xO Thin Films Research Articles

Phase segregation in MgxZn1–xO probed by optical absorption and photoluminescence at high pressure

The appearance of segregated wurtzite MgxZn1–xO with low Mg content in thin films with x > 0.3 affected by phase separation cannot be reliably probed with crystallographic techniques owing to its embedded nanocrystalline configuration. Here, we show a high-pressure approach which exploits the distinctive behaviors under pressure of wurtzite MgxZn1–xO thin films with different Mg contents to unveil phase segregation for x > 0.3. By using ambient conditions, photoluminescence (PL), and with optical absorption and PL under high pressure for x = 0.3, we show that the appearance of a segregated wurtzite phase with a magnesium content of x ∼ 0.1 is inherent to the wurtzite and rock-salt phase separation. We also show that the presence of segregated wurtzite phase in oversaturated thin films phase is responsible for the low-energy absorption tail observed above x = 0.3 in our MgxZn1–xO thin films. Our study has also allowed us to extend the concentration dependence of the pressure coefficient of the band gap from the previous limit of x = 0.13 to x ≈ 0.3 obtaining dEg/dP = 29 meV/GPa for wurtzite with x ≈ 0.3 and 25 meV/GPa for the segregated x ≈ 0.09 wurtzite phase.

The structural and multiferroic properties of Bi(1−x)Ce x Fe(1−x)(Mg0.5Ti0.5) x O3 thin films

Bi(1−x)Ce x Fe(1−x)(Mg0.5Ti0.5) x O3 (x = 0, 0.05, 0.10, 0.20, 0.25) thin films were synthesized by a sol–gel method. The structural, electrical and magnetic properties of Bi(1−x)Ce x Fe(1−x)(Mg0.5Ti0.5) x O3 (x = 0, 0.05, 0.10, 0.20, 0.25) thin films have been investigated. BiFeO3 thin film possessed a perovskite-type rhombohedral structure with space group R3c, and the average grain size decreased as the concentration of Ce, Mg and Ti co-doping increased. The leakage current densities of BiFeO3 co-doping with Ce, Mg and Ti ions exhibited a significant reduction compared with that of BiFeO3 thin film. Compared with the leakage current density of BiFeO3 thin film (4.19 × 10−4 A/cm2), the leakage current density of Bi0.80Ce0.20Fe0.80Mg0.10Ti0.10O3 thin film was reduced by about four orders of magnitude (1.05 × 10−8 A/cm2) under the electric field of 300 kV/cm. Well-defined ferroelectric loops were obtained and the polarization increased with the increase of Ce, Ti and Mg co-doping. The remanent polarization of Bi0.80Ce0.20Fe0.80Mg0.10Ti0.10O3 thin film (2P r ~34.9 μC/cm2) was approximately four times larger than that of pure BiFeO3 (2P r ~7.8 μC/cm2) under the applied field of 300 kV/cm. The magnetization under 10 kOe magnetic field of Bi(1−x)Ce x Fe(1−x)(Mg0.5Ti0.5) x O3 thin films increased with the increasing concentration of Ce, Mg and Ti co-doping. The improvement of magnetic behaviors for BiFeO3 co-doping with Ce, Mg and Ti ions thin films were observed significantly, cooperating with the enhanced ferroelectricity which indicated that BiFeO3 co-doping with Ce, Mg and Ti ions thin films will be the promising materials in the application to magnetoelectric devices.

Resonant Raman scattering study of BexZn1−xO thin films grown on sapphire by molecular beam epitaxy

Resonance Raman spectra of Be[Formula: see text]Zn[Formula: see text]O alloy materials were studied using 325 nm Laser. The research showed that the Raman spectra of Be[Formula: see text]Zn[Formula: see text]O alloys presents a dual-mode vibration. Compare Be[Formula: see text]Zn[Formula: see text]O alloy with ZnO single crystal, the A1 (LO) phonon vibration mode of Be[Formula: see text]Zn[Formula: see text]O alloy moved to the larger wave number direction. The position of A1 (LO) phonon vibration modes of Be[Formula: see text]Zn[Formula: see text]O and Be[Formula: see text]Zn[Formula: see text]O was 580 cm[Formula: see text] and 582 cm[Formula: see text], respectively. In addition, the temperature-dependent Raman spectroscopy was employed for Be[Formula: see text]Zn[Formula: see text]O, and the phonon mode frequency shift with temperature was studied in detail. Finally, the stability of the polar and nonpolar Be[Formula: see text]Zn[Formula: see text]O alloy materials was studied using resonance Raman spectroscopy. The results showed that the A1 (LO) phonon mode frequency of polar Be[Formula: see text]Zn[Formula: see text]O alloy remained in the same position, while the nonpolar Be[Formula: see text]Zn[Formula: see text]O alloys moved nearly 3.5 cm[Formula: see text] to larger direction after being placed in the air for two years. The reason may be that the stability of the nonpolar Be[Formula: see text]Zn[Formula: see text]O alloy is relatively poor upon interaction with molecule such as H2O, O2 in the air.

