Al2O3 Interface Research Articles

Secondary ion mass spectrometry and photoluminescence study on microstructural characteristics of chemically synthesized ZnO nanowalls

We report on the thermal effect on the microstructural modification of self-assembled ZnO nanowalls grown on Al substrates. The morphology of the as-synthesized nanowalls has been investigated using scanning electron microscopy, while the X-ray diffraction study ensures the formation of hexagonal wurtzite crystal structure of ZnO and also indicates the formation of Al2O3 layer at the interface of Al and ZnO nanowalls. The unidirectional vertical growth of ZnO wire-like network formed at an intermediate stage has been anticipated as the evolution mechanism of the nanowalls. Upon annealing in air and vacuum environment, significant crystallographic modification of the ZnO nanowalls has been observed through X-ray diffraction. The appearance of various native defects (e.g. oxygen vacancies, zinc interstitials, etc.) in the ZnO crystals of the annealed nanowalls has been evidenced through photoluminescence spectroscopy. The luminescence response of the nanowalls is found to be dependent on the surface adsorbant species like OH−, O2−, etc., whose existence on the nanowall surface have been detected through secondary ion mass spectrometry (SIMS). Again the oxygen deficiency in the vacuum-annealed nanowalls has also been confirmed through SIMS measurements. Apart from this microstructural alteration of the nanowalls, annealing also has aggravated the dominance of insulating nature of the Al2O3 interface.

Dielectric Breakdown of $\hbox{Al}_{2}\hbox{O}_{3}/ \hbox{HfO}_{2}$ Bi-Layer Gate Dielectric

We investigate the dielectric breakdown of aluminum oxide (Al2O3) interfacial layers compared with SiO2 used in the high- k gate dielectric stacks. We predict the maximum operating voltage for a ten-year lifetime, using extracted values of Weibull β, voltage power law exponent (n), and breakdown voltage. We show that the Al2O3 interface layer reliability is sufficient for it to be considered as a replacement for SiO2.

Laser-deposited Cu/α–Al2O3 nanocomposite: experiment and modeling

A Nd:YAG laser operating at a wavelength of 266 or 355 nm is used to deposit a thin layer of copper on the (0 0 0 1)α-Al2O3 surface. The formation process is precisely controlled by identification of time distribution of two characteristics: energy and flux density of particles incident on the substrate. For this purpose, the Cu-plasma expansion is described by means of an analytical hydrodynamic model whose self-similar solutions are fitted to the experimental plasma images and time-of-flight spectra. The obtained nanocomposite is examined by the aberration-corrected high-resolution transmission electron microscopy (Cs-HRTEM) method. The results reveal that copper crystals assume one main orientation relative to the substrate (1 1 1)[2 −1 −1]Cu\(\Vert\) (0 0 0 1)[−1 −1 2 0]α–Al2O3 and the formed interface has a specific microstructure. To reconstruct the phase boundary region, molecular dynamic (MD) and static (MS) simulations are carried out. The results show that strong bonding between copper and sapphire induces structural changes in the (1 1 1) Cu layer nearest the substrate and leads to formation of the system of partially dissociated dislocations in the next layer. In consequence, the Cu/α–Al2O3 interface becomes the semicoherent system. The lattice matching regions of the individual Cu layers are significantly lowered, which results in strong deformations along the closed packed planes. The reconstructed interface is used for Cs-HRTEM image simulation. A good accordance with the experimental results indicates that the MD model correctly maps the microstructure at the phase boundary of the synthesized nanocomposite.

First-principles study of the effect of S impurity on the adhesion of Fe/Al2O3 interface

The effect of S impurity on adhesion of Fe(110)/Al2O3(0001) interface is studied by the first-principles plane wave pseudopotential method within the density functional theory. It is shown that S impurity prefers to occupy the site of Fe3 at Fe(110)/Al2O3(0001) interface substitutionally due to the smallest interface segregation energy. The adhesion of Fe(110)/Al2O3(0001) interface is mainly governed by the interaction between Fe and O atoms on both sides of interface. The calculation results of the partial density of states, Mulliken overlap population and the electron density all suggest that Fe-O interaction is weakened by the segregated S impurity at Fe/Al2O3 interface, and the presence of S impurity gives rise to stronger electrostatic repulsion between Fe and O atoms across Fe/Al2O3 interface, which all leads to a reduced adhesion for Fe/Al2O3 interface. As a result, the results obtained by the first principles can give us a deeper understanding of the mechanism of a reduced interface adhesion and the oxidation film spallation by the segregation of S impurity at FeCrAl alloy interface.

