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Applied Physics Reviews — 2011

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Bleaching versus poling: Comparison of electric field induced phenomena in glasses and glass-metal nanocomposites

A. A. Lipovskii, V. G. Melehin, M. I. Petrov, Yu. P. Svirko, and V. V. Zhurikhina

J. Appl. Phys. 109, 011101 (2011); http://dx.doi.org/10.1063/1.3511746 (11 pages)

Online Publication Date: 11 January 2011

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By examining the electric field induced processes in glasses and glass-metal nanocomposites (GMN) we propose mechanism of the electric field assisted dissolution (EFAD) of metal nanoparticles in glass. We show that in both glass poling and EFAD processes, the strong (up to 1 V/nm) local electric field in the subanodic region is due to the presence of “slow” hydrogen ions bonded to nonbridging oxygen atoms in glass matrix. However, the origin of these hydrogen ions in glass and GMN is different. Specifically, when we apply the electric field to a virgin glass, the enrichment of the glass with hydrogen species takes place in the course of the poling. In GMN, the hydrogen ions have been incorporated into the glass matrix during metal nanoparticles formation via reduction in a metal by hydrogen, i.e., before the electric field was applied. The EFAD of metal nanoparticles resembles the electric field stimulated diffusion of metal film in glass (the important difference however is that in GMN, there is no direct contact of dissolving metal entity with anodic electrode). This similarity makes it possible to estimate the energy of thermal activated transition of silver atoms from a nanoparticle to glass matrix as ∼ 1.3 eV. Electroneutrality of the GMN requires emission of electrons from nanoparticles. Photoconductivity spectra of soda-lime glasses and the results of numerical calculations of band structure of fused silica, sodium disilicate and sodium-calcium-silicate glass enable us to evaluate the bandgap and the position of electron mobility edge in soda-lime glass. The evaluated values are ∼ 6 eV and ∼ 1.2 eV below vacuum level, respectively. The bent of the glass band structure in strong electric field permits a direct tunneling of Fermi electrons from silver nanoparticle (4.6 eV below the vacuum level) to the glass conductivity band. Evaluated in accordance with the Fowler–Nordheim equation the magnitude of electric field necessary to establish comparable electron emission and ion ejection rates is ∼ 0.27 V/nm, although other phenomena including polarization of the nanoparticles and tunneling of electrons thermally distributed above Fermi level, decreases this magnitude. We believe that the different mechanisms of ejection for electrons and ions should result in charging nanoparticles in EFAD process. The electron tunneling to localized OH− states and glass matrix relaxation process are also discussed.
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81.16.-c Methods of micro- and nanofabrication and processing
77.22.Ej Polarization and depolarization
64.75.Bc Solubility
72.40.+w Photoconduction and photovoltaic effects
71.23.Cq Amorphous semiconductors, metallic glasses, glasses
73.40.Gk Tunneling

Critical assessment of the issues in the modeling of ablation and plasma expansion processes in the pulsed laser deposition of metals

Deepak Marla, Upendra V. Bhandarkar, and Suhas S. Joshi

J. Appl. Phys. 109, 021101 (2011); http://dx.doi.org/10.1063/1.3537838 (15 pages)

Online Publication Date: 31 January 2011

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This paper presents a review on the modeling of ablation and plasma expansion processes in the pulsed laser deposition of metals. The ablation of a target is the key process that determines the amount of material to be deposited; while, the plasma expansion governs the characteristics of the deposited material. The modeling of ablation process involves a study of two complex phenomena: (i) laser-target interaction and (ii) plasma formation and subsequent shielding of the incoming radiation. The laser-target interaction is a function of pulse duration, which is captured by various models that are described in this paper. The plasma produced as a result of laser–target interaction, further interacts with the incoming radiation, causing the shielding of the target. The shielding process has been modeled by considering the various photon absorption mechanisms operative inside the plasma, namely: inverse Bremsstrahlung, photoionization, and Mie absorption. Concurrently, the plasma expands freely until the ablated material gets deposited on the substrate. Various models describing the plasma expansion process have been presented. The ability of the theoretical models in predicting various ablation and plasma characteristics has also been compared with the relevant experimental data from the literature. The paper concludes with identification of critical issues and recommendations for future modeling endeavors.
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81.15.Fg Pulsed laser ablation deposition
52.77.Dq Plasma-based ion implantation and deposition
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)

