Light Enhanced Electron Transduction and Amplified Sensing at a Nanostructure Modified Semiconductor Interface

Visible and UV light are demonstrated to significantly enhance the sensing properties of an n‐type porous silicon (PS) extrinsic semiconductor interface to which TiO₂ and titanium oxynitride (TiO_2‐xN_x) photocatalytic nanostructures are fractionally deposited. The acid/base chemistry of NH₃, a moderately strong base, and NO₂, a moderately strong acid, couples to the majority charge carriers of the doped semiconductor as the strong acid (TiO₂) enhances the extraction of electrons from NH₃, and the more basic TiO_2‐xN_x decreases the efficiency of electron extraction relative to the untreated interface. In contrast, NO₂ and a TiO₂ or TiO_2‐xN_x nanostructure‐decorated PS interface compete for the available electrons leading to a distinct time dependent electron transduction dynamics as a function of TiO₂ and TiO_2‐xN_x concentration. Only small concentrations of TiO₂ and its oxynitride and no self‐assembly are required to enhance the response of the decorated interface. With light intensities of less than a few lumens/cm²‐sterad‐nm, responses are enhanced by up to 150% through interaction with visible (and UV) radiation. These light intensities should be compared to the sun's radiation level, ≈500 lumens/cm²‐sterad‐nm suggesting the possibility of solar pumped sensors. The observed behavior in these systems is largely explained by the recently developed Inverse Hard/Soft Acid/Base (IHSAB) concept.     

Control of the Ti diffusion in Pt/Ti bottom electrodes for the fabrication of PZT thin film transducers

For the achievement of microactuators based on piezoelectric thin films, a Pt/Ti/Si bottom electrode is widely used. This study presents the experimental results for Ti out-diffusion in Pt and Si for both sputtered Pt/Ti and Pt/TiO_x electrodes. These results have been compared before and after a rapid thermal annealing (RTA). The diffusion has been characterized by secondary ion mass spectroscopy (SIMS) analysis using Cs⁺ as a primary ion source. The Pt orientation has been observed by XRD measurements. Ti thin films (20 nm) have been sputtered in pure Ar whereas TiO_x films have been obtained by reactive sputtering in a mixture of Ar/O₂ (90/10). Finally, the Pt (100 nm) has been sputtered without vacuum breaking. After RTA (400°C, 30 s, in N₂), the Pt film exhibited a (1 1 1) orientation for both Ti and TiO_x adhesion films. The roughness of the Pt film measured by AFM with TiO_x underlayer was 80% less than that of the Pt/Ti bi-layer. The TiO_x film, as shown by SIMS analysis, has drastically reduced the diffusion of Ti in both Pt and Si. This phenomenon is accompanied by a very low Pt roughness. These results are analyzed in terms of diffusion and regrowth mechanisms inside the Pt film.     

Evidence for Chemicals Intermingling at Silicon/Titanium Oxide (TiOx) Interface and Existence of Multiple Bonding States in Monolithic TiOx

Metal oxides (MOs) are used in photovoltaics and microelectronics as surface passivating layers and gate dielectrics, respectively. The effectiveness of MOs predominantly depends on their structure and the nature of the semiconductor/MO (S/MO) interface. While some efforts are made to analyze interface behavior of a few MOs, greater fundamental understanding on the interface and structural behaviors of emerging MOs is yet to be established for enhanced scientific and technological developments. Here, the structure of atomic layer deposited titanium oxide (TiO_x) and the nature of the c‐Si/TiO_x interface on the atomic‐ to nanoscale are probed. A new breed of mixed oxide (SiO_x+TiO_x) interfacial layer with a thickness of ≈1.3 nm at the c‐Si/TiO_x interface is discovered, and its thickness further increases to ≈1.5 nm after postdeposition annealing. It is observed that both as‐deposited and annealed monolithic TiO_x films comprise multiple bonding states at varying film thickness, with an oxygen‐deficient TiO_x layer located close to the mixed oxide/TiO_x interface. The stoichiometry of this layer improves when reaching the middle and near surface regions of the TiO_x layer, respectively. This work uncovers several critical structural and interface aspects of TiO_x, and thus creates opportunities to control and design improved photovoltaic and electronic devices for future development.     

