Effect of rapid thermal annealing on the structural and electrical properties of TiO2 thin films prepared by plasma enhanced CVD

Titanium oxide thin films were deposited on p-type Si(100), SiO₂/Si, and Pt/Si substrates by plasma enhanced chemical vapor deposition using high purity Ti(O-i-C₃H₇)₄ and oxygen. As-deposited amorphous TiO₂ thin films were treated by rapid thermal annealing (RTA) in oxygen ambient, and the effects of RTA conditions on the structural and electrical properties of TiO₂ films were studied in terms of crystallinity, microstructure, current leakage, and dielectric constant. The dominant crystalline structures after 600 and 800 ‡C annealing were an anatase phase for the TiO₂ film on SiO₂/Si and a rutile phase for the film on a Pt/Si substrate. The dielectric constant of the as-grown and annealed TiO₂ thin films increased depending on the substrate in the order of Si, SiO₂/Si, and Pt/ Si. The SiO₂ thin layer was effective in preventing the formation of titanium silicide at the interface and current leakage of the film. TEM photographs showed an additional growth of SiO_x from oxygen supplied from both SiO₂ and TiO₂ films when the films were annealed at 1000 ‡C in an oxygen ambient. Intensity analysis of Raman peaks also indicated that optimizing the oxygen concentration and the annealing time is critical for growing a TiO₂ film having high dielectric and low current leakage characteristics.     

Amorphous LiCoO2 thin films on Li1+x+yAlxTi2−xSiyP3−yO12 prepared by radio frequency magnetron sputtering for all-solid-state Li-ion batteries

Amorphous LiCoO₂ thin films were deposited on the NASICON-type glass ceramics, Li_1+x+yAl_xTi_2−xSi_yP_3−yO₁₂ (LATSP), by radio frequency (RF) magnetron sputtering below 180 °C. The as-deposited LiCoO₂ thin films were characterized by X-ray diffraction, scanning electron microscopy and atomic force microscope. All-solid-state Li/PEO₁₈-Li (CF₃SO₂)₂N/LATSP/LiCoO₂/Au cells were fabricated using the amorphous film. The electrochemical performance of the cells was investigated by galvanostatic cycling, cyclic voltammetry, potentiostatic intermittent titration technique and electrochemical impedance spectroscopy. It was found that the amorphous LiCoO₂ thin film shows a promising electrochemical performance, making it a potential application in microbatteries for microelectronic devices.     

SiC formation for a solar cell passivation layer using an RF magnetron co-sputtering system

In this paper, we describe a method of amorphous silicon carbide film formation for a solar cell passivation layer. The film was deposited on p-type silicon (100) and glass substrates by an RF magnetron co-sputtering system using a Si target and a C target at a room-temperature condition. Several different SiC [Si_1-xC_x] film compositions were achieved by controlling the Si target power with a fixed C target power at 150 W. Then, structural, optical, and electrical properties of the Si_1-xC_x films were studied. The structural properties were investigated by transmission electron microscopy and secondary ion mass spectrometry. The optical properties were achieved by UV-visible spectroscopy and ellipsometry. The performance of Si_1-xC_x passivation was explored by carrier lifetime measurement.     

Film thickness dependence of the dielectric properties of SrTiO3/BaTiO3 multilayer thin films deposited by double target RF magnetron sputtering

Four-layer SrTiO₃/BaTiO₃ thin films ((ST/BT)₄) with various thicknesses deposited on Pt/Ti/SiO₂/Si substrates at 500 °C by double target RF magnetron sputtering have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), profilometry, capacitance-voltage and current-voltage measurements. The XRD patterns reveal the frame formation of the sputter deposited (ST/BT)₄ with controlled modulation. The adhesion between the Pt bottom electrode layer and the BT layer is excellent. The dielectric constant of the (ST/BT)₄ multilayer thin film increases with increasing film thickness. The effects of temperature, frequency, and bias voltage on the dielectric constant of the (ST/BT)₄ multilayer thin films are discussed in detail. The leakage current density of the (ST/BT)₄ multilayer with a thickness of 450.0 nm is lower than 1.0 × 10⁻⁸ A/cm² for the applied voltage of less than 5 V, showing that the multilayer thin films with such a characteristic could be applied for use in dynamic random access memory (DRAMs) capacitors.     

