The Preparation and Characterization of Lithium Cobalt Oxide Thin Films by LPCVD
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L.T. KENNY, R.C. BREITKOPF, T.E. HAAS, and R.B. GOLDNER Department of Chemistry, Tufts University, Medford, MA 02155 [email protected]
ABSTRACT LPCVD thin films of lithium cobalt oxide (LiCoO 2) were grown using tertbutyllithium (t-BuLi) and cyclopentadienyl cobalt dicarbonyl (CoCp(CO) 2) as precursors in a cold-walled vertical reactor. Deposition rates were found to be as high as 25 A/s. Lithium to cobalt ratios (Li/Co) in the films were found to increase with increasing Ar carrier gas flow rates in the t-BuLi line while they were found to decrease with increasing system pressure. Electrochemical measurements show typical film open circuit voltages (OCV) for as-prepared films to be between 3.30 and 3.60 V vs. Li. Atomic Force Microscopy shows a striated, crystalline film formed at substrate temperatures (T,) as low as 400 C with grain sizes as large as 200 nm in length.
INTRODUCTION LiCoO 2 thin films have been of recent interest for their use as cathodic materials in thin film batteries and electrochromic displays [ 1,2,3,4]. Previous efforts to grow films using rf sputtering methods have produced microstructurally uniform films but with little control of Li/Co stoichiometry. [5]. We felt that fabrication of LiCoO 2 films using CVD would offer the significant potential advantages of improved stoichiometric control and faster deposition than the PVD (laser ablation [7] and rf sputtering [5]) methods previously reported. Fraghaud et al. have recently [6] grown LiCoO 2 films by hot-walled CVD using cobalt acetylacetonate (Co(acac) 3) and t-BuLi as precursors. They were able to form a crystalline phase above a Ts (substrate temperature) of 450 TC with little control of Li/Co at deposition rates as high as 10 lpm/hr. Here we describe efforts to control film stoichiometry and crystal structure using variations in deposition parameters.
EXPERIMENTAL A cold-walled reactor system employing two metal organic baths was used. A schematic of this experimental setup is pictured below (Figure 1).
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Mat. Res. Soc. Symp. Proc. Vol. 415 0 1996 Materials Research Society
LPCVD VERTICAL [cold wall] SYSTEM TE BUBBLER
VALVE
OX
PRESSURE yoe GAUGE
i Ar
QUARTZ , .
CyP-Co-(CO) ,
BUBBLER
FILTER
Bn "TBL
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MASSFLOW CONTROLLER PARTICULATE
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HEATER
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PROCESS
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Figure 1: Schematic of the LPCVD system. Note cold-walled substrate chamber.
Ultra High purity argon gas was purchased from Northeast Airgas and was used as a carrier gas to flow t-BuLi and Co(Cp)(CO) 2 through the bubblers. Mass flow controllers (Tylan Model FC-260, FC-280) were used to regulate flow rates of the carrier gases to within 10%. The chamber pressure was monitored with Tylan DH 80-21 capacitance manometers. Flow rates were varied from 15 to 80 SCCM on the Co line and from 40 to 325 SCCM on the Li line. 02 and N 20 were used as oxidants and were varied from 60 to 640 SCCM. Films were de
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