Tailoring Antibonding-Orbital Occupancy State of Selenium in Se-Enriched ReSe2+x Cocatalyst for Exceptional H2 Evolution of TiO2 Photocatalyst

Electron density regulation of active sites can realize an optimal hydrogen-binding strength, whereas the underlying regulation mechanism is still indistinct. Herein, a new concept of antibonding-orbital occupancy state is first proposed to unveil the fundamental influence mechanism of electron density on the Se H_ads bond strength for achieving first-rank adsorption energy toward atomic hydrogen by constructing Se-enriched surrounding to form electron-deficient Se^(2-δ)- active sites in ReSe_2+x nanodots. To this end, the Se-rich ReSe_2+x nanodots (0.3–1 nm) can be dexterously fabricated onto the TiO₂ to prepare Se-rich ReSe_2+x/TiO₂ by an ingenious one-step photosynthesis route. In a surprise, a large number of visual H₂ bubbles are continuously produced on the resultant ReSe_2+x/TiO₂(0.7 wt.%) with an ultrahigh rate of 12 490.4 µmol h⁻¹ g⁻¹ and an apparent quantum efficiency of 60.0%, which is 5.0 times higher than that of traditional ReSe₂/TiO₂, even comparable with benchmark Pt/TiO₂(0.7 wt.%). In situ/ex situ XPS characterizations coupled with density functional theory (DFT) calculations corroborate that a Se-enriched environment can induce the formation of electron-deficient Se^(2-δ)− and then reduce its antibonding-orbital occupancy state, thus increasing the stability of H 1s-p antibonding and accordingly reinforcing the Se H_ads bonds. This holistic study identifies the dominant role of antibonding-orbital occupancy states in the optimization of hydrogen-binding energy.     

TiO2 Coating for SnO2:F Films Produced by Filtered Cathodic Arc Evaporation for Improved Resistance to H+ Radical Exposure

Titanium dioxide thin films were deposited by filtered cathodic arc evaporation (FCAE) from a Ti target in an oxygen atmosphere onto (a) fluorine-doped tin oxide substrates SnO₂:F (FTO) and (b) glass microscope slides. The growth rate calculated from film thickness profilometry measurements was found to be approximately 0.8?nm/s. The films were highly transparent to visible light. x-Ray photoemission spectroscopy analysis of the Ti 2p electron binding- energy shift confirmed the presence of a TiO₂ stoichiometric compound. The results for the root-mean-square (RMS) surface roughness of the films deposited onto FTO substrates evaluated by atomic force microscopy suggested nanostructured film surfaces. When exposed to hydrogen plasma, TiO₂ films revealed insignificant changes in the optical spectra. The initial sheet resistance of the SnO₂:F layer was 14?Ω/sq. The deposition of the top TiO₂ layer (45?nm thick) over the FTO electrode resulted in an increase of the sheet resistance of 2?Ω/sq. In addition, the sheet resistance of the double-layer FTO/TiO₂ transparent conductive oxide (TCO) electrode increased by 1?Ω/sq as a result of H⁺ plasma exposure. Regardless of the TiO₂ film’s low conductivity, a thin protective layer could be coated onto FTO films (presumably 15?nm thick) due to their high transparency, offering high resistance to aggressive H⁺ plasma conditions. In this paper we show that ～50-nm-thick TiO₂ coating on FTO films provides sufficient protection against deterioration of transparency and conductivity due to hydrogen radical exposure.     

