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Weak localization, a semiclassical process established in disordered systems, has recently been used as an experimental probe of the classical dynamics of ballistic cavities. Through a systematic study of the classical and quantum behaviour of three ballistic cavities featuring large width to length aspect ratios, we highlight the important role of the entrance and exit leads. We present a novel ballistic weak localization process in which the contributing classical trajectories pass directly between the entrance and exit leads and are not shaped by the walls of the cavity but purely by the profiles of the leads. This demonstration of a small section of a cavity supporting a `local' weak localization process has important implications for studies which relate cavity geometry to the dynamics it supports.

Low-field magnetoresistance measurements were performed on a two-dimensional grid lateral surface superlattice (LSSL) fabricated on a GaAs/AlxGa1-xAs heterojunction. From a one-parameter fit to the two-dimensional weak-localization theory we have obtained the effective inelastic scattering length at different temperatures. By subtracting the weak-localization term, we have also investigated the electron-electron interactions in this system. Both localization and interaction effects saturate at low temperatures when the inelastic scattering length and thermal diffusion length are longer than the period of the LSSL. At higher temperatures, when the thermal diffusion length becomes comparable to the period of the LSSL, the electron-electron interaction term causes the conductivity to decrease logarithmically with temperature. These results can be explained in terms of coherent backscattering of the electrons in a fraction of the electron puddles formed below the superlattice gate. Disorder ensures that the cells showing these results are not neighbors, so that this effect is an ensemble average of many unit cells several periods apart.

[EN] We present a Monte Carlo time-domain study of nanostructured ballistic three-branch junctions (TBJs) excited by both step-function and Gaussian picosecond transients. Our TBJs were based on InGaAs 2-dimensional electron gas heterostructures and their geometry followed exactly the earlier experimental studies. Time-resolved, picosecond transients of both the central branch potential and the between-the-arms current demonstrate that the bandwidth of the intrinsic TBJ response reaches the THz frequency range, being mainly limited by the large-signal, intervalley scattering, when the carrier transport regime changes from ballistic to diffusive., ROOTHz (FP7-243845)

In this paper we investigate the effects of intravalley acoustic phonon scattering on the quantum transport and on the electrical characteristics of multigate silicon nanowire metal-oxide-semiconductor field-effect transistors. We show that acoustic phonons cause a shift and broadening of the local DOS in the nanowire, which modifies the electrical characteristics of the device. The influence of scattering on off-state and on-state currents is investigated for different values of channel length. In the ballistic transport regime, source-to-drain tunneling current is predominant, whereas in the presence of acoustic phonons, diffusion becomes the dominant current transport mechanism. A three-dimensional quantum mechanical device simulator based on the nonequilibrium Green's function formalism in uncoupled-mode space has been developed to extract device parameters in the presence of electron-phonon interactions. Electron-phonon scattering is accounted for by adopting the self-consistent Born approximation and using the deformation potential theory. (C) 2010 American Institute of Physics. (doi: 10.1063/1.3457848)

Phonon hydrodynamics is used to analyze the influence of porosity and of pore size on reduction in thermal conductivity in porous silicon, with respect to crystalline silicon. The expressions predict that the thermal conductivity is lower for higher porosity and for smaller pore radius, as a consequence of phonon ballistic effects. The theoretical results describe experimental data better than the assumption that they only depend on porosity.

