Controllable synthesis of molybdenum tungsten disulfide alloy for vertically composition-controlled multilayer

The effective synthesis of two-dimensional transition metal dichalcogenides alloy is essential for successful application in electronic and optical devices based on a tunable band gap. Here we show a synthesis process for Mo<inf>1-x</inf>W<inf>x</inf>S<inf>2</inf> alloy using sulfurization of super-cycle atomic layer deposition Mo<inf>1-x</inf>W<inf>x</inf>O<inf>y</inf>. Various spectroscopic and microscopic results indicate that the synthesized Mo<inf>1-x</inf>W<inf>x</inf>S<inf>2</inf> alloys have complete mixing of Mo and Watoms and tunable band gap by systematically controlled composition and layer number. Based on this, we synthesize a vertically composition-controlled (VCC) Mo<inf>1-x</inf>W<inf>x</inf>S<inf>2</inf> multilayer using five continuous super-cycles with different cycle ratios for each super-cycle. Angle-resolved X-ray photoemission spectroscopy, Raman and ultraviolet-visible spectrophotometer results reveal that a VCC Mo<inf>1-x</inf>W<inf>x</inf>S<inf>2</inf> multilayer has different vertical composition and broadband light absorption with strong interlayer coupling within a VCC Mo<inf>1-x</inf>W<inf>x</inf>S<inf>2</inf> multilayer. Further, we demonstrate that a VCC Mo<inf>1-x</inf>W<inf>x</inf>S<inf>2</inf> multilayer photodetector generates three to four times greater photocurrent than MoS<inf>2</inf>-and WS<inf>2</inf>-based devices, owing to the broadband light absorption. © 2015 Macmillan Publishers Limitedopen1

Layer-Controlled, Wafer-Scale, and Conformal Synthesis of Tungsten Disulfide Nanosheets Using Atomic Layer Deposition

The synthesis of atomically thin transition-metal disulfides (MS2) with layer controllability and large-area uniformity is an essential requirement for their application in electronic and optical devices. In this work, we describe a process for the synthesis of WS2 nanosheets through the sulfurization of an atomic layer deposition (AID) WO, film with systematic layer controllability and wafer-level uniformity. The X-ray photoemission spectroscopy, Raman, and photoluminescence measurements exhibit that the AID-based WS2 nanosheets have good stoichiometry, clear Raman shift, and bandgap dependence as a function of the number of layers. The electron mobility of the monolayer WS2 measured using a field-effect transistor (FET) with a high-k dielectric gate insulator is shown to be better than that of CVD-grown WS2, and the subthreshold swing is comparable to that of an exfoliated MoS2 FET device. Moreover, by utilizing the high conformality of the AID process, we have developed a process for the fabrication of WS2 nanotubes

Hydrophobicity of Rare Earth Oxides Grown by Atomic Layer Deposition

Rare earth oxide (REO) atomic layer deposition (ALD) processes are investigated for hydrophobic coatings. Thermal and plasma-enhanced ALD (PE-ALD) Er2O3 and Dy2O3 are developed using the newly synthesized Er and Dy precursors bis-methylcyclopentadienyl-diisopropyl-acetamidinate-erbium and bis-isopropylcyclopentadienyl-diisopropyl-acetamidinate-dysprosium, with H2O and O-2 plasma counter oxidants. The Er and Dy precursors show typical ALD growth characteristics with no nucleation incubation, indicating that they are suitable ALD precursors. The hydrophobicities of ALD-grown Er2O3 and D2O3 are investigated, together with those of ALD-grown Y(2)O(3)Z, La2O3, and CeO2 that were previously developed for high-k applications. All the ALD-grown REOs show high hydrophobicity, with water contact angles as high as 90 degrees. After annealing at 500 degrees C in air for 2h, hydrophobicity is degraded depending on the kind of material; this degradation is related to the hygroscopy of REOs. In addition, we demonstrate the fabrication of a superhydrophobic surface by depositing highly conformal ALD REO films on 3D Si nanowire nanostructures. The Si NWs are conformally coated with ALD Y2O3, yielding a surface with a water contact angle of about 158 degrees. The ALD REOs reported herein should find widespread applicability in the fabrication of robust hydrophobic coatings.close1

Heteroleptic Cyclopentadienyl-Amidinate Precursors for Atomic Layer Deposition (ALD) of Y, Pr, Gd, and Dy Oxide Thin Films

Thin films of rare-earth (RE) oxides (Y₂O₃, PrO_<i>x</i>, Gd₂O₃, and Dy₂O₃) were deposited by atomic layer deposition from liquid heteroleptic RE­(<i>ⁱ</i>PrCp)₂(<i>ⁱ</i>Pr-amd) precursors with either water or ozone as the oxygen source. Film thickness, crystallinity, morphology, and composition were studied. Saturation was achieved with Gd₂O₃ when O₃ was used as the oxygen source at 225 °C and with Y₂O₃ with both oxygen sources at as high temperature as 350 °C. The growth rates were 0.90–1.3 Å/cycle for these processes. PrO_<i>x</i> was challenging to deposit with both oxygen sources but with long, 20 s purges after the water pulses uniform films could be deposited. However, saturation was not achieved. With Dy₂O₃, uniform films could be deposited and the Dy­(<i>ⁱ</i>PrCp)₂(<i>ⁱ</i>Pr-amd)/O₃ process was close to saturation at 300 °C. The different oxygen sources had an effect on the crystallinity and impurity contents of the films in all the studied processes. Whether ozone or water was better choice for oxygen source depended on the metal oxide material that was deposited

