62 research outputs found
Non-monotonic magnetoresistance of two-dimensional electron systems in the ballistic regime
We report experimental observations of a novel magnetoresistance (MR)
behavior of two-dimensional electron systems in perpendicular magnetic field in
the ballistic regime, for k_BT\tau/\hbar>1. The MR grows with field and
exhibits a maximum at fields B>1/\mu, where \mu is the electron mobility. As
temperature increases the magnitude of the maximum grows and its position moves
to higher fields. This effect is universal: it is observed in various Si- and
GaAs- based two-dimensional electron systems. We compared our data with recent
theory based on the Kohn anomaly modification in magnetic field, and found
qualitative similarities and discrepancies.Comment: 4 pages 3 figure
Optical and transport properties of short period InAs/GaAs superlattices near quantum dot formation
We have investigated the optical and transport properties of MBE grown
short-period superlattices of InAs/GaAs with different numbers of periods (3 <=
N <= 24) and a total thickness 14 nm. Bandstructure calculations show that
these superlattices represent a quantum well with average composition
In_0.16Ga_0.84As. The electron wave functions are only slightly modulated by
the superlattice potential as compared to a single quantum well with the same
composition, which was grown as a reference sample. The photoluminescence, the
resistance, the Shubnikov-de Haas effect and the Hall effect have been measured
as a function of the InAs layer thickness Q in the range 0.33 <= Q <= 2.7
monolayers (ML). The electron densities range from 6.8 to 11.5x10^11 cm^-2 for
Q <= 2.0 ML. The photoluminescence and magnetotransport data show that only one
subband is occupied. When Q >= 2.7 ML quantum dots are formed and the metallic
type of conductivity changes to variable range hopping conductivity.Comment: 15 pages (incl.7 figures); pdf file; submitted to Semicond. Sci.
Techno
Electron transport and optical properties of shallow GaAs/InGaAs/GaAs quantum wells with a thin central AlAs barrier
Shallow GaAs/InGaAs/GaAs quantum well structures with and without a three
monolayer thick AlAs central barrier have been investigated for different well
widths and Si doping levels. The transport parameters are determined by
resistivity measurements in the temperature range 4-300 K and magnetotransport
in magnetic fields up to 12 T. The (subband) carrier concentrations and
mobilities are extracted from the Hall data and Shubnikov-de Haas oscillations.
We find that the transport parameters are strongly affected by the insertion of
the AlAs central barrier. Photoluminescence spectra, measured at 77 K, show an
increase of the transition energies upon insertion of the barrier. The
transport and optical data are analyzed with help of self-consistent
calculations of the subband structure and envelope wave functions. Insertion of
the AlAs central barrier changes the spatial distribution of the electron wave
functions and leads to the formation of hybrid states, i.e. states which extend
over the InGaAs and the delta-doped layer quantum wells.Comment: 14 pages, pdf fil
A Model for the Voltage Steps in the Breakdown of the Integer Quantum Hall Effect
In samples used to maintain the US resistance standard the breakdown of the
dissipationless integer quantum Hall effect occurs as a series of dissipative
voltage steps. A mechanism for this type of breakdown is proposed, based on the
generation of magneto-excitons when the quantum Hall fluid flows past an
ionised impurity above a critical velocity. The calculated generation rate
gives a voltage step height in good agreement with measurements on both
electron and hole gases. We also compare this model to a hydrodynamic
description of breakdown.Comment: 4 pages including 3 figure
Sn delta-doping in GaAs
We have prepared a number of GaAs structures delta-doped by Sn using the
well-known molecular beam epitaxy growth technique. The samples obtained for a
wide range of Sn doping densities were characterised by magnetotransport
experiments at low temperatures and in high magnetic fields up to 38 T.
Hall-effect and Shubnikov-de Haas measurements show that the electron densities
reached are higher than for other delta-dopants, like Si and Be. The maximum
carrier density determined by the Hall effect equals 8.4x10^13 cm^-2. For all
samples several Shubnikov-de Haas frequencies were observed, indicating the
population of multiple subbands. The depopulation fields of the subbands were
determined by measuring the magnetoresistance with the magnetic field in the
plane of the delta-layer. The experimental results are in good agreement with
selfconsistent bandstructure calculations. These calculation shows that in the
sample with the highest electron density also the conduction band at the L
point is populated.Comment: 11 pages text (ps), 9 figures (ps), submitted to Semicon. Science
Tech
A quantum-dot heterostructure transistor with enhanced maximum drift velocity of electrons
Electron transport in unipolar heterostructure transistors with quantum dots in strong electric fields
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