X-ray photoelectron spectroscopy characterization and morphology of MgO thin films grown on single-crystalline diamond (
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H. Murakami Superconductor Photonics Center, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan (Received 15 August 2001; accepted 25 April 2002)
The morphology and composition of MgO films grown on single-crystalline diamond (100) have been studied. MgO thin films were deposited in the substrate temperature range from room temperature (RT) to 723 K by means of electron beam evaporation using a MgO powder source. Atomic force microscopy images indicated that the film grown at RT without O2 supply was relatively uniform and flat whereas that deposited in oxygen ambient yielded higher growth rates and rough surface morphologies. X-ray photoelectron spectroscopy analyses demonstrate that the MgO film deposited at RT without O2 has the composition closest to that of the stoichiometric MgO and that a thin contaminant layer composed mainly of magnesium peroxide (before etching) or hydroxide (after etching) was unintentionally formed on the film surface, respectively. These results will be discussed in relation to the interaction among the evaporated species and intentionally supplied oxygen molecules at the growth front as well as the interfacial energy between diamond and MgO.
I. INTRODUCTION
The properties of ultrathin metal oxide films have received considerable attention because of their important applications to heterogeneous chemical catalysis and microelectronics.1–5 For example, highly oriented MgO thin films have been widely used as structural templates for textured growth of other oxide films such as ferroelectrics6,7 and high-temperature superconductors.8,9 Furthermore, it was recently reported that the insertion of an ultrathin insulating layer such as MgO or LiF between a low-work-function electrode and an electron transport layer in light-emitting diode structures led to a decrease of the onset voltage as well as an increase of the light emission efficiency.10 However, it is well known that such insertion of a different material or heterostructure growth often leads to significant problems such as alloying, defect creation, and nonuniform growth near the interfaces. So far, MgO thin-film fabrications have been performed on single-crystalline Si,11–13 GaAs,6,7 and metal substrates such as Ag(100)14 and Ru(100).15,16 In these studies, metal oxide films were prepared either by postoxidation of deposited metallic Mg surfaces or by metallic Mg deposition in an oxygen ambient. On one hand, postoxidation of a deposited Mg film usually resulted in 1914
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J. Mater. Res., Vol. 17, No. 8, Aug 2002
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an amorphous or incompletely oxidized film17–19 although this method possibly brought forth more information about the initial growth process of MgO films under controlled conditions.20 On the other hand, simultaneous deposition by electron beam evaporation usually led to formation of an essentially stoichiometric (1:1) MgO film. As a matter of fact, Wollschlager et al. demonstrated that very different compositions and structures were obtained from a post
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