Chelate route for the synthesis of PbZrxTi1−xO3 thin films

A chemical solution deposition route using the chelate approach was developed for the synthesis of PbZrxTi1−xO3 thin films. The route avoids the highly toxic 2-methoxyethanol used in previous formulations and offers the advantage of a simple and rapid solution synthesis without the need of distillation and refluxing strategies. The use of acetoin as chelating agent allows a transition between amorphous and final perovskite phases with no segregation of secondary phases during film growth. The obtained PbZrxTi1−xO3 films presented a thickness of ∼150 nm and exhibited good dielectric and ferroelectric properties. It was found that rapid thermal annealing facilitates the perovskite formation process and produces films with better ferroelectric properties than those sintered in conventional furnaces.

High electrochemical performance of Cu x Co3−x O4 nanostructured electrodes: the effect of spinel inversion and annealing temperature

CuxCo3 − xO4 thin films (with x = 0, 0.1, 0.3, and 0.5) were successfully deposited on glass and indium-doped tin oxide substrates by the sol-gel method. Despite the observed enhancement in crystallinity, a decrease in ion capacity was observed with the increasing of annealing temperature, which was attributed to the suppression of the diffusion of the electrolyte ions into the films. It was also deduced that Cu doping resulted in a significant increase in capacity with a maximum value of 2.92E-02 C/cm2 which was obtained for x = 0.3. The observed decrease in capacity after raising the x value from 0.3 to 0.5 was due to the formation of mixed spinel structure. The results also showed that the variation of optical absorption coefficient, optical band gap energy, electrical resistance, and doping density with x was consistent with the general characteristics of the formed mixed spinel structure when it exceeded x = 0.3.

As-grown enhancement of spinodal decomposition in spinel cobalt ferrite thin films by Dynamic Aurora pulsed laser deposition

Cobalt ferrite CoxFe3−xO4 thin films with composition within the miscibility gap were grown using Dynamic Aurora pulsed laser deposition. X-ray diffraction patterns reveal as-grown phase separation to Fe-rich and Co-rich phases with no post-deposition annealing. The interconnected surface microstructure of thin film shows that this phase separation occurs through spinodal decomposition enhanced by magnetic-field-induced ion-impingement. The lattice parameter variation of the thin films with the magnetic field indicates that the composition fluctuations can be enhanced further by increasing the magnetic field. Results show that spinodal decomposition enhancement by magnetic-field-induced ion-impingement is independent of the lattice-mismatch-induced strain. This approach can promote spinodal decomposition in any thin film with no post-deposition annealing process.

Valence state of Sm in single-crystalline EuO thin films

Samarium has two stable valence states, 2+ and 3+, which coexist in many compounds forming spatially homogeneous intermediate valence states. We study the valence state of samarium when incorporated in a single crystalline EuO thin film which crystallizes in a fcc structure similar to that of the intermediate valence SmO, but with a larger lattice constant. Due to the increased lattice spacing, a stabilization of the larger Sm2+ ion is expected. Surprisingly, the samarium incorporated in SmxEu1−xO thin films shows a predominantly trivalent character, as determined by x-ray photoelectron spectroscopy and magnetometry measurements. We infer that the O2− ions in the EuO lattice have enough room to move locally, so as to reduce the Sm-O distance and stabilize the Sm3+ valence.

The surface wettability and improved electrochemical performance of nanostructured CoxFe3 − xO4 thin film

The spin coated CoxFe3−xO4 thin films have been prepared with different cobalt value (x=0.3, 0.5 and 0.7). The X-ray diffraction analysis indicated the presence of cubic spinel phase in all samples. The chemical composition of the films was studied using X-ray photoelectron spectroscopy. The formation of granular morphology for sample with x=0.7 were observed by field emission scanning electron microscope (FESEM). The calculated fractal dimensions from AFM image changed from 2.45 to 2.32 by increasing the cobalt value. The optical band gap changed from 2.28 to 2.17eV by cobalt addition. The wetting properties of surface changed from hydrophilic to hydrophobic by increasing the cobalt content. Also the UV irradiation leads to contact angle reduction. The ion storage capacity of cobalt ferrite thin films was characterized using cyclic voltammetry in 1M KOH. The Co0.5Fe2.5O4 sample has the maximum specific capacitance (121.66mF/cm2).