Enhanced Performance of I2‐Free Solid‐State Dye‐Sensitized Solar Cells with Conductive Polymer up to 6.8%

An iodine‐free solid‐state dye‐sensitized solar cell (ssDSSC) is reported here, with 6.8% energy conversion efficiency—one of the highest yet reported for N719 dye—as a result of enhanced light harvesting from the increased transmittance of an organized mesoporous TiO₂ interfacial layer and the good hole conductivity of the solid‐state‐polymerized material. The organized mesoporous TiO₂ (OM‐TiO₂) interfacial layer is prepared on large‐area substrates by a sol‐gel process, and is confirmed by scanning electron microscopy (SEM) and grazing incidence small‐angle X‐ray scattering (GISAXS). A 550‐nm‐thick OM‐TiO₂ film coated on fluorine‐doped tin oxide (FTO) glass is highly transparent, resulting in transmittance increases of 8 and 4% compared to those of the bare FTO and conventional compact TiO₂ film on FTO, respectively. The high cell performance is achieved through careful control of the electrode/hole transport material (HTM) and nanocrystalline TiO₂/conductive glass interfaces, which affect the interfacial resistance of the cell. Furthermore, the transparent OM‐TiO₂ film, with its high porosity and good connectivity, exhibits improved cell performance due to increased transmittance in the visible light region, decreased interfacial resistance ( Ω ), and enhanced electron lifetime ( τ ). The cell performance also depends on the conductivity of HTMs, which indicates that both highly conductive HTM and the transparent OM‐TiO₂ film interface are crucial for obtaining high‐energy conversion efficiencies in I₂‐free ssDSSCs.     

3D Hexagonal (R‐3m) Mesostructured Nanocrystalline Titania Thin Films: Synthesis and Characterization

A straightforward and reproducible synthesis of crack‐free large‐area thin films of 3D hexagonal (R‐3m) mesostructured nanocrystalline titania (meso‐nc‐TiO₂) using a Pluronic triblock copolymer (P123)/1‐butanol templating system is described. The characterization of the films is achieved using a combination of electron microscopy (high‐resolution scanning electron microscopy and scanning transmission electron microscopy), grazing‐incidence small‐angle X‐ray scattering, in situ high‐temperature X‐ray diffraction, and variable‐angle spectroscopic ellipsometry. The mesostructure of the obtained films is found to be based upon a 3D periodic array of large elliptically shaped cages with diameters around 20 nm interconnected by windows of about 5 nm in size. The mesopores of the film calcined at 300 °C are very highly ordered, and the titania framework of the film has a crystallinity of 40 % being composed of 5.8 nm sized anatase crystallites. The film displays high thermal stability in that the collapse of the pore architecture is incomplete even at 600 °C. The accessible surface area of 3D hexagonal meso‐nc‐TiO₂ estimated by the absorption of methylene blue is nearly twice as large as that of 2D hexagonal meso‐nc‐TiO₂ at the same annealing temperature.     

Synthesis and characterization of TiO2-doped Polyaniline nanocomposites by chemical oxidation method

Polyaniline (PANI)/TiO₂ nanocomposite samples with various dopant percentages of TiO₂ were synthesized at room temperature using a chemical oxidative method. The samples were characterized by ultraviolet-visible spectrometer, Fourier transform infrared (FTIR) spectrometer, X-ray diffraction (XRD), scanning electron microscopy (SEM), EDAX and conductivity measurements. Incorporation of TiO₂ nanoparticles caused a slight red shift at 310 nm in the absorption spectra due to the interactions between the conjugated polymer chains and TiO₂ nanoparticles with π–π^? transition. FTIR confirmed the presence of TiO₂ in the molecular structure. In PANI/TiO₂ composites, two additional bands at 1623 cm^?1 and 1105 cm^?1 assigned to Ti–O and Ti–O^–C stretching modes were present. It can be concluded that Ti organic compounds are formed with an alignment structure of TiO₂ particles. XRD patterns revealed that, as the TiO₂ percentage was increased, the amorphous nature disappeared and the composites became more strongly oriented along the (1 1 0) direction, which showed the tetragonal structure of nanocrystalline TiO₂. SEM studies revealed the formation of uniform granular morphology with average grain size of 200 nm for (50%) PANI/TiO₂ nanocomposite samples.     

