X-RAY MEASUREMENTS OF LATTICE PARAMETER CHANGES IN ELECTRON-IRRADIATED SILICON

DENNIS DALE STUCKEY KAUFFMAN, Purdue University

Abstract

In this work a double-source, double-crystal X-ray spectrometer and a double-crystal X-ray topograph were constructed and refined. The spectrometer can compare the lattice parameters of two highly-perfect silicon crystals to an accuracy of 2 parts per billion. The topograph produces X-ray photographs displaying lattice parameter variations within a sample to a sensitivity of 200 parts per billion. Silicon samples doped with various concentrations of boron or arsenic were irradiated with 1-MeV electrons. The spectrometer and topograph were used to detect resultant changes in the lattice parameter. It was found that the lattice parameters of boron-doped crystals were smaller than the lattice parameter of a pure silicon crystal. Irradiation caused an increase in the lattice parameter initially proportional to the electron fluence. As irradiation continued the lattice dilation stopped before the parameter reached that of pure silicon. The difference between the final and original lattice parameters in the boron-doped samples was found to be proportional to the impurity concentration, with each boron atom contributing an increase of 0.16 atomic volumes to the crystal at saturation. After annealing to 520(DEGREES)C the lattice parameter of the most heavily doped sample returned to nearly its original value. It was also found that the boron impurities did not migrate significantly as a result of irradiation at 200 keV or 1 MeV. An arsenic-doped sample was also irradiated and showed similar expansion to a saturated state. The increase in crystal volume was found to be 0.02 atomic volumes per arsenic atom, but this value has a large uncertainty due to the unusually large change in the lattice parameter at saturation. Two high-perfection samples were also irradiated. They were undoped with resistivities in excess of 1500 ohm(.)cm. The lattice parameters of these samples showed no change to within several parts per billion after irradiation to a fluence of 4 x 10('18) electrons/cm('2). This is interpreted to mean that the damage produced by 1-MeV electrons is not stable at room temperature and must be trapped by impurity atoms in order to be retained by the crystal.

Degree

Ph.D.

Subject Area

Condensation

COinS