xO Thin Films Research Articles

Structural, Optical, and Electrical Transformations in FexSi1−xO Thin Films: The Role of Iron and Annealing

Structural, Optical, and Electrical Transformations in FexSi1−xO Thin Films: The Role of Iron and Annealing

Effects of silver nanoparticle doping on electrical, optical, and photovoltaic properties of PbZrxTi1−xO3 thin films synthesized by sol–gel

Effects of silver nanoparticle doping on electrical, optical, and photovoltaic properties of PbZrxTi1−xO3 thin films synthesized by sol–gel

A comparative investigation into the impact of Cd and Mg on the optoelectronic properties of ZnO thin films by spray pyrolysis for waveguide applications

This study examines the impact of Mg and Cd on ZnO waveguide properties using CdxZn1−xO and CdxZn1−xO thin films (x=0, 0.03, 0.05). X-ray diffraction confirms successful Mg incorporation in the ZnO wurtzite, while CdO segregates, increasing lattice parameters. UV–Vis measurements show opposing trends in band gap energy, increasing for Mg doping and decreasing for Cd doping. Photoluminescence data reveals varying defect characteristics consistent with band gap energy behavior. Through SEM, Mg increased grain size and slightly decreased the surface density. Cd-doped films exhibit no significant morphology change by AFM measurements. Hall measurements show both Cd and Mg doping increase resistivity tenfold. Cd causes a 94.5% decrease in mobility, whereas Mg leads to a substantial 2190 times increase. Charge carrier density decreases by 105 in Mg0.05Zn0.95O and increases threefold in Cd0.05Zn0.95O. M-lines measurements demonstrate birefringent behavior, positive for Mg doping and negative for Cd doping, and a decreasing refractive index.

ICEMS Characterization of NixFe3-xO4 (0.7 ≤ x ≤ 1.7) thin films grown by ion beam sputtering on (0001) Al2O3 substrates

We report here on the ICEMS characterization of nickel ferrite (NixFe3−xO4) thin films having different nickel contents grown on alumina substrates by Ion Beam Sputtering. The spectra corresponding to the films with nominal x = 0.7, 1.0 and 1.2 are characteristic of compounds crystallizing in a spinel-related structure showing two different magnetic sextets associated with Fe3+ located in the tetrahedral and octahedral sites of such structure. The spectra show an additional broad third sextet with a large isomer shift which suggests the occurrence of electron hopping between Fe2+ and Fe3+ ions sitting in the octahedral sites. With increasing nickel content, the linewidth of the sextets increases and their corresponding hyperfine magnetic fields decrease. This is an indication of an increase in structural disorder in the deposited films as their nickel concentrations increase. The cation distribution of the iron ions over the tetrahedral and octahedral sites appears also to depend on the nickel content. The film with x = 1.2 shows a significant increase in the fraction of octahedral iron ions as compared with the expected nominal value suggesting that, for this composition, some Ni2+ could also occupy tetrahedral sites. The Mössbauer spectrum corresponding to the film with x = 1.7 shows a magnetic pattern with very broad lines similar to those shown by amorphous or disordered materials. The average isomer shift is quite high (around 0.40 mms− 1) and characteristic of Fe3+ in octahedral oxygen coordination. This indicates that for the largest nickel content studied (x = 1.7), the film does not contain Fe3+ in tetrahedral environments suggesting that the spinel structure is no longer present. This correlates well with the X-Ray Diffraction data which indicate a structural change from spinel to a disordered rock-salt structure for this particular film with high nickel content.

