Effect of radiation on the characteristics of MIS structures containing rare-earth oxides

The results of experimental studies of the effect of γ-irradiation on the electrical properties of MIS structures containing the rare-earth oxides Y₂O₃, Dy₂O₃, Tb₂O₃, Gd₂O₃, and Lu₂O₃ are reported. The static characteristics (current-voltage, capacitance-voltage) and dynamic characteristics (transient characteristics, diagrams of oscillatory regimes) of the structures before and after irradiation with doses D=10⁴–10⁶ rad are examined. It is found that the irradiation dose D=10⁶ rad does not produce any substantial degradation of the characteristics of the structures. The radiation-induced changes observed in the experimental samples are consistent with existing data for MIS structures with SiO₂ as the insulator. Fiz. Tekh. Poluprovodn. 31, 885–888 (July 1997)     

Structural Determinants of the Sign of the Pyroelectric Effect in Quasi‐Amorphous SrTiO3 Films

The magnitude and direction of the permanent electric polarization in the non‐crystalline, polar phase (termed quasi‐amorphous) of SrTiO₃ in Si\SiO₂\Me\SrTiO₃\Me, (Me = Cr or W), Si\SrRuO₃\SrTiO₃, and Si\SrTiO₃ layered structures were investigated. Three potential sources of the polarization which appears after the material is pulled through a temperature gradient were considered: a) contact potential difference; b) a flexoelectric effect due to a strain gradient caused by substrate curvature; and c) a flexoelectric effect due to the thermally induced strain gradient that develops while pulling through the steep temperature gradient. Measurements show that options a) and b) can be eliminated from consideration. In most cases studied in this (Si\SrTiO₃, Si\SiO₂\Me\SrTiO₃\Me, M = Cr or W) and previous works (Si\BaTiO₃, Si\BaZrO₃), the top surface of the quasi‐amorphous phase acquires a negative charge upon heating. However, in Si\SrRuO₃\SrTiO₃ structures the top surface acquires a positive charge upon heating. On the basis of the difference in the measured expansion of the upper and lower surfaces of the SrTiO₃ layer in the presence and absence of SrRuO₃, we contend that the magnitude and direction of the pyroelectric effect are determined by the out‐of‐plane gradient of the in‐plane strain in the SrTiO₃ layer while pulling through the temperature gradient.     

Complex Band Structures and Lattice Dynamics of Bi2Te3‐Based Compounds and Solid Solutions

Bi₂Te₃‐based compounds and derivatives are milestone materials in the fields of thermoelectrics (TEs) and topological insulators (TIs). They have highly complex band structures and interesting lattice dynamics, which are favorable for high TE performance as well as strong spin orbit and band inversion underlying topological physics. This review presents rational calculations of properties related to TEs and provides theoretical guidance for improving the TE performance of Bi₂Te₃‐based materials. Although the band structures of these TE materials have been studied theoretically and experimentally for many years, there remain many controversies on band characteristics, especially the locations of band extrema and the exact values of bandgaps. Here, the key factors in the theoretical investigations of Bi₂Te₃, Bi₂Se₃, Sb₂Te₃, and their solid solutions are reviewed. The phonon spectra and lattice thermal conductivities of Bi₂Te₃‐based materials are discussed. Electronic and phonon structures and TE transport calculations are discussed and reported in the context of better establishing computational parameters for these V₂VI₃‐based materials. This review provides a useful guidance for analyzing and improving TE performance of Bi₂Te₃‐based materials.     

Material property, compatibility, and reliability issues in diamond-enhanced, GaAs-based plastic packages

