Bandgap calculation in Si quantum dot arrays using a genetic algorithm

Dipolar transformations of two-dimensional arrays of quantum dots are investigated theoretically. Homogeneous and non-homogeneous two-dimensional distributions are modeled by considering sections of the quantum dot array with different confinement potential. The dipolar transformations are tested by means of simulation of the dispersion of electrons of a beam traveling parallel to the plane of the quantum dot array. We calculate the electron energy loss function as a function of the temperature for different non-homogeneous distributions, considering several confinement potentials and coupling potentials among the quantum dots. Our results indicate that it would be possible by electron energy loss spectroscopy experiments to detect these dipolar transformations.

In this paper, a single-charge tunneling simulator, the Monte Carlo simulator of two-dimensional quantum dot array using standard single electron tunneling theory and Monte-Carlo method is introduced. The simulated results show that the two-dimensional quantum dot array has Coulomb charging effect at low temperatures. The quantum effect of the dot array exhibits great prospect in research and applications.

: Our research on electron-phonon and phonon-phonon processes in low-dimensional semiconductor nanostructures has been focused on the effects of a surfaces and interfaces on the electron energy momentum relaxation rates due to acoustic phonon emission, new specific mechanism of electron interaction with acoustic phonons in semiconductor heterostructures, and phonon-induced processes of charge can loosing phase coherence in heterostructures. Our research provides the necessary basis for the control of 1 dissipation and for the design of nanodevices with electron-phonon relaxation and dephasing rates required specific applications. .

Electron paramagnetic resonance and spin echo methods are used to probe the spin dynamics in two-dimensional quantum dot (QD) arrays with different shape of nanoclusters. Two types of QD structures were investigated: 1) with single shaped QDs (hut-clusters, having the ratio of height h to lateral size l, h/l = 0.1), and 2) with two groups of QDs, hut- and dome-clusters (h/l = 0.2). Both types of structures demonstrate the EPR signals from electrons localized in QD layers.The orientation dependence of EPR line width for first type structures is well described by model of spin relaxation through precession in the effective magnetic field, arising during tunneling between QDs due to structure-inversion-asymmetry. In the experiments on structures with dome-clusters the additional peculiarity, which can not be explained in the framework of precession model, is observed. The different orientation dependencies can be explained by different localization degree of electrons in investigated structures. Spin echo measurements give the longest spin decoherence time for structures with single shaped QDs.

We investigate theoretically the Zeeman effect on the electron and hole states in quantum dots. In frame of tight-binding approach, we propose a method of calculating the g factor for localized states. The principal values of the g factor for the ground electron and hole states in the self-assembled Ge / Si quantum dot are calculated. We find the strong g factor anisotropy — the components gxx, gyy are one order smaller than the gzz component, gzz=15.71, gxx=1.14, and gyy=1.76. The analysis of the wave function structure shows that the g factor of hole are mainly controlled by the contribution of the state with Jz=±(3/2), where Jz is the angular momentum projection on the growth direction of the quantum dot. The g factor of localized electron in Ge / Si quantum dot is close to 2: gzz=2.0004 and gxx=gyy=1.9976.

Recent experiments on semiconductor quantum dots have demonstrated the ability to utilize a large quantum dot to mediate superexchange interactions and generate entanglement between distant spins. This opens up a possible mechanism for selectively coupling pairs of remote spins in a larger network of quantum dots. Taking advantage of this opportunity requires a deeper understanding of how to control superexchange interactions in these systems. Here, we consider a triple-dot system arranged in linear and triangular geometries. We use configuration interaction calculations to investigate the interplay of superexchange and nearest-neighbor exchange interactions as the location, detuning, and electron number of the mediating dot are varied. We show that superexchange processes strongly enhance and increase the range of the net spin-spin exchange as the dots approach a linear configuration. Furthermore, we show that the strength of the exchange interaction depends sensitively on the number of electrons in the mediator. Our results can be used as a guide to assist further experimental efforts towards scaling up to larger, two-dimensional quantum dot arrays.

Improved technologies for growing semiconductor thin films of different structural and electronic properties, and even with layer thickness approaching atomic dimensions, have provided new opportunities for basic scientific studies and device applications. New fabrication technologies have also made it possible to reduce device dimensions to the point where quantum size effects must be considered in order to realistically describe the device operation and reliably predict their performance for electronic and optical applications. In this chapter, we shall present a partial summary of advances in low-dimensional semiconductor materials and their device applications.

