The incorporation of the group VII element Br in the III-V compounds GaAs, InAs and InP and its interaction with defects was studied by means of the perturbed angular correlation technique (PAC). In PAC experiments the lattice site location of the probe is controlled by the parent isotope. Based on theoretical calculations of Uliyashin et.al. [1], we assume that Br is dissolved on the group V site and thus acts as a double donor for the discussion of our results.
We reported earlier [2] about measurements in semi insulating (SI-)GaAs. After annealing at 1173K an 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. One model we proposed for the microscopic structure of this Br--defect complex was the formation of a relaxed state of the isolated probe atom. This (Br(Se)As-Gai) configuration is predicted to be stable in the Vacancy-Interstitial (V-I) model [3], if the double donor 77Br is in the neutral charge state.
To test the relevance of the V-I model for 77Br(77Se) in GaAs, we studied the appearance of the 299MHz interaction in dependance of the charge state of the probe. PAC experiments in various bulk doped GaAs crystals have been carried out for that purpose. Whereas in weakly n ((2...3,4)×1017 Te/cm3.) and p doped ((1...10)$×1016 Zn/cm3) samples no effect on the interaction parameters was detectable, a new quadrupole interaction was observed (νQ=165(2)MHz, EFG axially symmetric) in a highly p doped ((7...10)×1018 Zn/cm3) sample. For the well known EL2 and DX defects in GaAs, for which the V-I model was developped originally, photoexcitation at low temperatures induces a transition between the relaxed and the substitutional configuration of the donor. Thus we studied the influence of illumination with a halogen lamp or a HeNe-laser at 55K on the 299MHz interaction. No change in the interaction parameters was observed.
To study the influence of parameters like lattice constant, gap energy etc., the microscopic environment of the probe 77Br(77Se) was also investigated in other III-V semiconductors. While in InP after annealing at 1073K a high percentage of the probes were subjected to a uniform quadrupole interaction (νQ=170(2)MHz, EFG axially symmetric), in InAs all probes were found on undisturbed lattice sites after annealing at 973K.
[1] A.G. Uliyashin, Yu.A. Bunnay, V.E. Mahalakovskya, N.V. Shlopak, Mat. Sci. For. 83-87 (1992) 735
[2] E. Lohmann, Th. Schaefer, M. Wehner, R. Vianden, Mat. Sci. For. 143-147 (1994) 1155
[3] J. Dabrowski, M. Scheffler, Mat. Sci. For. 83-87 (1992) 735