Positron Probing of Defects on Nanosurfaces and Electronic Materials

Jun Xu

Oak Ridge National Laboratory

Positrons, anti-particles of electrons, constitute many unique techniques that are sensitive, selective, and specific to defects on materials. After being implanted into a solid, positrons are trapped in vacancy sites and annihilate with both valence and deeper level electrons of surrounding atoms. The annihilation photo peak is broadened from 511 keV due to the Doppler effect induced by the electron momentum. Doppler broadening reflects the vacancy concentration and size. The lifetimes of trapped positrons are inversely correlated to the densities of electrons in the trapping sites, a longer lifetime indicating a larger size of vacancy cluster since the electron density in these clusters is lower than that of a smaller-size vacancy cluster. Two examples in using positron probe to be presented are the following: MeV implantation of gold ions into MgO (100) followed by annealing is a method to form gold nano-particles for obtaining modified optical properties. Surface phenomena dominate nanomaterial properties because of their high surface-to-bulk ratio. We used advanced positron (anti-electron) spectroscopy to reveal clusters of four atomic vacancies located at the surface of gold nanoparticles embedded in the magnesia matrix. These surface clusters are expected to mediate electron transfer between the nanoparticles and the matrix, which allows one to control optical properties, such as the surface plasma resonance frequency. A method for preparing shallow dopant distributions via solid-phase epitaxial growth (SPEG) following amorphization by low-energy Si self-ion implantation leaves defects that can lead to unwanted dopant impurity diffusion. The double implant method for SPEG uses both low- and high-energy Si self-ion implantation to remove most of the interstitials. Nevertheless, we find that measurable crystalline imperfections remain following the SPEG annealing step. Measurements of defect profiles using variable-energy positron spectroscopy show that there are divacancy-impurity complexes in the SPEG layer and V6 and larger vacancy clusters near the SPEG-crystalline interface.