Structural and ferroelectric properties of epitaxial BaZrxTi1−xO3 thin films

Epitaxial BaZrxTi1−xO3 (BZTO) thin films with Zr contents of x = 0, x = 0.12 and x = 0.2 were grown by pulsed laser deposition on (0 0 1)-oriented single crystalline SrTiO3 substrates utilizing an additional conducting SrRuO3 buffer layer. It was found that the oxygen pressure during the deposition heavily influences the lattice constants and the microstructure of BZTO. A low of 0.01 mbar gives rise to a significant tetragonal distortion. Texture measurements reveal that an undisturbed epitaxial growth is only achieved for BZTO films prepared in 0.01 mbar oxygen. In contrast, the formation of twins was observed for higher . A detailed microstructural analysis indicates that the sample preparation in low prevents a preferential growth of columnar grains within the BZTO layers and leads to smoother film surfaces. BZTO thin films deposited with optimized deposition parameters show characteristic ferroelectric polarization behavior. The saturation polarization at room temperature declines with increasing Zr content and the characteristic ferroelectric hysteresis diminishes. Temperature-depended measurements of the relative permittivity reveal the existence of a broad transition range and a significant shift of the phase transition temperature to lower values for increasing Zr content.

Optical properties and UV photoresponse of Na2x Zn1−x O thin film

Na2xZn1−xO thin films were successfully synthesized on quartz glass substrates by sol–gel method. The effect of Na content on the microstructure, optical properties and UV photoresponse of the thin films was investigated using X-ray diffraction, optical absorbance, photoluminescence, and conductivity measurements. The results indicate that the Na0.04Zn0.98O thin film exhibits the strongest preferential c-axis orientation with a polycrystalline hexagonal wurtzite structure and has the largest optical band gap. An obvious decrease in crystal size has been observed with the increasing of Na content. A weak ultraviolet emission band at about 381 nm and a strong visible emission band have been observed in the photoluminescence spectra. The Na0.04Zn0.98O thin film has the shortest growth and decay time (45 and 18 s, respectively) and the largest photoresponse. The results indicate that the photoresponse can be effectively improved by means of moderate Na doping.

Effect of Zn substitution in (111)-textured ZnxFe3−xO4 thin films on magnetization dynamics

The strong (111)-textured ZnxFe3−xO4 thin films were grown on SiO2-terminated silicon substrates using pulsed laser deposition. X-ray diffraction and high resolution transmission electron microscopy clearly show the column growth structure along the film thickness direction in [111] crystalline orientation for every grain column. With the introduction of texture induced magnetic anisotropy in the free energy density model, we quantitatively determined the doping effect on the various anisotropies in (111)-textured ZnxFe3−xO4 films. The mechanisms of ferromagnetic resonance linewidth of ZnxFe3−xO4 thin films are investigated by the theoretical fitting, in which considering the contributions from intrinsic spin-orbit coupling, extrinsic compositions from two-magnon scattering and inhomogeneous broadening. It is found that the Gilbert damping coefficient α increases with increasing Zn substitution x from 0 to 0.3, indicating that the spin-orbit coupling of the ZnxFe3−xO4 thin films increases with the introduction of Zn impurities.

Band gap bowing in NixMg1-xO.

Epitaxial transparent oxide NixMg1−xO (0 ≤ x ≤ 1) thin films were grown on MgO(100) substrates by pulsed laser deposition. High-resolution synchrotron X-ray diffraction and high-resolution transmission electron microscopy analysis indicate that the thin films are compositionally and structurally homogeneous, forming a completely miscible solid solution. Nevertheless, the composition dependence of the NixMg1−xO optical band gap shows a strong non-parabolic bowing with a discontinuity at dilute NiO concentrations of x < 0.037. Density functional calculations of the NixMg1−xO band structure and the density of states demonstrate that deep Ni 3d levels are introduced into the MgO band gap, which significantly reduce the fundamental gap as confirmed by optical absorption spectra. These states broaden into a Ni 3d-derived conduction band for x > 0.074 and account for the anomalously large band gap narrowing in the NixMg1−xO solid solution system.

