REVIEW Semiconductor devices generally take advantage of the charge of electrons, whereas magnetic materials are used for recording information involving electron spin. To make use of both charge and spin of electrons in semiconductors, a high concentration of magnetic elements can be introduced in nonmagnetic III-V semiconductors currently in use for devices. Low solubility of magnetic elements was overcome by low-temperature nonequilibrium molecular beam epitaxial growth, and ferromagnetic (Ga,Mn)As was realized. Magnetotransport measurements revealed that the magnetic transition temperature can be as high as 110 kelvin. The origin of the ferromagnetic interaction is discussed. Multilayer heterostructures including resonant tunneling diodes (RTDs) have also successfully been fabricated. The magnetic coupling between two ferromagnetic (Ga,Mn)As films separated by a nonmagnetic layer indicated the critical role of the holes in the magnetic coupling. The magnetic coupling in all semiconductor ferromagnetic/nonmagnetic layered structures, together with the possibility of spin filtering in RTDs, shows the potential of the present material system for exploring new physics and for developing new functionality toward future electronics.

https://science.sciencemag.org/content/sci/281/5379/951.full.pdf

Making Nonmagnetic Semiconductors Ferromagnetic

Electron transport experiments on two lateral quantum dots coupled in series are reviewed. An introduction to the charge stability diagram is given in terms of the electrochemical potentials of both dots. Resonant tunneling experiments show that the double dot geometry allows for an accurate determination of the intrinsic lifetime of discrete energy states in quantum dots. The evolution of discrete energy levels in magnetic field is studied. The resolution allows one to resolve avoided crossings in the spectrum of a quantum dot. With microwave spectroscopy it is possible to probe the transition from ionic bonding (for weak interdot tunnel coupling) to covalent bonding (for strong interdot tunnel coupling) in a double dot artificial molecule. This review is motivated by the relevance of double quantum dot studies for realizing solid state quantum bits.

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Electron transport through double quantum dots

The goal of this paper is to review in brief the basic physics of single-election devices, as well as their-current and prospective applications. These devices based on the controllable transfer of single electrons between small conducting "islands", have already enabled several important scientific experiments. Several other applications of analog single-election devices in unique scientific instrumentation and metrology seem quite feasible. On the other hand, the prospect of silicon transistors being replaced by single-electron devices in integrated digital circuits faces tough challenges and remains uncertain. Nevertheless, even if this replacement does not happen, single electronics will continue to play an important role by shedding light on the fundamental size limitations of new electronic devices. Moreover, recent research in this field has generated some by-product ideas which may revolutionize random-access-memory and digital-data-storage technologies.

Single-electron devices and their applications

Recent research results pertaining to InN, GaN and AlN are reviewed, focusing on the different growth techniques of Group III-nitride crystals and epitaxial films, heterostructures and devices. The chemical and thermal stability of epitaxial nitride films is discussed in relation to the problems of deposition processes and the advantages for applications in high-power and high-temperature devices. The development of growth methods like metalorganic chemical vapour deposition and plasma-induced molecular beam epitaxy has resulted in remarkable improvements in the structural, optical and electrical properties. New developments in precursor chemistry, plasma-based nitrogen sources, substrates, the growth of nucleation layers and selective growth are covered. Deposition conditions and methods used to grow alloys for optical bandgap and lattice engineering are introduced. The review is concluded with a description of recent Group III-nitride semiconductor devices such as bright blue and white light-emitting diodes, the first blue-emitting laser, high-power transistors, and a discussion of further applications in surface acoustic wave devices and sensors.

https://iopscience.iop.org/article/10.1088/0022-3727/31/20/001/pdf

Growth and applications of Group III-nitrides

When an individual molecule1, nanocrystal2,3,4, nanotube5,6 or lithographically defined quantum dot7 is attached to metallic electrodes via tunnel barriers, electron transport is dominated by single-electron charging and energy-level quantization8. As the coupling to the electrodes increases, higher-order tunnelling and correlated electron motion give rise to new phenomena9,10,11,12,13,14,15,16,17,18,19, including the Kondo resonance10,11,12,13,14,15,16. To date, all of the studies of Kondo phenomena in quantum dots have been performed on systems where precise control over the spin degrees of freedom is difficult. Molecules incorporating transition-metal atoms provide powerful new systems in this regard, because the spin and orbital degrees of freedom can be controlled through well-defined chemistry20,21. Here we report the observation of the Kondo effect in single-molecule transistors, where an individual divanadium molecule20 serves as a spin impurity. We find that the Kondo resonance can be tuned reversibly using the gate voltage to alter the charge and spin state of the molecule. The resonance persists at temperatures up to 30 K and when the energy separation between the molecular state and the Fermi level of the metal exceeds 100 meV.