The atomistic tight binding simulator NEMO 3-D has previously been validated against the experimental data for quantum dots, wells, and wires in the InGaAlAs and SiGe material systems. Here, we demonstrate our new capability to compute optical matrix elements and transition strengths in tight binding. Systematic multi-million atom electronic structure calculations explore the quantum confined stark shift and the ground state optical transition rate for an electric field in the lateral (100) direction. The simulations treat the strain in a ~15 million atom system and the electronic structure in a subset of ~9 million atoms. The effects of the long range strain, the optical polarization anisotropy, the interface roughness, and the non- degeneracy of the p-states which are missing in continuum methods like effective mass approximation or kp are included. A significant red shift in the emission spectra due to an applied in- plane electric field indicating a strong quantum confined stark effect (QSCE) is observed. The ground state optical transition rate rapidly decreases with the increasing electric field magnitude due to reduced spatial overlap of ground electron and hole states.

Recent achievements in experimental and computational methods open the possibility of measuring and inverting the diffraction pattern from a finite object of submicrometer size. In this paper the possibilities of such experiments for two-dimensional arrays of quantum dots are discussed. The diffraction pattern corresponding to coherent and partial coherent illumination of a sample was generated. Test calculations based on the iterative algorithms were applied to reconstruct the shape of the individual islands in such a quantum structure directly from its diffraction pattern. It is demonstrated that, in the case of coherent illumination, the correct shape and orientation of an individual island can be obtained. In the case of partially coherent illumination, the correct shape of the particle can be obtained only when the coherence of the incoming beam is reduced to match the size of the island.

We present a finite-size scaling analysis of the entanglement in a two-dimensional arrays of quantum dots modeled by the Hubbard Hamiltonian on a triangular lattice. Using multistage block renormalization group approach, we have found that there is an abrupt jump of the entanglement when a first-order quantum phase transition occurs. At the critical point, the entanglement is constant, independent of the block size.

Spectrum of collective modes and possible symmetry breaking transitions in arrays of quantum dots

Using an exact diagonalization technique within a generalized Mott-Hubbard Hamiltonian, we predict the existence of a ground state persistent current in coherent two-dimensional semiconductor quantum dot arrays pierced by an external magnetic flux. The calculated persistent current, which arises from the nontrivial dependence of the ground state energy on the external flux, exists in isolated arrays without any periodic boundary condition. The sensitivity of the calculated persistent current to interaction and disorder is shown to reflect the intricacies of various Anderson-Mott-Hubbard quantum phase transitions in two dimensional systems.

Cathodoluminescence (CL) is a very powerful technique for studying the optical properties of semiconductor structures, especially low-dimensional structures (structures with sub-micron features). We will give a number of examples of studies, where CL has been used to study a variety of low-dimensional semiconductor structures. We will also compare the CL technique with other techniques for studies of luminescence with high spatial resolution.

We present here the results on electron transport and noise (diffusion) in low‐dimensional semiconductor structures obtained self‐consistently by the Monte Carlo technique. Electron scattering by confined and localized optical phonons in quasi‐one‐dimensional quantum wires and quasi‐two‐dimensional quantum wells is taken into account. In spite of the considerable differences in phonon spectra and their coupling with electrons, and in spite of electron quantization, the diffusivity (noise) and transport parameters in low‐dimensional structures only slightly differ from corresponding bulk material characteristics at room temperature. At low lattice temperatures the existence of several optical phonon modes in low‐dimensional structures causes electron streaming at several frequencies.

Recently, low dimensional semiconductor structures such as metallic nanoparticles, have attracted significant attention for photovoltaics. In this work we study the low-temperature formation of metallic nano-dots in an indium tin oxide (ITO) antireflection coating for solar cells after an appropriate hydrogen plasma treatment. Atomic force microscopy (AFM), scanning spreading resistance microscopy (SSRM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used for the analysis of the morphology, electrical properties and composition of ITO layers deposited on Corning glass at different temperatures and subsequently subjected to hydrogen plasma. Formation of highly conductive nano-structures at hydrogenation temperatures below 250degC is observed. It is shown that the nano-structures formation can be controlled by the time and the temperature of the hydrogenation