The probe nucleus 77Br(77Se) was incorporated into the III-V semiconductor InAs via the nuclear reaction 75As(α, 2n)77Br at the Bonn Isochronous Cyclotron and via ionimplantation at ISOLDE-CERN. First results on the α-irradiated n-type InAs material have been discussed in [1]. It has been shown that in Perturbed Angular Correlation (PAC) measurements a unique quadrupole interaction (QI) with an EFG (νQ=374 MHz) oriented along the ⟨111⟩-axis appears after annealing at 500°C for 120 s and disappears after a step at 650°C leaving behind a nearly undisturbed probe environment. These two observed probe surroundings can be identified as a relaxed and a substitutional configuration as predicted for DX-like-defects. Those defects should be possible for every donor, and double donor incorporated in the III-V semiconductors [2]. The substitutional case can be identified as a 77Br(77Se)-atom which occupies a cubic As-site, consequently no EFG is produced. In the relaxed situation a neigbhouring In-Atom of the substitutional 77Br(77Se)-atom is displaced along the ⟨111⟩-axis and an EFG along the ⟨111⟩-axis is produced. Theoretical calculations [2] predict, that the relaxed situation, which leads to the observable EFG, is only stabilized in the negative charge state in the case of a donor (Se) and in the neutral charge state in the case of a doubledonor (Br). In p-type InAs no uniform quadrupole interaction appears.
New PAC-measurements after isochronous annealing have been carried out with α-irradiated undoped InAs. There the same behaviour like in the n-type Material was observed. A unique QI (νQ=374 MHz) appears after annealing at 550°C 120 s and disappears after annealing at 700°C 120 s. Measurements after ion-implantation lead to the same results for all three different predoped materials.
The results can be interpreted as due to a change of the Fermi-level due to the α-irrradiation respectively the ion-implantation and annealing. Electric measurements on the samples (Hall-, four-point probe- measurements) after α-irrradiation and annealing lead to the result, that the produced defects during irradiation lead to an n-type doping effect which raises the Fermi-level. In case of most semiconductor materials (e.g. GaAs, Si) it is known, that irradiation defects lead to a p-type doping effect, only few materials are known in which the effect is n-type doping (e.g. InP). Further annealing lowers the Fermi-level and the possible charge states of the daughter nucleus 77Se (− −,−,0,+) are occupied and as predicted in the case of donors only in the negative charge state the relaxed situation, wich produces an EFG along the ⟨111⟩-axis, is stabilized. Se as the daughter nucleus of 77Br(77Se) has to be taken into consideration because the PAC-measurements are carried out in the Se electronic structure. Therefore the negative charge state of Selen could be determined out of the electric measurements and the knowledge at which annealing temperature the interaction frequency in the PAC-measurements appears. The estimated negative charge state lies in the conduction band from 13 meV above the bandgap up to 45 meV.
[1] M. Risse, R. Vianden, M. Wehner; Research report ISKP 60-61 (1997)
[2] J. Dabrowski, M. Scheffler; Mat. Sci. For. 83-87 (1992) 735