J. Oswald

Energy levels of interstitial manganese in silicon

Abstract Interstitial manganese in silicon can exist in four charge states which imply single acceptor and double donor behaviour. The corresponding level scheme with the acceptor level ( Mn − 0 ) at Ec-0.13eV, the first donor level ( Mn 0 + ) at Ec−0.45eV and the second donor level (Mn+/++) at Ev+0.30eV is established through EPR-controlled Hall measurements. Another deep donor level at Ec−0.54eV is produced by the donor acceptor pair (MnB) whereas a more shallow one around Ec-0.28eV was already attributed to a Mn4-cluster.

A new network model for the integer quantum Hall effect

Abstract A Landauer–Buttiker-type formulation of backscattering between pairs of opposite directed channels is used to describe the coupling at the nodes of a network. Physically, these nodes correspond to saddle points of a slowly varying lateral potential modulation in a 2D electron system in the high magnetic field regime. We show that the network can be solved without needing a transfer matrix as used by Chalker and Coddington. We use an exponential dependence of the coupling on the filling factor of the associated Landau level. We demonstrate that our network representation allows a quantitative modeling of almost every realistic sample geometry in the quantum Hall regime, including the effect of gate electrodes across a Hall bar.

Experimental identification of the energy level of substitutional manganese in silicon

A combination of deep level transient spectroscopy (DLTS) and electron spin resonance (ESR) measurements was used to determine the energy level of substitutional manganese in silicon. Samples of p‐type silicon were subjected to a copper‐manganese codiffusion. Successfully prepared samples show the typical ESR signal of substitutional manganese with a single positive charge. The Hamiltonian parameters g=2.029 and A=−62.7×10−4 cm−1 are different from those for negatively charged interstitial manganese. The DLTS measurements reveal an energy level of M1=0.39 eV above the valence‐band edge for the substitutional manganese. Because of the codiffusion of Cu also the previously reported levels C1=0.098 eV, C2=0.22 eV, and C3=0.41 eV were found. The combination of ESR and DLTS results allowed a conclusive identification of the defect level M1 and provided no evidence for ordinary amphoteric or negative U behavior in the lower half of the band gap. Furthermore, isothermal and isochronal annealing experiments were ...

A network model for the Hall insulator

A Landauer-Buttiker type back scattering between pairs of opposite directed channels is used to describe the node of a network at the saddle point of a slowly varying potential in the high magnetic field regime. We show that the network can be solved without needing a transfer matrix like used by e.g. Chalker and Coddington.

UNIVERSALITY IN THE CROSSOVER BETWEEN EDGE-CHANNEL AND BULK TRANSPORT IN THE QUANTUM HALL REGIME

We present a new theoretical approach for the integer quantum Hall effect, which is able to describe the inter-plateau transitions as well as the transition to the Hall insulator. We find two regimes (metallic and insulator like) of the top Landau level, in which the dissipative bulk current appears in different directions. The regimes are separated by a temperature invariant point.

A new model for the transport regime of the integer quantum Hall effect: The role of bulk transport in the edge channel picture

Based on a current balance between edge and bulk we obtain a modification of the Landauer–Buttiker formalism. The new aspect of this approach is an interpretation of edge channel (EC) backscattering in terms of a bulk current which couples the edges. This coupling is described by a novel backscattering parameter P, which is a function P(Δν) of the Landau level filling. We show, that the most important features of transport can be modeled already without requesting a specific function for P(Δν). In addition, a number of trends in Rxx and Rxy, from which most of them have been studied experimentally in the recent work of Shahar et al., can be reproduced by using a pure exponential function for P(Δν).

Anomalous magnetotransport in wide quantum wells

Abstract We present magneto transport experiments of quasi 3D PbTe wide quantum wells. A plateau-like structure in the Hall resistance is observed, which corresponds to the Subnikov de Haas oscillations in the same manner as known from the quantum Hall effect. The onsets of plateaux in R xy do not correspond to 2D filling factors but coincide with the occupation of 3D (bulk-) Landau levels. At the same time a non-local signal is observed which corresponds to the structure in R xx and R xy and fulfils exactly the Onsager-Casimir relation [ R ij , kl ( B ) = R kl , ij (− B )]. We explain the behaviour in terms of edge channel transport which is controlled by a permanent backscattering across a system of “percolative EC-loops” in the bulk region. Long range potential fluctuations with an amplitude of the order of the subband splitting are explained to play an essential role in this electron system.

Magnetotransport in wide parabolic PbTe quantum wells

The 3D and 2D behaviour of wide parabolic PbTe single quantum wells, which consist of PbTe p-n-p structures, are studied theoretically and experimentally. A simple model combines the 2D sub-band levels and the 3D Landau levels in order to calculate the density of states in a magnetic field perpendicular to the 2D plane. It is shown that at a channel width of about 300 nm, one can expect to observe 3D and 2D behaviour at the same time. Magnetotransport experiments in selectively contacted Hall bar samples are performed at temperatures down to T=50 mK, and at magnetic fields up to B = 17 T.

Systematic study of nonideal contacts in integer quantum Hall systems

In the present paper we investigate the influence of the contact region on the distribution of the chemical potential in integer quantum Hall samples as well as the longitudinal and Hall resistance as a function of the magnetic field. First we use a standard quantum Hall sample geometry and analyze the influence of the length of the leads where current enters/leaves the sample and the ratio of the contact width to the width of these leads. Furthermore we investigate potential barriers in the current-injecting leads and the measurement arms in order to simulate nonideal contacts. Second we simulate nonlocal quantum Hall samples with applied gating voltage at the metallic contacts. For such samples it has been found experimentally that both the longitudinal and Hall resistance as a function of the magnetic field can change significantly. Using the nonequilibrium network model we are able to reproduce most qualitative features of the experiments.

Novel non-local behaviour of quasi-3D wide quantum wells

We investigate the high-magnetic-field regime of wide quantum wells (WQW) for the case of a many-valley host semiconductor. The complete system is described within a modified Landauer - Buttiker formalism and we demonstrate that a parallel contribution of two electron systems in different valleys of the band structure can lead to an edge-channel-related non-local behaviour even in the 3D regime. From the obtained general result we also derive a simplified model which applies for cases of very different dissipation. It represents the most dissipative system by an ohmic resistor network and the least dissipative system by edge channels.