Molecules of benzene-1,4-dithiol were self-assembled onto the two facing gold electrodes of a mechanically controllable break junction to form a statically stable gold-sulfur-aryl-sulfur-gold system, allowing for direct observation of charge transport through the molecules. Current-voltage measurements at room temperature demonstrated a highly reproducible apparent gap at about 0.7 volt, and the conductance-voltage curve showed two steps in both bias directions. This study provides a quantative measure of the conductance of a junction containing a single molecule, which is a fundamental step in the emerging area of molecular-scale electronics.

https://www.eng.yale.edu/reedlab/publications/83%20BDT%20Science97.pdf

Conductance of a Molecular Junction

The properties of quasi-two-dimensional semiconductor quantum dots are reviewed. Experimental techniques for measuring the electronic shell structure and the effect of magnetic fields are briefly described. The electronic structure is analyzed in terms of simple single-particle models, density-functional theory, and "exact" diagonalization methods. The spontaneous magnetization due to Hund's rule, spin-density wave states, and electron localization are addressed. As a function of the magnetic field, the electronic structure goes through several phases with qualitatively different properties. The formation of the so-called maximum-density droplet and its edge reconstruction is discussed, and the regime of strong magnetic fields in finite dot is examined. In addition, quasi-one-dimensional rings, deformed dots, and dot molecules are considered. (Less)

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Electronic structure of quantum dots

Progress in semiconductor technology has enabled the fabrication of structures so small that they can contain just one mobile electron. By varying controllably the number of electrons in these 'artificial atoms' and measuring the energy required to add successive electrons, one can conduct atomic physics experiments in a regime that is inaccessible to experiments on real atoms.

Electrons in artificial atoms

The ongoing miniaturization of solid state devices often leads to the question: “How small can we make resistors, transistors, etc., without changing the way they work?” The question can be asked a different way, however: “How small do we have to make devices in order to get fundamentally new properties?” By “new properties” we particularly mean those that arise from quantum mechanics or the quantization of charge in units of eeffects that are only important in small systems such as atoms. “What kind of small electronic devices do we have in mind?” Any sort of clustering of atoms that can be connected to source and drain contacts and whose properties can be regulated with a gate electrode. Practically, the clustering of atoms may be a molecule, a small grain of metallic atoms, or an electronic device that is made with modern chip fabrication techniques. It turns out that such seemingly different structures have quite similar transport properties and that one can explain their physics within one relatively simple framework. In this paper we investigate the physics of electron transport through such small systems.

Electron Transport in Quantum Dots

The resonant tunneling diode (RTD) has been widely studied because of its importance in the field of nanoelectronic science and technology and its potential applications in very high speed/functionality devices and circuits. Even though much progress has been made in this regard, additional work is needed to realize the full potential of RTD's. As research on RTD's continues, we will try in this tutorial review to provide the reader with an overall and succinct picture of where we stand in this exciting field or research and to address the following questions: What makes RTD's so attractive? To what extent can RTD's be modeled for design purposes? What are the required and achievable device properties in terms of digital logic applications? To address these issues, we review the device operational principles, various modeling approaches, and major device properties. Comparisons among the various RTD physical models and major features of RTD's, resonant interband tunneling diodes, and Esaki tunnel diodes are presented. The tutorial and analysis provided in this paper may help the reader in becoming familiar with current research efforts, as well as to examine the important aspects in further RTD developments and their circuit applications.

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Resonant tunneling diodes: models and properties

We have investigated the magnetoresistance of a two-dimensional electron gas subjected to a periodic potential of variable amplitude. The periodic potential, of period 300 nm, is generated with use of a gate deposited over a layer of patterned resist, and its amplitude is controlled by the gate voltage. At low gate voltages, two series of oscillations periodic in inverse magnetic field are observed. One series, at low magnetic field, is due to the periodic potential and the other is the usual Shubnikov--de Haas oscillations. The application of a small gate voltage generates an increase in the amplitude of the low-field oscillations, followed by a quenching of these oscillations as the gate voltage is increased further. In addition, a low-field positive magnetoresistance is generated, becoming larger with increasing gate voltage. These effects are explained within a semiclassical model of electron transport. Also the Shubnikov--de Haas oscillations quench as the amplitude of the potential increases. This is explained in terms of a broadening of the Landau levels.

