Thin Al Oxide Layer Research Articles

Formation mechanisms of surface blistering in AA6xxx rolled products: microstructure characterisation, ultrasonic analysis, and rolling tests results

ABSTRACT The source of surface blister defect in 6xxx series Al rolled products was studied. A blister defect, aligned with the rolling direction, was observed on the surface of the AA6xxx Al rolled sheets. It was indicated that the porosities and small microstructural inhomogeneities inside the cast ingots, identified by ultrasonic tests and metallography analysis, were not the origin of these surface defects. Pilot scale rolling trials indicated that the blister defects were correlated with the longitudinal internal casting cracks. The short side cracks in the cast ingots, covered by a thin Al oxide layer, were not healed and remained partially open after the rolling process. The unhealed cracks served as the nucleation sites for H2 precipitation during the later stages of the thermomechanical process, which were subsequently resulted from surface blisters on the AA6xxx Al rolled sheets.

Surface properties of AlN and Al x Ga1−x N epitaxial layers characterized by angle resolved X-ray photoelectron spectroscopy

The AlN and AlxGa1−xN (x = 0.35, 0.73) epitaxial layers were grown on c-plane sapphire substrates by metal–organic chemical vapor deposition. The surface properties of these layers were quantitatively characterized by angle resolved X-ray photoelectron spectroscopy. The characterization results revealed that the air-exposed surfaces of the Al-containing AlN and AlGaN films were covered by a thin Al-oxide layer due to the large chemical affinity of aluminum to oxygen. The as-deposited AlN film was proved to be a nitrogen-deficient film. On the other hand, the Ga Auger effect for the as-deposited AlGaN film was found to be greatly suppressed by increasing Al composition. Moreover, the Al composition in the as-deposited AlGaN films was demonstrated to be non-uniform since Al atom is more easily oxidized than Ga atom, especially for the AlGaN film with relatively high Al content. Our work will be beneficial to understanding and controlling of the surface properties for Al-containing III-nitrides.

Graphene Transistors Are Insensitive to pH Changes in Solution

We observe very small gate-voltage shifts in the transfer characteristic of as-prepared graphene field-effect transistors (GFETs) when the pH of the buffer is changed. This observation is in strong contrast to Si-based ion-sensitive FETs. The low gate-shift of a GFET can be further reduced if the graphene surface is covered with a hydrophobic fluorobenzene layer. If a thin Al-oxide layer is applied instead, the opposite happens. This suggests that clean graphene does not sense the chemical potential of protons. A GFET can therefore be used as a reference electrode in an aqueous electrolyte. Our finding sheds light on the large variety of pH-induced gate shifts that have been published for GFETs in the recent literature.

Anti-Corroded Molybdenum Back Electrodes by Al Doping for CuIn1-xAlxSe2 Solar Cells

The effects of Al doping into Mo films, used as back electrodes of CuIn1-xAlxSe2 solar cells, were systematically investigated. The Mo1-XAlX films fabricated by co-sputtering were exposed in a damp-heat (DH) chamber with 85°C and 85% relativity humidity for 168 h to verify the reliability. The resistivity of the Mo79Al21 films after DH exposure was only slightly increased from 2 × 10−6 Ω m to 2.3 × 10−6 Ω m; however, the pure Mo films after DH exposure became insulating due to the formation of the MoOx layer. Based on the analyses of the x-ray diffraction and x-ray photoelectron spectrum, we suggest that the Al atoms are dissolved into the Mo films in the as-deposited state and segregate to the surface of Mo1-XAlX films during DH exposure. A thin Al-oxide layer is formed on the surface and behaves as an anti-corrosion layer, which enhances reliability. On the other hand, with increasing the Al content, increased amount of the Al2O3 layer may suppress the formation of the MoSe2 layer at the interface between Mo and CIS-based films. Consequently, the Mo film with the optimized Al doping, for example, the Mo79Al21 film may become a good candidate of the back electrode for CuIn1-xAlxSe2 solar cells to maintain good reliability as well as to achieve good adhesion and low series resistance.

Corrosion resistance of Al-alloying high Cr–ODS steels in stagnant lead–bismuth

Corrosion resistances of oxide dispersion strengthened ferritic steels containing 3.5 wt% Al and 14–17 wt% Cr were examined. Experiments were performed in stagnant lead–bismuth eutectic (LBE) containing 10 −6 and 10 −8 wt% oxygen at 550 and 650 °C. The maximum exposure duration was 5000 h. Observations of the cross-sections of exposed samples obtained by scanning electron microscopy and Auger electron spectroscopy suggested that a very thin Al oxide layer was formed continuously. The materials containing 3.5 wt% Al exhibited good corrosion resistance despite being exposed to 5000 h of LBE containing 10 −6 wt% at 650 °C. Furthermore, a favourable influence of adding minor amount of Zr was detected after 5000 h exposure to LBE containing 10 −8 wt% oxygen at 650 °C.