Interfacial energies and mass transport in the Ni(Al)–Al2O3 system: The implication of very low oxygen activities

Adhesion and capillary-driven mass transport at ceramic–metal interfaces play a very important role in the performance and durability of materials for many applications, and the influence of the oxygen activity is a critical issue. This work systematically investigates the variation of interfacial energies and atomic transport mechanisms at metal–oxide interfaces at very low oxygen activities by bonding Ni–Al alloys and pure polycrystalline alumina under controlled conditions in sessile drop experiments. The angles and the evolution of the grain boundary grooves were analyzed by scanning electron microscopy, atomic force microscopy and focused ion beam milling to calculate the interfacial and grain boundary energies and the transport rates at the metal–Al2O3 interface. In parallel, high-resolution structural and chemical analysis of selected grain boundaries was performed using advanced transmission electron microscopy. Our results confirm that all the interfacial energies (metal–Al2O3, Al2O3 surface and grain boundary energy) are smaller at reduced p(O2) than those of stoichiometric interfaces. The atomic transport at the metal–Al2O3 interface was found to decrease initially with decreasing p(O2) but increased significantly with a further decrease in the oxygen activity.

Self-aligned top-gate InGaZnO thin film transistors using SiO2/Al2O3 stack gate dielectric

Self-aligned top-gate amorphous indium–gallium–zinc oxide (a-IGZO) thin film transistors (TFTs) utilizing SiO2/Al2O3 stack thin films as gate dielectric are developed in this paper. Due to high quality of the high-k Al2O3 and good interface between active layer and gate dielectric, the resulting a-IGZO TFT exhibits good electrical performance including field-effect mobility of 9cm2/Vs, threshold voltage of 2.2V, subthreshold swing of 0.2V/decade, and on/off current ratio of 1×107. With scaling down of the channel length, good characteristics are also obtained with a small shift of the threshold voltage and no degradation of subthreshold swing.

Combined Flame and Electrodeposition Synthesis of Energetic Coaxial Tungsten-Oxide/Aluminum Nanowire Arrays

A nanostructured thermite composite comprising an array of tungsten-oxide (WO2.9) nanowires (diameters of 20-50 nm and lengths of >10 μm) coated with single-crystal aluminum (thickness of ~16 nm) has been fabricated. The method involves combined flame synthesis of tungsten-oxide nanowires and ionic-liquid electrodeposition of aluminum. The geometry not only presents an avenue to tailor heat-release characteristics due to anisotropic arrangement of fuel and oxidizer but also eliminates or minimizes the presence of an interfacial Al2O3 passivation layer. Upon ignition, the energetic nanocomposite exhibits strong exothermicity, thereby being useful for fundamental study of aluminothermic reactions as well as enhancing combustion characteristics.

In situ RBS measurements for the effect of swift heavy ion irradiation on metal–insulator interfaces

We have studied the effect of 200MeV Xe ion irradiation on Bi–Al2O3 and Au–Al2O3 interfaces by means of in situ Rutherford backscattering spectrometry (RBS). For the Bi–Al2O3 interface, we could observe a systematic change in RBS spectrum as a function of Xe ion fluence or the irradiation time. On the other hand, for Au–Al2O3 system, we could not observe any change in RBS spectrum. The experimental result confirmed our previous result that atomic mixing at Bi–Al2O3 interface surely occurs by the Xe ion irradiation. We analyzed the RBS spectra by the simulation software SIMNRA, and discussed the Bi–Al2O3 mixing quantitatively. From the irradiation time dependence of the mixing layer thickness, we estimated the effective diffusion constant for the atomic mixing at Bi–Al2O3 interface. Its value corresponds to the self-diffusion near the melting temperature of Al2O3. The result implies that Bi and Al2O3 are in molten forms during the ion irradiation, resulting in the atomic mixing at Bi–Al2O3 interface.

Nitrogen passivation at GaAs:Al2O3 interfaces

The effects of nitrogen passivation at interfaces between III–V semiconductors and high dielectric constant oxides are calculated using the GaAs:Al2O3 interface as a model. Nitrogen replaces As from interface sites. The N-N dimer is found to be unstable and breaks into N dangling bonds. The N dangling bond has states lying well below the bulk valence band edge, away from the band gap, due to the electronegativity of nitrogen. Thus nitrogen is an effective passivant because it removes the states of two candidate species away from the gap.

