A study of the electrical characteristics,before electroforming,of thin SiOx films with laterally-spaced electrodes

The sample structure comprises two laterally-spaced electrodes joined by a thin dielectric film. The electrodes make a blocking contact to the dielectric with a barrier height ～ 0.75 eV. The voltage-current characteristic is studied between 213 to 413 K. Below room temperature the effect of hopping conduction on such contacts is explained. At room temperature and at high fields where hopping conduction is expected to be less effective, the conduction is controlled by the contact. Above room temperature and at low fields localized state conduction (hopping) becomes effective again with activation energy ～ 0.07 eV and an estimated charge carrier mobility of 1.3×10^?2cm²V^?1sec^?1. The density of ionizable impurities is estimated to be in the range 1 to 6×10¹⁸cm^?3 and the density of surface states is of the order of 1×10¹³cm^?2.     

Europium precipitates in monocrystalline NaCl

As-grown and well aged at room temperature NaCl crystals doped with divalent europium were systematically studied by using optical, dielectrical and mechanical methods. It has been found that at room temperature the optical absorption spectrum of -these crystals is strongly concentration-dependent. The role of the dopant concentration has been also evidenced by photoluminescence (taken at liquid nitrogen temperature) thermally stimulated depolarization current techniques and the mechanical response of the as-grown crystals. The experimental results obtained have been explained by assuming that the increase of the dopant concentration is accompanied by structural changes of the Eu²⁺-related precipitates from impurity-vacancy dipoles and small aggregates to orthorhombic EuCl₂ Of PbC₂-type.     

Flow stress of Eu doped KCl

The flow stress of Eu doped single crystal KCl has been measured as a function of the dopant concentration and thermal history. Samples that have been fast cooled from temperatures near 1000 K show a flow stress that is linear in the concentration. This result is consistent with a model in which isolated Eu⁺⁺-K⁺ vacancy dipoles undergo a Snoek ordering. Crystals aged at room temperature first show an increased strength due to aggregation of the dipoles into small clusters. After nine months at room temperature precipitation has taken place and the flow stress is lower than the original value. Heating the crystal at 1010 K restores the original flow stress and, presumably, the original degree of isolation for the Eu⁺⁺-K⁺ vacancy dipoles.     

Photoconductivity and photomemory in ZnXCd1–XIn2S4 layered crystals

Deep acceptor levels related to the ionization states of a double negatively charged center, A^2?, which cause high sensitivity in Zn_XCd_1–XIn₂S₄ layered crystals grown by chemical vapor deposition, have been investigated. The energy location, the capture cross section ratio and its dependence on the temperature have been determined by photoconductivity and thermal quenching experiments. The origin of a higher sensitivity and a region for the charge storage effect extending toward room temperature has been analyzed in crystals grown at the limit of existence of the quaternary layered phase. A possible origin of such centers due to zinc vacancy whose density increases with the partial substitution of cadmium by zinc atoms has been assumed to prove theoretical predictions and experimental results.     

Phenomenology of the “explosive” crystallization of sputtered non-crystalline germanium films

Certain non-crystalline germanium films (> 10 μm in thickness) prepared by rf-sputtering crystallize “explosively” at room temperature when initiated by pricking the surface with a sharp point (or certain other methods). The propagation velocity of the crystallization at room temperature was found to be as fast a 1200 mm sec^?1 depending somewhat on the conditions of film preparation, thickness, etc. The density of several such crystallizable films was determined as 5.05 g cm^?3±1%. The crystallite size in the crystallized films, measured by X-ray broadening, was typically larger than 500 Å. A model for the crystallization process invokes a cascade energy transfer process, basically thermal in nature.     

Crystal structure,magnetic susceptibility and electrical conductivity of pure and NiO-doped MoO2 and WO2

Single crystals of MoO₂ and WO₂, pure and doped with NiO, were grown by chemical transport in Vycor ampoules using iodine as the transporting agent. From X-ray diffraction analyses it was found that all the crystals were monoclinic, and that doping up to 5% does not change the unit cell constants. The pure dioxide crystals are weakly paramagnetic χ_M < 100×10^?6 emu, but doping raises the magnetic susceptibility markedly: ～ 2500×10^?6 emu. Resistance studies show that these materials are metallic conductors, the room temperature specific resistivity being of the order of 10^?4 – 10^?3 Ωcm, decreasing by one order of magnitude at liquid nitrogen temperature. Doping also substantially lowers the conductivity.     

