Al2O3 Single Layers Research Articles

Comparative investigation of structure, mechanical properties, and oxidation resistance of Mo-Si-B and Mo-Al-Si-B coatings

Mo-Si-B and Mo-Al-Si-B coatings were deposited by DC magnetron sputtering of MoSiB and MoAlSiB composite targets fabricated by the self-propagating high-temperature synthesis method. The structure, element and phase composition of coatings were studied by means of scanning and transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive spectroscopy, and glow discharge optical emission spectroscopy. To evaluate their oxidation resistance, the coatings were annealed in air in the temperature range of 1200–1700°C during different time slots between 10min and 5h. The obtained results demonstrated that the Mo-Si-B coatings possess higher hardness, improved oxidation resistance and better thermal stability compared with their Mo-Al-Si-B counterparts. The 7-μm thick Mo-Si-B coatings were shown to successfully withstand oxidation during short-time exposure for 10min at a temperature as high as 1700°C due to the formation of protective silica scale. The oxidation of Mo-Al-Si-B coatings was accompanied by the diffusion of aluminum to the coating surfaces and the formation of a single Al2O3 layer at 1200–1300°C and a double Al2O3-SiO2 layer at 1500°C which were less protective against oxidation. The surface oxidation processes were also accompanied by phase transformations inside the oxygen-free part of both Mo-Si-B and Mo-Al-Si-B coatings with the formation of MoB and Mo5Si3 phases.

Impact of AlN layer sandwiched between the GaN and the Al2O3 layers on the performance and reliability of recessed AlGaN/GaN MOS-HEMTs

This work compares the performance and the reliability of recessed-gate AlGaN/GaN MOS-HEMTs with two different gate dielectrics: a single layer of Al2O3 and a bilayer of AlN/Al2O3. Although the normally-OFF operation is well observed in the AlN/Al2O3 device, the other characteristics are remarkable poor and directly prejudice the performance of the device. PBTI phenomenon was studied by using a pulse characterization technique in order to capture the fast charge trapping when a single stress pulse is applied to the gate contact. The physical origin of PBTI is ascribed to electron trapping and de-trapping in defect sites located in the gate dielectrics. The results during the stress phase show that the positive threshold voltage shift (ΔVth) in both devices follows a saturating log-time dependence model. From this analysis, the double stack exhibits a higher saturation threshold voltage and a faster trap time constant compared with the single layer device. The universal relaxation function fits well the experimental data during the relaxation phase at different stress pulse widths and clearly shows a faster defect discharge in the Al2O3 device.

Optical simulation, corrosion behavior and long term thermal stability of TiC-based spectrally selective solar absorbers

This work is a continuation of earlier work where TiC/Al2O3 spectrally selective solar absorbers were deposited onto stainless steel (SS). In this study, the optical simulation, corrosion behavior and long term thermal stability are investigated in detail. A series of TiC and Al2O3 single layers are fabricated by magnetron sputtering using the same deposition parameters as described in the previous work. The optical constants and thicknesses of glass, stainless steel, TiC and Al2O3 layers are obtained by CODE software using the suitable dielectric function model. Then, the structure model of SS/TiC/Al2O3 is constructed and the reflectance spectra of the SS/TiC/Al2O3 solar absorber is successfully simulated using CODE software. The anti-corrosion performance of SS/TiC/Al2O3 solar absorbers is evaluated through electrochemical potentiodynamic polarization measurement and salt spray test. The SS/TiC/Al2O3 solar absorbers have a good thermal stability in vacuum at 500°C for 100h. Raman analysis indicates that the increased sp2 bonds of the coating is the main reason for the degradation of the optical properties.

