Thickness Of Al2O3 Film Research Articles

Investigation of the atomic layer etching mechanism for Al2O3 using hexafluoroacetylacetone and H2 plasma.

Atomic layer etching (ALE) is required to fabricate the complex 3D structures for future integrated circuit scaling. To enable ALE for a wide range of materials, it is important to discover new processes and subsequently understand the underlying mechanisms. This work focuses on an isotropic plasma ALE process based on hexafluoroacetylacetone (Hhfac) doses followed by H2 plasma exposure. The ALE process enables accurate control of Al2O3 film thickness with an etch rate of 0.16 ± 0.02 nm per cycle, and an ALE synergy of 98%. The ALE mechanism is investigated using Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) simulations. Different diketone surface bonding configurations are identified on the Al2O3 surface, suggesting that there is competition between etching and surface inhibition reactions. During the Hhfac dosing, the surface is etched before becoming saturated with monodentate and other hfac species, which forms an etch inhibition layer. H2 plasma is subsequently employed to remove the hfac species, cleaning the surface for the next half-cycle. On planar samples no residue of the Hhfac etchant is observed by FTIR after H2 plasma exposure. DFT analysis indicates that the chelate configuration of the diketone molecule is the most favorable surface species, which is expected to leave the surface as volatile etch product. However, formation of the other configurations is also energetically favorable, which explains the buildup on an etch inhibiting layer. The ALE process is thus hypothesized to work via an etch inhibition and surface cleaning mechanism. It is discussed that such a mechanism enables thickness control on the sub-nm scale, with minimal contamination and low damage.

Direct Characterization of Buried Interfaces in 2D/3D Heterostructures Enabled by GeO2 Release Layer.

Inconsistent interface control in devices based on two-dimensional materials (2DMs) has limited technological maturation. Astounding variability of 2D/three-dimensional (2D/3D) interface properties has been reported, which has been exacerbated by the lack of direct investigations of buried interfaces commonly found in devices. Herein, we demonstrate a new process that enables the assembly and isolation of device-relevant heterostructures for buried interface characterization. This is achieved by implementing a water-soluble substrate (GeO2), which enables deposition of many materials onto the 2DM and subsequent heterostructure release by dissolving the GeO2 substrate. Here, we utilize this novel approach to compare how the chemistry, doping, and strain in monolayer MoS2 heterostructures fabricated by direct deposition vary from those fabricated by transfer techniques to show how interface properties differ with the heterostructure fabrication method. Direct deposition of thick Ni and Ti films is found to react with the monolayer MoS2. These interface reactions convert 50% of MoS2 into intermetallic species, which greatly exceeds the 10% conversion reported previously and 0% observed in transfer-fabricated heterostructures. We also measure notable differences in MoS2 carrier concentration depending on the heterostructure fabrication method. Direct deposition of thick Au, Ni, and Al2O3 films onto MoS2 increases the hole concentration by >1012 cm-2 compared to heterostructures fabricated by transferring MoS2 onto these materials. Thus, we demonstrate a universal method to fabricate 2D/3D heterostructures and expose buried interfaces for direct characterization.

Exploring Al2O3 blister evolution through cathodoluminescence and attenuated total reflectance infrared analyses

This work focuses on the morphological and optical evolution of Al2O3 thick films grown by atomic layer deposition on Si-SiO2 substrates. Blister formation has been the subject of extensive research in the literature; our work fills a crucial gap in the optical characterization of areas inside and outside blisters. Morphological studies were carried out by scanning electron microscopy; we found a reciprocal relationship between the density of the blisters and their diameter. The thickness and refractive index were studied by ellipsometry, revealing a systematic increase in the refractive index with increasing annealing temperature. In addition, we observed the hydrophobic behavior in all films using the water contact angle technique, which suggests that even with blisters, this material can be used in waterproof coatings. Using Auger spectroscopy, we confirmed that delamination occurs completely once the blisters are broken. In this work, we perform cathodoluminescence measurements outside and inside the ampoules. In the area outside the blisters, we observe emissions attributed to the F centers, and the change from the main peaks of 2.8 and 3.4 eV for the as-deposited film to the dominance of emissions centered at 3.4 and 3.7 eV is clearly observed. Furthermore, we observed a strong increase in the cathodoluminescence signal at higher annealing temperatures. On the other hand, we also observed the evolution of the blisters through the cathodoluminescence spectra; in that area, we observed the radical change in the spectrum once the blister is broken, giving rise to the SiO2 signals. We also observed this rupture through a new absorption band in the attenuated total reflectance IR spectra.

