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.