High Annealing Research Articles

Coronene‐Based 2D Networks by On‐Surface Skeletal Rearrangement of Sumanene Precursors

AbstractThe design of novel low‐dimensional carbon materials is at the forefront of modern chemistry. Recently, on‐surface covalent synthesis has emerged as a powerful strategy to synthesize previously precluded compounds and polymers. Here, we report a scanning probe microscopy study, complemented by theoretical calculations, on the sequential skeletal rearrangement of sumanene‐based precursors into a coronene‐based organometallic network by stepwise intra‐ and inter‐molecular reactions on Au(111). Interestingly, upon higher annealing, the formed organometallic networks evolve into two‐dimensional coronene‐based covalently linked patches through intermolecular homocoupling reactions. A new reaction mechanism is proposed based on the role of C−Au−C motifs to promote two stepwise carbon‐carbon couplings to form cyclobutadiene bridges. Our results pave avenues for the conversion of molecular precursors on surfaces, affording the design of unexplored two‐dimensional organometallic and covalent materials.

The Impact of Defect Density on Mechanical Characteristics of 4H-SiC Substrates

In this work the intrinsic and induced defects related to the mechanical strength of 4H-SiC wafer have been investigated by considering substrates having different dislocation density and subjected to different treatments such as thinning process and high temperature bulk and laser annealing. Three point bending test has been performed on die extracted from the substrates in order to calculate the stress σ the die can withstand at breakage (flexural strength). The variation of intrinsic defect density seems does not act to modify the material flexural strength. Conversely, a considerable correlation between the induced defect density and flexural strength has been found.

Ion Implantation into Nonconventional GaN Structures

Despite more than two decades of intensive research, ion implantation in group III nitrides is still not established as a routine technique for doping and device processing. The main challenges to overcome are the complex defect accumulation processes, as well as the high post-implant annealing temperatures necessary for efficient dopant activation. This review summarises the contents of a plenary talk, given at the Applied Nuclear Physics Conference, Prague, 2021, and focuses on recent results, obtained at Instituto Superior Técnico (Lisbon, Portugal), on ion implantation into non-conventional GaN structures, such as non-polar thin films and nanowires. Interestingly, the damage accumulation is strongly influenced by the surface orientation of the samples, as well as their dimensionality. In particular, basal stacking faults are the dominant implantation defects in c-plane GaN films, while dislocation loops predominate in a-plane samples. Ion implantation into GaN nanowires, on the other hand, causes a much smaller density of extended defects compared to thin films. Finally, recent breakthroughs concerning dopant activation are briefly reviewed, focussing on optical doping with europium and electrical doping with magnesium.

MOCVD of II-VI HRT/Emitters for Voc Improvements to CdTe Solar Cells

CdTe solar cells were produced using metal organic chemical vapour deposition (MOCVD), which employed a (Zn,Al)S (AZS) high resistant transparent (HRT) layer at the transparent conducting oxide (TCO)/Cd(Zn)S emitter interface, to enable the higher annealing temperature of 440 °C to be employed in the chlorine heat treatment (CHT) process. The AZS HRT remained intact with conformal coverage over the TCO after performing the high CHT annealing, confirmed by cross-section scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (STEM-EDX) characterisation, which also revealed the Cd(Zn)S emitter layer having been consumed by the CdTe absorber via interdiffusion. The more aggressive CHT resulted in large CdTe grains. The combination of AZS HRT and aggressive CHT increased open circuit voltage (Voc) and improved solar cell performance.

High Pressure Deuterium Passivation of Charge Trapping Layer for Nonvolatile Memory Applications.

