Diffusion of point defects on crystalline surfaces and in their bulk is an important and ubiquitous phenomenon affecting lm quality, electronic properties and device functionality. A complete understanding of these diffusion processes enables one to predict and then control those processes. Such understanding includes knowledge of the structural, energetic and electronic properties of these native and non-native point defect diffusion processes. Direct experimental observation of the phenomenon is difficult and microscopic theories of diffusion mechanisms and pathways abound. Thus, knowing the nature of diffusion processes, of specific point defects in given materials, has been a challenging task for analytical theory as well as experiment.
The recent advances in computing technology have been a catalyst for the rise of a third mode of investigation. The advent of tremendous computing power, breakthroughs
in algorithmic development in computational applications of electronic density functional theory now enables direct computation of the diffusion process. This thesis demonstrates such a method applied to several different examples of point defect diffusion on the (001) surface of gallium arsenide (GaAs) and the bulk of cadmium telluride (CdTe) and cadmium sulfide (CdS).
All results presented in this work are ab initio, total-energy pseudopotential calculations within the local density approximation to density-functional theory. Single particle wavefunctions were expanded in a plane-wave basis and reciprocal space kpoint sampling was achieved by Monkhorst-Pack generated k-point grids. Both surface and bulk computations employed a supercell approach using periodic boundary conditions.
Ga adatom adsorption and diffusion processes were studied on two reconstructions of the GaAs(001) surface including the c(4×4) and c(4×4)-heterodimer surface reconstructions. On the GaAs(001)-c(4×4) surface reconstruction, two distinct sets of minima and transition sites were discovered for a Ga adatom relaxing from heights of 3 and 0.5 A from the surface. These two sets show significant differences in the interaction of the Ga adatom with surface As dimers and an electronic signature of the differences in this interaction was identified. The energetic barriers to diffusion were computed between various adsorption sites.
On the GaAs(001)-c(4×4)-heterodimer reconstruction, structural and bonding features of the surface were examined including a comparison with the c(4×4) reconstruction. Minimum energy sites (MES) on the c(4×4)-heterodimer surface were located by mapping the potential energy surface for a Ga adatom. Barriers for diffusion of a Ga adatom between the neighboring MES were calculated by using top hopping- and exchange-diffusion mechanisms. Signature differences between electronic structures of top hopping- and exchange-diffusion mechanisms were studied for relevant atoms. A higher diffusion barrier was observed for the exchange mechanism compared to top hopping.
Diffusion profiles for native, adatom and vacancy, and non-native interstitial adatoms were investigated along the open [1 1 0] channel in bulk zinc-blende CdTe. This includes native Cd and S and non-native Cu, Ag, Au, Mo, P, Sb, O, S, and Cl. High symmetry Wyckoff positions were found to be the global minimum energy location for Cd, Ag, Mo, O and Cl interstitials. Adatoms of Cu, Au, P, Sb, S show an asymmetric shape of the energy diffusion barrier with two structurally equivalent minima and two energetically distinct maxima in the pathway. Adatoms of Mo, Ag and Cd interstitial and vacancy, show a symmetric diffusion barrier with two structurally unique minima and a maximum. Adatoms of O, Cl, and Te interstitial and vacancy, show a symmetric diffusion barrier with a unique maximum and minimum. Diffusion for Cu, Au, Te and S interstitials proceeds along the [1 1 0] channel in a near straight line path. Diffusion for Cd, Ag, O and Cl proceeds along two nearly straight line paths along [1 1 1] and [1 1 -1]. Diffusion for Mo, P and Sb is along the [1 1 0] channel deviating slightly from the straight line paths along [1 1 1] and [1 1 -1]. The diffusion barriers range from a low of 0.10 eV for a Ag interstitial to a high of 1.83 eV for a Cd vacancy. The barriers for Cu, Ag, Te, Cl and S are in agreement with the available experimental data. The symmetric or asymmetric nature of the diffusion path as well as the bond length and atomic coordination at the energetic extrema positions were found to influence the size of the diffusion energy barrier. In addition there exist electronic signatures in the local density of states for the bond breaking, difference in the hybridization and energy of occupied states between the global minimum and global maximum energy positions.
Diffusion pro les for native Cd and S, adatom and vacancy, and non-native interstitial adatoms of Te, Cu and Cl were investigated in bulk wurtzite CdS. The interstitial diffusion paths considered in this work were chosen parallel to c-axis as it represents the path encountered by defects di using from the CdTe layer. Because of the lattice mismatch between zinc-blende CdTe and hexagonal wurtzite CdS, the c-axis in CdS is normal to the CdTe interface. The global minimum and maximum energy positions in the bulk unit cell vary for different diffusing species. This results in a significant variation, in the bonding configurations and associated strain energies of different extrema positions along the diffusion paths for various defects. The diffusion barriers range from a low of 0.42 eV for an S interstitial to a high of 2.18 eV for a S vacancy. The computed 0.66 eV barrier for a Cu interstitial is in good agreement with experimental values in the range of 0.58 - 0.96 eV reported in the literature. There exists an electronic signature in the local density of states for the s- and d-states of the Cu interstitial at the global maximum and global minimum energy position.
The work presented in this thesis is an investigation into diffusion processes for semiconductor bulk and surfaces. The work provides information about these processes at a level of control unavailable experimentally giving an elaborate description into physical and electronic properties associated with diffusion at its most basic level. Not only does this work provide information about GaAs, CdTe and CdS, it is intended to contribute to a foundation of knowledge that can be extended to other systems to expand our overall understanding into the diffusion process.