The mechanical performance and reliability of various Al₂O₃ ceramics were evaluated with intended applications as insulator frames for radio frequency/microwave power packages. Considering variations in base-flange material properties (CuW, CuMoCu), base-flange thickness (0.040 in., 0.020 in.), assembly process material (AgCu, Cu) assembly process temperature (860°C, 1080°C), and Al₂O₃ insulator material (96%, 99%, 20% ZrO₂/Al₂O₃, ZTA), finite element analysis (FEA) spatially resolved the fabrication-induced stresses during the assembly of CuW/Al₂O₃ and CuMoCu/Al₂O₃ structures and showed the critical regions to be the frame corners at the metal base-flange/Al₂O₃ interface. Bend strengths (four-point) and Weibull distributions were determined for each Al₂O₃ material and coupled with the FEA-predicted stresses for the various package configurations and assembly processes, the failure probabilities (P_f ) for the various CuW/Al₂O₃ and CuMoCu/Al₂O₃ structures were calculated. The CuMoCu-based structures exhibited the greatest warp deformation (concave upward, +), and highest stress and failure probability for all Al₂O₃ insulator varieties. The CuW-based structures exhibited an order-of-magnitude lower warp deformation (concave downward, −), an order-of-magnitude lower stress in the Al₂O₃, and in excess of seven orders-of-magnitude lower failure probability than CuMoCu-based structures, for all Al₂O₃ insulator varieties. Confirmational experiments were conducted using AgCu and direct-bond copper (DBC) assembly processes for selected varieties of CuW/Al₂O₃ and CuMoCu/Al₂O₃ structures. Al₂O₃ failure sites were identified using radiographic, ultrasonic and optical techniques and were in good agreement with model predictions of suspect structures and failure location. The strength and reliability data were considered in conjunction with relative cost for the Al₂O₃ ceramics to select an optimum frame insulator for the application.     

The Contributions of Polar Nanoregions to the Dielectric and Piezoelectric Responses in Domain‐Engineered Relaxor‐PbTiO3 Crystals

The existence of polar nanoregions is the most important characteristic of relaxor‐based ferroelectric materials. Recently, the contributions of polar nanoregions to the shear piezoelectric property of relaxor‐PbTiO₃ (PT) crystals are confirmed in a single domain state, accounting for 50%–80% of room temperature values. For electromechanical applications, however, the outstanding longitudinal piezoelectricity in domain‐engineered relaxor‐PT crystals is of the most significance. In this paper, the contributions of polar nanoregions to the longitudinal properties in [001]‐poled Pb(Mg_1/3Nb_2/3)O₃‐0.30PbTiO₃ and [110]‐poled Pb(Zn_1/3Nb_2/3)O₃‐0.15PbTiO₃ (PZN‐0.15PT) domain‐engineered crystals are studied. Taking the [110]‐poled tetragonal PZN‐0.15PT crystal as an example, phase‐field simulations of the domain structures and the longitudinal dielectric/piezoelectric responses are performed. According to the experimental results and phase‐field simulations, the contributions of polar nanoregions (PNRs) to the longitudinal properties of relaxor‐PT crystals are successfully explained on the mesoscale, where the PNRs behave as “seeds” to facilitate macroscopic polarization rotation and enhance electric‐field‐induced strain. The results reveal the importance of local structures to the macroscopic properties, where a modest structural variation on the nanoscale greatly impacts the macroscopic properties.     

Increasing the Efficiency of Organic Dye‐Sensitized Solar Cells over 10.3% Using Locally Ordered Inverse Opal Nanostructures in the Photoelectrode

3D inverse opal (3D‐IO) oxides are very appealing nanostructures to be integrated into the photoelectrodes of dye‐sensitized solar cells (DSSCs). Due to their periodic interconnected pore network with a high pore volume fraction, they facilitate electrolyte infiltration and enhance light scattering. Nonetheless, preparing 3D‐IO structures directly on nonflat DSSC electrodes is challenging. Herein, 3D‐IO TiO₂ structures are prepared by templating with self‐assembled polymethyl methacrylate spheres on glass substrates, impregnation with a mixed TiO₂:SiO₂ precursor and calcination. The specific surface increases from 20.9 to 30.7 m² g^?1 after SiO₂ removal via etching, which leads to the formation of mesopores. The obtained nanostructures are scraped from the substrate, processed as a paste, and deposited on photoelectrodes containing a mesoporous TiO₂ layer. This procedure maintains locally the 3D‐IO order. When sensitized with the novel benzothiadiazole dye YKP‐88, DSSCs containing the modified photoelectrodes exhibit an efficiency of 10.35% versus 9.26% for the same devices with conventional photoelectrodes. Similarly, using the ruthenium dye N719 as sensitizer an efficiency increase from 5.31% to 6.23% is obtained. These improvements originate mainly from an increase in the photocurrent density, which is attributed to an enhanced dye loading obtained with the mesoporous 3D‐IO structures due to SiO₂ removal.     

