arXiv subjects
Publications and source records attributed to Andreas Wacker.
We present a capacitance-voltage study for arrays of vertical InAs nanowires. MOS capacitors are obtained by insulating the nanowires with a conformal 10nm HfO2 layer and using a top Cr/Au metallization as one of the capacitor's electrodes. The described fabrication and characterization technique enables a systematic investigation of the carrier density in the nanowires as well as of the quality of the MOS interface.
We study the rapid decrease of peak gain in resonant-phonon THz Quantum Cascade Lasers with increasing temperature. The effect of various microscopic scattering processes on the gain profile as a function of temperature is discussed. We argue that increased broadening, primarily due to increased impurity scattering, and not diminishing population inversion, is the main reason for the reduction of peak gain.
Electron transmission through nanosystems is blocked if there are no states connecting the left and the right reservoir. Electron-electron scattering can lift this blockade and we show that this feature can be conveniently implemented by considering a transport model based on many-particle states. We discuss typical signatures of this phenomena, such as the presence of a current signal for a finite bias window.
The optical properties of quantum dots embedded in a catalytically grown semiconductor nanowire are studied theoretically. In comparison to dots in a bulk environment, the excitonic absorption is strongly modified by the one-dimensional character of the nanowire phonon spectrum. In addition to pronounced satellite peaks due to phonon-assisted absorption, we find a finite width of the zero-phonon line already in the lowest-order calculation.
The method of nonequilibrium Greens functions allows for a spatial and energetical resolution of the electron current in Quantum Cascade Lasers. While scattering does not change the spatial position of carriers, the entire spatial evolution of charge can be attributed to coherent transport by complex wave functions. We discuss the hierarchy of transport models and derive the density matrix equations as well as the hopping model starting from the nonequilibrium Greens functions approach.
Various causes for negative differential conductance in transport through an interacting double quantum dot are investigated. Particular focus is given to the interplay between the renormalization of the energy levels due to the coupling to the leads and the decoherence of the states. The calculations are performed within a basis of many-particle eigenstates and we consider the dynamics given by the von Neumann-equation taking into account also processes beyond sequential tunneling. A systematic comparison between the levels of approximation and also with different formalisms is performed. It is found that the current is qualitatively well described by sequential processes as long as the temperature is larger than the level broadening induced by the contacts.
We suggest a new approach for transport through finite systems based on the Liouville equation. By working in a basis of many-particle states for the finite system, Coulomb interactions are taken fully into account and correlated transitions by up to two different contact states are included. This latter extends standard rate equation models by including level-broadening effects. The main result of the paper is a general expression for the elements of the density matrix of the finite size system, which can be applied whenever the eigenstates and the couplings to the leads are known. The approach works for arbitrary bias and for temperatures above the Kondo temperature. We apply the approach to standard models and good agreement with other methods in their respective regime of validity is found.
Gain in current-driven semiconductor heterostructure devices is calculated within the theory of nonequilibrium Green functions. In order to treat the nonequilibrium distribution self-consistently the full two-time structure of the theory is employed without relying on any sort of Kadanoff-Baym Ansatz. The results are independent of the choice of the electromagnetic field if the variation of the self-energy is taken into account. Excellent quantitative agreement is obtained with the experimental gain spectrum of a quantum cascade laser. Calculations for semiconductor superlattices show that the simple 2-time miniband transport model gives reliable results for large miniband widths at room temperature
Electric transport in semiconductor superlattices is dominated by pronounced negative differential conductivity. In this report the standard transport theories for superlattices, i.e. miniband conduction, Wannier-Stark-hopping, and sequential tunneling, are reviewed in detail. Their relation to each other is clarified by a comparison with a quantum transport model based on nonequilibrium Green functions. It is demonstrated how the occurrence of negative differential conductivity causes inhomogeneous electric field distributions, yielding either a characteristic sawtooth shape of the current-voltage characteristic or self-sustained current oscillations. An additional ac-voltage in the THz range is included in the theory as well. The results display absolute negative conductance, photon-assisted tunneling, the possibility of gain, and a negative tunneling capacitance.
Stationary electric transport in semiconductor nanostructures is studied by the method of nonequilibrium Green functions. In the case of sequential tunneling the results are compared with density matrix theory, providing almost identical results. Nevertheless, the method of Green functions is easier to handle due to the availability of an absolute energy scale. It is demonstrated, that the transport in complicated structures, like quantum cascade lasers, can be described in reasonable agreement with experiment.
We derive a theory for transmission through disordered finite superlattices in which the interface roughness scattering is treated by disorder averaging. This procedure permits efficient calculation of the transmission thr ough samples with large cross-sections. These calculations can be performed utilizing either the Keldysh or the Landauer-Büttiker transmission formalisms, both of which yield identical equations. For energies close to the lowest miniband, we demonstrate the accuracy of the computationally efficient Wannier-function approximation. Our calculations indicate that the transmission is strongly affected by interface roughness and that information about scale and size of the imperfections can be obtained from transmission data.
Theories describing electrical transport in semiconductor superlattices can essentially be divided in three disjoint categories: i) transport in a miniband; ii) hopping between Wannier-Stark ladders; and iii) sequential tunneling. We present a quantum transport model, based on nonequilibrium Green functions, which, in the appropriate limits, reproduces the three conventional theories, and describes the transport in the previously unaccessible region of the parameter space.
In this book article effects related to the vertical transport in weakly coupled multiple quantum wells are reviewed. A self-contained microscopical model for the calculation of the well-to-well currents without any fittings parameters is presented. The model exhibits the well-known peaks in the current-field relation in quantitative agreement with experiments. This local current-field relation is used as an input for the calculation of the transport in the extended structure consisting of many periods. Here both the formation of stationary field domains as well as self-sustained current oscillations are found in good agreement with experimental data. The underlying physics of these nonlinear phenomena is discussed in detail.
A theoretical and experimental study of transport in a low doped multiple quantum well structure shows that impurity bands are essential in understanding the electronic transport both with and without terahertz irradiation. A full self-contained model, which involves only the nominal parameters of the sample, agrees quantitatively with a wide range of experiments.
A microscopic calculation of the perpendicular current in doped multiple quantum wells is presented. Interface roughness is shown to affect the resonant transitions as well as to cause a nonresonant background current. The theoretical characteristics exhibit several branches due to the formation of electric field domains in quantitative agreement with experimental data.
We present a microscopic calculation of transport in strongly doped superlattices where domain formation is likely to occur. Our theoretical method is based on a current formula involving the spectral functions of the system, and thus allows, in principle, a systematic investigation of various interaction mechanisms. Taking into account impurity scattering and optical phonons we obtain a good quantitative agreement with existing experimental data from Helgesen and Finstad (J. Appl. Phys. 69, 2689, (1991)). Furthermore the calculated spectral functions indicate a significant increase of the average intersubband spacing compared to the bare level differences which might explain the experimental trend.