High-temperature Vacuum Annealing Research Articles

Lead-related quantum emitters in diamond

We report on quantum emission from Pb-related color centers in diamond following ion implantation and high temperature vacuum annealing. First-principles calculations predict a negatively-charged Pb-vacancy center in a split-vacancy configuration, with a zero-phonon transition around 2.3 eV. Cryogenic photoluminescence measurements performed on emitters in nanofabricated pillars reveal several transitions, including a prominent doublet near 520 nm. The splitting of this doublet, 2 THz, exceeds that reported for other group-IV centers. These observations are consistent with the PbV center, which is expected to have the combination of narrow optical transitions and stable spin states, making it a promising system for quantum network nodes.

On-chip sensor solution for hydrogen gas detection with the anodic niobium-oxide nanorod arrays

Two types of anodic niobium-oxide nanofilms were synthesized via anodization of an Al/Nb bilayer sputter-deposited onto a SiO2-coated Si wafer. Type I nanofilm was composed of a 200 nm thick NbO2 layer holding the upright-standing 650 nm long, 50 nm wide, and 70 nm spaced Nb2O5 nanorods, of 7·109 cm−2 density, whereas the Type II nanofilm had similarly long but bigger Nb2O5 nanorods, 100 nm wide, 220 nm spaced, and of 8·108 cm−2 density, aligned directly on a niobium metal without any buffering oxide layer, which was achieved for the first time. Each film was then incorporated in an advanced 3-D architecture and multilayer layout on a silicon chip comprising 33 microsensors, with variable sizes and tuned electrical characteristics, by combining the high-temperature vacuum or air annealing, sputter-deposition and lift-off photolithography to form Pt/NiCr top electrodes and a multifunctional SiO2 interlayer, chemical etching, laser dicing, and ultrasonic wire-bonding. The proposed on-chip sensor solution allowed for a sensitive, fast, and highly selective (toward NH3 and CH4) detection of hydrogen gas. Comprehensive gas sensing tests performed for Type II nanofilm ultimately confirmed the presence of a Schottky-type sensing mechanism, the contribution, however, being substantially weaker than that due to reactions over the surface of the oxide nanorods, especially when the rods show a transition from fully to partially depleted states when interacting with H2 gas. The film formation and chip fabrication technologies may be transferable to other PAA-assisted 1-dimensional metal-oxide nanomaterials suitable for on-chip gas sensing.

Role of Al2O3 Antireflective Layer on the Exceptional Durability of Mo–Si–N-Based Spectrally Selective Coatings in Air at High Temperature

Enhancing the stability of spectrally selective coatings (SSC) at high temperatures in air is necessary to push the concentrated solar power (CSP) technology to the next level. To avoid failures that might not be discovered for years, advanced knowledge of the chemical properties related to the degradation mechanisms with temperature is required. For this purpose, the Mo local environment is investigated here for Mo–Si3N4 and MoSi2–Si3N4 nanocomposites SSCs. The atomic short-range order around Mo proves that MoSi2 is the stable form that appears in high temperature vacuum annealing and that MoO3 formation is associated with optical degradation in air annealing. Deposition of Al2O3 capping layers with different techniques is found of paramount relevance in the long term performance of MoSi2–Si3N4 based SSCs. MoSi2–Si3N4 hybrid composite with Al2O3 capping layer shows exceptional functional stability even after 2900 h at 600 °C in air. Antireflective Al2O3 layer synthesized by atomic layer deposition (ALD) ...

Behavior of Plasma-doped Graphene upon High Temperature Vacuum Annealing

Behavior of Plasma-doped Graphene upon High Temperature Vacuum Annealing

Growth of SiC films on silicon substrate by cold implantation of carbon recoil atoms

A novel method of silicon carbide growth on silicon substrates is proposed. The method makes use of carbon recoil atoms implantation from a layer of molecules of carbon-containing gas, adsorbed on a cooled silicon surface bombarded by argon ions. A silicon carbide film is formed on the surface of a Si(1 1 1) substrate after high-temperature annealing in vacuum. The film properties are studied by IR-spectroscopy, XRD, AFM and optical microscopy methods. The studies have shown that the proposed method enables the growth of single-crystalline SiC films on the Si crystal surface.

Influence of Surface Faceting of RABiT-Type Metallic Substrate on Epitaxial Film Growth

Surface faceting is a well-known phenomenon occurring in either metallic or oxide materials. It is due to thermal etching favoured by high-temperature annealing in vacuum, and leads to the formation of steps or facets on the surface. The occurrence of surface faceting on cube-textured metallic substrates for YBCO coated conductor has been only seldom reported and its influence on the epitaxial growth of both buffer and superconducting layers has never been considered. In this paper, we studied the effect of the annealing conditions on the surface morphology of Ni- and Ni-Cu-based cube-textured metallic substrates and the influence of the obtained surface morphology on epitaxial film grown by both physical vapour and chemical solution deposition techniques. We confirm that annealing in reducing atmosphere suppresses both surface faceting and grain boundary grooving. Such a surface is beneficial for epitaxial film growth as it leads to smoother and featureless surfaces. On the other hand, from a microstructural point of view, film deposition on a faceted substrate shows a sharper orientation.