Enhancement of magnetic moment in ZnxFe3−xO4 thin films with dilute Zn substitution

Highly (111)-textured ZnxFe3−xO4 thin films were grown by pulsed laser deposition on silicon substrates. The spin and orbital magnetic moments of the ZnxFe3−xO4 thin films have been obtained by X-ray magnetic circular dichroism (XMCD) and sum rule analysis. The total magnetic moments thus extracted are in good agreement with the values obtained by vibrating sample magnetometer. Both the unquenched orbital moment and the ratio of orbital-to-spin moment first increase significantly with increasing Zn substitution at a low concentration range (0≤x≤0.1), and then decrease at a higher concentration (x = 0.3). The underlying site-specific doping mechanisms involved here have been elucidated by detailed analysis of the XMCD of ZnxFe3−xO4 films. Our work demonstrates a practical means to manipulate the spin-orbit coupling in the ZnxFe3−xO4 thin films via Zn impurity doping.

Effect of Substrate Surface Atom Constitution and The Migration Characteristics of Reactive Atoms on Crystal Structure of MgxZn1–xO Thin Films Deposited by PLD Method

MgxZn1–xO thin films were deposited on fused quartz substrate by PLD method with Mg0.4Zn0.6O source target, and the effect of the substrate surface atom constitution and the migration characteristics of reactive atoms on crystal structure of the MgZnO thin films were studied. Under oxygen-deficient condition, MgxZn1–xO thin film deposited mainly along (200) orientation of cubic structure lattice at temperature below 350 °C because of the alternatively constituted substrate surface by Si and O atoms and the weak horizontal migration of reactive atoms, mix-constituted substrate surface(Si/O areas and O-rich areas) and the intense horizontal migration of reactive atoms introduced phase separation in MgxZn1–xO thin films deposited at higher temperature. Single cubic structure (200) Mg0.38Zn0.62O thin films was made at 300 °C with Zn composition over theoretical limit in bulk materials, the band gap of which reached 4.2 eV and the MgZnO thin film could detect 330 nm near-UV light. And the available UV range of...

Inversion of absorption anisotropy and bowing of crystal field splitting in wurtzite MgZnO

The anisotropic optical properties of wurtzite MgxZn1−xO thin films (0≤x≤0.45) grown on m-plane ZnO substrates by plasma assisted molecular beam epitaxy are studied using spectroscopic ellipsometry at room temperature. The data analysis provides the dielectric functions for electric field polarizations perpendicular and parallel to the optical axis. The splitting between the absorption edges of the two polarization directions decreases between x = 0 and x = 0.24, while an inverted absorption anisotropy is found at higher Mg content, indicating a sign change of the crystal field splitting Δcr as for the spin orbit parameter. The characteristic energies such as exciton binding energies and band gaps are determined from the analysis of the imaginary parts of the dielectric functions. In particular, these data reveal a bowing parameter of b=−283 meV for describing the compositional dependence of the crystal field splitting and indicate Δcr=−327 meV for wurtzite MgO. The inverted valence band ordering of ZnO (Γ7-Γ9-Γ7) is found to be preserved with increasing Mg content, while the optical selection rules interchange.

Structural and linear and nonlinear optical properties of Zn_1-xAl_xO (0≤x≤010) thin films fabricated via pulsed laser deposition technique

The effect of Al doping on linear and nonlinear optical properties of pulsed laser deposited (PLD) Zn1-xAlxO (AZO) thin films is reported. The increase in 002 peak in X-ray diffraction pattern up to x = 0.05 is indicative of improvement in the wurtzite phase and was also confirmed by enhancement in E2 (high) peak at 438 cm−1 of Raman spectra. The increase in the blue shift in the band gap up to x = 0.05 suggests the Burstein Moss effect. The red shift in the band gap for x>0.05 can be interpreted due to the band gap renormalization as a result of many body interaction effects. The two photon absorption coefficients (β) and nonlinear refractive indices (n2) measured via z-scan measurement were found to be in the range of 3.4-8.0 cm/W and (1.0-5.2) × 10−4 cm2/W for x = 0.00-0.10 respectively. An attempt is made to correlate the linear and nonlinear optical behavior and crystal structure as a function of level of Aluminum doping (x).

