Atomic structure of AlN/Al2O3 interfaces fabricated by pulsed-laser deposition

The atomic structure of AlN/Al₂O₃ interface fabricated by pulsed laser deposition is characterized by high-resolution transmission electron microscopy (HRTEM) combined with systematic multi-slice HRTEM image simulations. It is found that the AlN film deposited on a (0001) Al₂O₃ substrate grows epitaxially with the orientation relationship of (0001)AlN//(0001) Al₂O₃ and [ ]AlN//[ ]Al₂O₃, with an atomically sharp interface. The observed interface showed best correspondence with the rigid structural model that AlN is terminated by Al at the interface, while the Al₂O₃ substrate is terminated by O. Detailed structural analysis indicates that Al sites at the interface are coordinated by both oxygen and nitrogen in this model, with similar coordination environment in AlN. This favored coordination state at the interface may stabilize the AlN/Al₂O₃ interface.     

Interfacial reactions between titanium film and single crystal α-Al2O3

Titanium is commonly used to join metals and ceramics by active metal brazing methods. In this work, titanium was sputter deposited on to single-crystal -Al₂O₃ substrates and the interfacial reactions between the titanium film and the Al₂O₃ substrate were studied. Al₂O₃ was reduced by titanium when samples were annealed at 973 and 1173 K for 300 s in an argon gas flow. Metallic aluminium was produced at the interface, and this diffused from the interface into the titanium film. At 1173 K, the intermetallic compound Ti₃Al and the intermediate titanium oxides, such as Ti₂O and TiO, were formed. The Al⁰ diffusion is important in stimulating interfacial reactions.     

Effects of interface structures on cracking in AlN(11\overline 2 0)/α-Al2O3 (1\overline 1 02) epitaxial films

The crystal structures and microstructures of AlN $(11\overline 2 0)$ /GaN $(11\overline 2 0)$ epitaxial films on just-cut and ±4° off-cut Al₂O₃ $(1\overline 1 02)$ substrates grown by metal organic chemical vapor deposition (MOCVD) are investigated using high-resolution X-ray diffractometry and transmission electron microscopy, and are compared with those of AlN $(11\overline 2 0)$ film on +4° off-cut Al₂O₃ $(1\overline 1 02)$ substrate. In the AlN/Al₂O₃(+4° off-cut) film and the AlN/GaN/Al₂O₃ (just-cut, ?4° off-cut) films, cracks parallel to the $[1\overline 1 00]$ _AlN direction and perpendicular to the interfaces of the films and the substrates are observed. The AlN/Al₂O₃ and AlN/GaN interfaces exhibit low crystallinity in which moiré fringes are observed. On the other hand, in the AlN/GaN/Al₂O₃(+4° off-cut) film, no cracks form. The GaN layer buffers the lattice mismatch between the AlN film and the Al₂O₃ substrate, and moiré fringes are not observed in the GaN/Al₂O₃ and AlN/GaN interfaces. On the basis of these results, the effects of the interface structures on cracking are discussed.     

Investigation of the aluminum oxide/Si(1 0 0) interface formed by chemical vapor deposition

Thin films of aluminum oxide were deposited using trimethylaluminum and oxygen. The deposition rate was found to decrease with increasing temperature. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to investigate the film/substrate interface. When dry O₂ was used during deposition, the film/substrate interface was free of any silicon dioxide or aluminum silicate phase. On annealing the as-deposited films in Ar, a layer of silicon dioxide film formed at the interface. XPS results indicated that the O/Al ratio in the as-deposited films was higher than that in stoichiometric Al₂O₃. However, the ratio was found to decrease in the annealed samples suggesting that excess oxygen present in the deposited films is responsible for the formation of interfacial silicon dioxide layer. Interfacial phase formation was observed in the as-deposited samples, when small amounts of ozone along with oxygen were used as the oxygen precursor.     