Magnetoresistance of a two-dimensional electron gas in a strong periodic potential.

The fabrication and operation of a 1μm × 1μm gated GaAs/(AlGa)As resonant tunnelling diode is described. By biasing the gate the I/V characteristic can be varied and hence the negative differential resistance of the diode can be controlled. Using a wafer with an appropriate doping profile ensures that the maximum depletion due to the gate will occur close to the (AlGa)As tunnel barriers. When a large negative bias is applied to the gate extra structure develops in the I/V characteristic which may be related to the modification of the sub-band structure in the well due to the lateral quantum confinement of electrons by the gate. The potential of this fabrication technique is also discussed for resonant tunnelling and vertical field effect transistors.

Gated resonant tunnelling devices

We have measured the magnetoresistance at various temperatures of a two-dimensional electron gas subjected to a one-dimensional periodic potential. A series of oscillations periodic in inverse magnetic field is observed at low fields due to a resonant drift of electrons in the periodic potential, while at higher fields the Shubnikov--de Haas (SdH) oscillations are observed. The low-field oscillations persist to a much higher temperature than the SdH oscillations. We argue that the low-field oscillations are quenched when the thermal smearing of the cyclotron orbit diameter is equal to the period of the potential. Using this simple model, we show that the temperature at which the low-field oscillations are quenched is larger than that for the SdH oscillations by a factor ${\mathit{k}}_{\mathit{F}}$a/2, where a is the period and ${\mathit{k}}_{\mathit{F}}$ the Fermi wave vector. In addition, an explicit expression for the temperature dependence of the low-field oscillations is calculated and compared with our experimental data. Excellent agreement is found between the predicted and observed temperature dependences.

Temperature dependence of magnetoresistance oscillations in a two-dimensional electron gas subjected to a periodic potential

We have investigated the current‐voltage [I(V)] characteristics of a gated GaAs/(AlGa)As resonant tunneling diode. As the negative gate voltage is progressively increased I(V) becomes asymmetric. In particular the peak‐to‐valley ratio in forward bias is decreased from ≂20 to ≂1, but in reverse bias remains constant ≂20. This arises from a lateral variation of the voltage drop across the emitter tunnel barrier, which in forward bias leads to a smearing of the resonance. We discuss the relationship between our experiment and the low peak‐to‐valley ratios of two‐terminal submicron resonant tunneling diodes observed by other groups.

Edge effects in a gated submicron resonant tunneling diode

We have measured the magnetoresistance of a two-dimensional electron gas subjected to a one-dimensional periodic potential using structures with periods 150, 300, and 500 nm. In all structures we observe a low-field positive magnetotresistance, which extends up to a critical field at which a new form of magnetic breakdown occurs. This critical field is found to be inversely proportional to the period of the potential, in agreement with a model based on the semiclassical dynamics of electrons. In addition, we have developed a quantum-mechanical formulation of this problem in which the electron streaming, which generates the positive magnetoresistance, arises from the quasibound states of the combined magnetic and periodic potentials. The tunneling coefficient of these states is calculated and found to be small in the limit where ${\mathit{h}}^{2}$/2${\mathit{ma}}^{2}$, the kinetic energy of electrons with a wavelength equal to a, the period of the potential, is much less than the amplitude of the potential, as is the case experimentally. The motion of the electrons can then be treated semiclassically.

M. W. Dellow

Papers

Magnetoresistance of a two-dimensional electron gas in a strong periodic potential.

Gated resonant tunnelling devices

Temperature dependence of magnetoresistance oscillations in a two-dimensional electron gas subjected to a periodic potential

Edge effects in a gated submicron resonant tunneling diode

Magnetic breakdown of a two-dimensional electron gas in a periodic potential.