A density functional study of point defects in nanoelectrical materials

Abstract

Materials used in technology, from the development of nano-electronics to the efficient conversion of solar energy to electrical power are under constant optimisation. The factors that govern the size of field effect transistors, such as leakage current through the gate or the on-state power consumption, and those that impact upon the efficiency of optoelectronic devices such as non-radiative recombination and device aging arising from defect migration are specific to the elements and compounds deployed in these devices. Defects, whether intentionally incorporated such as electrical doping, or unintentional contamination such as oxygen in silicon, may have a qualitative impact upon the materials properties (e.g. conductivity), or limits in processing that need to be accommodated in both the manufacture of devices, and in their operation. Device miniaturisation for CPUs and photovoltaic efficiency improvements are two areas developing in parallel for which a detailed understanding of the composition and evolution of defects, dopants and impurities are particularly important. In the project presented in this thesis, first principles density functional calcu- lations within a supercell approach have been performed, with particular focus on selected impurities in a range of technologically relevant materials, where the impu- rities are likely to be incorporated due to their presence in the growth environment. The host materials are divided into those relevant for nano-electronic devices and high capacitance structures, being the perovskite titanates (SrTiO3, BaTiO3 and PbTiO3), and material that has a role in the energy sector in photovoltaic cells, being cadmium telluride (CdTe). In the case of the titanate films, it is important to recognise that thin films are often grown using organic precursors, and therefore carbon contamination is of key importance. Although it is generally assumed that carbon is incorporated in such materials in the form of carbonates, the results of the calculations performed for this study challenge this, showing that a distribution of multiple sites occurs, depending upon growth conditions and the Fermi level. Observable calculated for these struc- tures, including their electrical properties and vibrational modes are presented to aid the identification of the carbon sites in future experimental studies. Critically, car- bon substitution at the Sr, Ba or Pb site leads to electrical effects not present for Ti substitution. The similarities and differences between the three titanates are reviewed. For CdTe, it has long been understood that oxygen is a common impurity grown in thin-films, occurring in high concentrations across a range of growth methods. Vibra- tional modes observed in experiment at 1096.78 and 1108.35 cm−1 have been variously assigned to oxygen-containing point defects, but most recently to SO2 molecules dis- solved in the lattice. However, the precise structure and location of these centres, as well as the electrical properties of the defect, are yet to be determined. As with the analysis of carbon in the titanates, density-functional simulations of SO2 in various locations in CdTe show that several possible structures are low in energy, with the equilibrium form depending upon growth conditions and the Fermi-level. It is shown that a plausible candidate for the vibrational centre is an interstitial species, based upon the frequencies and isotopic splittings.