Oxygen-deficient Films Research Articles

Effects of V doping on magnetic and optical properties of oxygen-deficient In2O3 thin films

The scope of the work was to study structural, optical and magnetic behaviors of vanadium (V) doped indium oxide thin films prepared by thermal evaporation. Thin films of vanadium doped oxygen deficient In2O3 were deposited on sapphire, corning glass and Si(100) substrates. EDAX analysis revealed a great reduction in oxygen was happen during evaporation and oxygen deficient undoped and V doped In2O3 thin films were formed. XRD characterization suggested that the present In2O3 portion was in the amorphous state and the observed crystalline peaks were indexed as tetragonal metallic indium. The undoped In2O3 had the lowest transmittance and the transmittance increased with increasing V doping. The optical band gap and refractive index values decreased with increasing V doping. The dependence of magnetization on magnetic field for V doped In2O3 thin films were studied at 300K and 5K. All the samples exhibited soft ferromagnetic behaviour. The parameters of the hysteresis loop were extracted. The achieved results may find usage in magneto-optical applications.

Optical investigation of oxygen defect states in SrTiO3 epitaxial thin films

We investigated oxygen defect states in SrTiO3 (STO) thin films by using photoluminescence spectroscopy and spectroscopic ellipsometry. The oxygen-deficient STO homoepitaxial films were fabricated by using different oxygen partial pressures and flow rates for controlling the amount of oxygen vacancies systematically. The visible emission and absorption increased simultaneously with increasing oxygen vacancy concentration in the STO thin films. Given the absolute values of the optical constants, we discussed the origins of the visible absorption features in the STO thin films, in relation to the defect formation and related electronic reconstruction.

Effect of Ag and Ti electrodes on capacitance tunability of oxygen-deficient SrTiO3 based MIM varactors prepared by plasma sputtering

The role of oxygen deficiencies on the electrical performance of SrTiO3 films is investigated by depositing on the stack of TiN/Ti/SiO2/Si films using radio-frequency (RF) and direct current (DC) magnetron sputtering systems operating in oxygen-deficient plasma environment. Electrodes of silver (Ag) and titanium (Ti) are used to observe the effects of their sizes on the electrical properties of the SrTiO3 films. It is found that the nonlinearity in capacitance–voltage graphs (capacitance tunability) of the varactor can be controlled by varying the electrode area. In addition, the material of the electrode plays a vital role in controlling the non-linearity in the capacitance of the oxygen-deficient SrTiO3 dielectric films with varying voltage. It is found that the silver top electrode instead of titanium, delivers better performance with high capacitance tunability, low leakage current and good quality factor. Post deposition annealing at 550 °C in nitrogen ambient, creates more oxygen deficiencies in SrTiO3 film thereby increasing the linearity of the capacitance–voltage graph and decreasing the capacitance tunability.

Tunable electrical resistivity of oxygen-deficient zinc oxide thin films

Oxygen-deficient zinc oxide thin films were deposited by radio frequency magnetron sputtering with a sintered zinc oxide ceramic target under an atmosphere of manipulated sputtering gas pressure (SGP). Under the designed deposition conditions, all the prepared films were of hexagonal würtzite structure with c-axis as the preferential growth orientation. With increasing SGP, the film thickness, deposition rate, grain size and atomic ratio of O to Zn in the films initially increased and then decreased. Interestingly, when the SGP was low, the main defects in the films were oxygen vacancies; when it was high, the dominant defects were interstitial zinc; but when it was in a moderate value, the film composition might be relatively close to the stoichiometric ZnO, possessing the least number of defects. As a result, the electrical resistivity of the films first increased and then dropped down as the SGP increased.