Evolution of structural phase transition from hexagonal wurtzite ZnO to cubic rocksalt NiO in Ni doped ZnO thin films and their electronic structures

This study thoroughly examines the collective influence of compositional variation and annealing temperature on the electronic structure of sol–gel derived NixZn1−xO (x = 0 to 1) thin films annealed at different temperatures (700 ° C, 800 ° C, and 900 ° C) using x-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy. A gradual structural phase transition from hexagonal wurtzite ZnO to cubic rocksalt NiO with increasing Ni concentration was revealed by x-ray diffraction (XRD). Grain growth was observed from scanning electron microscopy with increasing annealing temperature. Photoluminescence measurements indicate the presence of interstitial oxygen when Ni atoms are incorporated in the film. The Ni L 3,2 absorption edge shows an intensity enhancement in the white-line feature with increasing Ni concentration, evidencing the presence of higher oxidation states. Concurring results were observed by XPS where both Ni2+ and Ni3+ free ion multiplets are present in the Ni 2p core level spectrum for 20% and higher Ni concentration. O K and Zn L 3,2 XAS spectra demonstrated the e g-t 2g sub-band splitting at higher Ni concentration, triggered by band anti-crossing interaction and crystal field splitting. The extended x-ray absorption fine structure (EXAFS) simulation for the Zn K edge revealed a Zn–Zn/Ni bond length change for 60% Ni concentration. The thermal disorder factor increased up to 40% Ni concentration, and beyond that, it decreased due to stable NiO phase dominance in the alloy composite. Ni K edge EXAFS fitting indicated an insignificant change in the Ni-O and Ni–Ni/Zn bond lengths throughout the range of varying Ni concentrations. The thermal disorder factor increases with increasing annealing temperature, indicating a more disordered lattice. Such investigation is essential where the electronic properties of nanometer-sized materials determine the performance of functional devices. The present work critically elucidates the combined impact of compositional variations and annealing temperatures on electronic structures.

Influence of doping concentrations of Hf on structural and electrical properties of HfxZn1-xO thin films

HfxZn1−xO thin films (x=0, 2.5,3, 7.5, 10 and 15mol %) were deposited on glass substrates using Sol-gel process. The influence of the Hf concentration on the structural, electrical, and optical properties of the films was studied. It is found that Hf ions can be effectively doped into ZnO and all films crystallize in the hexagonal wurtzite structure with a preferred c-axis orientation. The lattice constants of HfxZn1−xO films increase with the Hf contents. The HfxZn1−xO thin films structures of high-Hf-content films remain after annealing at 600 °C for 20 min. The optical band gap increases with the Hf content, but it decreases with the annealing temperature. The reduction of bandgap partly results from grain growth, which is due to the quantum confinement effect of the small grains. Hf doping increases the resistivity of ZnO owing to the disorder of the material structure and the higher bandgap, which results in more carrier traps and less thermally excited carriers in the conduction bands. Two FTIR peaks centered at about 1432 and 1056 cm-1 coexist in the fluorescent spectra. With increasing the Hf contents, the intensity of fluorescent peaks enhances remarkably. The Minimum resistivity reached 6.1 * 10-2 Ὠ cm after annealing with 3% Hf content.

Optoelectronic properties of sprayed MnxZn1−xO optical waveguide thin films: Refractive index and birefringence tailoring

This study investigated the properties of MnxZn1−xO thin films with varying Mn concentrations (x = 0, 0.03, 0.05, 0.07, and 0.09) fabricated via spray pyrolysis on glass substrates. The films exhibited a wurtzite structure with (002) orientation and decreasing crystal size. By utilizing the first derivative technique, it was found that the optical gap energy increased linearly by 0.06 eV, starting at 3.24 eV. Photoluminescence spectra deconvolution revealed the presence of multiple point defects, particularly VO, which correlated with the increase in optical gap energy. The Hall effect-based electric measurements indicated a decline in charge carriers and mobility accompanied by an increase in magnetoresistance and resistivity. The SEM observations revealed a morphological transition from flaky to dome-shaped grains with a reduction in size. The waveguiding properties of the films were studied using the m-lines technique, which showed that all films were dual-mode waveguides. The ordinary and extraordinary refractive indices increased to 2.0019 and 2.0005, respectively with Mn doping, while the birefringence inversion began at Mn0.05Zn0.95O and remained constant at Mn content of 7 and 9 at.%. Overall, these findings provide valuable insights into the structural, optical, electrical, and waveguiding properties of MnxZn1−xO thin films, which could be useful for the development of new optoelectronic devices.