Structural,electric and multiferroic properties of Sm-doped BiFeO3 thin films prepared by the sol–gelprocess

Pure polycrystalline Bi_1−xSm_xFeO₃ (BSFO) (x=0–0.12) thin films were successfully prepared on FTO/glass substrates by the sol–gel method. The influence of Sm doping on the structure, dielectric, leakage current, ferroelectric and ferromagnetic properties of the BSFO films was investigated. X-ray diffraction analysis and FE-SEM images both reveal a gradual rhombohedra to pseudo-tetragonal phase transition with the increase of Sm dopant content. On one hand, a proper amount of Sm doping can decrease the leakage current densities of the BSFO thin films. On the other hand, excess Sm substitution for Bi will lead to multiphase coexistence in the film, the lattice inhomogeneity results in more defects in the film, which can increase the leakage current density. The result shows that defects in the complexes lead to electric domain back-switching in the BSFO_x=0.06 thin film, resulting in a decreased dielectric constant, leakage current and remanent polarization. The BSFO_x=0.09 thin film is promising in practical application because of its highest dielectric constant, remanent polarization and remanent magnetization of 203–185, 70 μC/cm² and 1.31 emu/cm³, respectively.     

Preparation and dielectric properties of dense and amorphous alumina film by sol–gel technology

Dense and crack-free aluminum oxide films were fabricated by sol–gel spin-coating technology. Aluminum nitrate (Al(NO₃)₃.9H₂O) was used as the precursor material. X-ray diffraction shows that the fabricated films are amorphous. X-ray photoelectron spectroscopy confirms that the thin films are alumina (Al₂O₃). Field-emission scanning electron microscopy images of the films reveal that the films are compact with a dense cross section. Dielectric measurements were carried out on samples with a metal–insulator–metal structure. The electrical characteristics of the films were affected by the thermal sintering temperature of the films. The leakage current density of the films decreases with the increase in the sintering temperature and increases with the increase in the measuring temperature. The leakage current shows a linear dependence on the voltage in the low-electric field-regime. The current density ascends to higher values due to the effect of space charges in the high-electric-field regime. The ionization energy of the top-electrode metals (Au, Pt or Ti–Au) has a strong effect on the leakage current.     

Effect of thermal treatment on the electrical properties of the sol–gel-derived (Zr,Sn)TiO4 thin films

The effect of heating temperatures on the electrical properties of sol–gel-derived (Zr,Sn)TiO₄ thin films deposited on a p-type (1 0 0) Si substrate was studied. The leakage currents of films with two different heating temperatures chosen to burn-out the solvent as a function of applied voltage were measured at different temperatures. The activation energies obtained from the Arrhenius plot of the leakage current density versus measured temperature for (Zr,Sn)TiO₄ films were then extracted. Additionally, microstructures of films with two different heating temperatures chosen to burn-out the solvent were analyzed by a conductive atomic force microscope (AFM) and an X-ray diffraction (XRD). Finally, the conductive mechanisms of leakage current and leakage current correlated to microstructures were also discussed.     

Enhanced resistive switching phenomena using low-positive-voltage format and self-compliance IrOx/GdOx/W cross-point memories

Enhanced resistive switching phenomena of IrO_x/GdO_x/W cross-point memory devices have been observed as compared to the via-hole devices. The as-deposited Gd₂O₃ films with a thickness of approximately 15 nm show polycrystalline that is observed using high-resolution transmission electron microscope. Via-hole memory device shows bipolar resistive switching phenomena with a large formation voltage of -6.4 V and high operation current of >1 mA, while the cross-point memory device shows also bipolar resistive switching with low-voltage format of +2 V and self-compliance operation current of <300 μA. Switching mechanism is based on the formation and rupture of conducting filament at the IrO_x/GdO_x interface, owing to oxygen ion migration. The oxygen-rich GdO_x layer formation at the IrO_x/GdO_x interface will also help control the resistive switching characteristics. This cross-point memory device has also Repeatable 100 DC switching cycles, narrow distribution of LRS/HRS, excellent pulse endurance of >10,000 in every cycle, and good data retention of >10⁴ s. This memory device has great potential for future nanoscale high-density non-volatile memory applications.     

Texture effect on the electrochemical properties of LiCoO2 thin films prepared by PLD

LiCoO₂ thin films were prepared by pulsed laser deposition (PLD) on Pt/Ti/SiO₂/Si (Pt) and Au/MgO/Si (Au) substrates, respectively. Crystal structures and surface morphologies of thin films were investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The LiCoO₂ thin films deposited on the Pt substrates exhibited a preferred (0 0 3) texture with smooth surfaces while the LiCoO₂ thin films deposited on the Au substrates exhibited a preferred (1 0 4) texture with rough surfaces. The electrochemical properties of the LiCoO₂ films with different textures were compared with charge-discharge, dQ/dV, and Li diffusion measurements (PITT). Compared with the (1 0 4)-textured LiCoO₂ thin films, the (0 0 3)-textured thin films exhibited relatively lower electrochemical activity. However, the advantage of the (1 0 4)-textured film only remained for a small number of cycles due to the relatively faster capacity fade. Li diffusion measurements showed that the Li diffusivity in the (0 0 3)-textured film is one order of magnitude lower than that in the (1 0 4)-textured film. As discussed in this paper, we believe that Li diffusion through grain boundaries is comparable to or even faster than Li diffusion through the grains.     

