Development and Implementation of New Volatile Cd and Zn Precursors for the Growth of Transparent Conducting Oxide Thin
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factor of 2-4. CTO is of particular interest as a compatible TCO layer in CdSe and CdTe photovoltaic devices, the properties of which are highly dependent on the nature of the TCO [4]. A potential complimentary route to this physical vapor deposition (PVD) approach to CTO films is MOCVD. As with other chemical vapor deposition techniques, MOCVD offers several attractions including scalability, conformal coverage, growth at higher pressures, and growth at lower growth temperatures. Cd-containing oxide thin films have not yet been grown using MOCVD, likely due to the lack of suitable Cd precursors [5]. Useful MOCVD precursors must be sufficiently volatile to efficiently deliver the metalcontaining species to the substrate, and sufficiently thermally stable to survive several heat/cool cycles during the film growth process. Dimethylcadmium (CdMe 2 ) meets these criteria and, despite its high toxicity, is frequently employed in the thin film growth of cadmium chalcogenide films [6]. However, attempts to grow CTO using CdMe2 and tetramethyltin were unsuccessful [7]. A review of the literature on Cd 2, compounds containing ligands typically employed for oxide precursors, such as P-diketonates and alkoxides, fails to reveal any viable candidates for Cd CVD. Most Cd2+ compounds containing such mono- or bidentate ligands are not discreet monomeric complexes, a common problem of small charge-to-radius ratio metal ions. For example, the solid state structure of bis(2,4-pentanedionato)cadmium (II) [Cd(acac) 2] shows that this complex is polymeric [8]. Our group has previously addressed this issue using tridentate ketoiminate ligands with lanthanide (La3+, Ce3+, Nd3+, Er3+) and alkaline earth (Ca 2 +, +Sr2 , Ba2 +) precursors or a combination of P3-diketonate and neutral polydentate amines with Mg2+ [9]. A similar approach seemed logical for Cd2+ and Zn 2+ [10]. While Zn(dpm) 2 (dpm = 2,2,6,6tetramethyl-3,5-heptanedionato) has been successfully employed as a precursor for MOCVD [2, 11] and is monomeric [12], it is a relatively high-melting solid (132-134°C) and remains a solid under typical film growth conditions. The details for the synthesis and characterization of improved Zn and Cd precursors are presented in this communication along with our preliminary film growth studies using these complexes.
P3-
EXPERIMENTAL Precursors Bis(f3-ketoininato)cadmium(II) complexes 2a-c. Representative procedurefor bis(4-N-2ethoxyethylimino-2-pentanonato)cadinium(II) (2b): In a N2 -filled glovebox, a 3-neck, 500 mL
round bottom flask fitted with a glass stopper, rubber septum, and reflux condenser is charged with Cd[N(TMS)2] 2 (24 mmol) [13]. The vessel is then removed from the glovebox, interfaced to a Schlenk line, and 200 mL toluene (dried over and distilled from molten Na) is added via syringe. 4-N-2-Ethoxyethylimino-2-pentanone (1b; 50 mmol; 2.1 equiv.) is injected and the solution stirred at 65°C for 2 h. The volatiles are next removed in vacuo, and 9.3 g (20 mmol; 86% yield) of pure 2b was obtained as a pale yellow solid via vacuum
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