Chemical Annealing Research Articles

Clean reconstructed InAs(1 1 1) A and B surfaces using chemical treatments and annealing

Well-ordered clean InAs(1 1 1) A and B surfaces have been prepared using HCl–isopropanol solutions and characterized using low-energy electron diffraction and photoemission spectroscopy. The as-treated surfaces are covered by a layer containing arsenic and small amounts of InCl x . Annealing induces desorption of the overlayer and reveals (2 × 2) and (1 × 1) structures on the A and B surfaces, respectively. For both surfaces, the surface components of the In 4 d and As 3 d reveal a charge transfer from the electropositive surface indium to the electronegative surface arsenic. The major advantage of this preparation method over conventional thermal cleaning is a significant reduction in the annealing temperature (≈250 °C) thereby avoiding anion evaporation.

Kinetics of ozone oxidation of oriented polypropylene and high-density polyethylene

The interrelation between the rate of ozone oxidation of oriented samples of PP and HDPE and the structural processes in the region of relaxation transitions at 50–70°C in PP and 40–60°C in HDPE was found. Within the studied temperature range, the annealing results in significant structural changes accompanied by the emergence of breaks in the temperature dependence of the rate of ozone oxidation. The chemical and temperature preannealing decreases the oxidation rate. After the chemical annealing at 96°C, the Arrhenius temperature dependence of the oxidation rate was observed.

Fabrication of integrated microchip for optical sensing by femtosecond laser direct writing of Foturan glass

By the one-continuous fabrication procedure of hollow microstructures using femtosecond (fs) laser direct writing followed by thermal treatment, successive chemical wet etching and additional annealing, three-dimensional integration of microoptics with microfluidics, i.e., a planoconvex microlens with a microfluidic chamber, in a single Foturan glass chip was achieved. Further integration of an optical waveguide was performed through internal refractive index modification by fs laser direct writing after the fabrication of the microlens and the microchamber. An “all-in-one” microchip that is highly effective for on-chip photonic biosensing can be manufactured by the present technique with easy assembly of each microcomponent and without any cumbersome processes for stacking and joining substrates. Experimental demonstration of photonic biosensing using the integrated microchip has revealed that fluorescence analysis and absorption measurement of liquid samples can be performed with efficiencies enhanced by factors of 8 and 3, respectively.

Fast Monte Carlo methodology for multivariate particulate systems—I: Point ensemble Monte Carlo

A fast Monte Carlo methodology for particulate processes is introduced. The proposed methodology combines concepts from discrete population balance equations and dynamic Monte Carlo simulations of chemical kinetics to construct a new jump Markov model that approximates the population balance dynamics. The Markov model consists of a definition of a new type of reaction channel, in which the reaction product is a stochastic process by itself. One feature of this model is that, although a coarse view of the process is taken, it still conserves the history of individual particles. This is a very important aspect for effective modeling of multivariate models, especially when part of the goal is to study the evolution of the internal states of the particles (i.e., composition, phase behavior, etc.). Numerical experiments show that this algorithm can improve the computational load of the exact method by orders of magnitude without sacrificing computational accuracy. The methodology is useful especially in stochastic optimization applications where many function calls (simulations) are required. Possible applications are optimization and dynamic optimization using an artificial chemical process algorithm, genetic algorithm, or simulated annealing among others.

Free energies of protein–protein association determined by electrospray ionization mass spectrometry correlate accurately with values obtained by solution methods

The advantages of electrospray ionization mass spectrometry (ESIMS) to measure relative solution-phase affinities of tightly bound protein-protein complexes are demonstrated with selected variants of the Bacillus amyloliquefaciens protein barstar (b*) and the RNAase barnase (bn), which form protein-protein complexes with a range of picomolar to nanomolar dissociation constants. A novel chemical annealing procedure rapidly establishes equilibrium in solutions containing competing b* variants with limiting bn. The relative ion abundances of the complexes and those of the competing unbound monomers are shown to reflect the relative solution-phase concentrations of those respective species. No measurable dissociation of the complexes occurs either during ESI or mass detection, nor is there any evidence for nonspecific binding at protein concentrations < 25 microM. Differences in DeltaDeltaG of dissociation between variants were determined with precisions < 0.1 kcal/mol. The DeltaDeltaG values obtained deviate on average by 0.26 kcal/mol from those measured with a solution-phase enzyme assay. It is demonstrated that information about the protein conformation and covalent modifications can be obtained from differences in mass and charge state distributions. This method serves as a rapid and precise means to interrogate protein-protein-binding surfaces for complexes that have affinities in the picomolar to nanomolar range.

