Electronic Properties Of Semiconductor Nanostructures Research Articles

Simulating the electronic properties of semiconductor nanostructures using multiband [formula omitted] models

The eight-band k·p formalism has been successfully applied to compute the electronic properties of a wide range of semiconductor nanostructures in the past and can be considered the backbone of modern semiconductor heterostructure modelling. However, emerging novel material systems and heterostructure fabrication techniques raise questions that cannot be answered using this well-established formalism, due to its intrinsic limitations. The present article reviews recent studies on the calculation of electronic properties of semiconductor nanostructures using a generalized multiband k·p approach that allows both the application of the eight-band model as well as more sophisticated approaches for novel material systems and heterostructures.

Rolled-Up Quantum Wells Composed of Nanolayered InGaAs/GaAs Heterostructures as Optical Materials for Quantum Information Technology

Strain-based band structure engineering is a powerful tool to tune the optical and electronic properties of semiconductor nanostructures. We show that we can tune the band structure of InGaAs semic...

A generalized plane-wave formulation of [formula omitted] formalism and continuum-elasticity approach to elastic and electronic properties of semiconductor nanostructures

We present a generalized and flexible plane-wave based implementation of the multiband k·p formalism to study the electronic properties of semiconductor nanostructures. All ingredients of the modeling process, namely the Hamiltonian, the nanostructure’s geometry and the required material parameters, are defined in human-readable input files that can be easily generated and modified. The generalized k·p model can contain an arbitrary number of directly treated bands as well as strain, piezoelectric, and external potentials. All calculations can be performed for arbitrary crystal structures. The nanostructure is described in terms of a real-space composition map that may contain an arbitrary number of base compounds and alloys. We demonstrate the applicability and flexibility of our implementation for the example of (111)-oriented, site-controlled InGaAs quantum dots, where a rotated eight-band k·p Hamiltonian is employed. As a second example, a 14-band k·p model that captures the bulk inversion asymmetry of the zinc-blende lattice is applied for the case of a pyramidal (001)-oriented InAs/GaAs quantum dot. Here we show that the explicit treatment of 14 bands removes the well known shortcoming of eight-band k·p models for (001)-oriented zinc-blende quantum dots which leads to artificially degenerate p-like electron states.

Structures and electronics of buried and unburied semiconductor interfaces

The structure of interfaces plays an important role in determining the electronic properties of semiconductor nanostructures. Here, such examples are shown and discussed using semiconductor nanostructures prepared by molecular beam epitaxy and colloidal synthesis.

Challenges in cross-sectional scanning tunneling microscopy on semiconductors

Cross sectional scanning tunneling microscopy (X-STM) has now become a well established method for the investigation of the structural and electronic properties of semiconductor nano-structures down to the scale of individual impurity atoms. Nevertheless, some aspects still remain challenging, for example in the sample preparation by cleavage as well as in the quantitative interpretation of the results. We present a brief overview of the techniques and geometries employed to cleave different semiconductors such as the lll-V materials, mainly GaAs, and the elementary semiconductors Si and Ge. Furthermore, we discuss the inevitable impact of the surface on the properties of the addressed impurities. This is mainly an issue when the surface reconstruction creates electronic surface states in the band gap. But also the unreconstructed GaAs(110) surface significantly modifies the symmetry of acceptor wave functions and the binding energy of donors and acceptors in the first few atomic layers. The impact of the tip will be addressed as a third quite important challenge, which is frequently neglected in the analysis of X-STM data. On surfaces with an unpinned Fermi level, the presence of the STM tip and the voltage applied to the tunneling contact significantly modifies the spectral positions of the observed electronic states and bands. Furthermore, different band bending situations open up qualitatively different tunneling paths to address individual electronic states in the sample. Detailed knowledge of the tunneling mechanism and of the tip properties, mainly apex radius and work function, is required in order to correctly extract the energetic levels from the tunneling spectroscopy data.

