Use of ultrathin ZnSe dipole layers for band offset engineering at Ge and Si homo/heterojunctions

Abstract

The ability to control semiconductor band discontinuities would allow solid devices to be specifically tailored so that efficiency and performance could be dramatically improved. This article reports the use of an ordered ZnSe monolayer to induce a valence band discontinuity at the Ge homojunction (0.38 eV), at the Ge–Si heterojunction (0.53 eV), and at the Si homojunction (∼0.2 eV). Soft x-ray photoemission was used to probe the interfaces as they were formed under ultrahigh vacuum conditions. The effect of overlayer band bending on the interpretation of band offset measurements is discussed. As the interfacial bonding and orientation of the dipole layer are key factors in determining the direction and magnitude of the band modification, x-ray standing wave measurements were performed on the Ge–ZnSe–Ge systems to identify the atomic structure of the junction. Se atoms were always found to bond to the Ge substrate in the a-top position, while the Zn atoms adopted the H3 sites, bonding to the overlayer. The results for these interfaces are interpreted in terms of the charge transfer; other factors such as strain and order are also addressed.