First-principles study of the valence band offset between silicon and hafnia

Interfaces between silicon and oxides are of great fundamental and technological interest. According to the International Technology Roadmap for Semiconductors, the SiO2-based gate oxide in metal-oxide-semiconductor MOS transistors used in computer processors should be replaced by a higher-K value dielectric material starting in 2008.1 Presently, off-state gate tunneling is increasing power consumption and degrading performance. Many obstacles exist for incorporating alternative gate dielectrics into MOS transistors. Finding dielectric materials with sufficiently high band offsets to limit gate tunneling is essential. Metal oxides, such as zirconia ZrO2 , hafnia HfO2 , and others, have generated interest. Recently, hafnia has emerged as the leading candidate for a replacement gate dielectric.2,3 In addition to having dielectric constants approximately five times SiO2, hafnia-based oxides are structurally stable on silicon during fabrication and operation. This interest in hafnia for gate dielectric applications has motivated a desire to understand the fundamental properties of hafnia-based bulk and interfacial systems. Several groups have previously investigated the bulk properties of hafnia. Bulk physical, electronic, and dielectric properties have been studied with first-principles densityfunctional theory4–7 and are well understood. While the Si/HfO2 heterojunctions have been investigated experimentally, they are not well understood. Several groups have completed theoretical studies of silicon-zirconia interfaces;8–11 however, less attention has been given to interfaces between silicon and hafnia.8,9 Moreover, in the two studies reporting valence band offsets, there is a 2 eV spread in the values reported. Overall, the physics controlling the silicon-hafnia heterojunction band diagram is poorly understood. In this paper, we present density-functional calculations for interfaces of siliconand hafnia-based oxides. We construct models of Si 001 slabs with varying layers of strained phases of hafnia on top. Two types of oxygen passivation are considered. The goal of the present study is to help elucidate the fundamental physics relevant to the Si 001 /HfO2 interface and its band diagram.