Post-spray modification of cold-sprayed Ni-Ti coatings by high-temperature vacuum annealing and friction stir processing

In this study, two post-spray treatments, i.e. high-temperature vacuum annealing (HTVA) and friction stir processing (FSP), were employed to modify the cold-sprayed (CS) Ni-Ti coatings, obtained from a mechanically blended powder. The phase transformation occurred at different HTVA temperatures, but with the formation of pores due to the Kirkendall effect. FSP proved to provide a new opportunity for the formation of intermetallic phases in the CSed Ni-Ti coating. A comparison of the microstructures, microhardness and tribological behaviors between the CSed and FSPed coatings, reveals that the microstructure of the modified Ni-Ti coating exhibits a mechanically alloyed layer with the in-situ synthesized Ni-Ti intermetallics, and no obvious defects are found. The microhardness of the improved layer is increased to 1003.5 ± 65.9 HV0.1, which is about 4.5 times compared to the CSed coating (222.5 ± 6.6 HV0.1). In addition, the mechanically alloyed layer by FSP contains multiplicate Ni-Ti intermetallic compounds, exhibiting better wear resistance than that of the CSed coating, thus proving the effectiveness of FSP on the microstructural modification and properties improvement for the CSed metal-metal composite coatings.

Structure and composition peculiarities and spectral-luminescent properties of colorless and pink Bi4Ge3O12 scintillation crystals

Czochralski-grown colorless and pink Bi4Ge3O12 (BGO) single crystals have been characterized by the X-ray diffraction, X-ray absorption, and luminescence and optical spectroscopy. Additional high-temperature vacuum annealing of the as-grown colorless crystal at 900–950°C using a special reducing agent leds to the appearance of pink coloration and modification of its properties. The refinement of actual compositions of both BGO crystals showed the distribution of the Bi3+ions over two, tetrahedral and octahedral, sites, the Bi content in the colorless crystal being greater and lower in the octahedral and tetrahedral sites, respectively, compared with the colored crystal. A small amount of Bi5+ions was revealed in the tetrahedral sites of the colorless crystal. Oxygen vacancies, as a part of the color center responsible for the crystal coloration, were found in the structure of pink sample. The pink BGO crystal is characterized by photoluminescence in the near infra-red spectral range.

Influence of defects on electrical properties of electrodeposited co-doped ZnO nanocoatings

We present a systematic investigation of the electrical properties of undoped and Co-doped ZnO nanostructures at room temperature as an extensive study of the role of defects in ZnO. The ZnO nanostructures were fabricated by the electrodeposition method at low bath temperature (80 °C) and the Co concentration was varied from 0.01 to 0.2 mM. Electrical properties of the undoped and Co-doped ZnO nanostructures were studied in detail. The carrier concentration increases while the mobility reduces with increase in Co-concentration. The resistivity increases with an increase in Co-concentration and the reason is correlated with the defects in ZnO. In order to understand more details of the role of defects in the present I–V characteristic behavior of the Co-doped ZnO, high temperature vacuum annealing of ZnO sample was carried out. Electrical, optical and magnetic properties of the high temperature vacuum annealed ZnO were studied in detail. Photoluminescence spectroscopy (PL) results revealed more information of the defect levels which act as scattering centers for the carriers. Co-doping as well as annealing at high temperature in vacuum environment tunes the defects in ZnO and which influence the optical, magnetic and electrical behavior of the ZnO nanostructures.

Subsurface hydrogen bonds at the polar Zn-terminated ZnO(0001) surface

The role of hydrogen and other defects in the stabilization of polar oxide interfaces is a matter of significant fundamental and practical interest. Using experimental (scanning tunneling microscopy, x-ray photoelectron spectroscopy) and theoretical (density functional theory) surface science techniques, we find that the polar Zn-terminated ZnO(0001) surface becomes excessively Zn deficient during high-temperature annealing (780 K) in ultrahigh vacuum (UHV). The Zn vacancies align themselves into rows parallel to the $[10\overline{1}0]$ direction, and the remaining surface Zn ions alternately occupy wurtzite (hcp) and zinc-blende (fcc) lattice positions, giving a characteristic ``striped'' $c(\sqrt{12}\ifmmode\times\else\texttimes\fi{}\sqrt{12})\mathrm{R}30$\ifmmode^\circ\else\textdegree\fi{} surface morphology with three types of rows: wurtzite Zn, zinc-blende Zn, and Zn vacancies. Interstitial H plays a central role in such a reconstruction, as it helps to compensate the excessive Zn deficiency. We propose a model in which hydrogen occupies positions in half of the vacancy rows to form hydroxide ions that can participate in hydrogen bonds in the O subsurface layer as a result of the mixed wurtzite/zinc-blende stacking.