High-performance varistors simply by hot-dipping zinc oxide thin films in Pr6O11: Influence of temperature

High-performance ZnO-Pr6O11 thin-film varistors were fabricated simply by hot-dipping oxygen-deficient zinc oxide thin films in Pr6O11 powder. The films had a composition of ZnO0.81 and a thickness of about 200 nm, which were deposited by radio frequency magnetron sputtering a sintered zinc oxide ceramic target. Special attention was paid on the temperature dependence of the varistors. In 50 min with hot-dipping temperature increased from 300–700 °C, the nonlinear coefficient (α) of the varistors increased, but with higher temperature it decreased again. Correspondingly, the leakage current (IL) decreased first and then increased, owing mainly to the formation and destroying of complete zinc oxide/Pr6O11 grain boundaries. The breakdown field (E1mA) decreased monotonously from 0.02217 to 0.01623 V/nm with increasing temperature (300–800 °C), due to the decreased number of effective grain boundaries in the varistors. The varistors prepared at 700 °C exhibited the optimum nonlinear properties with the highest α = 39.29, lowest IL = 0.02736 mA/cm2, and E1mA = 0.01757 V/nm. And after charge-discharge at room temperature for 1000 times, heating at 100 or 250 °C for up to 100 h, or applying at up to 250 °C, the varistors still performed well. Such nanoscaled thin-film varistors will be very promising in electrical/electronic devices working at low voltage.

Reprint of “Theoretical description of metal/oxide interfacial properties: The case of MgO/Ag(001)”

We compare the performances of different DFT functionals applied to ultra-thin MgO(100) films supported on the Ag(100) surface, a prototypical system of a weakly interacting oxide/metal interface, extensively studied in the past. Beyond semi-local DFT-GGA approximation, we also use the hybrid DFT-HSE approach to improve the description of the oxide electronic structure. Moreover, to better account for the interfacial adhesion, we include the van de Waals interactions by means of either the semi-empirical force fields by Grimme (DFT-D2 and DFT-D2*) or the self-consistent density functional optB88-vdW. We compare and discuss the results on the structural, electronic, and adhesion characteristics of the interface as obtained for pristine and oxygen-deficient Ag-supported MgO films in the 1–4ML thickness range.

Impact of oxygen stoichiometry on electroforming and multiple switching modes in TiN/TaOx/Pt based ReRAM

We have investigated the material and electrical properties of tantalum oxide thin films (TaOx) with engineered oxygen contents grown by RF-plasma assisted molecular beam epitaxy. The optical bandgap and the density of the TaOx films change consistently with oxygen contents in the range of 3.63 to 4.66 eV and 12.4 to 9.0 g/cm3, respectively. When exposed to atmosphere, an oxidized Ta2O5-y surface layer forms with a maximal thickness of 1.2 nm depending on the initial oxygen deficiency of the film. X-ray photoelectron spectroscopy studies show that multiple sub-stoichiometric compositions occur in oxygen deficient TaOx thin films, where all valence states of Ta including metallic Ta are possible. Devices of the form Pt/Ta2O5-y/TaOx/TiN exhibit highly tunable forming voltages of 10.5 V to 1.5 V with decreasing oxygen contents in TaOx. While a stable bipolar resistive switching (BRS) occurs in all devices irrespective of oxygen content, unipolar switching was found to coexist with BRS only at higher oxygen contents, which transforms to a threshold switching behaviour in the devices grown under highest oxidation.

Semiconductor/Piezoelectrics Hybrid Heterostructures with Highly Effective Gate-Tunable Electrotransport and Magnetic Behaviors.

We report the epitaxial growth of oxygen deficient titanium dioxide thin films on 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) single crystals and realized highly effective in situ electrostatic manipulation of electrotransport and magnetism of TiO2-δ films via gate voltages. Upon the polarization switching in the PMN-PT, the carrier density of the TiO2-δ film could be reversibly modified, resulting in a large nonvolatile resistivity modulation by ∼51% at T = 300 K, approximately 4-12 times larger than that of other transition-metal oxide film/PMN-PT structures. By taking advantage of in situ manipulation of the carrier density via gate voltages, we found that competition between the trap of electrons by the Ti3+-VO pairs and that by the positive polarization charges at the interface results in a significant resistivity relaxation upon the polarization switching, and revealed that magnetization is inversely correlated with the carrier density of the TiO2-δ film. Such hybrid structures combining materials with dissimilar functionalities may have potential applications in multifunctional devices which can take advantage of the useful and unique properties of both materials.

