Oxygen Ion Drift‐Induced Complementary Resistive Switching in Homo TiOx/TiOy/TiOx and Hetero TiOx/TiON/TiOx Triple Multilayer Frameworks

Developing a means by which to compete with commonly used Si‐based memory devices represents an important challenge for the realization of future three‐dimensionally stacked crossbar‐array memory devices with multifunctionality. Therefore, oxide‐based resistance switching memory (ReRAM), with its associated phenomena of oxygen ion drifts under a bias, is becoming increasingly important for use in nanoscalable crossbar arrays with an ideal memory cell size due to its simple metal–insulator–metal structure and low switching current of 10–100 μA. However, in a crossbar array geometry, one single memory element defined by the cross‐point of word and bit lines is highly susceptible to unintended leakage current due to parasitic paths around neighboring cells when no selective devices such as diodes or transistors are used. Therefore, the effective complementary resistive switching (CRS) features in all Ti‐oxide‐based triple layered homo Pt/TiO_x/TiO_y/TiO_x/Pt and hetero Pt/TiO_x/TiON/TiO_x/Pt geometries as alternative resistive switching matrices are reported. The possible resistive switching nature of the novel triple matrices is also discussed together with their electrical and structural properties. The ability to eliminate both an external resistor for efficient CRS operation and a metallic Pt middle electrode for further cost‐effective scalability will accelerate progress toward the realization of cross‐bar ReRAM in this framework.     

Epitaxial TiOx Surface in Ferroelectric BaTiO3: Native Structure and Dynamic Patterning at the Atomic Scale

Surfaces and interfaces of ferroelectric oxides exhibit enhanced functionality, and therefore serve as a platform for novel nano and quantum technologies. Experimental and theoretical challenges associated with examining the subtle electro‐chemo‐mechanical balance at metal‐oxide surfaces have hindered the understanding and control of their structure and behavior. Here, combined are advanced electron‐microscopy and first‐principles thermodynamics methods to reveal the atomic‐scale chemical and crystallographic structure of the surface of the seminal ferroelectric BaTiO₃. It is shown that the surface is composed of a native <2 nm thick TiO_x rock‐salt layer in epitaxial registry with the BaTiO₃. Using electron‐beam irradiation, artificial TiO_x sites with sub‐nanometer resolution are successfully patterned, by inducing Ba escape. Therefore, this work offers electro‐chemo‐mechanical insights into ferroelectric surface behavior in addition to a method for scalable high‐resolution beam‐induced chemical lithography for selectively driving surface phase transitions, and thereby functionalizing metal‐oxide surfaces.     

Light‐modulated TiOx Interlayer Dipole and Contact Activation in Organic Solar Cell Cathodes

Understanding working mechanisms of selective interfacial layers and the underlying energetics of the organic semiconductor/electrode interface is an issue of primary concern for improving organic solar cell technologies. TiO _x interlayers are used here to tune the selectivity of the cathode contact to electrons by controlled UV‐light activation. The S‐shaped kink observed for deactivated titania interlayers completely disappears after 2 min of UV‐light exposure yielding high fill factor (≈60%) and adequate efficiencies. UV‐light activation of complete cells alters the work function of the oxide that decreases about 650 mV as observed by Kelvin probe measurements. Capacitive techniques reveals a light‐intensity dependent shift in flat‐band voltage of up to 1.2 V under 1 sun illumination (without UV) in the case of deactivated TiO _x interlayers. An increase in the magnitude of the light‐modulated dipole present at the oxide layer accounts for that voltage shift. Although the sign of the interface dipole would favor the extraction of electrons, the concomitant modification of the band bending in the organic semiconductor hinders an efficient extraction of carriers at positive voltages and originates the S‐shaped characteristics. After contact activation, the dipole strength does not change with the light intensity.     

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.     

