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SERVICES
1. ION BEAM ANALYSIS
In IBA, beams of charged particles are focused on a target resulting in
various interactions between the atoms in the target and the charged particles
in the beam. The interactions usually take the form of Coulombic interactions,
excitations or nuclear reactions. The radiation that emerges from the
interaction (scattered particles from Coulombic interactions, emitted
photons from excited atoms and reaction products from nuclear reactions)
is detected and their properties such as energy, are measured yielding
information on the composition of the target and distribution of the elements
in the target.
1.1 ANALYSIS TECHNIQUES
There are many techniques of IBA, each using characteristic properties
of each element (e.g. mass, charge of nucleus or electromagnetic radiation
emitted or absorbed) to determine the composition, concentration and distribution
of various elements in materials. The following analysis techniques are
routinely used at the Materials Research Group:
Rutherford Backscattering Spectrometry (RBS)
RBS is based on Rutherford's experiment which lead to the discovery of
the nucleus of the atom. Today RBS, is a powerful tool for determining
elemental information, for example in the characterization of thin films.
Helium ions (alpha particles) are accelerated to energies between 1 and
4 MeV. These alpha particles are then are focused on the sample to be
analyzed.
Measurements are done in a vacuum chamber where an area of a few square
millimeters is analyzed. Up to ten samples can be loaded into the chamber
for standard RBS measurements. A silicon detector tilted at 165° detects
the backscattered alphas from the sample. The chamber can be fitted with
a cold-trap for liquid nitrogen that is used for in-situ heating measurements
to obtain temperature dependent information on changes in the sample.
RBS analysis is used mostly for determining the composition and the depth
distribution of elements but by aligning the crystallographic axes of
the sample to the incoming alpha particles, RBS channeling analysis provides
information about the crystal structure of the sample. RBS is a nondestructive
and multielemental analysis technique.
Proton Induced X-Ray Emission (PIXE)
Charged particles (protons, alpha particles or heavy ions) are used to
create inner-shell vacancies in the atoms of the specimen. Filling the
vacancies by electrons from the outer shells leads to the emission of
characteristic X-rays (and/or Auger electrons) and this forms the basis
for a highly sensitive elemental analysis. Protons of 1-4 MeV energy are
most often used. Their slowing down in matter is smooth and well characterized,
with little scattering and deflection. The process is therefore easy to
quantify. X-ray production yields are high and continuum background in
PIXE is low. Therefore the detection limits are about two orders of magnitude
better than with electron beams. PIXE spectra are usually collected in
energy-dispersive mode and all elements with atomic numbers above 10 (Na
and above) can in principle be detected at once. The characteristic X-rays
of lighter elements are absorbed in the windows of routinely used Si(Li)
or HPGe detectors. Typically reported sensitivities are 10-20 ppm for
Na to Cl and 1-10 ppm for Ca and heavier elements. No information related
to chemical identity, coordination chemistry or oxidation state of a particular
element could be directly obtained.
The GeoPIXE software package is used for PIXE analysis and quantitative
imaging. For point PIXE analysis GUPIX software can also be used.
FURTHER
READING:
S.A.E. Johansson, J.L. Campbell, and K.G. Malmqvist, Particle Induced
X-ray Emission Spectrometry, Wiley, New York (1995).
LINKS:
GeoPIXE software package:
www.nmp.csiro.au
GUPIX:
www.physics.uoguelph.ca
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Scanning
Transmission Ion Microscopy (STIM)
The loss
of energy by particles (ions) passing through a thin specimen depends
on the elemental composition and thickness (areal density). The energies
of transmitted ions and their number are measured using a semiconductor
detector positioned behind the specimen.
FURTHER
READING:
M.B.H. Breese, D.N. Jamieson, and P.J.C. King. Materials Analysis Using
a Nuclear Microprobe. Wiley, New York (1996).
(this book
can be recommended for any ion beam technique)
Elastic Recoil Detection Analysis
(ERDA) and Heavy Ion Elastic Recoil Detection Analysis (HI-ERDA)
The quantitative and sensitive analysis of light elements in thin films
is in general a non-trivial task in materials science, since there are
only a few techniques available to get reliable and accurate profiles.
Elastic recoil detection (ERD) using energetic heavy ions is such a technique.
Recoild ions scattered off a thin film by an energetic heavy ion beam
impinging the surface at glancing angle are dtected under forward directions
and anlysed for their nuclear charge or mass and energy. A sensitivity
in the ppm reggion with a depth resolution of some 10 nm and a depth range
of 1 micron is obtained in standard ERD set-ups.
Conditions for ERD analyses are currently being investigated with respect
to the technical and physical limits. As a second phase experiments are
planned to characterize various thin films within different fields of
physical and technological contexts. A time of flight (TOF) detection
system in coincidence with energy analysis will be used and is currently
under development.
2. NUCLEAR MICROPROBE
Contact Dr. Wojciech Przybylowicz for further details.
