Growth Morphology and Electronic Structure of Ultra-Thin TaO x Films on Ag(100)
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using molecular beam epitaxy (MBE) techniques (e.g., thermal evaporation of tantalum in an oxygen atmosphere). The main drawback of this technique is the formation of an interfacial SiOz layer.J6] This can be reduced by depositing tantalum without oxygen, followed by oxidation of the tantalum film.[5] However, this results in a film with a much higher number of structural defects. In fact, for all of these growth techniques, there will be some degree of SiO2 formation at the interface. Although tantalum oxide shows promise for replacing 802 for many thin film applications, the basic electronic properties of the various crystallographic forms of tantalum oxide are not well understood. This is primarily due to the fact that high quality bulk crystals of tantalum oxide are not commercially available. In addition, the insulating nature of tantalum oxide would make it difficult to study bulk crystals using either electron or ion spectroscopies. One technique that has been used to study the properties of insulating metal oxides is to grow the metal oxide as a thin film on a noble-metal, single-crystal substrate.[8] The noble-metal substrate can act as a template to induce order in the metal oxide overlayer, without forming a mixed interfacial oxide. If the metal oxide film is thin enough (less than -5 nm), any excess charge induced during ion or electron spectroscopies will tunnel into the metallic substrate. In this study, the growth morphology and electronic properties of tantalum oxide films on a Ag(100) substrate have been examined. Since silver does not oxidize at the oxygen pressures and temperatures used for the growth of the TaO, films, the effect of the ordered Ag(100) substrate on the crystal structure of the TaOx films can be determined directly. There are three stable crystal forms for tantalum oxide: TaO, TaO 2, and Ta 2O5 . The most stable form of tantalum oxide is generally considered to be Ta,0 5 . It crystallizes in the vanadium oxide (V 20 5) structure. This forms an orthorhombic lattice with lattice parameters of a0 = 6.2 A, b0 = 3.7 A, co = 3.9 A.[9] The crystal structure of TaO 2 is the rutile structure. This forms a tetragonal lattice with lattice parameters of ao = 4.7 A and c. = 3.1 A.[10] The crystal structure of TaO is the rocksalt structure, which is a face centered cubic (fcc) lattice with a two atom basis. Its lattice parameter is a0 = 4.4 A.[10]. Either TaO or TaO 2 can be formed when growing tantalum oxide under oxygen deficient conditions. The primary goal of this study is to examine the properties of Ta,0 5 . This proves somewhat problematic when trying to find a lattice matched noble-metal substrate since there are no noble metals with an orthorhombic symmetry. Since silver is a fcc crystal with a lattice parameter of a0 = 4.1 A, the closest match of Ta2O5 with Ag(100) would be with the b-c plane of the Ta20 5 that is rotated by 45*. This would result in a tensile strain of the overlayer of -7%. EXPERIMENT These experiments were performed at the CAMD synchrotron in Baton Rouge, Louisiana.
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