Bonding Configurations at Epitaxial CaF 2 /Si Interfaces
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BONDING CONFIGURATIONS AT EPITAXIAL CaF 2/Si INTERFACES J.L. BATSTONE* and JULIA M. PHILLIPS AT&T Bell Laboratories, 600 Mountain Ave., Murray Hill, NJ 07974, USA *Present address: Department of Materials Science and Engineering, University of Liverpool, P.O. Box 147, Liverpool, L69 3BX, UK ABSTRACT Determination of the bonding configuration at the epitaxial CaF 2/Si(1 11) interface has revealed CaSi bonds in the interfacial plane. Removal of an unstable layer of F from the interface occurs during a rapid thermal anneal. In comparison, the CaF 2/Si(100) interface is not smooth, making assignment of interface bonding configurations ambiguous. After a rapid thermal anneal, evidence for an intermediate interfacial phase is observed on Si (100). This phase does not occur on Si (111). INTRODUCTION The observation that cubic CaF2 will grow epitaxially on both Si(100) and Si(l 11) substrates' has led to the possibility of incorporating epitaxial insulators in three-dimensional device integration, either as the insulator in a metal-insulator-semiconductor (MIS) field effect transistor or as the dielectric layer for Silicon-on-Insulator where epitaxial Si can be grown on CaF 2 to give a Si/CaFJ/Si structure. In this paper we review our understanding of the CaFJSi(1 11) interface. In addition, results are presented from CaF2(100) films where a rapid thermal anneal (RTA) is seen to have a detrimental effect on interface quality. The CaF2/Si films were grown by sublimation of CaF2 from a Knudsen effusion cell onto clean ptype Si(11l) and Si(100) at temperatures -700"C for Si(11) and ~600"C for Si(100), in an ultrahigh vacuum molecular beam epitaxy chamber. Details of the growth conditions have been described elsewhere.3 An improvement in crystaflinity occurred during RTA which was performed in an Ar ambient at temperatures of -1 100°C for times -30-60 sec. Examination of -5000A thick CaF 2 films was performed using transmission electron microscopy (TEM) in a JEOL 4000EX operating at 200kV to minimize electron-beam induced decomposition of the CaF 2. Characterization of the electrical characteristics at the interfaces was performed by capacitancevoltage (C-V) techniques. Metal contacts were evaporated onto the CaF 2 to form a MIS diode to 4 detect both the density of interface states and the ability to form an inversion layer. CaF2 on Si (111) We have recently presented results which show a correlation between electrical properties and the bonding configurations at the epitaxial CaF 2/Si(1 11) interface before and after RTA. 5 High resolution electron microscopy (HREM) was used to infer the interface structure using a 6 combination of image simulations using multislice algorithms and rigid shift measurements. Figure 1 shows ball and stick models of proposed interface bonding configurations. As-grown films of CaF2 on Si(1 11), -5000A thick contain eight-fold coordinated Ca atoms with direct Ca-Si bonding at the interface, as illustrated in Figure la. Thus the interfacial Ca atoms have their bulklike eight-fold coordination. It i
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