A New Quality Metric for III–V/High-k MOS Gate Stacks Based on the Frequency Dispersion of Accumulation Capacitance and the CET

This letter proposes a metric to assess the quality of high-k dielectrics on III–V substrates and a benchmarking methodology for the gate stack qualification in the region of MOS device operation above threshold voltage, ${V}_{t}$ . The metric is based on a capacitive equivalent thickness (CET) - normalized frequency dispersion ( $\text{D}_{\mathrm {eff}}$ ) evaluated in the accumulation region of capacitance–voltage (C–V) measurements of III–V MOS devices. ${D} _{\mathrm {eff}}$ is found to be CET independent, which allows for a preliminary assessment of the dielectric quality by using relatively thick layers. Several gate stacks, single layer or bi-layer, including those with Al2O3, and the recently reported ASM-imec interfacial layer (IL) with HfO2 are evaluated and compared against Si MOS devices. Using the proposed technique, a clear difference between the various deposition processes is observed. The results indicate that the quality of a single layer Al2O3 or a bi-layer stack of Al2O3/HfO2 on InGaAs is significantly lower compared with Si gate stacks while the ASM-imec IL yields a gate-stack with good performance on the proposed quality metric. In addition, these results correlate well with the reliability performance of the studied gate stacks.

A composite layer of atomic-layer-deposited Al2O3 and graphene for flexible moisture barrier

In this study, a composite layer of atomic-layer-deposition-grown Al2O3 (ALD Al2O3) on chemical-vapor-deposition-grown graphene (CVD graphene) was fabricated for encapsulation of an organic light-emitting diode (OLED). NO2 functionalization was adopted to improve the nucleation and film quality of the ALD Al2O3 on CVD graphene, and the resulting Al2O3 on NO2-functionalized graphene exhibited increased film density and decreased porosity and roughness compared to that without functionalization; these improved properties satisfy the requirements for encapsulation layers. In addition, the water vapor barrier property of the Al2O3 and graphene composite layer was improved compared with that of a single Al2O3 layer; a single graphene layer did not exhibit the barrier property because of inherent defects. According to the laminate theory, the calculated water vapor transmission rate of graphene in the composite layer was about 2 × 10−3 g/m2·day, which is much greater than that of the single graphene layer without Al2O3 because of the combinational effects of the Al2O3 layer. Furthermore, the permeability of graphene was almost unchanged before and after the application of bending stress because of its mechanical durability and flexibility. This ALD Al2O3/graphene composite is expected to be a promising encapsulation layer for future flexible OLED devices.

A mechanically enhanced hybrid nano-stratified barrier with a defect suppression mechanism for highly reliable flexible OLEDs.

Understanding the mechanical behaviors of encapsulation barriers under bending stress is important when fabricating flexible organic light-emitting diodes (FOLEDs). The enhanced mechanical characteristics of a nano-stratified barrier were analyzed based on a defect suppression mechanism, and then experimentally demonstrated. Following the Griffith model, naturally-occurring cracks, which were caused by Zn etching at the interface of the nano-stratified structure, can curb the propagation of defects. Cross-section images after bending tests provided remarkable evidence to support the existence of a defect suppression mechanism. Many visible cracks were found in a single Al2O3 layer, but not in the nano-stratified structure, due to the mechanism. The nano-stratified structure also enhanced the barrier's physical properties by changing the crystalline phase of ZnO. In addition, experimental results demonstrated the effect of the mechanism in various ways. The nano-stratified barrier maintained a low water vapor transmission rate after 1000 iterations of a 1 cm bending radius test. Using this mechanically enhanced hybrid nano-stratified barrier, FOLEDs were successfully encapsulated without losing mechanical or electrical performance. Finally, comparative lifetime measurements were conducted to determine reliability. After 2000 hours of constant current driving and 1000 iterations with a 1 cm bending radius, the FOLEDs retained 52.37% of their initial luminance, which is comparable to glass-lid encapsulation, with 55.96% retention. Herein, we report a mechanically enhanced encapsulation technology for FOLEDs using a nano-stratified structure with a defect suppression mechanism.

Characteristics of Al2O3/ZrO2 laminated films deposited by ozone-based atomic layer deposition for organic device encapsulation

We investigated the characteristics of 100nm-thick Al2O3/ZrO2 laminated films grown by ozone (O3)-based atomic layer deposition (ALD) at low temperature (100°C) as thin film encapsulation (TFE). Calcium (Ca) test was performed at a relative humidity (RH) of 50% with a temperature of 50°C to derive the water vapor transmission rates (WVTRs) of aluminum oxide (Al2O3)-, zirconium oxide (ZrO2)-, and Al2O3/ZrO2-laminated films. Al2O3/ZrO2-laminated films exhibited better permeation barrier properties than Al2O3 or ZrO2 single layer films. In Al2O3/ZrO2 laminated films, permeation barrier properties improved as the number of alternating layers increased. Permeation barrier properties were closely related to the number of interfaces because of the ZrAlxOy phase that formed at the interface between the Al2O3 and ZrO2 sublayers. The ZrAlxOy phase was denser than single layers of Al2O3 and ZrO2 and had a homogeneous amorphous structure. The formation of a ZrAlxOy phase in Al2O3/ZrO2 laminated film effectively improved the permeation barrier properties of the film.

