Brian G. Willis, University of Delaware, Colburn Laboratory, 150 Academy Street, Newark, DE 19716, D.B. Skliar, Chemical Engineering, University of Delaware, Colburn Laboratory, 150 Academy Street, Newark, DE 19716, and A. Mathew, Materials Science and Engineering, University of Delaware, DuPont Hall, Newark, DE 19716.
Over the past 10 years, there has been significant progress in the heteroepitaxy of perovskite oxide layers such as SrTiO3 on silicon substrates using molecular beam epitaxy (MBE) methods. These perovskite oxide materials have numerous useful properties including ferroelectricity, pyroelectricity, and piezoelectricity that could be useful for novel electronic, optical, or sensing devices. From basic surface chemistry studies, it has been found that alkaline earth metals form a good template for epitaxial oxide growth on silicon due to their ability to stabilize the surface and prevent silicon oxidation. Because of the unfavorable capital and operating costs, as well as slow growth rates (nm/hr) and low throughput of MBE growth methods, it would be advantageous if a chemistry based growth technique such as chemical vapor deposition or atomic layer deposition (ALD) could be discovered. This paper will present ultra-high vacuum (UHV) surface chemistry investigations of three possible process routes to epitaxial oxides through the growth of an alkaline earth template layer using chemical reactions. UHV-scanning tunneling microscopy (STM) data combined with density functional theory calculations have been used to investigate the surface chemistry of the protonated beta-diketonate ligand dpmH (dipivaloylmethane), which is used to volatilize the strontium metal-organic precursor (Sr(dpm)2). Analysis of the data lead to the conclusion that dpmH reacts with the Si(100)-2x1 surface through a dative bonded intermediate followed by a O-H cleavage as the dominant pathway. The strong interaction with the surface leads to undesirable carbon incorporation and a c(4x4) re-structuring of the surface at elevated temperatures. A second pathway using water termination of Si(100)-2x1 to hydroxylate the surface has been investigated with UHV-STM, x-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and reflection high-energy electron diffraction to analyze the H2O/Si(100) surface chemistry and the reaction of HfCl4 as a model compound. Analysis of the data show the successful growth of an abrupt metal-oxide/silicon epitaxial interface, but the process window is limited by the kinetics of hydroxyl decomposition on Si(100). UHV-STM data of the hydroxyl decomposition/ oxidation reaction will be presented and it is argued that oxidation occurs locally from reaction at islands of OH clusters. The implications of this reaction for the ALD of metal oxides will be discussed. Finally, a third reaction chemistry using alkaline earth catalytic de-oxidation of the SiO2/Si(100) surface is also found to a promising route for epitaxial oxides. Auger electron spectroscopy, Rutherford backscattering spectrometry, and low energy electron diffraction data will be presented to demonstrate the successful metal-organic growth of a 3x reconstructed Sr/Si(100). Issues related to ALD of the alkaline earth compounds will also be discussed.