Atomic layer deposition of ZnO on MoS2 and WSe2

Delayed atomic layer deposition (ALD) of ZnO, i.e., area selective (AS)-ALD, was successfully achieved on silicon wafers (Si\SiO2) terminated with tris(dimethylamino)methylsilane (TDMAMS). This resist molecule was deposited in a home-built, near-atmospheric pressure, flow-through, gas-phase reactor. TDMAMS had previously been shown to react with Si\SiO2 in a single cycle/reaction and to drastically reduce the number of silanols that remain at the surface. ZnO was deposited in a commercial ALD system using dimethylzinc (DMZ) as the zinc precursor and H2O as the coreactant. Deposition of TDMAMS was confirmed by spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), and wetting. ALD of ZnO, including its selectivity on TDMAMS-terminated Si\SiO2 (Si\SiO2\TDMAMS), was confirmed by in situ multi-wavelength ellipsometry, ex situ SE, XPS, and/or high-sensitivity/low-energy ion scattering (HS-LEIS). The thermal stability of the TDMAMS resist layer, which is an important parameter for AS-ALD, was investigated by heating Si\SiO2\TDMAMS in air and nitrogen at 330 °C. ALD of ZnO takes place more readily on Si\SiO2\TDMAMS heated in the air than in N2, suggesting greater damage to the surface heated in the air. To better understand the in situ ALD of ZnO on Si\SiO2\TDMAMS and modified (thermally stressed) forms of it, the ellipsometry results were plotted as the normalized growth per cycle. Even one short pulse of TDMAMS effectively passivates Si\SiO2. TDMAMS can be a useful, small-molecule inhibitor of ALD of ZnO on Si\SiO2 surfaces.

Comparisons of alumina barrier films deposited by thermal and plasma atomic layer deposition

ABSTRACTTo enhance cycleability of LiMn2O4 at elevated-temperature we use atomic layer deposition (ALD) method to deposit a variety of ultrathin and highly conformal amphoteric oxide (ZnO, ZrO2, Al2O3) coatings for surface modification of LiMn2O4 electrodes. High-resolution transmission electron microscopic (HRTEM) images of ZnO, ZrO2 and Al2O3 ALD coated LiMn2O4 particles demonstrate the high qualities of ALD coatings with respect to remarkable conformity, homogeneity and uniformity. Two types of ALD-modified LiMn2O4 electrodes are fabricated: one is ALD-coated LiMn2O4 composite electrode, the other is electrode composed of ALD-coated LiMn2O4 particles and uncoated carbon/poly-vinylidenefluoride (PVDF) network. All LiMn2O4 electrodes modified with 6 oxide ALD layers (as thin as ∼1 nm) reveal significantly enhanced electrochemical performances than bare electrodes at both 25°C and 55°C. After 100 electrochemical cycles at 1 C at 55°C, the electrode consisting of LiMn2O4 particles coated with 6 ZnO ALD layers remains the highest capacity of 56.1 mAh/g, higher than 51.1 mAh/g of ZrO2 coated LiMn2O4 particles, 45.8 mAh/g of Al2O3 coated LiMn2O4 particles and 27.0 mAh/g of the bare composite electrode as well as 44.5 mAh/g of the composite electrode coated with 6 ZnO ALD layers. These results indicate that ZnO ALD coating is the most effective protective film for improved cycling stability, followed by ZrO2 and Al2O3. It is also found that amphoteric oxide coating on LiMn2O4 particles is more effective to enhance the cycleability of LiMn2O4 than coating on composite electrode. Furthermore, for coating either on composite electrode or on LiMn2O4 particles, the effect of ALD coating on improving capacity retention and increasing specific capacity of LiMn2O4 is more phenomenal at elevated temperature than at room temperature.

A mesoporous atomic layer deposition (ALD) double-shell electrode, Al2O3 (insulating core)//ALD ZnO|ALD TiO2, on a fluorine-doped tin oxide (FTO) conducting substrate was explored for a photoanode assembly, FTO//Al2O3 (insulating core)//ALD ZnO|ALD TiO2|-chromophore-catalyst, for light-driven water oxidation. Photocurrent densities at photoanodes based on mesoporous ALD double-shell (ALD ZnO|ALD TiO2|) and ALD single-shell (ALD ZnO|, ALD TiO2|) electrodes were investigated for O2 evaluation by a generator-collector dual working electrode configuration. The high photocurrent densities obtained based on the mesoporous ALD ZnO|ALD TiO2 photoanode for O2 evolution arise from a significant barrier to back electron transfer (BET) by the optimized tunneling barrier in the structure with the built-in electric field at the ALD ZnO|ALD TiO2 interface. The charge recombination is thus largely decreased. In the films, BET following injection has been investigated through kinetic nanosecond transient absorption spectra, and the results of energy band analysis are used to derive insight into the internal electronic structure of the electrodes.