In this study, the deuterium passivation effect of silicon nitride (Si3N4) on data retention characteristics is investigated in a Metal-Nitride-Oxide-Silicon (MNOS) memory device. To focus on trap passivation in Si3N4 as a charge trapping layer, deuterium (D2) high pressure annealing (HPA) was applied after Si3N4 deposition. Flat band voltage shifts (ΔVFB) in data retention mode were compared by CV measurement after D2 HPA, which shows that the memory window decreases but charge loss in retention mode after program is suppressed. Trap energy distribution based on thermal activated retention model is extracted to compare the trap density of Si3N4. D2 HPA reduces the amount of trap densities in the band gap range of 1.06–1.18 eV. SIMS profiles are used to analyze the D2 profile in Si3N4. The results show that deuterium diffuses into the Si3N4 and exists up to the Si3N4-SiO2 interface region during post-annealing process, which seems to lower the trap density and improve the memory reliability.

Kondo effect and superconductivity in niobium with iron impurities

Kondo effect is an interesting phenomenon in quantum many-body physics. Niobium (Nb) is a conventional superconductor important for many superconducting device applications. It was long thought that the Kondo effect cannot be observed in Nb because the magnetic moment of a magnetic impurity, e.g. iron (Fe), would have been quenched in Nb. Here we report an observation of the Kondo effect in a Nb thin film structure. We found that by co-annealing Nb films with Fe in Argon gas at above 400 ^{circ }C for an hour, one can induce a Kondo effect in Nb. The Kondo effect is more pronounced at higher annealing temperature. The temperature dependence of the resistance suggests existence of remnant superconductivity at low temperatures even though the system never becomes superconducting. We find that the Hamann theory for the Kondo resistivity gives a satisfactory fitting to the result. The Hamann analysis gives a Kondo temperature for this Nb–Fe system at sim 16 K, well above the superconducting transition onset temperature 9 K of the starting Nb film, suggesting that the screening of the impurity spins is effective to allow Cooper pairs to form at low temperatures. We suggest that the mechanism by which the Fe impurities retain partially their magnetic moment is that they are located at the grain boundaries, not fully dissolved into the bcc lattice of Nb.

Insights into temperature influence on analog/RF and linearity performance of a Si/Ge heterojunction asymmetric double gate dopingless TFET

The charge-plasma based dopingless (DL) tunnel field effect transistor (TFET) is considered as an emerging TFET structure resulting from its immunity against random dopant fluctuations and not requiring high thermal budgets and expensive annealing methods for fabrication. But, temperature sensitivity is a major concern to predict the reliability of a device as the bandgap of semiconductor material changes under the influence of temperature variations when used in a system. Therefore, in this manuscript, the effect of variations in temperature (200–500 K) are investigated on the analog/RF and linearity characteristics of a Si/Ge hetero-junction (HJ) asymmetric double gate (ADG) DLTFET and abbreviated as HJ-ADG-DLTFET in the entire manuscript. In this context, Silvaco ATLAS simulator is used to evaluate DC and Analog/RF performance parameters such as $$I_{D}-V_{G}$$ characteristics, transconductance ( $$g_{m}$$ ), cut off frequency ( $$f_{T}$$ ) and transconductance generation factor (TGF) considering effect of temperature variations. Furthermore, linearity parameters such as second- and third-order voltage intercept point ( $$\mathrm{VIP}_{2}, \mathrm{VIP}_{3}$$ ), 1-dB compression point, third-order input-interception point ( $$\mathrm{IIP}_{3}$$ ) and intermodulation distortion $$(\mathrm{IMD}_{3})$$ are also evaluated considering temperature variations as these FoM are significant for linear and analog/RF applications.

Growth of 4H-SiC recombination-enhancing buffer layer with Ti and N co-doping and improvement of forward voltage stability of PiN diodes

