A precursor originally synthesized for the chemical vapor deposition of metallic nickel, Ni(dmamp)2 (dmamp=1-dimethylamino-2-methyl-2-propanolate, -OCMe2CH2NMe2), has been adopted as a nickel source for the atomic layer deposition of nickel oxide (NiO) using water (H2O) as the oxygen source. The precursor is a solid at room temperature, but readily sublimes at 90 °C. The self-limiting atomic layer deposition (ALD) process by alternate surface reactions of Ni(dmamp)2 and H2O was confirmed from thickness measurements of the NiO films grown with varying Ni(dmamp)2 supply times and numbers of the Ni(dmamp)2-H2O ALD cycles. The ALD temperature window for this precursor was found to be between 90 and 150 °C. Under optimal reaction conditions, the growth rate of the NiO films was ∼0.8Å∕cycle. The NiO films deposited on Si(001) at 120 °C were characterized by x-ray diffraction, x-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy. The x-ray diffraction patterns showed no distinct peaks for NiO, indicating that the films deposited at this temperature were amorphous. X-ray photoelectron spectroscopy analysis showed the films to be stoichiometric with no detectable amount of carbon impurities. For a film with the thickness of 810 Å (with 1000 ALD cycles) the root-mean-square surface roughness was only ∼4Å as measured by atomic force microscopy. To elucidate the ALD mechanism of the Ni precursor with water, a quadrupole mass analyzer was employed with D2O as the oxygen source in lieu of H2O. Interestingly, unlike the usual ALD fashion, the Ni(dmamp)2 precursor does not seem to decompose but only coordinatively bond to the OH-terminated surface when it was introduced. Next, the Ni(dmamp)2-surface species decompose to produce a hydroxylated nickel oxide surface and the alcohol dmampH when water was supplied.

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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...