Ultra high purity silicon is advantageously modified by as low as 5x1014 cm-3 nitrogen. Such a doping level was proved to drastically impact grown-in and process-induced defects, enhances the denuded zone intended for making devices, improves impurity gettering, and increases the gate oxide integrity in metal oxide silicon devices. Interestingly, with such a low nitrogen level wafer toughness is significantly increased. However, nitrogen doping alters standard wafer heat treatment processes through the modification of the early stages of point defect clustering dynamics.
In this thesis, the basic interactions of light element impurities, particularly N and O, with point defects and crystal defects in silicon are scrutinized in order to understand the mechanisms of extended defect nucleation and growth in N doped silicon. Experimental data are used with molecular dynamics and quantum mechanics calculations for modeling defect formation. Various thermal annealing have been utilized to produce diverse conditions for defect interactions. Defect type, size distribution, nanoscale and atomic structure, and composition have been determined with emphasis on the depth dependence. Nanoscale analysis of defects probed at different depths allowed to build models of point defect dynamics from the extended defect formation history. Defect nucleation during crystal growth was qualitatively discussed and defect precursors were mapped on the crystal hot zones showing point defect clustering stages during solidification. This was based on results from the atomistic modeling of atomic structure of chemical complex ground states, the thermodynamic parameters close to the melting point, and the adsorption/desorption of point defects by stable chemical complexes. It was found that N2 is a stable mobile species, VN2 is an active metastable complex, and V2N2 is an immobile stable nucleus for oxygen precipitation but not for vacancy clustering. The formation energy of VN2 was found positive by DFT calculations, which negates the spontaneous formation of isolated complex. However, the formation energy is reduced to about kBT/2 near the melting point by coupling to one oxygen atom, which activates the formation of VN2, while weakly bound to the oxygen. The calculated thermal stability of a wide range of prominent chemical complexes was cross-checked with the signature of experimentally proven viable ones. Furthermore, IR absorption line intensities in annealed wafers were obtained as a function of depth by high spatial resolution synchrotron FTIR, which allowed having N-V-O depth profiles. These appeared in good agreement with that of the oxynitride precipitate profiles by OPP and SIMS. Such an agreement represents a strong support for both chemical complex spectroscopic identification and calculated thermodynamic parameters.
At the macroscopic level, nitrogen appeared to slowly athermally segregate under compressive stresses to dislocations and wafer surface; the segregation is accelerated at high temperature. In addition, nitrogen was found to couple with oxygen to form oxynitride zones and it segregates to precipitate interfaces making N-rich shells. Finally, silicon mechanical properties measured by nanoindentation of a variety of substrates appeared to well correlate with the dislocation pinning by light elements such as N, O, and C. The locking mechanism was correlated to dislocation interaction with impurity atmospheres simulated using elasticity theory, the size effect model, and Tersoff inter-atomic potential.