“Bipolar degradation” phenomenon has severely impeded the development of 4H-SiC bipolar devices. Their defect mechanism is the expansion of Shockley-type stacking faults from basal plane dislocations under the condition of electron-hole recombination. To suppress the “bipolar degradation” phenomenon, not only do the basal plane dislocations in the 4H-SiC drift layer need eliminating, but also a recombination-enhancing buffer layer is required to prevent the minority carriers of holes from reaching the epilayer/substrate interface where high-density basal plane dislocation segments exist. In this paper, Ti and N co-doped 4H-SiC buffer layers are grown to further shorten the minority carrier lifetime. Firstly, the dependence of Ti doping concentration on TiCl<sub>4</sub> flow rate in 4H-SiC epilayers is determined by using single-dilution gas line and double-dilution gas line. Then the p<sup>+</sup> layer and p<sup>++</sup> layer in PiN diode are obtained by aluminum ion implantation at room temperature and 500 ℃ followed by high temperature activation annealing. Finally, 4H-SiC PiN diodes with a Ti, N co-doped buffer layer are fabricated and tested with a forward current density of 100 A/cm<sup>2</sup> for 10 min. Comparing with the PiN diodes without a buffer layer and with a buffer layer only doped with high concentration of nitrogen, the forward voltage drop stability of those diodes with a 2 μm-thick Ti, N co-doped buffer layer (Ti: 3.70 × 10<sup>15</sup> cm<sup>–3</sup> and N: 1.01 × 10<sup>19</sup> cm<sup>–3</sup>) is greatly improved.

Preparation of Fe-Al alloy layer on CLAM steel

Reduced activation ferritic/martensitic steel is one of the candidate materials for tritium breeding module in the fusion reactor. In order to control the permeability of tritium in an acceptable range, coating with low hydrogen isotope permeability, known as tritium permeation barrier, is usually prepared on the surface of such structural materials. The FeAl/Al<sub>2</sub>O<sub>3</sub> is the first choice of tritium permeation barrier for many countries, because of its fine performance of high permeation reduction factor, corrosion resistance and high-temperature resistance. The surface morphology and microstructure of Fe-Al infiltrated layer have important influence on the quality of Al<sub>2</sub>O<sub>3</sub> coating. In this study, Al coating on the surface of CLAM steel is prepared by electroplating of aluminum from AlCl<sub>3</sub>-EMIC. Then the Fe-Al infiltrated layer is obtained by diffusion between Al and substrate by annealing. The effects of annealing time and temperature on the microstructure of Fe-Al infiltrated layer are studied by X-ray diffraction, scanning electron microscope and energy dispersive spectrometer. The results show that 20-μm-thick aluminum coating is obtained on the CLAM steel surface by electroplating. The Al coating is uniform and compact, and the size of its surface columnar grain decreases with electroplating current density increasing. Annealing results show that neither hole nor gap is observed between the Fe-Al infiltrated layer and the substrate. In addition, the infiltrated layer is found to be tightly bound to the substrate with a thickness ranging from 7 μm to 45 μm, depending on the annealing parameters. At the initial stage of annealing, Cr enriched Fe-Al alloy is formed evidently. However, such a Cr enrichment disappears at higher annealing temperature or longer annealing time due to diffusion. The surface of infiltrated layer changes from aluminum-rich phase to aluminum-poor phase, and its thickness increases with annealing time or temperature rising. The temperature dependence of the growth rate of Fe-Al infiltrated layer can be described by Arrhenius equation. At this time, the Arrhenius activation energy of aluminization on CLAM steel is calculated to be 78.48 kJ/mol. At 640 ℃ and 760 ℃, the growth of Fe-Al infiltrated layer is controlled by the grain boundary as well as the volume diffusion. When the reasonable thickness and microstructure of Fe-Al alloy layer are used and annealing time or temperature keeps as low as possible, the optimal annealing temperature and time are 700 ℃/10 h, respectively.

Effect of annealing temperature on the spectroscopic and photoluminescence properties of CdO-ZnO nanocomposites

The annealing temperature of CdO-ZnO (3:1) particles that were derived by using a sol–gel precipitation method has been optimized in order to improve the photoluminescence (PL) efficiency. The CdO-ZnO particles have been characterized by Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), impedance spectroscopy and UV–VIS spectroscopy. The transition point was found near 32°C with DSC for the highest annealing temperature (600°C). The impedance (Z) and phase angle (θ) as functions of frequency were recorded and the CdO:ZnO particles annealed at 450°C has the lowest 8.2–6.8 kΩ impedance. The reflectance spectra in the range of 300–800 nm were recorded and the optical bandgap was found varied from 2.76 to 2.99 eV. The excitation and emission PL spectra were obtained and the sample annealed at 600°C showed an intense blue emission with respect to the other samples.