A 20 band sp3d5s* spin-orbit-coupled, semi-empirical, atomistic tight-binding (TB) model is used with a semi-classical, ballistic, field-effect-transistor (FET) model, to theoretically examine the bandstructure carrier velocity and ballistic current in silicon nanowire (NW) transistors. Infinitely long, uniform, cylindrical and rectangular NWs, of cross sectional diameters/sides ranging from 3nm to 12nm are considered. For a comprehensive analysis, n-type and p-type metal-oxide-semiconductor (NMOS and PMOS) NWs in [100], [110] and [111] transport orientations are examined. In general, physical cross section reduction increases velocities, either by lifting the heavy mass valleys, or significantly changing the curvature of the bands. The carrier velocities of PMOS [110] and [111] NWs are a strong function of diameter, with the narrower D=3nm wires having twice the velocities of the D=12nm NWs. The velocity in the rest of the NW categories shows only minor diameter dependence. This behavior is explained through features in the electronic structure of the silicon host material. The ballistic current, on the other hand, shows the least sensitivity with cross section in the cases where the velocity has large variations. Since the carrier velocity is a measure of the effective mass and reflects on the channel mobility, these results can provide insight into the design of NW devices with enhanced performance and performance tolerant to structure geometry variations. In the case of ballistic transport in high performance devices, the [110] NWs are the ones with both high NMOS and PMOS performance, as well as low on-current variations with cross section geometry variations., Comment: 31 pages, 7 figures, 1 table

We analyze the effects of disorder on the intrinsic anisotropic magnetoresistance (AMR) of a spin-polarized two-dimensional electron gas with a Rashba-type spin-orbit interaction in the linear response theory. We show that the AMR vanishes unless the lifetime is spin dependent, analogous with the anomalous and spin Hall effects, which indicates a strong similarity between the anomalous and spin Hall effects and AMR. Furthermore, we perform numerical simulations of the ballistic and diffusive AMR.

The electron beam induced self-consistent charge transport in layered insulators (here, bulk alumina covered by a thin silica layer) is described by means of an electron-hole flight-drift model and an iterative computer simulation. Ballistic secondary electrons and holes, their attenuation and drift, as well as their recombination, trapping, and detrapping are included. Thermal and field-enhanced detrapping are described by the Poole–Frenkel effect. Furthermore, an additional surface layer with a modified electric surface conductivity is included which describes the surface leakage currents and will lead to particular charge incorporation at the interface between the surface layer and the bulk substrate. As a main result, the time-dependent secondary electron emission rate σ(t) and the spatial distributions of currents j(x,t), charges ρ(x,t), field F(x,t), and potential V(x,t) are obtained. For bulk full insulating samples, the time-dependent distributions approach the final stationary state with j(x,t)=const=0 and σ=1. In the case of a measurable surface leakage current, the steady stationary state is reached for σ

