Reducing contact resistance of MoS2-based field effect transistors through uniform interlayer insertion via atomic layer deposition.

Molybdenum disulfide (MoS2), a semiconducting two-dimensional layered transition metal dichalcogenide (2D TMDC), with attractive properties enables the opening of a new electronics era beyond Si. However, the notoriously high contact resistance (RC) regardless of the electrode metal has been a major challenge in the practical applications of MoS2-based electronics. Moreover, it is difficult to lower RC because the conventional doping technique is unsuitable for MoS2 due to its ultrathin nature. Therefore, the metal-insulator-semiconductor (MIS) architecture has been proposed as a method to fabricate a reliable and stable contact with low RC. Herein, we introduce a strategy to fabricate MIS contact based on atomic layer deposition (ALD) to dramatically reduce the RC of single-layer MoS2 field effect transistors (FETs). We utilize ALD Al2O3 as an interlayer for the MIS contact of bottom-gated MoS2 FETs. Based on the Langmuir isotherm, the uniformity of ALD Al2O3 films on MoS2 can be increased by modulating the precursor injection pressures even at low temperatures of 150 °C. We discovered, for the first time, that film uniformity critically affects RC without altering the film thickness. Additionally, we can add functionality to the uniform interlayer by adopting isopropyl alcohol (IPA) as an oxidant. Tunneling resistance across the MIS contact is lowered by n-type doping of MoS2 induced by IPA as the oxidant in the ALD process. Through a highly uniform interlayer combined with strong doping, the contact resistance is improved by more than two orders of magnitude compared to that of other MoS2 FETs fabricated in this study.

Role of a cyclopentadienyl ligand in a heteroleptic alkoxide precursor in atomic layer deposition.

Alkoxide precursors have been highlighted for depositing carbon-free films, but their use in Atomic Layer Deposition (ALD) often exhibits a non-saturated growth. This indicates no self-limiting growth due to the chain reaction of hydrolysis or ligand decomposition caused by ß-hydride elimination. In the previous study, we demonstrated that self-limiting growth of ALD can be achieved using our newly developed precursor, hafnium cyclopentadienyl tris(N-ethoxy-2,2-dimethyl propanamido) [HfCp(edpa)3]. To elucidate the growth mechanism and the role of cyclopentadienyl (Cp) ligand in a heteroleptic alkoxide precursor, herein, we compare homoleptic and heteroleptic Hf precursors consisting of N-ethoxy-2,2-dimethyl propanamido (edpa) ligands with and without cyclopentadienyl ligand-hafnium tetrakis(N-ethoxy-2,2-dimethyl propanamido) [Hf(edpa)4] and HfCp(edpa)3. We also investigate the role of a Cp ligand in growth characteristics. By substituting an alkoxide ligand with a Cp ligand, we could modify the surface reaction during ALD, preventing undesired reactions. The last remaining edpa after Hf(edpa)4 adsorption can undergo a hydride elimination reaction, resulting in surface O-H generation. In contrast, Cp remains after the HfCp(edpa)3 adsorption. Accordingly, we observe proper ALD growth with self-limiting properties. Thus, a comparative study of different ligands of the precursors can provide critical clues to the design of alkoxide precursors for obtaining typical ALD growth with a saturation behavior.

Ionic Liquid-Mediated Route to Atomic Layer Deposition of Tin(II) Oxide via a C-C Bond Cleavage Ligand Modification Mechanism.

Atomic layer deposition (ALD) is a technologically important method to grow thin films with high conformality and excellent thickness control from vapor phase precursors. The development of new thermal ALD processes can be limited by precursor reactivity and stability: reaction temperature and precursor design are among the few variables available to achieve higher reactivity in gas-phase reactions, unlike in solution synthesis, where the use of solvent and/or a catalyst can promote a desired reaction. To bridge this synthesis gap between vapor-phase and solution-phase, we demonstrate the use of an ultrathin coating layer of a vapor phase-compatible solvent─an ionic liquid (IL)─on our growth substrate to perform ALD of SnO. Successful SnO deposition is achieved using tin acetylacetonate and water, a process that otherwise would require a stronger counter-reactant such as ozone. The presence of the layer of IL allows a solvent-mediated reaction mechanism to take place on the growth substrate surface. We report a growth per cycle of 0.67 Å/cycle at a deposition temperature of 100 °C in an IL comprising 1-ethyl-3-methylimidazolium hydrogen sulfate. Characterization of the ALD films confirms the SnO film composition, and 1H and 13C NMR are used to probe the solvent-mediated ALD reaction, suggesting a solvent-mediated addition-elimination-type mechanism which breaks a C-C bond in acetylacetonate to form acetone and acetate. Density functional theory calculations show that the IL solvent is beneficial to the proposed solvent-mediated mechanism by lowering the C-C bond cleavage energetics of acetylacetonate compared to the vapor phase. A general class of ligand modification reactions for thermal ALD is thus introduced in this work.

Hydrogen Barriers Based on Chemical Trapping Using Chemically Modulated Al2O3 Grown by Atomic Layer Deposition for InGaZnO Thin-Film Transistors.

