Showing papers in "Physica Status Solidi-rapid Research Letters in 2013"

Floquet topological insulators

Abstract: Topological insulators represent unique phases of matter with insulating bulk and conducting edge or surface states, immune to small perturbations such as backscattering due to disorder. This stems from their peculiar band structure, which provides topological protections. While conventional tools (pressure, doping etc.) to modify the band structure are available, time periodic perturbations can provide tunability by adding time as an extra dimension enhanced to the problem. In this short review, we outline the recent research on topological insulators in non-equilibrium situations. Firstly, we introduce briefly the Floquet formalism that allows to describe steady states of the electronic system with an effective time-independent Hamiltonian. Secondly, we summarize recent theoretical work on how light irradiation drives semi-metallic graphene or a trivial semiconducting system into a topological phase. Finally, we show how photons can be used to probe topological edge or surface states. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

III–V nanowire photovoltaics: Review of design for high efficiency

Abstract: 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)

Review of 3D topological insulator thin‐film growth by molecular beam epitaxy and potential applications

Abstract: Thin films of V–VI compound semiconductors (Bi2Se3, Bi2Te3 and Sb2Te3) have been synthesized recently as three-dimensional topological insulators (TIs). Although these materials have been used as thermoelectric materials for many years, for future studies and applications of the topological surface states, a major bottleneck remains the lack of high-quality bulk materials that have very few defects and the Fermi level can be moved to inside the bulk bandgap. In this paper, we review the use of molecular beam epitaxy (MBE) technique to achieve high-quality TI materials. Furthermore, the use of layered growth in MBE affords us the fabrication of heterostructures, such as quantum wells and superlattices. Thus, it may further enable additional studies and applications, similar to those of conventional semiconductor heterostructures but with the novel properties of TI. We explore the growth mechanism, providing a detail discussion on the growth parameters of thin-film synthesis by MBE. Then we discuss more complex cases, such as functional doping, heterostructures and superlattices. Potential new properties in such quantum structures are discussed. Finally, we give an outlook on this material system for both fundamental studies and applications. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Screening and interlayer coupling in multilayer MoS2

Abstract: The two-dimensional layered semiconducting di-chalcogenides are emerging as promising candidates for post-Si-CMOS applications owing to their excellent electrostatic integrity and the presence of a finite energy bandgap, unlike graphene. However, in order to unravel the ultimate potential of these materials, one needs to investigate different aspects of carrier transport. In this Letter, we present the first comprehensive experimental study on the dependence of carrier mobility on the layer thickness of back-gated multilayer MoS2 field-effect transistors. We observe a non-monotonic trend in the extracted effective field-effect mobility with layer thickness which is of relevance for the design of high-performance devices. We also discuss a detailed theoretical model based on Thomas–Fermi charge screening and interlayer coupling in order to explain our experimental observations. Our model is generic and, therefore, is believed to be applicable to any two-dimensional layered system. A model explaining the experimental findings related to screening and interlayer coupling in multilayer MoS2. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Raman scattering and disorder effect in Cu2ZnSnS4

Abstract: The Raman spectra of surface regions of bulk Cu2ZnSnS4 (CZTS) samples with different Cu and Zn cation content were obtained and the differences in the spectra are attributed to statistical disorder effects in the cation sublattice. This disorder in the Cu and Zn sublattices may initiate a change of the crystal symmetry from kesterite-type to space group. The investigated CZTS crystals grown at high temperature are characterised by the co-existence of regions with different composition ratio of Cu/(Zn + Sn) which results in kesterite and disordered kesterite phases. The presence of a disordered phase with symmetry is reflected in the appearance of a dominant broadened A-symmetry peak at lower frequency than the peak of the main A-symmetry kesterite mode at 337 cm–1. We suppose that due to a small energy barrier between these phases the transition from one phase to the other can be stimulated by optical excitation of Cu2ZnSnS4. The analysis of the Raman spectra measured under different excitation conditions has allowed obtaining first (to our knowledge) experimental evidence of the existence of such optically induced structural transition in CZTS. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Efficiency enhancement of Cu(In,Ga)Se2 thin‐film solar cells by a post‐deposition treatment with potassium fluoride