The Effect of Pore Sealing in a Multilayer Si–O–Zr/Al2O3 Coating Designed to Protect Aluminium from Corrosion

This study deals with the combination of two corrosion protection strategies for aluminium: barrier protection (provided by a 3.8 μm thick hybrid sol–gel coating) and aluminium pore sealing via the use of a 100 nm thick layer of aluminium oxide. A Si–O–Zr hybrid sol–gel coating (TMZ) was synthesised by combining two separately prepared sols (i) tetraethyl orthosilicate and 3-methacryloxypropyl trimethoxysilane and (ii) zirconium(IV) n-propoxide chelated with methacrylic acid. The synthesis of the Si–O–Zr hybrid sol–gel was evaluated at various stages using real-time infrared spectroscopy. A 100 nm thick Al2O3 film was prepared via thermal atomic layer deposition at 160 °C using trimethyl aluminium and water as precursors. The coating and film properties were assessed via focused ion beam/scanning electron microscopy coupled with energy-dispersive X-ray spectrometry. Sealing with the Al2O3 film did not affect the microstructure and composition of the underlying sol–gel coating. The coating’s corrosion performance in 0.1 M NaCl solution was evaluated using electrochemical impedance spectroscopy. Compared to individual coatings, the multilayer TMZ/Al2O3 coating ensured prolonged (more than three weeks) durable corrosion protection for the aluminium. The impedance magnitude increased by two orders compared to the uncoated substrate (|Z|10 mHz from 16 kΩ cm2 to almost 830 MΩ cm2). Thus, the pore sealing of the sol–gel coating using an ALD alumina film produced a protective multilayer coating system, with |Z|10 mHz remaining above 5 MΩ cm2 after four weeks in NaCl solution.

Lasing in Zn-doped GaAs nanowires on an iron film

In this work, we demonstrate optically pumped lasing in highly Zn-doped GaAs nanowires (NWs) lying on an iron film. The conically shaped NWs are first covered with an 8 nm thick Al2O3 film to prevent atmospheric oxidation and mitigate band-bending effects. Multimode and single-mode lasing have been observed for NWs with a length greater or smaller than 2 μm, respectively. Finite difference time domain calculations reveal a weak electric field enhancement in the Al2O3 layer at the NW/iron film interface for the lasing modes. The high Zn acceptor concentration in the NWs provides enhanced radiative efficiency and enables lasing on the iron film despite plasmonic losses. Our results open avenues for integrating NW lasers on ferromagnetic substrates to achieve new functionalities, such as magnetic field-induced modulation.

An Al2O3/Ni–Al tritium permeation barrier coating for the potential application in thorium-based molten salt reactor

The Al2O3/Ni–Al composite coating was formed on the outside surface of GH3535 superalloy by pack aluminizing and subsequent in-situ oxidation, and deuterium permeation tests were carried out to assess the tritium permeation barrier performance of the coating. Scanning Electron Microscopy (SEM) showed that the surface of Al2O3 films was dense, homogeneous, and crack-free. Transmission Electron Microscopy (TEM) photos manifested that the interface between γ-Al2O3 film and Ni–Al interlayer was distinct without micropores, and the thickness of Al2O3 film with the structure of γ-phase is approximately 80 nm. The Arrhenius equation for permeability of GH3535 was obtained as Φ=1.37·10−7·exp(−57.92(KJ mol−1)/RT) by deuterium permeation tests. The deuterium permeation reduction factor of Al2O3/Ni–Al coating kept 2–3 orders of magnitudes at a temperature from 400°C to 700°C. The D-permeation resistant performance of this coating still has the potential to be further improved.