Thermally Stable Ag Thin Films Modified with Very Thin Al Oxide layers

Ag has long been of interest for use as a low-resistivity electrode material, but the problem of agglomeration caused by annealing must be resolved. One of the solutions is alloying Ag, but this often causes an increase in electrical resistivity. In this study, thermal stability of Ag thin films modified with thin Al oxide layers was investigated from the viewpoints of surface morphology and resistivity. It was found that agglomeration was suppressed in the multilayer films even after annealing at 600 °C, similarly to Ag(Al) alloy films, which were investigated previously. As a result, it was suggested that the presence of the Al oxide layers at the film surface and interface played a major role in suppressing agglomeration in Ag(Al) films. Moreover, the resistivity of the multilayer film was 1.8 µΩ cm, which was much lower than that of alloy films. Consequently, to achieve both agglomeration suppression and low resistivity, the structural modification of the Ag films using Al oxide layers is more useful than alloying the films.

Agglomeration behavior of Ag films suppressed by alloying with some elements

Agglomeration behavior of co-sputtered Ag–Au, Ag–Cu and Ag–Al thin films was investigated. The results showed that Ag–Au films agglomerated during annealing in a similar way to pure Ag films, but a degree of suppression of this effect was observed in Ag–Cu and Ag–Al films. In particular, no void formation was observed in Ag–Al films for annealing temperatures up to 600 °C. Different mechanisms are proposed to explain agglomeration suppression and the prevention of Ag atom migration, including the precipitation Cu atoms in Ag–Cu films, and the formation of very thin Al-oxide layers at the film surface and at the interface with the substrate. The resistivity of Ag–Au film increased by annealing due to the agglomeration, but that of Ag–Cu and Ag–Al films did not. However, the resistivity of as-deposited Ag–Al film was too high to be reduced enough by annealing.

Magnetic tunnel junctions with Hf oxide and modified Hf oxide tunnel barriers

Magnetic tunnel junctions (MTJ’s) with Hf oxide and modified Hf oxide barriers were fabricated by ozone oxidation. The tunnel magnetoresistance (TMR) ratio in Hf oxide junction was 13% at room temperature and 21% at 77 K. In order to understand the low TMR ratio in MTJ’s with Hf oxides compared to those with Al oxides, tunnel barriers were modified by inserting a thin Al oxide layer of 0.3 nm at the interfaces between ferromagnetic electrodes and Hf oxide insulating layers. As the Al layer of 0.3 nm was inserted at top and bottom interfaces, the TMR ratio was restored to the value of the junctions with Al oxides. This implies that the polarization of CoFe contacted with Al oxide is larger than that of CoFe contacted with Hf oxide and the low TMR ratio in MTJ’s with Hf oxides may be attributed to the reduction of spin polarization of the CoFe electrodes due to CoFe/Hf oxide interface interaction.

Structural features of mechanically alloyed Al–PbO and Al–PbO–WO3 composites

Mechanical alloying, cold compression and hot extrusion were used to prepare Al–PbO–WO3 and Al–PbO composites. The as-extruded and annealed samples were examined by means of transmission electron microscopy and X-ray analysis. Heavily deformed matrix containing very fine particles of basic components and some remaining Al-grains were observed in the as-extruded materials. Because of high affinity of oxygen to aluminum the heavily deformed matrix was chemically unstable on annealing at high temperatures. A thin Al-oxide layer around W-rich particles resulted from chemical reaction between aluminum matrix and WO3-particles in samples annealed for 1–2 h at 873 K. Reduction of PbO-particles within the Al-matrix was found to result in growth of cavities and coarsening of Pb-particles within pores. Colonies of very fine oxides within pores and Pb-particles located close to neighboring Al-matrix were often observed in annealed samples.

Investigation of oxidation process on plasma treated thin Al-films by GIXR and GIXRD

The sensitivity of grazing incidence X-ray reflectometry (GIXR) for amorphous as well as crystalline layers and for layer formations in the nm-range combined with information about structural states by grazing incidence X-ray diffractometry (GIXRD) enable these methods to study interfaces and surface layers. Thin aluminium layers (30–60 nm) were deposited on Si by thermal evaporation. These Al-layers were plasma oxidized for 5 h with interruptions every hour. The plasma treatments were carried out in a 2.45 GHz microwave discharge (SLAN-slot antenna) with different plasma types (Ar, O2) at low temperatures. In this way a thin oxide layer additional to the native alumina on the top was formed. The properties of the oxide layers such as thickness, roughness and density were studied by GIXR. The chemical composition was investigated by GIXRD and XPS. In addition AFM was used for surface topography observations. All these methods were applied before and after the plasma treatment. The properties of the thin Al-oxide layers were found to depend on the type of plasma but in all cases plasma leads to a smoothing of the films as well as an increasing of the thickness of the upper alumina layer.

Critical current uniformity and stability of Nb/Al-oxide-Nb Josephson junctions

The critical current stability of Nb Josephson junctions using thin Al oxide layers as the tunneling barrier has been investigated with respect to repeated thermal cycling, aging, and annealing at elevated temperatures as a function of time. The Nb electrodes and in situ thermally oxidized Al barriers were deposited by dc magnetron sputtering prior to the lithographic formation of the individual junctions. The arrays of junctions were formed by standard photolithography techniques, reactive ion etching, and anodization. A population of 400 20-μm-square junctions were thermally cycled from room temperature to 6 K 4880 times with no failures due to shorts and no change in the average critical currents. These same unpassivated junctions were annealed at elevated temperatures for various periods of time until the critical currents had decreased by 50% at temperatures near 275 °C.