Materials technology
at ANU

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last modified
November 2002

Characterising semiconductors
with radioactive probes

ANU possesses specialist expertise in the area of using radioactive probe atoms to characterise semiconductors, a hyperfine technique known as Perturbed Angular Correlations or PAC. The radioactive atoms are created with the giant 14UD particle accelerator, part of the Heavy Ion Facility operated by the Department of Nuclear Physics.

The fabrication of semiconductor devices is increasingly dependent on the semiconductors being implanted (or doped) with ions. The process involves the semiconductor wafer being placed in an ion implanter where charged ions are fired at it. As a result, a portion of the ions becoming lodged in the semiconductor crystal. The implanted ions change the material properties of the semiconductor. The nature of the change depends on what type and concentration of ions are used, and what disorder is created in the semiconductor during the process of implantation.

Understanding what disorder is introduced to the crystal lattice of the semiconductor during implantation is critical to producing semiconductors with predictable behaviour. A variety of techniques can be used to characterise the crystal lattice including, for example, studies of X-ray absorption (EXAFS), backscattering of nuclei (RBS) and the use of radioactive probe atoms to study the internal electronic and magnetic structure of the lattice (PAC). No single method provides a full characterisation, however, together they provide a wealth of information on the properties of the crystal structure of semiconductor.

The nuclear hyperfine method of Perturbed Angular Correlations (PAC) uses radioactive atoms at very low concentrations implanted into the semiconductor being studied. The method relies on the change in the radiation pattern observed when an excited nucleus decays. Differences in these patterns reflect the different microscopic structure found at the site of the radioactive nucleus.

The method employed at the Heavy Ion Facility works like this: a beam of carbon ions is accelerated by the 14UD tandem accelerator. The beam strikes a thin (2.5 um) foil of rhodium foil. This produces a small amount of radioactive indium atoms that spray out the back of the foil and strike a semiconductor sample mounted some 2 cm behind the target foil. Up to 60% of all the radioactive indium nuclei that leave the target foil can be collected on the semiconductor sample, coming to rest within 1 to 2.5 microns of the sample's surface.

After implantation with the radioactive atoms, the semiconductor is thermally annealed (at 1100degrees Celsius for 10 seconds) to repair any damage to the crystal lattice caused by the implantation process. Off-line PAC measurements on these annealed samples confirm that the samples are returned to an undamaged state.

The semiconductor, now containing radioactive probe atoms, is then implanted with dopant nuclei in the 1.7 MeV ion implanter in the Department of Electronic Materials Engineering.
Radiation spectra being emitted by the probe nuclei are then measured using an array of four barium-fluoride scintillation detectors arranged in a plane around the sample.

Crystal defects in the semiconductor lattice, introduced during implantation, produce large electric field gradients around the probe nucleus. These electric field gradients perturb the distribution of radiation being emitted by the radioactive probe atoms. By measuring the amount of perturbation, it's possible to map the local electronic environment around the defects.
Another way of introducing radioactive probe nuclei into semiconductor materials is by using commercially available radioactive isotopes that are incorporated into the semiconductor in purpose-built ion implanters. The Australian Defence Force Academy (ADFA) in collaboration with ANU has developed a 150kV ion implanter which is housed at ADFA. The implanter was completed in 1998, and will shortly be ready for use with radioactive isotopes.

More information: Dr Aidan Byrne