Liquid Injection MOCVD of Rare-Earth Oxides Using New Alkoxide Precursors
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Liquid Injection MOCVD of Rare-Earth Oxides Using New Alkoxide Precursors Paul A. Williams1, Anthony C. Jones1, 2, Helen C. Aspinall2, Jeffrey M. Gaskell2, Paul R. Chalker3, Paul A. Marshall3, Yim F. Loo2 and Lesley M. Smith1 1 Epichem Limited, Power Road, Bromborough, Wirral, Merseyside, CH62 3QF, UK 2 Department of Chemistry, University of Liverpool, Liverpool, L69 7ZD, UK 3 Department of Materials Science and Engineering, University of Liverpool, Liverpool, L69 3BX, UK ABSTRACT High purity lanthanum oxide and praseodymium oxide thin films (C< 1 at.-%) have been deposited by liquid injection MOCVD using the volatile alkoxide precursos [La(mmp)3] and [Pr(mmp)3] in toluene-solution (mmp = OCMe2CH2OMe). 1H NMR solution studies have shown that the addition of donor species, such as tetraglyme (CH3O(CH2CH2O)4CH3) or mmpH prevent molecular aggregation and help stabilise the precursors.
INTRODUCTION Rare-earth oxides, such as lanthanum oxide [1] and praseodymium oxide [2] are promising alternative high-κ gate dielectric layers in Si-based field effect transistors. They are good insulators due to their large band-gaps (3.9 eV for Pr2O3), have high dielectric constants (La2O3 κ = 27, Pr2O3 κ = 26-30) and are more thermodynamically stable on silicon than other high-κ materials such as ZrO2 and HfO2. Metal-organic chemical vapour deposition (MOCVD) is an attractive technique for the deposition of metal oxides [3], having a number of advantages over other deposition techniques. These include the potential for large area growth, good film uniformity, and excellent conformal step coverage at device dimensions less than 2µm. Despite this, there have been very few reports of the successful MOCVD of rare-earth oxides, mainly due to a lack of suitable precursors with appropriate physical properties and decomposition characteristics. For instance, La2O3 films grown from the β-diketonate precursors [La(thd)3] (thd = 2,2,6,6-tetramethylheptane-3,5-dionate) and La(acac)3 (acac = acetylacetonate) showed heavy carbon contamination [4,5]. [Pr(hfac)3] (hfac = hexafluoroacetylacetonate) has been investigated [6], but led to the deposition of the oxyfluoride, rather than the pure oxide, and although- Pr2O3, was grown by MOCVD using [Pr(thd)3], the high growth temperatures used (750oC) are unsuited to high-κ dielectric processing. There is thus an urgent requirement for new, improved rare-earth oxide MOCVD precursors. Metal alkoxides have been widely used in the MOCVD of metal oxides, and generally allow lower growth temperatures than the more thermally stable metal β-diketonate precursors [3]. However, there are few reports in the literature into the use of rare-earth alkoxide precursors in MOCVD. This is because the large ionic radius of the highly positively charged lanthanide (III) ions leads to the formation of bridging intermolecular metal-oxygen bonds, resulting in many of the simple alkoxide complexes being polymeric or oligomeric, with a corresponding low volatility. However, the sterically hindered donor functionalis
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