Quantum ballistic transport in nanostructures of paraelectric PbTe
G. Grabecki
Institute of Physics, Polish Academy of Sciences, al. Lotników 32/46, PL-02-668 Warszawa, Poland
It is well known that one of the fundamental limits for nanostructuring is the discrete
character of the electric charge. One of its consequences is the distortion of the nanostructure
potentials by random fluctuations produced either by unintentional defects introduced during the
processing or heteroepitaxial growth, or by artificially incorporated doping impurities. The latter
cannot be completely avoided as they provide the free carriers necessary for the device operation.
Even in modulation doping devices, where the active region is spatially separated from the
ionised impurities, long-range tails of the Coulomb potentials give rise to potential fluctuations
that account for low-angle scattering of the free carriers. In this presentation, we review our
experimental studies on nanostructures of lead telluride. The main result is demonstration that
suppression of the Coulomb potential fluctuations may be achieved using a medium characterised
by a strong lattice polarizability. This is the case of PbTe, which combines excellent
semiconductor properties (low effective mass, high carrier mobility, large Zeeman splitting and
controllability of the carrier concentration) with paraelectric behaviour that leads to a huge
dielectric constant  = 1350 at 4.2 K.
We have developed nanostructurization methods of PbTe using as an initial material,
MBE-grown modulation-doped PbTe quantum wells embedded by Pb0.92Eu0.08Te:Bi barriers.
Electron transport measurements performed on 1D PbTe wires showed precise zero-magnetic
field conductance quantization. This was free of conductance resonances down to 50 mK, even
for a 2 nm thin spacer layer and a background ionised impurity concentration of about 1017 cm-3
[1]. This extraordinary result was fully supported by theoretical modelling. Moreover, we have
detected spin resolved half-integer quantisation plateaux already in a magnetic field as low as 0.2
T. This is a consequence of the large Zeeman splitting in PbTe that reaches 4 meV/T. Because for
typical devices the spin splitting in fields of 1 T is larger than the energy spacing between 1D
subbands, our nanostructures constitute an efficient spin filter, in which the entirely spinpolarised electron current can be carried by several electric subbands [2]. Following our work on
GaAs/(Al,Ga)As [3], we extend these filtering capabilities to even lower magnetic fields by
depositing micromagnets of ferromagnetic metals in the channel vicinity. Finally, in order to
develop an electrical spin detector as well as to fabricate an electrically controlled source of
entangled electron pairs, we investigate nanojunctions of PbTe with various superconducting
metals. We find that the interface In/PbTe is transparent making it possible to observe
pronounced conductance maxima associated with the Andreev reflection and the proximity
effect.
This work has been supported by ERATO Semiconductor Spintronics Project of JST and
by EC Project SPINTRA No. ERAS-CT-2003-980409.
[1] G. Grabecki et al., Phys. Rev. B 72, 125332 (2005), and references therein.
[2] G. Grabecki et al., Physica E 13, 649 (2001).
[3] J. Wróbel et al., Phys. Rev. Lett. 93, 246601 (2004).