Nanostructured ZnxMn3‒xO4 thin films by pulsed laser deposition: A spectroscopic and electrochemical study towards the application in aqueous Zn-ion batteries

Aqueous Zn-ion batteries (AZIBs) represent a safe and sustainable technology amongst the post-lithium systems, though the poor understanding of the material behaviour at the cathode prevents the full development of efficient AZIBs. ZnMn2O4 (ZMO) has been considered one of the cathode candidates owing to its analogy to the well-established LiMn2O4 cathode for lithium-ion batteries, however its electrochemical mechanism in the presence of Zn ions in aqueous environment is unclear and still debated. In this work, we synthesised nanostructured ZMO thin films by Pulsed Laser Deposition (PLD) and we evaluated, through extensive characterization by microscopic, spectroscopic, and diffraction techniques, how the deposition and annealing conditions affect the film properties. The self-supported nature and the high degree of control down to the nanoscale make a thin film an ideal model system to study the electrochemistry of the material in aqueous solution and to emphasize the impact of the film properties on its electrochemical response. We highlighted the crucial role of the oxygen pressure in the modulation of the film porosity and the combined effect of deposition pressure and annealing temperature to produce a film with tailored properties in terms of morphology, crystallinity, and Zn stoichiometry. A complex redox mechanism involving multiple concurrent reactions and the formation of zinc hydroxide sulphate hydrate (ZHS) was reported, as well as the influence of the film porosity on the voltammetric behaviour of the film at higher scan rate. Our results confirm the intricate electrochemical mechanism of the ZMO material, which does not merely involve the Zn2+ insertion/extraction but also the crucial participation of Mn2+ from the electrolyte, and pave the way for the nanoscale design of engineered ZMO-based electrodes.

DFT Simulations of ZnxMg1–XO Solid Solutions for Solar-Blind UV Sensors: Evaluation of Electronic Structure and Phase Stability

Abstract In this research, density functional theory accompanied with linear combination of atomic orbitals (LCAO) method is applied to study the atomic and electronic structure of wurtzite and rocksalt ZnxMg1−xO pseudobinary compounds in their bulk phases. Calculated band gaps of ZnxMg1−xO solid solutions under study are further validated by means of spectroscopic ellipsometry and optical absorption. In agreement with an experiment, it is predicted that increase of Zn content in ZnxMg1−xO leads to narrowing of its band gap for both wurtzite and rocksalt phases. The calculated infra-red (IR) spectra show that the IR peaks are shifted towards larger frequencies along with decrease of Zn content. Presence of imaginary phonon frequencies in rocksalt ZnxMg1−xO of x > 0.625 allows us to suggest that it is necessary to use properly oriented substrates for epitaxial growth to overcome polycrystallinity inZnxMg1−xO thin films at concentration x = 0.4 – 0.6.