Electromechanical phenomena in semiconductor nanostructures

L. C. Lew Yan Voon and M. Willatzen

J. Appl. Phys. 109, 031101 (2011); http://dx.doi.org/10.1063/1.3533402 (24 pages)

Online Publication Date: 9 February 2011

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Electromechanical phenomena in semiconductors are still poorly studied from a fundamental and an applied science perspective, even though significant strides have been made in the last decade or so. Indeed, most current electromechanical devices are based on ferroelectric oxides. Yet, the importance of the effect in certain semiconductors is being increasingly recognized. For instance, the magnitude of the electric field in an AlN/GaN nanostructure can reach 1–10 MV/cm. In fact, the basic functioning of an (0001) AlGaN/GaN high electron mobility transistor is due to the two-dimensional electron gas formed at the material interface by the polarization fields. The goal of this review is to inform the reader of some of the recent developments in the field for nanostructures and to point out still open questions. Examples of recent work that involves the piezoelectric and pyroelectric effects in semiconductors include: the study of the optoelectronic properties of III-nitrides quantum wells and dots, the current controversy regarding the importance of the nonlinear piezoelectric effect, energy harvesting using ZnO nanowires as a piezoelectric nanogenerator, the use of piezoelectric materials in surface acoustic wave devices, and the appropriateness of various models for analyzing electromechanical effects. Piezoelectric materials such as GaN and ZnO are gaining more and more importance for energy-related applications; examples include high-brightness light-emitting diodes for white lighting, high-electron mobility transistors, and nanogenerators. Indeed, it remains to be demonstrated whether these materials could be the ideal multifunctional materials. The solutions to these and other related problems will not only lead to a better understanding of the basic physics of these materials, but will validate new characterization tools, and advance the development of new and better devices. We will restrict ourselves to nanostructures in the current article even though the measurements and calculations of the bulk electromechanical coefficients remain challenging. Much of the literature has focused on InGaN/GaN, AlGaN/GaN, ZnMgO/ZnO, and ZnCdO/ZnO quantum wells, and InAs/GaAs and AlGaN/AlN quantum dots for their optoelectronic properties; and work on the bending of nanowires have been mostly for GaN and ZnO nanowires. We hope the present review article will stimulate further research into the field of electromechanical phenomena and help in the development of applications.
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82.45.-h Electrochemistry and electrophoresis
65.40.gk Electrochemical properties
77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials
77.70.+a Pyroelectric and electrocaloric effects
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Tunable, continuous-wave Terahertz photomixer sources and applications