Herein, amorphous molybdenum oxide films are constructed by thermal atomic layer deposition (T‐ALD) and plasma‐enhanced atomic layer deposition (PE‐ALD). The physical and chemical properties of molybdenum oxide films prepared by the two methods are systematically compared by means of film growth law, atomic force microscope, scanning electron microscope, etc. The results show that the amorphous molybdenum oxide physical phase prepared by both ALD methods is MoO3. Compared with T‐ALD MoO3, the growth rate of MoO3 thin films prepared by PE‐ALD is higher. Compared to PE‐ALD MoO3, the MoO3 films prepared by T‐ALD did not have nucleation delayed to a laminar growth mode, resulting in smoother deposited films and contained less impurity carbon. The MoO3 prepared by PE‐ALD contains 7.4% impurity carbon. This carbon‐doped film significantly improves the conductivity of the MoO3 film and shows good electrochemical activity. As expected, the MoO3 films prepared by PE‐ALD show good electrocatalytic oxygen evolution reaction. The overpotential is only 259 mV at 10 mA cm−2 and continues to evolution oxygen for 60 h with almost no attenuation, indicating that carbon doping significantly improves the catalytic intrinsic activity and stability of MoO3.

Comparison between thermal and plasma enhanced atomic layer deposition processes for the growth of HfO2 dielectric layers

The wide applications of ultrathin group IV metal oxide films (TiO2, ZrO2 and HfO2) probably expose materials to potentially reactive etchants and solvents, appealing for extraordinary chemical stability and corrosion resistance property. In this paper, TiO2 ultrathin films were deposited on Si at 200 °C while ZrO2 and HfO2 were grown at 250 °C to fit their growth temperature window, by thermal atomic layer deposition (TALD) and plasma-enhanced ALD (PEALD). A variety of chemical liquid media including 1 mol/L H2SO4, 1 mol/L HCl, 1 mol/L KOH, 1 mol/L KCl, and 18 MΩ deionized water were used to test and compare chemical stability of all these as-deposited group IV metal oxides thin films, as well as post-annealed samples at various temperatures. Among these metal oxides, TALD/PEALD HfO2 ultrathin films exhibit the best chemical stability and anti-corrosion property without any change in thickness after long time immersion into acidic, alkaline and neutral solutions. As-deposited TALD ZrO2 ultrathin films have slow etch rate of 1.06 nm/day in 1 mol/L HCl, however other PEALD ZrO2 ultrathin films and annealed TALD ones show better anti-acid stability, indicating the role of introduction of plasma O2 in PEALD and post-thermal treatment. As-deposited TiO2 ultrathin films by TALD and PEALD are found to be etched slowly in acidic solutions, but the PEALD can decrease the etching rate of TiO2 by ~41%. After post-annealing, TiO2 ultrathin films have satisfactory corrosion resistance, which is ascribed to the crystallization transition from amorphous to anatase phase and the formation of 5% Si-doped TiO2 ultrathin layers on sample surfaces, i.e. Ti-silicate. ZrO2, and TiO2 ultrathin films show excellent corrosion endurance property in basic and neutral solutions. Simultaneously, 304 stainless steel coated with PEALD-HfO2 is found to have a lower corrosion rate than that with TALD-HfO2 by means of electrochemical measurement. The pre-treatment of plasma H2 to 304 stainless steel can effectively reduce interfacial impurities and porosity of overlayers with significantly enhanced corrosion endurance. Above all, the chemical stability and anti-corrosion properties of IV group metal oxide coatings can be improved by using PEALD technique, post-annealing process and plasma H2 pre-treatment to substrates.

