INVESTIGADORES
ERRICO Leonardo Antonio
congresos y reuniones científicas
Título:
Temperature dependence of electronic defect in semiconductors: How ab initio calculations can complement PAC experiments
Autor/es:
L. A. ERRICO, M. RENTERÍA, A. G. BIBILONI, G. N. DARRIBA.
Lugar:
La Habana, Cuba
Reunión:
Simposio; XVII Latin American Symposium on Solid State Physics (SLAFES 2004); 2004
Institución organizadora:
-
Resumen:
The electronic charge density r(r) in a solid and its temperature dependence can be studied by
measuring a physical magnitude - the electric-field gradient tensor (EFG)
that is very sensitive to small changes
in r(r). The Time-Differential Perturbed-Angular-Correlation
technique (PAC) is specially suited to study the temperature dependence of the
EFG because its sensitivity is temperature independent. In metallic systems,
the temperature dependence of the EFG is relatively well understood in terms of
lattice vibrations. On the other hand, in the case of semiconductors, several
behaviours have been observed, which have been explained (with more or less
success) in terms of host properties or processes induced by the presence of
the hyperfine probes (generally impurities in the host). In
particular, the wide gap semiconductors that
crystallize in the cubic structure of the mineral bixbyite Mn2O3
have been subject of systematic studies with 111Cd tracers [1]. In
these compounds, the presence of dynamic hyperfine interactions, originated in
the electron-capture decay
after-effects, and a strong positive linear temperature dependence of the EFG
appear selectively, depending on the fact that if the host cations present
closed electronic shells (such as Sc, Y, and In) or if it has incomplete
electronic shells (as is the case of the 4f-orbitals of the rare-earths) [1].
Lutetium is the only rare-earth that presents a
closed-shell electronic structure in its 3+ oxidation state and its sesquioxide
does not undergo a phase transition like others in the group, converting this
oxide into an interesting laboratory to check the models proposed to explain
the phenomena mention above. The experimentally observed anomalous step-like EFG temperature dependence at 111Cd sites in Lu2O3
was explained in the frame of a two-state model that
considers an extremely fast fluctuation between two static EFG configurations
[2], which enabled the experimental determination of an
acceptor energy level introduced by the Cd impurity in the band-gap of the
semiconductor and the estimation of the oxygen vacancy density in the sample.
For a long time, point-charge model in combination
with Sternheimer factors [REF] has been used to interpret experimental EFG
results in solids through the assignment of some effective charges to the ions
of the crystal. These calculations, however, depend on empirical parameters and
often deviate significantly from the experimental values. To interpret the experimental
EFG and extract the electronic structure a system, ab initio
calculations which do not need external parameters (like the Sternheimer ones),
are indispensable. About 15 years ago, Blaha et al. have developed such
first-principles calculations using the Full-Potential Linearized Augmented
Plane Wave (FLAPW) method, which is one of the most accurate methods for
band-structure calculations of solids in the framework of the Density
Functional Theory. Recently, increasing computer power and progress in method
developing have made it possible to applied the FLAPW method to fairly
complicated systems like doped semiconductors [3] and, in particular, to oxide
semiconductors with diluted impurities [4], enabling the description of
structural and electronic properties on these complex. More recently, we have
showed that an ab initio calculation can be used to explain the
temperature dependence of the EFG at Cd impurities in TiO2 [5]. In
the present communication we have applied the FLAPW method to the case of Lu2O3
doped with Cd. Our results show that we can reproduce not only the experimental
results at RT but also the temperature dependence of the EFG. In this new
insight, the EFG thermal dependence arises from the ionization of an impurity
acceptor level introduced in the band-gap of the semiconductor, in good
agreement with the previously proposed
two-state model.
[1] J. Shitu et al., Mod..
Phys. Lett.
B 12, 281 (1998).
[2] L. Errico et al., Hyp. Interact. 120/121, 457 (1999).
[3] S. Lany et al., PRB 62, R2259 (2000).
[4] L. A. Errico et al., PRL 89, 55503 (2002).
[5] L. A. Errico, Hyp. Interact., in press (2004).