Lattice Location of Low-Z Impurities in Medium-Z Targets Using Ion-Induced X-Rays

Abstract

The common method of Rutherford backscattering and channeling of light energetic ions is in general not suited to lattice location studies of impurity atoms having a mass similar to or lower than the host. While specific nuclear reactions are sometimes available they usually require high beam doses and yield high backgrounds of scattered particles. In two such situations we have used ion induced x-ray yields to determine lattice location viz. for ³²S and ³¹P implants in Ge single crystals. In the course of this work we have had to identify and optimize a number of experimental parameters, in particular how the beam type affects (a) ψ_1/2,x_min and crystal damage rates, (b) x-ray yields (P-K, S-K, Ge-L and Ge-K), target bremsstrahlung and recoil-induced molecular x-ray intensities. Choice of detector geometry, aperture and window also proved to be important. Detection limits for P and S are now certainly better than 1 x 10¹⁴ atoms.cm^-2 in a thick Ge target for 0.5 MeV proton excitation. We have found that a room temperature implant of 40 keV ³¹P annealed at 450^oC is highly (93%) substitutional in Ge for a dose of 0.7 x 10¹⁵ ions.cm^-2, but shows a much lower fraction at 2.7 x 10¹⁵ ions.cm. Lattice location of S implanted into Ge parallels the pattern from Group VI impurities implanted and annealed in Si, showing ≤ 50% x_min. values for the S signals in <110> and <111> directions. A different distribution for S is implied by <100> channeling data and <111> and <110> angular scans.

This is a condensed version of a longer two-part paper that has been submitted to the Journal of Applied Physics.