We have employed plasma-enhanced and thermal atomic layer deposition (ALD) within the temperature range of 50–150°C for the deposition of ultra-thin (10–50 nm) Al2O3 films on 100Cr6 steel and aluminium Al2024-T3 alloys. [Al(CH3)3] was used as the precursor with either an O2 plasma or water as co-reactants. Neutral salt spray tests showed that the thicker films offered the best corrosion-resistance. Using cyclic voltametry, the 50 nm films were found to be the least porous (<0.5%). For 10 nm thick films, plasma-enhanced ALD afforded a lower porosity and higher film density than thermal ALD. ToF-SIMS measurements on 100Cr6 showed that the main ‘bulk’ of the films contained very few impurities, but OH and C were observed at the interfaces. TEM confirmed that the films were conformal on all substrates and the adhesion was excellent for the films deposited by plasma-enhanced ALD but not for thermal ALD, as delamination was observed. On the basis of these and other results, the prospects of the application of ALD films for corrosion protection, and the use of plasma-enhanced ALD to promote their nucleation, is discussed.

Ferroelectric HfZrO2 (HZO) formed by atomic layer deposition (ALD) has been widely studied due to its composition control of material contents and stable ferroelectric properties. However, the effect of various ALD methods on ferroelectric switching dynamics has not been thoroughly investigated. We conduct a comparative study on the differences in ferroelectric (FE) domain wall motion under electrical cycling stress between two ALD methods: thermal ALD (THALD) and plasma-enhanced ALD (PEALD). The extraction of activation energy from fatigue rate and FE switching speed analysis results shows that PEALD HZO has inherent defects during the deposition step, and the FE switching speed of PEALD HZO degraded faster under cycling stress than that of THALD HZO. XPS analysis results show that under cycling stress, oxygen vacancies are formed faster in PEALD than in THALD HZO. Furthermore, dynamic domain phase analysis shows that the electric fields required for switching in the relaxation to creep (E1) change in THALD HZO by +46%, while the electrical fields required for the transition from creep to flow (E2) rarely change under the cycling stress. However, E1 and E2 values of PEALD HZO change by +19% and -10%, respectively, depending on the cycling stress.

We report combining plasma-enhanced atomic layer deposition (PEALD) ZnO thin film transistors (TFTs) with a self-aligned-gate process to fabricate high speed circuits. Sputter deposited ZnO films have been widely studied, but there have been few reports of dense thin films and high performance devices and circuits. Atomic layer deposition (ALD) of ZnO has been shown to be a very uniform and conformal process, but ALD ZnO films typically have high background carrier concentration and require doping compensation for enhancement-mode TFTs. PEALD provides the uniform conformal coating of ALD and enhancement-mode devices from uncompensated films. Our PEALD ZnO TFTs have linear field-effect mobility >20 cm /V-s and saturation field-effect mobility >30 cm /V-s and PEALD ring oscillators with beta ratio ~5, channel length ~2.8 ¿m, and ~1.5 ¿m gate-source/drain overlap operate at ~25 ns/stage. Recently, scaled indium-gallium zinc oxide ring oscillator circuits on silicon substrates (0.5 ¿m channel length, and 0.5 ¿m overlap) were reported to operate at ~7 ns/stage with a saturated-load inverter design, and < 1 ns/stage with a novel bootstrapped inverter design. The speed of our previous PEALD circuits was largely limited by the parasitic capacitance between the gate and drain, and a self-aligned-gate process would provide higher speed devices and circuits. Here we have report a simple self-aligned-gate process for ZnO TFTs and high speed circuits.

Thermal atomic layer deposition (TALD) and plasma atomic layer deposition (PALD) were used for producing thin NiO x films from nickel(II) acetylacetonate Ni(acac)2, employing different oxidizing agents (deionized water H2O, ozone O3, and molecular oxygen O2). The films were deposited at 300 °C (TALD) and 220 °C (PALD) over glass substrates; their physical and chemical properties were considerably influenced by the choice of oxidizing agents. In particular, ALD(H2O) samples had a low growth per cycle (GPC) and a high concentration of defects. The best NiO x parameters were achieved with PALD(O2), featuring high GPC (0.07 nm/cycle), high optical transparency in the visible region, electrical resistivity (1.18 × 104 Ω·cm), good carrier concentration (8.82 × 1013 cm-3), and common mobility (5.98 cm2/V·s). The resulting NiO x films are polycrystalline and homogeneous in thickness and composition. According to ultraviolet photoelectron spectroscopy (UPS), work function φ and the valence band maximum E V can be tuned by the choice of the coreactant employed, with variations of up to ∼1 eV between TALD and PALD synthesis. Our results suggest that PALD permits one to achieve a better energy band alignment of NiO x and CsFAMAPbBrI perovskite, which is promising for solar cell applications.

