Ultrahigh Sensitivity Detection of Terahertz Waves Using Quantum Dots.

Electrons in a single electron transistor (SET) are transported one by one from source to drain based on the coulomb blockade mechanism. The transport rate is sensitively influenced by the presence of event a single electron charge located near the quantum dot. Based on this characteristic, we propose a Double Quantum Dot (DQD) SET to manipulate the presence of an electron in Quantum Dot (QD) as an implementation of a single-electron logic concept. The existence of an electron in the QD is used to represent logic 0 (no electron in QD) or logic 1 (an electron in QD). The logic states are sensed by a SET charge detector. Design of circuit based on DQD and SET charge detector are simulated by using SIMON 2.0 software. From the simulation, we have successfully developed a two-bit logic circuit by controlling the presence of an electron in DQD. We found that the existence of an electron in QD can be controlled by application of a gate voltage larger than 190µV. Gate should be separated from QD by a nontunnel capacitor of 500 aF. No larger than 1 aF of interdot tunnel capacitance is required to isolate the QD from one to another. The existence of an electron in QD is successfully detected by SET based charge detector.

Scaling of semiconductor devices has pushed CMOS devices close to fundamental limits. The remarkable success story of Moore's law during the last 40 years, predicting the evolution of electronic device performances related to miniaturization, has always been respected. However, electron device challenges are now much more complex. In order to keep Moore's law living, device scaling-down is not enough. So called material and geometry technology boosters have been introduced. High-k dielectrics, silicon-on-insulator substrates or strained silicon are some booster examples, used at sub-micron scale. This work proposes an investigation of a hybrid CMOS/SET technology, based on gate-all-around silicon nanowires. It is shown that a silicon nanowire can serve two device purposes: first as innovative 3D metal-oxide-semiconductor field effect transistor (MOSFET), and second as single electron transistor (SET). The SET is a solid-state electronic device, which controls the transport of a unique or a few electrons. SET functions are not limited by its nano-scaled structure. The smaller is the better. SET consists of a small conductive quantum dot, called island, connected to two reservoirs acting as drain and source by tunnel junctions. The size of the island should be as small as possible – typical values ranging from 1 to 4nm – in order to have room temperature operation. Electron transport from source to island, and from island to drain is controlled by a capacitively coupled gate. The nano-scale size of the island makes the carrier electrostatic repulsion efficient. This effect is called Coulomb Blockade. SET advantages are an ultra-reduced size and a very low power consumption, while SET challenges are variability related to dimension control and background charge effect, room temperature operation and a reduced fan-out. The first chapter of this thesis introduces ideas of up-to-date MOS and SET devices and shows how to combine MOS and SET to obtain original electronic functions. The second chapter is a discussion of the nanowire as a technology platform for the integration of SET devices and also presents TCAD simulation results. Key contributions are reported in chapter three. This is the original and complete description of the different top-down processes used for the integration of a gate-all-around MOS/SET platform. Samples integrated at the EPFL Center of Microtechnology (CMI) are presented. Different approaches have been studied and used in order to overcome lithographic limitations, and to have a fast and reliable integration at moderate cost. This includes auto-aligned techniques, focused ion beam prototyping, and top-down local-SOI and true-SOI nanowires. The local-SOI technique based on silicon nano-channel wires has given the best results and offers a lot of flexibility in term of wire cross-sections and shapes. Gate-all-around silicon nanowires, obtained by sacrificial etching and self-limited oxidation, with a circular 5nm diameter cross-section have been characterized. Chapter four is the validation by measurements of both SET and MOS devices. Excellent room temperature characteristics have been observed in MOS structures, while both ID-VG Coulomb oscillations and ID-VD Coulomb gap are observed on smaller structures at cryogenic temperature (T<20K). The process described has many advantages in comparison with bottom-up grown wires, such an excellent crystallinity of the channel and ohmic source-drain contacts. Finite elements simulations have also highlighted the influence of carrier mobility, channel length and cross-section shape in our defined nanowire structures. The last chapter of this report concludes this work and gives perspectives on the near future. The hybrid combination of silicon nanowire MOSFET and SET can definitely be an appealing approach in order to bridge the gap between emerging nanoelectronic devices and more traditional CMOS.

