Graphitization of a Polycrystalline Diamond under High-Fluence Irradiation with Noble Gas and Nitrogen Ions

Abstract

To analyze the process of the ion-induced graphitization of a polycrystalline diamond, the surfacelayer conductivity and microstructure are studied experimentally after high-fluence irradiation with Ne⁺, Ar⁺, N⁺, and ions with energies of 20–30 keV at irradiation and heat-treatment temperatures ranging from 30 to 720°R in vacuum. After irradiation with argon ions at room temperature and subsequent heat treatment, the resistivity ϱ of a modified layer decreases exponentially with increasing treatment temperature T_ht and reaches the graphite value ϱ at Tht = 700°R. Such a temperature T_ht is insufficient for surface-layer graphitization by nitrogen ions. The increase in the diamond temperature under irradiation leads to a decrease in the ion-induced thermal graphitization temperature T_g by several hundred degrees. It is found that the temperature T_g is almost coincident with the corresponding temperature T_a of the dynamic annealing of radiation-induced damage in graphite. Analysis of the irradiated layer using Raman spectroscopy reveals the heterogeneous structure of the modified layer containing graphite and amorphous phases, the ratio between which correlates with the layer resistivity. Under argon-ion irradiation at diamond temperatures of 500°R or more, an increase in ϱ of the irradiated layer is observed, which is related to the formation of nanocrystalline graphite. This effect is not observed under nitrogen-ion irradiation.