Özet KÜÇÜK SİSTEMLERDE BALİSTİK İLETİM VE TÜNELLEME A. Erkan Tekman Fizik Doktora Tez Yöneticisi: Prof. Dr. Salim Çıracı Ekim 1990 Mezoskopik sistemlerde balistik iletim ve tünelleme yoğun madde fiziğinin önemli konularından biri haline gelmiştir. Kuantum nokta bağlantıları ve tarama tünelleme mikroskobu bu çalışmaların temel deney araçlarıdır ve küçük sistemlerin pekçok özelliğinin anlaşılmasında kullanılmışlardır. Bu çalışmada küçük sistemlerde balistik iletim ve tünelleme kuramsal olarak ele alındı. Dar bağlantılarda balistik iletıim çeşitli durumlar için incelendi. Düzgün bağlantılarda iletkenliğin kuantalaştığı ve uzun kanallar için (2e2/h)m katlan olarak değiştiği bulundu. Dalgaların girişimi sonucu kuantalaşmış basamaklar üzerinde bir rezonans yapısı gözlendi. Rezonans yapısının deneysel sonuçlarda görülmemesi sıcaklık ve/veya bağlantının yuvarlatılması sonucu ortaya çıkan adiyabatik iletim ile açıklandı. Kanal içindeki bozukluklardan elastik saçılma sonucu kuantalaşmış basamakların bozulduğu bulundu. Bağlantının merkezinde çekici bozukluklar ya da genişleme olmasının bağlı durumların oluşmasına ve rezonant tünellemeye yol açacağı öngörüldü. Tarama tünelleme mikroskobundaki ölçümün dar bağlantılarla pekçok ortak noktası olduğu gösterildi. Tünellemeden nokta bağlantıya geçişe uç-yüzey metkileşimi sonucu etkin potansiyel eşiğinin yok olmasının yol açtığı bulundu. Soy ve basit metallerde pozisyona bağlı uç-yüzey etkileşiminin potansiyel eşiğinin modülasyonuna ve atomsal yükselti farklarının gözlenmesine yol açtığı öngörüldü. Nokta kaynaklardan elektronların odaklanmış alan saçınımı sistematik olarak incelendi. Paralel sıkıştırma sonucu oluşan etkin potansiyelin ve boru benzeri açıklıktaki adiyabatik olmayan iletimin odaklanmaya yol açtığı bulundu. Dengedışı iletim özellikleri Keldysh'in Green fonksiyonu tekniği ile araştırıldı. Elastik ve inelastik saçılımın etkileri tek boyutlu bir geometride çalışıldı. Gerilim ve akım ölçüm bağlantılarının önemi ve çok bağlantılı ölçümler için Landauer formülleri incelendi. Anahtar sözcükler: Mezoskopik fizik, kuantum nokta bağlantıları, balistik iletim, tünelleme, kuantalaşmış iletkenlik, adiyabatik iletim, resonant tünelleme, tarama tünelleme mikroskobu, odaklanmış alan saçınımı, dengedışı kuantum iletimi, Keldysh tekniği, Landauer formülleri. iv Abstract BALLISTIC TRANSPORT AND TUNNELING IN SMALL SYSTEMS A. Erkan Tekman Ph. D. in Physics Supervisor: Prof. Salim Çıracı October 1990 Ballistic transport and tunneling of electrons in mesoscopic systems have become one of the most important subjects of condensed matter physics. The quantum point contacts and scanning tunneling microscope form the basic experimental tools in this area and have been used for understanding many features of small systems. In this work ballistic transport and tunneling in small systems are investigated theoretically. Ballistic transport through narrow constrictions is investigated for a variety of configurations. It is found that for a uniform constriction the conductance is quantized in units of the quantum of conductance (2e2/h) for long channels. The interference of waves in the constriction gives rise to the resonance structure superimposed on the quantized steps. The lack of the resonance structure in the experimental results are attributed to temperature effects and/or adiabatic transport due to tapering of the constriction. It is shown that elastic scattering by an impurity distorts the quantization of conductance. Novel resonant tunneling effects due to formation of bound states are predicted for an attractive impurity or a local widening at the center of the constriction.It is shown that the probing in scanning tunneling microscopy have very much in common with narrow constrictions. The transition from tunneling to point contact regime is explained by the vanishing effective potential barrier as a result of tip sample interaction. For noble and simple metals it is conjectured that lateral position dependent interaction between the tip and sample leads to corrugation of the potential barrier and in turn to atomic corrugation observed by scanning tunneling microscopy. The focused field emission of electrons from point sources is analyzed in a systematical way. The effective barrier due to the lateral confinement and nonadiabatic transport through the horn-like opening are found to be responsible for focusing. The nonequilibrium nature of transport is investigated by use of Keldysh Green's function technique. The effects of elastic and inelastic scattering are analyzed in a strictly one-dimensional geometry. The features of voltage and current probes are studied and the Landauer formulae are examined for multiprobe measurements. Keywords: Mesoscopics, quantum point contacts, ballistic transport, tun neling, quantized conductance, adiabatic transport, resonant tunneling, scanning tunneling microscopy, focused field emission, nonequilibrium quantum transport, Keldysh technique, Landauer formulae. n 189