Low-temperature, high-stability, flexible thin-film transistors with a novel ScxIn1−xO3 semiconductor

Low-temperature scandium (Sc) incorporating In2O3 (ScxIn1−xO3) thin films as oxide semiconductors were developed and investigated. And flexible thin-film transistors (TFTs) based on ScxIn1−xO3 channel layers were fabricated on polyethylene naphthalate substrate with the highest process temperature of only 150 °C. The ScxIn1−xO3 TFT displayed high on/off ratio of 107 with a turn-on voltage (Von) of only −0.1 V, a subthreshold swing of 0.21 V dec−1, and a mobility of 17.5 cm2 V−1 s−1. Furthermore, the flexible ScxIn1−xO3 TFTs exhibit high stability under both positive bias stability and negative bias stability, which was ascribed to little change of the lattice parameter of In2O3 after incorporation with Sc (since the radius of Sc3+ is similar to that of In3+). More interestingly, the flexible ScxIn1−xO3 TFTs exhibited relatively good stability under negative bias illumination stability compared to those of IGZO TFTs, which was ascribed to fewer oxygen vacancies due to the strong bonding strength of Sc-O. Finally, the transfer curves of the ScxIn1−xO3 TFTs showed only a few changes under a curvature radius of larger than 20 mm.

Electrochromism of non-stoichiometric NiO thin film: as single layer and in full device

Electrochromic properties, known as a reversible modulation of the optical properties under an applied voltage, of NiO thin films are discussed in respect of the film stoichiometry. Using radio-frequency magnetron sputtering, non-stoichiometric “NiO” thin films of good crystallinity were grown at room temperature from low oxygen partial pressure [i.e., above 2 % P(O2/Ar + O2)]. A further increase in oxygen partial pressure leads to conductive brownish films containing a large amount of Ni3+. 2 %-Ni1−x O thin films exhibit significant EC performance in lithium-based electrolyte with a transmittance modulation of 25 %. If it is generally accepted that this optical modulation is due to an insertion of small cations, the presence of additional surface phenomena is also shown. The cycling of full device, based on the association of WO3 and “NiO” in temperature up to 60 °C and down to −35 °C confirms expected increase and decrease in capacity while surprisingly the optical switch from a transparent to a neutral gray color appears slightly modified.

Photoresponse characteristics of p-Si/n-CuxIn1−xO heterojunction diode prepared by sol-gel spin coating

In the present work, we report on the fabrication and detailed electrical characterization of p-Si/n-CuxIn1−xO heterojunction prepared via the deposition of nanocrystalline CuxIn1−xO thin films on p-type silicon substrate by sol-gel method using spin coating technique. X-ray diffraction and Raman spectroscopy results revealed the polycrystalline nature of CuxIn1−xO thin films consisting diffractions peaks and vibration modes, respectively, corresponding to CuO and In2O3. Field-emission scanning electron microscopy showed compact surface morphology while UV–vis absorption spectra exhibited sharp absorption between 300 and 425nm along with a long tail extending in the visible region. The current-voltage (I-V) characteristics of the fabricated heterojunction demonstrated obvious rectifying behavior in the dark and under illumination. The heterojunction exhibited low reverse leakage current (~10−7), and upon illumination, the forward current and rectification ratio of the junction was improved while the forward threshold voltage lowered. By fitting the experimental data we have observed that the forward current conduction is dominated by the space charge limited current mechanism.

Cationic structure and charge transport in sol–gel-derived nickel-cobaltite thin films

Conductive nickel-cobaltite (NixCo3−xO4) thin films were synthesized on Al2O3 substrates by using a sol–gel process for varying Ni composition x (≤ 1.0). X-ray diffraction data indicated that the polycrystalline NixCo3−xO4 samples have cubic spinel (Fd $$\bar{3}$$ m) structure with the lattice constant increasing slightly with increasing Ni composition (by 0.5 % for x = 1.0 compared to that of Co3O4). X-ray photoelectron spectroscopy data indicated that the Ni ions in nickel-cobaltite have multi-valence (Ni2+ and Ni3+). The temperature-dependent electrical resistivity (ρ) data of the nickel-cobaltite samples were measured in the 4–300 K range in comparison with Co3O4. At all temperatures (T), the resistivity of NixCo3−xO4 decreased gradually with increasing x down to ρ = 1.9 × 10−2 Ω cm for x = 1.0 at 300 K (reduction by a factor of ~10−3 from that of Co3O4). The decrease in ρ by the Ni doping in Co3O4 can be understood in terms of the polaronic d-electron hopping between octahedral Ni2+ and Ni3+ ions in the spinel lattice. The analysis on ρ(T) for NixCo3−xO4 exhibited a linearity between ln(ρ) and $$T^{ - 1/4}$$ below ~120 K, implying a variable-range hopping.