Analysis methods using absorption, ESR and RBS

Optical methods of studying defects have the advantage that if each defect has characteristic energy levels which lie within the forbidden energy gap, then they show separable optical absorption and luminescence bands. Higher photon energy absorption generally monitors electronic transitions, whereas infra-red absorption records vibrational spectra. Many of the optical transitions which result from the presence of impurities have energies in the visible part of the spectrum and consequently the defects are referred to as colour centres. Examples of colour centres are widespread and include the impurities which give colouration to ruby and sapphire or stained glass. They are the basis for photographic and photochromic materials, and frequently involve a mixture of impurity and intrinsic defects. Whilst analysis of absorption bands may determine defect symmetry and inter-relationships of different colour centres, it is unusual to be able to confirm precise models of defect sites solely from the absorption data. In this respect the processes which involve hyperfine interactions such as Electron Spin Resonance (ESR), Electron Nuclear Double Resonance (ENDOR), Mossbauer spectroscopy or spin precession techniques provide more specific answers if they can be applied. In the ion implantation literature there are frequent presentations of data from Rutherford Back-scattering Spectrometry (RBS) to give the depth distributions of impurities or damage in the target material. In part, RBS appears to be popular for implantation analysis because it requires a high energy ion accelerator, which is normally a feature of an implantation laboratory. The information is useful but, like electron microscopy, it rarely gives precise details of individual defect arrangements.