Systematic PAC-measurements of implantation induced uncorrelated defects indicate that even very small differences in defect concentrations cause distinct changes in the PAC-spectra of the semiconductor material InP [1]. The usual descriptions of these spectra with Gauss- or Lorentz distributed frequencies cannot begin to characterize defect properties satisfactorily. To achieve a better understanding of those properties a program was implemented that calculates PAC-spectra of random defect distributions in a simulated single crystal for probe atom with spin 5/2.
The simulation principle consists of simulating a semiconductor diamond lattice crystal with randomly distributed, uncorrelated point defects. A Monte-Carlo-routine is responsible for the number and distribution of these point defects around a probe atom (e.g. 111In) at the origin of a cubic volume (ca. 500.000 Si-atoms). The resulting electric field gradient (EFG) of the defect constellation can be calculated in the point-charge-model from which the perturbed angular correlation function can be derived. This step is repeated as often as needed. Averaging over all of these calculated functions leads us to the total perturbed angular correlation function.
Spectra of simulated defect distributions for several different defect/probe atom ratios were calculated and when compared to measured spectra of uncorrelatedly disturbed single crystals it is possible to derive information about screening effects, defect charge and - concentration. Measurements of this kind are currently being done.
[1] Bezakova et al, Appl. Phys. Lett. 75 (1999) 1923