Abstract: Alkali-free Cu(In,Ga)Se2(CIGS) absorbers grown on Mo-coated alumina (Al2O3) substrates were doped with potassium (K) after CIGS growth by a potassium fluoride (KF) post-deposition treatment (PDT). The addition of K to the absorber leads to a strong increase in cell efficiency from 10.0% for the K-free cell to 14.2% for the K-doped cell, mainly driven by an increase in the open-circuit voltage Voc and the fill factor FF, and to an increase in the net charge carrier density. Hence K doping by KF-PDT is comparable to doping with Na. J –V characteristics of a CIGS solar cell with KF-PDT (red solid line) and of a K-free reference cell without treatment (black dashed line). The solar cell treated with K shows a strong increase in Voc and FF compared to the solar cell without K. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dual behavior of excess electrons in rutile TiO2

Abstract: The behavior of electrons in the conduction band of TiO2 and other transition-metal oxides is key to the many applications of these materials. Experiments seem to produce conflicting results: optical and spin-resonance techniques reveal strongly localized small polarons, while electrical measurements show high mobilities that can only be explained by delocalized free electrons. By means of hybrid functional calculations we resolve this apparent contradiction and show that small polarons can actually coexist with delocalized electrons in the conduction band of TiO2, the former being energetically only slightly more favorable. We also find that small polarons can form complexes with oxygen vacancies and ionized shallow-donor impurities, explaining the rich spectrum of Ti3+ species observed in electron spin resonance experiments. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Topological insulators and thermoelectric materials

Abstract: Topological insulators (TIs) are a new quantum state of matter which have gapless surface states inside the bulk energy gap [1–4]. Starting with the discovery of two-dimensional TIs, the HgTe-based quantum wells [5, 6], many new topological materials have been theoretically predicted and experimentally observed. Currently known TI materials can possibly be classified into two families [7], the HgTe family and the Bi2Se3 family. The signatures found in the electronic structure of a TI also cause these materials to be excellent thermoelectric materials [8–10]. On the other hand, excellent thermoelectric materials can be also topologically trivial. Here we present a short introduction to topological insulators and thermoelectrics, and give examples of compound classes where both good thermoelectric properties and topological insulators can be found. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Self-induced growth of GaN nanowires by molecular beam epitaxy: A critical review of the formation mechanisms

Abstract: GaN nanowires, also called nanocolumns, have emerged over the last decade as promising nanosized building blocks for a wide variety of optoelectronic devices. In contrast to other III–V semiconductors, GaN nanowires have the ability to grow catalyst-free within the self-induced approach by plasma-assisted molecular beam epitaxy, which does not require the use of any foreign materials or patterned substrate. The self-induced growth has accordingly been considered as a valuable growth mode to form GaN nanowires on a wide number of substrates such as Si, Al2O3, diamond or SiC. The formation mechanisms have extensively been investigated and are specifically reviewed here from the very onset of the nucleation phase through the elongation phase to the coalescence process. A general approach of the self-induced growth of GaN nanowires is gained. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The role of stacking faults for the formation of shunts during potential-induced degradation of crystalline Si solar cells

Abstract: 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 SiNx anti-reflective layer, resulting in PID. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nanofunctional gallium oxide (Ga2O3) nanowires/nanostructures and their applications in nanodevices

Abstract: A brief review on beta gallium oxide (β-Ga2O3) nanowires (NWs) and nanostructures (NS) is presented in this article. β-Ga2O3 is a wide-bandgap (Eg ∼ 4.9 eV) semiconductor and can be doped with n- and p-type dopants, which can lead to applications in many functional devices. Here, we will first discuss briefly the properties of β-Ga2O3 in bulk form. Then we will describe the growth of β-Ga2O3 NWs/NS using various techniques including thermal CVD, MOCVD and laser ablation. The present status of research in the area of nanowire growth will be highlighted in this section. Then we will describe the luminescence properties of β-Ga2O3 NWs such as photoluminescence (PL) and cathodoluminescence (CL). The origin of various peaks in the PL and CL spectra of β-Ga2O3 NWs/ NS will be presented, with reference to various experimental studies carried out recently. In the final section, we will describe various applications of β-Ga2O3 NWs in nanodevices such as field effect transistors (FET), gas sensors and deep-UV photodetectors. Finally, we will give a conclusion and future perspective of the research in the area of this important semiconducting oxide. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Topological insulator nanostructures