Cracking mechanism in AlN(11―20)/α-Al2O3(1―102) heteroepitaxial films grown by MOCVD

The cracking mechanism in AlN(1120)/-Al₂O₃(1102) heteroepitaxial film grown by MOCVD is discussed. The crystal structure and microstructure of an AlN/Al₂O₃ film and an AlN/GaN/Al₂O₃ film are compared using high-resolution X-ray diffractometry, optical microscopy, scanning electron microscopy, and transmission electron microscopy. In the AlN/Al₂O₃ film, cracks parallel to the [1100]_AlN direction and perpendicular to the interface of the film and the substrate are observed. The cracks do not propagate to the AlN film surface. The tips of the cracks are widest in the AlN film, and the cracks narrow as they penetrate deeply into the substrate. On the other hand, in the AlN/GaN/Al₂O₃ film, no cracks are observed. A concave curvature is observed in the AlN film with cracks on the Al₂O₃ substrate along the [0001]_AlN direction, whereas a convex curvature is observed in the AlN film without cracks. On the basis of these results, the cracks, formed in the AlN film due to the tensile stress along the [0001]_AlN direction during the epitaxial growth, propagate to the AlN film surface and into the Al₂O₃ substrate. On the other hand, in the AlN/GaN/Al₂O₃ film, it seems that the GaN buffer layer suppresses the tensile stress; as a consequence, no cracks occur.     

Effects of aluminum doping on lanthanum oxide gate dielectric films

This work reports a novel method for improving the electrical properties of lanthanum gate oxide (La₂O₃) by using aluminum doping and rapid thermal annealing (RTA) techniques. In the bulk of the Al-doped La₂O₃ film together with 600 °C RTA, we found that the aluminum atoms were incorporated into the oxide network and the film was transformed into lanthanum aluminate complex oxide. At the interface, a thin Al₂O₃ layer was formed. This interfacial Al₂O₃ layer suppressed the out-diffusion of substrate Si, the formation of interfacial silicate layer and silicide bonds. These effects resulted in a significant reduction on the bulk and interface trap densities and hence the gate leakage current.     

Growth of aligned carbon nanotubes on ALD-Al2O3 coated silicon and quartz substrates

The smooth surface of the amorphous Al₂O₃ film on either silicon or quartz, coated by atomic layer deposition (ALD), was changed to rough surface by annealing in either air or hydrogen at high temperature (745°C) due to the formation of nanosized pinholes and micrometre pimples during the crystallisation of the amorphous Al₂O₃. The rough surface makes the growth of long carbon nanotubes (CNTs) by chemical vapour deposition impossible. Nevertheless, we were able to develop new catalyst recipes for successful growth of vertically aligned CNTs on ALD-Al₂O₃ coated silicon and quartz substrates. The lengths of the CNTs reached 90?µm on silicon substrates and 180?µm on quartz substrates. Furthermore, it is observed that the adhesion of CNTs on silicon substrates is much stronger than that on quartz substrates.     

Low resistivity transparent conducting Al-doped ZnO films prepared by pulsed laser deposition

A series of AZO films were grown on glass substrates by a method of pulsed laser deposition using a split target divided into AZO (Al₂O₃: 1 wt.%) and AZO (Al₂O₃: 2 wt.%). The film deposition took place at a substrate temperature of 230 °C within a magnetic field applied perpendicularly to the plume. To suppress the droplet generation caused by the intense laser energy, the laser energy density was reduced to 0.75 J/cm² (15 mJ). For an approximately 280-nm thick-AZO film grown at a target-to-substrate distance of 25 mm, we obtained the lowest resistivity of 8.54×10⁻⁵ Ω·cm and an average transmittance of more than 88% in the visible range. In the cross-sectional TEM observation and XRD spectra, the regularity in the crystal growth was generated immediately from the interface between the substrate and the film.     