S. Preu, G. H. Döhler, S. Malzer, L. J. Wang, and A. C. Gossard

J. Appl. Phys. 109, 061301 (2011); http://dx.doi.org/10.1063/1.3552291 (56 pages)

Online Publication Date: 22 March 2011

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This review is focused on the latest developments in continuous-wave (CW) photomixing for Terahertz (THz) generation. The first part of the paper explains the limiting factors for operation at high frequencies ∼ 1 THz, namely transit time or lifetime roll-off, antenna (R)-device (C) RC roll-off, current screening and blocking, and heat dissipation. We will present various realizations of both photoconductive and p-i-n diode–based photomixers to overcome these limitations, including perspectives on novel materials for high-power photomixers operating at telecom wavelengths (1550 nm). In addition to the classical approach of feeding current originating from a small semiconductor photomixer device to an antenna (antenna-based emitter, AE), an antennaless approach in which the active area itself radiates (large area emitter, LAE) is discussed in detail. Although we focus on CW photomixing, we briefly discuss recent results for LAEs under pulsed conditions. Record power levels of 1.5 mW average power and conversion efficiencies as high as 2 × 10−3 have been reached, about 2 orders of magnitude higher than those obtained with CW antenna-based emitters. The second part of the paper is devoted to applications for CW photomixers. We begin with a discussion of the development of novel THz optics. Special attention is paid to experiments exploiting the long coherence length of CW photomixers for coherent emission and detection of THz arrays. The long coherence length comes with an unprecedented narrow linewidth. This is of particular interest for spectroscopic applications, the field in which THz research has perhaps the highest impact. We point out that CW spectroscopy systems may potentially be more compact, cheaper, and more accurate than conventional pulsed systems. These features are attributed to telecom-wavelength compatibility, to excellent frequency resolution, and to their huge spectral density. The paper concludes with prototype experiments of THz wireless LAN applications. For future telecommunication systems, the limited bandwidth of photodiodes is inadequate for further upshifting carrier frequencies. This, however, will soon be required for increased data throughput. The implementation of telecom-wavelength compatible photomixing diodes for down-conversion of an optical carrier signal to a (sub-)THz RF signal will be required.
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84.30.Qi Modulators and demodulators; discriminators, comparators, mixers, limiters, and compressors
85.60.Dw Photodiodes; phototransistors; photoresistors
84.40.Ba Antennas: theory, components and accessories
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Understanding junction breakdown in multicrystalline solar cells

Otwin Breitenstein, Jan Bauer, Karsten Bothe, Wolfram Kwapil, Dominik Lausch, Uwe Rau, Jan Schmidt, Matthias Schneemann, Martin C. Schubert, Jan-Martin Wagner, and Wilhelm Warta

J. Appl. Phys. 109, 071101 (2011); http://dx.doi.org/10.1063/1.3562200 (10 pages)

Online Publication Date: 12 April 2011

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Extensive investigations on industrial multicrystalline silicon solar cells have shown that, for standard 1 Ω cm material, acid-etched texturization, and in absence of strong ohmic shunts, there are three different types of breakdown appearing in different reverse bias ranges. Between −4 and −9 V there is early breakdown (type 1), which is due to Al contamination of the surface. Between −9 and −13 V defect-induced breakdown (type 2) dominates, which is due to metal-containing precipitates lying within recombination-active grain boundaries. Beyond −13 V we may find in addition avalanche breakdown (type 3) at etch pits, which is characterized by a steep slope of the I-V characteristic, avalanche carrier multiplication by impact ionization, and a negative temperature coefficient of the reverse current. If instead of acid-etching alkaline-etching is used, all these breakdown classes also appear, but their onset voltage is enlarged by several volts. Also for cells made from upgraded metallurgical grade material these classes can be distinguished. However, due to the higher net doping concentration of this material, their onset voltage is considerably reduced here.
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88.40.jj Silicon solar cells
88.40.H- Solar cells (photovoltaics)
85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)

Injectorless quantum cascade lasers

Simeon Katz, Augustinas Vizbaras, Ralf Meyer, and Markus-Christian Amann

J. Appl. Phys. 109, 081101 (2011); http://dx.doi.org/10.1063/1.3566072 (9 pages)

Online Publication Date: 28 April 2011

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This review focuses on recent progress on injectorless quantum cascade lasers, an increasingly attractive approach in comparison to the “classical” injectorbased concepts. This particularly holds for the wavelength range between 7 and 12 μm, where fundamental vibrational modes of many important molecules exist, so that sensor systems for medical, industrial and military applications highly benefit from these laser sources. The atmospheric transmission window between 8 and 12 μm, with very low damping, also enables free space applications like communication, military countermeasures, and environmental sensors. Injectorless devices operate closer to the original design principle for intersubband lasers as suggested by Suris and Kazarinov [Sov. Phys. Semicond. 5, 707 (1971)]. Therefore, a short description of their features is given in comparison to injectorbased devices. Within recent years, injectorless devices have seen rapid improvement in performance. Best injectorless devices reach threshold current densities of 450 A/cm2 at 300 K, a factor of 1.6 smaller than that for the best injectorbased devices. Their output efficiency has also increased from 2% to more than 7% within the last 2 years, reaching comparable levels and making the injectorless device concept competitive and very attractive for applications.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Resolution of the Abraham-Minkowski debate: Implications for the electromagnetic wave theory of light in matter