Abstract: Topological insulators provide a gateway to investigate fundamental quantum behaviours of exotic quasi-particles as well as the promise to revolutionize modern technology. This is due to their unique surface states that are robust against time reversal perturbation and exhibit unique spin-momentum locking property. Enhancing the surface state signal in proportion to the bulk of the material is critical to study the surface states and for future electronics applications. This can be achieved by making topological insulators into nanostructures, which have large surface to volume ratios thus maximizing the topological surface state signal. This article reviews the recent progress made in topological insulator nanostructures including chemical and physical synthesis techniques to make topological insulator nanostructures, notable transport experiments, and current efforts to protect the surface states from degradation. Lastly, future transport studies using topological insulator nanostructures and various chemical methods to tune their materials properties for diverse electronic applications are suggested. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Group III nitride core–shell nano‐ and microrods for optoelectronic applications

Abstract: In the past few years, tremendous progress has been demonstrated on epitaxial growth and processing of group III nitride nano- and microrods (NAMs). This has also enabled the fabrication of optoelectronic devices based on NAMs as active elements. However, their efficiency is still far behind the performance of conventional GaN-based light emitting diodes (LEDs). This Review presents the most recent activities on the growth and processing of NAMs exhibiting a core–shell geometry, i.e. structures which consist of an active region shell layer wrapped around a three-dimensional (3D) core, which allow for an enormous increase in active area compared to planar technology. The most common growth approaches using metalorganic vapour phase epitaxy are described and evaluated with particular regard to their potential for solid state lighting applications. Examples for the unique properties of 3D NAMs are presented including their excellent crystalline quality. Furthermore, factors limiting the overall performance of 3D core–shell LEDs are revealed and the potential of overcoming these limitations are discussed. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Look fast: Crystallization of conjugated molecules during solution shearing probed in-situ and in real time by X-ray scattering

Abstract: High-speed solution shearing, in which a drop of dissolved material is spread by a coating knife onto the substrate, has emerged as a versatile, yet simple coating technique to prepare high-mobility organic thin film transistors. Solution shearing and subsequent drying and crystallization of a thin film of conjugated molecules is probed in situ using microbeam grazing incidence wide-angle X-ray scattering (μGIWAXS). We demonstrate the advantages of this approach to study solution based crystal nucleation and growth, and identify casting parameter combinations to cast highly ordered and laterally aligned molecular thin films. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Topological insulators from the perspective of first‐principles calculations

Abstract: Topological insulators are new quantum states with helical gapless edge or surface states inside the bulk band gap. These topological surface states are robust against weak time-reversal invariant perturbations without closing the bulk band gap, such as lattice distortions and non-magnetic impurities. Recently a variety of topological insulators have been predicted by theories, and observed by experiments. First-principles calculations have been widely used to predict topological insulators with great success. In this review, we summarize the current progress in this field from the perspective of first-principles calculations. First of all, the basic concepts of topological insulators and the frequently-used techniques within first-principles calculations are briefly introduced. Secondly, we summarize general methodologies to search for new topological insulators. In the last part, based on the band inversion picture first introduced in the context of HgTe, we classify topological insulators into three types with s–p, p–p and d–f, and discuss some representative examples for each type. Surface states of topological insulator Bi2Se3 consist of a single Dirac cone, as obtained from first-principles calculations. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

From the adiabatic theorem of quantum mechanics to topological states of matter

Abstract: Owing to the enormous interest the rapidly growing field of topological states of matter (TSM) has attracted in recent years, the main focus of this review is on the theoretical foundations of TSM. Starting from the adiabatic theorem of quantum mechanics which we present from a geometrical perspective, the concept of TSM is introduced to distinguish gapped many body ground states that have representatives within the class of non-interacting systems and mean field superconductors, respectively, regarding their global geometrical features. These classifying features are topological invariants defined in terms of the adiabatic curvature of these bulk insulating systems. We review the general classification of TSM in all symmetry classes in the framework of K-Theory. Furthermore, we outline how interactions and disorder can be included into the theoretical framework of TSM by reformulating the relevant topological invariants in terms of the single particle Green's function and by introducing twisted boundary conditions, respectively. We finally integrate the field of TSM into a broader context by distinguishing TSM from the concept of topological order which has been introduced to study fractional quantum Hall systems. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Silicon nanowires – a versatile technology platform