Experiments are reported on the transport properties if high freqeuncy phonons in semiconductors in particular on their spectral properties in the range of hundreds of Ghz. First we have developped a technique of phonon spectrometry based on time of flight measurements of the chromatic dispersion of ballistic phonons. Detailed informations on their interaction with the electron system were obtained. In germanium, the resonnant absorption of phonons between singlet and triplet electronic levels of the Sb impurity and the corresponding selection rules for the phonon polarization were measured with accuracy. By tuning the splitting energy with an uniaxial pressure, we could determine the energy spectrum of the transmitted phonons. In degenerate n-type InSb crystals we have shown the absence of couping between transverse modes and the conduction electrons. For longitudinal modes the interaction exhibits a sharp high frequency cut-off as the phonon wavevector reaches the Fermi sphere diameter 2k_F. This cut-off combined with the threshold phonon detection of a superconductiong tunnel junction was used to achieve a 2kF - spectroscopy of longitudinal phonons in the InSb crystal with no adjustable parameter. New resuts were also obtained on vey high frequency phonon generation by direct laser pulse excitation.; Les expériences présentées portent sur l'étude du transport de phonons de haute fréquence dans les semiconducteurs, en particulier leur analyse spectrale. Tout d'abord nous avons développé une technique de spectrométrie de phonons basée sur la mesure par temps de vol de la dispersion chromatique des phonons balistiques dans la gamme de centaines de GHz. Des informations précises sur l'interaction électron-phonon dans les semiconducteurs ont été obtenues : dans le germanium, l'absorption résonnante de phonons entre les niveaux singulet et triplet de l'état de base de l'impureté Sb et les règles de sélection sur les polarisations de phonons s'y attachant ont été vérifiées avec précision. La séparation accordable des niveaux d'énergie avec une pression uniaxiale nous a permis de déterminer la composition spectrale des impulsions de phonons transmis. Dans InSb (dégénéré) de type n, nous avons mis en évidence la non-interaction entre modes transverses et électrons de conduction. Dans le cas des modes longitudinaux, l'interaction est caractérisée par une coupure abrupte à haute fréquence lorsque le vecteur d'onde atteint le diamètre de la sphère de Fermi (2kF). Cette coupure, combinée à la détection à seuil de jonctions tunnel supraconductrices a été mise à profit pour réaliser une spectroscopie à 2kF, sans paramètre ajustable, des impulsions de phonons transmises dans le cristal. Des résultats sur la génération de phonons par excitation laser directe sont également présentés.

Room-temperature (RT) ballistic transport of electrons is experimentally observed and theoretically investigated in III-nitrides. This has been largely investigated at low temperatures in low band gap III-V materials due to their high electron mobilities. However, their application to RT ballistic devices is limited by their low optical phonon energies, close to KT at 300 K. In addition, the short electron mean-free-path at RT requires nanoscale devices for which surface effects are a limitation in these materials. We explore the unique properties of wide band-gap III-nitride semiconductors to demonstrate RT ballistic devices. A theoretical model is proposed to corroborate experimentally their optical phonon energy of 92 meV, which is similar to 4x larger than in other III-V semiconductors. This allows RT ballistic devices operating at larger voltages and currents. An additional model is described to determine experimentally a characteristic dimension for ballistic transport of 188 nm. Another remarkable property is their short carrier depletion at device sidewalls, down to 13 nm, which allows top-down nanofabrication of very narrow ballistic devices. These results open a wealth of new systems and basic transport studies possible at RT.

In this work we investigate the performance of silicon nanowire and carbon-nanotube FETs at their extreme miniaturization limits. The model self-consistently solves the Schroedinger and Poisson equations using the Quantum Transmitting Boundary Method (QTBM) formalism. We compare the subthreshold slope, the drain-induced barrier lowering and the Ion/Ioff ratio versus diameter and gate length. The performance comparison demonstrates that the nanowire FET provides a better scaling trend at very low size despite its weaker gate control on the device electrostatics.

In this paper we present the observation of ballistic transport in strained silicon cavities defined by etching on a silicon germanium heterostructure, demonstrated by magnetic focusing of conductance of the cavity at T = 50 mK. Numerical simulations, based on a novel approach which allows to include an arbitrary degree of decoherence in mesoscopic transport, show that magnetoconductance features can be related to the semiclassical orbits by means of the local density of states in the cavity. ©2004 IEEE.

In graphene, the extremely fast charge carriers can be controlled by electron-optical elements, such as waveguides, in which the transmissivity is tuned by the wavelength. In this work, charge carriers are guided in a suspended ballistic few-mode graphene channel, defined by electrostatic gating. By depleting the channel, a reduction of mode number and steps in the conductance are observed, until the channel is completely emptied. The measurements are supported by tight-binding transport calculations including the full electrostatics of the sample.