Rapid Thermal Annealing for Surface Optimisation of ZnO Substrates for MBE-Grown Oxide Two-Dimensional Electron Gases

Two-dimensional electron gases (2DEGs) at the ZnO/ZnMgO interface are promising for applications in spintronics and quantum computing due to the combination of low spin-orbit coupling and high electron mobility. Growing high mobility 2DEGs requires high quality substrates with low impurity densities. In this work we demonstrate a ZnO substrate sample treatment combining high temperature rapid thermal annealing and chemical etching to improve the surface quality for the subsequent growth of 2DEGs. This process enables the growth of a 2DEG with low-temperature mobility of 4.8×104 cm2V−1s−1. An unannealed control sample shows a scattering rate at least three times greater than the annealed sample.

Effect of Simplified-Single-Step Microwave Annealing in O2 Ambient for High Performance Solution-Processed In–Ga–Zn–O Thin Film Transistors

In this study, we propose a simplified-single-step microwave annealing (S₃-MWA) technique in an O₂ ambient, which is a low thermal budget heat treatment method, for the application in solutionprocessed amorphous indium-gallium-zinc oxide (a-IGZO) thin films. For the application of solutionprocessed a-IGZO films in electronic devices, a multi-step post deposition annealing (PDA) process, which involves baking at low temperatures to vaporize the solvent, and high temperature conventional thermal annealing to remove defects in the film, is essential. To simplify the multi-step PDA process, we studied the possibility of reducing the thermal process temperature and time by replacing it with a single-step PDA process using microwave equipment. The electrical properties were compared to investigate the effect of the annealing method and ambient on solution-processed a-IGZO thin film transistors (TFTs). As a result, the S₃-MWA-processed a-IGZO TFTs were found to exhibit superior electrical characteristics in comparison with the conventional PDA-processed devices. It was found that the O₂ ambient process not only shortened the annealing time of S₃-MWA but also improved the electrical properties. Furthermore, the S₃-MWA was superior to the conventional PDA in the evaluation of device reliability under a gate bias stress test. The S₃-MWA process in the O₂ ambient was also responsible for improving the reliability of solution-processed a-IGZO TFTs. Therefore, we confirmed that the proposed S₃-MWA in the O₂ ambient is a more effective and promising technique than conventional PDA for the low thermal budget treatment of solution-processed a-IGZO TFTs.

Manipulations on mechanical properties of multilayer folded graphene by annealing temperature: a molecular dynamics simulation study

Annealing is a commonly used fabrication technology of graphene-assembled materials, which serves as an efficient method to control material properties. In graphene-assembled materials, the multilayer folded configuration of graphene has been widely observed due to the two dimensional characteristic of graphene. However, the manipulation on the mechanical properties of graphene-assembled materials by annealing has not been fully understood yet, especially considering the effect of folded microstructures. In this paper, we focus on the effect of annealing temperature on the mechanical properties of multilayer folded graphene. The dependences of elastic modulus, tensile strength, ultimate strain and fracture toughness on the annealing temperature have been systematically studied by molecular dynamics simulations. Moreover, the mechanisms behind the manipulations by annealing temperature have been revealed combining the structural evolutions obtained from the simulations. Our results indicate that the multilayer folded graphene after annealing under higher temperature exhibits significant reinforcement on its elastic modulus and tensile strength, while its ultimate strain drops instead. The fracture toughness is enhanced only within a certain range of annealing temperature. The controllable mechanical properties are attributed to the formation of interlayer covalent bonds between carbon atoms belonging to adjacent layers during the annealing processing. With the annealing temperature increases, more interlayer crosslinks are observed from simulations, which greatly strengthens the interlayer interaction. For the cases with lower annealing temperature, the folded graphene can be unfolded easily then finally flattened under tensile stretch, and the structural failure originates from the interlayer slippage in the folded area. However, for the cases with higher annealing temperature, the unfolding deformation is prevented since the folded graphene is blocked by much denser interlayer crosslinks, and the origins of structural failure transforms to the intralayer fracture in graphene plane. Considering the intralayer covalent bond interaction is far more powerful than the interlayer van der Waals interaction, the higher annealing temperature will bring higher elastic modulus and tensile strength due to the change on the structural failure mode, but it will sacrifice the ductility at the same time due to the blocked unfolding process of folded area. It is confirmed in our study that the annealing is an effective approach for the synthetic modulation on the stiffness, strength, ductility and toughness of multilayer folded graphene.

