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

The ability to control semiconductor band discontinuities would allow solid devices to be specifically tailored so that efficiency and performance cou ld be dramatically improved. This article reports the use of an ordered ZnS e 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 (similar to 0.2 eV). Soft x-ray photoemission was used to probe the interf aces as they were formed under ultrahigh vacuum conditions. The effect of o verlayer band bending on the interpretation of band offset measurements is discussed. As the interfacial bonding and orientation of the dipole layer a re key factors in determining the direction and magnitude of the band modif ication, x-ray standing wave measurements were performed on the Ge-ZnSe-Ge systems to identify the atomic structure of the junction. Se atoms were alw ays found to bond to the Ge substrate in the a-top position, while the Zn a toms adopted the H-3 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. (C) 1999 American Vacuum Socie ty. [S0734-211X(99)07504-6].