Lattice location of ion-implanted 6He in diamond

TL;DR

This study investigates the lattice position and diffusion behavior of ion-implanted 6He in diamond using beta emission channeling, revealing preferred tetrahedral interstitial sites and estimating migration activation energy.

Contribution

It provides experimental evidence of 6He occupying tetrahedral interstitial sites in diamond and estimates its migration energy, aligning with theoretical predictions.

Findings

6He predominantly occupies tetrahedral interstitial sites in diamond.

Implantation at high temperature reduces interstitial fraction, indicating diffusion.

Estimated activation energy for He migration is around 1.63-2.89 eV.

Abstract

We report on the lattice location of the short-lived ion implanted nuclear probe 6He (t1/2=807 ms) in diamond, which was performed using the beta emission channeling method at CERN's ISOLDE facility. 6He was implanted with 30 keV into a single-crystalline artificial diamond sample kept at a temperature ranging from 30 deg C up to 800 deg C. By means of comparing the measured emission channeling patterns along different crystallographic directions with simulated yields for a variety of possible sites, we conclude that the implanted 6He occupies tetrahedral (T) interstitial sites, in agreement with theoretical predictions that T sites should be the preferred position of He in diamond. Implantation at 800 deg C resulted in a drop in the tetrahedral interstitial fraction by 20%, which we interpret as the onset of diffusion, 6He thus being able to change to lattice sites of low crystallographic symmetry, or reach the surface of the sample or escape to the bulk during its lifetime. We estimate the activation energy for interstitial migration of He to be around 1.63-2.89 eV, which agrees with theoretical predictions of 1.41 eV, 1.97 eV, 2.35 eV and 2.36 eV from the literature. Activation energies around 2 eV would mean that simple interstitial He cannot be stable in diamond on geological time scales, thus to remain inside, it should be bound to some defect in the material or exist in another form such as within inclusions of other minerals or liquids, or possibly small He bubbles.