Watermelon‐Like Structured SiOx–TiO2@C Nanocomposite as a High‐Performance Lithium‐Ion Battery Anode

A unique watermelon‐like structured SiO_x–TiO₂@C nanocomposite is synthesized by a scalable sol–gel method combined with carbon coating process. Ultrafine TiO₂ nanocrystals are uniformly embedded inside SiO_x particles, forming SiO_x–TiO₂ dual‐phase cores, which are coated with outer carbon shells. The incorporation of TiO₂ component can effectively enhance the electronic and lithium ionic conductivities inside the SiO_x particles, release the structure stress caused by alloying/dealloying of Si component and maximize the capacity utilization by modifying the Si–O bond feature and decreasing the O/Si ratio (x‐value). The synergetic combination of these advantages enables the synthesized SiO_x–TiO₂@C nanocomposite to have excellent electrochemical performances, including high specific capacity, excellent rate capability, and stable long‐term cycleability. A stable specific capacity of ≈910 mAh g^?1 is achieved after 200 cycles at the current density of 0.1 A g^?1 and ≈700 mAh g^?1 at 1 A g^?1 for over 600 cycles. These results suggest a great promise of the proposed particle architecture, which may have potential applications in the improvement of various energy storage materials.     

Evaluating the Critical Thickness of TiO2 Layer on Insulating Mesoporous Templates for Efficient Current Collection in Dye‐Sensitized Solar Cells

In this paper, a way of utilizing thin and conformal overlayer of titanium dioxide on an insulating mesoporous template as a photoanode for dye‐sensitized solar cells is presented. Different thicknesses of TiO₂ ranging from 1 to 15 nm are deposited on the surface of the template by atomic layer deposition. This systematic study helps unraveling the minimum critical thickness of the TiO₂ overlayer required to transport the photogenerated electrons efficiently. A merely 6‐nm‐thick TiO₂ film on a 3‐μm mesoporous insulating substrate is shown to transport 8 mA/cm² of photocurrent density along with ≈900 mV of open‐circuit potential when using our standard donor‐π‐acceptor sensitizer and Co(bipyridine) redox mediator.     

Optical and Electrical Properties of TixSi1‐xOy Films

Ti_xSi_1xO_y (TSO) thin films are fabricated using plasma‐enhanced atomic layer deposition. The Ti content in the TSO films is controlled by adjusting the sub‐cycle ratio of TiO₂ and SiO₂. The refractive indices of SiO₂ and TiO₂ are 1.4 and 2.4, respectively. Hence, tailoring of the refractivity indices from 1.4 to 2.4 is feasible. The controllability of the refractive index and film thickness enables application of an antireflection coating layer to TSO films for use as a thin film solar cell. The TSO coating layer on an Si wafer dramatically reduces reflectivity compared to a bare Si wafer. In the measurement of the current‐voltage characteristics, a nonlinear coefficient of 13.6 is obtained in the TSO films.     

Atomic layer deposited TiOx/AlOx nanolaminates as moisture barriers for organic devices

Atomic layer deposited nanolaminates of alternating AlO_x and TiO_x thin-films are investigated as moisture barriers for organic electronic devices. Direct encapsulation on organic light emitting diodes (OLEDs) is tested in aging experiments and compared to calcium corrosion tests of equivalent barrier films. This allows for a direct assessment of moisture barrier performance in simple as well as more complex systems. Thickness variations are performed for the nanolaminate single and total layer thickness, with an optimum single layer thickness of 1–2 nm observed. This correlates to the maximum number of dyads once completely closed single layers are produced. For large single layer thickness and low dyad count, strong lateral diffusion from the edges occurs in the OLEDs, which likely correlates to poor mechanical stability. At optimum single layer thickness, barriers remain mechanically and chemically stable up to 100 nm total thickness. OLEDs encapsulated with such nanolaminate barriers show no significant degradation after 2500 h of continuous aging.     