Wide gap p-type NiO-Ga2O3 alloy via electronic band engineering

Gallium oxide (Ga2O3) has gained significant interest in recent years due to its wide bandgap and related unique properties, making it suitable for many high power and deep ultra-violet optoelectronic and photodetection devices. Nevertheless, Ga2O3 can only be doped effectively n-type, and its full potential in device applications is severely limited by the lack of a reliable p-type material. Here, we report on modifying electronic bands of Ga2O3 through alloying with NiO to achieve p-type conducting Ga2O3-NiO alloy (NixGa1−xO) thin films. We find that room temperature sputter-deposited stoichiometric and O-rich alloys with low Ni content (x < 0.22) have an amorphous structure. In contrast, films with higher Ni content (x ≳ 0.22) are polycrystalline with the rocksalt (RS) NiO structure. O-rich RS-alloys are p-type with resistivity ~20 Ω-cm (for x~0.6) and decreases to< 10 Ω-cm with increasing x. Optically, p-type O-rich films with x ≥ 0.46 have strong sub-gap absorption in the low energy region (˂ 3.5 eV) due to a high concentration of Ni vacancies VNi and this results in a low transmittance which also decreases with x from ~70 % (x = 0.3) to ~ 40 % (x = 1). The band gap Eg of the alloy films exhibits a wide tunability with a monotonic decrease with increasing x from 4.98 (x = 0) to 3.53 (x = 1). The Eg for the RS alloys follows the virtual crystal approximation with a small bandgap bowing of 0.36 eV and an extrapolated Eg of 4.6 eV for RS Ga2O3. Furthermore, we find that the amorphous and RS crystalline alloys have a type II (staggered) band offset (ΔEV~1.8 eV and ΔEC~1.4 eV) with a stepwise upshift of the valence band maximum (VBM) position from ~8–6.2 eV below the vacuum level at the amorphous to RS transition (x~0.2). This significant uplift of the VBM is believed to be responsible for the measured p-type conductivity of the O-rich RS alloys. It also suggests that deep acceptors in Ga2O3 would become shallow in these alloys and hence effective acceptor doping to further improve their p-type conductivity is possible.

Bandgap tunability and local structure of MgxZn1–xO (0 ≤ x ≤ 1) thin films grown by RF magnetron co-sputtering

Bandgap tunability and local structure of MgxZn1–xO (0 ≤ x ≤ 1) thin films grown by RF magnetron co-sputtering

Reset First Resistive Switching in Ni1−xO Thin Films as Charge Transfer Insulator Deposited by Reactive RF Magnetron Sputtering

Reset-first resistive random access memory (RRAM) devices were demonstrated for off-stoichiometric Ni1−xO thin films deposited using reactive sputtering with a high oxygen partial pressure. The Ni1−xO based RRAM devices exhibited both unipolar and bipolar resistive switching characteristics without an electroforming step. Auger electron spectroscopy showed nickel deficiency in the Ni1−xO films, and X-ray photoemission spectroscopy showed that the Ni3+ valence state in the Ni1−xO films increased with increasing oxygen partial pressure. Conductive atomic force microscopy showed that the conductivity of the Ni1−xO films increased with increasing oxygen partial pressure during deposition, possibly contributing to the reset-first switching of the Ni1−xO films.

Controlling electrical and optical properties of wurtzite CdxZn1−xO with high Cd contents via native defects manipulation by low-temperature annealing

Bandgap energies in wurtzite (WZ) structured CdxZn1−xO alloys are known to decrease with increasing Cd content (x). Our previous work demonstrated that WZ-CdxZn1−xO alloys with a high Cd content of x ∼ 0.6 and a low gap of 2 eV can be stabilized by oxygen interstitials when grown in an O-rich environment. However, such O-rich WZ-CdxZn1−xO alloys have poor electrical properties due to compensating native defects. In this work, we synthesized pure WZ phase CdxZn1−xO thin films with different Cd contents by magnetron sputtering in an oxygen-rich environment. Changes in structural, electrical, and optical properties of these O-rich wurtzite CdxZn1−xO after rapid thermal annealing were investigated. While alloys with a low Cd composition of 0.2 can maintain a pure wurtzite structure up to 500 °C, phase separation occurs at a lower annealing temperature of ∼400 °C for Cd-rich (x = 0.6) films. Isochronal and isothermal annealing studies reveal the kinetics of native defects in these alloys. Highly mobile hydrogen interstitial donor defects, oxygen interstitials, and more stable cation vacancies outdiffuse sequentially as the annealing temperature increases from &amp;lt;300 to &amp;gt;400 °C. By exploiting the difference in the energy barrier between acceptor defects removal and phase separation, a pure wurtzite phase alloy with a low bandgap of 2 eV and decent electrical properties was realized by annealing O-rich WZ-Cd0.6Zn0.4O at 300 °C with an extended annealing duration of &amp;gt;100 s. These results demonstrate a practical way to obtain low-gap oxide semiconductors with strong optical absorption and controllable electrical conductivities.

