Optical Properties and Surface Roughness of Ion Implanted Single Crystal Sapphire

Abstract

It has been shown that metal ion implantation can harden single crystal sapphire and introduce compressive stresses at the surface, which may lead to an increase in the fracture toughness of the material. This may significantly affect the usefulness of this material as a shatter-resistant optical window in missile applications. In this study, we have examined the extent to which sapphire can be implanted without severely degrading its optical quality by ion beam defect production. Optically-polished single-crystal c-axis sapphire was implanted with 150 keV Cr⁺, Ti⁺, and Si⁺ ions to doses of 0.3 – 3.0 × 10¹⁷ ions/cm² at both room temperature and at 800 °C, to measure the optical effect of in situ annealing. Rutherford backscattering spectrometry showed evidence of implant species migration only in the case of Ti implanted at high temperatures; all other implant profiles were Gaussian. The optical transmittance of the sapphire was examined using visible spectrum transmission and Fourier Transform Infrared Spectroscopy. Si implantation resulted in a 10 % reduction in infrared transmittance at the highest ion dose, but this was reduced to 6 % when the implantation was done at high temperature. Both Cr and Ti implantation reduced the sapphire IR transmittance (by 16 % and 42 %, respectively) , but the effect of implantation temperature on transmittance was different. High temperature during implantation increased the transmittance of Cr-implanted samples, but further darkened Ti-implanted samples. Photon tunneling and atomic force microscopy showed that both Ti and Cr implantation roughened the surface of the material.