Low temperature void growth and resistivity decay in thin evaporated gold films

A transmission electron microscope study revealed that a high density (10¹⁵?10¹⁷ cm^?3) of small (< 50 Å) voids are present in as-deposited gold films. These voids were found to grow at room temperature. This paper reports the growth of pre-existing voids in thin evaporated gold films at room temperature. The voids were imaged with the use of high resolution defocus contrast. The results of room temperature void growth are compared with the measured electrical resistance decay. A possible mechanism for the long term (a few hours to over 300 h) resistivity decay and void growth is discussed.     

Fabry–Pérot Oscillation and Room Temperature Lasing in Perovskite Cube‐Corner Pyramid Cavities

Recently, organometal halide perovskite‐based optoelectronics, particularly lasers, have attracted intensive attentions because of its outstanding spectral coherence, low threshold, and wideband tunability. In this work, high‐quality CH₃NH₃PbBr₃ single crystals with a unique shape of cube‐corner pyramids are synthesized on mica substrates using chemical vapor deposition method. These micropyramids naturally form cube‐corner cavities, which are eminent candidates for small‐sized resonators and retroreflectors. The as‐grown perovskites show strong emission ≈530 nm in the vertical direction at room temperature. A special Fabry–Pérot (F–P) mode is employed to interpret the light confinement in the cavity. Lasing from the perovskite pyramids is observed from 80 to 200 K, with threshold ranging from ≈92 µJ cm^?2 to 2.2 mJ cm^?2, yielding a characteristic temperature of T₀ = 35 K. By coating a thin layer of Ag film, the threshold is reduced from ≈92 to 26 µJ cm^?2, which is accompanied by room temperature lasing with a threshold of ≈75 µJ cm^?2. This work advocates the prospect of shape‐engineered perovskite crystals toward developing micro‐sized optoelectronic devices and potentially investigating light–matter coupling in quantum optics.     

An examination of tellurium ion-implanted GaAs by transmission electron microscopy

The effect of implanting high energy tellurium ions into single crystal GaAs has been investigated by transmission electron microscopy. Results from annealing experiments on specimens implanted at room temperature show that two anneal stages centred near 200 and 600° C occur, the actual anneal temperature being dose dependent. Non-crystalline surface layers were formed after implanting doses in the range 5×10¹³ to 5×10¹⁵ ions cm^?2 at room temperature. However, implanting similar doses at 180° C caused no change in crystallinity. Thus the low temperature anneal stage which is the epitaxial recrystallization of the non-crystalline material, does not occur for the higher implant temperature. The high temperature anneal stage is associated with the annealing out of tiny defects such as twins and/or stacking faults leaving a single crystal containing only dislocation loops. The anneal stages observed by electron microscopy correlate with changes in sheet resistivity and Rutherford backscattering measurements on similar material. It is suggested that changes in dislocation loop size and density at high temperatures are coincident with the attainment of electrical activity.     

The density and thermal conductivity of solid nitrogen and carbon dioxide

The density and thermal conductivity of solid nitrogen and carbon dioxide have been measured. The solids were condensed from the gas phase under cryopumping conditions over a wide range of deposition rates and temperatures. The density and thermal conductivity of solid nitrogen increased with increasing deposition rate and temperature but carbon dioxide showed different behaviour. At a deposition rate of 400 mg s^?1 m^?2 and a temperature of 108 K the density and thermal conductivity fell sharply, the density reaching a value which was only 20% of that of the bulk crystals.     