Optoelectronic and birefringence properties of weakly Mg-doped ZnO thin films prepared by spray pyrolysis

MgxZn1−xO thin films were deposited on glass substrate with x varied between 0.01 and 0.05 by spray pyrolysis at a temperature of 450 °C. The structural investigation showed that all thin films had ZnO wurtzite structure with a preferred (002) orientation. The gap energy was calculated using Tauc’s plot, and it decreased over the Mg content by 0.07 eV. The charge carriers’ density dropped by an order of 105 as Mg content increased whereas the resistivity and the mobility increased. SEM observations revealed a significant difference between undoped and doped thin films. A 632.8 nm laser source prism coupler revealed 2 optical modes for every thin film in each Transverse Electric and Transverse Magnetic mode, and the birefringence of the Mg-doped films was positive. Both ordinary and extraordinary refractive indices were found to decrease as the Mg content increased. Great intention has been paid to the relation between the refractive, charge carriers’ density, and the optical band gap.

Structural, Magnetic, and Optical Properties of Mn2+ Doping in ZnO Thin Films

MnxZn1−xO thin films (x = 0%, 1%, 3%, and 5%) were grown on corning glass substrates using sol–gel technique. Single-phase hexagonal wurtzite structure was confirmed using X-ray diffraction. Raman analysis revealed the presence of Mn content with an additional vibrational mode at 570 cm−1. The surface morphology of the samples was observed by scanning electron microscopy which suggested that the grain size increases with an increase in Mn concentration. The optical bandgap increases with increasing Mn concentration due to a significant blueshift in UV–visible absorption spectra. The alteration of the bandgap was verified by the I–V measurements on ZnO and Mn-ZnO films. The various functional groups in the thin films were recorded using FTIR analysis. Magnetic measurements showed that MnxZn1−xO films are ferromagnetic, as Mn induces a fully polarised state. The effect of Mn2+ ions doping on MnxZn1−xO thin films was investigated by extracting various parameters such as lattice parameters, energy bandgap, resistivity, and magnetisation. The observed coercivity is about one-fifth of the earlier published work data which indicates the structure is soft in nature, having less dielectric/magnetic loss, and hence can be used as ultra-fast switching in spintronic devices.

Structural Insights of Electrical Aging in PZT Thin Films as Revealed by In Situ Biasing X-ray Diffraction.

Electrical aging in lead zirconate titanate (PbZrxTi1−xO3) thin films has been intensively studied from a macroscopic perspective. However, structural origins and consequences of such degradation are less documented. In this study, we have used synchrotron radiation to evaluate the behavior of ferroelectric domains by X-ray diffraction (XRD). The sample was loaded with an AC triangular bias waveform between ±10 V with a number of cycle varying from one up to 108. At each step of the aging procedure, XRD spectra had been collected in situ during the application of an electric field on a capacitor. The fine analysis of the (200) pseudo-cubic peak structure allows to separate the evolution of the volume of a/c tetragonal and rhombohedral domains along the electrical biasing. Throughout the aging, both intrinsic and extrinsic responses of tetra and rhombohedral domains are altered, the behavior depending on the observed phase. This methodology opens up new perspectives in the comprehension of the aging effect in ferroelectric thin film.

Structural, microstructural and optical properties of perovskite CCNbxTi1−x O (x = 0.02) thin film explored for photovoltaic applications

CCNbxTi1−xO(CCTO-Nb) thin film was prepared by modified sol-gel route owing to its obvious benefit over traditional sol-gel process. The prepared gel was used for preparing film by doctor’s blade technique. The material was observed to be crystallized in perovskite phase in space group no. Im3. The rietveld refinement confirms its phase and the morphological study reveal the formation of transformation of microstructure from grain to bead to rod like structure and then to fibrous form. From mapping it is seen that the grains are uniformly distributed. Mean surface roughness and RMS surface roughness were estimated from atomic force microscopy (AFM) study. The different optical properties like, band gap, optical conductivity, interband transition strength, oscillation energy etc. are estimated in this study. This shall throw an insight to the applicability of the material in photovoltaic applications.

