preferential growth direction of the grains in 〈111〉. For H ⊥ z, they show a very narrow hysteresis loop with a low coercivity (about 100 Oe) and a very low remanence squareness S of 0.08. So the combination of piezoelectric InP and magnetostrictive Ni forms a very promising multiferroic composite. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

By combination of chemical etching, atomic layer deposition (ALD) and galvanic deposition it becomes possible to produce Ni nanowires (NWs) with an ultra-high aspect ratio (∼1000), embedded in a semi-insulating, piezoelectric, single-crystalline InP matrix. The Letter discusses the structural and magnetic properties of these Ni NWs. They are crystalline with a preferential growth direction of the grains in 〈111〉. For H ⊥ z, they show a very narrow hysteresis loop with a low coercivity (about 100 Oe) and a very low remanence squareness S of 0.08. So the combination of piezoelectric InP and magnetostrictive Ni forms a very promising multiferroic composite.

We have studied the stable end phase formed in amorphous germanium (a-Ge) films that have been subjected to a pressure-induced phase transformation under indentation loading using a large (20 µm) spherical indenter. After indentation the samples have been annealed at room temperature to remove any residual unstable R8 and BC8 phases. Raman spectroscopy indicates a single broad peak centred around 292 cm^–1 and we have used first principles density functional perturbation theory calculations and simulated Raman spectra for nano-crystalline diamond cubic germanium (DC-Ge) to help identification of the final phase as hexagonal diamond germanium (HEX-Ge). Transmission electron microscopy and selected area diffraction analysis confirmed the presence of a dominant HEX-Ge end phase. These results help explain significant inconsistencies in the literature relating to indentation-induced phase transitions in DC- and a-Ge. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

By applying pressure to amorphous germanium with a spherical diamond tip, the authors show that transformations to dense germanium phases can be readily induced. Upon unloading, these high-pressure phases further transform to a stable hexagonal diamond phase. This is the first clear observation of such a germanium phase following indentation and helps resolve prior inconsistencies in the literature.

We report on the sputter deposition of copper oxide sulfides Cu₂O_1–xS_x up to a composition of Cu₂O_0.61S_0.39. For higher sulfur contents the films first become amorphous and then change to Cu₂S. Within the range 0 < x < 0.39 the cubic crystal structure is maintained and the lattice constant changes linearly with the sulfur content. The composition of the films is measured by energy dispersive X-ray fluorescence and suggests the formation of a random alloy. The transmission spectra show a clear red shift of the order of 450 meV (from Cu₂O to Cu₂O_0.61S_0.39) with an average transmission of 70%. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this Letter, the alloying effects of Cu₂O by sulphur were investigated. The random alloy formation is demonstrated by EDX and XRD measurements and showed that up to a composition of x > 0.39 of Cu₂O_1–xS_x the cubic crystal structure is stable. With increasing sulphur content the absorption edge shifts to the infrared, thus possibly improving the efficiency maximum of photovoltaic cells based on Cu₂O.

ZnO:Al thin films with a low electrical resistivity were grown by magnetron sputtering on sapphire substrates. The cross-plane thermal conductivity (κ = 4.5 ± 1.3 W/mK) at room temperature is almost one order of magnitude lower than for bulk materials. The thermoelectric figure of merit ZT at elevated temperatures was estimated from in-plane power factor and the cross-plane thermal conductivity at room temperature. It is expected that the thermal conductivity drops with increasing temperature and is lower in-plane than cross-plane. Consequently, the thin film ZT is at least three times higher than for bulk samples at intermediate temperatures. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The thermal conductivity of ZnO:Al thin films at room temperature (4.5 ± 1.3 W/mK) is almost one order of magnitude lower compared to the value for bulk samples. This leads to an improved thermoelectric figure of merit ZT of at least 0.04 estimated at 640 K, which is three times higher than for bulk samples.

A resonance splitting effect is investigated in a system composed of two cavities coupled by two unidirectional waveguides. Both theoretical analysis and numerical calculations demonstrate that the resonance splitting (indicating a coupling between the cavities) is independent of the phase shift between the cavities, which is in contrast to previous research where reciprocal waveguides are used. Moreover, this splitting can be tunable by an external magnetic field. Our findings offer a possibility to realize effective coupling between remote on-chip resonators, which is highly demanded in the next-generation photonic circuits.

(© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Remote coupling is highly demanded in the next-generation photonic circuits. However, multi-reflections between the cavities are unavoidable when the cavities are connected by reciprocal waveguides, which bring complex effects to the coupled-cavity system. In this Letter, the reciprocal waveguides are replaced by nonreciprocal waveguides. In contrast to previous research, the strength of coupling becomes independent of the phase shift introduced by waveguides and it changes little with increasing spacing. Thus, it is possible to achieve an effective coupling among distant cavities via unidirectional waveguides.