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- Modification of the energy spectrum by doping
- For a number of years, one of fields of research of our group has been directed to
investigation of the doping-induced modification of impurity spectrum of IV-VI narrow gap
semiconductors, mainly based on the lead chalcogenides. In some cases the energy spectrum
of a host material remains unchanged, but doping results in an appearance of a deep
impurity level with a high density of states that pins the Fermi level. The diagram of a
relative position of the actual bands and the impurity level under variation of alloy
composition, temperature, external pressure and magnetic field has been received for the
following alloys:
- Pb1-xSnxTe(In)
- Pb1-xSnxTe(Ga)
- Pb1-xMnxTe(In)
- Pb1-xMnxTe(Ga)
- Pb1-xGexTe(In)
- Pb1-xGexTe(Ga)
- Pb1-x-ySnxGeyTe(In)
- Pb1-xSnxTe1-ySey(In)
- Pb1-xSnxTe1-ySy(In)
- PbTe(Cr)
- PbTe(Yb)
- PbTe(Yb, Ga)
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- A range of new effects has been observed, such as pressure-induced successive dielectric
- metal - dielectric transitions, Hall voltage saturation in the Fermi level pinning
regime, and many others.
- Relative key publications:
- B.A.Akimov, A.V.Dmitriev, D.R.Khokhlov,L.I.Ryabova. Carrier transport and
non-equilibrium phenomena in doped PbTe and related materials. Phys. Stat. Sol. (a), 137
9 (1993).
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Contact person: Boris Akimov
- The problem of DX-centers in semiconductors attracts a lot of experimental and
theoretical efforts for more than 15 years. The main feature of these centers is strong
electron-lattice coupling leading to formation of a barrier in the
configuration-coordinate space between the states of a system with the different number of
localized electrons. Besides, strong polarization of a lattice in the impurity environment
results in some cases in the negative-U character of impurity centers.
- Until recently investigation of the DX-centers was restricted to the rather wide-bandgap
III-V and II-VI semiconductors. Now it appears that features of impurity centers arising
in the narrow-gap IV-VI semiconductors doped with some impurities, usually of the group
III, are quite similar to the features of DX-centers. For instance, persistent
photoconductivity and long-term relaxation processes are observed at low temperatures.
However there is a considerable difference between the cases of the "classical"
DX-centers in III-V's and II-VI's, and the "DX-like" centers in IV-VI. First of
all, the negative-U behaviour is observed in IV-VI independently on the Fermi level
position, in contrast to the "classical" DX-centers which are negative-U only
when the Fermi level is pinned within the gap. Besides that, the metastable one-electron
local state in deep in IV-VI, whereas it is shallow in III-V and II-VI. Finally, this
metastable local state is separated by potential barriers in the configuration-coordinate
space from both ground two-electron local state and the state with delocalized electrons.
This final circumstance allows non-equilibrium electrons to accumulate in the metastable
local state. It results in an appearance of a range of strong and unusual non-equilibrium
effects in the group III-doped IV-VI, such as giant negative magnetoresistance,
localization of the non-equilibrium free careers in the ultrahigh magnetic fields,
microwave stimulation of the quantum efficiency up to ~ 100, and many others.
- Relative key publications:
- 1. B.A.Akimov, A.V.Dmitriev, D.R.Khokhlov, L.I.Ryabova. Carrier transport and
non-equilibrium phenomena in doped PbTe and related materials. Phys. Stat. Sol. (a), 137
9 (1993).
- 2. D.R.Khokhlov, B.A.Volkov. Mixed valence, electrical activity and metastable states in
doped IV-VI compounds: experiment and theory. Proc. 23 Int. Conf. Phys. Semicond., Berlin,
Germany, July 21-26, 1996, ed. M.Scheffer, R.Zimmermann, World Sci., 4
2941 (1996).
Contact person: Dmitriy Khokhlov
- A special attention is attracted to the semimagnetic semiconductors based on IV-VI.