Link between Structural and Optical Properties of CoxFe3–xO4 Nanoparticles and Thin Films with Different Co/Fe Ratios

CoxFe3–xO4 nanoparticles (x = 0.4 to x = 2.5) and thin films (x = 0.9 to x = 2.2) are analyzed by Raman, absorption, and photoluminescence spectroscopy to link structural and optical properties to different cobalt to iron (Co/Fe) ratios. Raman spectroscopy shows that with decreasing Co content, the crystal structure changes from a predominantly normal cubic spinel phase to a mixed inverse spinel phase. This finding is supported by absorption spectroscopy that points out that inter valence charge transfer (IVCT) processes between octahedrally coordinated Co2+ and Fe3+ cations become more prominent with increasing Fe content. Independent of the Co/Fe ratio, CoxFe3–xO4 nanoparticles show a broad photoluminescence (PL) band with a maximum at around 510 nm. Time-resolved photoluminescence spectroscopy shows subnanosecond lifetimes and temperature-resolved photoluminescence experiments reveal that the green PL increases with decreasing temperature (300 to 10 K) while showing no temperature-dependent shift in energy. It is proposed that this green PL originates from OH-groups on the particles’ surface.

Optical properties of in vacuo lithiated nanoporous WO3:Mo thin films as determined by spectroscopic ellipsometry

Incorporation of Mo into pure WO3 thin films is of interest for electrochromic devices, as it improves colour neutrality in the dark state. Existing literature still lacks reliable quantitative data on the complex dielectric function of such coatings. In this study, we deposited WO3 and MoxW1−xO3 thin films by magnetron sputtering and subsequently characterised them by X-ray photoelectron spectrometry (XPS), grazing incidence X-ray diffraction (GIXRD) and scanning electron microscopy (SEM). In vacuo-lithiation was performed to incorporate lithium inside the thin films. The lithiated films were evaluated with variable-angle spectroscopic ellipsometry (VASE) from 340 to 990 nm and with transmission measurements from 330 to 2100 nm. The ellipsometric data were analysed with a straightforward model composed of the sum of Tauc-Lorentz and Lorentz oscillators dispersion laws. One Lorentz oscillator positioned around 1.3 eV is associated with the reduction of W6+ to W5+ by lithium incorporation. One Lorentz oscillator positioned around 2.3 eV is associated with additional states in the band-gap due to the presence of Mo in the lithiated film. The proposed model allows a better comprehension of the involved electronic transitions and allows to determine the refractive index and extinction coefficient of the materials precisely.

Alloying induced disorder and localized excitonic states in ternary BexZn1−xO thin films

II-VI ternary alloy is a promising material for advanced optoelectronic devices like ultraviolet (UV) photodetectors, light emitting diodes and laser diodes with high efficiency and low fabrication cost. The precise control over the doping element, concentration and crystal quality allows careful tuning of their band structures for novel optoelectronic properties. In this work, we report a prominent two-mode behavior in ternary BexZn1−xO alloy using resonant Raman spectroscopy as a probe. By combining with the excitonic localization and extrinsic Fröhlich interaction, we present a theoretical analysis on the resonant Raman scattering with different Be concentrations and tunable measured temperatures. Frequency blue shifts caused by compressed stress and two-mode phonons (ZnO-like and BeO-like A1(LO) phonons) scattering up to third orders revealed in this novel ternary alloy. In addition, alloying induced disorder and electronic states in the forbidden band result in the broadening of line width and the enhancement of LO-phonon oscillator strength. As a result, a high performance BeZnO based photoconductive type UV photodetector was realized, indicating the potential of BeZnO as a promising material for optoelectronic devices.