Lattice Dynamics of the Rhombohedral Polymorphs of CaSi2.

The structures of two trigonal-rhombohedral CaSi2 polymorphs (space group R3̅m) were studied by X-ray diffraction and polarized Raman scattering spectroscopy. Raman-active even-parity vibrational modes of A1g and Eg are unambiguously identified and assigned to the specific lattice eigenmodes. Experimental data are found to be in very good agreement with those predicted by density functional theory lattice dynamics calculations. The transformation of 6R structural modification of CaSi2 into its 3R polymorph, by high-temperature annealing in vacuum is also reported.

Impact of Post-Oxide Deposition Annealing on Resistive Switching in HfO<sub>2</sub>-Based Oxide RRAM and Conductive-Bridge RAM Devices

In this letter, we report the impact of post-oxide deposition annealing on the performance of HfO2-based resistive random access memory (RRAM) devices, namely, oxygen-ion-based oxide RRAM (OxRRAM) and Cu-ion-based conductive-bridge RAM (CBRAM) devices. Considerable degradation of the ON/OFF ratio and switching properties was observed in the OxRRAM devices after high-temperature annealing in vacuum, which is attributed to a considerable amount of oxygen vacancies created in the switching layer. However, the substantial improvement in device performance, such as stable switching, high switching cycles, decreased set/reset voltages, and near-100% device yield, were obtained in the CBRAM devices during the post-HfO2 deposition annealing in vacuum.

Raman and XPS analyses of pristine and annealed N-doped double-walled carbon nanotubes

N-doped single/multi-walled carbon nanotubes (CNTs) were studied for long time from synthesis to properties. However, the stability of N in the CNT lattice still needs further developments. In this work, to obtain more stable N-doped CNTs, concentric double-walled (DW) CNTs with more N were synthesized using benzylamine as C and N source. In order to test the stability of N-doped DWCNTs, high-temperature annealing in vacuum was performed. By XPS and Raman spectroscopic measurements, we found that the N-doped DWCNTs are still stable under 1500 $\,^{\circ}\mathrm{C}$: the graphitic N does not change at all, the molecular N is partly removed, and the pyridinic N ratio greatly increases by more than two times. The reason could be that the N atoms from the surrounded N-contained materials combine into the CNT lattice during the annealing. Compared with the undoped DWCNTs, no Raman frequency shift was observed for the RBM, the G-band, and the G'-band of the N-doped DWCNTs.

N‐V‐related fluorescence of the monoenergetic high‐energy electron‐irradiated diamond nanoparticles

Fluorescent diamond nanoparticles (FND) have recently been introduced as promising luminescent probes for bioimaging to compete with more commonly used fluorophores and quantum dots. In this work, we investigate the formation of NV color centers in diamond nanocrystallites using monoenergetic electrons. A large quantity (1.4 g) of FNDs has been irradiated in the cyclic relativistic electron accelerator (Microtron) with the surface charge up to 3C/cm2 using collimated accelerated electrons extracted with monochromatic energies 6−25 MeV. The nitrogen‐vacancy (NV) color centers have been activated by the high temperature vacuum annealing followed by the oxidation and sonification to remove sp2 carbon from the surface and to form stable colloid solutions with the concentration 1 mg/ml and the electro‐kinetic (zeta) potential about −35 mV. The steady state fluorescence spectra show that the fluorescence yield increases linearly with the surface charge irradiation.

Investigation of thermal stability and reliability of HfO2 based resistive random access memory devices with cross-bar structure

The effect of the annealing treatment of a HfO2 resistive switching layer and the memory performance of a HfO2-based resistive random access memory (cross-bar structure) device were investigated. Oxygen is released from HfO2 resistive switching layers during vacuum annealing, leading to unstable resistive switching properties. This oxygen release problem can be suppressed by inserting an Al2O3 thin film, which has a lower Gibbs free energy, between the HfO2 layer and top electrode to form a Ti/Al2O3/HfO2/TiN structure. This device structure exhibited good reliability after high temperature vacuum annealing and post metal annealing (PMA) treatments. Moreover, the endurance and retention properties of the device were also improved after the PMA treatment.