Doping with magnetic impurities provides a modification of the semiconductor energy
spectrum in the magnetic field. Until recently most of experimental activities have been
attracted to investigation of magnetic impurities that are electrically neutral and do not
form impurity levels in the proximity of the actual bands, but only modify the energy
spectrum of a semiconductor. Eu and Mn are the most popular dopants of this kind. They are
most commonly used for formation of barrier spacers in the IV-VI heterostructures where
the barrier parameters and, respectively, the wavefunction in the well, are magnetic field
- dependent. On the other hand, even for Eu and Mn modification of the energy spectrum is
well understood for the 3-D case, whereas much less is known on their behaviour in the 2-D
structures. Even less information is available for the other magnetic impurities -
transition metals and rare-earth elements. The progress in fabrication of a high-quality
IV-VI - based heterostructures is facing the problem of high carrier scattering in the
barrier region. Doping of a barrier layer with Gd or Cr may provide an effective solution
of the problem since introduction of these elements in PbTe increases drastically the free
carrier mobility. However the physics involved is not clearly understood, and almost
nothing is known on Gd and Cr behaviour in the 2-D case. Finally, nothing is known on the
electronic structure of the IV-VI semiconductors that are doped with two different
magnetic impurities. It is obvious that for small dopant concentration the action of these
impurities should be additive, but for higher impurity concentration the effects of
interaction of different impurity atoms may be considerable. Another important direction
of research is investigation of impurity states arising in IV-VI upon doping. The
"DX-like" behaviour of impurity centers has been observed in the lead
telluride-based alloys doped with the rare-earth element - Yb and the transition metal -
Cr. Both of them are magnetic impurities, and this circumstance introduces a great deal of
specifics in the physics involved and, consequently, in the effects observed. For
instance, position of the pinned Fermi level EF depends on the amount of Yb in
an alloy probably due to magnetic interaction of impurity centers. Besides that, the value
of EF may be tuned by variation of a magnetic field. This result opens new
possibilities both for the fundamental and applied aspects of the problem, such as
observation of a magnetic field - induced acceptor - donor transition, construction of a
magnetic field - tuneable photovoltaic infrared photodetector, and many others.
- Relative key publications
- 1. B.A.Akimov, P.V.Verteletski, V.P.Zlomanov, L.I.Ryabova, O.I.Tananaeva, N.A.Shirokova.
Shubnikov - de Haas oscillations in PbTe(Cr). Sov. Phys. Semicond., 23
151 (1989).
- 2. B.A.Akimov, E.N.Korobeinikova, L.I.Ryabova, M.E.Tamm. Influence of Tm on lead
telluride properties. Sov. Phys. Semicond., 25 208 (1991).
- 3. B.A.Akimov, A.V.Nikorich, L.I.Ryabova, N.A.Shirokova. Metal - dielectric transition
in Pb1-xMnxTe(In) solid solutions. Sov. Phys. Semicond., 23
636 (1989).
Contact person: Ludmila I. Ryabova
- We have started the joint research with the University of Linz, Austria (Prof. G.Bauer's
group), the University of Leoben, Austria (Dr. J.Oswald's group) and the Swiss Federal
Institute of Technology, Zurich (Dr. H.Zogg's group) based on the following idea. Quality
of semiconductor two-dimensional structures as well as performance of respective devices
are defined to a great extent by existence of appropriate substrate material for any
particular semiconductor system. In most of the cases the semiinsulating substrate is
required. The problem may be easily solved for the III-V and II-VI semiconductors, where
materials of the same group may be used as a substrate. This approach allows to overcome
difficulties arising from the lattice mismatch and from different temperature expansion
coefficients of a substrate and a 2-D layer. In the case of IV-VI semiconductors the
above-mentioned idea has not been used so far due to high values of free carrier
concentration in undoped crystals. In fact, IV-VI semiconductors grow with high deviation
from stoechiometry resulting from the specifics of the phase diagrams, and all the growth
defects are electrically active, so one cannot obtain a semiinsulating undoped IV-VI
substrate. At the same time investigations of the two-dimensional IV-VI semiconductor
structures look very attractive. Indeed, the characteristic energies of the IV-VI
electronic spectra are much less than for the III-V semiconductors, therefore one could
expect more pronounced effects under the action of external factors, such as electric and
magnetic fields, temperature and pressure. These effects become even more interesting due
to the proximity of a phase transition resulting in high values of a dielectric constant,
as well as large spin gaps in most of the IV-VI. Besides that, in some cases doping of
IV-VI semiconductors may modify substantially features of a material. The main idea of our
research is to use the possibilities arising from doping of the lead telluride-based
alloys with some of the group III impurities and/or with the rare-earth elements in order
to get the semiinsulating substrate material free from the disadvantages mentioned above,
for the IV-VI two-dimensional structures. PbTe(Ga) seems to be a good candidate as a
substrate material for the IV-VI heterostructures: it is well matched in the lattice
parameter and in its temperature coefficient, and it is semiinsulating, but if some
concentration of free electrons is required, it may be easily generated by infrared
radiation. Another attractive possibility is to use the lead telluride-based alloys
containing the rare-earth elements as a substrate material. One of the advantages of this
approach is the possibility of tuning the substrate parameters by the external magnetic
field.
Contact person: Dmitriy Khokhlov
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