“Zero-charge” SiO2/Al2O3 stacks for the simultaneous passivation of n+ and p+ doped silicon surfaces by atomic layer deposition

To achieve high conversion efficiencies, advanced silicon solar cell architectures such as interdigitated back contact solar cells demand that defects at both the n+ and p+ doped Si surfaces are passivated simultaneously by a single passivation scheme. In this work, corona charging experiments show that the fixed charge density Qf is a key parameter governing the passivation of both surface types. Alternatively, Qf can be controlled from strongly negative to even positive values by carefully tuning the SiO2 interlayer thickness in SiO2/Al2O3 stacks prepared by atomic layer deposition (ALD). This control in Qf allows for a superior passivation of n+ Si surfaces by SiO2/Al2O3 stacks compared to a single layer Al2O3. For instance, for SiO2 interlayer thicknesses of ~3–14nm, the recombination parameter of an n+ Si surface having a high surface doping concentration Ns of 2×1020cm−3 was reduced from J0n+=81fA/cm2 to J0n+=50fA/cm2. Simulations predict that the SiO2/Al2O3 stacks outperform Al2O3 passivation layers particularly on n+ Si surfaces having a moderate Ns in the range of 1018–1020cm−3. On p+ Si surfaces, J0p+≤54fA/cm2 was achieved for all ALD SiO2 interlayer thicknesses investigated (i.e., 1–14nm). The SiO2/Al2O3 stacks presented in this work are compatible with SiNx capping and subsequent high-temperature firing steps, which are typically used in solar cell processing. Furthermore, the results were successfully reproduced in an industrial ALD batch reactor using a low-temperature process. This makes ALD SiO2/Al2O3 stacks a promising candidate for the simultaneous passivation of n+ and p+ Si surfaces in solar cells.

Atomic layer deposition of Al2O3 and Al2O3/TiO2 barrier coatings to reduce the water vapour permeability of polyetheretherketone

We demonstrate significantly enhanced barrier properties of polyetheretherketone (PEEK) against water vapour penetration by depositing Al2O3 or Al2O3/TiO2 nanofilms grown by atomic layer deposition (ALD). Nanoindentation analysis revealed good adhesion strength of a bilayer Al2O3/TiO2 coating to PEEK, while the single layer Al2O3 coating displayed flaking and delamination. We identified three critical design parameters for achieving the optimum barrier properties of ALD Al2O3/TiO2 coatings on PEEK. These are a minimum total thickness dependent on the required water vapour transmission rate, the use of an Al2O3/TiO2 bilayer coating and the application of the coating to both sides of the PEEK film. Using these design parameters, we achieved a reduction in moisture permeability of PEEK of over two orders of magnitude while maintaining good adhesion strength of the polymer–thin film system.

Improvement and characterization of high-reflective and anti-reflective nanostructured mirrors by ion beam assisted deposition for 944 nm high power diode laser

Single-layer and multi-layer coatings were applied on the surface of diode laser facets as mirrors. This thin film mirrors were designed, deposited, optimized and characterized. The effects of mirrors on facet passivation and optical properties of InGaAs/AlGaAs/GaAs diode lasers were investigated. High-Reflective (HR) and Anti-Reflective (AR) mirrors comprising of four double-layers of Al2O3/Si and a single layer of Al2O3, respectively, were designed and optimized by Macleod software for 944nm diode lasers. Optimization of Argon flow rate was studied through Alumina thin film deposition by Ion Beam Assisted Deposition (IBAD) for mirror improvement. The nanostructured HR and AR mirrors were deposited on the front and back facet of the laser respectively, by IBAD system under optimum condition. Atomic Force Microscope (AFM), Vis-IR Spectrophotometer, Field Emission Scanning Electron Microscopy (FESEM) and laser characterization Test (P–I) were used to characterize various properties of mirrors and lasers. AFM images show mirror’s root mean square roughness is nearly 1nm. The Spectrophotometer results of the front facet transmission and the back facet reflection are in good agreement with the simulation results. Optical output power (P) versus driving current (I) characteristics, measured before and after coating the facet, revealed a significant output power enhancement due to optimized AR and HR optical coatings on facets.