Correlation of optical emission and pressure generated upon ignition of fully-dense nanocomposite thermite powders

Ignition was studied experimentally for four nanocomposite materials prepared by Arrested Reactive Milling (2Al·3CuO, 2.35Al·Bi2O3, 4Al·Fe2O3, and 8Al·MoO3). Thin layers of the prepared powders were coated on an electrically heated Ni–Cr filament and ignited at heating rates between 200 and 17,000K/s in a miniature vacuum chamber. The ignition was monitored based on both photodiode and pressure transducer signals recorded simultaneously. Ignition temperatures were found to be weakly dependent of the heating rate and similar for all thermites (in the range of ∼800–950K) except for that using Bi2O3 as an oxidizer, which ignited at lower temperatures. Both maximum pressure and the rate of pressure rise (dP/dt) decreased with increasing heating rate. The onsets of pressure pulses produced by ignited thermites occurred prior to those of the respective emission pulses, with the effect increasing at higher heating rates. It was observed that 2.35Al·Bi2O3 did not ignite at heating rates above 5000K/s in vacuum, while it readily ignited when heated in air. A qualitatively similar observation was made for 8Al·MoO3, which could not be ignited in vacuum when heating rates reached 13,000K/s. The results are interpreted to suggest that the low-temperature redox reactions occurring prior to ignition modify the oxidizers, making them less stable and prone to rapid decomposition upon further heating. Ignition, triggered by a change in transport properties of the growing interfacial Al2O3 layer is not directly affected by the oxygen release from the decomposing oxides. Both oxygen release and growth of the interfacial Al2O3 layer are directly affected by the rates of low-temperature redox reactions preceding ignition.

Boron Laser Doping through High Quality Al2O3 Passivation Layer for Localized B-BSF PERL Solar Cells

A new doping process is investigated by using Excimer Laser Annealing (ELA) on various Atomic Layer Deposition (ALD)-Al2O3/PECVD-SiNx(B) passivation stack. In a first part the passivation quality of dielectric stacks is investigated on lowly doped p-type Si substrate. Similar passivation level is highlighted with Boron containing SiNx as compared to un-doped SiNx layer when a thin interfacial Al2O3 layer is first deposited on silicon.In a second part laser doping of the silicon substrate is highlighted by sheet resistance (Rsh) decrease. Pulse energy and pulse number influence the diffusion of Boron and Aluminum atoms from the dielectric stack into the silicon. Electro Chemical Voltage (ECV) profiles confirmed p+ region formation. XPS analysis confirmed the presence of both doping atoms in the p+ region. It is suggested that Al is rather bonded to N and O than to Si atoms while B plays a major role in the doping mechanism of the Si lattice.

The nature of the aluminum–aluminum oxide interface: A nanoscale picture of the interfacial structure and energy-level alignment

We propose a new structural model for the Al(111)/Al2O3(0001) interface based on density functional theory calculations. The ultrathin interface structure is shown to consist of two Al layers, one that is oxide-like and the other metal-like. Our model interface reproduces the barrier height to the oxide conduction band edge and predicts the oxide overlayer to lower the metal work function by 0.49eV.

Electron barrier height at CuxTe1 − x/Al2O3 interfaces of conducting bridge memory stacks

We addressed the possible variation of the electron barrier at the Cu0.6Te0.4/Al2O3 interface as compared to the pure Cu electrode case. The experiments were performed using structures fabricated by atomic layer deposition of an Al2O3 insulator on (100)Si or thermally oxidized (100)Si, followed by sputtering of Cu or Cu0.6Te0.4 top electrodes. These structures were used to observe electron photoemission from the metal layer into the oxide in order to determine the energy barrier for electrons between the Fermi level of the metal and the conduction band of the insulating layer. The electron barrier height at interfaces of Cu and Cu0.6Te0.4 electrodes is found to be the same but appears to be sensitive to the oxide type which is ascribed to the formation of a polarization layer.

