Oxides Thin Films

Analysis of the total surface energy γ T and its three components as established by the van Oss-Chaudhury-Good Theory (vOCG) is conducted via Three Liquid Contact Angle Analysis (3LCAA). γ T is correlated with the composition of the top monolayers (ML) obtained from High-Resolution Ion Beam Analysis (HR-IBA). Control of γ T enables surface engineering for wafer bonding (Nano-Bonding TM) and/or epitaxial growth. Native oxides on boron-doped p-Si(100) are found to average γ T of 53 ± 1.4 mJ/m 2) and are always hydrophilic. An HF in methanol or aqueous HF etch for 60 s always renders Si(100) hydrophobic. Its γ T decreases by 20% to 44 ± 3 mJ/m 2 in HF in methanol etch and by 10% to 48 ± 3 mJ/m 2 in aqueous HF. On the contrary, GaAs(100) native oxides are found to always be hydrophobic. Tellurium n+-doped GaAs(100) yields an average of γ T of 37 ± 2 mJ/m 2 , 96% of which is due to the Lifshitz-Van der Waals molecular interactions (γ LW = 36 ± 1 mJ/m 2). However, hydrophobic GaAs(100) can be made highly hydrophilic. After etching, γ T increases by almost 50% to 66 ± 1.4 mJ/m 2. 3LCAA shows that the γ T increase is due to electron acceptor and donor interactions, while the Lifshitz-van der Waals energy γ LW remains constant. IBA combining the 3.039 ± 0.01 MeV oxygen nuclear resonance with <111> channeling, shows that oxygen on Si(100) decreases by 10% after aqueous HF etching, from 13.3 ± 0.3 monolayers (ML) to 11.8 ± 0.4 ML 1 hour after etch.Te-doped GaAs(100) exhibits consistent oxygen coverage of 7.2 ± 1.4 ML, decreasing by 50% after etching to a highly hydrophilic surface with 3.6 ± 0.2 oxygen ML. IBA shows that etching does not modify the GaAs surface stoichiometry to within 1%. Combining 3LCAA with HR-IBA provides a quantitative metrology to measure how GaAs and Si surfaces can be altered to a different hydroaffinity and surface termination.

In our previous work, we investigated the use of ion beam deposition (IBD) to grow epitaxial films at temperatures lower than those used in thermal processing (less than 500°C). Presently, we have applied IBD to the growth of dense (6.4×102 2 atom/cm3) silicon dioxide thin films at 400°C. Through these experiments we have found several clues to the microscopic processes leading to the formation of thin film phases by low energy ions. Using Monte‐Carlo simulations, we have found that low energy collision cascades in silicon have unique features such as a high probability of relocation events that refill vacancies as they are created. Our results show that the combination of a low defect density in low energy collision cascades with the high mobility of interstitials in covalent materials can be used to athermally generate atomic displacements tha can lead to ordering. These displacements can lead to epitaxial ordering at substrate temperatures below the minimum temperature necessary for molecular beam epitaxy (550°C). It can also lead to the formation of high quality silicon dioxide at temperatures well below that of thermal oxidation in silicon (i.e. <850°C). A growth model which we derive from these observations provides a fundamental understanding of how atomic collisions can be used to induce epitaxy or compound formation at low temperatures.

""Three different HF:alcohol solutions are investigated to etch native Si0 2 and passivate Si(100) surfaces with H which can the be desorbed at low temperature (T < 600'C). The resulting passivated Si(100) surfaces are compared using as a reference Si(100) passivated by a standard aqueous HF: solution (1:98 parts of HF: H2 0). After a modified RCA cleaning, Si(100) etched by HF:Methanol, HF:IPA, or HF:Ethanol, is characterized by Ion Beam Analysis (IBA), Tapping Mode Atomic Force Microscope (TMAFM), and Fourier Transform Infrared Spectroscopy (FTIR). The absolute coverage of 0 and C is measured by nuclear reaction analysis (NRA) combined with ion channeling at 3.05 MeV for 0 and 4.265 MeV for C. Hydrogen is measured via the elastic recoil detection (ERD) of 4He2+at 2.8 MeV.
Compared to aqueous HF, HF:alcohol passivates Si(100) leaving a lower 0 residue by an average factor of 0.62 and a similar C residue. H coverage is higher by an average factor of 1.43. Surface coverages are found to be reproducible in average by 1.4 x 1014 atoms/cm2 for C, and by 1.25 x 1014 atoms/cm2 for 0 when measured by IBA on samples identically processed. H coverage is reproducible within 5.5% when measured by ERD.
Selective area analysis by TMAFM shows that an increasing number of particulates is responsible for the apparent increase in root-mean -square (nns) surface roughness when the rms is measured over a whole image. Taking this effect into account, all passivated surfaces exhibit similar roughness when compared to the original Si(100) surface with little difference between
alcohols and with the reference aqueous HF solution. FTIR in the attenuation total reflection (ATR) mode detected SiHx species mostly as a
dihydride. Both IBA and FTIR detected significant levels of oxygen on surfaces passivated HF in alcohol and aqueous HF. This indicates that while Si(100) exhibits more H when passivated with HF in alcohol and can be desorbed at lower temperature than when treated with aqueous HF, H is
not bonded to Si only but likely bonds into a more complex surface termination, such as SiOH.""