Area-selective atomic layer deposition of Al2O3 using inkjet-printed inhibition patterns and lift-off process

Area-selective atomic layer deposition (AS-ALD) has been studied as an alternative method for metal oxide ALD film patterning in the microelectronics industry. To perform AS-ALD, area-deactivation or -activation processes should be implemented in advance using selective surface modification through a photography-based lift-off process and printing techniques. This study introduces a novel approach for Al2O3 AS-ALD using inkjet-printed inhibition patterns. ALD-inhibition patterns with two-layer structures of fluorocarbon (FC) thin film and photoresist (PR) were patterned by inkjet printing. Low surface energy FC thin films were used to block the Al2O3 nucleation and growth during the ALD process, and PR was patterned to easily remove ALD-inhibition patterns using a lift-off process. To demonstrate AS-ALD, an Al2O3 thin film approximately 10 nm thick was deposited via an in-house built system with a multiple-slit gas source. The proposed AS-ALD method was evaluated for topological analysis using atomic force microscopy and surface composition analysis using a time of flight secondary ion mass spectrometry after Al2O3 deposition and a lift-off process. Finally, a 6 nm thick Al2O3 film was selectively patterned by the lift-off process using an inkjet-printed 1.28 μm thick FC-covered PR inhibition pattern.

Unique reflection from birefringent uncoated and gold-coated InP nanowire crystal arrays.

We demonstrate unique reflective properties of light from bare and gold-coated InP nanowire (NW) photonic crystal arrays. The undoped wurtzite InP nanowire arrays are grown by selective area epitaxy and coated with a 12-nm thick Al2O3 film to suppress atmospheric oxidation. A nominally 10-nm thick gold film is deposited around the NWs to investigate plasmonic effects. The reflectance spectra show pronounced Fabry-Perot oscillations, which are shifted for p- and s-polarized light due to a strong intrinsic birefringence in the NW arrays. Gold-coating of the NW array leads to a significant increase of the reflectance by a factor of two to three compared to the uncoated array, which is partially attributed to a plasmon resonance of the gold caps on top of the NWs and to a plasmonic antenna effect for p-polarized light. These interpretations are supported by finite-difference-time-domain simulations. Our experiments and simulations indicate that NW arrays can be used to design micrometer-sized polarizers, analyzers, and mirrors which are important optical elements in optoelectronic integrated circuits.

Deposit and etchback approach for ultrathin Al2O3 films with low pinhole density using atomic layer deposition and atomic layer etching

­Ultrathin Al2O3 atomic layer deposition (ALD) films with low pinhole density were fabricated using a deposit and etchback approach. This strategy was able to avoid the pinholes that originated during nonuniform nucleation of Al2O3 ALD films. In this method, an Al2O3 ALD film was deposited to a thickness greater than the desired thickness to reduce the number of pinholes and form a more continuous Al2O3 ALD film. Subsequently, the Al2O3 ALD film was etched back to a smaller thickness using thermal Al2O3 atomic layer etching (ALE). The number of pinholes in the resulting Al2O3 ALD film was measured by the percentage yield of metal-insulator-metal (MIM) capacitors based on an Ag/Al2O3/Al structure that did not have an electrical short. The device yield was improved using the deposition and etchback approach. For example, using device areas of 0.01 mm2, Al2O3 ALD films that were grown to 5 nm in the MIM capacitor gave a yield of 30%–40%. In contrast, Al2O3 ALD films that were grown to 24 nm and then etched back to 5 nm to form the MIM capacitor provided a yield of 65%–75%. This increase in yield of approximately 100% indicates that the deposit and etchback approach can improve the yield of MIM devices based on ultrathin Al2O3 ALD films. Although this method has been previously suggested to improve the quality of ultrathin films, this report is believed to be the first demonstrated application of using the deposit and etchback approach for device fabrication. Additional experiments revealed that a portion of the yield improvement can be attributed to the fluorination of the Al2O3 ALD films that produced a volume expansion when forming AlF3. This expansion may produce a compressive stress that helps to close the pinholes. The dielectric constant was also measured for Al2O3 ALD films versus Al2O3 film thickness. The dielectric constant was the same for as-deposited Al2O3 ALD films and Al2O3 ALD films that were first grown to 24 nm and then etched back to smaller thicknesses. This agreement indicates that the dielectric constant can be understood in terms of a series capacitor model and that Al2O3 ALE does not affect the electrical properties of the Al2O3 films.

