In earlier experiments [1,2], we studied the incorporation of the halogen Br and its interaction with defects in several III-V compounds by means of the perturbed angular correlation technique (PAC). In semi insulating (SI) GaAs, after annealing at 1123K a uniform axially symmetric electric field gradient (EFG) was observed at the site of 100% of the probes, giving rise to a quadrupole interaction frequency of νQ=299(2) MHz. The principal component Vzz of the EFG was determined to be oriented along the 〈111〉 crystal axis.
The structural model we proposed for this Br-defect complex [1,2] is based on the assumption that Br is dissolved on the group V site in III-V semiconductors. Due to theoretical calculations [3], the isolated substitutional double-donor BrAs could form a relaxed configuration. In this so-called Vacancy-Interstitial (VI-)model, a neighbouring Ga atom is displaced along the ⟨111⟩-crystal axis, thus destroying the cubic symmetry around the Br probe, which remains on the As site.
The assumption that Br is incorporated on the group V site in III-V compounds is based for theoretical calculations on F in GaAs [4]. So far, experimental support for this was only given by the results of our PAC investigations in InAs: here, after annealing at 923K, all probe nuclei were found in an undisturbed cubic environment.
In order to determine the lattice site location of Br in GaAs experimentally, the emission channeling technique [5] was applied. The investigations made use of the isotope 82Br, which was used as a probe nucleus for emission channeling experiments for the first time. 82Br decays with a halflife of 35,34h by electron emission Emax=444keV to 82Kr. In first experiments the probe nuclei were implanted into GaAs and InAs with an energy of 60 keV (dose 3·1013 cm-2) at the Bonn Isotope Separator. Subsequently the samples were treated identically to those used in the PAC experiments. Angle dependent channeling spectra ("scans") were recorded around the ⟨100⟩ and the ⟨110⟩-crystal axis. In order to reduce the background, only β- particles with energies above 200 keV were accepted.
In the ⟨100⟩ as well as in the ⟨110⟩ direction a channeling effect (flux peak) was observed. This is expected for a substitutional lattice site occupation of the emitters [5]. A quantitative analysis of the data, comparing the experimental effect to the effect predicted by theoretical calculations in the Manybeam-formalism [6], shows agreement under the assumption that 80±15% of the implanted Br atoms occupy substitutional lattice sites. A destinction, which sublattice is being occupied is not possible since the continuum potentials of neighbouring ⟨110⟩ atomic rows in the Ga and As sublattices are almost equal. The result of the channeling measurements is consistent with the VI-model described above. However, channeling experiments are unable to detect the presence of a defect (the vacancy left behind by the displaced Ga atom) in the vicinity of the probe atom, as long as it does not cause a displacement of the emitter from the substitutional site.
Further measurements, in order to improve the precision of the data on the lattice site location of Br in GaAs, are currently in progress. In these experiments an increase of the channeling effect will be achieved by reducing the Br--implantation engery, thus avoiding multiple scattering of the emitted β-particles.
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[2] M. Wehner, R. Vianden, D. Forkel-Wirth, S. G. Jahn, Hyp. Int. C1 (1996) 215
[3] J. Dabrowski, M. Scheffler, Mat. Sci. For.83-87 (1992) 735
[4] A.G. Uliyashin, Yu.A. Bunnay, V.E. Mahalakovskya, N.V. Shlopak, Mat. Sci. For. 83-87 (1992) 1009
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[6] H. Hofsäss, U. Wahl, S.G. Jahn, Hyp. Int. 84 (1994) 24