This work was supported by DAAD under contract No. 424/AUS
The intent of this work was to determine the influence of statistically distributed defects in a cubic single crystal on the shape of PAC-spectra. As the behaviour of these PAC-spectra also depend on the number of defects existing in the samples we wanted to study different concentrations of defects. One way of investigating this would be to use different samples implanted with different doses. Another way would be to control the number of defects in one sample. Therefore we predoped Si with an acceptor very low in the bandgap. In this case we used stable In (0.016 above the valence band). Doping with acceptors will lower the Fermi level so that at room temperature a certain amount of acceptors will be ionized i.e. charged and appear as defects in PAC measurements. By lowering the temperature we freeze out these free charge carriers and the number of defects caused by ionized states will decrease thereby changing the PAC-spectra.
The samples were prepared by doping them with implantations starting with a high energy and decreasing steadily to achieve an evenly distributed concentration over a broad region of depth in the samples. For the Si samples we used a series of 6 implantations with energies ranging from 7 to 0.25 MeV with steadily decreasing doses. The samples were then annealed in a rapid thermal annealing (RTA) procedure. After this the samples were implanted with the PAC-probe 111In. This is done by recoil implantation with an average energy of around 4 MeV. The real energies range from 0 to 7.5 MeV, so the In reaches exactly the region where we predoped the sample.
In order to do PAC measurements at low temperatures a special sample holder had to be built. The copper finger we used as sample holder was especially formed to suit our needs. The air flow cooling the finger was first led through a heat exchanger, after that it came through an ewer filled with liquid nitrogen, flowed through the copper finger and was then led back to heat exchanger to precool the following air. The lowest temperature reached in this way was −180°C which was measured by a thermocouple attached to the copper finger. To be able to keep the temperatures between −180°C and room temperature stable the copper finger was also encircled by a heater. The whole finger with heater and thermocouple was additionally kept in an evacuated glass tube.