Rietveld analysis of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> thin films obtained by RF-sputtering

Calcium copper titanate, CaCu₃Ti₄O₁₂, CCTO, thin films with polycrystalline nature have been deposited by RF sputtering on Pt/Ti/SiO₂/Si (100) substrates at a room temperature followed by annealing at 600 °C for 2 h in a conventional furnace. The crystalline structure and the surface morphology of the films were markedly affected by the growth conditions. Rietveld analysis reveal a CCTO film with 100 % pure perovskite belonging to a space group Im3 and pseudo-cubic structure. The XPS spectroscopy reveal that the in a reducing N₂ atmosphere a lower Cu/Ca and Ti/Ca ratio were detected, while the O₂ treatment led to an excess of Cu, due to Cu segregation of the surface forming copper oxide crystals. The film present frequency -independent dielectric properties in the temperature range evaluated, which is similar to those properties obtained in single-crystal or epitaxial thin films. The room temperature dielectric constant of the 600-nm-thick CCTO films annealed at 600 °C at 1 kHz was found to be 70. The leakage current of the MFS capacitor structure was governed by the Schottky barrier conduction mechanism and the leakage current density was lower than 10^?7 A/cm² at a 1.0 V. The current–voltage measurements on MFS capacitors established good switching characteristics.     

Defects and acceptor centers in ZnO introduced by C+-implantation

ZnO single crystals were implanted with 280 keV C⁺ to a dose of 6 × 10¹⁶ cm^?2. Positron annihilation measurements reveal a large number of vacancy clusters in the implanted sample. They further agglomerate into larger size or even microvoids after annealing up to 700 °C, and are fully removed at 1200 °C. X-ray diffraction, photoluminescence, and Raman scattering measurements all indicate severe damage introduced by implantation, and the damaged lattice is partially recovered after annealing above 500 °C. From room temperature photoluminescence measurements, an additional peak at around 3.235 eV appears in the implanted sample after annealing at 1100 °C, which is much stronger than that of the free exciton. From the analysis of low temperature photoluminescence spectra, this peak is mostly a free electron to acceptor (e,A⁰) line which is probably associated with C _O .     

Pyroelectric properties of a new alanine- and phosphate-substituted triglycine sulphate (ATGSP) crystal

Phosphorous- and alanine-doped triglycine sulphate (ATGSP) crystals have been grown with partial substitution of H₂SO₄ with H₃PO₄. ATGSP crystals have lower permittivity (?_r ≈ 30) and higher pyroelectric coefficient (6.5 × 10^?4 C/K m²) compared to those of pure TGS. In the doping range used, the higher pyroelectric coefficient is traced to a significantly larger spontaneous polarization P_s (≈ 5 μC/cm² at room temperature). Tangent δ is below 0.01 over the whole frequency range from 100 Hz to 100 kHz.     

Thermal behaviour of helium in silicon carbide: Influence of microstructure

We have studied the microstructure dependence of He bubble formation in silicon carbide. Helium accumulation in SiC was performed by 500 keV ³He implantation at room temperature with a fluence of 5 × 10¹⁵ cm^?2. Depth concentration profiles have been investigated in 6H-SiC single crystals and α-SiC polycrystals by NRA spectrometry. Cross-sectional TEM samples have been imaged to study bubble formation. After annealing at 1300 °C, results clearly demonstrate an influence of grain boundaries on He retention yield in α-SiC polycrystals while helium is totally released from single crystals. Polycrystals also display the formation of intragranular overpressurized bubbles while no bubbles are observed in single crystals. Interpretations are proposed on the basis of the nature of He traps.     

Mechanical properties of nanostructed diamond-like carbon films synthesized by low energy cluster beam deposition

Mechanical properties of carbon-films obtained by low energy neutral cluster beam deposition (LECBD) have been measured using the nanoindentation technique. Three selected size distributions centered around C₂₀, C₆₀ and C₉₀₀ have been deposited on various substrates at room temperature. The films with a nanostructured morphology (grain size around 15 to 25 nm) conserve a memory of the specific character of the free clusters which is sp³ for C₂₀, sp² for C₉₀₀ and intermediate (sp^2.3) for C₆₀. The densities of the films (0.8 to 1.1 g.cm⁻³) are lower than most polymers but their hardnesses (3 to 12 GP a) are comparable to those of many DLC-films. These results are discussed taking into account the particular structures of the free clusters and the nucleation and growth mechanism specific for the LECBD technique, which corresponds nearly to a random stacking of incident clusters.     

