A positron beam study of hydrogen confined in nano-cavities in crystalline silicon

Abstract

The Positron Beam Doppler Broadening (DB) and Two Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR) techniques have been applied to study the behavior of hydrogen confined in nanometer size cavities in crystalline silicon. A layer of subsurface cavities was successfully created by He + ion implantation at 33 keV to a dose of 3 × 10 16 cm −2 followed by an annealing in vacuum at 800 °C for 30 min. Hydrogen was introduced by implanting 28 keV hydrogen ions to doses of 1, 3 and 6 × 10 16 cm −2, respectively, followed by annealing from 200 to 900 °C. Results for the low dose H implantation show two main features. Below 400 °C the hydrogen dissociates from the small defects and re-trapping of hydrogen in the pre-existing cavities takes place. It is not until about 700 °C that the hydrogen is released from the cavities. For the higher dose a similar behavior is observed although the defects created by the H implantation itself start to play a role in the initial redistribution of the hydrogen. A novel method to fill the cavities by in-diffusion from a deuterium ambient (16 h at 550 °C in 10 bar) was successfully tested. In this case, the release of deuterium was found to occur in a narrower temperature interval starting at 600 °C. The gas release from the cavities is quantitatively understood using literature data for the of heat of solution and migration- and adsorption enthalpies of H in Si.