Ultrathin Al2O3 Layer Research Articles

Mechanical, structural, and optical properties of PEALD metallic oxides for optical applications.

Structural, optical, and mechanical properties of Al2O3, SiO2, and HfO2 materials prepared by plasma-enhanced atomic layer deposition (PEALD) were investigated. Residual stress poses significant challenges for optical coatings since it may lead to mechanical failure, but in-depth understanding of these properties is still missing for PEALD coatings. The tensile stress of PEALD alumina films decreases with increasing deposition temperature and is approximately 100 MPa lower than the stress in thermally grown films. It was associated with incorporation of -OH groups in the film as measured by infrared spectroscopy. The tensile stress of hafnia PEALD layers increases with deposition temperature and was related to crystallization of the film. HfO2 nanocrystallites were observed even at 100°C deposition temperature with transmission electron microscopy. Stress in hafnia films can be reduced from approximately 650 MPA to approximately 450 MPa by incorporating ultrathin Al2O3 layers. PEALD silica layers have shown moderate stress values and stress relaxation with the storage time, which was correlated to water adsorption. A complex interference coating system for a dichroic mirror (DCM) at 355 nm wavelength was realized with a total coating thickness of approximately 2 μm. Severe cracking of the DCM coating was observed, and it propagates even into the substrate material, showing a good adhesion of the ALD films. The reflectance peak is above 99.6% despite the mechanical failure, and further optimization on the material properties should be carried out for demanding optical applications.

Surface chemistry and initial growth of Al2O3 on plasma modified PTFE studied by ALD

This paper reports on the growth of ultra-thin Al2O3 layers deposited on polytetrafluoroethylene (PTFE) surfaces pre-treated by atmospheric pressure ambient air plasma. Al2O3 thin films were deposited by a thermal atomic layer deposition (ALD) process using trimethylaluminium and water at 100°C. The concentration of elements and the bonding structures in layers resulting from up to 350 ALD cycles was measured by X-ray photoelectron spectroscopy. It was disclosed that the concentration of oxygen-containing groups increased initially as surface carbon contamination was oxidised, then decreased as the surface composition approached pure PTFE. In general, the plasma-treated samples showed significantly faster nucleation; 300 cycles were necessary for the formation of a complete film on untreated PTFE, whereas only 150 cycles were needed to create uniform films on plasma-pre-treated PTFE surfaces. The swiftest nucleation behaviour was associated with the highest number of oxygen-containing groups on the surface rather than complete removal of contamination. As demonstrated here, the state of a plasma-activated surface may easily be studied by means of deposition of atomic layers by the ALD method.

Eliminated Phototoxicity of TiO2 Particles by an Atomic-Layer-Deposited Al2 O3 Coating Layer for UV-Protection Applications.

We demonstrate the conformal coating of an ultrathin Al2 O3 layer on TiO2 nanoparticles through atomic layer deposition by using a specifically designed rotary reactor to eliminate the phototoxicity of the particles for cosmetic use. The ALD reactor is modified to improve the coating efficiency as well as the agitation of the particles for conformal coating. Elemental and microstructural analyses show that ultrathin Al2 O3 layers are conformally deposited on the TiO2 nanoparticles with a controlled thickness. Rhodamine B dye molecules on Al2 O3 -coated TiO2 exhibited a long life time under UV irradiation, that is, more than 2 h, compared to that on bare TiO2 , that is, 8 min, indicating mitigation of photocatalytic activity by the coated layer. The effect of carbon impurities in the film resulting from various deposition temperatures and thicknesses of the Al2 O3 layer on the photocatalytic activity are also thoroughly investigated with controlled experimental condition by using dye molecules on the surface. Our results reveal that an increased carbon impurity resulting from a low processing temperature provides a charge conduction path and generates reactive oxygen species causing the degradation of dye molecule. A thin coated layer, that is, less than 3 nm, also induced the tunneling of electrons and holes to the surface, hence oxidizing dye molecules. Furthermore, the introduction of an Al2 O3 layer on TiO2 improves the light trapping thus, enhances the UV absorption.

Highly Repeatable and Recoverable Phototransistors Based on Multifunctional Channels of Photoactive CdS, Fast Charge Transporting ZnO, and Chemically Durable Al2O3 Layers.

