Structural,ferromagnetic,Optical And Electrical Properties Of Epitaxial SnO₂-based Semiconductor Films

Developing semiconductor thin films with desirable properties for practical applications and exploring their new properties for advanced applications have become the global research focus.With distinct physical and chemical properties and thus wide applications in spintronics and photocatalysis,SnO2 has attracted significant research interest in the recent decades.However,the origin of ferromagnetism is still a controversy.In particular,more applications will exist in the visible light region if their band gap is reduced.For this reason,it is crucial to understand their physical properties and closely correlate their ferromagnetism with their optical properties.In this dissertation,epitaxial SnO2 ? Sn0.94K0.06O2 and Sn1-xAlxO2 thin films were fabricated by using magnetron sputtering,and the structural,ferromagnetic,optical and electrical properties were systematically studied.1.Epitaxial SnO2 thin films were fabricated by direct-current magnetron sputtering,and the effect of mismatch strain on the structural and optical properties has been investigated.Tensile strain exists in the thin films and increases with decreasing the film thickness.Moreover,the optical band gap is significantly narrowed by increasing the tensile strain in the bc plane.This work indicates that the strain engineering is an effective approach to modulate the optical band gap of SnO2.2.Epitaxial Sn0.94K0.06O2 thin films were deposited by radio-frequency magnetron sputtering.Sn O2 buffer layers were used to alter the biaxial tensile strain induced by lattice mismatch in Sn0.94K0.06O2.When the tensile strain in the bc plane becomes larger,more K ions tend to occupy substitutional sites,with more holes being created.By enlarging the strain,the hole carrier concentration is increased while the optical band gap is narrowed.Besides,the saturation magnetization of the films increases as enlarging the strain,indicating the tensile strain can be used to significantly enhance the d0 ferromagnetism induced by holes.3.Epitaxial Sn1-xAlxO2 thin films were prepared by radio-frequency magnetron sputtering.The role of acceptor and donor defects on the structural,optical and magnetic properties of the films were investigated.Al doping introduces acceptor defects(AlSn)at low doping concentration while donor ones(Ali)in a concentration range from 8 to 10 at.%.The band gap is firstly narrowed by hole-doping and then widened by electrons introduced by oxygen vacancies and Ali.Air-annealing absorbs oxygen and makes Al ions transformed from Al Sn to Ali,corresponding to a decrease(increase)in the band gap when most Al ions occupy the substitutional(interstitial)sites.