Abstract: Silicon nanowires offer unique properties like inherent small diameters, quasi-one-dimensional current transport and the flexibility to combine materials that cannot be combined in bulk or thin film structures. Based on these properties electron devices, sensors as well as solar cells and lithium batteries can be envisioned that significantly outperform their thin film or bulk counterparts. The possibility to form silicon nanowires in a top-down process using bulk silicon or silicon-on-insulator substrates, gives this technology the potential for a seamless integration into integrated electronic systems. This Review gives an overview of important device applications of silicon nanowires. Starting with nanowire fabrication, the different device concepts will be introduced and their most important features are reported.

Thermal conductivity of silicon nanowires: From fundamentals to phononic engineering

Abstract: Silicon nanowires have attracted great interest in recent years due to their ideal interface compatibility with silicon-based electronic technology and their various potential applications, such as energy harvest and generation, and thermal management. A variety of theoretical and experimental studies have been conducted to understand the thermal properties of silicon nanowires. In this review, we summarize the recent progress in this field from the perspective of both theoretical calculations and experiments. First, we introduce the fundamental physics underlying the thermal conduction of silicon nanowires. Then, the various approaches to manipulate the thermal conductivity of silicon nanowires are discussed. Finally, based on the understanding of dominant scattering mechanisms in different phonon frequency regimes, we present the basic concept of phononic engineering to control the thermal conductivity of silicon nanowires. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Advances in semiconductor nanowire growth on graphene

Abstract: Here we review the recent research activities on the epitaxial growth of semiconductor nanowires (NWs) on graphene substrates. Semiconductor NWs with quasi one-dimensional structure have become an active research field due to their various interesting physical properties and potentials for future electronic and optoelectronic device applications, such as transistors, sensors, solar cells, light emitting diodes, and lasers. At almost the same time graphene, a two-dimensional material made of carbon, was discovered and has gained an ever increasing interest during the last few years owing to its remarkable properties, including excellent electrical conductivity, optical transparency, and mechanical strength and flexibility. A hybrid structure by epitaxially growing semiconductor NWs on graphene could provide a new avenue for the development of future advanced NW-based flexible electronic and optoelectronic devices. We address the challenges for the growth of semiconductor NWs on graphene, with a special focus on the III–V semiconductors, and highlight some potential applications of the NW/graphene hybrid system. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural, electronic and magnetic properties of quaternary half-metallic Heusler alloy CoFeCrAl

Abstract: We have investigated the structural, electronic and magnetic properties in quaternary CoFeCrAl Heusler alloy. Rietveld refinement and Mossbauer spectroscopy of samples quenched from 973 K show absence of the A2 phase and presence of a highly ordered B2-type structure. Full-potential linearized augmented plane wave calculations using the experimental lattice constant show that there is an energy gap of 0.41 eV around the Fermi level in the bands for minority spin electrons. Partial substitution of Co by Fe in Co2CrAl thus brings excellent structural ordering while retaining the high Curie temperature, spontaneous magnetization and half-metallicity necessary for application as spin valves. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Circular dichroism in angle-resolved photoemission spectroscopy of topological insulators

Abstract: United States. Dept. of Energy (Office of Basic Energy Sciences, Grant number DE-FG02-08ER46521)

Structural and electronic properties of highly doped topological insulator Bi2Se3 crystals

Abstract: 1 Department of Physics, Purdue University, 525 Northwestern Ave., West Lafayette, IN 47907, USA 2 Birck Nanotechnology Center, Purdue University, 1205 W. State St., West Lafayette, IN 47907, USA 3 Joseph Henry Laboratories, Department of Physics, Princeton University, Jadwin Hall, Princeton, NJ 08544, USA 4 Princeton Institute for Science and Technology of Materials, Princeton University, 70 Prospect Ave., Princeton, NJ 08544, USA 5 School of Electrical and Computer Engineering, Purdue University, 465 Northwestern Ave., West Lafayette, IN 47907 USA