Plasmonic Heterostructure TiO2‐MCs/WO3−x‐NWs with Continuous Photoelectron Injection Boosting Hot Electron for Methane Generation

Nonmetallic plasmonic heterostructure TiO₂‐mesocrystals/WO_3?x‐nanowires (TiO₂‐MCs/WO_3?x‐NWs) are constructed by coupling mesoporous crystal TiO₂ and plasmonic WO_3?x through a solvothermal procedure. The continuous photoelectron injection from TiO₂ stabilizes the free carrier density and leads to strong surface plasmon resonance (SPR) of WO_3?x, resulting in strong light absorption in the visible and near‐infrared region. Photocatalytic hydrogen generation of TiO₂‐MCs/WO_3?x‐NWs is attributed to plasmonic hot electrons excited on WO_3?x‐NWs under visible light irradiation. However, utilization of injected photoelectrons on WO_3?x‐NWs has low efficiency for hydrogen generation and a co‐catalyst (Pt) is necessary. TiO₂‐MCs/WO_3?x‐NWs are used as co‐catalyst free plasmonic photocatalysts for CO₂ reduction, which exhibit much higher activity (16.3 µmol g^?1 h^?1) and selectivity (83%) than TiO₂‐MCs (3.5 µmol g^?1 h^?1, 42%) and WO_3?x‐NWs (8.0 µmol g^?1 h^?1, 64%) for methane generation under UV–vis light irradiation. A photoluminescence study demonstrates the photoelectron injection from TiO₂ to WO_3?x, and the nonmetallic SPR of WO_3?x plays a great role in the highly selective methane generation during CO₂ photoreduction.     

Analysis of the stress and interfacial reactions in Pt/Ti/SiO2/Si for use with ferroelectric thin films

The microstructure of the Pt/Ti/SiO₂/Si structure has been investigated by scanning and transmission electron microscopy. Pt films of 100 nm thickness deposited by sputtering or evaporation onto unheated substrates gave complete coverage of the underlying Ti layer and showed a granular and faceted structure with grains ∼20 nm in diameter. They did not exhibit hillocks or surface TiO_x formation. X-ray diffraction was used to examine the film stress through use of the sin²ψ method with bulk values for the elastic constants (v=0.39, E=162 GPa). The as-deposited sputtered film had a compressive stress of ∼540 MPa, while the evaporated films had tensile stresses of ∼630 MPa. The films then received a 400°C rapid thermal anneal (RTA) for 90 s and a subsequent RTA of 650°C for 30s. Further investigation of the film stresses and microstructure were made after each annealing step. After the low temperature anneal, the film stress for the sputtered film became tensile. Plan-view sections examined by transmission electron microscopy (TEM) showed that the as-deposited sputtered films were dense but became porous after annealing. Initially, the evaporated films had a less dense microstructure, but were more stable with annealing. Little change in the stress for the evaporated film was observed after this initial low temperature annealing step. Additional annealing of the evaporated and sputtered samples caused complete consumption of the Ti layer including some TiO_x formation from the underlying SiO₂ layer and marked interaction with the Pt; however, little change in the stress was found. The surface of the Pt film revealed larger grains, but otherwise remained unaffected. The underlying phase changes were minimized once the Ti layer had reacted with the Pt. Due to the ratio of the layers, Pt:Ti of 2:1, the surface of the Pt was unaffected.     

Resistive switching characteristics of ultra-thin TiOx

We investigated the resistive switching characteristics of Ir/TiO_x/TiN structure with 50 nm active area. We successfully formed ultra-thin (4 nm) TiO_x active layer using oxidation process of TiN BE, which was confirmed by X-ray Photoelectron Spectroscopy (XPS) depth profiling. Compared to large area device (50 μm), which shows only ohmic behavior, 250 and 50 nm devices show very stable resistive switching characteristics. Due to the formation and rupture of oxygen vacancies induced conductive filament at Ir and TiO_x interface, bipolar resistive switching was occurred. We obtained excellent switching endurance up to 10⁶ times with 100 ns pulse and negligible degradation of each resistance state at 85 °C up to 10⁴ s.     