The oxygen-deficient TiO2 films deposited by a dual magnetron sputtering

This paper describes the deposition of the oxygen-deficient TiO2 films by the dual magnetron sputtering with different magnetic field configurations. The XRD survey and calculations of optical band gap demonstrate that the TiO2 films are polycrystalline with a mixture of anatase and rutile phases and have a low content of unbound Ti atoms. SEM and AFM investigations result in a columnar structure of the TiO2 films in the case of the mirror magnetic field configuration sputtering. For the closed configuration magnetic field, more intense ion bombardment of the growing films leads to the films densification. Optical and photocatalytic properties of the TiO2 films are strongly depended on the magnetic field configuration and determined by the films structure. The obtained results are interesting to modify the deposition technology of low-e coatings and solar materials.

Theoretical description of metal/oxide interfacial properties: The case of MgO/Ag(001)

We compare the performances of different DFT functionals applied to ultra-thin MgO(100) films supported on the Ag(100) surface, a prototypical system of a weakly interacting oxide/metal interface, extensively studied in the past. Beyond semi-local DFT-GGA approximation, we also use the hybrid DFT-HSE approach to improve the description of the oxide electronic structure. Moreover, to better account for the interfacial adhesion, we include the van de Waals interactions by means of either the semi-empirical force fields by Grimme (DFT-D2 and DFT-D2*) or the self-consistent density functional optB88-vdW. We compare and discuss the results on the structural, electronic, and adhesion characteristics of the interface as obtained for pristine and oxygen-deficient Ag-supported MgO films in the 1–4ML thickness range.

Highly nonlinear varistors from oxygen-deficient zinc oxide thin films by hot-dipping in Bi2O3: Influence of temperature

Highly nonlinear varistors were fabricated by hot-dipping oxygen-deficient zinc oxide (ZnO1−x, x<1) thin films in Bi2O3. The ZnO1−x films were deposited on conducting silicon chips by radio frequency magnetron sputtering of a sintered zinc oxide ceramic target. Then the films were hot-dipped at a temperature from 200 to 600°C in Bi2O3. With the increase in hot-dipping temperature, the nonlinear coefficient (α) of the film varistors increases first and then decreases, and the leakage current (IL) correspondingly decreases initially and then increases, owing mainly to the formation and destroying of complete ZnO1−x/Bi2O3 grain boundaries and the roundness change of the ZnO1−x grains; and the varistor voltage (E1mA) decreases from 0.0268 to 0.0137V/nm, due to the decreased number of effective grain boundaries in the materials. The film varistors prepared by hot-dipping at 400°C exhibit the optimum nonlinear properties with the highest α=15.1, lowest IL=0.0223mA/cm2, and E1mA=0.0176V/nm. Such nanoscaled film varistors will be very promising in electrical/electronic devices working in low-voltage.

Exotic orange to green luminescence conversion in vacuum annealed ZnO:Sn thin films

Post deposition annealing is an important processing which can modify the properties of thin films. Even though the properties of Sn doped ZnO (ZnO:Sn) thin films have been extensively studied, here observed an unaccustomed photoluminescence conversion from orange band for the as-deposited film to green band for the films annealed at high temperatures under high vacuum. This strange luminescence property could be attributed to the transition from oxygen rich film to oxygen deficient film under vacuum annealing. The crystallinity and transmittance in the visible and near infrared region improved with increase in annealing temperature. The optical energy gap red-shifted with heat treatment due the stress arisen in the film during annealing. Further, the carrier concentration and mobility increased with annealing but a reduction in mobility was observed for the film annealed at 700°C due to increased ionized impurity scattering. Consequently the resistivity decreased with increase in annealing temperature.