Smoothing surface roughness using Al2O3 atomic layer deposition

Al2O3 atomic layer deposition (ALD) was used to smooth the roughness on silicon wafers obtained prior to chemical mechanical polishing (CMP). The initial silicon wafers had an average RMS surface roughness of 3.3 nm as determined by atomic force microscopy (AFM) measurements. AFM line scans also measured an average lateral spacing of ≈490 nm between the surface asperities. The RMS roughness decreased and the average lateral spacing increased progressively with number of Al2O3 ALD cycles. After 3000 Al2O3 ALD cycles that deposit an Al2O3 film thickness of 370 nm, the RMS roughness reduced to 1.5 nm and the average lateral spacing between the surface asperities increased to ≈890 nm. Additional Al2O3 ALD cycles produced little change in the RMS roughness or average lateral spacing. The efficiency of the smoothing decreased when the lateral distance between the surface asperities was much larger than the Al2O3 ALD film thickness. Power spectral density (PSD) analysis revealed that the ALD smoothing was most effective for surface topographical features with lateral spacings in the range of 10s to 100s of nanometers. Reflectivity studies of silver films deposited on the silicon wafers also demonstrated that Al2O3 ALD smoothing improved the optical performance of reflective mirrors.

PHOTOELECTRON SPECTROSCOPY STUDIES ON Al2O3 FILMS ON p-GaN(0001)

In order to determine its electronic and chemical properties, the Al2O3/p-GaN(0001) interface is studied in situ by the X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS). Using physical vapor deposition (PVD) method, the Al2O3 film is deposited step by step under ultra-high vacuum (UHV) onto p-GaN(0001) surface covered with residual native Ga oxide. Prior to the first Al2O3 layer evaporation, binding energy of the Ga 3[Formula: see text] substrate line is equal to 20.5[Formula: see text]eV. The PVD method of deposition leads to an amorphous Al2O3 film formation. For the final 12.0[Formula: see text]nm thick Al2O3 film binding energy of the Al 2[Formula: see text] line is set at 76.0[Formula: see text]eV and for the O 1[Formula: see text] line at 532.9[Formula: see text]eV. The valence band offset (VBO) and the conduction band offset (CBO) of the Al2O3/p-GaN(0001) interface are determined to be equal to [Formula: see text]1.6[Formula: see text]eV and 1.8[Formula: see text]eV, respectively.

Lasing from InP Nanowire Photonic Crystals on InP Substrate

Abstract2D photonic crystal (PhC) lasing from an InP nanowire array still attached to the InP substrate is demonstrated for the first time. The undoped wurtzite InP nanowire array is grown by selective area epitaxy and coated with a 10 nm thick Al2O3 film to suppress atmospheric oxidation and band‐bending effects. The PhC array displays optically pumped lasing at room temperature at a pulsed threshold fluence of 14 µJ cm−2. At liquid nitrogen temperature, the array shows lasing under continuous wave excitation at a threshold intensity of 500 W cm−2. The output power of the single mode laser line reaches values of 470 µW. Rate equation calculations indicate a quality factor of Q ≈ 1000. Investigations near threshold reveal that lasing starts from isolated islands within the pumped region before coherently merging into a single homogeneous area with increasing excitation power. This field emits a lasing mode with an average off‐normal angle of ≈6°. Single mode lasing with the nanoarray still attached to the InP substrate opens new design opportunities for electrically pumped PhC laser light sources.

(Invited) In-Situ and Combinatorial Techniques for Spatial ALD

Atmospheric-pressure spatial atomic layer deposition (AP-SALD) and chemical vapor deposition (AP-CVD) have been developed in recent years as scalable techniques for the rapid deposition of oxide thin films on different substrates for a variety of applications. The atmospheric nature of these techniques facilitates the integration of characterization tools and the modification of the experimental setup to produce novel materials. Here we report in-situ electrical and optical characterization methods that have been developed for our AP-SALD/CVD system, as well as new techniques to deposit oxide films with nanoscale thickness gradients. The in-situ electrical measurements are enabled by a custom-designed, flexible printed circuit board substrate and the optical measurements are performed via reflectometry techniques. The thickness, resistance, and optical constants of prototypical AP-SALD films (ZnO and Al2O3) were monitored during depositions, providing insight into film nucleation and growth. The nanoscale thickness gradient films facilitate combinatorial high-throughput screening of devices, where a multitude of devices with varying film thicknesses can be fabricated on a single substrate. This combinatorial approach was applied to study the role of Al2O3 film thickness as an insulating layer in quantum-tunneling metal-insulator-metal diodes and as an encapsulation layer in metal halide perovskite solar cells.