Passivation of aluminium–n+ silicon contacts for solar cells by ultrathin Al2O3 and SiO2 dielectric layers

Abstract: Ultra-thin thermally grown SiO2 and atomic-layer-deposited (ALD) Al2O3 films are trialled as passivating dielectrics for metal–insulator–semiconductor (MIS) type contacts on top of phosphorus diffused regions applicable to high efficiency silicon solar cells. An investigation of the optimum insulator thickness in terms of contact recombination factor J0_cont and contact resistivity ρc is undertaken on 85 Ω/□ and 103 Ω/□ diffusions. An optimum ALD Al2O3 thickness of ∼22 A produces a J0_cont of ∼300 fAcm–2 whilst maintaining a ρc lower than 1 mΩ cm2 for the 103 Ω/□ diffusion. This has the potential to improve the open-circuit voltage by a maximum 15 mV. The thermally grown SiO2 fails to achieve equivalently low J0_cont values but exhibits greater thermal stability, resulting in slight improvements in ρc when annealed for 10 minutes at 300 °C without significant changes in J0_cont. The after-anneal J0_cont reaches ∼600 fAcm–2 with a ρc of ∼2.5 mΩ cm2 for the 85 Ω/□ diffusion amounting to a maximum gain in open-circuit voltage of 6 mV. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of nitrogen doping on the thermal conductivity of GeTe thin films

Abstract: The 3ω method was employed to determine the effect of nitrogen doping (5 at.%) on the thermal conductivity of sputtered thin films of stoichiometric GeTe (a material of interest for phase change memories). It was found that nitrogen doping has a detrimental effect on the thermal conductivity of GeTe in both phases, but less markedly in the amorphous (–25%) than in the crystalline one (–40%). On the opposite, no effect could be detected on the measured thermal boundary resistance between these films and SiO2, within the experimental error. Our results agree with those obtained by molecular dynamic simulation of amorphous GeTe. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Bismuth-based candidates for topological insulators: Chemistry beyond Bi2Te3

Abstract: The present contribution considers chemical aspects relevant for established and candidate topological insulators (TIs) based on the element bismuth. We provide an overview of selected bismuth-containing compounds with topological protection, place them among structurally related compounds and, proceeding from the comparison, propose further families and guidelines for the search of new candidate TIs. Owing to the unique electronic properties and structural flexibility, bismuth demonstrates an overwhelming diversity of structural motifs, including low-dimensional ones. Bismuth acts both as an electron donor and acceptor interacting with other elements, thus initiating a refined interplay of electron delocalization and localization that results in a wide range of properties – from an insulator or a semiconductor to a metal or a semimet al. Due to the bonding abilities of Bi 6p-orbitals an isolated pure-bismuth layer is bound to be corrugated. Yet compounds exist with planar decorated honeycomb nets of bismuth and transition-metal atoms that allow for testing theoretical predictions that nets with such geometry could support a TI phase. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Mechanical and electronic properties of stoichiometric silicene and germanene oxides from first-principles

Abstract: 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)

Magnetotransport and induced superconductivity in Bi based three-dimensional topological insulators

Abstract: The surface of a three-dimensional (3D) topological insulator is conducting and the topologically nontrivial nature of the surface states is observed in experiments. It is the aim of this paper to review and analyze experimental observations with respect to the magnetotransport in Bi-based 3D topological insulators, as well as the superconducting transport properties of hybrid structures consisting of superconductors and these topological insulators. The helical spin-momentum coupling of the surface state electrons becomes visible in quantum corrections to the conductivity and magnetoresistance oscil-lations. An analysis will be provided of the reported magnetoresistance, also in the presence of bulk conductivity shunts. Special attention is given to the large and linear magnetoresistance. Superconductivity can be induced in topological superconductors by means of the proximity effect. The induced supercurrents, Josephson effects and current–phase relations will be reviewed. These materials hold great potential in the field of spintronics and the route towards Majorana devices

Ferroelectricity from iron valence ordering in rare earth ferrites

Abstract: 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)