High‐Performance Micro‐Solid Oxide Fuel Cells Fabricated on Nanoporous Anodic Aluminum Oxide Templates

Micro‐solid oxide fuel cells (μ‐SOFCs) are fabricated on nanoporous anodic aluminum oxide (AAO) templates with a cell structure composed of a 600‐nm‐thick AAO free‐standing membrane embedded on a Si substrate, sputter‐deposited Pt electrodes (cathode and anode) and an yttria‐stabilized zirconia (YSZ) electrolyte deposited by pulsed laser deposition (PLD). Initially, the open circuit voltages (OCVs) of the AAO‐supported μ‐SOFCs are in the range of 0.05 V to 0.78 V, which is much lower than the ideal value, depending on the average pore size of the AAO template and the thickness of the YSZ electrolyte. Transmission electron microscopy (TEM) analysis reveals the formation of pinholes in the electrolyte layer that originate from the porous nature of the underlying AAO membrane. In order to clog these pinholes, a 20‐nm thick Al₂O₃ layer is deposited by atomic layer deposition (ALD) on top of the 300‐nm thick YSZ layer and another 600‐nm thick YSZ layer is deposited after removing the top intermittent Al₂O₃ layer. Fuel cell devices fabricated in this way manifest OCVs of 1.02 V, and a maximum power density of 350 mW cm^?2 at 500 °C.     

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.     

Structural characterization of oxide layers thermally grown on 3C-SiC films

The oxidation of 3C-SiC films deposited on off-oriented Si(001) substrates by reactive magnetron sputtering has been studied. The oxidation was carried out using dry conditions at a temperature of 1200°C. The composition of the oxide layer was investigated by Auger electron spectroscopy (AES). The oxide layer was found to contain no C except for the region very close to the interface, and the stoichiometry was found to be close to that of SiO₂. Cross-sectional transmis-sion electron microscopy (XTEM) showed the oxide layer to be completely amorphous, dense, and homogeneous with a uniform thickness. High-resolution XTEM imaging showed an atomically sharp SiO₂/SiC interface.     

Thermal Stability Study of Cu(MoN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>) Seed Layer on Barrierless Si

This study reports the good thermal stability of a sputtered Cu(MoN _x ) seed layer on a barrierless Si substrate. A Cu film with a small amount of MoN _x was deposited by reactive co-sputtering of Cu and Mo in an Ar/N₂ gas mixture. After annealing at 560°C for 1 h, no copper silicide formation was observed at the interface of Cu and Si. Leakage current and resistivity evaluations reveal the good thermal reliability of Cu with a dilute amount of MoN _x at temperatures up to 560°C, suggesting its potential application in advanced barrierless metallization. The thermal performance of Cu(MoN _x ) as a seed layer was evaluated when pure Cu is deposited on top. X-ray diffraction, focused ion beam microscopy, and transmission electron microscopy results confirm the presence of an ∼10-nm-thick reaction layer formed at the seed layer/Si interface after annealing at 630°C for 1 h. Although the exact composition and structure of this reaction layer could not be unambiguously identified due to trace amounts of Mo and N, this reaction layer protects Cu from a detrimental reaction with Si. The Cu(MoN _x ) seed layer is thus considered to act as a diffusion buffer with stability up to 630°C for the barrierless Si scheme. An electrical resistivity of 2.5 μΩ cm was obtained for the Cu/Cu(MoN _x ) scheme after annealing at 630°C.     

Surface passivation of high‐efficiency silicon solar cells by atomic‐layer‐deposited Al2O3

Atomic‐layer‐deposited aluminium oxide (Al₂O₃) is applied as rear‐surface‐passivating dielectric layer to passivated emitter and rear cell (PERC)‐type crystalline silicon (c‐Si) solar cells. The excellent passivation of low‐resistivity p‐type silicon by the negative‐charge‐dielectric Al₂O₃ is confirmed on the device level by an independently confirmed energy conversion efficiency of 20·6%. The best results are obtained for a stack consisting of a 30 nm Al₂O₃ film covered by a 200 nm plasma‐enhanced‐chemical‐vapour‐deposited silicon oxide (SiO_x) layer, resulting in a rear surface recombination velocity (SRV) of 70 cm/s. Comparable results are obtained for a 130 nm single‐layer of Al₂O₃, resulting in a rear SRV of 90 cm/s. Copyright © 2008 John Wiley & Sons, Ltd.     

