Joy Mukherjee,^a Sudip Bhowmick,^a Sabyasachi Karmakar,^b Dipak Bhowmik,^a Biswarup Satpati,^b Satyajit Hazra,^b Prasanta Karmakar^a
aIon Beam Development and Application Section, RIB Group, Variable Energy Cyclotron Centre, HBNI, 1/AF Bidhannagar, Kolkata 700064, India
^bSaha Institute of Nuclear Physics, HBNI, 1/AF Bidhannagar, Kolkata 700 064, India

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⁺+N₂⁺) 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 (∼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.