Stability of HfO2 / SiOx /Si surficial films at ultralow oxygen activity

Abstract

HfO2 / SiOx /Si layer structures have been fabricated using atomic layer deposition and subsequently annealed at partial oxygen pressures close to the equilibrium pressure of bulk SiO2. These ultralow oxygen partial pressures at around 10-40 atm have been achieved by a solid state buffer method. An interfacial SiOx layer with an approximate thickness of 1 nm is shown to be part of the equilibrium configuration of HfO2 on Si even at an oxygen activity eight orders of magnitude below the equilibrium pressure of bulk SiO2. Elimination of the silica-rich interfacial layer in any process permitting approach toward thermodynamic equilibrium seems highly improbable. By comparison with the case of pure SiOx on Si, it is shown that HfO2 acts as a "surfactant" to SiOx leading to a smooth film structure, while the pure SiOx on Si exhibits island formation under the same conditions. Furthermore, the oxidation state of Si in SiOx and the silicate composition at the HfO2 / SiOx interface can be tailored by the partial oxygen pressure. © 2008 The American Physical Society.