Pseudopotential Theory of Electronic Excitations in Semiconductor Nanostructures

The calculation of the optical and electronic properties of semiconductor nanostructures is still based for the most part on highly approximated, continuum-like models such as the effective-mass approximation, which do not take into account the atomistic structure of the nanostructures. We present here an atomistic pseudopotential approach to the calculation of excited states in semiconductor nanostructures. The single-particle Schroedinger equation is solved using O(N) methods. The electronic excited states (such as excitons, charged excitons, multi-excitons, etc.) are then calculated by solving the many-particle Schroedinger equation in a basis set of Slater determinants obtained by promoting one or more electrons from the valence band to the conductions band (configuration interaction expansion). Applications of this method to predict the excitonic fine structure and the optical emission spectra of neutral and charged excitons, bi-excitons, and tri-excitons in CdSe colloidal nanocrystals are presented.

State-of-the-art eigensolvers for electronic structure calculations of large scale nano-systems

The band edge states determine optical and electronic properties of semiconductor nano-structures which can be computed from an interior eigenproblem. We study the reliability and performance of state-of-the-art iterative eigensolvers on large quantum dots and wires, focusing on variants of preconditioned CG, Lanczos, and Davidson methods. One Davidson variant, the GD+k (Olsen) method, is identified to be as reliable as the commonly used preconditioned CG while consistently being between two and three times faster.

Microscopic modeling of the optical properties of semiconductor nanostructures

A brief overview of a consistent microscopic approach to model the optical and electronic properties of semiconductor nanostructures is presented. Coupled semiconductor Bloch and Maxwell equations are used to investigate the performance of semiconductor microcavity structures, photonic band gap systems, and lasers. The predictive potential of the microscopic theory is demonstrated for several examples of practical importance. Optical gain and output characteristics are computed for modern vertical external cavity surface emitting laser structures. It is shown how design flexibilities can be used to optimize the device performance. Nanostructures are proposed where semiconductor quantum wells are embedded in one-dimensional photonic crystals. For field modes spectrally below the photonic band edge it is shown that the optical gain and absorption can be enhanced by more than one order of magnitude over the value of the homogeneous medium. The increased gain can be used for laser action by placing quantum wells and a suitably designed photonic crystal structure inside a microcavity.

High pressure as a tool to tune electronic coupling in self‐assembled quantum dot nanostructures

AbstractHigh pressure provides an efficient way of controlling the electronic properties of semiconductor nanostructures. We use pressure to tune electronic coupling in stacks of self‐assembled quantum dots (SAQDs). The coupling occurs as soon as the distance and the height of the barriers between the dots are sufficiently small to permit interaction between the wavefunctions of electrons in adjacent dots. We demonstrate experimental evidence that application of pressure reduces and may quench the coupling. This happens because, under pressure, the inter‐dot barriers become higher and hence less transparent for electron wavefunctions. The effect was revealed by means of photoluminescence spectroscopy and confirmed by theoretical modelling. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Boundary-element method for the calculation of electronic states in semiconductor nanostructures.

We have developed a boundary-element method to treat the single-particle electronic properties of semiconductor nanostructures that consist of piecewise homogeneous materials of arbitrary shapes. Green's-function techniques are used to derive integral equations that determine these electronic properties. These equations involve integrals over the boundaries between the homogeneous regions, and they are discretized and solved numerically. In effect, this approach changes a partial differential equation with boundary conditions in d independent variables into an integral equation in d-1 independent variables, which leads to its efficiency. For bound states these methods are used to calculate eigenenergies, for scattering states to calculate differential cross sections, and for both bound and scattering states to calculate spectral density functions and wave functions. For such systems, we show that this method generally provides improved calculational efficiency as compared to alternative approaches such as plane-wave expansions, finite-difference methods, or finite-element methods and that it is more effective in treating highly excited states than are these methods. Illustrative examples are given here for several systems whose potentials are functions of two variables, such as quantum wires or patterned two-dimensional electron gases. \textcopyright{} 1996 The American Physical Society.

Electronic properties of semiconductor nanostructures probed by scanning tunneling microscopy

We demonstrate the novel application of the scanning tunneling microscope to find and probe the electronic transport properties of single semiconductor nanostructures at temperatures from room temperature to as low as 1.5 K. The current-voltage characteristics of InGaAs/InAlAs resonant tunneling nanostructures clearly show negative differential resistance. From the dependence of the current on cross-sectional area the lateral depletion is estimated to be 300 Å.