Rational Engineering of Multilayered Co3O4/ZnO Nanocatalysts through Chemical Transformations from Matryoshka‐Type ZIFs

Hybrid metal oxides with multilayered structures exhibit unique physical and chemical properties, particularly important to heterogeneous catalysis. However, regulations of morphology, spatial location, and shell numbers of the hybrid metal oxides still remain a challenge. Herein, binary Co₃O₄/ZnO nanocages with multilayered structures (up to eight layers) are prepared via chemical transformation from diverse Matryoshka‐type zeolitic imidazolate frameworks (ZIFs) via a straightforward and scalable calcination method. More importantly, the obtained ZIF‐derived metal oxides (ZDMOs) with versatile layer numbers exhibit remarkable catalytic activity for both gas‐phase CO oxidation and CO₂ hydrogenation reactions, which are directly related to the sophisticated shell numbers (i.e., Co₃O₄‐terminated layers or ZnO‐terminated layers). Particularly, in situ reflectance infrared Fourier transform spectroscopy (DRIFTS) results indicate that the promotional effects of the multilayered structures indeed exist in CO₂ hydrogenation, wherein the key reaction intermediates are quite different for five‐layer and six‐layer ZDMOs. For instance, *HCOO is the predominant intermediate over the six‐layer ZDMO; on the contrary, *H₃CO is the crucial species over the five‐layer ZDMO. The ZnO/Co₃O₄ interface should be the active sites for CO₂ hydrogenation to *HCOO and *H₃CO species, which are ultimately converted to the products (CH₄ or methanol). Accordingly, the work here provides a convenient way to facilely engineer multilayered Co₃O₄/ZnO nanocomposites with precisely controlled shell numbers for heterogeneous catalysis applications.     

CdHgTe heterostructures on large-area Si(310) substrates for infrared photodetector arrays of the short-wavelength spectral range

Heteroepitaxial structures n-Cd _x Hg_{1 ? x} Te for the near-infrared spectral range (x ≈ 0.4) are grown by molecular beam epitaxy on Si(310) substrates 72.6 and 100 mm in diameter. High composition homogeneity over the structure area is attained; the variation in x for 100-mm wafers is 0.015–0.025. During growth, the mercury-cadmium telluride (MCT) layers are doped with In with the concentration (0.5–3) × 10¹⁵ cm^?3. The magnetic-field dependences of the Hall effect are studied in the range of magnetic fields 0.05–1.0 T at liquid-nitrogen and room temperatures. The experimental values of the electron mobility at room temperature are close to the calculated ones, while at liquid-nitrogen temperature, they are lower than the calculated mobilities. The possible causes of this phenomenon such as the influence of the MCT transition layer at the interface with the CdTe buffer layer and lattice defects of MCT are discussed. The variation in the concentration and mobility of charge carriers in MCT structures after activation annealing are studied.     

Experimental and simulation study for impact of different halides on the performance of planar perovskite solar cells

MeA-PbX₃ and MeA-PbI₂X (where MeA=CH₃NH₃; X=I, Br, Cl) systems have been synthesized using grinding processing. Plainly, the crystal structures of the perovskite materials were altered with the variation of the halide ions. Meanwhile, the band gap energy was enhanced from 1.5 eV for MeA-PbI₃ to 2.1 and 2.8 eV for MeA-PbBr₃ and MeA-PbCl₃ as a result of substitution by halide Br and Cl, respectively. The intensity peaks of different perovskite structures were confirmed by photoluminescence (PL). Furthermore, the following energy parameters, heat of formation, high occupied molecular orbital (HOMO) and low unoccupied molecular orbital (LUMO) were evaluated using hyperchem system software. Herein, we performed a device modeling and theoretical study on planar perovskite solar cells without a hole transporting material (HTM) using a solar cell simulation program (wxAMPS) as an update of the popular solar cell simulation tool (AMPS; Analysis of Microelectronic and Photonic Structures). Simulation and experimental design of MeA-PbX₃ and MeA-PbI₂X (where MeA=CH₃NH₃; X=I, Br, Cl) systems were investigated. The cells without HTM have been suggested to enhance the low cost and simple assembly of organic-inorganic lead halide perovskite based solar cells. MeA-PbBr₃ with HTM-free solar cells was achieved a high PCE of 13.96% in simulation program compared to 3.88% as experimental one.     

Development and photoelectric properties of In/<Emphasis Type="Italic">p</Emphasis>-Ag<Subscript>3</Subscript>AsS<Subscript>3</Subscript> surface-barrier structures

Homogeneous p-Ag₃AsS₃ bulk single crystals with rhombic structure have been grown by planar crystallization from melts with atomic composition corresponding to this ternary compound. Photosensitive surface-barrier structures based on the interface between the surface of these crystals and thin films of pure indium are fabricated for the first time. The photosensitivity of fabricated structures is studied in natural and linearly polarized light. Photosensitivity spectra of In/p-Ag₃AsS₃ structures are measured for the first time and used to determine the nature and energy of interband transitions in p-Ag₃AsS₃ crystals. The phenomenon of natural photopleochroism is studied for surface-barrier structures grown on oriented p-Ag₃AsS₃ single crystals. It is concluded that Ag₃AsS₃ single crystals can be used in photoconverters of natural and linearly polarized light.     

