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.
S.A.E. Johansson, J.L. Campbell, and K.G. Malmqvist, Particle Induced X-ray Emission Spectrometry, Wiley, New York (1995).
GeoPIXE software package:
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.
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.
Contact Dr. Mlungisi Nkosi for further details.
2. Nuclear Microprobe
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.
Contact Dr. Christopher Mtshali for further details.
3. Sample Preparation
3.1 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 Dr Mlungisi Nkosi for further details.
3.2 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 Mlungisi Nkosi 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.
- 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
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.