"The water affinity of Si-based surfaces is quantified by contact angle measurement and surface free energy to explain hydrophobic or hydrophilic behavior of silicone, silicates, and silicon surfaces. Surface defects such as dangling bonds, surface free energy including Lewis acid-base and Lifshitz-van der Waals components are discussed. Water nucleation and condensation is further explained by surface topography. Tapping mode atomic force microscopy (TMAFM) provides statistical analysis of the topography of these Si-based surfaces. The correlation of the above two characteristics describes the behavior of water condensation at Si-based surfaces. Surface root mean square roughness increasing from several Å to several nm is found to provide nucleation sites that expedite water condensation visibly for silica and silicone. Hydrophilic surfaces have a condensation pattern that forms puddles of water while hydrophobic surfaces form water beads. Polymer adsorption on these surfaces alters the water affinity as well as the surface topography, and therefore controls condensation on Si-based surfaces including silicone intraocular lens (IOL). The polymer film is characterized by Rutherford backscattering spectrometry (RBS) in conjunction with 4.265 MeV 12C(α, α)12C, 3.045 MeV 16O(α, α)16O nuclear resonance scattering (NRS), and 2.8 MeV elastic recoil detection (ERD) of hydrogen for high resolution composition and areal density measurements. The areal density of hydroxypropyl methylcellulose (HPMC) film ranges from 1018 atom/cm2 to 1019 atom/cm2 gives the silica or silicone surface a roughness of several Å and a wavelength of 0.16 ± 0.02 μm, and prevents fogging by forming a complete wetting layer during water condensation.
Keywords: ERD, NRS, RBS, AFM, polymer film, silicone, silica, Si(100), contact angle, surface free energy, surface topography, intraocular implant."

Dr. Herbots, Belgian-born, Ph.D. graduate in applied physics of Catholic University of Louvain and a microelectronics specialist, made progress in the course of three years as an Oak Ridge Re- search Associate and four years as an MIT professor (1987-1991). At MIT she developed new methods for synthesizing thin film layers on chips at low temperatures, and for creating films that can’t be grown using thermal processes.

She joined Arizona State University in 1991,  to further develop a novel invention she had made at MIT. Called Combined Ion and Molecular Beam Deposition or CIMD, it is a  tech- nique for depositing materials on semiconductors that combines a so- phisticated evaporation system (molecular beam deposition) with an easily controlled ion beam.
But a life-threatening accident curtailed her research
After building a CIMD clean room at Arizona State University and getting research off to a good start, Nicole Herbot’s work, almost her life, came to an abrupt end in May 1994.  This is where the history of her research in ordered nanophases of SiO2 on Si(100) began. She had to rebuild her scientific understanding of matter,  interfaces, phase structures and nanophases after losing her short memory and developing "petit mal" (a milder form of epilepsy characterized by short or longer "absences" where the patient loses awareness of surroundings without loss of consciousness). The episodes increase with fatigue, mild illnesses and colds, to several a day.  They result in post-"absences" exhaustion, typical of epilepsy, where the patient needs to sleep for a period of time concomittant with the duration of the absences to recover.  The condition precludes driving.
The long hours spent on bed rest recovering from the petit mal absences gave her the time to reflect on the continuing mystery of SiO2/Si(100) extraordinary properties in microelectronics.  This is how her "Tsunami" model of structural collapse as Si(100) crystals oxidizes into an amorphous film came to be, and a new concept to prevent structure loss and thus grow heteroepitaxial SiO2 on Si(100) was conceived, and led to several patents (2003, 2010), PhD theses and papers.

