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Struktur Vortrag: Mi., 18:20–18:40 M-V19
Electrical and structural properties of DX defects in CdTe
Muhammed Türker 1 , Jörg Kronenberg 1 , Manfred Deicher 1 , Herbert
Wolf 1 , Karl Johnston 1 , Thomas Wichert 1 , ISOLDE Collaboration 2
1 Technische Physik, Universität des Saarlandes, D-66041 Saarbrücken, Germany –
2 CERN, CH-1211 Geneva 23, Switzerland
In some semiconductors the achievable doping levels are limited by the formation of
intrinsic defects which leads to the effect of electrical “self-compensation” of dopants.
In AlGaAs and CdTe, donors are electrically compensated by the formation of so-called
DX centers. In the case of In donors in CdTe and In concentrations exceeding 10 18
cm −3 , these DX centers are created. A characteristic feature of DX centers is the
so-called “persistent photoconductivity (PPC)”, a metastable state of the DX-center,
that is formed by illumination at low temperatures thereby increasing the carrier concentration.
The DX-center and the PPC effect are theoretically explained by the “large
lattice relaxation model” [1]. In this model, the DX center is formed by the relaxation
of the In donor towards an interstitial lattice site thereby generating a neighboring Cd
vacancy. Then, the PPC effect consists in a relaxation of the In donor back to a substitutional
lattice site resulting in an electrically reactivation of the donor. In this model,
the lattice relaxation involves changes of the In-Te distance of more then 1 ˚A thereby
locally breaking the cubic symmetry of the CdTe lattice. Such a lattice perturbation
should create an electric field gradient (EFG) observable by perturbed angular correlation
spectroscopy (PAC). Using the probe 111 In/ 111 Cd, an EFG assigned to the DX
defect has been reported characterized by νQ = 21 MHz and η = 0 [2]. This assignment
is based on theoretical calculations of the respective EFG. At the same time, these calculations
predict a short lived stability of the DX center even after the decay of 111 In to
111 Cd, the isotope at which the actual PAC measurements take place. We performed
PAC measurements using 111 In/ 111 Cd and simultaneously measured the conductivity
of the same samples as a function of temperature with and without illumination. Below
150 K, the samples showed a PPC effect that was accompanied by an increase of about
20 % of the carrier concentration. But, this effect is not accompanied by any changes
of the observed EFG. Possible explanations of the observed EFG, originally assigned to
the DX center will be discussed. We will also present first results on the experimental
detection of DX centers in CdTe by PAC using the radioactive isotope 117 Cd decaying
to 117 In; this isotope is both a donor in CdTe and a PAC probe atom. In this type of
experiment, the DX centers including In donors are formed after the decay of 117 Cd
and are still present during the PAC measurement performed at an excited nuclear
state of 117 In.
This work has been supported by the Bundesministerium für Bildung und Forschung
(BMBF) under Contract No. 05 KK1TSB/5.
[1] C.H. Park and D.J. Chadi, Phys. Rev. B 52 (1995) 11884.
[2] S. Lany, H. Wolf, and Th. Wichert, Phys. Rev. Lett. 92 (2004) 225504.