Atomic-scale probing of metallic and semiconductor nanostructures

Polarization selective magneto-optical study on the coupled quantum dots using resonant excitation

We investigate a one-component, quasi–zero-dimensional, quantum plasma exposed to a parabolic potential and an applied magnetic field in the symmetric gauge. If the size of such a system as can be realized in the semiconducting quantum dots is on the order of the de Broglie wavelength, the electronic and optical properties become highly tunable. Then the quantum size effects challenge the observation of many-particle phenomena such as the magneto-optical absorption, Raman intensity, and electron energy loss spectrum. An exact analytical solution of the problem leads us to infer that these many-particle phenomena are, in fact, dictated by the generalized Kohn's theorem in the long-wavelength limit. Maneuvering the confinement and/or the magnetic field furnishes the resonance energy capable of being explored with the FIR, Raman, or electron energy loss spectroscopy. This implies that either of these probes should be competent in observing the localized magnetoplasmons in the system. A deeper insight into the physics of quantum dots is paving the way for their implementation in diverse fields such as quantum computing and medical imaging.

Magnetooptical study of shallow donors in InSb under high pressures

Circularly polarized luminescence of CdMnSe quantum dots in magnetic fields up to 5 T is studied for nominal Mn concentrations of 0%, 1%, and 2% by using a photoelastic modulator technique. The exciton g factors as well as spin relaxation times were determined from the polarized luminescence taking into account the exciton lifetimes, which were also extracted by means of time-resolved photoluminescence spectroscopy. For quantum dots without Mn and with 2% Mn exciton g factors of −1.62 and +1.32, respectively, were found. The quantum dots with 1% Mn show a vanishing small value of g for small excitation powers. For this structure the polarization properties are dominated by the optical orientation. Interestingly, for the 1% Mn quantum dots with increasing excitation power considerable changes of the polarization and the exciton g factor were observed which are interpreted in terms of heating effects. From the power dependence indirect heating via phonons and above a critical value direct heating due to photocarriers were identified to result in drastic changes of the circular polarized quantum dot emission.

Real-time sensing of (spin-dependent) single-electron tunneling is fundamental to electrical readout of qubit states in spin quantum computing. Here, we demonstrate the feasibility of detecting such single-electron tunneling events using an atomically planar charge sensing layout, which can be readily integrated in scalable quantum computing architectures with phosphorus-donor-based spin qubits in silicon (Si:P). Using scanning tunneling microscopy (STM) lithography on a Si(001) surface, we patterned a single-electron transistor (SET), both tunnel and electrostatically coupled to a coplanar ultrasmall quantum dot, the latter consisting of approximately four P donors. Charge transitions of the quantum dot could be detected both in time-averaged and single-shot current response of the SET. Single electron tunneling between the quantum dot and the SET island on a time-scale (τ ∼ ms) two-orders-of-magnitude faster than the spin-lattice relaxation time of a P donor in Si makes this device geometry suitable for projective readout of Si:P spin qubits. Crucial to scalability is the ability to reproducibly achieve sufficient electron tunnel rates and charge sensitivity of the SET. The inherent atomic-scale control of STM lithography bodes extremely well to precisely optimize both of these parameters.

We propose to employ a selective formation method to embed an isolated self assembled quantum dot into a n-i-n junction to implement single electron transistors (SETs). The absorption and emission spectrum of SETs are theoretically studied by the Keldysh Green function method. The electronic levels and Coulomb interactions of electrons of InAs quantum dot (QD) are evaluated by an effective mass model. It is found that Coulomb interaction and level mixing in the many body open system lead to double peaks associated with the intraband transitions involving two lowest levels of the QD. We can electrically control the SETs as a single-photon source and double-photon source for 10 μm wavelength. The single photon source can be used in the application of quantum cryptograph, and the double-photon source can be utilized in the teleportation.

Energy levels of an impurity atom and its binding energy in a quantum dot with or without electron-phonon interactions are obtained by the second-order perturbation theory. The dot is confined laterally by a parabolic potential in quantum-well structures. The energy correction is expressed as a function of the strength of lateral confinement, the applied magnetic field, and the thickness of the quantum dot in question. It is shown that the binding energy depends sensitively on the thickness if it is of the order of the polaron size or less. In the case of thicker quantum dots, the finite thickness reduces the binding energy by approximately 10%.

We investigate the possibility to implement low-energy consuming binary logic gates by means of charge sensing in electrostatically defined quantum dots (QDs) formed in single electron transistors (SETs). In the proposed logic devices, the input signals are represented by the voltage levels at the SET's gate electrodes, and the output is determined by the electrical current in the charge sensor (CS) circuit. In contrast to previous proposals of SET-based logic, the output signal does not depend on the electrical current through SET, but is defined by its charge configuration in the Coulomb blockade regime. Such a logic implementation suppresses the shot noise without increasing energy consumption.