We use ion beam analysis to probe the structure and interface of ultra-thin thermal oxide films grown on (1x1) Si(100) surfaces prepared using the Herbots-Atluri [U.S. patent No. 6,613,677,
(Sept. 2,2003)] wet chemical clean. We discover that these oxide layers are structurally registered with the substrate lattice with no interfacial structural disorder. Registry of Si atoms is most pronounced along <111>directions relative to the Si substrate, consistent with a beta-cristobalite epitaxial phase.This structurally registered phase transitions to an amorphous structure approximately 2 nm fromthe interface. ©
2006 American Institute of Physics

DOI: 10.1063/1.2358835

Due to their large magneto-optic responses, rare-earth-doped yttrium iron garnets, Y 3 Fe 5 O 12 (YIG), are highly regarded for their potential in photonics and magnonics. Here, we consider the case of Ce-doped YIG (Ce-YIG) thin films, in which substitutional Ce 3+ ions are magnetic because of their 4f 1 ground state. In order to elucidate the impact of Ce substitution on the magnetization of YIG, we have carried out soft x-ray spectroscopy measurements on Ce-YIG films. In particular, we have used the element specificity of x-ray magnetic circular dichroism to extract the individual magnetization curves linked to Ce and Fe ions. Our results show that Ce doping triggers a selective charge transfer from Ce to the Fe tetrahedral sites in the YIG structure. This, in turn, causes a disruption of the electronic and magnetic properties of the parent compound, reducing the exchange coupling between the Ce and Fe magnetic moments and causing atypical magnetic behavior. Our work is relevant for understanding magnetism in rare-earth-doped YIG and, eventually, may enable a quantitative evaluation of the magneto-optical properties of rare-earth incorporation into YIG.

Lanthanide-doped vanadate thin films offer (i) a promising platform for luminescence-based noncontact temperature sensing; (ii) ratiometric/self-referencing absolute measurements; (iii) exceptional repeatability and reversibility for multirun uses and a long life cycle; (iv) 2% K−1 maximum temperature sensitivity (among the highest recorded for inorganic nanothermometers); (v) a temperature resolution greater than 0.5 K; and (vi) the potential for high-resolution 2D temperature mapping.

The performance of Ti0.3W0.7 and Nb thin films as diffusion barriers for Au was investigated by Rutherford Backscattering Spectrometry (RBS) and Auger Electron Spectroscopy(AES). The films were sputter deposited in Ar:N2 (70:30 vol%) or pure Ar on amorphous SiO2. They were annealed in air at temperatures ranging from 250 °C up to 750 °C for several hours. In‐depth profiles revealed an onset of oxidation of the barriers at 520 °C for Nb and about 600 °C for TiW. Barrier oxidation and extensive diffusion could be correlated. Distinct diffusion behavior as a function of temperature was established between TiW and Nb. A Nb multilayer structure was found to provide the best reliability as the barrier and as the adhesion layer.

© 1984 American Vacuum Society
Received 15 September 1983 Accepted 31 October 1983
"

New dielectric materials based on SiGe have been formed at room temperature by direct ion beam oxidation and nitridation. Si0.8Ge0.2 layers were deposited by molecular beam epitaxy on Si(100) and then exposed to a low-energy ion beam of O-18-2+ to form oxides and N-14-2+ to form nitrides. The ion energies investigated ranged from 100 eV to 1 keV. Thin films of SiGe oxide and SiGe nitride were formed at all energies used as evidenced by in situ x-ray photoelectron spectroscopy analysis. They were found to be insulating by ex situ scanning electron microscopy observations. During the ion beam processing, the Ge content of the alloy layer decreases, due to preferential sputtering of Ge and the Ge compounds. However, as the ion energy is decreased, the concentration of Ge in the alloy remains closer to the original content. The thermal stability of these new SiGe dielectrics was also assessed up to 500-degrees-C.

The paper contains the investigation results on the structure and phase composition of the laser radiation area (LRA) of the U8 and U10 steels over its entire thickness. In the present study, the laser surface hardening of both U8 (ASTM-W1-7) and U10 (ASTM-W1-9) steels in the air was performed by exploiting a quasi-CW fiber laser with a 130 W power and 3 mm/s processing speed. The phase composition of the oxide layer formed as a result of laser treatment (LT) in air, as well as the structure of the oxide-metal interface on the surface of U8 (ASTM-W1-7) and U10 (ASTM-W1-9) carbon tool steels were studied by X-ray photoelectron spectroscopy (XPS). It was established that the thickness of the completely oxidized surface layers for U8 and U10 steels is 38.7 nm and 99 nm, respectively. The composition of the oxides of the steel surface after LT was determined. The presence of a wüstite-based film on U8 steel evidences the low wear properties of the LRA surface, while the thicker oxide layer of the modified U10 steel which contains Fe2O3 and Fe3O4 oxides with better strength properties, on the contrary, provides U10 steel surface with higher wear resistance. It was found that the wear rate of U10 steel modified surface decreases by more than two times, while the given value for U8 steel reduces no more than 17%. The paper reports the metallographic examination of the LRA structure. It was shown that the wear-resistant structural components that appeared after laser treatment lead to an increase in the deformation properties of steels. The maximum microhardness value of the LRA is 710 HV 0.1 for U8 steel and 750 HV 0.1 for U10 steel.