Growth of crystalline grains in microcrystalline silicon films

Growth of crystalline grains has been studied using atomic hydrogen plasma exposure on thick$(\ensuremath{\sim}190\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ microcrystalline silicon films $(\ensuremath{\mu}\mathrm{c}\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H})$ with a low crystalline volume fraction $(\ensuremath{\sim}7%)$. Raman spectra reveal that the intensity near $520\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ increases after hydrogen exposure. Cross-sectional transmission electron micrographs of $\ensuremath{\mu}\mathrm{c}\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}$ films exposed to atomic hydrogen show the growth of crystalline grains extending from surface to bulk, approximately $100\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. These results suggest that crystal formation in $\ensuremath{\mu}\mathrm{c}\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}$ films is likely to be caused by chemical annealing.

Improving stability of amorphous silicon using chemical annealing with helium

We report on the growth and properties of amorphous silicon (a-Si:H) materials and devices prepared using a chemical annealing technique. The chemical annealing technique consists of a layer-by-layer growth, where a thin a-Si:H layer is followed by annealing in a plasma beam of inert ions (helium). We show that high quality materials and devices can be fabricated using chemical annealing with helium. Infrared absorption data show that chemical annealing significantly improves the hydrogen microstructure and this improved hydrogen microstructure leads to improved stability of both materials and devices.

Nucleation and growth study of atomic layer deposited HfO2 gate dielectrics resulting in improved scaling and electron mobility

HfO 2 films have been grown with two atomic layer deposition (ALD) chemistries: (a) tetrakis(ethylmethylamino)hafnium (TEMAHf)+O3 and (b) HfCl4+H2O. The resulting films were studied as a function of ALD cycle number on Si(100) surfaces prepared with chemical oxide, HF last, and NH3 annealing. TEMAHf+O3 growth is independent of surface preparation, while HfCl4+H2O shows a surface dependence. Rutherford backscattering shows that HfCl4+H2O coverage per cycle is l3% of a monolayer on chemical oxide while TEMAHf+O3 coverage per cycle is 23% of a monolayer independent of surface. Low energy ion scattering, x-ray reflectivity, and x-ray photoelectron spectroscopy were used to understand film continuity, density, and chemical bonding. TEMAHf+O3 ALD shows continuous films, density &amp;gt;9g∕cm3, and bulk Hf–O bonding after 15 cycles [physical thickness (Tphys)=1.2±0.2nm] even on H-terminated Si(100). Conversely, on H-terminated Si(100), HfCl4+H2O requires 50 cycles (Tphys∼3nm) for continuous films and bulk Hf–O bonding. TEMAHf+O3 ALD was implemented in HfO2∕TiN transistor gate stacks, over the range 1.2nm⩽Tphys⩽3.3nm. Electrical results are consistent with material analysis suggesting that at Tphys=1.2nm HfO2 properties begin to deviate from thick film properties. At Tphys=1.2nm, electrical thickness scaling slows, gate current density begins to deviate from scaling trendlines, and no hard dielectric breakdown occurs. Most importantly, n-channel transistors show improvement in peak and high field electron mobility as Tphys scales from 3.3 to 1.2nm. This improvement may be attributed to reduced charge trapping and Coulomb scattering in thinner films. Scaled HfO2 enables 1nm equivalent oxide thickness and 82% of universal SiO2 mobility.

Role of Hydrogen in the Grain Growth in Microcrystalline Silicon Films

AbstractThick microcrystalline silicon (mc-Si:H) films were exposed to atomic hydrogen plasma at substrate temperature of 220°C after deposition. The microstructure of μc-Si:H films after exposure was characterized using Raman back scattering spectroscopy and transmission electron microscopy (TEM). Raman spectra reveal that the intensity near 520 cm−1 significantly increases after hydrogen exposure, indicating an increase of crystallinity in the films. TEM micrographs of μc-Si:H films exposed to atomic hydrogen also show an increase in the size of grains and a growth of crystalline grains ranging from surface to bulk. These results suggest that crystalline grain formation in μc-Si:H films is likely to be caused by chemical annealing.

The atomic surface structure of SrTiO 3(0 0 1) in air studied with synchrotron X-rays

The atomic surface structure of single terminated SrTiO 3(0 0 1) (1 × 1) is investigated employing surface X-ray diffraction. In order to obtain these surfaces a special treatment is needed consisting of chemical etching and annealing. Since this is done in an aqueous and subsequently oxygen environment, after which the crystals are kept at ambient conditions, the surface is studied in air. Crystal truncation rods are measured and several models that are proposed in literature in recent years are tested against the experimental data. These models include surface rumpling, low temperature-like distortions, strontium adatom and lateral displacement distortions for both TiO 2 and SrO-terminated surfaces. None of these models represents the data very accurately. A much better fit to the experimental results is obtained by using a model in which a TiO 2-terminated crystal is covered by an oxygen layer.

