Development of Improved Precursors for the MOCVD of Bismuth Titanate
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Development of Improved Precursors for the MOCVD of Bismuth Titanate Paul A. Williams,1 Anthony C. Jones,1, 2 Neil L. Tobin,2 Paul A. Marshall,3 Paul R. Chalker,3 Hywel O. Davies1 and Lesley M. Smith.1 1
Epichem Limited, Power Road, Bromborough, Wirral, Merseyside, CH62 3QF, UK Department of Chemistry and Surface Science Research Centre, University of Liverpool, Liverpool, L69 7ZD, UK 3 Department of Materials Science and Engineering, University of Liverpool, Liverpool, L69 3BX, UK 2
ABSTRACT Bismuth titanate thin films have been grown by liquid injection MOCVD using Bi(mmp)3 in combination with the new Ti precursors Ti(OPri)2(mmp)2 and Ti(mmp)4 (mmp = OCMe2CH2OMe). Films were grown on Si(100) substrates over the temperature range 300 – 600oC, and were shown to consist predominantly of the Bi4Ti3O12 phase at substrate temperatures > 500oC. INTRODUCTION The bismuth–titanium–oxide ternary system contains a number of technologically important phases with potentially useful dielectric, ferroelectric and electro-optic properties. Consequently, some of the bismuth titanates, such as Bi4Ti3O12 have potential applications in non-volatile memory devices [1] and ferroelectric-effect transistors [2]. MOCVD is a promising technique for the deposition of bismuth titanate films offering the potential for large area growth, and having the advantages of good composition control, high film uniformity, good doping control and excellent conformal step coverage. The availability of suitable precursors with appropriate physical properties and decomposition characteristics is an essential requirement of the MOCVD process. In particular, in the growth of multi component oxides, such as Bi4Ti3O12 or Bi2Ti4O11, co-precursors should deposit oxide films in similar temperature regimes, otherwise film uniformity across the substrate will be adversely affected. Good precursor compatibility is especially important in liquid injection MOCVD, which is being increasingly used for the MOCVD of oxides using precursors with low vapour pressures. This technique uses a single heated evaporator, and often uses a “cocktail” of precursors in a single solution, and so it is important that precursors evaporate at similar temperatures to avoid a build up of un-evaporated material. It is also essential that precursors are stable in solution and do not undergo any unfavourable reactions with co-precursors. A variety of precursor combinations have been used for the MOCVD of Bi4Ti3O12. These include BiPh3 in combination with Ti(OEt)4 [3], Ti(OBun)4 [4] or Ti(OPri)4 [5], and BiMe3 with Ti(OPri)4[6]. For Bi4Ti3O12 in common with other Bi-containing oxides, such as SrBi2(TaxNb1x)2O9 , the Bi precursor has a number of disadvantages which has restricted progress in the development of an MOCVD process. For instance, BiPh3 has a significantly higher thermal stability than Ti alkoxides, leading to a general increase in the Bi/Ti ratio with increasing substrate temperature [5], which can cause problems in controlling compositional uniformity across the substrate
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