Study Of Atomic Layer Deposition Research Articles

(Invited) Enhanced Photocatalysis on TiO2-Passivated III-V Compound Semiconductors and 2D Material Heterostructures for Water Splitting and CO2 Reduction

We report enhanced photocatalysis for H2 evolution and CO2 reduction using TiO2-passivated InP and GaAs photocathodes.1-4 The TiO2 layer makes the InP semiconductor photochemically stable. This represents a major step forward in photocatalysis, which has typically been limited to metal oxide materials. In addition to making these surfaces stable, the TiO2 film, deposited by atomic layer deposition (ALD), also provides a substantial enhancement in the efficiency of H2 evolution. We find that passivating GaAs with just a few nm of TiO2 produces a shift in the onset potential of H2 evolution by +0.35 V at 1 mA/cm2 and enhances the photocurrent by 32-fold over bare GaAs (at 0 V vs. RHE). Here, thinner TiO2 films produce a larger enhancement than thicker films, which correlates with the higher density of O-vacancies (i.e., Ti3+ surface states) observed in these thinner films using X-ray photoemission spectroscopy (XPS). While TiO2 films 1-5nm thick produce large enhancements, no enhancement is observed for TiO2 thicknesses above 10 nm, which are crystalline and, therefore, considerably more insulating than thinner amorphous TiO2 films.Using transient reflectance spectroscopy (TRS), we measure the photoexcited carrier dynamics in a GaP/TiO2 photoelectrode, as well as the electrostatic field dynamics at this semiconductor-liquid interfaces in situ under various electrochemical potentials.5 Here, the electrostatic fields at the surface of the semiconductor are measured via Franz−Keldysh oscillations (FKO). These spectra reveal that the nanoscale TiO2 protection layer enhances the built-in field and charge separation performance of GaP photoelectrodes.We also demonstrated plasmon-resonant enhancement of photocatalysis on monolayer WSe2.6 In addition, we measured the stacking dependence and resonant interlayer excitation of monolayer WSe2/MoSe2 heterostructures for photocatalytic energy conversion.7 Qiu, J., G. Zeng, M. Ge, S. Arab, M. Mecklenburg, B. Hou, C. Shen, A.V. Benderskii and S.B. Cronin, Correlation of Ti3+ states with photocatalytic enhancement in TiO2-passivated p-GaAs. Journal of Catalysis, 337, 133 (2016).Qiu, J., G.T. Zeng, M.A. Ha, M.Y. Ge, Y.J. Lin, M. Hettick, B.Y. Hou, A.N. Alexandrova, A. Javey and S.B. Cronin, Artificial Photosynthesis on TiO2-Passivated InP Nanopillars. Nano Letters, 15, 6177-6181 (2015).Qiu, J., G.T. Zeng, M.A. Ha, B.Y. Hou, M. Mecklenburg, H.T. Shi, A.N. Alexandrova and S.B. Cronin, Microscopic Study of Atomic Layer Deposition of TiO2 on GaAs and Its Photocatalytic Application. Chemistry of Materials, 27, 7977-7981 (2015).Wang, Y. and S.B. Cronin, Performance Enhancement of TiO2-encapsulated Photoelectrodes Based on III–V Compound Semiconductors, in Ultrathin Oxide Layers for Solar and Electrocatalytic Systems. 2022, Royal Society of Chemistry. p. 103-134.Xu, Z.H., B.Y. Hou, F.Y. Zhao, Z. Cai, H.T. Shi, Y.W. Liu, C.L. Hill, D.G. Musaev, M. Mecklenburg, S.B. Cronin and T.Q. Lian, Nanoscale TiO2 Protection Layer Enhances the Built-In Field and Charge Separation Performance of GaP Photoelectrodes. Nano Letters, 21, 8017-8024 (2021).Chen, J., C.S. Bailey, Y. Hong, L. Wang, Z. Cai, L. Shen, B. Hou, Y. Wang, H. Shi, J. Sambur, W. Ren, E. Pop, and S.B. Cronin., Plasmon-Resonant Enhancement of Photocatalysis on Monolayer WSe2. ACS Photonics, 6, 787 (2019).Chen, J., C.S. Bailey, D. Cui, Y. Wang, B. Wang, H. Shi, Z. Cai, E. Pop, C. Zhou and S.B. Cronin, Stacking Independence and Resonant Interlayer Excitation of Monolayer WSe2/MoSe2 Heterostructures for Photocatalytic Energy Conversion. ACS Applied Nano Materials, DOI:10.1021/acsanm.9b01898 (2020).