Highly repeatable and recoverable phototransistors were explored using a "multifunctional channels" structure with multistacked chalcogenide and oxide semiconductors. These devices were made of (i) photoactive CdS (with a visible band gap), (ii) fast charge transporting ZnO (with a high field-effect mobility), and (iii) a protection layer of Al2O3 (with high chemical durability). The CdS TFT without the Al2O3 protection layer did not show a transfer curve due to the chemical damage that occurred on the ZnO layer during the chemical bath deposition (CBD) process used for CdS deposition. Alternatively, compared to CdS phototransistors with long recovery time and high hysteresis (ΔVth = 19.5 V), our "multi-functional channels" phototransistors showed an extremely low hysteresis loop (ΔVth = 0.5V) and superior photosensitivity with repeatable high photoresponsivity (52.9 A/W at 400 nm). These improvements are likely caused by the physical isolation of the sensing region and charge transport region by the insertion of the ultrathin Al2O3 layer. This approach successfully addresses some of the existing problems in CdS phototransistors, such as the high gate-interface trap site density and high absorption of molecular oxygen, which originate from the polycrystalline CdS.

Influence of NH3 annealing on the chemical states of HfO2/Al2O3 stacks studied by X-ray photoelectron spectroscopy

The effect of NH3 annealing on the chemical properties and thermal stability of ultrathin HfO2/Al2O3 film grown on silicon substrate by atomic layer deposition are investigated as a function of post deposition annealing temperature. X-ray photoelectron spectroscopy shows that nitrogen incorporation can be approached by a NH3 annealing treatment and its composition evidently increases after annealing at 700 °C. Nitrogen atoms are found to bond to hafnium, aluminum and silicon atoms with annealing temperature above 800 °C, respectively. In addition, ultrathin Al2O3 layer is demonstrated as a robust barrier layer for preventing silicon diffusion.

Improved Cycle Life and Stability of Lithium Metal Anodes through Ultrathin Atomic Layer Deposition Surface Treatments

Improving the cycle life and failure resistance of lithium metal anodes is critical for next-generation rechargeable batteries. Here, we show that treating Li metal foil electrodes with ultrathin (∼2 nm) Al2O3 layers using atomic layer deposition (ALD) without air exposure can prevent dendrite formation upon cycling at a current density of 1 mA/cm2. This has the effect of doubling the lifetime of the anode before failure both under galvanostatic deep discharge conditions and cyclic plating/stripping of symmetric Li–Li cells. The ALD treated electrodes can be cycled for 1259 cycles before failure occurs, which is attributed to improved electrode morphology resulting from homogeneous Li ion flux across the electrode/electrolyte interface.

Silver Nanorods Wrapped with Ultrathin Al2O3 Layers Exhibiting Excellent SERS Sensitivity and Outstanding SERS Stability.

Silver nanostructures have been considered as promising substrates for surface-enhanced Raman scattering (SERS) with extremely high sensitivity. The applications, however, are hindered by the facts that their morphology can be easily destroyed due to the low melting points (~100 °C) and their surfaces are readily oxidized/sulfured in air, thus losing the SERS activity. It was found that wrapping Ag nanorods with an ultrathin (~1.5 nm) but dense and amorphous Al2O3 layer by low-temperature atomic layer deposition (ALD) could make the nanorods robust in morphology up to 400 °C, and passivate completely their surfaces to stabilize the SERS activity in air, without decreasing much the SERS sensitivity. This simple strategy holds great potentials to generate highly robust and stable SERS substrates for real applications.

High temperature thermal stability studies of ultrathin Al2O3 layers deposited on native oxide and sulphur passivated InGaAs surfaces

High resolution synchrotron radiation core level photoemission measurements have been used to undertake a comparative study of the high temperature stability of ultrathin Al2O3 layers deposited by atomic layer deposition (ALD) on both sulphur passivated and native oxide covered InGaAs. The residual interfacial oxides between sulphur passivated InGaAs and the ultrathin Al2O3 layer can be substantially removed at high temperature (up to 700°C) without impacting on the InGaAs stoichiometry while significant loss of indium was recorded at this temperature on the native oxide InGaAs surface.

Non-magnetic organic/inorganic spin injector at room temperature

Spin injection into solid-state devices is commonly performed by use of ferromagnetic metal electrodes. Here, we present a spin injector design without permanent magnet; rather, the spin selectivity is determined by a chiral tunneling barrier. The chiral tunneling barrier is composed of an ultrathin Al2O3 layer that is deposited on top of a chiral self-assembled monolayer (SAM), which consists of cysteine or oligopeptide molecules. The experimentally observed magnetoresistance can be up to 20% at room temperature, and it displays an uncommon asymmetric curve as a function of the applied magnetic field. These findings show that the spin injector transmits only one spin orientation, independent of external magnetic field. The sign of the magnetoresistance depends on the handedness of the molecules in the SAM, which act as a spin filter, and the magnitude of the magnetoresistance depends only weakly on temperature.