The aluminum current collector with honeycomb-like surface and thick Al2O3 film increased durability and enhanced safety for lithium-ion batteries

Durability and safety are main factors contributing to the market requirement of lithium-ion batteries (LIBs) in practical applications. The improvement of current collector has been proven as an effective approach to enhance comprehensive performance of LIBs. To achieve a sufficient electrical contact between the current collector and active materials, honeycomb-like surface of aluminum current collector is etched by direct current in sulfuric acid and phosphoric acid mixed solution. At the same time, a dense anodic aluminum oxide film is formed on the surface of aluminum current collector, which can protect LIBs from corrosion and thermal runaway. Experimental results show that the adhesion between the active material and aluminum current collector was improved by 23% after anodization. The corrosion resistance of alumina foil was promoted significantly in ethyl methyl carbonate and ethylene carbonate electrolyte with LiPF6. The corrosion current peak reduces to 0.022 mA/cm2 from 0.267 mA/cm2 after surface treatment. The capacity retention rate of the LiCoO2 electrode with an oxidation-treated aluminum current collector is 6.3% higher than with untreated aluminum foil at 5C after 500 cycles. What’s more, the nail penetration demonstrates that the aluminum current collector we prepared can reduce the temperature of the full batteries surface when the nail pierced, which improved the safety performance of LIBs.

Microstructure and mechanical properties of (B4C+Al2O3)/Al composites designed for neutron absorbing materials with both structural and functional usages

To meet the demand for the new generation of neutron absorber materials (NAMs) for the dry storage of the spent nuclear fuels, (B4C + Al2O3)/Al composites were fabricated by powder metallurgy technique using ultrafine Al powders. The composites designed with various fabricating parameters and fabricated at various sintering temperatures were characterized by electron microscopy and mechanically tested. The sample sintered at 450 °C shows the best strength-ductility balance at 350 °C (106.2 MPa in ultimate tensile strength and 9.6% in elongation). Addition of B4C particles and increase of the Al2O3 film thickness could enhance the strength of the composites at room temperature but showed no obvious effect on the strength at 350 °C. When sintering temperature of the composites increased from 450 °C to 550 °C, the transformation of amorphous Al2O3 lamellae to γ-Al2O3 particles led to deterioration of the strength of the composites. Based on the analyses of both high-temperature deformation mechanism and strengthening mechanism, it was considered that the amorphous Al2O3 could pin the grain boundaries and prevent them from gliding, which was the main factor to significantly increase the high-temperature strength. Based on the results, a strategy to design the aluminium matrix NAMs with excellent high-temperature strength was proposed.

Passivation effect for current collectors enables high-voltage aqueous sodium ion batteries

The narrow electrochemical stability window (1.23 V) of electrolyte has hindered the development of aqueous sodium ion batteries greatly, especially the choice of electrode materials. Here, through theoretical calculation and experimental proof, it's found that the deposition of oxide films on both Al cathode and Ti anode current collectors exhibits strong passivation effect. When the thickness of Al2O3 film on Al is about 3 nm and the thickness of TiO2 film on Ti is about 5 nm, the electrochemical stability window of aqueous electrolyte can be expanded to 3.5 V. Therefore, TiS2 with low working potential (1.5 V versus Na+/Na) can be used as anode material in aqueous sodium-ion battery for the first time. We assembled a full cell coupling TiS2 anode with Prussian blue cathode in 15 M NaClO4 aqueous electrolyte. The full cell of 2.6 V demonstrates cycle life up to 1000 times with high energy density (100 Wh kg−1) and high rate capability (30 C).