B. A. Kemp

J. Appl. Phys. 109, 111101 (2011); http://dx.doi.org/10.1063/1.3582151 (17 pages)

Online Publication Date: 1 June 2011

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A century has now passed since the origins of the Abraham-Minkowski controversy pertaining to the correct form of optical momentum in media. Experiment and theory have been applied at both the classical and quantum levels in attempt to resolve the debate. The result of these efforts is the identification of Abraham’s kinetic momentum as being responsible for the overall center of mass translations of a medium and Minkowski’s canonical or wave momentum as being responsible for translations within or with respect to a medium. In spite of the recent theoretical developments, much confusion still exists regarding the appropriate theory required to predict experimental outcomes and to develop new applications. In this paper, the resolution of the longstanding Abraham-Minkowski controversy is reviewed. The resolution is presented using classical electromagnetic theory and logical interpretation of experiments disseminated over the previous century. Emphasis is placed on applied physics applications: modeling optical manipulation of cells and particles. Although the basic interpretation of optical momentum has been resolved, there is still some uncertainly regarding the complete form of the momentum continuity equation describing electromagnetics. Thus, while a complete picture of electrodynamics has still yet to be fully interpreted, this correspondence should help clarify the state-of-the-art view.
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41.20.Jb Electromagnetic wave propagation; radiowave propagation
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ZnO Schottky barriers and Ohmic contacts

Leonard J. Brillson and Yicheng Lu

J. Appl. Phys. 109, 121301 (2011); http://dx.doi.org/10.1063/1.3581173 (33 pages)

Online Publication Date: 23 June 2011

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ZnO has emerged as a promising candidate for optoelectronic and microelectronic applications, whose development requires greater understanding and control of their electronic contacts. The rapid pace of ZnO research over the past decade has yielded considerable new information on the nature of ZnO interfaces with metals. Work on ZnO contacts over the past decade has now been carried out on high quality material, nearly free from complicating factors such as impurities, morphological and native point defects. Based on the high quality bulk and thin film crystals now available, ZnO exhibits a range of systematic interface electronic structure that can be understood at the atomic scale. Here we provide a comprehensive review of Schottky barrier and ohmic contacts including work extending over the past half century. For Schottky barriers, these results span the nature of ZnO surface charge transfer, the roles of surface cleaning, crystal quality, chemical interactions, and defect formation. For ohmic contacts, these studies encompass the nature of metal-specific interactions, the role of annealing, multilayered contacts, alloyed contacts, metallization schemes for state-of-the-art contacts, and their application to n-type versus p-type ZnO. Both ZnO Schottky barriers and ohmic contacts show a wide range of phenomena and electronic behavior, which can all be directly tied to chemical and structural changes on an atomic scale.
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73.40.Ns Metal-nonmetal contacts
73.30.+y Surface double layers, Schottky barriers, and work functions
81.65.Cf Surface cleaning, etching, patterning
61.72.Cc Kinetics of defect formation and annealing
85.40.Ls Metallization, contacts, interconnects; device isolation
73.25.+i Surface conductivity and carrier phenomena

The third-order nonlinear optical coefficients of Si, Ge, and Si1−xGex in the midwave and longwave infrared