Healing performance of Ti2AlC ceramic studied with in situ microcantilever bending

In situ microcantilever bending tests were carried out to evaluate the healing efficiency of pre-notched Ti2AlC ceramic after annealing at 1200 °C for 1.5 h. Microcantilevers of different orientations were fabricated with focused ion beam method at different locations, i.e. in a singular Ti2AlC grain, at a grain boundary or at the Ti2AlC–Al2O3 interface after healing. Ti2AlC microcantilever shows an anisotropic bending strength (ranging between 9.6 GPa and 4.6 GPa depending on the precise crystallographic orientations) that is closely related to the different atomic bonds in the layered structure. After healing, the Ti2AlC–Al2O3 microcantilevers exhibit almost the same strength of about 5.2 GPa, i.e. slightly higher than the cleavage strength (4.6 GPa) of the initial Ti2AlC microcantilevers. It suggests that the orientation of the matrix grain has no significant influence on the strength of healed microcantilevers. Furthermore, it turns out that the strength of the microcantilever with a healed grain boundary is at least twice the strength of the initial Ti2AlC cantilever with a grain boundary. It is concluded that the oxidation dominated self-healing mechanism of Ti2AlC ceramics can result in a perfect recovery of mechanical performance. The paper shows that the in situ microcantilever bending test provides a quantitative method for the evaluation of the strength of self-healing ceramics.

Surface plasmon polariton enhanced electroluminescence and electron emission from electroformed Al-Al2O3-Ag diodes

Electroforming of Al-Al2O3-Ag diodes results in voltage-controlled negative resistance (VCNR) in the current-voltage (I-V) curves. Electroluminescence (EL) and electron emission into vacuum (EM) develop simultaneously. The temperature dependence between 200 and 300 K of VCNR, EL, and EM of Al-Al2O3-Ag diodes with anodic Al2O3 thicknesses between 12 and 41 nm has been studied. I-V curves and VCNR are slightly temperature dependent. The voltage for the onset of EL, VEL, is between ∼1.3 and ∼1.9 V for the range of Al2O3 thicknesses, with small temperature dependence. The density of defects in anodic Al2O3 is >1.5 × 1020 cm−3. Defect conduction bands that form from excited states of F- or F+-centers, oxygen vacancies in Al2O3, determine the value of the barrier height at the Al-Al2O3 interface, ϕA, and they control EM. EM is anomalous. The threshold voltage for EM, VEM, is ∼1.9 to ∼2.5 V for the range of Al2O3 thicknesses, which is less than the work function of Ag, 4.6 eV. EM at 300 K is ∼10−9 A. As temperature is lowered, EM drops to ∼10−12 A at TD ≅ 290 K and recovers to ∼10−9 A at TR ≅ 260 K. The particular values of TD and TR depend on sample preparation and Al2O3 thickness. The source of anomalous EM is electrons that tunnel through the high field region at the Al-Al2O3 interface into defect conduction bands. They gain energy and momentum by combining with surface plasmon polaritons (SPPs) that are generated at the Al2O3-Ag interface by EL photons. EL from Al-Al2O3-Ag diodes with 12 nm or 15 nm of Al2O3 is much larger than EL from diodes with thicker Al2O3 layers. The conducting channel of electroformed diodes with the thinnest Al2O3 acts as a microcavity in which the electromagnetic field due to SPPs stimulates EL from defect centers by the Purcell effect.

Hf‐Doped Ni–Al2O3 Interfaces at Equilibrium

In this study, a series of solid‐state dewetting experiments of pure and Hf‐doped Ni films on sapphire and HfO2 substrates were conducted in order to measure the change in interfacial energy of the Ni–Al2O3 interface in the presence of Hf, and to study Hf interfacial segregation. It was found that Hf oxidizes under the conditions of the experiment (P(O2)=10−20 atm), Hf does not segregate to the Ni–Al2O3 interface, and that the interface energy of Ni–HfO2 is 2.7 ± 0.4 J/m2 vs 2.16 ± 0.2 J/m2 for the Ni–Al2O3 interface. This contradicts several theoretical studies that predict that Hf segregates to the interface to stabilize it thermodynamically. The solubility of Hf in bulk Ni was found to be significantly lower than the value reported in the equilibrium phase diagram.