IR emission and electrical conductivity of Nd/Nb-codoped TiOx (1.5 < x < 2) thin films grown by pulsed-laser deposition

The effect of the co-doping with Nd and Nb on electrical and optical properties of TiOx films is reported. The role of oxygen vacancies on the physical properties is also evidenced. The films are grown by pulsed-laser deposition onto (001) sapphire and (100) silicon substrates. The substrate temperature was fixed at 700°C. To obtain either stoichiometric (TiO2) or highly oxygen deficient (TiOx with x<1.6) thin films, the oxygen partial pressure was adjusted at 10−1 and 10−6mbar, respectively. 1%Nd-1%Nb, 1%Nd-5%Nb and 5%Nd-1%Nb co-doped TiO2 were used as bulk ceramic target. Composition, structural and morphological properties of films determined by Rutherford backscattering spectroscopy, X-ray diffraction and scanning electron microscopy, are correlated to their optical (UV–vis transmission and photoluminescence) and electrical properties (resistivity at room temperature). The most intense Nd3+ emission in the IR domain is obtained for stoichiometric films. Codoping Nd-TiOx films by Nb5+ ions is found to decrease the photoluminescence efficiency. The oxygen pressure during the growth allows to tune the optical and electrical properties: insulating and highly transparent (80% in the visible range) Nd/Nb codoped TiO2 films are obtained at high oxygen pressure, while conductive and absorbent films are grown under low oxygen pressure (10−6mbar).

Effect of Oxygen-deficiencies on Resistance Switching in Amorphous YFe0.5Cr0.5O3-d films.

Herein, we demonstrate the contribution of the oxygen-deficiencies on the bipolar resistance switching (RS) properties of amorphous-YFe0.5Cr0.5O3−d (a-YFCO) films. The a-YFCO films were prepared under various oxygen pressures to tune the concentration of oxygen-deficiencies in the films. The XPS data verify that the oxygen-deficiencies increase with decreasing oxygen pressure. The RS property becomes more pronounced with more oxygen-deficiencies in a-YFCO films. Based on the Ohmic conduction measurements in the low resistance state, we confirm that the RS mechanism is related to the migration of oxygen-deficiencies. The enhanced RS and long retention in a-YFCO suggest a great potential for applications in nonvolatile memory devices.

Switching of magnetic easy-axis using crystal orientation for large perpendicular coercivity in CoFe2O4 thin film

Perpendicular magnetization and precise control over the magnetic easy axis in magnetic thin film is necessary for a variety of applications, particularly in magnetic recording media. A strong (111) orientation is successfully achieved in the CoFe2O4 (CFO) thin film at relatively low substrate temperature of 100 °C, whereas the (311)-preferred randomly oriented CFO is prepared at room temperature by the DC magnetron sputtering technique. The oxygen-deficient porous CFO film after post-annealing gives rise to compressive strain perpendicular to the film surface, which induces large perpendicular coercivity. We observe the coercivity of 11.3 kOe in the 40-nm CFO thin film, which is the highest perpendicular coercivity ever achieved on an amorphous SiO2/Si substrate. The present approach can guide the systematic tuning of the magnetic easy axis and coercivity in the desired direction with respect to crystal orientation in the nanoscale regime. Importantly, this can be achieved on virtually any type of substrate.

Hafnium carbide formation in oxygen deficient hafnium oxide thin films

On highly oxygen deficient thin films of hafnium oxide (hafnia, HfO2−x) contaminated with adsorbates of carbon oxides, the formation of hafnium carbide (HfCx) at the surface during vacuum annealing at temperatures as low as 600 °C is reported. Using X-ray photoelectron spectroscopy the evolution of the HfCx surface layer related to a transformation from insulating into metallic state is monitored in situ. In contrast, for fully stoichiometric HfO2 thin films prepared and measured under identical conditions, the formation of HfCx was not detectable suggesting that the enhanced adsorption of carbon oxides on oxygen deficient films provides a carbon source for the carbide formation. This shows that a high concentration of oxygen vacancies in carbon contaminated hafnia lowers considerably the formation energy of hafnium carbide. Thus, the presence of a sufficient amount of residual carbon in resistive random access memory devices might lead to a similar carbide formation within the conducting filaments due to Joule heating.