— To ensure good adhesion between a 200 nm thick silicon dioxide layer and a 4.5 μm thick hardcoat polymeric coating, a better understanding of mechanisms of adhesion at this interface is needed. To reach this purpose, focus is placed on two axes: characterizing mechanical properties of materials composing the system and in parallel, finding an applicable and effective method to quantify adhesion. Small dimension of SiO 2 thin film makes it challenging to accurately characterize it. Hence the use of both nano-indentation and AFM to attempt assessment of SiO 2 thin film elastic modulus Ef; taking into account limitations and uncertainty associated with each technique. Elastic modulus of SiO 2 thin film determined by nano-indentation is roughly 50 GPa on a wafer substrate and 15 GPa on a lens substrate. As for AFM, modulus measured is approximately 56 GPa on a wafer substrate and 22 GPa on a lens substrate. This highlights significant influence of substrate for both techniques. Impact on mechanical properties between SiO 2 thin films under different intrinsic stresses was also investigated. Results suggest that higher density of SiO 2 thin film leads to higher elastic modulus. To quantify adhesion, micro-tensile and micro-compression tests were performed. Micro-tensile experiments give ultimate shear strengths of hardcoat-substrate interface ranging from 9 to 14 MPa. Values of energy release rates of SiO 2 / Hardcoat, range from 0.1 J/m² to 0.5 J/m², depending on moduli values found on wafer or lens substrate. Keywords— Adhesion, mechanical properties, oxide thin film, polymer substrate.

The characterization of high temperature superconductor grain boundary junctions under a magnetic field in the mT range is reported. The devices were obtained by patterning narrow stripes in a YBa2Cu3O72x ~YBCO! film deposited on SrTiO3 and MgO symmetric bi-crystal substrates.
To allow the investigation of devices having different current density, two grain boundary disorientations were considered: 24° and 45° for SrTiO3 and 24° for MgO. Current-voltage characteristics as a function of the applied field for several temperatures have been collected. The experimental data are discussed on the basis of the electrical parameters obtained for the different substrates. The periodic modulation of the Josephson critical current with the magnetic field
indicates that the behavior of YBCO grain boundary junctions can be approximated by the standard overlap junctions model. A comparison between the experimental variation of the critical current with theoretical behavior allows for the determination of the current density distribution in the grain boundary as a function of the ratio of the largest junction dimension L and the Josephson penetration depth lJ . The nature of the barrier and the transport mechanism across the grain boundary is,
therefore, investigated; good agreement between the experimental results and the expected behavior of a superconductor-insulator-superconductor structure where the barrier is intrinsically defective is
observed. ©1997 American Institute of Physics.[S0021-8979(97)03213-1]

"Ion beam deposition (IBD) is the direct formation of thin films using a low-energy (tens of eV) mass-analyzed ion beam. The process allows depositions in which the energy, isotopic species, deposition rate, defect production, and many other beam and sample parameters can be accurately controlled. This paper will review recent research at ORNL on the IBD process and the effects of deposition parameters on the materials properties of deposited thin films, epitaxial layers, and isotopic heterostructures. A variety of techniques including ion scattering/channeling, cross-sectional transmission electron microscopy, scanning electron microscopy, and Auger spectroscopy has been used for analysis. The fabrication of isotopic heterostructures of 74Ge and 30Si will be discussed, as well as the fabrication of metal and semiconductor overlayers on Si and Ge. The use of IBD for low-temperature epitaxy of 30Si on Si and 76Ge on Ge will be presented. The use of self-ion sputter cleaning and in situ reactive ion cleaning as methods for preparing single-crystal substrates for epitaxial deposition will be discussed. Examples of IBD formation of oxides and suicides on Si at low temperatures will also be presented.

∗
Present address: Massachusetts Institute of Technology Cambridge, MA, USA.
Copyright © 1989 Published by Elsevier B.V."