For atomic layer deposition (ALD) of doped, ternary, and quaternary materials achieved by combining multiple binary ALD processes, it is often difficult to correlate the material properties and growth characteristics with the process parameters due to a limited understanding of the underlying surface chemistry. In this work, in situ Fourier transform infrared (FTIR) spectroscopy was employed during ALD of zinc-oxide, tin-oxide, and zinc-tin-oxide (ZTO) with the precursors diethylzinc (DEZ), tetrakis(dimethylamino)tin (TDMASn), and H2O. The main aim was to investigate the molecular basis for the nucleation delay during ALD of ZTO, observed when ZnO ALD is carried out after SnO2 ALD. Gas-phase FTIR spectroscopy showed that dimethylamine, the main reaction product of the SnO2 ALD process, is released not only during SnO2 ALD but also when depositing ZnO after SnO2, indicating incomplete removal of the ligands of the TDMASn precursor from the surface. Transmission FTIR spectroscopy performed during ALD on SiO2 powder revealed that a significant fraction of the ligands persist during both SnO2 and ZnO ALD. These observations provide experimental evidence for a recently proposed mechanism, based on theoretical calculations, suggesting that the elimination of precursor ligands is often not complete. In addition, it was found that the removal of precursor ligands by H2O exposure is even less effective when ZnO ALD is carried out after SnO2 ALD, which likely causes the nucleation delay in ZnO ALD during the deposition of ZTO. The underlying mechanisms and the consequences of the incomplete elimination of precursor ligands are discussed.

Only a few iron precursors that can be used in the atomic layer deposition (ALD) of iron oxides have been examined thus far. This study aimed to compare the various properties of FeOx thin films deposited using thermal ALD and plasma-enhanced ALD (PEALD) and to evaluate the advantages and disadvantages of using bis(N,N'-di-butylacetamidinato)iron(II) as an Fe precursor in FeOx ALD. The PEALD of FeOx films using iron bisamidinate has not yet been reported. Compared with thermal ALD films, PEALD films exhibited improved properties in terms of surface roughness, film density, and crystallinity after they were annealed in air at 500 °C. The annealed films, which had thicknesses exceeding ~ 9 nm, exhibited hematite crystal structures. Additionally, the conformality of the ALD-grown films was examined using trench-structured wafers with different aspect ratios.

Interfacial engineering of fiber-reinforced composites is critical to control material properties, such as mechanical strength and toughness. In this study, we utilize atomic layer deposition (ALD) as a method to conformally coat structural fiber surfaces and study the impact of these interlayers on their mechanical adhesion to polymer-matrix materials. ALD of Al2O3, ZnO, and TiO2 were applied to Kevlar® and carbon fibers, and microbond testing was performed using droplets of PMMA and Epoxy. It was observed that the mechanical force required for debonding of the polymer droplet from the coated fiber surfaces depended on the composition and thickness of the coating. Post-mortem scanning electron microscopy and energy dispersive X-ray spectroscopy for elemental analysis indicated that the ALD films remained adhered to the droplet, suggesting that the ALD/fiber interface limited the mechanical properties of the interphase region. Additionally, ALD of ZnO was demonstrated to prevent fiber degradation from ultraviolet (UV) and high-temperature thermal treatments, demonstrating a pathway towards multi-functional composite interphase engineering by ALD.

In this paper, we present the current research efforts on the atomic layer deposition (ALD) ZnO based TFT devices carried out in our laboratory. ZnO thin film deposition was carried out by two different ALD processes; thermal ALD using water as a reactant and plasma-enhanced ALD using oxygen plasma as a reactant. The film properties were comparatively studied showing large difference in terms of electrical properties. For thermal ALD ZnO, carrier concentrations were too high to fabricate well-operated ZnO TFTs. To control the carrier concentration, nitrogen doping was utilized based on NH4OH reactant. Meanwhile, for PE-ALD, highly resistive films were obtained at low growth temperature below 200 °C. To reduce the resistivity to a proper level for the fabrication of TFTs, UV-light exposure was used. At properly controlled conditions, high performance TFT devices were fabricated based on these processes. ZnO TFTs were also fabricated on flexible substrates and the initial research was carried out on the effects of device bending on device properties.