Compatibility between alumina fibres and aluminium

An investigation is carried out on the interfacial wetting behaviour and reactions between aluminium and alumina fibres (85mass% Al₂O₃ and 15mass% SiO₂). Aluminium is coated onto alumina fibres by a vacuum evaporation technique and the surface of the fully coated fibres and the edge of the partially coated fibres are examined by scanning electron microscope after heat treatments at various temperatures. Within a temperature regime between 943 and 1273 K, occurrence of such interfacial reactions as 4Al(I) + Al₂O₃(s) 3Al₂O₃(g) and 4Al(I) + 3SiO₂(s) 2Al₂O₃(s) + 3Si(s) are detected. It is found that molten aluminium can cover the alumina fibre surface but it peels off near the edge of the coating film on a partially coated fibre, showing the very weak interface cohesion. This is ascribed to the lack of a stable compound formation at the interface. Results of tensile test show that the strength of the coated fibres is degraded after heat-treating at above the melting point of aluminium. The culprits for the tensile failure of alumina fibres are evaluated by the Weibull distribution theory.     

Oxidation behavior of AlN substrate at low temperature

The oxidation onset and the kinetics of polycrystalline AlN substrates were studied by measuring the weight percent of oxygen in the surface layer and the surface roughness with energy dispersive X-ray spectroscopy (EDX) and atomic force microscopy (AFM), respectively. The oxidation started in the temperature range 800–900 °C and the entire surface of the AlN substrate was covered with an Al₂O₃ oxide layer below 1100 °C. The oxidation kinetics followed a linear rate law below 1000 °C and a parabolic rate law above 1100 °C. Above 1100 °C, the surface roughness increased abruptly by the irregular shape of overgrown oxide, which might enhance the adhesion of metal to the AlN surface in a metallization process. With an increase of the oxidation temperature above 1200 °C, the oxide layer split during cooling due to the thermal expansion mismatch between the AlN matrix and the Al₂O₃ oxide layer.     

Composition of alumina films grown on Si at low temperature with catalytic CVD

The X-ray photoelectron spectroscopy (XPS) measurements have been used to reveal the compositions of alumina (Al₂O₃) films formed on Si wafers using tri-methyl aluminium (TMA) and molecular oxygen (O₂) with catalytic chemical vapour deposition (Cat-CVD). The atomic ratio (O/Al) for Al₂O₃ samples formed at substrate temperature of 200-400 °C has been obtained to be 1.4 which is close to stoichiometry. The increase of growth rate at substrate temperatures below 200 °C and above 400 °C can be attributed to formation of aluminum oxides with non-stoichiometry and metallic aluminum incorporated in the films resulting from deficient oxygen. Angle resolved XPS measurements have revealed that the alumina/Si interface with no SiO₂ film has been obtained at substrate temperatures below 200 °C.     

Bonding of Al to Al2O3 via Al–Cu eutectic method

The Al oxidation layer in the manufactures of direct aluminum bonded Al₂O₃ substrates (DAB) has been a long-term trouble for industries. In this work we propose a new method for fabricating the DAB substrates with no requirement of high vacuum or active O₂-getters. The new method comprises two stages: (i) Cu-film is bonded onto Al₂O₃ ceramic surface via DBC method; (ii) Al foil is joined to the DBC substrate by Al–Cu eutectic method at 600 °C in pure N₂ atmosphere. KF–AlF₃ flux was used to disrupt the Al–oxide layer on the surface of Al foil. The wetting ability was significantly enhanced due to the diffusion of Cu into Al and the dissolving of Al. The final contact angle is achieved of 22.10°. Microstructure and composition of the interface between Al and Al₂O₃ substrate were analyzed. The XRD, SEM and EDS results show that two new phases Al₂Cu and CuAlO₂ were formed, leading to a strong bonding along the interface. The thermal cycling reliability and adhesion strength of DAB substrates were also evaluated. The results show that the DAB substrates can satisfy application requirements completely.     