The thesis describes the computational study of structural, electonic and transport properties of monolayer transition metal dichalcogenides (TMDs) in the stable 2H and the metastable 1T' phases. Several aspects have been covered by the study including the electronic properties of the topological quantum spin Hall (QSH) state in the 1T' monolayer phase as well as the effects of strain, periodic line defects, interfaces and edges of monolayer TMDs. The electronic properties of the bulk monolayer phases were described by the ab-initio density functional theory framework while the electronic and transport properties of 1D defects were calculated using the non-equilibrium Green's function formalism. A specific focus was made on the transport of spin-polarized charge carriers across line defects in the monolayer 2H phase. Subject to energy, pseudomomentum and spin conservation, the size of the transport gap is governed by both bulk properties of a material and symmetries of a line defect. Outside the transport gap energy region, the charge carriers are discriminated with respect to their spin resulting in the spin polarization of the transmitted current. Next, the properties of the metastable monolayer 1T' phase were studied. The presence of a sufficiently large band gap is crucial to observe the QSH phase in the family of materials by probing the topological boundary states. The meV-order band gaps of the 1T' phase of monolayer TMDs were found to be sensitive to materials' lattice constants suggesting the control of the band gap size by strain. In particular, the electronic band structure and the size of the band gap in monolayer 1T'-WSe2 were found to be in agreement with spectroscopy studies. The topologically protected states at the edges of the monolayer 1T' phase as well as at the boundaries between the topological 1T' phase and the trivial 2H phase of monolayer TMDs were studied. Specific atomic structure configurations were suggested to observe experimentally the topological protection of the charge carrier transport against back-scattering. Finally, in the context of lateral semiconducting device engineering, the electronic and transverse transport properties of 2H-1T' phase boundaries as well as the dimerization defects in the 1T' phase were investigated. Both kinds of defects considered exhibit a relatively large transmission probability for the charge carriers crossing the defects. However, the differences between the shapes of bulk bands of the two phases open a sizeable transport gap for charge carriers crossing periodic domain boundaries between the monolayer 2H and 1T' phases. The calculated formation energies of dimerization defects were found to be relatively low suggesting their high concentration in real samples of monolayer 1T'-TMDs. Additionally, the thesis includes studies of magnetic dopants on the surface of Bi2Te3 and atomic vacancies in monolayer 2H-MoSe2 where the electronic properties of point defects were calculated and compared to experimental results. The two possible adsorption sites of Fe on the surface of Bi2Te3 both show a large out-of-plane magnetic anisotropy in agreement with experiments. The calculated local electronic properties of Se vacancies in monolayer 2H-MoSe2 show the presence of in-gap states which are not observed in experiment. Nevertheless, the combination of theoretical and experimental scanning tunneling microscopy images allowed the unambiguous identification of the vacancy defect.

The scaling of device technologies poses new challenges, not only in circuit design, but also in device modeling, especially because of the short-channel effects and the emergence of novel phenomena like ballistic transport. Nonetheless, it enables the design of ultra low-power analog and Radio Frequency (RF) circuits by allowing to push the operating points intomoderate and eventually weak inversion regions, which are increasingly becoming the preferred regions of operation for such applications. Even though modern compact models have evolved to adequately model the short-channel effects in all regions of operation, there is a lack of simpler models that (a) reliably predict the physics of downscaled devices while (b) remaining continuous through moderate inversion and (c) aid the designer’s intuition through simple designmethodologies. In this work, we extend the EKV charge based model to include the velocity saturation effect for weak inversion operation. Using the simple analytical model hence developed, we propose a design methodology for low-power analog circuit design. Then, we focus our attention on ballistic transport in MOSFETs, that is expected to dominate in the deeply scaled devices. Again, despite the extensive body of work available in the literature, most models remain deeply rooted in physics, consisting of fairly complicated equations, that are of little use for an intuitive understanding and design. In addition, the quasi-ballistic devices, which lie on the continuumbetween the ballistic and the diffusive devices, pose their own modeling challenges: a model for the quasi-ballistic devices would have to remain continuous between the ballistic and diffusive regimes. Most of the published works, based on the carrier flux transport over the source-channel potential barrier approach, seem to ignore the electrostatics in the rest of the channel. The shape of the electrostatic potential in the channel is approximated through polynomial functions, which is adequate for the very short-channel devices but not scalable to long channel quasi-ballistic devices. In this work, we study the role of the gate and the electrostatics in a ballistic channel by drawing on the insights gained from Monte-Carlo simulations on quasi-ballistic and ballistic doublegate MOSFETs. We propose a simple semi-empirical model of the channel charge, using which we develop an analytical model for the channel potential, both of which could be used as precursors to a scalable compact model that would encompass the ballistic, quasi-ballistic and drift-diffusion regimes.

