Alteration and interrogation of ultra-thin layer of silicon by reactive molecular ion implantation

We report the engineering of ultrathin top layer of Si substrate by low energy (14 keV) reactive ion bombardment. The physicochemical nature of the layer is interrogated and the optical properties are explored. Irradiation of nitric oxide (NO+) and mixture of carbon monoxide & nitrogen (CO+ + N-2(+)) ions on Si surface at normal incidence leads to formation of multiple chemical phases of Si with varying ion distribution within the layer. It is found that the chemistry of the topmost layer (similar to 5 nm) is massively modified by the reactive projectiles, while implanted rich layer is formed at a depth of 25 to 37 nm followed by an intermediate amorphous Si layer of 20 nm thickness. The Ultra Violet-Infrared (UV-IR) response of the modified layer reveals high sensitivity in the UV region. The change in optical bandgap, estimated from Kubelka-Munks theory, indicates the presence of multiple direct and indirect bandgaps. The potential applications of such chemically modified multi-layered ultra-thin Si are addressed here.