Studies of hydrogen doped lithium nitride

Single crystals of hydrogen-doped lithium nitride have been grown and relative estimates of dopant concentrations obtained from infra-red transmission studies. This technique together with nuclear reaction analyses has also been used to characterize surface layers of lithium hydroxide/lithium carbonate which form readily on the crystals when exposed to the atmosphere. Complex plane analysis of a.c. conductivity data in the temperature range 25–200°C has enabled the separation of bulk crystal and surface layer properties. A value of 2.8 10^?3 Ω^?1 cm^?1 at 25°C, with an activation energy of 0.23 ev, for ionic transport perpendicular to the c-axis, within the bulk of the crystal, has been achieved.     

Few‐Layer Black Phosphorus Carbide Field‐Effect Transistor via Carbon Doping

Black phosphorus carbide (b‐PC) is a new family of layered semiconducting material that has recently been predicted to have the lightest electrons and holes among all known 2D semiconductors, yielding a p‐type mobility (≈10⁵ cm² V^?1 s^?1) at room temperature that is approximately five times larger than the maximum value in black phosphorus. Here, a high‐performance composite few‐layer b‐PC field‐effect transistor fabricated via a novel carbon doping technique which achieved a high hole mobility of 1995 cm² V^?1 s^?1 at room temperature is reported. The absorption spectrum of this material covers an electromagnetic spectrum in the infrared regime not served by black phosphorus and is useful for range finding applications as the earth atmosphere has good transparency in this spectral range. Additionally, a low contact resistance of 289 Ω µm is achieved using a nickel phosphide alloy contact with an edge contacted interface via sputtering and thermal treatment.     

High‐Coulombic‐Efficiency Carbon/Li Clusters Composite Anode without Precycling or Prelithiation

Lithium metal has attracted much research interest as a possible anode material for high‐energy‐density lithium‐ion batteries in recent years. However, its practical use is severely limited by uncontrollable deposition, volume expansion, and dendrite formation. Here, a metastable state of Li, Li cluster, that forms between LiC₆ and Li dendrites when over‐lithiating carbon cloth (CC) is discovered. The Li clusters with sizes in the micrometer and submicrometer scale own outstanding electrochemical reversibility between Li⁺ and Li, allowing the CC/Li clusters composite anode to demonstrate a high first‐cycle coulombic efficiency (CE) of 94.5% ± 1.0% and a stable CE of 99.9% for 160 cycles, which is exceptional for a carbon/lithium composite anode. The CC/Li clusters composite anode shows a high capacity of 3 mAh cm^?2 contributed by both Li⁺ intercalation and Li‐cluster formation, and excellent cycling stability with a signature sloping voltage profile. Furthermore, the CC/Li clusters composite anode can be assembled into full cells without precycling or prelithiation. The full cells containing bare CC as the anode and excessive LiCoO₂ as the cathode exhibit high specific capacity and good cyclic stability in 200 cycles, stressing the advantage of controlled formation of Li clusters.     

Temperature dependence of ferrobielastic switching in quartz

Using the piezoelectric effect, the variation of coercive stress with temperature for ferrobielastic switching in synthetic quartz crystals has been measured under unaxial load applied in a direction favoring Dauphiné twinning. It is found that the coercive stress decreases with increasing temperature from about 4×10⁸ Pa at room temperature to less than 10⁷ Pa near 300°C.     

Hierarchically Nanoperforated Graphene as a High Performance Electrode Material for Ultracapacitors

High performance is reported for a symmetric ultracapacitor (UC) cell made up of hierarchically perforated graphene nanosheets (HPGN) as an electrode material with excellent values of energy density (68.43 Wh kg^?1) and power density (36.31 kW kg^?1). Perforations are incorporated in the graphite oxide (GO) and graphene system at room temperature by using silica nanoparticles as template. The symmetric HPGN‐based UC cell exhibits excellent specific capacitance (Cs) of 492 F g^?1 at 0.1 A g^?1 and 200 F g^?1 at 20 A g^?1 in 1M H₂SO₄ electrolyte. This performance is further highlighted by galvanostatic charge–discharge study at 2 A g^?1 over a large number (1000) of cycles exhibiting 93% retention of the initial Cs. These property features are far superior as compared to those of symmetric UC cells made up of only graphene nanosheets (GNs), i.e. graphene sheets without perforations. The latter exhibit Cs of only 158 F g^?1 at 0.1 A g^?1 and the cells is not stable at high current density.