Recently, NiO film of metal-oxide-metal (MOM) structure 1a has drawn much attention because it shows reproducible resistance switching phenomena applicable to next generation nonvolatile resistive random access memory (ReRAM) device. The resistance switching phenomenon, that is, a drastic change in resistance between a high-resistance state (OFFstate) and low resistance state (ON-state) in current-voltage (I-V) characteristic of MOM structure, has been observed in binary oxides such as NiO, 1 TiO 2 , 2 Nb 2 O 5 , 3 Cr-doped SrZrO 3 , 4 Pr 1-x CaMnO 3 , 5 and chalcogenide semiconductors although exact origin was unsolved. Among these materials, the binary oxide is the first consideration for the application due to the relatively simple fabrication process.In the case of a device fabrication, the self-limiting process of atomic layer deposition (ALD) may be very important for the growth of NiO films because the expected structure of ReRAM device requires the filling process of NiO on the hole or trench-patterned 3-dimensional substrates. Therefore, it is worthwhile to find the optimal ALD process for NiO film growth by using suitable Ni precursors. In general, most of Ni compounds are in solid state at room temperature although highly volatile liquid precursor is highly desirable to obtain conformal film qualities in ALD processes.A number of nickel precursors have been synthesized and examined for preparing Ni and NiO films using metal organic chemical vapor deposition (MOCVD) and ALD during the past years. The ligands for the nickel precursors are carbonyl, 6 β-diketonates, 7 cycolpentadienyl, 8 and alkoxides, 9 N,N'-dialkylacetamidinates, 10 phosphites, 11 and combination of cyclopentadienyl and allyl.12 Many nickel precursors have their drawbacks such as low volatilities and toxicity, and carbon contaminations in the prepared films. Except Ni(CO) 4 , all of them are solids at room temperature, although cyclopentadienylallylnickel for nickel thin films is a liquid at 10 ºC.12 HubertPfalzgraf et al. prepared a volatile Ni(OCHMeCH 2 NMe 2 ) 2 which was sublimed at 93-95 °C under 0.01 mmHg.13 Recently, we have synthesized nickel dialkylamino alkoxide complexes [Ni(dmamp) 2 , Ni(emamp) 2 , and Ni(deamp) 2 ] which are solids at room temperature but they are volatile and thermally stable.
14In this study, novel volatile and liquid nickel precursors were synthesized by using ligands with unsymmetrical substituents and employed with water to deposit NiO films by ALD. In addition, the resistance switching of Pt/NiO/Pt MIM structure was also investigated.Metal complexes with low melting point, that is, liquid precursors are highly desirable to keep a constant vapor pressure and to avoid particle problems during the deposition process. As our ongoing studies to develop volatile and stable precursors applicable to chemical deposition processes such as CVD and ALD, we have introduced dialkylamino alkoxide ligands which contain alkoxide and donor-functionalized amino group to saturate the vacant sites of the central metal io...