New understanding of hardening mechanism of TiN/SiNx-based nanocomposite films

In order to clarify the controversies of hardening mechanism for TiN/SiN_x-based nanocomposite films, the microstructure and hardness for TiN/SiN_x and TiAlN/SiN_x nanocomposite films with different Si content were studied. With the increase of Si content, the crystallization degree for two series of films firstly increases and then decreases. The microstructural observations suggest that when SiN_x interfacial phase reaches to a proper thickness, it can be crystallized between adjacent TiN or TiAlN nanocrystallites, which can coordinate misorientations between nanocrystallites and grow coherently with them, resulting in blocking of the dislocation motions and hardening of the film. The microstructure of TiN/SiN_x-based nanocomposite film can be characterized as the nanocomposite structure with TiN-based nanocrystallites surrounded by crystallized SiN_x interfacial phase, which can be denoted by nc-TiN/c-SiN_x model (''c’ before SiN_x means crystallized) and well explain the coexistence between nanocomposite structure and columnar growth structure within the TiN/SiN_x-based film.     

Resistive switching memory characteristics of Ge/GeOx nanowires and evidence of oxygen ion migration

The resistive switching memory of Ge nanowires (NWs) in an IrO_x/Al₂O₃/Ge NWs/SiO₂/p-Si structure is investigated. Ge NWs with an average diameter of approximately 100 nm are grown by the vapor–liquid-solid technique. The core-shell structure of the Ge/GeO_x NWs is confirmed by both scanning electron microscopy and high-resolution transmission electron microscopy. Defects in the Ge/GeO_x NWs are observed by X-ray photoelectron spectroscopy. Broad photoluminescence spectra from 10 to 300 K are observed because of defects in the Ge/GeO_x NWs, which are also useful for nanoscale resistive switching memory. The resistive switching mechanism in an IrO_x/GeO_x/W structure involves migration of oxygen ions under external bias, which is also confirmed by real-time observation of the surface of the device. The porous IrO_x top electrode readily allows the evolved O₂ gas to escape from the device. The annealed device has a low operating voltage (<4 V), low RESET current (approximately 22 μA), large resistance ratio (>10³), long pulse read endurance of >10⁵ cycles, and good data retention of >10⁴ s. Its performance is better than that of the as-deposited device because the GeO_x film in the annealed device contains more oxygen vacancies. Under SET operation, Ge/GeO_x nanofilaments (or NWs) form in the GeO_x film. The diameter of the conducting nanofilament is approximately 40 nm, which is calculated using a new method.     

Microstructure,optical and electrical properties of sputtered HfTiO high-k gate dielectric thin films

The microstructure, optical and electrical properties of HfTiO high-k gate dielectric thin films deposited on Si substrate and quartz substrate by RF magnetron sputtering have been investigated. Based on analysis from x-ray diffraction (XRD) measurements, it has been found that the as-deposited HfTiO films remain amorphous regardless of the working gas pressure. Meanwhile, combined with characterization of ultraviolet-visible spectroscopy (UV–vis) and spectroscopy ellipsometry (SE), the deposition rate, band gap and optical properties of sputtered HfTiO gate dielectrics were determined. Besides, by means of the characteristic curves of high frequency capacitance–voltage (C–V) and leakage current density–voltage (J–V), the electrical parameters, such as permittivity, total positive charge density, border trap charge density, and leakage current density, have been obtained. The leakage current mechanisms are also discussed. The energy band gap of 3.70 eV, leakage current density of 1.39×10⁻⁵ A/cm² at bias voltage of 2 V, and total positive charge density and border trap charge density of 9.16×10¹¹ cm⁻² and 1.3×10¹¹ cm⁻², respectively render HfTiO thin films deposited at 0.6 Pa, potential high-k gate dielectrics in future CMOS devices.     