Multi-excitonic (N=1,2 and 3) quantum dots in magnetic field: Analytical mapping of correlations (exchange) by multipole expansion

Preface V. Chandrasekhar, et al. Introduction N.O. Birge, et al. Finite Size Effects in Kondo Alloys. Effect of Disorder on the Kondo Behavior of Thin Cu(Mn) Films T.M. Jacobs, N. Giordano. The Kondo Effect and Weak Localization P. Phillips, I. Martin. Surface Magnetic Anisotropy of Kondo Impurities Induced by Spin-Orbit Scattering O. Ujsaghy. Finite Size Effects in Spin Glass Alloys. Thermopower of Mesoscopic Spin Glasses C. Strunk, et al. Shape-Induced Magnetic Anisotropy in Dilute Magnetic Alloys V.N. Gladilin, et al. Zero-Bias Transport Anomaly in Metallic Nanobridges H.B. Weber, et al. Conductance Noise and Irreversibility in Diluted Magnetic Semiconductors J. Jaroszynski, et al. Point Contact Spectroscopy and Tunnelling Spectroscopy of Kondo Impurities. Enhancement of Kondo Temperature in Nanometer-Size Point Contacts I.K. Yanson, et al. Scanning Tunnelling Spectroscopy of a Single Kondo Impurity R. Berndt, W.-D. Schneider. Two-Level Systems and Dephasing in Thin Metal Structures. Two-Channel Kondo Effect from Tunneling Impurities G. Zarand. Electron Decoherence at Zero Temperature P. Mohanty. Probing Interactions in Mesoscopic Gold Wires F. Pierre, et al. Kondo Effect in Non-Equilibrium J. Kroha. Kondo Effect in Quantum Dots. Tunneling Through a Quantum Dot A. Schiller. Electron Transport Through Quantum Dots: An Unusual Kondo Effect S. De Franceschi, et al. The Kondo Effect in a Single-Electron Transistor D. Goldhaber-Gordon, et al. Contributions Related to Poster Presentations. Flux Dependent Dielectric Response of Stacked Nanoscopic Rings K.-H. Ahn, P. Fulde.High-Frequency Response of Two-Level Systems in NixNb1-x Metallic Point Contacts O.P. Balkashin, et al. Giant Magnetoresistance of a Single Interface J. Balogh, et al. Fermi Edge Singularities in Transport Through Quantum Dots E. Bascones, et al. Theory of Magnetoresistance in Films of Dilute Magnetic Alloys L. Borda. Current and Shot Noise in a Ferromagnetic Double Tunnel Junction with an Atomic Size Spacer B.R. Bulka, et al. Positive Domain-Wall Magnetoresistance of Ferromagnetic Point Contacts J. Caro, et al. Enhancement of Kondo Effect Due to Spin-Singlet-Triplet Competition in Quantum Dots M. Eto, Y.V. Nazarov. Diagrammatic Theory of the Anderson Impurity Mod with Finite Coulomb Interaction K. Haule, et al. Generalized Conductance Sum Rule in Atomic Break Junctions S. Kirchner, et al. Is CeNiSn a Kondo Semiconductor? Y.G. Naidyuk, et al. The Influence of Single Magnetic Impurities on the Conductance of Quantum Microconstrictions A. Namiranian, et al. 7 Additional Chapters.