Effect of Carrier Leakage on Optimal AR Coatings in Midinfrared Interband Cascade Lasers

Variation of leakage current in interband cascade lasers (ICLs) with different carrier concentrations achieved by coating the facet with antireflections (AR) coating was experimentally studied. Single-layer Al2O3 and double-layer ZnS-YF3, ZnS-SiO2, and Ti2O5-SiO2 AR coatings are applied to midinfrared (mid-IR) ICL devices to achieve reflectance ranging from 0.15 to $7\times 10^{-4} $ . By monitoring the laser performance before and after coating, it was observed that, for lower reflectivity coatings, the leakage current appreciably rises with increasing carrier concentration, thus diminishing the slope efficiency improvements and, in fact, degrading the slope efficiency at very low reflectance. It was observed that the ratio of leakage to threshold current could increase by 17% for high carrier concentrations at very low AR coating values. The results also allow for experimental optimization of AR coatings for increased slope efficiency in mid-IR ICLs. Therefore, in this paper, we propose an optimal value for the AR coatings in order to maximize the power in ICLs.

Oxidation precursor dependence of atomic layer deposited Al2O3 films in a-Si:H(i)/Al2O3 surface passivation stacks.

In order to obtain a good passivation of a silicon surface, more and more stack passivation schemes have been used in high-efficiency silicon solar cell fabrication. In this work, we prepared a-Si:H(i)/Al2O3 stacks on KOH solution-polished n-type solar grade mono-silicon(100) wafers. For the Al2O3 film deposition, both thermal atomic layer deposition (T-ALD) and plasma enhanced atomic layer deposition (PE-ALD) were used. Interface trap density spectra were obtained for Si passivation with a-Si films and a-Si:H(i)/Al2O3 stacks by a non-contact corona C-V technique. After the fabrication of a-Si:H(i)/Al2O3 stacks, the minimum interface trap density was reduced from original 3 × 1012 to 1 × 1012 cm−2 eV−1, the surface total charge density increased by nearly one order of magnitude for PE-ALD samples and about 0.4 × 1012 cm−2 for a T-ALD sample, and the carrier lifetimes increased by a factor of three (from about 10 μs to about 30 μs). Combining these results with an X-ray photoelectron spectroscopy analysis, we discussed the influence of an oxidation precursor for ALD Al2O3 deposition on Al2O3 single layers and a-Si:H(i)/Al2O3 stack surface passivation from field-effect passivation and chemical passivation perspectives. In addition, the influence of the stack fabrication process on the a-Si film structure was also discussed in this study.

A flexible transparent gas barrier film employing the method of mixing ALD/MLD-grown Al2O3 and alucone layers.

Atomic layer deposition (ALD) has been widely reported as a novel method for thin film encapsulation (TFE) of organic light-emitting diodes and organic photovoltaic cells. Both organic and inorganic thin films can be deposited by ALD with a variety of precursors. In this work, the performances of Al2O3 thin films and Al2O3/alucone hybrid films have been investigated. The samples with a 50 nm Al2O3 inorganic layer deposited by ALD at a low temperature of 80°C showed higher surface roughness (0.503 ± 0.011 nm), higher water vapor transmission rate (WVTR) values (3.77 × 10−4 g/m2/day), and lower transmittance values (61%) when compared with the Al2O3 (inorganic)/alucone (organic) hybrid structure under same conditions. Furthermore, a bending test upon single Al2O3 layers showed an increased WVTR of 1.59 × 10−3 g/m2/day. However, the film with a 4 nm alucone organic layer inserted into the center displayed improved surface roughness, barrier performance, and transmittance. After the bending test, the hybrid film with 4 nm equally distributed alucone maintained better surface roughness (0.339 ± 0.014 nm) and barrier properties (9.94 × 10−5 g/m2/day). This interesting phenomenon reveals that multilayer thin films consisting of inorganic layers and decentralized alucone organic components have the potential to be useful in TFE applications on flexible optical electronics.

