Dyfuzja 3D domieszek w GaN - mechanizm i rola defektów

Diffusion of atoms in the crystal lattice is a very complex area of research, because movements of atoms depend not only on temperature, but also on their charge state (and this depends on the Fermi level of the crystal) and on presence of defects (point- and extended-defects). The literature data for GaN in this area is very limited because of three reasons: i) Most of the research is done for GaN layers grown on foreign substrates (sapphire and silicon). Such layers contain a very high dislocation density of more than 108cm-2 and these dislocations mask all other factors influencing the atom diffusion. ii) GaN is grown at relatively low temperatures- about 1000oC, whereas the melting point is of about 2500oC. Such material contains a high concentration of point defects (vacancies, interstitials, impurities) which influence the atom diffusion. These concentrations depend very strongly on growth conditions (temperature, pressure, flows of reactants) and are different for samples from almost every laboratory. iii) The classical semiconductors are cubic and in this case the diffusion coefficients are almost isotropic. In the case of GaN, which is hexagonal, we deal with very strong built-in electric fields (even a few MV/cm) which can be a driving force for the diffusion. The objective of this project is to study diffusion of main dopants (Mg, Be, Si, O) in GaN along different crystallographic directions. The principal axes of the hexagonal crystal, i.e. [0001] c axis, [-1100] a-axis and [1123] m axis have been chosen. These directions are perpendicular to the principal slip planes in the hcp structure: basal, prismatic and pyramidal, respectively. We will be able to provide a number of new information on diffusion because of the following world- unique technologies and analytical methods: i) The partner of our consortium Institute of High Pressure Physics (Unipress) has an access to the GaN crystals of the world-lowerst dislocation density (104 cm-2). Thus, we will be able to compare dopant diffusion in materials with different dislocation density. ii) The Unipress is also one of very few academic labs using two epitaxic techniques for growing GaN: MOVPE (Metalorganic Chemical Vapour Phase Epitaxy) and MBE (Molecular Beam Epitaxy). Using those methods we will be able to get materials with very different point defects. iii) The third Unipress unique technology is high pressure annealing. At high (10 kbars) pressure of nitrogen we will be able to anneal GaN at temperatures of about 1300oC, not accessible at 1 atmosphere. iv) The second partner, Lukasiewicz Research Network- Institute of Electron Technology (ITE), is equipped with implantation system which will enable us to introduce dopants in a controlled way into GaN crystals of various orientations. v) The Project coordinator, Lukasiewicz Research Network- Institute of Electronic Materials Technology (ITME) is equipped with the SIMS (Secondary Ion Beam Spectrometry) which has a depth resolution of a fraction of nanometer, and a lateral resolution of a few microns. Such SIMS system is used only by a few large companies, not by academic laboratories. The goal of this project is to get advantage of those world-unique technologies and analytical methods for creating microscopic models of dopant diffusion-mechanisms in GaN. In the project, we will introduce dopants by growth (MOVPE and MBE) and implantation into various GaN layers (of different orientations, of different defects and of different doping levels). The samples will be annealed at various conditions of temperature, time and atmosphere. The samples will be examined using a variety of analytical methods monitoring not only dopant diffusion but also changes in crystal microstructure. An important issue will be a development of sample capping protecting against interaction with the atmosphere during annealing. All experimental results will be confronted with the theoretical calculations. The project belongs to “curiosity driven research”, but will also provide important information for nitride semiconductor technologists.