Electrons In Semiconductor Nanostructures Research Articles

Plasmon-induced carrier polarization in semiconductor nanocrystals.

Spintronics 1 and valleytronics 2 are emerging quantum electronic technologies that rely on using electron spin and multiple extrema of the band structure (valleys), respectively, as additional degrees of freedom. There are also collective properties of electrons in semiconductor nanostructures that potentially could be exploited in multifunctional quantum devices. Specifically, plasmonic semiconductor nanocrystals3-10 offer an opportunity for interface-free coupling between a plasmon and an exciton. However, plasmon-exciton coupling in single-phase semiconductor nanocrystals remains challenging because confined plasmon oscillations are generally not resonant with excitonic transitions. Here, we demonstrate a robust electron polarization in degenerately doped In2O3 nanocrystals, enabled by non-resonant coupling of cyclotron magnetoplasmonic modes 11 with the exciton at the Fermi level. Using magnetic circular dichroism spectroscopy, we show that intrinsic plasmon-exciton coupling allows for the indirect excitation of the magnetoplasmonic modes, and subsequent Zeeman splitting of the excitonic states. Splitting of the band states and selective carrier polarization can be manipulated further by spin-orbit coupling. Our results effectively open up the field of plasmontronics, which involves the phenomena that arise from intrinsic plasmon-exciton and plasmon-spin interactions. Furthermore, the dynamic control of carrier polarization is readily achieved at room temperature, which allows us to harness the magnetoplasmonic mode as a new degree of freedom in practical photonic, optoelectronic and quantum-information processing devices.

Image charge models for accurate construction of the electrostatic self-energy of 3D layered nanostructure devices

Efficient analytical image charge models are derived for the full spatial variation of the electrostatic self-energy of electrons in semiconductor nanostructures that arises from dielectric mismatch using semi-classical analysis. The methodology provides a fast, compact and physically transparent computation for advanced device modeling. The underlying semi-classical model for the self-energy has been established and validated during recent years and depends on a slight modification of the macroscopic static dielectric constants for individual homogeneous dielectric regions. The model has been validated for point charges as close as one interatomic spacing to a sharp interface. A brief introduction to image charge methodology is followed by a discussion and demonstration of the traditional failure of the methodology to derive the electrostatic potential at arbitrary distances from a source charge. However, the self-energy involves the local limit of the difference between the electrostatic Green functions for the full dielectric heterostructure and the homogeneous equivalent. It is shown that high convergence may be achieved for the image charge method for this local limit. A simple re-normalisation technique is introduced to reduce the number of image terms to a minimum. A number of progressively complex 3D models are evaluated analytically and compared with high precision numerical computations. Accuracies of 1% are demonstrated. Introducing a simple technique for modeling the transition of the self-energy between disparate dielectric structures we generate an analytical model that describes the self-energy as a function of position within the source, drain and gated channel of a silicon wrap round gate field effect transistor on a scale of a few nanometers cross-section. At such scales the self-energies become large (typically up to ~100 meV) close to the interfaces as well as along the channel. The screening of a gated structure is shown to reduce the self-energy relative to un-gated nanowires.

Eigenstate modelling in arbitrary shaped nanostructres with gradual heterointerfaces

We consider the problem of evaluating the energy levels and envelope wavefunctions of electrons in semiconductor nanostructures with a smooth confining potential, such as inter- diffused quantum dots or quantum wells. We show how Gaussian functions can be used to model various confinement potentials in 1D, 2D and 3D. We develop a fast and accurate numerical algorithm for solving the one-particle Schrodinger equation with a position-dependent effective mass for this type of potential. We examine the convergence and accuracy of our algorithm. This method can be used to analyze experimental data, such as the PL spectra of quantum wells, wires and dots and the charge carrier transport through various nanostructures. GaAs/Al0.3Ga0.7As system is used in calculations.

Quantum nanostructures in strongly spin-orbit coupled two-dimensional systems

Recent progress in experimental studies of low-dimensional systems with strong spin-orbit coupling poses a question on the effect of this coupling on the energy spectrum of electrons in semiconductor nanostructures. It is shown in the paper that this effect is profound in the strong coupling limit. In circular quantum dots a soft mode develops, in strongly elongated dots electron spin becomes protected from the effects of the environment, and the lower branch of the energy spectrum of quantum wires becomes nearly flat in a wide region of the momentum space.

