Positron diffusion in germanium

Abstract

The temperature dependence of the diffusion constant of positrons in Ge single crystals is discussed. Experimental data on the fraction (fp, ) of 0 -5-keV incident positrons reemitted from the surface as positronium are presented for Ge over the temperature range 300& T & 1020 K. The positron diffusion constant is deduced, at various temperatures, from an analysis using a onedimensional diffusion model for fp, which is determined as a function of incident positron energy E. The magnitude and temperature dependence of the positron diffusion constant are compared with the predictions of the conventional weak carrier-phonon scattering theory that is used to describe the mobilities of electrons and holes in the elemental semiconductors.Conventional carrier-phonon scattering does not adequately represent the observed temperature dependence of the positron diffusion constant, and we investigate some possible origins of these discrepancies. Quantitative calculations identify several of these possibilities as implausible. We tentatively conclude that some form of positron-lattice coupling is the dominant interaction governing the positron motion.

Theory of positrons in solids and on solid surfaces

“…However, in the case of semiconductors with a narrow gap, this does not hinder thermalization appreciably; i.e. , the thermalization time is of the same order as in metals (Jorch et al , 1984). In the case of wide-band-gap insulators, electron-hole excitations are no longer possible when the positron energy reaches the region of a few eV's.…”

Fraction of Positronium Formation at Semiconductor Surface

“…Figure 1 shows a plot of calculated results of Ps fraction (f ps) as a function of incident energy of positron for two temperatures in the case of Ge(110). The calculated results are compared with the experimental observations of Jorch et al [5]. One can see from these graphs that the general trends of the experimental curves are well reproduced.…”

“…Considering the exponential type of implantation profile and a = AEn, where E [keV] is the energy of the incident positrons. The value of n is taken to be 1.6 as per experimental observations [5]. The solution of Eq.…”

“…The first few studies were bulk Doppler broadening measurements of the positron mobility in solid state Ge and Si by Mills and Pheiffer [4] yielding values for positron diffusion coefficients (D+) that were comparable with other solids. These measurements were followed by positron beam studies of D+ vs. temperature in Ge by Jorch et al [5], measuring the fraction of positronium formation. In this work, it was observed that D+ did not follow any simple model.…”

Positronium Fraction and Positron Life Time at Ni(110) and Ge(110) Surfaces

“…2), the positronium fraction Ps is constant at low temperatures up to 600 K and then it increases almost linearly with temperature up to 1100 K and after that it becomes nearly constant. These results have been compared with the available experimental results of Jorch and Lynn [13] in Table III. We have found that the calculated results agree well with the experimental ones.…”