Plasma-induced damage and annealing repairing in ALD-Al2O3/PECVD-SiNx stacks

We study the effect of plasma-enhanced chemical vapor deposition (PECVD) SiNx process to atomic layer deposited (ALD) Al2O3 films on crystalline silicon surface passivation. The plasma-induced damage on Al2O3 films is affected by the methods of PECVD process (direct or microwave), and the states (as-deposited or annealed) and the thickness of ALD-Al2O3 films. The passivation degradation may be related to the doping of Si, N and H atoms into Al2O3 films during PECVD process, and is mainly manifested in the form of affecting interface defect density rather than negative fixed charge density. In particular, the annealed thick Al2O3 films with direct-PECVD SiNx process show more severe passivation degradation. Anyhow, to the great degree, the passivation quality can be repaired and further improved by post-annealing. Low temperature long time annealing can effectively increase the lifetime of the ALD-Al2O3/PECVD-SiNx stacks, while it cannot be simply replaced by a rapid high temperature firing. By post-annealing repairing, an absolute 0.13% increase in efficiency can be achieved for p-PERC solar cells.

Annealing Impact on Interface Properties of Sprayed Al2O3-Based MIS Structure for Silicon Surface Passivation

Aluminum oxide (Al2O3) films of different thicknesses were deposited on quartz and silicon (100) substrates by an ultrasonic spray method from a solution of aluminum acetylacetonate dissolved in N,N-dimethylformamide with different molar concentrations. The optical, morphological and electrical properties were investigated. Increasing the molar concentration leads to a refractive index decrease, an increase in the optical band gap from 5.26 eV to 5.52 eV and a change in the surface roughness of the films. The electrical parameters at the Al2O3/Si interface such as the flat band voltage (VFB), effective charge density (Qeff) and interface trap density (Dit) were explored as a function of the molar concentration, film thickness and heat treatment. The latter, done by two annealing processes, namely, the post deposition annealing (PDA) and post metallization annealing (PMA) on the structure, lead to remarkable interface properties. It was found that the positive flat band voltage VFB shift is correlated with the generation of negative effective charge during PMA. A decrease of the Dit distribution in the PMA samples with no significant effect in the case of PDA samples was clearly observed for different molar concentrations. Furthermore, as the Al2O3 film thickness decreases, Dit decreases in both PDA and PMA samples while the relatively high density Qeff and its negative charge polarity were obtained for thinner films. A noticeable passivation effect on the Al2O3/Si interface has been confirmed on samples that underwent the annealing process. These findings related specifically to the interface properties are promising for silicon surface passivation, in particular for solar cells applications.

Fracture behavior of metal oxide/silver nanowire composite electrodes under cyclic bending

The mechanical responses under cyclic bending of Ag nanowire composite electrodes coated with various metal oxide films of different thicknesses were explored, in an attempt to evaluate their applicability to flexible transparent electrodes. Al2O3, HfO2, and TiO2 films were deposited onto Ag nanowire electrodes by atomic layer deposition at a low temperature of 100 °C, and cyclic bending tests with in situ resistance measurements were conducted for up to 300,000 cycles. Thicker metal oxide films resulted in greater increases in the resistances of the composite electrodes under cyclic bending due to the reduced fracture strength of the films. Regardless of the type of metal oxide, however, similar tendencies were observed in the resistance changes in response to cyclic bending. It was found that the critical thickness calculation based on the Griffith theory for the evaluation of the brittle-to-ductile transition was not applicable to the metal oxide/Ag nanowire composite electrodes. By correlating the mechanical, ambient, and thermal reliability test results with those of optoelectrical property evaluations, the optimal thickness of Al2O3 film, as a representative metal oxide film, was determined to be 3–5 nm for Ag nanowire composite flexible transparent electrodes.

Effect of Al2O3 film on silver paste contact formation at silicon surface

We have studied the role of aluminum oxide (Al2O3) film on silver (Ag) paste contact formation at boron doped p-type silicon surface. By increasing the thickness of Al2O3 film in Al2O3/SiNx stack, the contact resistance between Ag paste and Si surface was observed to decrease to a minimum at Al2O3 film in thickness of 4 nm then increase again as the thickness increasing. Microstructure analysis via scanning electron microscopy (SEM) imaging demonstrated the number of Ag particles embedded in interfacial frits which is found to be responsible for the behavior of contact resistance. X-ray photoemission spectroscopy (XPS) data revealed out that Al2O3 incorporated into glassy phase during co-firing process. We suggested Al2O3 films affected silver paste contact formation by modifying chemical composition and properties of interfacial glassy phase.