Kinetics versus Charge Separation: Improving the Activity of Stoichiometric and Non-Stoichiometric Hematite Photoanodes Using a Molecular Iridium Water Oxidation Catalyst

Oxygen-deficient iron oxide thin films, which have recently been shown to be highly active for photoelectrochemical water oxidation, were surface-functionalized with a monolayer of a molecular iridium water oxidation cocatalyst. The iridium catalyst was found to dramatically improve the kinetics of the water oxidation reaction at both stoichiometric and nonstoichiometric α-Fe2O3-x surfaces. This was found to be the case in both the dark and in the light as evidenced by cyclic voltammetry, Tafel analysis, and electrochemical impedance spectroscopy (EIS). Oxygen evolution measurements under working conditions confirmed high Faradaic efficiencies of 69–100% and good stability over 22 h of operation for the functionalized electrodes. The resulting ∼200–300 mV shift in onset potential for the iridium-functionalized sample was attributed to improved interfacial charge transfer and oxygen evolution kinetics. Mott–Schottky plots revealed that there was no shift in flat-band potential or change in donor density fo...

Nano-crystallization in ZnO-doped In2O3 thin films via excimer laser annealing for thin-film transistors

In a previous work, we reported the high field effect mobility of ZnO-doped In2O3 (IZO) thin film transistors (TFTs) irradiated by excimer laser annealing (ELA) [M. Fujii et al., Appl. Phys. Lett. 102, 122107 (2013)]. However, a deeper understanding of the effect of ELA on the IZO film characteristics based on crystallinity, carrier concentrations, and optical properties is needed to control localized carrier concentrations for fabricating self-aligned structures in the same oxide film and to adequately explain the physical characteristics. In the case of as-deposited IZO film used as the channel, a high carrier concentration due to a high density of oxygen vacancies was observed; such a film does not show the required TFT characteristics but can act as a conductive film. We achieved a decrease in the carrier concentration of IZO films by crystallization using ELA. This means that ELA can form localized conductive or semi-conductive areas on the IZO film. We confirmed that the reason for the carrier concentration decrease was the decrease of oxygen-deficient regions and film crystallization. The annealed IZO films showed nano-crystalline phase, and the temperature at the substrate was substantially less than the temperature limit for flexible films such as plastic, which is 50°C. This paves the way for the formation of self-aligned structures and separately formed conductive and semi-conductive regions in the same oxide film.

Subtle local structural variations in oxygen deficient niobium germanate thin film glasses as revealed by x-ray absorption spectroscopy

The local electronic and crystal structure of niobium-lead-germanate, Nb2O5-PbO- GeO2 (NPG), glass thin films on silicon substrates were probed by XANES and EXAFS. NPG glasses are promising candidates for applications in nonlinear optical devices because they exhibit interesting optical characteristics such as high nonlinear third order optical susceptibility. In this work NPG glasses were prepared with pulsed laser deposition method with varying oxygen partial pressure to induce thin films with different oxygen stoichiometry. Previously, it was shown that oxygen stoichiometry has a very important effect to produce unusual high optical susceptibility. Detailed EXAFS and XANES analyses in a series of NPG thin films revealed the subtle variations in the local environment around Nb atoms and the Nb oxidation states caused by oxygen deficiencies.

Tuning the magnetic properties ofLa0.67Sr0.33CoO3−δfilms by oxygen pressure

In this paper, oxygen-deficient $\mathrm{L}{\mathrm{a}}_{0.67}\mathrm{S}{\mathrm{r}}_{0.33}\mathrm{Co}{\mathrm{O}}_{3}$ (LSCO) thin films are prepared by adjusting the oxygen pressure during the deposition. As oxygen vacancies are introduced into the LSCO films, the out-of-plane lattices are elongated, as revealed by x-ray diffraction and high-resolution transmission electron microscopy (HRTEM). Modulated bright and dark stripes observed in the HRTEM images of the oxygen-deficient LSCO films are induced by cation ordering and oxygen vacancy ordering, respectively. Magnetic measurements and first-principles calculations show that the oxygen vacancies depress the magnetism of the LSCO films greatly. We also demonstrate that doping Sr into $\mathrm{LaCo}{\mathrm{O}}_{3}$ and removing oxygen from LSCO have a similar effect on the magnetic properties, which is correlated with a derived formula.