The nuclear microprobe (NMP) was installed on the 0° beam line of
the single-ended 6MV Van de Graaff accelerator in 1991. The NMP system
is now highly automated with most of the NMP being computer-controlled.
Since its installation it has been successfully used in the analysis of
samples from fields including archaeology, biology, geology, materials
science and medicine.
The microprobe target chamber is a modified version of the standard Oxford
Microbeams chamber. A custom-made lid has been installed that allows for
stepper motor control of the target ladder in the X-, Y- and Z-directions.
3. SAMPLE PREPARATION
3.1 CRYO-PREPARATION OF SAMPLES
Contact Dr. Jolanta Mesjasz-Przybylowicz for further details.
3.2 TARGET PREPARATION
We have a vacuum chamber using a titanium boat to heat a metal for the
evaporation of thin layers of metals on a target. We have used it for
Aluminium, Gold and Lithium. We also have prepared thick targets. 0.5mm,
1mm and 3mm Lithium targets using a 5-ton press in an Argon environment
and then enclosing the Lithium using 10µm Havar foil. We also have
a vacuum chamber to Carbon coat an insulated target using a hated 3mm
Graphite rod.
Contact Mr Karl Springhorn for further details.
3.5 THIN FILM PREPARATION AND MODIFICATION
A laboratory
for thin film deposition houses a high vacuum (HV) electron beam evaporator
system. The evaporator is designed for thin metal film deposition for
research and development. This system provides the capability for the
evaporation of high melting point materials. The main components of the
system include a sample changer designed to hold six 60mm diameter sample
holders, a pumping unit system consisting of rotary pump, turbo pump and
ion pumps and three crucibles which enable more than one layer to be deposited.
The electron gun (e-gun) consists of a tungsten filament which produces
electrons that are focused on the material by a magnetic field. Two vacuum
furnaces are also available for annealing, viz., a thin tube and thick
tube furnace. They are used to induce reactions between a thin film layer
and a substrate. Both furnaces are fitted with turbo pumps and cryopanel.
The thin tube furnace can reach a maximum temperature of 900°C and
the thick tube furnace can reach 1500°C.
Contact Dr. Chris Theron for further details.
3.4 PLASMA SOURCE ION IMPLANTATION LABORATORY
The PSII facility at the MRG has a 1m3 volume plasma chamber and a 30kV,
8A (peak) implantation power supply. Both RF and glow discharge plasma
generation is supported, and the modular design of the power supply allows
for relatively easy upgrading of parameters.
The chamber has multiple viewports, numerous electrical feedthroughs and
highly mobile stepper-motor controlled Langmuir probes.
Research to date has included the implantation of N into steel for hardening
purposes and the implantation of H into Si for wafer cleaving. At present,
the MRG is looking for a young, dynamic post-doctoral candidate to lead
this facility into a research path of their devising. See Available
(MSc/PhD/Post-Doc) Projects for more details.
Contact Dr. Chris Theron for further details.
4. X-RAY DIFFRACTION LABORATORY
Recently, an X-ray Diffractometry Facility was established at the Materials
Research. This facility has been jointly funded by the National Research
Foundation iThemba LABS, University of Cape Town, University of Stellenbosch
and Uuniversity of the Western Cape. Two Bruker (previously Siemens) diffractometers
have been acquired, one a Multi-purpose (powder) diffractometer and the
other a High-resolution diffractometer. The establishment of this facility
will provide researchers locally and nationally with access to modern
state-of-the-art x-ray diffraction equipment. X-ray diffractometry is
the only method that permits the direct identification of any crystalline
material based on their unique crystal structure. Such materials include
minerals, metals, alloys, semiconductors, polycrystalline materials, superconductors,
polymers, textile fibers, gemstones, proteins, as well as any other synthetic
inorganic and organic crystals.
EQUIPMENT
· D8 Powder Diffractometer with a theta-theta goniometer suitable
for analysis of powder, liquid, etc., samples
· D8 High-Resolution Diffractometer suitable for semiconductor
and materials research
· 1/4 Eulerian Cradle with 7 degrees of freedom (theta,2theta,phi,chi,
x, y, z) on D8 High-Resolution Diffractometer. Accessories includes: Göbel
mirror for parallel X-ray beam, Ge 4-bounce channel-cut, reflectometry
stage, Soller slits, LiF monochromator, vacuum stage for semiconductor
wafers, etc.
· 9-position sample changer on D8 Powder Diffractometer
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D8
Advance Powder Diffractometer |
D8
Discover High - Resolution Diffractometer |
Current
Applications of XRD include:
· qualitative and quantitative phase analysis
· characterisation of texture and stress
· crystallite size determination
· thin films characterisation
· examination of perfect epitaxial layers
· determination of lattice-mismatch in epitaxial layers
· examination of amorphous and polycrystalline layers
· high-resolution reflectometry studies for determination of layer
thickness, density, surface and interfacial roughness
· high accuracy lattice parameter determination, etc.
Contact Dr. Remy Bucher for further details. You may also fill in the XRD_user (427k) proposal form and email it to xrd@tlabs.ac.za
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