Atomistic structure and energetics of interface between Mn-doped γ-Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> and MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript>

The interface between an Mn-doped γ-gallium oxide (Ga₂O₃) thin film and an MgAl₂O₄ (001) substrate has been investigated using high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and first-principles calculations. A high-quality Mn-doped γ-Ga₂O₃ film with a defective spinel structure has been epitaxially grown by pulsed laser deposition. The γ-Ga₂O₃ crystal shows an uniform tetragonal distortion with a tetragonality of 1.05 throughout the film thickness of 75 nm. HRTEM and HAADF-STEM observations reveal that the γ-Ga₂O₃ and MgAl₂O₄ crystals form a coherent interface without any interfacial layers or precipitates. The atomistic structure and energies are theoretically evaluated for the interfaces with two types of termination plane, i.e., Mg- and Al₂O₄-termination of MgAl₂O₄. The cation sublattice is found to be continuous for both interfaces despite the defective spinel structure of Mn-doped γ-Ga₂O₃ with some vacant cation sites. The Al₂O₄-termination shows a lower interfacial energy than the Mg-termination under most conditions of the chemical potentials. This behavior is attributed to the energetic preference of the Mn–Al₂O₄ local configuration at the interface.     

Metallization development for AIN/W cofired substrate at low temperature

In this paper, metallization properties of AlN/W cofired substrate sintered at 1650°C were studied. The adhesion strength between W and AlN ceramic was improved by some methods, which including addition of several kinds of oxides mixture or metallic particles into W thick film ink. When MgO-Al₂O₃-SiO₂ was used, glass bonding increased the adhesion strength and when metallic particles were added, W conductor resistance decreased obviously.     

Nickel and copper deposition on Al2O3 and SiC particulates by using the chemical vapour deposition–fluidized bed reactor technique

A chemical vapour deposition–fluidized bed reactor technique was developed to perform metal deposition on ceramic particulates. Experiments of nickel and copper deposition on Al₂O₃ and SiC particulates were conducted. Argon was used as the carrier gas to fluidize the ceramic particulates. The metal–H–Cl system was selected for the chemical vapour deposition. The volumetric ratios of the inlet gas were 3.5% HCl, 20.0% H₂, and 76.5% Ar. The deposition reactions were carried out at four different temperatures: 500, 600, 700 and 800 °C. Successful deposition of metallic nickel and copper on the ceramic particulates was observed. It was also noticed that the deposition rates varied with the types of substrates and deposited metals. This revised version was published online in November 2006 with corrections to the Cover Date.     

Nanoscale adhesion,friction and wear studies of biomolecules on silane polymer-coated silica and alumina-based surfaces

Proteins on biomicroelectromechanical systems (BioMEMS) confer specific molecular functionalities. In planar FET sensors (field-effect transistors, a class of devices whose protein-sensing capabilities we demonstrated in physiological buffers), interfacial proteins are analyte receptors, determining sensor molecular recognition specificity. Receptors are bound to the FET through a polymeric interface, and gross disruption of interfaces that removes a large percentage of receptors or inactivates large fractions of them diminishes sensor sensitivity. Sensitivity is also determined by the distance between the bound analyte and the semiconductor. Consequently, differential properties of surface polymers are design parameters for FET sensors. We compare thickness, surface roughness, adhesion, friction and wear properties of silane polymer layers bound to oxides (SiO₂ and Al₂O₃, as on AlGaN HFETs). We compare those properties of the film–substrate pairs after an additional deposition of biotin and streptavidin. Adhesion between protein and device and interfacial friction properties affect FET reliability because these parameters affect wear resistance of interfaces to abrasive insult in vivo. Adhesion/friction determines the extent of stickage between the interface and tissue and interfacial resistance to mechanical damage. We document systematic, consistent differences in thickness and wear resistance of silane films that can be correlated with film chemistry and deposition procedures, providing guidance for rational interfacial design for planar AlGaN HFET sensors.     

Al2O3 particle rounding in molten copper and Cu8wt%Al

We investigate processing-microstructure relationships in the production of Al₂O₃ particle reinforced copper composites by solidification processing. We show that during production of the composites by gas-pressure infiltration of packed Al₂O₃ particle preforms with liquid Cu or with liquid Cu8wt%Al at either 1,150 or 1,300 °C, capillarity-driven transport of alumina can cause rounding of the Al₂O₃ particles. We use quantitative metallography to show that the extent of particle rounding increases markedly with temperature and with the initial aluminum concentration in the melt. An analysis of the thermodynamics and kinetics governing the transport of alumina in contact with molten copper, considering both interfacial and volume diffusion, leads to propose two mechanisms for the rounding effect, namely (i) variations in the equilibrium concentration of oxygen in the melt as affected by the initial aluminum concentration, or (ii) segregation of aluminum to the interface with the ceramic.     