Effects of He+ and H+ Co-Implantation with High Energy on Blisters and Craters of Si and SiO2-On-Si Wafers

In this paper, effects of He+ and H+ co-implantation with high implantation energy on surface blisters and craters at different annealing conditions are systematically investigated. Surface morphology as well as defect microstructure are observed and analyzed by various approaches, such as scanning electron microscopy (SEM), optical microscopy (OM), atomic force microscopy (AFM), and Raman spectroscopy. It is found that after 500 °C annealing and above for 1 h, surface blisters and exfoliation are observed for Si and SiO2-on-Si wafers except for the samples implanted with only He+ ions. AFM images reveal that the heights of blisters in Si and SiO2-on-Si wafers are 432 nm and 397 nm respectively and the thickness of transfer layer is at the depth of about 1.4 μm, which is consistent with the projected range of He+ and H+ ions. Raman spectroscopy demonstrates that higher annealing temperature can lead to a stronger intensity of the VH2 peak. Under the same implantation parameters, surface morphology of Si and SiO2-on-Si wafers is different after annealing process. This phenomenon is discussed in detail.

State analysis of fluorine-doped SnO2 (FTO) by 57Fe Mössbauer spectroscopy

Fe doped FTO powders were prepared by a chemical precipitation method. Mossbauer spectra of FTO with several concentrations of Fe dopants were decomposed into two doublets (D1 and D2). A doublet with a small electric field gradient (D1) is assigned to Fe3+ in a small distorted site of the rutile structure, and another doublet (D2) to Fe3+ in a large distorted site due to F− ion and oxygen defect. Furthermore, with increase of the annealing temperature the isomer shifts tended to decrease. It suggests that Fe3+ bonds are more covalently bonded than ionic at the higher annealing temperatures. The surface layer of grains were also characterized by 57Fe CEMS. The defect structures of Fe doped FTO were estimated using the oxygen defect models of tin oxide [Mudarra Navarro et al., J. Phys. Chem. C, 119, 5596 (2015)].

The properties of the earth abundant Cu2SnS3 thin film prepared by spray pyrolysis and rapid thermal annealing route

Ternary compound Cu2SnS3 (CTS) was fabricated by simple, low cost and high efficient spray pyrolysis and rapid thermal annealing route. The influences of annealing temperature and annealing duration on the properties of the prepared CTS thin film were investigated in detail. X-ray diffraction spectrometer, Scanning Electron Microscope, UV–Vis–IR spectrophotometer and I–V test system were employed for the analysis of the structural, morphological, optical and photoelectric properties of the prepared CTS thin film, respectively. The characterization results show that the CTS thin film prepared under the optimal annealing condition of 500 °C and 15 min presents as tetragonal structure with preferential (1,1,2), bandgap value of 1.44 eV, crystal grain size of around 80 nm and photoelectric conversion efficiency as high as 1.24%.

Peculiarities of Phase Formation in TbBO3 during Isothermal Annealing

Detailed X-ray studies of changes in the structure of terbium orthoborate TbBO3 in the course of successive high-temperature isothermal annealing of the initial mixture in the form of an amorphous precursor and in the form of a homogenized mixture of microcrystalline Tb7O12 and B2O3 powders have been carried out. It is shown that the formation of TbBO3 crystals in both cases occurs through the formation of intermediate two-phase states. Particularly, the triclinic high-temperature ν-TbBO3 phase is formed already at the first crystallization stages (about 600°C) almost simultaneously with the equilibrium vaterite modification of TbBO3 (sp. gr. P63/mmc) during low-temperature annealing of an amorphous precursor, which transforms into the vaterite modification at an annealing temperature of 850°C. The phase of monoclinic terbium trioxoborate TbB3O6 is formed at the first stages of phase formation (about 800°C) almost simultaneously with the vaterite phase when the feedstock is annealed in the form of a homogenized mixture of microcrystalline Tb7O12 and B2O3 powders. It also transforms into the vaterite modification at an annealing temperature of 950°, which is preserved up to the highest annealing temperatures in the experiment (1200°C). A hypothetical explanation of the formation of such two-phase states during low-temperature annealing of the feedstock and their disappearance at higher annealing temperatures is proposed.