Fabrication of Si heterojunction solar cells using P-doped Si nanocrystals embedded in SiNx films as emitters

Si heterojunction solar cells were fabricated on p-type single-crystal Si (sc-Si) substrates using phosphorus-doped Si nanocrystals (Si-NCs) embedded in SiN_x (Si-NCs/SiN_x) films as emitters. The Si-NCs were formed by post-annealing of silicon-rich silicon nitride films deposited by electron cyclotron resonance chemical vapor deposition. We investigate the influence of the N/Si ratio in the Si-NCs/SiN_x films on their electrical and optical properties, as well as the photovoltaic properties of the fabricated heterojunction devices. Increasing the nitrogen content enhances the optical gap E₀₄ while deteriorating the electrical conductivity of the Si-NCs/SiN_x film, leading to an increased short-circuit current density and a decreased fill factor of the heterojunction device. These trends could be interpreted by a bi-phase model which describes the Si-NCs/SiN_x film as a mixture of a high-transparency SiN_x phase and a low-resistivity Si-NC phase. A preliminary efficiency of 8.6% is achieved for the Si-NCs/sc-Si heterojunction solar cell.     

Deposition of amorphous carbon films from C60 fullerene sublimated in electron beam excited plasma

C₆₀ fullerene clusters are used as a carbon source for amorphous carbon films deposition in an electron beam excited plasma. C₆₀ clusters are sublimated by heating a ceramic crucible containing the C₆₀ powders up to 850 °C, which is located in a highly vacuumed process chamber. The sublimated fullerene powders are injected to the electron beam excited argon plasma and dissociated to be active species that are propelled toward the substrates. Consequently, the carbon species condense as a thin film onto the negatively biased substrates that are immersed in the plasma. Deposition rates of approximately 1.0 μm/h and the average surface roughness of 0.2 nm over an area of 400 μm² are achieved. Decomposition of the C₆₀ fullerene after injecting into the plasma is confirmed by optical emission spectroscopy that shows existence of small carbon species such as C₂ in the plasma. X-ray diffraction pattern reveals that the microstructure of the film is amorphous, while fullerene films deposited without the plasma show crystalline structure. Raman spectroscopic analysis shows that the films deposited in the plasma are one of the types of diamond-like carbon films. Different negative bias voltages have been applied to the substrate holder to examine the effect of the bias voltage to the properties of the films. The nano-indentation technique is used for hardness measurement of the films and results in hardness up to about 28 GPa. In addition, the films are droplet-free and show superior lubricity.     

Annealing-temperature-modulated optical,electrical properties,and leakage current transport mechanism of sol–gel-processed high-k HfAlOx gate dielectrics

Deposition of HfAlO_x gate dielectric films on n-type Si and quartz substrates by sol-gel technique has been performed and the optical, electrical characteristics of the as-deposited HfAlO_x thin films as a function of annealing temperature have been investigated. The optical properties of HfAlO_x thin films related to annealing temperature are investigated by ultraviolet-visible spectroscopy (UV–vis) and spectroscopy ellipsometry (SE). By measurement of UV–vis, average transmission of all the HfAlO_x samples are about 85% owing to their uniform composition. And the increase in band gap has been observed with the increase of annealing temperature. Moreover, the increase of refractive index (n) and density with the increase of annealing temperature are obtained by SE measurements. Additionally, the electrical properties based on Al/Si/HfAlO_x/Al capacitor are analyzed by means of the high frequency capacitance-voltage (C-V) and the leakage current density-voltage (J-V) characteristics. Results have shown that 400 °C-annealed sample demonstrates good electrical performance, including larger dielectric constant of 12.93 and lower leakage current density of 3.75×10⁻⁷ A/cm² at the gate voltage of 1 V. Additionally, the leakage current conduction mechanisms as functions of annealing temperatures are also discussed systematically.     

Investigation of Tb-doping on structural transition and multiferroic properties of BiFeO3 thin films

Pure BiFeO₃ (BFO) and Bi_1−xTb_xFeO₃ (BTFO) thin films were successfully prepared on FTO (fluorine doped tin oxide) substrates by the sol–gel spin-coating method. The effects of Tb-doping on the structural transition, leakage current, and dielectric and multiferroic properties of the BTFO thin films have been investigated systematically. XRD, Rietveld refinement and Raman spectroscopy results clearly reveal that a structural transition occurs from the rhombohedral (R3c:H) to the biphasic structure (R3c:H+R-3m:R) with Tb-doping. The leakage current density of BTFO_x=0.10 thin film is two orders lower than that of the pure BFO, i.e. 5.1×10⁻⁷ A/cm² at 100 kV/cm. Furthermore, the electrical conduction mechanism of the BTFO thin films is dominated by space-charge-limited conduction. The two-phase coexistence of BTFO_x=0.10 gives rise to the superior ferroelectric (2P_r=135.1 μC/cm²) and the enhanced ferromagnetic properties (M_s=6.3 emu/cm³). The optimal performance of the BTFO thin films is mainly attributed to the biphasic structure and the distorted deformation of FeO₆ octahedra.     