Core/Shell PbSe/PbS QDs TiO2 Heterojunction Solar Cell

Quasi type‐II PbSe/PbS quantum dots (QDs) are employed in a solid state high efficiency QD/TiO₂ heterojunction solar cell. The QDs are deposited using layer‐by‐layer deposition on a half‐micrometer‐thick anatase TiO₂ nanosheet film with (001) exposed facets. Theoretical calculations show that the carriers in PbSe/PbS quasi type‐II QDs are delocalized over the entire core/shell structure, which results in better QD film conductivity compared to PbSe QDs. Moreover, PbS shell permits better stability and facile electron injection from the QDs to the TiO₂ nanosheets. To complete the electrical circuit of the solar cell, a Au film is evaporated as a back contact on top of the QDs. This PbSe/PbS QD/TiO₂ heterojunction solar cell produces a light to electric power conversion efficiency (η) of 4% with short circuit photocurrent (J_sc) of 17.3 mA/cm². This report demonstrates highly efficient core/shell near infrared QDs in a QD/TiO₂ heterojunction solar cell.     

Well‐Ordered Large‐Pore Mesoporous Anatase TiO2 with Remarkably High Thermal Stability and Improved Crystallinity: Preparation,Characterization, and Photocatalytic Performance

Thermally‐stable, ordered mesoporous anatase TiO₂ with large pore size and high crystallinity has been successfully synthesized through an evaporation‐induced self‐assembly technique, combined with encircling ethylenediamine (EN) protectors to maintain the liquid crystal mesophase structure of TiO₂ primary particles, followed by calcination at higher temperature. The structures of the prepared mesoporous TiO₂ are characterized in detail by small‐angle and wide‐angle X‐ray diffraction, Raman spectra, N₂ adsorption/desorption isotherms, and transmission electron microscopy. Experimental results indicate that the well‐ordered mesoporous structure could be maintained up to 700 °C (M700) and also possesses large pore size (10 nm), high specific BET surface area (122 m² g^?1), and high total pore volumes (0.20 cm³ g^?1), which is attributed to encircling EN protectors for maintaining the mesoporous framework against collapsing, inhibiting undesirable grain growth and phase transformation during the calcination process. A possible formation mechanism for the highly stable large‐pore mesoporous anatase TiO₂ is also proposed here, which could be further confirmed by TG/FT‐IR in site analysis and X‐ray photoelectron spectroscopy. The obtained mesoporous TiO₂ of M700 exhibit better photocatalytic activity than that of Degussa P25 TiO₂ for degradation of highly toxic 2,4‐dichlorophenol under UV irradiation. This enhancement is attributed to the well‐ordered large‐pore mesoporous structure, which facilitates mass transport, the large surface area offering more active sites, and high crystallinity that favors the separation of photogenerated electron‐hole pairs, confirmed by surface photovoltage spectra.     

Enhanced Performance of Fullerene n‐Channel Field‐Effect Transistors with Titanium Sub‐Oxide Injection Layer

Enhanced performance of n‐channel organic field‐effect transistors (OFETs) is demonstrated by introducing a titanium sub‐oxide (TiO_x) injection layer. The n‐channel OFETs utilize [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PC₆₁BM) or [6,6]‐phenyl‐C₇₁ butyric acid methyl ester (PC₇₁BM) as the semiconductor in the channel. With the TiO_x injection layer, the electron mobilities of PC₆₁BM and PC₇₁BM FET using Al as source/drain electrodes are comparable to those obtained from OFETs using Ca as the source/drain electrodes. Direct measurement of contact resistance (R_c) shows significantly decreased R_c values for FETs with the TiO_x layer. Ultraviolet photoelectron spectroscopy (UPS) studies demonstrate that the TiO_x layer reduces the electron injection barrier because of the relatively strong interfacial dipole of TiO_x. In addition to functioning as an electron injection layer that eliminates the contact resistance, the TiO_x layer acts as a passivation layer that prevents penetration of O₂ and H₂O; devices with the TiO_x injection layer exhibit a significant improvement in lifetime when exposed to air.     

