History of the Depth-Profiled Positron Spectrometer

The Depth-Profiled Positron Spectrometer has been in use for well over ten years at the University of Michigan. In that time it has proven not only to be a useful tool for studying the interactions of positrons at or near surfaces of condensed matter, but also a unique and powerful probe for condensed matter and thin film research.

Many positron related signals are available for study. The energy distribution of annihilation gamma rays contain information about the momentum distrubution of electrons in the target. This forms the basis of Doppler Broadening Spectroscopy (DBS). The time delay between implantation and annihilation and the number of annihilating positrons as a function of the time delay forms the basis of Positron Annihilation Lifetime Spectroscopy (PALS). The energy distribution of positrons that make it back to the surface before annihilation and are spontaneously ejected form the basis of Reemission Positron Spectroscopy (RPS). These techniques are described in detail in the tutorial section. Each has its own niche of applicability.

In the early days the Spectrometer was heavily employed to study the behavior of positrons at and near the surfaces of single crystals. Since positrons have an electric charge opposite that of electrons, it is possible for some materials to have a negative work function, i.e. positrons that make it back to the surface before annihilation can be spontaneously ejected from the sample. Early work on the Spectrometer involved measuring the values of these negative work functions and positron affinities.

The Spectrometer was also employed to study the growth characteristics of cobalt and nickel silicides which are of great importance to the semiconductor industry. It was found that silicide films grown by thermal reaction techniques have a relatively high number of positron trapping defects. The growth details of various stoichiometries of silicides was studied using positrons and positron affinities were also measured.

More recently the Spectrometer has been used to study thin polymer films. Positrons have been used successfully in the past to study many polymers however most of this work involved high-energy positrons from radioactive sources which are implanted deeply into the samples, disallowing the posibility of thin polymer film research. The Spectrometer, being capable of depth-profiling, was used very successfully to study the structure of thin polymer films and work continues in this area with planned research involving polymer nanofoams and laminates used by the semiconductor industry.

Another area of research undertaken has been to study the characteristics of thin passivating films on metals. These films are extremely important to the usefulness of such metals in industrial applications and their dissolution and subsequent breakdown resulting in pitting and corrosion is a topic of much concern. In collaboration with the Pennsylvania State University, we have surveyed passivating films on titanium and aluminum with the goal of determining the usefulness of positrons as a probe.

In addition to the above work, the Spectrometer has been used to study chalcogenides and amorphous silicon, important for industrial electro-optical applications such as readable/writeable CD's and solar cells, graphite fibers and other materials. The thrust of much of the later work involving the Spectrometer has exploited the positrons high sensitivity to defects and the ability of the Spectrometer to combine this high defect sensitivity with depth-profiling has resulted in a very unique and powerful tool that will be useful for years to come.