Interfacial reactions in the liquid diffusion couples of Mg/Ni,Al/Ni and Al/(Ni)-Al2O3 systems

The interfacial reactions between various molten metals and solid plates were investigated in this diffusion couple study. The molten metals were pure magnesium, pure aluminium, aluminium-rich Al-Mg alloy, and aluminium-rich Al-Cu alloys, and the solid plates were pure nickel plate, alumina plate, and nickel-plated alumina plate. The interfacial reactions in the diffusion couples were determined by using optical microscopy, scanning electron microscopy and electron probe microanalysis in regard to the formation of intermetallic phases, the dissolution rates of the nickel plates, and the morphology of the interfaces. Mg₂Ni phase was found in the pure Mg/Ni plate diffusion couples, and the Al₃Ni and Al₃Ni₂ phases were observed in the pure Al/Ni plate and Al-alloys/Ni plate diffusion couples. In the Al-Cu alloy/Ni-plated alumina plate diffusion couple, Al₂O₃ formed at the interface, while spinel particles were found in the diffusion couples of Al-7.4wt% Mg alloy/Ni-plated alumina plate. Experimental difficulty was encountered in preparing the diffusion couples with alumina plate, and a gap existing at the interface prohibited reactions between the molten metal with alumina plate.     

Untersuchung der Haftmechanismen kaltgasgespritzter Al‐Schichten auf Al2O3

The metallization of ceramics by means of cold gas spraying has been in the focus of numerous publications in the recent past. However, the bonding mechanisms of metallic coatings on non‐ductile substrates are still not fully understood. Former investigations of titanium coatings on corundum revealed that a combination of recrystallisation induced by adiabatic shear processes and heteroepitaxial growth might be responsible for the high adhesions strengths of coatings applied on smooth ceramic surfaces. In the present work, it is intended to examine the interface area of cold gas sprayed aluminum on alumina substrates. Besides a variation of powder fraction and substrate temperature, it is investigated if a downstream heat treatment has an influence on tensile strength and hardness of the coatings. The splat formation of single particles is investigated by means of scanning electron microscopy, while a high resolution transmission electron microscope is used to examine the Al/Al₂O₃ interface. First results suggest that mechanical clamping is the primary bonding mechanism on polycrystalline coatings with a sub‐micrometer‐scaled surface roughness, while heteroepitaxial growth is the main bonding mechanism for Al coatings on single‐crystalline, atomically smooth sapphire (α‐Al₂O₃) substrates. Heteroepitaxy is promoted by deformation‐induced recrystallisation of the cold gas‐sprayed aluminum.     

Electromechanical Behavior of Al/Al2O3 Multilayers on Flexible Substrates: Insights from In Situ Film Stress and Resistance Measurements

A series of Al and Al/Al₂O₃ thin-film multilayer structures on flexible polymer substrates are fabricated with a unique deposition chamber combining magnetron sputtering (Al) and atomic layer deposition (ALD, Al₂O₃, nominal thickness 2.4–9.4 nm) without breaking vacuum and thoroughly characterized using transmission electron microscopy (TEM). The electromechanical behavior of the multilayers and Al reference films is investigated in tension with in situ X-ray diffraction (XRD) and four-point probe resistance measurements. All films exhibit excellent interfacial adhesion, with no delamination in the investigated strain range (12%). For the first time, an adhesion-promoting naturally forming amorphous interlayer is confirmed for thin films sputter deposited onto polymers under laboratory conditions. The evolution of Al film stresses and electrical resistance reveal changes in the deformation behavior as a function of oxide thickness. Strengthening of Al is observed with increasing oxide thickness. Significant embrittlement can be avoided for oxide layer thicknesses ≤2.4 nm.