Inserting Sn Nanoparticles into the Pores of TiO2−x–C Nanofibers by Lithiation

Tin holds promise as an anode material for lithium‐ion batteries (LIBs) because of its high theoretical capacity, but its cycle life is limited by structural degradation. Herein, a novel approach is exploited to insert Sn nanoparticles into the pores of highly stable titanium dioxide–carbon (TiO_2?x–C) nanofiber substrates that can effectively localize the postformed smaller Sn nanoparticles, thereby address the problem of structural degradation, and thus achieve improved anode performance. During first lithiation, a Li_4.4Sn alloy is inserted into the pores surrounding the initial Sn nanoparticles in TiO_2?x–C nanofibers by its large volume expansion. Thereafter, the original Sn nanoparticle with a diameter of about 150 nm cannot be recovered by the delithiation because of the surface absorption between inserted Sn nanoparticles and the TiO_2?x–C substrate, resulting in many smaller Sn nanoparticles remaining in the pores. Batteries containing these porous TiO_2?x–C–Sn nanofibers exhibit a high capacity of 957 mAh g^?1 after 200 cycles at 0.1 A g^?1 and can cycle over 10 000 times at 3 A g^?1 while retaining 82.3% of their capacity, which represents the longest cycling life of Sn‐based anodes for LIBs so far. This interesting method can provide new avenues for other high‐capacity anode material systems that suffer from significant volume expansion.     

Low sublimation temperature cesium pivalate complex as an efficient electron injection material for organic light-emitting diode devices

Cesium pivalate ((CH₃)₃CCOOCs) has been synthesized and applied as an electron injection material for organic light-emitting diodes, which showed low sublimation temperature of 180 °C. Typical bilayer structure of ITO/NPB (60 nm)/Alq₃ (50 nm)/EIL/Al was used to evaluate the electron injection efficacy of (CH₃)₃CCOOCs, the results showed (CH₃)₃CCOOCs/Al exhibits better electron injection than LiF/Al cathode and the power efficiency was improved by about 19% at current density of 50 mA/cm². More interestingly, in the typical three layer OLED structure ITO/2-TNATA (60 nm)/NPB (10 nm)/Alq₃:2% C545T (40 nm)/MADN (15 nm)/(CH₃)₃CCOOCs (2 nm)/Al, the maximum current efficiency is up to 20 cd/A with Commission Internationale d’Eclairage (CIE_x,_y) color coordinates of (x = 0.30, y = 0.65) at current density of 140 mA/cm², which indicates that the non-aromatic alkali metal complex can also have good match with the chemically stable compound and exhibit good electron injection properties.     

Dual‐Function Scattering Layer of Submicrometer‐Sized Mesoporous TiO2 Beads for High‐Efficiency Dye‐Sensitized Solar Cells

Submicrometer‐sized (830 ± 40 nm) mesoporous TiO₂ beads are used to form a scattering layer on top of a transparent, 6‐µm‐thick, nanocrystalline TiO₂ film. According to the Mie theory, the large beads scatter light in the region of 600–800 nm. In addition, the mesoporous structure offers a high surface area, 89.1 m² g^?1, which allows high dye loading. The dual functions of light scattering and electrode participation make the mesoporous TiO₂ beads superior candidates for the scattering layer in dye‐sensitized solar cells. A high efficiency of 8.84% was achieved with the mesoporous beads as a scattering layer, compared with an efficiency of 7.87% for the electrode with the scattering layer of 400‐nm TiO₂ of similar thickness.     