In this thesis the optical properties of self-assembled III-V semiconductor nanostructures are investigated by analyzing the photoluminescence (PL) with and without an external magnetic field. The studied PL is a result of the recombination of electrons and holes confined in the nanostructures. In this thesis we directly correlate the size, shape, composition and topology of the nanostructures to their magnetic and optical properties and study the influence of an electrical contact in the close proximity of these nanostructures. One of the most interesting nanostructures are quantum dots (QDs), which are able to confine charge carriers in all three dimensions. Both the exciton g-factor gex and the exciton diamagnetic coefficient ad determine the behavior of excitons confined in a QD in a magnetic field and provide further insight in the energy level structure of these nanostructures. The exciton Zeeman splitting is proportional to gex and is a result of the different spin configurations of the electron and hole, which constitute the exciton. The diamagnetic shift is a result of the additional confinement provided by the magnetic field B, and is proportional adB2, where ad scales with the lateral extend of the quantum dot. PL experiments on a large ensemble of InAs/GaAs QDs, as well as on individual InAs/GaAs QDs are performed. For these dots ad and gex have been determined. Importantly, there is a trend between gex and the emission energy E0: for larger emission energy a more negative value of the exciton g-factor is observed. From the power and temperature dependence of the ensemble PL it is shown that the sample consists of QDs with different height, where the highest dots correspond to the smallest emission energies. Moreover, QDs with larger ad have a more positive g-factor. From this it is inferred that quantum dots with an overall larger size have a less negative value of gex. By implementing the QDs in a Schottky device in close proximity to an electrical contact it is possible to tune the charged state of the exciton in the dot. We show that charge-tunable InAs/GaAs quantum dots also allow to study many-body interactions between a localized QD state and a Fermi sea of electrons. As a result of these many-body interactions new optical transitions are observed in the PL spectra. The same chargetunable quantum dots are also investigated in a magnetic field. Instead of the positive diamagnetic shift observed for the majority of the quantum dots, two different types of negative diamagnetic shift are reported. The shallow character of our quantum dots causes a negative quadratic diamagnetic shift for the highly negatively charged exciton complexes in line with predictions for shallow quantum dots. The second type of negative diamagnetic shift is observed even for the neutral exciton and is strongly linearly dependent on the magnetic field. We also investigated quantum dots grown from different semiconductor materials. InAs/InP quantum dots are studied using AFM, X-STM, macro-PL and micro-PL. Macro PL and X-STM measurements show that the studied InAs/InP dots have a multimodal height distribution. Single quantum dot luminescence, carried out on a large number of dots, shows a strong correlation between exciton g-factor, diamagnetic coefficient and emission energy. In fact, the strong dependence of gex on the emission energy results in a sign change of the exciton g-factor. In correspondence with what we found on the InAs/GaAs QDs, we find that dots having a smaller overall size will have a more negative gex as compared to quantum dots of larger overall size. We also show that for several quantum dots the exciton g-factor is quenched. A theoretical model is used to calculate the effect of the quantum dot size on the g-factor. The model is both qualitatively and quantitatively in good agreement with the experimental obtained results. Nanostructures of a different topology are self-assembled InAs/GaAs quantum rings (QRs). Using magnetization measurements on a large ensemble of quantum rings we demonstrate the presence of the Aharonov-Bohm (AB) effect in these nanostructures. Moreover, a model based on the X-STM measurements on these nanostructures reproduces the magnetic field position of this oscillation. The optical properties of these QRs are investigated for magnetic fields up to 30T for a large ensemble of quantum rings and for individual rings. Although the excitonic AB effect is suppressed in these nanostructures, the ring character of our nanostructures gives rise to non-equidistant energy level splittings and into a magnetic field induced splitting of each excited state into two states in the ensemble PL. This is different to what has been observed in measurements on quantum dots. The calculations based on the same model show a qualitative agreement with the experimental data, and allow us to identify the different PL peaks. Furthermore, analyzing exciton lines with high resolution revealed an anomalous quadruplet splitting, which is different as the one reported for InAs/GaAs quantum dots. Although the origin of this splitting is still unclear, it is most likely related to the singlet to triplet transition of singly charged excitons. Finally the magneto-luminescence experiments on type II self-assembled InP/GaAs quantum dots are discussed. For these dots the hole is located outside the quantum dot, whereas the electron is confined inside the quantum dot creating a type II exciton. The magneto-luminescence of the ensemble of dots did not resolve any oscillatory behavior related to the AB effect in contrast to experiments reported on the same sample. Decisive magneto-luminescence measurements on individual dots did also not reveal any AB related phenomena within the experimental resolution of 40 µeV. These results show the necessity to study subtle magneto-luminescence properties of nanostructures on an individual dot level, rather than on a large ensemble of dots.

In this paper theoretical concepts based on analytical models for Single Electron Transistor (SET) are presented. It has been observed that the fabrication technology has reached on its limits for the MOS feature size, beyond which further scaling of the channel is not achievable. Due to which SET is considered as the future of the complex IC fabrication by replacing MOS technology, having small quantum dot or island, instead of channel. Working of the SET is based on the Coulomb blockade principle which is the heart of the technology. Quantum mechanics is the physics used for explaining the tunnelling of single electron of SET, which says that the energy levels are quantized not continuous. Main reason behind the advancement in SET is the market requirement of the low power, high density and fast switching devices. These are possible by SET; no doubt speed is an issue for the memories designed with SET, with its low gain and high input impedance. In the review, mathematical model including tunnelling effect in junction, coulomb blockade, current equation, free energy equation based on Shcrodinger's wave equation are presented.