Thermal instability of HfO2 on InP structure with ultrathin Al2O3 interface passivation layer

AbstractWe studied the thermal stability of HfO2 on an InP structure when an Al2O3 interface passivation layer (PL) was introduced. In contrast to the thick (∼4 nm) Al2O3‐PL, an almost complete disappearance of the thin (∼1 nm) Al2O3‐PL was observed after a post‐deposition anneal at 600 °C. Based on various chemical and electrical analyses, this was attributed to the intermixing of the thin Al2O3‐PL with HfO2, which might have been accompanied by the out‐diffusion of a substantial amount of substrate elements. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Interface barriers at the interfaces of polar GaAs(111) faces with Al2O3

Internal photoemission measurements of barriers for electrons at interfaces between GaAs(111) and atomic-layer deposited Al2O3 indicate that changing the GaAs polar crystal face orientation from the Ga-terminated (111)A to the As-terminated (111)B has no effect on the barrier height and remains the same as at the non-polar GaAs(100)/Al2O3 interface. Moreover, the presence of native oxide on GaAs(111) or passivation of this surface with sulphur also have no measurable influence on the GaAs(111)/Al2O3 barrier. These results suggest that the orientation and composition-sensitive surface dipoles conventionally observed at GaAs surfaces are effectively compensated at GaAs/oxide interfaces.

Electroforming and Ohmic contacts in Al-Al2O3-Ag diodes

Electroforming of metal-insulator-metal (MIM) diodes is a non-destructive dielectric breakdown process that changes the diode from its initial high resistance state (HRS) to a low resistance state (LRS). After electroforming, resistance switching memories (RSMs) use voltages to switch from HRS to LRS and back. Many MIM combinations are proposed for use in RSMs. In many cases conduction in the LRS is nearly temperature independent at low temperatures; an Ohmic contact with a barrier to electron injection of ∼0 eV results from electroforming. Electroforming of Al-Al2O3-Ag diodes with amorphous anodic Al2O3 thicknesses between 12 and 41 nm has been studied. Two anodizing electrolytes have been used; 0.1 M ammonium pentaborate (bor-H2O) and a solution of 0.1 M of ammonium pentaborate per liter of ethylene glycol (bor-gly). Polarization of Al2O3 and negative charge in Al2O3 are much larger when Al2O3 is formed in bor-H2O solution than when Al is anodized in bor-gly solution. Electroforming of Al-Al2O3-Ag diodes results in an Ohmic contact at the Al-Al2O3 interface, voltage-controlled negative resistance (VCNR) in the current-voltage (I–V) characteristics, electroluminescence (EL), and electron emission into vacuum (EM) from filamentary conducting channels. Two distinct modes of electroforming occur for Al-Al2O3-Ag diodes. α-forming occurs for 2.5 V ≲ VS ≲ 5 V, where VS is the applied voltage. It is characterized by an abrupt current jump with the simultaneous appearance of EL and EM. β-forming occurs for VS ≳ 7 V. I–V curves, EL, and EM develop gradually and are smaller than for α-forming. Electroforming occurs more readily for diodes with Al2O3 formed in bor-H2O that have greater defect densities. Fully developed I-V curves have similar VCNR, EL, and EM after α-forming or β-forming. A model is proposed in which excited states of F-centers, oxygen vacancies in amorphous anodic Al2O3, form defect conduction bands. Electroforming that results in an Ohmic contact requires injection of positive charge at the Al-Al2O3 interface. α-forming is the result of ionization of F-center recombination centers with energies that are close to the Al Fermi level. Hole injection by high-field ionization of valence band states of Al2O3 causes β-forming.

Influence of Ag Impurity Doping on the Microstructure and Hardness of Alumina Dispersion Strengthened Copper Alloys

Dispersion strengthened copper alloys (DSC) with high strength and high conductivity are widely used in automobile and electronics industries. In the present investigation the influence of Ag impurity doping on the mechanical properties of the alumina dispersion strengthened copper alloys was studied by mechanical alloying, and the influence mechanism was discussed subsequently. Alumina dispersion strengthened copper alloys powder was fabricated by high-energy ball milling. XRD results showed that the high-energy ball milling can dissolve Al2O3 into Cu matrix. As-milled powder was consolidated by cold-pressing and sintered at 860oC, 900oC, 940oC and 980oC, respectively. It was shown that the addition of Ag can significantly improve the hardness of Cu alloys. SEM characterization showed that the size of Al2O3 particles in Cu-Al2O3 sintered at 980oC was about 200nm, which is bigger than the initial size (about 150±30nm) of Al2O3 particles; and the size of Cu-Al2O3-Ag sintered at 980oC and 860oC was about 100nm and 30nm respectively. The difference in alumina size was considered as the result of Ag segregation at Cu/ Al2O3 interface. The segregation of Ag at Cu/ Al2O3 interface can suppress the growth of Al2O3, which will significantly improve the hardness of Cu alloys.