Enhanced photocatalytic activity of graphene oxide/titania nanosheets composites for methylene blue degradation

In the present work, we report enhanced photocatalytic degradation of methylene blue dye in aqueous solution by using ultra-thin anatase TiO₂ nanosheets (NSs) combined with graphene oxide (GO) as a photocatalyst. The two-dimensional ultra-thin anatase TiO₂ NSs are fabricated via chemical exfoliation. By completely delaminating a lepidocrocite-type layered protonic titanate H_xTi_2−x/4□_x/4O₄·H₂O (x=0.7, □: vacancy) into individual layers through ion exchange with tetrabutylammonium (TBA⁺) cations, well-dispersed ultra-thin colloidal Ti_0.91O₂ NSs with a lateral size up to a few micrometers are obtained. Subsequent acid treatment induces colloidal Ti_0.91O₂ to reassemble and precipitate into a gelation form, followed by thermal annealing to convert the Ti_0.91O₂ gelation into anatase TiO₂ nanosheets as photocatalyst for methylene blue degradation. TiO₂ NSs show a high photocatalytic degradation efficiency of 53.2% due to the ultra-thin thickness for facile electron transfering and large surface area for methylene blue absorption. Moreover, photocatalytic effect can be further improved by simply adding GO suspension to achieve colloidal self-assembly of GO and TiO₂ NSs. An optimal GO content of 3 wt% further increases the photocatalytic degradation efficiency to 91.2% due to faster electron–hole seperation and improved surface area provided by GO. This work provides a simple but effective approach by combing graphene oxide with TiO₂ nanosheets synthesized via the exfoliation method for methylene blue degradation.     

A New Phase Boundary in (Bi1/2Na1/2)TiO3−BaTiO3 Revealed via a Novel Method of Electron Diffraction Analysis

A new phase boundary is revealed in (1–x)(Bi_1/2Na_1/2)TiO₃?xBaTiO₃, the most extensively studied lead‐free piezoelectric solid solution. This discovery results from a novel method of electron diffraction analysis, which allows the precise determination of oxygen octahedra tilting in multi‐domain perovskite ferroelectrics. The study using this method supports the recently proposed Cc symmetry for pure (Bi_1/2Na_1/2)TiO₃, and, more importantly, indicates the crystal structure evolves into the R3c symmetry with the addition of BaTiO₃, forming a Cc/R3c phase boundary at x = 3–4%. In the poling field E_pol versus composition x phase diagram for polycrystalline ceramics, this phase boundary exists with E_pol below 5.5 kV mm^?1; the Cc phase is transformed to the R3c phase during poling at higher fields. The results reported here provide the microstructural origin for the previously unexplained strain behavior and clarify the low‐BaTiO₃‐content phase relationship in this popular lead‐free piezoelectric system.     

High indium content graded channel GainAs/AlinAs pseudomorphic MODFETs

We report on the electrical and microstructural properties of InP/Ga_xIn ₁ -xAs/Al_0.48In_0.52As modulation doped layers having compositionally graded active channels with different channel thicknesses. The layers were grown by solid source molecular beam epitaxy on Fe-doped InP substrates. The undoped GaInAs two dimensional electron gas channel layers were grown having indium compositions graded fromx = 0.53 at the substrate buffer tox= 0.65 at the heterointerface by varying the Ga cell temperature during growth. Active channel thicknesses of 20 nm and 30 nm were compared with lattice matched layers. Transmission electron microscope image analysis indicates no misfit dislocations in these structures. Hall-effect measurements at 300 K show an increase in the mobility from 8,380 cm²/Vs for the lattice matched layer to 12,500 cm²/Vs for the 30 nm pseudomorphic layer. Small gate-length, 0.25 μn, MODFETs were fabricated to determine effective velocity values from transconductance (g _m ) and current gain (h ₂₁ ) measurements. The peak dc extrinsicg _m increased from 367 mS/mm for the lattice matched layer to 668 mS/mm for the 30 nm pseudomorphic layer. The effective electron carrier velocity increased from 1.57 × 10⁷ cm/s for the lattice matched layer to 1.88 × 10⁷ cm/s for the 30 nm pseudomorphic layer. Our results show that compositional grading is a useful technique to obtain thick pseudomorphic layers with good transport properties.