Highly Stable Colloidal “Giant” Quantum Dots Sensitized Solar Cells

Colloidal quantum dots (QDs) are widely studied due to their promising optoelectronic properties. This study explores the application of specially designed and synthesized “giant” core/shell CdSe/(CdS)_x QDs with variable CdS shell thickness, while keeping the core size at 1.65 nm, as a highly efficient and stable light harvester for QD sensitized solar cells (QDSCs). The comparative study demonstrates that the photovoltaic performance of QDSCs can be significantly enhanced by optimizing the CdS shell thickness. The highest photoconversion efficiency (PCE) of 3.01% is obtained at optimum CdS shell thickness ≈1.96 nm. To further improve the PCE and fully highlight the effect of core/shell QDs interface engineering, a CdSe_x S_1?x interfacial alloyed layer is introduced between CdSe core and CdS shell. The resulting alloyed CdSe/(CdSe_x S_1?x )₅/(CdS)₁ core/shell QD‐based QDSCs yield a maximum PCE of 6.86%, thanks to favorable stepwise electronic band alignment and improved electron transfer rate with the incorporation of CdSe_x S_1?x interfacial layer with respect to CdSe/(CdS)₆ core/shell. In addition, QDSCs based on “giant” core/CdS‐shell or alloyed core/shell QDs exhibit excellent long‐term stability with respect to bare CdSe‐based QDSCs. The giant core/shell QDs interface engineering methodology offers a new path to improve PCE and the long‐term stability of liquid junction QDSCs.     

New Strategy for Polysulfide Protection Based on Atomic Layer Deposition of TiO2 onto Ferroelectric‐Encapsulated Cathode: Toward Ultrastable Free‐Standing Room Temperature Sodium–Sulfur Batteries

The room temperature (RT) sodium–sulfur batteries (Na–S) hold great promise for practical applications including energy storage and conversion due to high energy density, long lifespan, and low cost, as well based on the abundant reserves of both sodium metal and sulfur. Herein, freestanding (C/S/BaTiO₃)@TiO₂ (CSB@TiO₂) electrode with only ≈3 wt% of BaTiO₃ additive and ≈4 nm thickness of amorphous TiO₂ atomic layer deposition protective layer is rational designed, and first used for RT Na–S batteries. Results show that such cathode material exhibits high rate capability and excellent durability compared with pure C/S and C/S/BaTiO₃ electrodes. Notably, this CSB@TiO₂ electrode performs a discharge capacity of 524.8 and 382 mA h g^?1 after 1400 cycles at 1 A g^?1 and 3000 cycles at 2 A g^?1, respectively. Such superior electrochemical performance is mainly attributed from the “BaTiO₃‐C‐TiO₂” synergetic structure within the matrix, which enables effectively inhibiting the shuttle effect, restraining the volumetric variation and stabilizing the ionic transport interface.     

Novel Diffusion‐Barrier Materials Against Oxygen for High‐Density Dynamic Random Access Memory Capacitors