Understanding the physical properties of CdCl2 treated thin CdSe films for solar cell applications

Nowadays the traditional post CdCl2 thermal treatment to Cd-based thin films is an emerging route to improve the crystallinity and to passivate the grain boundaries which results into augmentation in physical properties of absorber layer. Therefore, in this communication an impact of CdCl2 thermal treatment on physical properties of e-beam deposited CdSe absorber layers has been investigated. The XRD results reveal that these films have dominant reflection (111) plane with cubic phase upto 320 °C and then cubic phase transformed into hexagonal phase with partial oxidation at higher annealing. The optical absorbance is found to be good and transmittance is observed to be very low in the visible region. Surface topological study indicates that the surface roughness is improved with the treatment while the observed Cd and Se along with Cl peaks in EDAX spectra to elemental analysis confirm the deposition of films and CdCl2 treatment. The ohmic nature of the films is visible in current-voltage behavior within the voltage range of −2 and +2 V. The chloride activated CdSe films processed at 320 °C might be anticipated as an efficient absorber-layer for the fabrication of Cd-based solar cells.

Effect of Annealing on Microstructure, Grain Growth and Hardness of Nanocrystalline Cu-Zr Alloy Prepared by Cryogenic Ball Milling

Nanocrystalline Cu-0.75 at.%Zr alloy was synthesized by high energy ball milling under cryogenic temperature. To investigate the influence of 0.75 at.%Zr addition on thermal stabilization of nanocrystalline state of Copper, milled powder was annealed up to T/Tm = 0.79 for 1h in an inert atmosphere. The microstructural changes of both milled and annealed powders were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Mechanical properties were determined in terms of hardness. It was found that addition of 0.75 at.%Zr can stabilize grain size at higher temperature, i.e., ~ 32 nm at 800oC (T/Tm = 0.79). The hardness of Cu-0.75 at.%Zr at 800oC was found to decrease by only ~ 13% as opposed to a 65% decrease in pure copper from cryomilled condition. The thermal stability of Cu-0.75 at.%Zr system at high temperatures was attributed to the kinetic stabilization, i.e., grain boundary pinning by intermetallic phases. Thermal stability contributions were assessed by thermodynamic models elicits added Zr is not sufficient for stabilization, rather kinetic stabilization (by intermetallic pinning of grain boundary) became active at higher annealing temperature.

Thermal annealing evolution to physical properties of ZnS thin films as buffer layer for solar cell applications

The conventional CdS window layer in solar cells is found to be hazardous for the environment due to toxic nature of the cadmium. Therefore, in order to seek an alternative, a study on effect of post-annealing treatment on physical properties of e-beam evaporated ZnS thin films has been carried out where films of thickness 150 nm were deposited on glass and indium tin oxide (ITO) substrates. The post annealing treatment was performed in air atmosphere within the temperature range from 100 °C to 500 °C. X-ray diffraction analysis reveals that the films on glass substrate are found to be amorphous at low temperature annealing (≤300 °C) while have α-ZnS hexagonal phase (wurtzite structure) at higher annealing. The patterns also show that the possibility of oxidation is increased significantly at temperature 500 °C which leads to decrease in direct band gap from 3.28 eV to 3.18 eV except films annealed at 300 °C (i.e. 3.39 eV). The maximum transmittance is found about 95% as a result of Doppler blue shift while electrical analysis indicated almost ohmic behavior between current and voltage and surface roughness is increased with post-annealing treatment.