Properties of the c-Si/Al2O3 interface of ultrathin atomic layer deposited Al2O3 layers capped by SiNx for c-Si surface passivation

This work presents a detailed study of c-Si/Al2O3 interfaces of ultrathin Al2O3 layers deposited with atomic layer deposition (ALD), and capped with SiNx layers deposited with plasma-enhanced chemical vapor deposition. A special focus was the characterization of the fixed charge density of these dielectric stacks and the interface defect density as a function of the Al2O3 layer thickness for different ALD Al2O3 deposition processes (plasma-assisted ALD and thermal ALD) and different thermal post-deposition treatments. Based on theoretical calculations with the extended Shockley–Read–Hall model for surface recombination, these interface properties were found to explain well the experimentally determined surface recombination. Thus, these interface properties provide fundamental insights into to the passivation mechanisms of these Al2O3/SiNx stacks, a stack system highly relevant, particularly for high efficiency silicon solar cells. Based on these findings, it was also possible to improve the surface passivation quality of stacks with thermal ALD Al2O3 by oxidizing the c-Si surface prior to the Al2O3 deposition.

Nitrogen doped p-type ZnO films and p-n homojunction

We demonstrate the ZnO homojunction fully obtained by the atomic layer deposition technique at low temperature growth of 100–130 °C. For the n-type partner of the junction we used undoped ZnO film obtained at 130 °C, while nitrogen doped ZnO acted as the p-type partner of the junction. Nitrogen was introduced into the ZnO film during the ALD process by using ammonia water as an oxygen precursor and diethylzinc as a zinc precursor. The p-type conductivity of ZnO was activated by the subsequent annealing of the ZnO:N film in an oxygen or nitrogen atmosphere. The initial rectification ratio of 102 at ±2 V was raised to 4 · 104 by inserting an ultrathin Al2O3 layer between p- and n-type ZnO. The resulting rectification ratio is among the best parameters reported for a ZnO homojunction so far.

Enhanced electrochemical stability of high-voltage LiNi0.5Mn1.5O4 cathode by surface modification using atomic layer deposition

We report improved cycling performance of high-voltage LiNi0.5Mn1.5O4 cathode for lithium-ion batteries by modifying its surface with ultrathin layer coating of Al2O3 (<1 nm) using atomic layer deposition (ALD) technique. Four and six layers of ALD Al2O3 were coated directly on the porous LiNi0.5Mn1.5O4 nanoparticle (~200 nm) slurry composite. Electrochemical characterization results show that LiNi0.5Mn1.5O4 electrode with four and six layers of ALD Al2O3 maintained 92 and 98 % of their initial capacity after 200 cycles, respectively, in comparison to bare LiNi0.5Mn1.5O4 electrode which showed 84 % capacity retention. We show that the ALD Al2O3 coated cathode surface is protected from undesired side reactions occurring at the electrode/electrolyte interface. For the first time, we used XPS studies to investigate the surface chemistry of ultrathin ALD Al2O3 coated LiNi0.5Mn1.5O4 and confirmed that the ultrathin ALD Al2O3 layer reduces side reactions involving organic components of the electrolyte decomposition product in high-voltage cathodes.

High temperature thermal stability investigations of ammonium sulphide passivated InGaAs and interface formation with Al2O3 studied by synchrotron radiation based photoemission

High resolution synchrotron radiation core level photoemission measurements have been used to undertake a comparative study of the high temperature thermal stability of the ammonium sulphide passivated InGaAs surface and the same surface following the atomic layer deposition (ALD) of an ultrathin (∼1nm) Al2O3 layer. The solution based ex situ sulphur passivation was found to be effective at removing a significant amount of the native oxides and protecting the surface against re-oxidation upon air exposure. The residual interfacial oxides which form between sulphur passivated InGaAs and the ultrathin Al2O3 layer can be substantially removed at high temperature (up to 700°C) without impacting on the InGaAs stoichiometry while significant loss of indium was recorded at this temperature on the uncovered sulphur passivated InGaAs surface.