Optical mapping of the boundary of a two-dimensional electron gas by a nearfield optical microscopy

We constructed a dilution-refrigerator-based near-field scanning optical microscope (NSOM) which operates at temperatures down to 200 mK. Our NSOM enables us to investigate spatial properties of the electrons in semiconductor nanostructures with a subwavelength spatial resolution. We report on a result of the optical mapping of the boundary of a two-dimensional electron gas in a gated Hall-bar structure of a GaAs/AlGaAs modulation-doped single heterojuntion.

Reliability of the Heitler-London approach for the exchange coupling between electrons in semiconductor nanostructures

We calculate the exchange coupling $J$ between electrons in a double-well potential in a two-dimensional semiconductor environment within the Heitler-London (HL) approach. Two functional forms are considered for the single-well potential. We show that by choosing an appropriate and relatively simple single-electron variational wave function it is possible, within the HL approach, to significantly improve the estimates for $J$. In all cases the present scheme overcomes the artifacts and limitations at short interdot distances, previously attributed to the HL method, where unphysical triplet ground states have been found, and leads to an overall agreement with analytic interpolated expressions for $J$ obtained for a donor-type model potential.

Far Infrared Coherent Synchrotron Edge Radiation at ANKA

The region of the electromagnetic spectrum between 0.3 and 20 THz is a “frontier region” of spectroscopy in physics, chemistry, biology, material sciences, and medicine [1]. Radiation in the THz range allows the investigation and excitation of phenomena with time constants of about 1 ps. Examples on this time scale are atomic electron orbits in highly excited Rydberg states, rotations of small molecules and modes of collective oscillations of proteins and polar liquids like water. Resonances of electrons in semiconductor nanostructures and band gaps of superconductors can equally be studied with THz radiation. The THz region lies above the frequencies of traditional electronics but below the range of optical and infrared generators. Until now, possibilities to generate sufficiently intense and brilliant radiation at the wavelengths in question were scarce and the region was therefore named the “THz gap” (Figure 1). In accelerators such highly intense coherent THz radiation can be generated under special conditions when the electron bunch length is comparable to the wavelength of the emitted radiation [2,3]. Since the emission of long wavelengths is suppressed by shielding effects of the vacuum chamber, the bunch length needs to be sufficiently short in order to yield observable intensities.

Spin interferometry with electrons in nanostructures: A road to spintronic devices

The wave nature of electrons in semiconductor nanostructures results in spatial interference effects similar to those exhibited by coherent light. The presence of spin–orbit coupling renders interference in spin space and in real space interdependent, making it possible to manipulate the electron’s spin state by addressing its orbital degree of freedom. This suggests the utility of electronic analogs of optical interferometers as blueprints for new spintronics devices. We demonstrate the usefulness of this concept using the Mach–Zehnder interferometer as an example. Its spin-dependent analog realizes a spin-controlled field-effect transistor without magnetic contacts and may be used as a quantum logical gate.

Observation of optical phonon instability induced by drifting electrons in semiconductor nanostructures

We have experimentally proven the Cerenkov generation of optical phonons by drifting electrons in a semiconductor. We observe an instability of the polar optical phonons in nanoscale semiconductors that occurs when electrons are accelerated to very high velocities by intense electric fields. The instability is observed when the electron drift velocity is larger than the phase velocity of optical phonons and rather resembles a “sonic boom” for optical phonons. The effect is demonstrated in p–i–n semiconductor nanostructures by using subpicosecond Raman spectroscopy.

Kinetic modelling of electron tunneling processes in quantum dots coupled to field-effect transistors

Transport of electrons in semiconductor nano-structures exhibits many features that are a consequence of quantum confinement and Coulomb blockade. A quantum dot coupled to a metal-oxide-semiconductor transistor's channel region is one example of such a structure with utility as a dense semiconductor memory. The memory state of this unit cell is a function of the number of electrons stored in the quantum dot and is sensed by the conduction in the channel. We describe a kinetic approach, based on a master equation, for modelling the injection and ejection of electrons into and from the quantum dot, and compare numerical results with experimental results for the silicon/silicon dioxide system where such memory structures have been achieved.

Quantum hydrodynamics and virial theorems

This paper elucidates how many-particle quantum hydrodynamic equations underlie the virial formula, including its surface pressure term. It shows the relation of the field variables to the particle Wigner function, proposes a many-particle ‘‘liquid’’ model to approximate these variables, and discusses its application to electrons in semiconductor nanostructures.