Gallium Nitride (GaN) and all III-Nitride compounds have revolutionized the world with the development of the blue light emitting diode (LED). In addition, GaN-based epi-structures, such as AlGaN/GaN, enable the fabrication of high electron mobility transistors (HEMTs) for several applications handling high power. In the radio frequency (RF) domain, power amplifiers (PAs) based on GaN HEMTs are currently used for base stations, radars and, in the future, will replace low power amplifiers, currently made with silicon, ensuring much smaller footprint and reduced cost. Despite the presence of several commercial GaN HEMT-based devices, performances, especially in frequency, are quite far from the predicted theoretical limits. A lot of research focuses in using etched AlGaN/GaN HEMT nanowires (NWs) as channel for ultrascaled metal semiconductor field-effect transistors (MESFETs) to reduce short channel effects, arising when shrinking the gate length to increase the operating frequency, and linearity issues. In this thesis, we perform a transverse study of AlGaN/GaN NWs starting from the investigation of electron transport and ending with the proposal of new device architectures for improved device performance. Electron transport is studied in mesoscopic devices based on GaN HEMT NWs. Ballistic transport is observed even at room temperature and at large bias, thanks to the high mobility of electrons and large optical phonon energy. Quenched Hall effect and quantum physics phenomena due to phase coherence are observed at cryogenic temperature under a strong magnetic field. Finally, the sidewall depletion width, an important parameter for designing devices based on NWs, of AlGaN/GaN is extracted, resulting in one of the smallest values (19.5 nm) among all III-V HEMTs. Due to the increased parasitic capacitance in NW-based MESFETs, we also explored more exotic gating techniques consisting of side and in-plane gates (IPGs). IPGFETs and metal IPGFETs (M-IPGFETs) are fabricated, characterized and simulated. A record current density of 1.4 A/mm and transconductance of 665 mS/mm are measured, together with a breakdown voltage larger than 300 V. Simulations of the device capacitance yield values as low as few aF, resulting in possible cut-off frequencies up to 0.89 THz. Such value, together with the large breakdown voltage, push GaN IPGFETs beyond the theoretical limit of GaN for RF applications and paves the way for future device architectures. NWs are also used to noticeably improve performance of zero-bias RF detectors. We developed for the first time a field-effect rectifier (FERs) where the channel is constituted by an array of GaN-HEMT etched NWs. The much better gate control over NWs resulted in a current curvature (30.1 1/V) close to the theoretical limit of Schottky diodes (38.7 1/V). The cut-off frequency of these devices is up to 140 GHz, which, together with the large measured responsivity (3000 V/W), make NW-FERs very promising for stand-alone applications as well as for integration in GaN Monolithic Microwave Integrated Circuits (MMICs) for improving overall performance and add new applications.

We use phonon hydrodynamics with a surface slip flow as a simplified macroscopic model accounting for a reduction in lateral thermal conductivity in nanosystems. For high Knudsen numbers, the corresponding effective thermal conductivity decreases linearly with the radius or the width, in contrast with the quadratic dependence predicted by usual phonon hydrodynamics. The linear dependence is accounted for by the surface slip flow. The difference in the expressions for the surface tangential flow in the hydrodynamic and the diffusive regimes is commented on and the influence of boundary conditions on the form of the effective thermal conductivity is explored.