In the last few years the use of radioactive isotopes for the investigation of defects in semiconductors has been quite successful in connection with nuclear techniques (e.g. PAC (Perturbed Angular Correlation), Mößbauer spectroscopy). Besides, the continuation of originally "non-nuclear" experimental methods like DLTS, Hall effect and PL to radioactive nuclei have attracted considerable interest [1,2,3,4].
The Hall effect applied here, allows to measure changes of the electrical sample properties induced by the radioactive decay. The elemental conversion connected with the radioactive decay can lead to time dependent, exponential changes of the carrier concentration and resistivity according to the half-life of the implanted parent nucleus. Thus, a transmutation doping is observable provided the new atom has a different doping characteristic compared to the parent atom. Consequently, the half-life can serve as a "fingerprint", so that the changes of the electrical sample properties with time can unambiguously be assigned to the radioactive decay.
Hall effect and resistivity measurements on p-GaAs implanted with 67Ga — (78.3h) → 67Zn and semiinsulating GaAs implanted with 71As — (64h)→ 71Ge — (11.2d) → 71Ga were performed. In either case, time dependent changes of the carrier concentration and resistivity according to the half-life of the involved probe atoms could be observed.
As substrate material for the implantation of 67Ga+ we used GaAs preimplanted with 1·1013 Ge/cm2 at 160 keV, so that the material is p-type. Commercially available 67Ga was then postimplanted at an energy of 80 keV to a dose of about 8·1011 cm-2.
The 71AS-implantation was performed at the ISOLDE implanter at CERN, Geneva. Semiinsulating GaAs was implanted at 60 keV to a dose of 4·1012 cm-2. In both cases the lattice damage caused by the ion implantation was removed by rapid thermal annealing (RTA) in a graphite strip heater at 900°C for 30 sec.
In the case of 67Ga, a transmutation from the electrically neutral Ga to the single acceptor ZnGa occurs. The radioactive decay of the isoelectronic parent atoms does not disturb the transformation to the stable single acceptors, so that these acceptors are electrically active on regular lattice sites.
For 71As a transformation into the single acceptor GeAs is observed, which increases the p-type doping of the sample. However, the finally resulting GaAs does not act as a double acceptor, but seems to have a compensating effect. This may be due to the formation of Gai.