Nick K. Hon, Richard Soref, and Bahram Jalali

J. Appl. Phys. 110, 011301 (2011); http://dx.doi.org/10.1063/1.3592270 (8 pages)

Online Publication Date: 14 July 2011

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Using a combination of semiconductor theory and experimental results from the scientific literature, we have compiled and plotted the key third-order nonlinear optical coefficients of bulk crystalline Si and Ge as a function of wavelength (1.5−6.7 μm for Si and 2–14.7 μm for Ge). The real part of third-order nonlinear dielectric susceptibility (χ(3)′), the two-photon absorption coefficient (βTPA), and the Raman gain coefficient (gR), have been investigated. Theoretical predictions were used to curve-fit the experimental data. For a spectral range in which no experimental data exists, we estimate and fill in the missing knowledge. Generally, these coefficient-values appear quite useful for a host of device applications, both Si and Ge offer large χ(3)′ and gR with Ge offering the stronger nonlinearity. In addition, we use the same theory to predict the third-order nonlinear optical coefficients of Si1−xGex alloy. By alloying Si and Ge, device designers can gain flexibility in tuning desired optical coefficients in between the two fundamental components based upon their application requirements.
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42.65.An Optical susceptibility, hyperpolarizability
78.30.Am Elemental semiconductors and insulators
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Vertically aligned carbon based varactors

Farzan A. Ghavanini, Peter Enoksson, Stefan Bengtsson, and Per Lundgren

J. Appl. Phys. 110, 021101 (2011); http://dx.doi.org/10.1063/1.3583536 (14 pages)

Online Publication Date: 25 July 2011

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This paper gives an assessment of vertically aligned carbon based varactors and validates their potential for future applications. The varactors discussed here are nanoelectromechanical devices which are based on either vertically aligned carbon nanofibers or vertically aligned carbon nanotube arrays. A generic analytical model for parallel plate nanoelectromechanical varactors based on previous works is developed and is used to formulate a universal expression for their voltage-capacitance relation. Specific expressions for the nanofiber based and the nanotube based varactors are then derived separately from the generic model. This paper also provides a detailed review on the fabrication of carbon based varactors and pays special attention to the challenges in realizing such devices. Finally, the performance of the carbon based varactor is assessed in accordance with four criteria: the static capacitance, the tuning ratio, the quality factor, and the operating voltage. Although the reported performance is still far inferior to other varactor technologies, our prognosis which stems from the analytical model shows a promise of a high quality factor as well as a potential for high power handling for carbon based varactors.
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84.32.Tt Capacitors
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
81.07.Oj Nanoelectromechanical systems (NEMS)
85.35.Kt Nanotube devices
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Will we exceed 50% efficiency in photovoltaics?

Antonio Luque

J. Appl. Phys. 110, 031301 (2011); http://dx.doi.org/10.1063/1.3600702 (19 pages)

Online Publication Date: 8 August 2011

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Solar energy is the most abundant and reliable source of energy we have to provide for the multi-terawatt challenge we are facing. Although huge, this resource is relatively dispersed. High conversion efficiency is probably necessary for cost effectiveness. Solar cell efficiencies above 40% have been achieved with multijunction (MJ) solar cells. These achievements are here described. Possible paths for improvement are hinted at including third generation photovoltaics concepts. It is concluded that it is very likely that the target of 50% will eventually be achieved. This high efficiency requires operating under concentrated sunlight, partly because concentration helps increase the efficiency but mainly because the cost of the sophisticated cells needed can only be paid by extracting as much electric power form each cell as possible. The optical challenges associated with the concentrator optics and the tools for overcoming them, in particular non-imaging optics, are briefly discussed and the results and trends are described. It is probable that optical efficiency over 90% will be possible in the future. This would lead to a module efficiency of 45%. The manufacturing of a concentrator has to be addressed at three levels of integration: module, array, and photovoltaic (PV) subfield. The PV plant as a whole is very similar than a flat module PV plant with two-axes tracking. At the module level, the development of tools for easy manufacturing and quality control is an important topic. Furthermore, they can accommodate in different position cells with different spectral sensitivities so complementing the effort in manufacturing MJ cells. At the array level, a proper definition of the nameplate watts, since the diffuse light is not used, is under discussion. The cost of installation of arrays in the field can be very much reduced by self aligning tracking control strategies. At the subfield level, aspects such as the self shadowing of arrays causes the CPV subfields to be sparsely packed leading to a ground efficiency, in the range of 10%, that in some cases will be below that of fixed modules of much lower cell efficiency. All this taken into account, High Concentration PV (HCPV) has the opportunity to become the cheapest of the PV technologies and beat the prevalent electricity generation technologies. Of course the way will be paved with challenges, and success is not guaranteed.
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88.40.hj Efficiency and performance of solar cells
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Erratum: “Patterned piezo-, pyro-, and ferroelectricity of poled polymer electrets” [J. Appl. Phys. 108, 011101 (2010)]