Influence of the microstructure of n-type Si:H and passivation by ultrathin Al2O3 on the efficiency of Si radial junction nanowire array solar cells

In this work, p-type solar-grade Si (100) wafers with a thickness of 180μm were used to fabricate Al-doped ZnO(AZO)/n-type Si:H/i-Si:H/c-Si(p) nanowire (NW) array solar cells in which the Si:H and Al2O3 layers were deposited by plasma-enhanced chemical deposition and atomic layer deposition, respectively. To realize a good coverage of Si:H layers on the Si NWs, excessively thin Si:H layers were avoided. Moreover, since the NW array gives rise to significant light trapping, the light absorption by the Si:H layers could not be neglected. It is known that compared to amorphous Si:H, the charge mobility and light absorption are improved in the two-phase Si:H with nanocrystalline grains dispersed in the amorphous matrix. Therefore, it is important to investigate the influence of the microstructures of n-type Si:H on the performance of solar cells. On the other hand, in contrast to the Al2O3 passivation of planar Si solar cells, in this study, ultrathin Al2O3 layers were deposited on Si NWs without post-deposition annealing. We also discuss the passivation behavior of these ultrathin Al2O3 layers on n-type Si:H and the balance between the surface passivation and the role of these layers as tunnel barriers.

Role of ultrathin Al2O3 layer in organic/inorganic hybrid gate dielectrics for flexibility improvement of InGaZnO thin film transistors

We investigated flexible amorphous InGaZnO (a-IGZO) thin film transistors (TFTs) on a polyimide (PI) substrate by using organic/inorganic hybrid gate dielectrics of poly-4vinyl phenol (PVP) and ultrathin Al2O3. IGZO TFTs were fabricated with hybrid PVP/Al2O3 gate dielectrics having Al2O3 layers of different nanoscale thicknesses, which were deposited by atomic layer deposition (ALD). The electrical characteristics of the TFTs with the organic/inorganic hybrid gate dielectrics were measured after cyclic bending up to 1,00,000 cycles at the bending radius of 10mm. The ultrathin Al2O3 layer in the hybrid gate dielectrics improved the mechanical flexibility and protected the organic gate dielectric against damage during the sputter deposition of the IGZO layer. Finite elements method (FEM) simulations along with the structural characterization of the cyclically bent device showed the importance of optimizing the thickness of the Al2O3 layer in the hybrid gate dielectrics to obtain mechanically stable and flexible a-IGZO TFTs.

Ultrathin Surface Modification By Atomic Layer Deposition on High Voltage Cathode LiNi0.5Mn1.5O4 for Lithium Ion Batteries

Atomic layer deposition (ALD) has been employed to modify the surface of high voltage cathode LiNi0.5Mn1.5O4 by coating ultrathin Al2O3 layer on the electrodes. The ultrathin layer of Al2O3 can suppress the undesirable reactions during cycling, while maintaining the electron and ion conductivity of the electrode. The ALD Al2O3 coated LiNi0.5Mn1.5O4 showed remarkable improvement over bare LiNi0.5Mn1.5O4. After 200 cycles, the ALD Al2O3 coated LiNi0.5Mn1.5O4 showed 91% capacity retention when the bare LiNi0.5Mn1.5O4 can only maintain 75% in the same testing condition. In addition, the ALD coated LiNi0.5Mn1.5O4 maintained 63% capacity retention after as long as 900 cycles. At an elevated temperature of 55 ºC, the ALD Al2O3 coated LiNi0.5Mn1.5O4 still delivered 116 mAh/g at the 100th cycle, in comparison, the capacity for bare LiNi0.5Mn1.5O4 decreased to 98 mAh/g. According to the results from charge/discharge test and AC impedance test, the improvement is ascribed to the reduced overpotental and Li ion surface diffusion impedance. The promising results demonstrate the potential of developing high energy and long life lithium ion batteries with highly scalable LiNi0.5Mn1.5O4 preparation and broadly applicable ALD process.

The Use of Feature Parameters to Asses Barrier Properties of ALD coatings for Flexible PV Substrates