Efficient fabrication method for nitrogen-vacancy color centers for a biomarker

Nitrogen-vacancy (NV) color centers in nanodiamond (ND) are fabricated through three steps, high-energy ion irradiation on ND, high-temperature (800 ℃) annealing in vacuum and lowtemperature (500 ℃) annealing in air. Raman measurements show that a surface-graphite layer is formed after annealing at 800 ℃ in vacuum while the graphite formed during ion irradiation is below the detection limit. Annealing at 500 ℃ in air is a crucial step in removing the graphite layer from ND that is ion-irradiated and 800 ℃-annealed. A strong luminescence from NV color center is observed only after annealing at 500 ℃ in air.

Ultra-smooth and lattice relaxed ZnO thin films

The crystal structure and quality of ZnO thin films were enhanced by high temperature vacuum annealing. High quality ZnO thin films have been grown on a-plane sapphire substrate by radio frequency (rf) magnetron sputtering method at a substrate temperature of 600°C. A remarkable improvement in the epilayer quality were established by in situ high temperature annealing. The film quality, smoothness, the in plane stress, and the degree of epitaxy of the films have been evaluated. The crystalline quality was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman spectroscopy analyses. An extremely smooth ZnO films were achieved at higher annealing temperatures with root mean square roughness of 0.3nm. The transverse optical mode A1(TO) observed in all the samples and the longitudinal optical mode A1(LO) appeared only at higher annealing temperatures over 800°C in the micro-Raman scattering measurements. The strain of c-axis were relaxed and the lattice parameter was comparable to that of bulk ZnO at high annealing temperature of 900°C.

Photocatalytic activity of anatase and rutile TiO2 epitaxial thin film grown by pulsed laser deposition

Epitaxial rutile-TiO2(011) and anatase-TiO2(001) films have been grown by pulsed laser deposition on Al2O3(1¯102) and LaAlO3(100), respectively. For low growth rates of ~0.0012nm/s and growth temperatures of 600°C high quality films are obtained. In particular the anatase films exhibit 100nm wide flat terraces for 10–15nm film thickness. Atomic steps of both ~0.25 and ~0.5nm are measured, suggesting the existence of two different terminations, although only a 4×1 reconstruction is observed in reflection high-energy electron diffraction. The rutile films exhibit coherent twin domain boundaries, which is likely the reason for smaller terrace sizes compared to anatase films. Photocatalytic activity of the films is measured by decomposition of methyl orange solution under UV-irradiation. The anatase films are about twice as active as the rutile films. For both films the activity does not change significantly for films of 10nm and thicker. For rutile films the activity is also compared to that of single crystal rutile TiO2(011) samples. Both rutile-films and single crystals exhibit very similar activity, indicating the good quality of the films. Post-growth annealing of the films in air at 600°C decreases the photocatalytic activity of the films. On the other hand, reduction of thin films by vacuum annealing to 800°C does not change the photocatalytic activity of the films significantly compared to the as grown films. This is in contrast to single crystal rutile samples whose photocatalytic activity for the decomposition of methyl orange almost doubles after reduction by high-temperature annealing in vacuum compared to the stoichiometric bulk-samples. The differences between films and bulk samples may hint differences in defect formation due to the substrate interface.

Investigation of Commercially Pure Titanium Structure during Accumulation and Release of Hydrogen by Means of Positron Lifetime and Electrical Resistivity Measurements

In this work the goal was to study the behavior of titanium-hydrogen system with hydrogen concentration of 1 wt. % at high temperature vacuum annealing by means of positron lifetime (PL) spectroscopy and electrical resistivity measurements. The range of hydrogen concentrations under investigation requires the γ phase formation. The study is related with the X-ray diffraction (XRD) and thermal desorption spectra (TDS) analysis. The registered positron lifetime spectra were analyzed by multiexponential decomposition into two components. Correlation of changes of the parameters of the PL spectrum and the electrical resistivity is revealed in this paper.

High Temperature Vacuum Annealing and Hydrogenation Modification of Exfoliated Graphite Nanoplatelets

Highly active defect sites on the edges of graphene automatically capture oxygen from air to form various oxygen groups. A two-step procedure to remove various oxygen functional groups from the defect sites of exfoliated graphene nanoplatelets (GNPs) has been developed to reduce the atomic oxygen concentration from 9.5% to 4.8%. This two-step approach involves high temperature vacuum annealing followed by hydrogenation to protect the reduced edge carbon atoms from recombining with the atmospheric oxygen. The reduced GNPs exhibit decreased surface resistance and graphitic potential-dependent capacitance characteristics compared to the complex potential-dependent capacitance characteristics exhibited by the unreduced GNPs as a result of the removal of the oxygen functional groups present primarily at the edges. These reduced GNPs also exhibit high electrochemical cyclic stability for electrochemical energy storage applications.