Xunlin Qiu

J. Appl. Phys. 110, 059905 (2011); http://dx.doi.org/10.1063/1.3638069 (1 page)

Online Publication Date: 13 September 2011

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Abstract Unavailable
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99.10.Cd Errata
77.80.-e Ferroelectricity and antiferroelectricity
77.84.Jd Polymers; organic compounds
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Electrical transport mechanisms in three dimensional ensembles of silicon quantum dots

I. Balberg

J. Appl. Phys. 110, 061301 (2011); http://dx.doi.org/10.1063/1.3637636 (26 pages)

Online Publication Date: 29 September 2011

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In this review, we try to derive a comprehensive understanding of the transport mechanisms in three dimensional ensembles of Si quantum dots (QDs) that are embedded in an insulating matrix. This understanding is based on our systematic electrical measurements as a function of the density of Si nanocrystallites as well as on a critical examination of the available literature. We conclude that in ensembles of low density QDs, the conduction is controlled by quantum confinement and Coulomb blockade effects while in the high density regime, the system behaves as a simple disordered semiconductor. In between these extremes, the transport is determined by the clustering of the QDs. In view of the clustering, two types of transitions in the electrical and optical properties of the system are identified. In order to understand them, we introduce the concept of “touching.” The application of this concept enables us to suggest that the first transition is a local carrier deconfinement transition, at which the concentration of the non “touching” QDs reaches its maximum, and that the other transition is associated with the onset of percolation in a continuous disordered network of “touching” QDs. It is hoped that our conclusions for the entire possible density range will provide guidance for the discussion and understanding of the transport in ensembles of semiconductor QDs in general and in ensembles of Si and Ge QDs in particular.
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73.63.Kv Quantum dots
78.67.Hc Quantum dots
73.23.Hk Coulomb blockade; single-electron tunneling
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Adaptive oxide electronics: A review

Sieu D. Ha and Shriram Ramanathan

J. Appl. Phys. 110, 071101 (2011); http://dx.doi.org/10.1063/1.3640806 (20 pages)

Online Publication Date: 5 October 2011

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Novel information processing techniques are being actively explored to overcome fundamental limitations associated with CMOS scaling. A new paradigm of adaptive electronic devices is emerging that may reshape the frontiers of electronics and enable new modalities. Creating systems that can learn and adapt to various inputs has generally been a complex algorithm problem in information science, albeit with wide-ranging and powerful applications from medical diagnosis to control systems. Recent work in oxide electronics suggests that it may be plausible to implement such systems at the device level, thereby drastically increasing computational density and power efficiency and expanding the potential for electronics beyond Boolean computation. Intriguing possibilities of adaptive electronics include fabrication of devices that mimic human brain functionality: the strengthening and weakening of synapses emulated by electrically, magnetically, thermally, or optically tunable properties of materials.In this review, we detail materials and device physics studies on functional metal oxides that may be utilized for adaptive electronics. It has been shown that properties, such as resistivity, polarization, and magnetization, of many oxides can be modified electrically in a non-volatile manner, suggesting that these materials respond to electrical stimulus similarly as a neural synapse. We discuss what device characteristics will likely be relevant for integration into adaptive platforms and then survey a variety of oxides with respect to these properties, such as, but not limited to, TaOx, SrTiO3, and Bi4-xLaxTi3O12. The physical mechanisms in each case are detailed and analyzed within the framework of adaptive electronics. We then review theoretically formulated and current experimentally realized adaptive devices with functional oxides, such as self-programmable logic and neuromorphic circuits. Finally, we speculate on what advances in materials physics and engineering may be needed to realize the full potential of adaptive oxide electronics.
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87.85.J- Biomaterials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
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Photoluminescence of deep defects involving transition metals in Si: New insights from highly enriched 28Si