Ion beam-assisted pulsed laser deposition with an Ar-oxygen ion mixture was used to prepare Ba 0.6 Sr 0.4 Ti 0.7 Zr 0.3 O 3 (BSTZ) thin films on Pt-coated Si substrates. The ion beam with an anode voltage of 600 V was effective to reduce the thermal budget, i.e., to achieve similar crystallinity with approximately 100 C lower deposition temperature compared to the cases without ionization. It was revealed that the dielectric properties (relative dielectric constant e r and its electric field tunability), out-of-plane lattice parameter of (001)-oriented grains (a 001 ), and the existence of (110)-oriented grains are correlated with one another. Elongation of a 001 was suppressed, resulting in large e r values comparable with that of a ceramic bulk of the same composition, in the BSTZ films that contain (110)-oriented grains. Less volume of amorphous BSTZ region is supposed to be playing an important role for the bulklike properties of these BSTZ films.

Intermetallic compound (IMC) layer growth at dissimilar metal interfaces, controlled by diffusion, usually follows a parabolic law. However, in the present study, the growth of Al 3 Ti layer on the Al-Ti interface at a low annealing temperature (T A , 500 °C) obviously didn't follow a parabolic law. A very long incubation stage of Al 3 Ti layer growth was observed in the Al-Ti diffusion couple annealed at 500 °C. Residual oxide layers and an oxygen rich layer were experimentally observed on Al-Ti and Al 3 Ti-Ti interface regions using high resolution STEM-EDS. The very long incubation stage is probably caused by the oxide layers and the oxygen rich layer on the interface region, which act as a diffusion barrier and as a source of oxygen atoms. The length of the incubation stage became shorter and even unobservable with increasing temperature probably due to the higher reaction rate between titanium oxides and aluminum at higher temperatures.

Atomically sharp oxide heterostructures often exhibit unusual physical properties that are absent in the constituent bulk materials. The interplay between electrostatic boundary conditions, strain and dimensionality in ultrathin epitaxial films can result in monolayer-scale transitions in electronic or magnetic properties. Here we report an atomically sharp antiferromagnetic-to-ferromagnetic phase transition when atomically growing polar antiferromagnetic LaMnO 3 (001) films on SrTiO 3 substrates. For a thickness of five unit cells or less, the films are antiferromagnetic, but for six unit cells or more, the LaMnO 3 film undergoes a phase transition to a ferromagnetic state over its entire area, which is visualized by scanning superconducting quantum interference device microscopy. The transition is explained in terms of electronic reconstruction originating from the polar nature of the LaMnO 3 (001) films. Our results demonstrate how new emergent functionalities can be visualized and engineered in atomically thick oxide films at the atomic level.

The atomic structure of "pinwheel" TiO ∼1.2 ultrathin oxide (w-TiO-UTO) layer and its reaction with gold are studied by scanning tunneling microscopy (STM) imaging and density functional theory (DFT) calculations. The UTO film was formed as an encapsulation layer on the top facet (111) of stripe-like Rh nanoparticles supported on a TiO 2 (110) substrate. For proposing a structural model, the previous STM, photoelectron (XPS), and ion scattering spectroscopy (LEIS) results were also taken into account. DFT calculations were carried out within the generalized gradient approximation (GGA-PBE) in the frame of the Quantum Espresso code. A Rh(111) slab of four layers with a TiO 1.14 overlayer and a Rh−Ti−O stacking sequence were used. In the starting model, the ratio between hcp and fcc sites filled with Ti atoms was 1.54 (the same value for O atoms was 2.2) on the top of Rh layers. The simulation of the STM images of the relaxed structure was done following the Tersoff− Hamann approximation. The main structural characteristics obtained experimentally were successfully reproduced in the simulation results: (i) the chemical contrast appeared as a pinwheel structure and (ii) compared with an ideal hexagonal lattice, characteristic local distortions were found in the UTO film. In harmony with the experimental results, the DFT calculations of the adsorption of a single Au atom on a w-TiO-UTO layer indicated that there is a characteristic site preference within the unit cell of the UTO film. This feature was also experimentally demonstrated for the early stage of the deposition of Au at room temperature, suggesting a moderate template effect adjusted by the pinwheel structure. This work demonstrates clearly that the lack of the so-called "nanoholes" does not completely cancel the template effect because the periodic lattice strain in itself substitutes their role. Moreover, the weaker modulation of Au/TiO-UTO bond permits the formation of one atomic layer thick 2D gold nanoparticles at 300 K.

The samples of the tool steel P6M5 were modified by means of laser irradiation (hereinafter-LO). The chemical composition of the sample surface before and after the LO was studied using the Auger and X-ray photoelectron spectroscopy (hereinafter-AES and XPS respectively). It was found that while the steel is exposed to LO, the thick oxide layer consisting mainly of the Fe 2 O 3 oxide is formed. It was established that the modification with LO leads to increasing of wear resistance and durability of the R6M5 steel because of a double reduction of the friction coefficient.

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