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Atomic layer deposition (ALD) is a thin-film fabrication method that can be used to deposit films with precise thickness controllability and uniformity. The low deposition temperature of ALD, however, often interrupts the facile crystallization of films, resulting in inferior optical and electrical properties. In this study, the extremely localized crystallization of TiO2 thin films was demonstrated by per-cycle plasma treatment during the plasma-enhanced ALD process. By layering crystalline and amorphous films, a phase-gradient TiO2 film with precisely modulated optical and electrical properties was fabricated. Moreover, the ratio between the amorphous and crystalline layer thicknesses for a high dielectric constant and low leakage current density was optimized.

In this study, we demonstrated the seed layer mediated growth of high-quality BaTiO3 (BTO) thin films using a two-step radio frequency (RF) magnetron sputtering process. Since the as-grown BTO thin films obtained by RF magnetron sputtering at the deposition temperatures of 300–500 °C were amorphous with a low dielectric constant of 20, it is necessary to develop a fabrication process for obtaining crystalline high-k BTO thin films without sacrificing other film properties such as morphology and leakage current. First, it was revealed that ex-situ post-deposition annealing (PDA) at high temperatures in the 700–800 °C range led to the crystallization of BTO films and a high dielectric constant of 121. However, the film morphology deteriorated significantly during PDA, and consequently, a high leakage current was observed due to the rough and discontinuous surface containing voids and micro-cracks. To achieve an excellent leakage current characteristic as well as a high dielectric constant for a crystalline BTO thin film, in-situ crystallization was carried out through local epitaxial growth using a crystalline seed layer. The crystalline BTO seed layer was formed by annealing a 5-nm-thick amorphous BTO film at 700 °C on which the in-situ crystallized BTO main layer was deposited at 500 °C. The in-situ crystallization method resulted in a smooth and uniform surface and a high dielectric constant of 113. In addition, the in-situ crystallized BTO film exhibited a low leakage current density of 10−6 A/cm2 (at 0.8 V) displaying an improvement by a factor of 103 compared to the ex-situ crystallized BTO film.

The continuous device scaling of dynamic random access memories has been increasing demands for the development of dielectric materials with high dielectric constants and low leakage currents. In this study, we developed MgO thin films through atomic layer deposition (ALD) using bis(ethylcyclopentadienyl)magnesium combined with two different reactants, H2O or O2 plasma, and compared the physical and chemical characteristics of MgO thin films produced by thermal ALD (Th-ALD) and plasma-enhanced ALD (PEALD). The films were deposited in a temperature range of 200–400 °C, and self-limited surface reactions were observed for both ALD processes. Th-ALD MgO films showed the oxygen deficient composition, while more stoichiometric MgO films were achieved by PEALD process. To evaluate the electrical characteristics of the MgO films, the metal–insulator–metal capacitors were fabricated. The electrical characteristics of the MgO film, such as the dielectric constant and leakage current, were compared according to the reactant type. The bulk-limited leakage current conduction mechanisms of the Th-ALD and PEALD MgO thin films were also investigated.

In2O3 films were deposited by atomic layer deposition (ALD) using a newly synthesized heteroleptic In precursor, In(DMAMP)2(OiPr), and O3 at 150–300 °C. Self-limiting growth characteristics were exhibited for a wide ALD temperature range of 200–300 °C and growth rate of 0.029–0.033 nm/cycle. At a low temperature of 150 °C, the amorphous In2O3 film was deposited, while polycrystalline In2O3 films were achieved at 200–300 °C. The In2O3 films grown in this ALD temperature range had high densities of 7.0–7.2 g/cm3, which are comparable to those of bulk In2O3. At all growth temperatures (150–300 °C), no carbon or nitrogen impurities were detected, suggesting high reactivity of the In(DMAMP)2(OiPr) precursor. The ALD In2O3 films showed n-type electronic property with high electron concentrations of 1.6 × 1020–3.6 × 1020/cm3 and a Hall mobility of 31–39 cm2/V·s.

