Epitaxial Electronic Oxides on Semiconductors Using Pulsed-Laser Deposition
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D. P. Norton, J. D. Budai, and M. F. Chisholm Solid State Division Oak Ridge, National Laboratory, Oak Ridge, TN 37831-6056 ABSTRACT We describe the growth and properties of epitaxial (001) CeO2 on a (001) Ge surface using a hydrogen-assisted pulsedlaser deposition method. Hydrogen gas is introduced during film growth to eliminate the presence of the GeO2 from the semiconductor surface during the initial nucleation of the metal oxide film. The hydrogen partial pressure and substrate temperature are selected to be sufficiently high such that the germanium native oxides are thermodynamically unstable. The Gibbs free energy of CeO2 is larger in magnitude than that of the Ge native oxides, making it more favorable for the metal oxide to reside at the interface in comparison to the native Ge oxides. By satisfying these criteria, the metal oxide/semiconductor interface is shown to be atomically abrupt with no native oxide present. Preliminary structural and electrical properties are reported. INTRODUCTION Metal/oxide/semiconductor (MOS) structures are key elements in microelectronic applications. [1,2] Various semiconductor materials would be attractive for MOS-type device applications given a method to form well-defined oxide/semiconductor interfaces suitable for functional structures. For example, Ge and SiGe alloys are attractive semiconductor materials for electronic applications, possessing higher carrier mobilities and thermal conductivities than that of silicon. Germanium possesses a simple cubic crystal structure with µ n = 3900 cm2/V-sec, µ p = 1800 cm2/V-sec, and a thermal conductivity of 0.6 W/cm-K. Unfortunately, the native germanium oxides are not suitable for MOS-type device structures.[3,4] The formation of stable metal oxides on Ge could prove instrumental in the development of Ge and/or SiGe alloy integrated circuits. For applications involving sensors, photovoltaics, and optoelectronics, the formation of well-defined metal oxide/semiconductor interfaces for semiconductor materials other than silicon is vital to current and future device architectures. In many cases, one would prefer to have a well-defined metal oxide/semiconductor structure devoid of any native oxide at the interface, as the presence of native oxide at the interface limits the performance of these structures. This has been demonstrated for the case of silicon, in which molecular beam epitaxy was used to grow a crystalline oxide as a monolithic, commensurate structure on silicon.[5] In addition, a method to form oxides that are epitaxial on semiconductors would enable the integration of various epitaxial oxide materials and device structures with semiconductor electronics by providing a crystalline oxide template for additional epitaxial oxide film growth. In this paper, we report on the epitaxial growth and properties of CeO2 on (001) Ge using pulsed-laser deposition (PLD). Hydrogen is introduced as a background gas during the film nucleation in order to eliminate the native GeO2 on the Ge surface and achieve epitaxy.[6,7] The use of hy
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