Experimental investigations of superconductivity in quasi-two-dimensional epitaxial copper oxide superlattices and trilayers

Epitaxial trilayer and superlattice structures grown by pulsed laser ablation have been used to study the superconducting-to-normal transition of ultrathin (one and two c-axis unit cells) YBa₂Cu₃O_7−x layers. The normalized flux-flow resistances for several epitaxial structures containing two-cell-thick YBa₂CU₃O,_7−x films collapse onto the “universal” curve of the Ginzburg-Landau Coulomb gas (GLCG) model. Analysis of normalized resistance data for a series of superlattices containing one-cell-thick YBa₂Cu₃O_7−x layers also is consistent with the behavior expected for quasi-two-dimensional layers in a highly anisotropic, layered three-dimensional superconductor. Current-voltage measurements for one of the trilayer structures also are consistent with the normalized resistance data, and with the GLCG model. Scanning tunneling microscopy, transmission electron microscopy, and electrical transport studies show that growth-related steps in ultrathin YBa₂Cu₃O_7−x layers affect electrical continuity over macroscopic distances, acting as weak links. However, the perturbation of the superconducting order parameter can be minimized by utilizing hole-doped buffer and cap layers, on both sides of the YBa₂Cu₃O_7−x layer, in trilayers and superlattices. These results demonstrate the usefulness of epitaxial trilayer and superlattice structures as tools for systematic, fundamental studies of high-temperature superconductivity.     

Synchrotron study of the formation of nanoclusters in Al2O3/SiO x /Al2O3/SiO x /…/Si(100) multilayer nanostructures

Al₂O₃/SiO _x /Al₂O₃/SiO _x /…/Si(100) multilayer nanoperiodic structures (MNS) are studied by X-ray absorption near-edge structure spectroscopy (XANES). Experimental XANES spectroscopy spectra are obtained using synchrotron radiation. The formation of Si nanoclusters in the surface layers of the structures during their high-temperature annealing is observed. The structures featured intense size-dependent photoluminescence in the wavelength region near 800 nm. At the same time, it is shown that the formation of aluminum silicates is possible. The inversion effect of the intensity of the XANES spectra during the interaction of synchrotron radiation with MNSs is revealed.     

Modeling the Electrical Conduction in Epoxy–BaTiO3 Nanocomposites

Epoxy–BaTiO₃ nanocomposites are widely used as the dielectric material in embedded planar capacitors. To maximize the effective dielectric constant of this nanocomposite, the loading of BaTiO₃ is kept as high as possible, but at high loadings of BaTiO₃ the magnitude of undesirable leakage current in the dielectric also increases. This paper investigates the conduction mechanism in epoxy–BaTiO₃ nanocomposites. Further, the effects of BaTiO₃ loading and the size of BaTiO₃ particles on the electrical conduction are investigated and also modeled. To investigate the conduction mechanism, capacitor structures (Cu/dielectric/Cu) with nanocomposite dielectric were fabricated using the colloidal process. The loading and size of BaTiO₃ particles were varied in the nanocomposite dielectric. Once the capacitor structures were fabricated, the leakage current was measured across the capacitor dielectric as a function of temperature and voltage. The leakage current data were checked for any consistency with the standard conduction models using regression analysis, and the dominant conduction mechanism was identified. Finally, the activation energy of the dominant conduction mechanism was trended as a function of BaTiO₃ loading and particle size both experimentally and theoretically.     

Optimization and performance of Al2O3/GaN metal–oxide–semiconductor structures

We investigate electrical properties of Ni/Al₂O₃/GaN metal–oxide–semiconductor (MOS) structures having different pre-treatment of GaN surface by O₂, Ar and NH₃, combined with various temperature of annealing. MOS and reference Ni/GaN Schottky contact are characterized using current–voltage and capacitance–voltage methods. MOS structures compared with the Schottky contact ones show leakage current reduction for all types of processing, from 3 to 5 orders of magnitude in reverse direct. We observed substantial influence of the pre-treatment on electrical parameters of MOS structures.     