M. Steger, A. Yang, T. Sekiguchi, K. Saeedi, M. L. W. Thewalt, M. O. Henry, K. Johnston, H. Riemann, N. V. Abrosimov, M. F. Churbanov, A. V. Gusev, A. K. Kaliteevskii, O. N. Godisov, P. Becker, and H.-J. Pohl

J. Appl. Phys. 110, 081301 (2011); http://dx.doi.org/10.1063/1.3651774 (25 pages)

Online Publication Date: 31 October 2011

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Deep luminescence centers in Si associated with transition metals have been studied for decades, both as markers for these deleterious contaminants, as well as for the possibility of efficient Si-based light emission. They are among the most ubiquitous luminescence centers observed in Si, and have served as testbeds for elucidating the physics of isoelectronic bound excitons, and for testing ab-initio calculations of defect properties. The greatly improved spectral resolution resulting from the elimination of inhomogeneous isotope broadening in the recently available highly enriched 28Si enabled the extension of the established technique of isotope shifts to the measurement of isotopic fingerprints, which reveal not only the presence of a given element in a luminescence center, but also the number of atoms of that element. This has resulted in many surprises regarding the actual constituents of what were thought to be well-understood deep luminescence centers. Here we summarize the available information for four families of centers containing either four or five atoms chosen from (Li, Cu, Ag, Au, Pt). The no-phonon transition energies, their isotope shifts, and the local vibrational mode energies presented here for these deep centers should prove useful for the still-needed theoretical explanations of their formation, stability and properties.
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78.55.Ap Elemental semiconductors
71.55.Cn Elemental semiconductors
61.72.jn Color centers
63.20.-e Phonons in crystal lattices
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Schottky barriers in carbon nanotube-metal contacts

Johannes Svensson and Eleanor E. B. Campbell

J. Appl. Phys. 110, 111101 (2011); http://dx.doi.org/10.1063/1.3664139 (16 pages)

Online Publication Date: 8 December 2011

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Semiconducting carbon nanotubes (CNTs) have several properties that are advantageous for field effect transistors such as high mobility, good electrostatics due to their small diameter allowing for aggressive gate length scaling and capability to withstand high current densities. However, in spite of the exceptional performance of single transistors only a few simple circuits and logic gates using CNTs have been demonstrated so far. One of the major obstacles for large scale integration of CNTs is to reliably fabricate p-type and n-type ohmic contacts. To achieve this, the nature of Schottky barriers that often form between metals and small diameter CNTs has to be fully understood. However, since experimental techniques commonly used to study contacts to bulk materials cannot be exploited and studies often have been performed on only single or a few devices there is a large discrepancy in the Schottky barrier heights reported and also several contradicting conclusions. This paper presents a comprehensive review of both theoretical and experimental results on CNT-metal contacts. The main focus is on comparisons between theoretical predictions and experimental results and identifying what needs to be done to gain further understanding of Schottky barriers in CNT-metal contacts.
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73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Ns Metal-nonmetal contacts
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Gauge fields in spintronics

T. Fujita, M. B. A. Jalil, S. G. Tan, and S. Murakami

J. Appl. Phys. 110, 121301 (2011); http://dx.doi.org/10.1063/1.3665219 (29 pages)

Online Publication Date: 22 December 2011

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We present an overview of gauge fields in spintronics, focusing on their origin and physical consequences. Important topics, such as the Berry gauge field associated with adiabatic quantum evolution as well as gauge fields arising from other non-adiabatic considerations, are discussed. We examine the appearance and effects of gauge fields across three spaces, namely real-space, momentum-space, and time, taking on a largely semiclassical approach. We seize the opportunity to study other “spin-like” systems, including graphene, topological insulators, magnonics, and photonics, which emphasize the ubiquity and importance of gauge fields. We aim to provide an intuitive and pedagogical insight into the role played by gauge fields in spin transport.
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85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
72.25.-b Spin polarized transport
73.20.-r Electron states at surfaces and interfaces
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