Atomic layer deposition (ALD) of SnO and SnO 2 thin films was successfully demonstrated over a wide temperature range of 70–300 °C using a divalent Sn-precursor, bis( N- ethoxy-2,2-dimethyl propanamido)tin (Sn(edpa) 2 ). The regulated growth of the SnO 2 and SnO films was realized by employing O 2 -plasma and H 2 O, respectively. Pure SnO 2 and SnO films were deposited with negligible C and N contents at all the growth temperatures, and the films exhibited polycrystalline and amorphous structures, respectively. The SnO 2 films presented a high transmittance of > 85% in the wavelength range of 400–700 nm and an indirect band gap of 3.6–4.0 eV; meanwhile, the SnO films exhibited a lower transmittance of > 60% and an indirect band gap of 2.9–3.0 eV. The SnO 2 films exhibited n-type semiconducting characteristics with carrier concentrations of 8.5 × 10 16 –1.2 × 10 20 cm −3 and Hall mobilities of 2–26 cm 2 /V s. By employing an alternate ALD growth of SnO and SnO 2 films, SnO 2 /SnO multilayer structures were successfully fabricated at 120 °C. The in-situ quadrupole mass spectrometry analysis performed during ALD revealed that the oxidation of chemisorbed Sn-precursor occurs dominantly during the Sn(edpa) 2 /O 2 -plasma ALD process, resulting in the production of combustion by-products, whereas the Sn(edpa) 2 /H 2 O ALD process was governed by a ligand exchange reaction with the maintenance of the original oxidation state of Sn 2+ .

Carbon nanofibers (CNFs) are used as active materials for electrodes in various energy devices, such as lithium ion secondary batteries, supercapacitors, and fuel cells. Recent studies have shown that nanoscale coatings on carbon nanotubes increase the output and lifespan of these devices owing to the improvement of their mechanical and chemical properties. Among various coating methods, atomic layer deposition (ALD) can adjust the thickness of the coating layer conformally without any directional growth. Therefore, ALD can coat particles with high aspect ratios, such as CNFs, even at nanometer levels of thickness. In this work, we grew two different morphologies of a SnO2 layer on CNF. We used two types of ALD equipment: flow-type ALD (static ALD), and fluidized bed reactor-type ALD (dynamic ALD). Static ALD could form a discontinuous SnO2, while a uniform SnO2 layer was formed by pre-inserting a layer of Al2O3. On the other hand, dynamic ALD formed a uniform SnO2 layer without pre-insertion of an Al2O3 layer. X-ray photoelectron spectroscopy analysis revealed that both Sn4+ and Sn2+ were present in SnO2 on the CNF deposited by static ALD, probably due to the formation of an interfacial layer between the SnO2 and CNF. When the dynamic ALD method was used, only Sn4+ was present in the SnO2 on CNF. Cyclic voltammetry analysis was performed to characterize the electrochemical properties of the SnO2-coated CNF as an electrode on a direct methanol fuel cell. It was revealed that the discontinuous SnO2 on CNF deposited by static ALD showed the highest current efficiency as well as enhanced electrocatalytic stability.

Aluminum oxide (AlO x ) thin films were synthesized by mist-chemical vapor deposition (mist-CVD) using aluminum acetylacetonate (Al(acac) 3 ) dissolved in an aqueous solvent mixture of acetone and water. Nitrogen gas was used to purge the precursor solution and growth rates between 7.5–13.3 nm/min were achieved at substrate temperatures of 250–350 °C. The AlO x layers deposited at temperatures below 350 °C exhibit 3–5 at% residual carbon levels, however those grown at 350 °C exhibit only 1–2 at% carbon impurity. Reasonable dielectric properties were obtained in the latter, with a dielectric constant (κ) of ~ 7.0, breakdown field of ~ 9 MV/cm and relatively low leakage current density of ~ 8.3×10 −10 A/cm 2 .