Effects of Cl-Based Ligand Structures on Atomic Layer Deposited HfO₂

Atomic layer deposition (ALD) of HfO₂ is a key technology for the application of high dielectric constant gate dielectrics ranging from conventional Si devices to novel nanodevices. The effects of the precursor on the growth characteristics and film properties of ALD HfO₂ were investigated by using hafnium tetrachloride (HfCl₄) and bis­(ethylcyclopentadienyl)hafnium dichloride (Hf­(EtCp)₂Cl_2, Hf­(C₂H₅C₅H₄)₂Cl₂) with O₂ plasma reactant. The growth characteristics were significantly affected even by simply changing the precursor. Theoretical calculations utilizing geometrical information on the precursor and density functional theory revealed that the steric demands of the precursor ligands have a dominant effect on the different growth characteristics rather than the reaction probability of the precursor on the surface. The chemical compositional analysis results showed that the Cl residue in the HfO₂ films was reduced by using Hf­(EtCp)₂Cl₂ due to the lower number of Cl atoms in each Hf precursor molecule and the relieved bridge formation of Hf–Cl–Hf bridge on the surface compared to HfCl₄. The electrical property measurement results showed significantly improved insulating properties in HfO₂ using Hf­(EtCp)₂Cl₂ compared to HfCl₄ due to the low concentration of Cl residue in the film. These results provide broad insights to researchers who are interested in the fabrication of high quality dielectric layers to achieve better device performance and overcome physical limitations in the nanoscale regime

Layer-Controlled, Wafer-Scale, and Conformal Synthesis of Tungsten Disulfide Nanosheets Using Atomic Layer Deposition

The synthesis of atomically thin transition-metal disulfides (MS₂) with layer controllability and large-area uniformity is an essential requirement for their application in electronic and optical devices. In this work, we describe a process for the synthesis of WS₂ nanosheets through the sulfurization of an atomic layer deposition (ALD) WO₃ film with systematic layer controllability and wafer-level uniformity. The X-ray photoemission spectroscopy, Raman, and photoluminescence measurements exhibit that the ALD-based WS₂ nanosheets have good stoichiometry, clear Raman shift, and bandgap dependence as a function of the number of layers. The electron mobility of the monolayer WS₂ measured using a field-effect transistor (FET) with a high-k dielectric gate insulator is shown to be better than that of CVD-grown WS₂, and the subthreshold swing is comparable to that of an exfoliated MoS₂ FET device. Moreover, by utilizing the high conformality of the ALD process, we have developed a process for the fabrication of WS₂ nanotubes

Comparison of the Atomic Layer Deposition of Tantalum Oxide Thin Films Using Ta(N^<i>t</i>Bu)(NEt₂)₃, Ta(N^<i>t</i>Bu)(NEt₂)₂Cp, and H₂O

The growth characteristics of Ta₂O₅ thin films by atomic layer deposition (ALD) were examined using Ta­(N^<i>t</i>Bu)­(NEt₂)₃ (TBTDET) and Ta­(N^<i>t</i>Bu)­(NEt₂)₂Cp (TBDETCp) as Ta-precursors, where ^<i>t</i>Bu, Et, and Cp represent <i>tert</i>-butyl, ethyl, and cyclopentadienyl groups, respectively, along with water vapor as oxygen source. The grown Ta₂O₅ films were amorphous with very smooth surface morphology for both the Ta-precursors. The saturated ALD growth rates of Ta₂O₅ films were 0.77 Å cycle^–1 at 250 °C and 0.67 Å cycle^–1 at 300 °C using TBTDET and TBDETCp precursors, respectively. The thermal decomposition of the amido ligand (NEt₂) limited the ALD process temperature below 275 °C for TBTDET precursor. However, the ALD temperature window could be extended up to 325 °C due to a strong Ta–Cp bond for the TBDETCp precursor. Because of the improved thermal stability of TBDETCp precursor, excellent nonuniformity of ∼2% in 200 mm wafer could be achieved with a step coverage of ∼90% in a deep hole structure (aspect ratio 5:1) which is promising for 3-dimensional architecture to form high density memories. Nonetheless, a rather high concentration (∼7 at. %) of carbon impurities was incorporated into the Ta₂O₅ film using TBDETCp, which was possibly due to readsorption of dissociated ligands as small organic molecules in the growth of Ta₂O₅ film by ALD. Despite the presence of high carbon concentration which might be an origin of large leakage current under electric fields, the Ta₂O₅ film using TBDETCp showed a promising resistive switching performance with an endurance cycle as high as ∼17 500 for resistance switching random access memory application. The optical refractive index of the deposited Ta₂O₅ films was 2.1–2.2 at 632.8 nm using both the Ta-precursors, and indirect optical band gap was estimated to be ∼4.1 eV for both the cases