Mechanistic studies of atomic layer deposition on oxidation catalysts - AlOx and POx deposition.

Atomic layer deposition is a rising technique for catalyst synthesis and modification. Typically, the focus of ALD in catalysis is on supported metal nanoparticles. Here, the authors give mechanistic insights into the ALD of oxides on redox active catalysts by a combination of in situ analytics, such as XPS, DRIFTS and gravimetric measurements. Phosphorus oxide and aluminum oxide were deposited on divanadium pentoxide powder in a fixed bed reactor. In contrast to the generally accepted concepts, the first half cycle does not proceed over surface hydroxyl groups but involves redox chemistry between the precursor and the vanadium atoms, as shown by 31P-SSNMR and XPS. For POx deposition, a temperature step from 150 °C in the first half cycle to 450 °C in the second half cycle is needed to obtain linear mass gain per cycle as the remaining ligands are combusted and reduced vanadium atoms are reoxidized. Homogeneous deposition was confirmed by STEM-EDX and XRD showing no additional phases, despite performing up to 10 ALD cycles. Even the well-known process of alumina ALD confirms the involvement of reduction-oxidation reactions between the ALD precursor and the substrate V2O5. However, redox chemistry can be suppressed for alumina ALD at low temperatures of 50 °C. Therefore, this study shows that ALD on oxidation catalysts is complex and thus the developed ALD processes are unusual compared to ALD on typical supports, such as SiO2 or Al2O3.

The Endocyclic Carbon Substituent of Guanidinate and Amidinate Precursors Controlling Atomic Layer Deposition of InN Films

Indium nitride (InN) is an interesting material for future high-frequency electronics due to its high electron mobility. The problematic deposition of InN films currently prevents full exploration of InN-based electronics. We present studies of atomic layer deposition (ALD) of InN using In precursors with bidentate ligands forming In–N bonds: tris(N,N-dimethyl-N′,N″-diisopropylguanidinato)indium(III), tris(N,N′-diisopropylamidinato)indium(III), and tris(N,N′-diisopropylformamidinato)indium(III). These compounds form a series were the size of the substituent on the endocyclic position decreases from −NMe2 to −Me and to −H, respectively. We show that when the size of the substituent decreases, the InN films deposited have a better crystalline quality, of better optical quality, lower roughness, and an In/N ratio closer to unity. From quantum chemical calculations, we show that the smaller substituents lead to less steric repulsion and weaker bonds between the ligand and In center. We propose that these effects render a more favored surface chemistry for the nitridation step in the ALD cycle, which explains the improved film properties.

Microscopic Study of Atomic Layer Deposition of TiO2 on GaAs and Its Photocatalytic Application

We report a microscopic study of p-GaAs/TiO2 heterojunctions using cross-sectional high resolution transmission electron microscopy (HRTEM). The photocatalytic performance for both H2 evolution and CO2 reduction of these heterostructures shows a very strong dependence on the thickness of the TiO2 over the range of 0–15 nm. Thinner films (1–10 nm) are amorphous and show enhanced catalytic performance with respect to bare GaAs. HRTEM images and electron energy loss spectroscopy (EELS) maps show that the native oxide of GaAs is removed by the TiCl4 atomic layer deposition (ALD) precursor, which is corrosive. Ti3+ defect states (i.e., O vacancies) in the TiO2 film provide catalytically active sites, which improve the photocatalytic efficiency. Density functional theory (DFT) calculations show that water molecules and CO2 molecules bind stably to these Ti3+ states. Thicker TiO2 films (15 nm) are crystalline and have poor charge transfer due to their insulating nature, while thinner amorphous TiO2 films are conducting.

Density Functional Theory Study of Atomic Layer Deposition of Zinc Oxide on Graphene.