Passivation of phosphorus diffused silicon surfaces with Al2O3: Influence of surface doping concentration and thermal activation treatments

Thin layers of Al2O3 are well known for the excellent passivation of p-type c-Si surfaces including highly doped p+ emitters, due to a high density of fixed negative charges. Recent results indicate that Al2O3 can also provide a good passivation of certain phosphorus-diffused n+ c-Si surfaces. In this work, we studied the recombination at Al2O3 passivated n+ surfaces theoretically with device simulations and experimentally for Al2O3 deposited with atomic layer deposition. The simulation results indicate that there is a certain surface doping concentration, where the recombination is maximal due to depletion or weak inversion of the charge carriers at the c-Si/Al2O3 interface. This pronounced maximum was also observed experimentally for n+ surfaces passivated either with Al2O3 single layers or stacks of Al2O3 capped by SiNx, when activated with a low temperature anneal (425 °C). In contrast, for Al2O3/SiNx stacks activated with a short high-temperature firing process (800 °C) a significant lower surface recombination was observed for most n+ diffusion profiles without such a pronounced maximum. Based on experimentally determined interface properties and simulation results, we attribute this superior passivation quality after firing to a better chemical surface passivation, quantified by a lower interface defect density, in combination with a lower density of negative fixed charges. These experimental results reveal that Al2O3/SiNx stacks can provide not only excellent passivation on p+ surfaces but also on n+ surfaces for a wide range of surface doping concentrations when activated with short high-temperature treatments.

Anti-Corrosion Property of Atomic Layer Deposition Films for Anodized Aluminum

Single and multilayer of Al2O3 and TiO2 were fabricated on anodized aluminum by atomic layer deposition. The effect of processing parameters on pore sealing and anti-corrosion property was studied. It is concluded that the micropores on anodized aluminum could be sealed by atomic layer deposition oxide films through SEM and staining experiments. The anti-corrosive property is affected mainly by the film thickness and process temperature. The acidic drip and salt spray test shows that the anti-corrosion property is enhanced as the film thickness increase. With the similar thickness, Al2O3/TiO2 multilayer film has the best anti-corrosion property, while the single Al2O3 layer appears the poorest. The complementary roles between two basic materials result in the enhanced application property.

Growth behavior and improved water–vapor-permeation-barrier properties of 10-nm-thick single Al2O3 layer grown via cyclic chemical vapor deposition on organic light-emitting diodes

We have investigated the growth behavior and water vapor permeation barrier properties of cyclic chemical vapor deposition (C-CVD)-grown 10-nm-thick single layer of Al2O3. Al2O3 layers grown by C-CVD showed a high density of 3.298g/cm3 and were amorphous without grain boundaries. A deposition rate of 0.46nm/cycle was obtained. The C-CVD system was self-limiting, as in the case of atomic layer deposition, which enables precise control of the thickness of the Al2O3 layer. A water vapor transmission rate of 1.51×10−5(g/m2)/day was obtained from a Ca degradation test performed at 85°C and 85% relative humidity. Moreover, the performance of organic light-emitting diodes, passivated by a C-CVD-grown 10-nm-thick Al2O3 single layer, was not affected after 24,000h of turn-on time; this is strong evidence that C-CVD-grown Al2O3 layers effectively prevent water vapor from diffusing into the active organic layer.