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Kondo resonance in a single-molecule transistor

We study atomiclike properties of artificial atoms by measuring Coulomb oscillations in vertical quantum dots containing a tunable number of electrons starting from zero. At zero magnetic field the energy needed to add electrons to a dot reveals a shell structure for a two-dimensional harmonic potential. As a function of magnetic field the current peaks shift in pairs, due to the filling of electrons into spin-degenerate single-particle states. When the magnetic field is sufficiently small, however, the pairing is modified, as predicted by Hund’s rule, to favor the filling of parallel spins. [S00319007(96)01418-4]

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Shell Filling and Spin Effects in a Few Electron Quantum Dot.

Studies of the ground and excited states in semiconductor quantum dots containing 1 to 12 electrons showed that the quantum numbers of the states in the excitation spectra can be identified and compared with exact calculations. A magnetic field induces transitions between the ground and excited states. These transitions were analyzed in terms of crossings between single-particle states, singlet-triplet transitions, spin polarization, and Hund’s rule. These impurity-free quantum dots allow “atomic physics” experiments to be performed in magnetic field regimes not accessible for atoms.

https://science.sciencemag.org/content/sci/278/5344/1788.full.pdf

Excitation Spectra of Circular, Few-Electron Quantum Dots

Reduction of quantized conductance at low temperatures observed in 2 to 10 μm-long quantum wires

Vertically coupled dots can be employed to study the filling of electrons in quantum dot molecules. When the dots are quantum mechanically strongly coupled, the electronic states in the system are not localized, and the Coulomb diamonds and addition energy spectra of the molecule resemble those of a single artificial atom. When the dots are quantum mechanically weakly coupled, the electronic states in the system are usually localized, although the dots can be electrostatically coupled, and this leads to a pairing of conductance peaks.

Quantum dot molecules

We have measured electron transport through a vertical quantum dot containing a tunable number (between 0 and 40) of electrons. Over a region of the magnetic field the electrons are spin polarized and occupy successive angular momentum states. This is the maximum-density-droplet ( MDD) state. The stability region where the MDD is the ground state decreases for increasing electron number. The instability of the MDD and other transitions in this high B region are accompanied by a redistribution of charge which abruptly changes the area of the electron droplet. [S0031-9007(99)08873-0] PACS numbers: 73.20.Dx, 71.45.Gm, 73.40.Hm In this Letter we study quantum dots [1,2] in the quantum Hall regime and exploit the fact that dots contain a tunable and well-defined number of electrons. In particular, we consider the spin-polarized, maximumdensity-droplet ( MDD) state at high magnetic fields that corresponds to filling factor n › 1 in a large two dimensional electron gas. The stability of this state is set by a balance of forces [3‐6] acting on this finite electron system, namely, the inward force of the confining potential, the repulsive force of the direct Coulomb interaction between electrons, and a binding force due to the exchange interaction. In addition, Zeeman energy and correlation effects are important. As the magnetic field and the electron number are changed, the relative strengths of these forces are tuned, which induces transitions between the MDD and other states. Earlier these transitions were measured in the linear response regime [1,7]. We report detailed measurements in linear and nonlinear transport showing that the transitions are accompanied by a significant redistribution of chargein this many-body system. Our vertical quantum dot is made from a double barrier resonant tunneling structure with an InGaAs well, AlGaAs barriers, and n-doped GaAs source and drain contacts [8]. The device is processed in the shape of a submicron circular pillar with a diameter of 0.54 mm, and a self-aligned gate surrounds it. We discuss data taken on one particular device, but comparable results have been obtained on several devices. A magnetic field, B, is applied perpendicular to the plane in which the electrons are confined. The energy spectrum of the quantum dot is derived from transport experiments at a temperature of 100 mK in the Coulomb blockade regime. A dc source-drain voltage, VSD, is applied and the current, I, is measured versus gate

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Takashi Honda

Papers

Shell Filling and Spin Effects in a Few Electron Quantum Dot.

Excitation Spectra of Circular, Few-Electron Quantum Dots

Reduction of quantized conductance at low temperatures observed in 2 to 10 μm-long quantum wires

Quantum dot molecules

Maximum-density droplet and charge redistributions in quantum dots at high magnetic fields