The dissociation of zinc ions (Zn2+) from vapor-phase zinc acetylacetonate, Zn(C5H7O2)2, or Zn(acac)2 and its adsorption onto graphene oxide via atomic layer deposition (ALD) were studied using a quantum mechanics approach. Density functional theory (DFT) was used to obtain an approximate solution to the Schrödinger equation. The graphene oxide cluster model was used to represent the surface of the graphene film after pre-oxidation. In this study, the geometries of reactants, transition states, and products were optimized using the B3LYB/6-31G** level of theory or higher. Furthermore, the relative energies of the various intermediates and products in the gas-phase radical mechanism were calculated at the B3LYP/6-311++G** and MP2/6-311 + G(2df,2p) levels of theory. Additionally, a molecular orbital (MO) analysis was performed for the products of the decomposition of the Zn(acac)2 complex to investigate the dissociation of Zn2+ and the subsequent adsorption of H atoms on the C5H7O2 cluster to form acetylacetonate enol. The reaction energies were calculated, and the reaction mechanism was accordingly proposed. A simulation of infrared (IR) properties was performed using the same approach to support the proposed mechanism via a complete explanation of bond forming and breaking during each reaction step.

A comparative study of atomic layer deposition of Al2O3 and HfO2 on AlGaN/GaN

The atomic layer depositions of Al2O3 and HfO2 on AlGaN/GaN were systematically studied. The band alignments of Al2O3/AlGaN and HfO2/AlGaN were investigated using in situ X-ray photoelectron spectroscopy. A conduction band offset of 1.8 and 1.1 eV were observed for Al2O3/AlGaN and HfO2/AlGaN, respectively. The Al2O3 and HfO2 dielectric layers were found to reduce the leakage current as expected with respect to metal/AlGaN/GaN Schottky diodes, but neither changes the surface states of AlGaN. The positive ionized surface donor states density and average interface state density (D it ) below the AlGaN conduction band edge (0.34 < E C − E T < 0.50 eV), extracted from capacitance voltage (C–V) curves, were ~5 × 1013 cm−2 and 1.1 × 1014 cm−2 eV−1, respectively.

Atomistic kinetic Monte Carlo study of atomic layer deposition derived from density functional theory

To describe the atomic layer deposition (ALD) reactions of HfO2 from Hf(N(CH3)2)4 and H2O, a three-dimensional on-lattice kinetic Monte-Carlo model is developed. In this model, all atomistic reaction pathways in density functional theory (DFT) are implemented as reaction events on the lattice. This contains all steps, from the early stage of adsorption of each ALD precursor, kinetics of the surface protons, interaction between the remaining precursors (steric effect), influence of remaining fragments on adsorption sites (blocking), densification of each ALD precursor, migration of each ALD precursors, and cooperation between the remaining precursors to adsorb H2O (cooperative effect). The essential chemistry of the ALD reactions depends on the local environment at the surface. The coordination number and a neighbor list are used to implement the dependencies. The validity and necessity of the proposed reaction pathways are statistically established at the mesoscale. The formation of one monolayer of precursor fragments is shown at the end of the metal pulse. Adsorption and dissociation of the H2O precursor onto that layer is described, leading to the delivery of oxygen and protons to the surface during the H2O pulse. Through these processes, the remaining precursor fragments desorb from the surface, leaving the surface with bulk-like and OH-terminated HfO2, ready for the next cycle. The migration of the low coordinated remaining precursor fragments is also proposed. This process introduces a slow reordering motion (crawling) at the mesoscale, leading to the smooth and conformal thin film that is characteristic of ALD.

Structural properties of as deposited and annealed ZrO2 influenced by atomic layer deposition, substrate, and doping

Thin ZrO2 films are of high interest as high-k material in dynamic random access memory (DRAM), embedded dynamic random access memory, and resistive random access memory as well as for gate oxides. Actually, ZrO2 is predicted to be the key material in future DRAM generations below 20 nm. Profound knowledge of pure and doped ZrO2 thin films, especially of the structural properties, is essential in order to meet the requirements of future devices. This paper gives a detailed overview about the structural properties of ZrO2 films in dependence of various process parameters. The study of atomic layer deposition (ALD) growth mechanisms of ZrO2 on a TiN-substrate in comparison to a Si-substrate covered with native oxide exhibits significant differences. Furthermore, the structural properties crystallinity, surface roughness, and film stress are studied after the ALD deposition in dependence of the process parameters deposition temperature, layer thickness, and underlying substrate. Remarkable dependencies of the ZrO2 crystallization temperatures on the substrates are figured out. The structural properties after various annealing steps are monitored as well. The influence of doping by SiO2 and Al2O3 is studied, which is primarily used to keep the thin films amorphous during deposition.