In this study, the excellent hydrogen barrier properties of the atomic-layer-deposition-grown Al2O3 (ALD Al2O3) are first reported for improving the stability of amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs). Chemical species in Al2O3 were artificially modulated during the ALD process using different oxidants, such as H2O and O3 (H2O-Al2O3 and O3-Al2O3, respectively). When hydrogen was incorporated into the H2O-Al2O3-passivated TFT, a large negative shift in Vth (ca. -12 V) was observed. In contrast, when hydrogen was incorporated into the O3-Al2O3-passivated TFT, there was a negligible shift in Vth (ca. -0.66 V), which indicates that the O3-Al2O3 has a remarkable hydrogen barrier property. We presented a mechanism for trapping hydrogen in a O3-Al2O3 via various chemical and electrical analyses and revealed that hydrogen molecules were trapped by C-O bonds in the O3-Al2O3, preventing the inflow of hydrogen to the a-IGZO. Additionally, to minimize the deterioration of the pristine device that occurs after a barrier deposition, a bi-layered hydrogen barrier by stacking H2O- and O3-Al2O3 is adopted. Such a barrier can provide ultrastable performance without degradation. Therefore, we envisioned that the excellent hydrogen barrier suggested in this paper can provide the possibility of improving the stability of devices in various fields by effectively blocking hydrogen inflows.

Optical Reflection from Unforbidden Diffraction of Block Copolymer Templated Gyroid Films.

We present material substitutions and optical characterization of block copolymer (BCP)-templated gyroid structures that are obtained from a volume-asymmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA). In addition to the structural analyses reported earlier, we elucidate the optical responses to the nonaffine gyroid planes, in which the PMMA channels are complexed with Al2O3 by sequential infiltration synthesis and the organic components are further eliminated to produce an inorganic air-Al2O3 gyroid film. Grazing-incidence small-angle X-ray scattering measurements show that three-dimensional gyroid lattices are retained in both in-plane and out-of-plane directions through these material substitution processes. Our BCP-templated gyroid films respond to the middle UV wavelength from 200 to 300 nm, and peculiar optical reflectance peaks correlate with the unforbidden {110} diffraction spots. Together with the red- and blue-shifts of the reflectance peaks by the component substitutions, the air-Al2O3 gyroid structure reveals the high-amplitude spectrum due to the large refractive-index difference between channel and matrix.

Synthesis of a Hybrid Nanostructure of ZnO-Decorated MoS2 by Atomic Layer Deposition.

We introduce the synthesis of hybrid nanostructures comprised of ZnO nanocrystals (NCs) decorating nanosheets and nanowires (NWs) of MoS2 prepared by atomic layer deposition (ALD). The concentration, size, and surface-to-volume ratio of the ZnO NCs can be systematically engineered by controlling both the number of ZnO ALD cycles and the properties of the MoS2 substrates, which are prepared by sulfurizing ALD MoO3. Analysis of the chemical composition combined with electron microscopy and synchrotron X-ray techniques as a function of the number of ZnO ALD cycles, together with the results of quantum chemical calculations, help elucidate the ZnO growth mechanism and its dependence on the properties of the MoS2 substrate. The defect density and grain size of MoS2 nanosheets are controlled by the sulfurization temperature of ALD MoO3, and the ZnO NCs in turn nucleate selectively at defect sites on MoS2 surface and enlarge with increasing ALD cycle numbers. At higher ALD cycle numbers, the coalescence of ZnO NCs contributes to an increase in areal coverage and NC size. Additionally, the geometry of the hybrid structures can be tuned by changing the dimensionality of the MoS2, by employing vertical NWs of MoS2 as the substrate for ALD ZnO NCs, which leads to improvement of the relevant surface-to-volume ratio. Such materials are expected to find use in newly expanded applications, especially those such as sensors or photodevices based on a p-n heterojunction which relies on coupling transition-metal dichalcogenides with NCs.

Surface Energy Change of Atomic-Scale Metal Oxide Thin Films by Phase Transformation.

Fine-tuning of the surface free energy (SFE) of a solid material facilitates its use in a wide range of applications requiring precise control of the ubiquitous presence of liquid on the surface. In this study, we found that the SFE of rare-earth oxide (REO) thin films deposited by atomic layer deposition (ALD) gradually decreased with increasing film thickness; however, these changes could not be understood by classical interaction models. Herein, the mechanism underlying the aforesaid decrease was systematically studied by measuring contact angles, surface potential, adhesion force, crystalline structures, chemical compositions, and morphologies of the REO films. A growth mode of the REO films was observed: layer-by-layer growth at the initial stage with an amorphous phase and subsequent crystalline island growth, accompanied by a change in the crystalline structure and orientation that affects the SFE. The portion of the surface crystalline facets terminated with (222) and (440) planes evolved with an increase in ALD cycles and film thickness, as an amorphous phase was transformed. Based on this information, we demonstrated an SFE-tuned liquid tweezer with selectivity to target liquid droplets. We believe that the results of this fundamental and practical study, with excellent selectivity to liquids, will have significant impacts on coating technology.

Reaction Mechanism of Area-Selective Atomic Layer Deposition for Al2O3 Nanopatterns.

The reaction mechanism of area-selective atomic layer deposition (AS-ALD) of Al2O3 thin films using self-assembled monolayers (SAMs) was systematically investigated by theoretical and experimental studies. Trimethylaluminum (TMA) and H2O were used as the precursor and oxidant, respectively, with octadecylphosphonic acid (ODPA) as an SAM to block Al2O3 film formation. However, Al2O3 layers began to form on the ODPA SAMs after several cycles, despite reports that CH3-terminated SAMs cannot react with TMA. We showed that TMA does not react chemically with the SAM but is physically adsorbed, acting as a nucleation site for Al2O3 film growth. Moreover, the amount of physisorbed TMA was affected by the partial pressure. By controlling it, we developed a new AS-ALD Al2O3 process with high selectivity, which produces films of ∼60 nm thickness over 370 cycles. The successful deposition of Al2O3 thin film patterns using this process is a breakthrough technique in the field of nanotechnology.