This paper reports on the recent work carried out as part of the EU funded NanoMend project. The project seeks to develop integrated process inspection, cleaning, repair and control systems for nano-scale thin films on large area substrates.In the present study flexible photovoltaic films have been the substrate of interest. Flexible PV films are the subject of significant development at present and the latest films have efficiencies at or beyond the level of Si based rigid PV modules. These flexible devices are fabricated on polymer film by the repeated deposition, and patterning, of thin layer materials using roll-to-roll processes, where the whole film is approximately 3um thick prior to encapsulation. Whilst flexible films offer significant advantages in terms of mass and the possibility of building integration (BIPV) they are at present susceptible to long term environmental degradation as a result of water vapor transmission through the barrier layers to the CIGS (Copper Indium Gallium Selenide CuInxGa(1-x)Se2) PV cells thus causing electrical shorts and efficiency drops. Environmental protection of the GIGS cell is provided by a thin (40nm) barrier coating of Al2O3. The highly conformal aluminium oxide barrier layer is produced by atomic layer deposition (ALD) where, the ultra-thin Al2O3 layer is deposited onto polymer thin films before these films encapsulate the PV cell. The surface of the starting polymer film must be of very high quality in order to avoid creating defects in the device layers. Since these defects reduce manufacturing yield, in order to prevent them, a further thin polymer coating (planarization layer) is generally applied to the polymer film prior to deposition.The presence of surface irregularities on the uncoated film can create defects within the nanometre-scale, aluminium oxide, barrier layer and these are measured and characterised. This paper begins by reporting the results of early stage measurements conducted to characterise the uncoated and coated polymer film surface topography using feature parameter analysis. The measurements are carried out using a Taylor Hobson Coherence Correlation Interferometer an optical microscope and SEM. Feature parameter analysis allows the efficient separation of small insignificant defects from large defects. The presence of both large and insignificant defects is then correlated with the water vapour transmission rate as measured on representative sets of films using at standard MOCON test. The paper finishes by drawing conclusions based on analysis of WVTR and defect size, where it is postulated that small numbers of large defects play a significant role in higher levels of WVTR.

Control of Al2O3/InAlN interface by two-step atomic layer deposition combined with high-temperature annealing

An attempt was made to control the Al2O3/InAlN interface by the phase change of the Al2O3 layer formed by atomic layer deposition (ALD). The electrical properties of an InAlN metal–oxide–semiconductor (MOS) diode with a sufficiently thick ALD-Al2O3 layer deteriorated following conventional postdeposition annealing (PDA) at 850 °C, which is sufficiently high for microcrystallization of the ALD-Al2O3 layer. However, X-ray photoelectron spectroscopy showed no evidence of an interface disorder in the ultrathin ALD-Al2O3/InAlN structure annealed at 850 °C. Two-step ALD interrupted by annealing at 850 °C right after the formation of the initial ultrathin Al2O3 layer improved the electrical properties of the MOS diode with reduced interface state density (Dit) and leakage current. A weak crystallization of the ultrathin Al2O3 layer was confirmed by transmission electron microscopy. Improvement of the interface disorder by high-temperature annealing is discussed as the origin of the Dit reduction.

Ultrathin Surface Modification by Atomic Layer Deposition on High Voltage Cathode LiNi0.5Mn1.5O4 for Lithium Ion Batteries

AbstractAtomic layer deposition (ALD) has been used to modify the surface of the high‐voltage cathode LiNi0.5Mn1.5O4 by coating ultrathin Al2O3 layers on the electrodes. The ultrathin layer can suppress the undesirable reactions during cycling while retaining the electron and ion conductivity of the electrode. The Al2O3‐coated LiNi0.5Mn1.5O4 showed remarkable improvement over bare LiNi0.5Mn1.5O4. After 200 cycles, the Al2O3‐coated cathode showed 91 % capacity retention whereas the bare LiNi0.5Mn1.5O4 can only maintain 75 % under the same testing conditions. In addition, the Al2O3‐coated LiNi0.5Mn1.5O4 retained 63 % of its capacity 900 cycles. At an elevated temperature of 55 °C, the Al2O3‐coated LiNi0.5Mn1.5O4 still delivered 116 mAh g−1 at the 100th cycle; in comparison, the capacity for bare LiNi0.5Mn1.5O4 decreased to 98 mAh g−1. According to the results from charge/discharge and AC impedance experiments, the improvement is ascribed to the reduced overpotential and Li ion surface diffusion impedance. The promising results demonstrate the potential of developing high‐energy lithium ion batteries with a long cycle life by using a highly scalable preparation method for LiNi0.5Mn1.5O4 and the broadly applicable ALD process.

Influence of RF-sputtering power on formation of vertically stacked Si1−xGex nanocrystals between ultra-thin amorphous Al2O3 layers: structural and photoluminescence properties

In this work, we investigate the structural and photoluminescence (PL) properties of (SiGe+Al2O3)/Al2O3 multi-layer films with layer thicknesses in the range of a few nanometres. The films were prepared by magnetron sputtering deposition at room temperature followed by an annealing process to promote the formation of small SiGe nanocrystals (NCs) (∼3 to 5 nm) embedded between ultra-thin (∼6 nm thickness) Al2O3 layers. Our results show that the structural and compositional properties of the films can be tuned by changing the RF-power. It is found that nearly spherical and well confined isolated SiGe NCs (∼5 nm) are obtained for an RF-power value of 80 W. The PL properties of the films were studied and optical emission in the blue visible wavelength region was observed.