INFRARED SPECTROSCOPY STUDIES OF DEFECTS IN SEMICONDUCTORS  The Physics of Defects in Semiconductors  Rapid advances in electronics change our lives at home, in the office, in the working places and throughout our society. The basic materials for electronics are semiconductors. Application of semiconductors for electronic and optical devices require generally the introduction and control of imperfections in the crystal lattice. These imperfections are usually foreign atoms incorporated, intentionally or otherwise during growth, either as impurities in the crystal matrix and/or as lattice defects introduced during the processing of the semiconductor material. Their presence has beneficial but mostly detrimental effects in the behavior of the material with regard to various applications. The need to control these defects is, therefore, evident and this in turn necessitates full knowledge and understanding of them. This has led to a separate branch of the physics of Semiconductors, that is the Physics of Defects in Semiconductors, which in itself is a specific branch of the physics of defects in materials, aiming at the detailed understanding of the atomic configuration of the defects, their nature and chemical structure and also the mechanisms that control their introduction, their thermal stability and motion in the crystal lattice, their interactions, etc. The more we know about the defects, the conditions of their generation, their removal and passivation, the better we can improve the yield and properties of the devices. Research activity in the field of defects We study defects in crystals both from a purely scientific point of view, aiming at getting deeper insights in Solid State Physics but also for technological purposes. For basic research purposes, the understanding of a defect in a semiconductor requires its observation and characterization, which involves electrical and optical studies realized mainly through spectroscopic techniques, and its identification which could be achieved by magnetic resonances and/or localized vibrational modes spectroscopies. In addition to the above, and for purposes of applied research, the completion of the picture requires defect control which involve knowledge of the conditions of a defect creation, of its thermal stability, its annealing characteristics and finally the full picture of the influence of the defect on device performance. In general, a qualitative and quantitative description of a defect in a semiconductor entails the knowledge of the following properties and parameters of the defect: concentration , electronic structure, change state, energy levels in the gap, thermal and optical rates of carrier capture and emission, the mechanisms of the carrier capture and emission, its symmetry, any induced lattice relaxation, its type (if it is donor or acceptor), etc. It is also required the knowledge of the defect chemical identity, its atomic configuration and structure, the defect’s introduction rate, the formation mechanisms, the diffusion mechanisms, the defect’s interaction mechanisms, the kinetics, as well as the parameters which describe these mechanisms ,and so on. Optical characterization of semiconductors Electromagnetic radiation, due to its interaction with matter, has become an important tool in probing experimentally the properties of semiconductors. The x-rays, for example, with their short wavelengths, are essential in examining atomic lattices of the crystals. Larger wavelengths in the spectral ranges of ultraviolet, visible, infrared and microwaves help to examine i) the properties of crystal lattice through the interaction of light with phonons, ii) the electronic band structure and the behavior of free electrons and holes through the absorption of light, iii) the behavior of impurities and defects which interact with light through their own vibrations in the lattice or/and through electronic excitations. In the case of microelectronic applications the electromagnetic radiation is used to measure the dimension of the producing microstructures and explore the interfaces formed between the various components. Thus the optical characterization of semiconductors in very important in understanding their basic properties both from the point of view of the physics of semiconductors but also for assessing their ability to the various applications of technology. To this end a number of optical techniques has been developed as for example Photoluminescence, Raman and Infrared spectroscopy. Infrared spectroscopy is the technique used by our group in studying defects in semiconductors. Infrared spectroscopy studies of defects in Semiconductors Infrared spectroscopy involves the use of light to probe the presence, the properties and generally the behavior of defects in crystals via absorption measurements. There are three different absorption mechanics for the IR radiation: lattice or phonon absorption, electronic absorption and free carrier absorption. a) The lattice or phonon absorption is the result of the interactive coupling between the motions of thermally induced vibrations of the constituent atoms of the substrate crystal lattice and the incident radiation. The presence of a defect in a crystal destroys the symmetry of the lattice and modifies the modes of vibration. The defects usually introduce new vibrational modes with higher frequencies than those of the host crystal vibrational modes and which are localized both in real space and frequency space. They are called local vibrational modes (LVM). LVM spectroscopy is an important tool for studying defects in Semiconductors. It could provide valuable information about the symmetry, the structure, the lattice location and the concentration of a defect. LVM spectroscopy with perturbations such as polarization of the probe light, uniaxial and hydrostatic stress and isotope substitution has been highly successful in identifying the structure and composition of a large number of defect complexes: It is particularly useful for qualitative and quantitative analysis. Notice that each vibrational peak is a fingerprint of the defect. Furthermore, the analysis of the spectrum can be made quantitative by suitable calibration which allow the estimation of the concentration of the defect. b) Electronic absorption is the result of the interaction between the incident radiation and the motions of electrons or holes within the material. The energy of the irradiation should be sufficient to cause the transition of the electron between the valence and the conduction band. Electronic transitions could also occur between the valence or the conduction band and the donor or acceptor states in the band gap of a semiconductor. In the cases of localized energy states caused by the presence of defects electronic absorption can occur by excitation of electrons into higher energy states. c) In free carriers absorption transitions occur within any one energy band. It gives rise to a broad continuum absorption that can make semiconductor samples opaque in a frequency range of interest for the case of large free carriers densities in the materials. In defect studies, the effect of free carriers absorption in the spectra should be removed and a number of methods can be used to reduce the free carriers concentrations. Notice that vibrational and electronic excitation energies of various structures in crystals and especially in semiconductors lie mostly in the energy range of a 0-5000cm-1 which correspond to light in the infrared region of the electromagnetic spectrum. When a range of IR frequencies passes through a crystal some of the frequencies that match with those of defects will be absorbed and those that do not will be transmitted. Thus, some of the frequencies will be almost completely absorbed and others will pass unimpeded. The spectrum obtained is characteristic of the material and the defects present. Infrared spectroscopy is an important experimental technique that gives detailed information about defects. In conjunction with other techniques could lead to the understating of the defects present in a material and therefore to their control for the purpose of improving the device yield in various applications. Two types of infrared spectroscopy are generally used. The Dispersive Spectroscopy where the relating instruments work by splitting the IR radiation into its component frequencies and looking at each one individually. The dispersive instruments look at intensities of various frequencies but the detector cannot determine the frequency hitting it. Fourier Transform Infrared Spectroscopy (FTIR) is another way of looking at the samples using infrared radiation. FTIR instruments get around the above problem that limits dispersive instruments. FTIR modulates infrared frequencies down to audio frequencies where the detectors can track both intensity and frequency. FTIR spectrometer has generally larger resolutions and can run more easily in the frequency range 0-400cm-1 were signals form electronic excitation usually occur. In conclusion infrared spectroscopy is a significant experimental method in studying defects in semiconductors. Its main advantage is that it is non –destructive technique. Infrared radiation can pass through the samples without destroying or changing them. It requires little sample preparation and the analysis time is short. Importantly, the method has strong theoretical support, since the subject of the electromagnetic interaction with crystals has been studied thoroughly. This allows a proper interpretation of the results without obscure points and misunderstandings.