A new design concept for diffusion barriers in high‐density memory capacitors is suggested, and both RuTiN (RTN) and RuTiO (RTO) films are proposed as sacrificial oxygen diffusion barriers. The newly developed RTN and RTO barriers show a much lower sheet resistance than various other barriers, including binary and ternary nitrides (reported by others), up to 800 °C, without a large increase in the resistance. For both the Pt/RTN/TiSi_x/n⁺⁺poly‐plug/n⁺ channel layer/Si and the Pt/RTO/RTN/TiSi_x/n⁺⁺poly‐plug/n⁺ channel layer/Si contact structures, contact resistance—the most important electrical parameter for the diffusion barrier in the bottom electrode structure of capacitors—was found to be as low as 5 kohm, even after annealing up to 750 °C. When the RTN film was inserted as a glue layer between the bottom Pt electrode layer and the TiN barrier film in the chemical vapor deposited (Ba,Sr)TiO₃ (CVD–BST) simple stack‐type structure, the RTN glue layer was observed to be thermally stable to temperatures 150 °C higher than that to which the TiN glue layer is stable. Moreover, the capacitance of the physical vapor deposited (PVD)–BST simple stack‐type structure adopted TiN glue layer initially degraded after annealing at 500 °C, and, thereafter, completely failed. In the case of the RTN and RTO/RTN glue layers, however, the capacitance continuously increased up to 550 °C. Thus, the new RTN and RTO films, which act as diffusion barriers to oxygen, are very promising materials for achieving high‐density capacitors.     

CuOx/a‐Si:H heterojunction thin‐film solar cell with an n‐type µc‐Si:H depletion‐assisting layer

We showed that thin n‐type CuO_x films can be deposited by radio‐frequency magnetron reactive sputtering and demonstrated the fabrication of n‐CuO_x/intrinsic hydrogenated amorphous silicon (i‐a‐Si:H) heterojunction solar cells (HSCs) for the first time. A highly n‐doped hydrogenated microcrystalline Si (n‐µc‐Si:H) layer was introduced as a depletion‐assisting layer to further improve the performance of n‐CuO_x/i‐a‐Si:H HSCs. An analysis of the external quantum efficiency and energy‐band diagram showed that the thin depletion‐assisting layer helped establish sufficient depletion and increased the built‐in potential in the n‐CuO_x layer. The fabricated HSC exhibited a high open‐circuit voltage of 0.715 V and an efficiency of 4.79%. Copyright © 2015 John Wiley & Sons, Ltd.     

The effect of Zn1−xSnxOy buffer layer thickness in 18.0% efficient Cd‐free Cu(In,Ga)Se2 solar cells

The influence of the thickness of atomic layer deposited Zn_1−xSn_xO_y buffer layers and the presence of an intrinsic ZnO layer on the performance of Cu(In,Ga)Se₂ solar cells are investigated. The amorphous Zn_1−xSn_xO_y layer, with a [Sn]/([Sn] + [Zn]) composition of approximately 0.18, forms a conformal and in‐depth uniform layer with an optical band gap of 3.3 eV. The short circuit current for cells with a Zn_1−xSn_xO_y layer are found to be higher than the short circuit current for CdS buffer reference cells and thickness independent. On the contrary, both the open circuit voltage and the fill factor values obtained are lower than the references and are thickness dependent. A high conversion efficiency of 18.0%, which is comparable with CdS references, is attained for a cell with a Zn_1−xSn_xO_y layer thickness of approximately 13 nm and with an i‐ZnO layer. Copyright © 2012 John Wiley & Sons, Ltd.     

Study of nitrogen impact on VFB-EOT roll-off by varying interfacial SiO2 thickness

Flat band voltage (V_FB) roll-off in long channel devices at thin equivalent oxide thickness (EOT) is studied on SiO₂/nitrided-HfSiO stacks. V_FB increases when SiO₂ interfacial layer thickness decreases, and charges pumping (CP) frequency sweep analysis shows higher trap density near Si/SiO₂ interface. Based on this observation, an atomic diffusion model is introduced. Higher concentration of nitrogen atom in the HfSiO(N) layer diffuses to the Si/SiO₂ interface through the SiO₂ layer in thinner SiO₂ device, and accumulates near Si/SiO₂ interface which can introduce higher density of interfacial traps. Lifetime extracted from negative bias temperature instability (NBTI), and mobility are also degraded in thinner SiO₂ devices due to the higher interfacial trap density.The V_FB roll-off can be improved by lowering nitrogen concentration in the HfSiO(N) layer from optimizing plasma nitridation pressure, decreasing post deposition anneal temperature, or using defect absorbing layer on the high-k oxide.