The probe 77Br(77Se) was implanted into the III-V compound semiconductor InAs via the nuclear reaction 75As(α,2n)77Br at the Bonn Isochronous Cyclotron. Based on theoretical calculations aiming at a description of the properties of the EL2 and DX defects in GaAs, Dabrowski and Scheffler [2] proposed that a double donor incorporated on a group V site (like e.g. BrAs) can exist in two states similar to the DX centre in GaAs. One state would be a Br-atom ocupying an undisturbed cubic As-site. This situation would produce no electric field gradient (EFG). During Perturbed Angular Correlation (PAC) measurements indeed an vanishing EFG for 100$% of the Br probes was observed in highly n-doped InAs (1.3·1018cm-3) after annealing at 750°C for 120s [1].
In the other state a neighbouring Ga atom of the substitutional Br probe is displaced along the ⟨111⟩ crystal axis, producing an unique large EFG. Such an EFG was observed in most other III-V compound semiconductors like GaAs, GaP, GaSb, InP, InSb and in slightly n-doped InAs (4.2·1016cm-3) after annealing at 650°C for 120 s [3]. However, InAs is the first material in which both situations have been observed.
These further PAC experiments were carried out with better characterized lightly n-doped InAs (5.7...6.7*1016cm-3). After annealing at 500°C for 120s ≈10% of the probe atoms were subjected to a uniform quadrupole interaction (QI) with νQ=376 MHz. This fraction increases up to 18% after annealing at 650°C. The principal component VZZ of the EFG was determined to be oriented along the ⟨111⟩ crystal axis, analogous to the EFG predicted by the microscopic model. The QI disappears after an annealing step at 750°C for 120s, where 68% of the probe atoms are in an undisturbed environment. This is the first time, that in one sample both probe states have been observed.
The results can be interpreted as due to a change of the Fermi-level due to the α-irradiation (the produced defects act like electron traps and have a p-doping effect) and the annealing (the defects disappear and the Fermi-level returns to its original position). During the transition the different possible charge-states of Bromine (++,+,0,-) are occupied and only in the neutral charge state the relaxed situation, which produced an EFG in the ⟨111⟩ direction, is stabilized.
To verify this interpretation some measurements with p-doped InAs (6.21*1017cm-3) have been carried out. During an isochronal annealing program no QI could be observed, the probe atoms are in a 90% undisturbed environment after 750oC for 120s.
In undoped InAs a measurement after annealing at 750°C for 120 s showed ≈80% of the Br probes on cubic sites. These results are in accordance with the interpretation.
[1] M. Wehner, P. Friedsam, S. G. Jahn, D. Forkel-Wirth, Mat. Sci. For. 196-201 (1995) 1419
[2] J. Dabrowski, M. Scheffler, Mat. Sci. For. 83-87 (1992) 735
[3] M. Wehner, PhD thesis, 1997