Study of Atomic Layer Deposition of ZnO on a Polar Oxide Substrate by In Situ Quartz Crystal Microbalance

ZnO thin films were grown using the atomic layer deposition (ALD) technique with diethyl zinc (DEZ) and deionized water as precursors. A viscous-flow atomic layer deposition reactor has been designed and constructed for the ZnO growth. An in-situ quartz crystal microbalance (QCM) has been interfaced with the reactor to monitor the ALD film growth. Using this ALD reactor we have grown ZnO films on oxidized Si(001) and on MgO(111) polar oxide substrates. QCM was used to monitor the film growth rate as function of precursor pulse time and temperature. In the ALD temperature window (130 to 170oC), the growth rate was found to be about 2.3 Aå/cycle. Ex-situ atomic force microscope (AFM) studies on ZnO thin films reveal surface roughness increases with number of ALD cycles.

Study of Atomic Layer Deposition of ZnO on Polar Oxide Substrate by In Situ Quartz Crystal Microbalance

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Fabrication of Solid Oxide Fuel Cells via Thin Film Techniques

Fabricating electrolyte with thin film is one of various methods of lowering the operation temperature for solid oxide fuel cell (SOFC). Because the major polarization of SOFCs is ohmic loss, many groups have tried to fabricate a thin film electrolyte – some proton conducting material that has the conductivity of a couple of orders higher than oxygen conducting one. We have investigated the synthesis of Membrane Electrode Assembly (MEA) including Yttria-doped Barium Zirconate(BYZ) thin film electrolyte via thin-film processes such as Pulsed Laser Deposition and Sputtering. Another approach to lower the operation temperature of SOFCs is the bi-layer structure electrolyte. The functional layer via thin film deposition could guarantee minimum power density loss and stable operation. After the study of Atomic Layer Deposition (ALD) condition to deposit Yttria Stabilized Zirconia (YSZ) on Gadonilia doped Cerate (GDC) substrate, GDC/YSZ bi-layered electrolyte button cells showed higher OCV and larger limiting current with 100nm YSZ ultra thin film. The performance improvement might be attributed to the function of electron blocking and cutting off the reducible gas.

Low temperature atomic layer deposition of titania thin films

This paper presents a comprehensive study of atomic layer deposition of TiO 2 films on silicon and polycarbonate substrates using TiCl 4 and H 2O as precursors at temperatures in the range 80–120 °C. An in-situ quartz crystal microbalance was used to monitor different processing conditions and the resultant films were characterised ex-situ using a suite of surface analytical tools. In addition, the contact angle and wettability of as-deposited and UV irradiated films were assessed. The latter was found to reduce the contact angle from ≥ 80° to < 10°. Finally, the effect of surface pre-treatment on film toughness and adhesion was investigated and the results show a significant improvement for the pre-treated films.

Comparative Studies of Atomic Layer Deposition and Plasma-Enhanced Atomic Layer Deposition Ta2O5 and the Effects on Electrical Properties of In situ Nitridation

Ta2O5 and TaOxNy thin films were deposited by atomic layer deposition (ALD) from Ta(NMe2)5 (PDMAT) with water, oxygen plasma, and nitrogen added oxygen plasma. The film properties were comparatively investigated focusing on the electrical properties from metal oxide semiconductor capacitor structure with 10 nm Ta2O5 or TaOxNy. The results show that plasma-enhanced ALD (PE-ALD) Ta2O5 film has better electrical properties including lower interface state density and leakage current than thermal ALD. Moreover, PE-ALD TaOxNy shows the best properties, indicating the beneficial effects of in situ nitridation. Especially, time dependent dielectric breakdown was significantly improved up to 4000 times of thermal ALD Ta2O5. These results show that, intentional in situ nitrogen incorporation with good electrical properties was successfully achieved by PE-ALD using nitrogen–oxygen mixture.