Low temperature radiative properties of materials used in cryogenics

Radiative heat transfer between two parallel surfaces, a sample surface and a black surface, was measured. One of the surfaces was cooled with liquid helium to about 5 K and the other one was step by step heated to temperatures ranging between 30 and 140 K. As a result, the total hemispherical absorptivity and emissivity of the sample surface were determined in dependence on the temperature of the heat radiation. Aluminium samples were made of Al sheet, Al foil and aluminized mylar. Further measurements were performed on sheets of aluminium alloy, Cu, zinc brass and stainless steel. The influence of different types of sample treatment such as chemical and mechanical surface finishing and material annealing on the radiative properties is presented.

The effects of 1,3-cyclohexanebis(methylamine) modification on gas transport and plasticization resistance of polyimide membranes

We have identified that 1,3-cyclohexanebis(methylamine) (CHBA) can be effectively used as a new cross-linking agent for the chemical modification of polyimide membranes. We have also observed that the combined effects of diamino cross-linking and thermal annealing would significantly change the chemical compositions, micro-structure, gas sorption, gas transport properties and plasticization resistance of polyimide membranes. The membrane's physicochemical changes after diamino cross-linking and thermal annealing were characterized by FTIR-ATR, XPS, gel content, UV, SEM-EDX and TGA and the possible reaction mechanisms during chemical modification and annealing have been proposed. Interestingly, it is found that the chemical reactions between diamino and polyimides are reversible during thermal annealing and the membrane micro-structures are modified and charge transfer complexes (CTCs) are formed during the post-treatments. The gas sorption and gas transport properties of membranes before and after modifications are reported and discussed. The gas sorption concentration continuously decreases with an increase in the degree of cross-linking and the subsequent thermal annealing. Experimental results illustrate that thermal annealing not only improves CO 2/CH 4 selectivity of the cross-linked membranes but also greatly enhances the plasticization resistance by the formation of CTCs. The critical plasticization pressure is significantly improved from about 300 psia of the original samples to more than 720 psia of the cross-linked and 200 °C thermal treated samples. Based on the experimental results, a novel approach to enhance plasticization resistance of polyimide membranes by means of diamino cross-linking and followed by thermal annealing is elucidated.

Nanocrystalline Si Films and Devices Produced Using Chemical Annealing with Helium

AbstractWe report on growth of nanocrystalline Si:H films and devices using a layer-by-layer growth technique, where the growth of a thin amorphous layer by PECVD is followed by chemical annealing in a Helium plasma. The films and devices were grown using a remote, low pressure ECR plasma process. It was found that the structure of the films grown using the layerby-layer technique depended critically upon whether the annealing was done with hydrogen or helium, and the time taken to do the annealing. When the annealing was done in a hydrogen plasma, the films remained amorphous; in contrast, when the annealing was done in helium, and the annealing time was increased to 20 seconds from 10 seconds, the films became crystalline. The crystallinity of the films was confirmed using Raman spectroscopy and x-ray diffraction. The result obtained here shows that it is not necessary to have a high hydrogen dilution to obtain nanocrystalline films. Rather, the amount of hydrogen already present in an amorphous film is enough to cause crystallization, provided that enough ion flux and perhaps energy are available for converting the amorphous structure to a crystalline structure. Proof - of - concept p+nn+ junction devices were fabricated in these chemically annealed materials, and they showed classical nanocrystalline Si solar cell type behavior.

Atomic force microscopy studies of SrTiO3 (001) substrates treated by chemical etching and annealing in oxygen

The SrTiO3 (001) substrates treated by chemical etching in NH4F-buffered HF solution and annealing in oxygen ambient have been studied by an atomic force microscopy (AFM). The SrTiO3 substrates with TiO2-termineted and atomically smooth surfaces and single unit cell steps have been obtained. The surface morphologies of SrTiO3 substrates strongly depend on the treated conditions and the quality of the substrates.

Surface treatment for forming unit-cell steps on the (0 0 1) KTaO3 substrate surface

A method for the preparation of unit-cell-high steps on the (100) surface of KTaO3 has been developed. The effects of various surface treatments on the (100) KTaO3 surface, specifically the chemical etching and annealing characteristics, are discussed. Surface step formation was observed for KTaO3 single-crystals that were subjected to buffered HF etching followed by annealing in air. The resulting surface morphology of (001) KTaO3 was examined using atomic force microscopy (AFM). The role of etching time and annealing temperatures in determining the resulting step structure, roughness, and particulate formation on the KTaO3 surface is discussed.