X-ray Photoelectron Spectroscopy Study of the Passivation of NiAl(100) by Water Vapor

The oxidation of NiAl(100) surfaces by water vapor is studied using X-ray photoelectron spectroscopy (XPS) to elucidate the effect of temperature and vapor pressure on the surface passivation mechanism of the NiAl alloy. The water-vapor oxidation at ambient temperature (25 °C) results in self-limiting Al(OH)3/Al2O3 bilayer film growth to a less extent of the limiting thickness regimes, in which the growth of the inner Al2O3 layer occurs via dehydration of the outer Al(OH)3 layer. The growth of the passivating overlayer at the ambient temperature depletes Al and forms a Ni-rich layer at the oxide/alloy interface that impedes supply of Al atoms to the outer surface for Al(OH)3 formation via the hydration reaction, whereby resulting in a more Al-deficient structure of the outer Al(OH)3 layer upon increasing the vapor pressure. In contrast, the water-vapor oxidation at 300 °C results in Al2O3 single-layer film growth to a larger limiting thickness without involving the transient hydroxide phase of Al(OH)3. It is shown that increasing the oxidation temperatures results in the formation of a more compact Al2O3 film owning to the enhanced bulk diffusion rate that maintains an adequate supply of Al atoms to the oxide/alloy interface to sustain the oxide film growth to the full extent of the limiting thickness.

Deposition temperature dependence of material and Si surface passivation properties of O3-based atomic layer deposited Al2O3-based films and stacks

The material composition and the Si surface passivation of aluminum oxide (Al2O3) films prepared by atomic layer deposition using Al(CH3)3 and O3 as precursors were investigated for deposition temperatures (TDep) between 200 °C and 500 °C. The growth per cycle decreased with increasing deposition temperature due to a lower Al deposition rate. In contrast the material composition was hardly affected except for the hydrogen concentration, which decreased from [H] = 3 at. % at 200 °C to [H] < 0.5 at. % at 400 °C and 500 °C. The surface passivation performance was investigated after annealing at 300 °C–450 °C and also after firing steps in the typical temperature range of 800 °C–925 °C. A similar high level of the surface passivation performance, i.e., surface recombination velocity values <10 cm/s, was obtained after annealing and firing. Investigations of Al2O3/SiNx stacks complemented the work and revealed similar levels of surface passivation as single-layer Al2O3 films, both for the chemical and field-effect passivation. The fixed charge density in the Al2O3/SiNx stacks, reflecting the field-effect passivation, was reduced by one order of magnitude from 3·1012 cm−2 to 3·1011 cm−2 when TDep was increased from 300 °C to 500 °C. The level of the chemical passivation changed as well, but the total level of the surface passivation was hardly affected by the value of TDep. When firing films prepared at of low TDep, blistering of the films occurred and this strongly reduced the surface passivation. These results presented in this work demonstrate that a high level of surface passivation can be achieved for Al2O3-based films and stacks over a wide range of conditions when the combination of deposition temperature and annealing or firing temperature is carefully chosen.

Homogeneous Al2O3 Multilayered Films Deposited on Poly(ethylene terephthalate) by Facing-Target Sputtering System for Thin-Film Passivation of Organic Light-Emitting Diodes

Homogeneous multilayered barrier films were fabricated by means of reactive and nonreactive processes using Al2O3 with the facing-target sputtering (FTS) system. The multilayered films showed 60% improved barrier performance and their fabrication was 30% faster than that of single Al2O3 layers of the same thickness. The water vapor transmission rate was increased up to the order of 10-4 g·m-2·d-1 from a three-pair system of reactive and nonreactive sputtered Al2O3 bilayers.

Al2O3/TiO2 stack layers for effective surface passivation of crystalline silicon

For silicon surface passivation, we investigate stack layers consisting of a thin Al2O3 layer and a TiO2 capping layer deposited by means of thermal atomic layer deposition (ALD). In this work, we studied the influence of different thermal post-deposition treatments and film thickness for the activation of passivating ALD Al2O3 single layers and Al2O3/TiO2 stack layers. Our experiments show a substantial improvement of the passivation for the Al2O3/TiO2 stack layers compared to a thin single Al2O3 layer. For the stacks, especially with less than 10 nm Al2O3, a TiO2 capping layer results in a remarkably lower surface recombination. Effective fixed charge density of Al2O3/TiO2 stack layers increases after TiO2 deposition and O2 annealing. It is also demonstrated that the enhanced surface passivation can be mainly related to a remarkably low interface defect density of 1.1 × 1010 eV−1 cm−2, whereas post-TiO2 heat treatment in O2 ambience is not beneficial for the passivation of silicon, which is attributed to increasing interface defect density of stack layers.