In Situ Auger Electron Spectroscopy Study of Atomic Layer Deposition: Growth Initiation and Interface Formation Reactions during Ruthenium ALD on Si−H, SiO2, and HfO2 Surfaces

Growth initiation and film nucleation in atomic layer deposition (ALD) is important for controlling interface composition and achieving atomic-scale films with well-defined composition. Ruthenium ALD is studied here using ruthenocene and oxygen as reactants, and growth initiation and nucleation are characterized on several different growth surfaces, including SiO2, HfO2, and hydrogen terminated silicon, using on-line Auger electron spectroscopy and ex-situ X-ray photoelectron spectroscopy. The time needed to reach the full growth rate (typically approximately 1 A per deposition cycle) is found to increase as the surface energy of the starting surface (determined from contact angle measurements) decreased. Growth starts more readily on HfO2 than on SiO2 or Si-H surfaces, and Auger analysis indicates distinct differences in surface reactions on the various surfaces during film nucleation. Specifically, surface oxygen is consumed during ruthenocene exposure, so the nucleation rate will depend on the availability of oxygen and the energetics of surface oxygen bonding on the starting substrate surface.

Atomic layer deposition in porous structures: 3D photonic crystals

This paper reports recent results from studies of atomic layer deposition for the infiltration of three-dimensional photonic crystals. Infiltration of ZnS:Mn and TiO 2 are reported for SiO 2-based opal templates. It has been demonstrated that high filling fractions can be achieved and that the infiltrated material can be of high crystalline quality as assessed by photoluminescence measurements. The highly conformal and uniform coatings obtained in these studies are shown to contribute significantly to the photonic band gap properties. These investigations show the advantages of atomic layer deposition (ALD) as a flexible and practical pathway for attaining high performance photonic crystal structures and optical microcavities.

In Situ Quartz Crystal Microbalance and Quadrupole Mass Spectrometry Studies of Atomic Layer Deposition of Aluminum Oxide from Trimethylaluminum and Water

Reaction mechanisms in the atomic layer deposition of Al2O3 from Al(CH3)3 and water were studied with a quartz crystal microbalance at 150−350 °C and with a quadrupole mass spectrometer at 150−400 °C. The growth rate was the highest at 250 °C. At lower temperatures the growth was limited due to kinetic reasons and at higher temperatures due to lower amount of surface −OH groups. About half of the ligands were released during the Al(CH3)3 pulse and the other half during the water pulse. The reaction temperature had no marked effect on the growth mechanisms, in the temperature range studied.

Comparative study of atomic layer deposition and low-pressure MOCVD of copper sulfide thin films

Thin films of copper sulfide (Cu x S) have been prepared on fluor-doped tin dioxide (SnO 2 :F) coated glass and on uncoated glass by atomic layer deposition (ALD) and low-pressure chemical vapor deposition (LPCVD). In both cases, copper(II) bis-tetramethylheptanedionate (Cu(thd) 2 ) and H 2 S are used. Cu x S films form between 125 and 300 °C with ALD and between 127 and 505 °C with LPCVD. Deposition rates are obtained between 1 and 109 A/min by LPCVD and between 0.15 and 0.40 A/cycle by ALD. Self-limited grow by ALD is achieved up to 250 °C at 2 mbar. It is found that in both processes the temperature determines the crystalline phase of the films. CuS is obtained at low deposition temperatures when using either ALD or LPCVD. A phase transition occurs at 175 °C from pure CuS into pure Cu 9 S 5 with ALD. In the LPCVD process the phase composition of the films starts to change at 230 °C from pure CuS into a mixture of CuS and Cu 9 S 5 . Pure Cu 9 S 5 is obtained between 285 and 315 °C. The morphology of LPCVD films are fine-structured with an average grain size of 100 nm for films of 500 nm, while 40 nm thinn ALD films consist of grains with an average grain size of ∼100 nm grown in columns perpendicular to the surface.