Spin-coating for fabrication of high-entropy high-k (AlTiVZrHf)Ox films on Si for advanced gate stacks

Spin-coating was performed to fabricate amorphous high-entropy oxide (HEO) (AlTiVZrHf)O_x films on Si substrates. The films were evaluated through X-ray photoelectron spectroscopy and through transmission electron microscopy (TEM)-based energy-dispersive spectroscopy (EDS) mapping, which respectively revealed all constituent elements and the homogenous distributions and no aggregation of those elements. TEM analysis revealed four distinct layers—a crystalline Al metal gate, an amorphous Al₂O₃ interfacial layer, amorphous (AlTiVZrHf)O_x, and native amorphous SiO₂—in a patterned metal–oxide–semiconductor (MOS) gate stack on the Si substrate. The resulting stack exhibited a low leakage current density (J_L) and no frequency dispersion in its capacitance–voltage characteristics after undergoing forming gas annealing. The obtained dielectric constant (k ≈ 32) for the HEO film is promising for advanced gate stacks and transistor applications. Although the film exhibited minor nanocrystallinity and the stack exhibited no interfacial Al₂O₃ layer after rapid thermal annealing at 900 °C, a low J_L, distinct layers, and a clearly defined interface between the (AlTiVZrHf)O_x and SiO₂ were observed, indicating no substantial diffusion among these layers. EDS mapping revealed the homogenous distribution of each constituent element without aggregation. Medium-entropy-oxide films and their MOS devices were fabricated for comparison; however, they exhibited inferior performance.     

Comparison of resistive switching characteristics using copper and aluminum electrodes on GeOx/W cross-point memories

Comparison of resistive switching memory characteristics using copper (Cu) and aluminum (Al) electrodes on GeO_x/W cross-points has been reported under low current compliances (CCs) of 1 nA to 50 μA. The cross-point memory devices are observed by high-resolution transmission electron microscopy (HRTEM). Improved memory characteristics are observed for the Cu/GeO_x/W structures as compared to the Al/GeO_x/W cross-points owing to AlO_x formation at the Al/GeO_x interface. The RESET current increases with the increase of the CCs varying from 1 nA to 50 μA for the Cu electrode devices, while the RESET current is high (>1 mA) and independent of CCs varying from 1 nA to 500 μA for the Al electrode devices. An extra formation voltage is needed for the Al/GeO_x/W devices, while a low operation voltage of ±2 V is needed for the Cu/GeO_x/W cross-point devices. Repeatable bipolar resistive switching characteristics of the Cu/GeO_x/W cross-point memory devices are observed with CC varying from 1 nA to 50 μA, and unipolar resistive switching is observed with CC >100 μA. High resistance ratios of 10² to 10⁴ for the bipolar mode (CCs of 1 nA to 50 μA) and approximately 10⁸ for the unipolar mode are obtained for the Cu/GeO_x/W cross-points. In addition, repeatable switching cycles and data retention of 10³ s are observed under a low current of 1 nA for future low-power, high-density, nonvolatile, nanoscale memory applications.     

Coexistence of memory resistance and memory capacitance in TiO2 solid-state devices

This work exploits the coexistence of both resistance and capacitance memory effects in TiO₂-based two-terminal cells. Our Pt/TiO₂/TiO _x /Pt devices exhibit an interesting combination of hysteresis and non-zero crossing in their current-voltage (I-V) characteristic that indicates the presence of capacitive states. Our experimental results demonstrate that both resistance and capacitance states can be simultaneously set via either voltage cycling and/or voltage pulses. We argue that these state modulations occur due to bias-induced reduction of the TiO _x active layer via the displacement of ionic species.     

Study on Resistance Switching Properties of Na0.5Bi0.5TiO3 Thin Films Using Impedance Spectroscopy

The Na_0.5Bi_0.5TiO₃ (NBT) thin films sandwiched between Au electrodes and fluorine-doped tin oxide (FTO) conducting glass were deposited using a sol–gel method. Based on electrochemical workstation measurements, reproducible resistance switching characteristics and negative differential resistances were obtained at room temperature. A local impedance spectroscopy measurement of Au/NBT was performed to reveal the interface-related electrical characteristics. The DC-bias-dependent impedance spectra suggested the occurrence of charge and mass transfer at the interface of the Au/NBT/FTO device. It was proposed that the first and the second ionization of oxygen vacancies are responsible for the conduction in the low- and high-resistance states, respectively. The experimental results showed high potential for nonvolatile memory applications in NBT thin films.