Tunable Visible Photoluminescence from ZnO Thin Films Through Mg‐Doping and Annealing.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Preparation and properties of clean Si 3N 4 surfaces

In situ chemical methods for preparing atomically-clean surfaces of Si 3N 4 thin films in ultra-high vacuum (UHV) have been studied using X-ray and ultraviolet photoemission, electron energy loss and Auger electron spectroscopies. Prior to the UHV studies, the films (grown ex situ on Si(1 0 0) wafers by low-pressure chemical vapor deposition) were characterized using primarily infrared reflection–absorption spectroscopy. A combination of annealing in NH 3 to remove C and deposition of Si (followed by thermal desorption) to remove O is found to be an effective cleaning procedure. Other potential cleaning methods, such as annealing in UHV without in situ chemical treatment and annealing in a flux of H atoms, were also considered and found to be only partly effective. The clean surfaces are disordered but show no evidence of SiSi bonding (which would indicate N vacancies) in the Si LVV Auger spectrum or in surface-sensitive Si 2p photoemission data. Evidence for surface-related features is seen in the N 1s photoemission and in energy loss spectra in the region of valence excitations; however, no indication of occupied surface states near the valence band maximum is seen in ultraviolet photoemission spectra. Preliminary results for O 2 chemisorption show adsorbate-induced features in the Si 3N 4 band gap and also evidence for changes in surface potential due to adsorption.

Tunable Visible Photoluminescence from ZnO Thin Films through Mg-Doping and Annealing

Visible photoluminescence (PL) from ZnO has been found to be tunable in a wide range from blue to green and orange through chemical doping and annealing. Mg-doped, (Al, Li)-doped, and undoped ZnO thin films were deposited on glass substrates by a metal-organic decomposition method at temperatures around 600 °C. The films were annealed under different atmospheres, including air, oxygen, nitrogen, and hydrogen/nitrogen. X-ray diffraction analysis and field-emission scanning electron microscope observations revealed that the films consisted of large ZnO grains 50−100 nm in size. When the Mg-doped ZnO films were annealed in nitrogen or hydrogen/nitrogen, unusual blue or bluish-white PL, respectively, was observed in response to an ultraviolet light excitation. We confirmed the band-gap broadening (approximately 0.25 eV) of the Mg-doped ZnO films as compared to that of the undoped films through observation of the absorption edge. The blue-related PL therefore appeared to be caused by energetic shifts of the valence band and/or the conduction band of ZnO. Films annealed in the oxidizing atmospheres, on the other hand, showed yellow/orange PL. We ascribed this PL to electronic transitions between shallow and deep defect levels. Yellow PL was also observed in the (Al, Li)-doped ZnO films, suggesting that shallow donor/acceptor levels due to extrinsic defects also contributed to the yellow PL.

Chemical annealing of oxygen hole centers in bulk glasses

Chemical annealing of the oxygen hole centers in bulk glasses by using hydrogen (or deuterium) has been extensively reported in the literature. Hydrogen chemically anneals these defect centers by reacting with them to form hydroxyl species. We have here presented a reaction–diffusion model to predict the chemical annealing rates of these defect centers in bulk glasses. The model considers diffusion of hydrogen (or its isotopes, e.g., deuterium) through the glass and its simultaneous reaction with the oxygen hole defect centers, resulting in the formation of the hydroxyl species in the glass. The predictions from the model are in excellent agreement with the experimental data presented in [J. Appl. Phys. 60 (12) (1986) 4325]. The model also establishes the precise connection between the dependence of effective hydrogen diffusivity on its intrinsic diffusivity, temperature, hydrogen and defect concentration. We also discuss methods for understanding chemical annealing behavior at high temperatures, where hydrogen not only reacts with the oxygen hole centers, but also with the silica matrix. For the case where hydrogen interaction with the silica matrix is the dominant reaction, a reaction–diffusion model is presented and the equilibrium constant (between 500 and 900 °C) for the reaction is estimated by comparing the predictions from the model with the hydroxyl profiles reported in [Appl. Phys. Lett. 47 (3) (1985) 328; J. Appl. Phys. 61 (12) (1987) 5447].

Rigid amorphous silicon network from hydrogenated and fluorinated precursors in ECR-CVD

Abstract Structural rigidity in Si-network prepared from hydrogenated and fluorinated silicon precursors produced by low-pressure electron-cyclotron-resonance plasma has been studied. Chemical annealing of the Si-network by the atomic H of the plasma promotes the solid-state reactions for hydrogen elimination and network modification to obtain a rigid structure. However, Si-network produced from SiH 4 feedstock keeps on maintaining a mostly amorphous feature up to a large extent, on the contrary, a radical transformation of the network towards a more rigid microcrystalline structure happens to take place easily when SiF 4 is used as the source gas. F acts as an efficient scavenger of H from the network and enhances the network relaxation process significantly with the assistance of atomic hydrogen, and produces a rigid amorphous silicon network while used in limited dose for structural modulation.