Ionization potential dependent air exposure effect on the MoO3/organic interface energy level alignment

We reported an ionization potential (IP) dependent air exposure effect on the MoO₃/organic interface energy level alignment by carrying out in situ ultraviolet photoelectron spectroscopy and synchrotron light based X-ray photoelectron spectroscopy investigations. The electronic structures at MoO₃/organic interfaces comprising various π-conjugated small organic molecules with different IP on MoO₃ substrate have been systematically investigated. For the molecules with low IP, MoO₃/organic interface electronic structures remained almost unchanged after air exposure. In contrast, for the molecules with high IP, the highest occupied molecular orbital (HOMO) leading edge (or hole injection barrier) increases gradually with the increasing molecule IP after air exposure. For the MoO₃/copper-hexadecafluorophthalocyanine (F₁₆CuPc, IP: ∼6.58 eV) interface, air exposure can induce a significant downward shift of the HOMO level as large as ∼0.80 eV.     

Deep-level trapping centers in heterostructures for GaN field-effect transistors

Structural and electrophysical properties of heteroepitaxial gallium nitride layers on a sapphire substrate that are grown via the molecular beam epitaxy (MBE) method are studied. The parameters of deep-level trapping centers are determined by the method of the thermostimulated capacitor discharge; the degree of perfection of the film and substrate are determined by the two-crystal X-ray spectrometry method. The following structures are studied: i-GaN (1–2 μm)/GaN 〈Si〉(0.1−0.4 μm) and multilayer structures (Al_0.3Ga_0.7N-GaN-Al_0.3-Ga_0.7N-GaN-Al₂O₃) grown via the MBE method on a sapphire substrate. The effect of reactive ion etching on the energy spectrum of deep-level trapping centers in gallium nitride is studied. The obtained results are used to calculate the energy spectrum of defects in gallium nitride structures. Original Russian Text ? M.S. Andreev, L.E. Velikovskii, T.S. Kitichenko, T.G. Kolesnikova, A.P. Korovin, V.G. Mokerov, S.N. Yakunin, 2007, published in Radiotekhnika i Elektronika, 2007, Vol. 52, No. 7, pp. 880–887.     

Substrate effect on the interfacial electronic structure of thermally-evaporated CH3NH3PbI3 perovskite layer

The impact of substrate work function on the interfacial electronic structure of thermally-evaporated CH₃NH₃PbI₃ perovskite films on various substrates have been systematically investigated using in-situ ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). On substrates with work function lower than ∼4.43 eV, a Fermi level pinning effect of the lowest unoccupied molecular orbital (LUMO) is observed, resulting in the near zero electron extraction barrier for the CH₃NH₃PbI₃ perovskite solar cells. On the other hand, when substrates with high work function are used, even exceed the highest occupied molecular orbital (HOMO) of CH₃NH₃PbI₃, an almost constant hole extraction barrier of ∼0.88 eV is observed, indicating that the efficiency of hole extraction at these interfaces are low. In order to understand the low hole extraction efficiency at interfaces between CH₃NH₃PbI₃ and these high work function electrodes, the evolution of electronic structures at the interface between CH₃NH₃PbI₃ and MoO₃ is further investigated. The charge transfer and dipole formation between CH₃NH₃PbI₃ and MoO₃ are deduced from the UPS and XPS results, and the energy level alignment between CH₃NH₃PbI₃ and MoO₃ is discussed.     

Surface-barrier In/p-CuGa3Te5 and In/p-CuGa5Te8 structures: Fabrication and properties

Single crystals of the ternary CuGa₃Te₅ and CuGa₅Te₈ ternary compounds are grown and their properties are studied. The photosensitive In/p-CuGa₃Te₅ and In/p-CuGa₅Te₈ structures were formed on homogeneous crystals for the first time. Photoelectric properties of new structures were studied and the parameters of the structures and of new semiconductors were determined; it is concluded that these structures can be used in broadband photoconverters of nonpolarized radiation.     

Characteristics of GdGaO grown by MBE

Ga₂O₃/GdGaO dielectric stacks have been grown on GaAs for MOSFETs. This paper highlights variations in the characteristics of GdGaO as the Gd flux, Ga₂O flux and substrate temperature are changed. The growth rate, composition, crystallinity are discussed and the sheet resistance of final MOSFET structures are presented. The Gd compositional variation with depth is examined using Rutherford back scattering (RBS) and electron energy loss spectroscopy (EELS).