To understand electrical/dielectric phenomena and the origins of bistable resistive switching, impedance spectroscopy was applied to NiO thin films prepared through atomic layer deposition. The dc current-voltage characteristics of the NiO thin films were also determined. Frequency-dependent characterizations indicated that the switching and memory phenomena in NiO thin films did not originate from the non-Ohmic effect at the electrode/NiO interfaces but from the bulk-related responses, i.e., from an electrocomposite where highly conducting components are distributed in the insulating NiO matrix. Low dielectric constants and bias-independent capacitance appeared to corroborate the bulk-based responses in resistive switching in NiO thin films.

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An atomic layer deposition (ALD) process for SrTiO 3 (STO) thin film growth was developed using a newly designed and synthesized heteroleptic Sr-precursor, {Sr(demamp)(tmhd)} 2 (demampH = 1-{[2-(dimethylamino)ethyl](methyl)amino}-2-methylpropan-2-ol, tmhdH = 2,2,6,6-tetramethyl-3,5-heptanedione), which offered an intermediate reactivity toward oxygen between Sr(tmhd) 2 and Sr( i Pr 3 Cp) 2 . Because of the appropriate reactivity of {Sr(demamp)(tmhd)} 2 toward oxygen, the abnormal initial growth behavior (due to interaction between the Sr-precursor and active oxygen contained in the underlying oxidized Ru layer) became negligible during the growth of the SrO and STO films on the Ru electrode, which allowed the growth of the SrO and STO films to be highly controllable with a moderate growth rate. Using Ti(CpMe 5 )(OMe) 3 as the Ti-precursor and O 3 as the oxygen source in the TiO 2 ALD subcycle, the ALD process of the STO film revealed a growth rate of 0.05 nm/cycle and ∼85% of step coverage in terms of the thickness and cation composition on a capacitor hole structure with an aspect ratio of 10 (opening diameter of 100 nm and depth of 1 μm). The minimum achievable equivalent oxide thickness (t ox ) with a low leakage current (<10 −7 A/cm 2 at 0.8 V) was limited to 0.46 nm. The damage effect on the underlying Ru electrode by the prolonged ALD process time appears to affect the limited scalability of t ox . ■ INTRODUCTIONSrTiO 3 (STO) has been considered to be a promising candidate for a dielectric layer in the next-generation dynamic random access memory (DRAM) capacitors because of its high permittivity (∼300 in bulk material) compared with that of other dielectric materials, such as HfO 2 and ZrO 2 . Many studies have reported a high dielectric constant of >100 for metal− insulator−metal (MIM) capacitors that contain an STO insulator, in which the insulators are thinner than 20 nm. 1−6 Considering the extremely tiny three-dimensional (3D) structure of the DRAM capacitors, 7 atomic layer deposition (ALD) appears to be the only feasible thin film growth technique that can fulfill the stringent requirements of thickness and composition step coverage in the DRAM capacitors. Despite the acute requirement for a suitable ALD process of the STO films, the development has been hindered for two main reasons: first, the lack of a suitable Sr-precursor for feasible STO ALD, although that for Ti is abundant; second, because of the low temperature of the ALD, ensuring a suitable crystalline quality of the STO film is challenging in general. Because such problems and possible solutions have already been extensively reviewed in previous reports from the authors' group, 3,7,8 more recent reports that are directly related to the present work are described in this section.The first viable report in this field was published by the Helsinki group in the late 1990s. Vehkamaki et al. deposited STO films with Sr( i Pr 3 Cp) 2 (Pr and Cp are propyl and cyclopentadienyl group, respectively) and Ti(O i Pr) 4 [TTIP] as Sr-and Ti-precursors,...

Ruthenium (Ru) and ruthenium oxide (RuO 2 ) thin films were grown by atomic layer deposition (ALD) using a novel zerovalent (1,5-hexadiene)(1-isopropyl-4-methylbenzene)Ru complex and O 2 as the Ru precursor and oxidant, respectively. The self-limiting growth mode for the Ru and RuO 2 ALD processes was achieved while varying the Ru precursor and O 2 feeding time. Metallic Ru films were deposited at growth temperatures of 230−350°C, while the temperature window for the growth of the RuO 2 film was limited to <230°C. At 270°C, the growth per cycle (GPC) of Ru ALD was 0.076 nm/cycle, and the incubation times of Ru on SiO 2 and TiN substrates were considerably short (3 cycles on SiO 2 , negligible on TiN) compared to that of Ru ALD from a high-valent Ru precursor and O 2 . The resistivity of the Ru film was as low as 29−36 μΩ·cm at growth temperatures of 270−350°C. On the other hand, the RuO 2 film was grown at a low temperature of 200°C and showed a GPC of 0.15 nm/cycle with a resistivity of ∼270 μΩ·cm. In situ quadruple mass spectrometry analysis of the CO 2 byproduct revealed that the amount of subsurface oxygen extracted during the Ru pulse half-cycle affected the resultant film phase, either Ru or RuO 2 , which was strongly influenced by the growth temperature. ■ INTRODUCTIONThe growth of nanoscale ruthenium (Ru) and ruthenium oxide (RuO 2 ) thin films has been spotlighted due to their promising characteristics such as low resistivity (Ru ∼7 μΩ·cm, RuO 2 ∼30 μΩ·cm), excellent chemical and thermal stabilities, high work functions (Ru ∼4.7 eV, RuO 2 ∼5.1 eV), and catalytic functionality. 1−15 These properties have enabled Ru-based thin films to be employed for energy device applications as a catalyst, in microelectronics as an electrode for dynamic random access memory (DRAM) capacitors, and as a seed layer for Cu electroplating. 2−7 Although a variety of deposition techniques for fabricating Ru and RuO 2 thin films have been used such as sputtering, pulse laser deposition, and chemical vapor deposition, atomic layer deposition (ALD) is the most appropriate method to grow uniform and conformal film over a 3-dimensional substrate with very precise composition/thickness controllability in nanotechnology applications. For Ru ALD, meanwhile, the choice of the Ru precursor is very important because not only the growth characteristics but also the film properties are highly affected by the Ru precursor used. For example, Ru(thd) 3 (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate), Ru-(Cp) 2 (Cp = cyclopentadienyl), Ru(EtCp) 2 (EtCp = ethylcyclopentadienyl), and 2,4-(dimethylpentadienyl)-(ethylcyclopentadienyl)Ru are the most widely utilized Ru precursors in conjunction with O 2 as an oxidant. 8−12 Ru ALD using these precursors, however, showed extremely retarded nucleation and consequently resulted in long incubation cycles (e.g., >300 cycles on SiO 2 and 500 cycles on TiN when Ru(thd) 3 and O 2 were employed) at temperatures of 275−400°C . This poor nucleation behavior hinders the practical use of the Ru precursors listed ...

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.