The work presented in this thesis has been carried out under the ongoing programme on the study of the as yet uninvestigated characteristics of the transition metal-related defects in silicon in the Semiconductor Physics Laboratory. Pd- and Pt-related defects in silicon had a1ready been characterised in this Laboratory using deep-level transient spectroscopy. Now the same technique has been employed to study the defects in silver- and gold-doped silicon. The emission rate signatures, activation energies, and the values for capture cross-sections of main silver-and gold-related defect levels are reported in literature. A detailed survey of the available information on these and other characteristics of TM-related defects, is presented in Chapter 2. From a review of the available data it emerges that investigations of the effects of irradiation and thermal annealing on the silver- and gold-related defects in silicon are either scarce or lacking in the literature. The response of the TM-related defects to irradiation and annealing are expected to provide a deeper insight into the nature of the defects. During this work, the effects of alpha irradiation on silver-related defects in p- and n-type silicon and gold-related defects in n-type silicon have been studied. Isochronal thermal annealing characteristics of the TM-related defects before and after irradiation have formed another focus of our study. In addition to the characterisation of silver- and gold-related defects, some striking new aspects of the Fe-related deep levels in silicon have also been unravelled by our study.
The samples used for these measurements were either linearly graded (Ag-doped p-type silicon) or step junctions (Ag- and Au-doped n-type silicon). 5.48 MeV alpha-particles from 241 Am were used for irradiation of the samples. The linearly graded nature of the n + p junctions used allowed the acceptor(Ec - 0.54 eV) and donor (Ev + 0.35 eV) levels associated with silver to be studied simultaneously. The acceptor level was found to be stable against thermal annealing performed upto~ 400 °C, while the donor level showed a decrease beyond 300 °c Important effects related with rapid thermal quenching of samples were revealed by a comparison of our data with the corresponding data on identical virgin (unquenched) samples. Irradiation by α-particles led to interesting new insights into the interaction of silver-related defects and the radiation-induced defects. New aspects of the kinetics and formation mechanism of the carbon-related defects in Si were brought to light as a result of the room temperature isothermal annealing and higher temperature isochronal annealing studies on the irradiated samples with and without silver. Irradiation also produces interesting effects on the Ag-related levels - while the acceptor level Ag(A) remained more or less stable, the donor level Ag(D) showed a small but systematic increase in concentration with a.-radiation dose. These observations, together with the different annealing behaviours, suggested that the two levels due to silver are not coupled states of the same defect.
New levels, in addition to the well known silver-related deep donor and acceptor levels, have been found during the investigations on silver-doped n-type silicon. Isochronal thermal annealing before and after irradiation provide interesting insights on interactions between the TM-related and radiation-induced defects and on the nature of the silver-related levels. In particular, the two newly observed prominent silver-related levels exhibit a complementary annealing behaviour, suggesting a mutual thermal transformation. The presence of silver is seen to produce a significant change in the annealing characteristics of the prominent radiation induced A-centre defect.
Thermal annealing characteristics of the well-known gold acceptor up to temperatures as high as about 600 °c were measured during pre-irradiation annealing of the gold-doped samples. This level was found to be very stable, supporting the substitutional-gold model for the acceptor centre. A slight reduction in the gold-acceptor concentration is observed due to irradiation, contrary to an earlier reported study. A noticeable suppression of the thermal stability of this level is also observed after irradiation. The presence of gold leads to a significant enhancement of the anneal-out temperature of the well-known A-centre defect. A deep level at Ec- 0.34 eV, ascribed to the Fe-Au complex, is found to be enhanced by irradiation. We find no evidence to support the recently proposed gold-divacancy complex model of the gold acceptor - rather, our results strongly support the isolated substitutional impurity model for this centre.
In addition to the effects indicated above, irradiation of above mentioned TM-doped silicon samples with alpha-particles produced the usual radiation-induced levels. A number of new annealed-in levels were also observed after thennal annealing. Emission rate signatures. activation energies and annealing characteristics of these annealed-in defects are reported in detail in this thesis.
The interstitial iron (Fei) donor in thermally quenched p-type silicon and its interaction with α-radiation and 4d and 5d transition metals forms the last and a very interesting part of our study. This centre was revealed by minority cattier injection in our samples with no deliberate iron doping. A number of its characteristics including the emission rate signatures, its variation with thermal quenching temperature, its production on dissociation of iron-boron complex with minority carrier injection. its decay on room temperature storage after production and its (room temperature) isothermal and higher temperature isochronal annealing behaviours have been studied. These measurements definitively identify it with the inadvertent iron impurity in our samples. A significant reduction in the concentration of iron interstitial centre was observed after irradiation with alpha particles. A striking new property of the iron interstitial donor revealed by our investigations on p-silicon samples doped with transition metal impurities Ag (4d), Pd (4d), Au (5d) and Pt(5d)is a complete bleach out of the DLTS signal of the Fei donor in the presence of these impurities, leading to the suggestion that iron has a strong tendency to form complexes with these transition metal impurities.