Yawei Li

Structural, electronic band transition and optoelectronic properties of delafossite CuGa1−xCrxO2 (0 ≤ x ≤ 1) solid solution films grown by the sol–gel method

Pure phase CuGa1−xCrxO2 (0 ≤ x ≤ 1) films were prepared on (001) sapphire substrates by the sol–gel method. The structure, vibration modes, and compositions of the films were analyzed by X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. It was found that the Cr-substituting induced the increase of the film’s roughness, changed the film’s internal structure, and made more crystal defects and grain boundaries. Due to the interatomic potential becoming weaker between Cu and O atoms with increasing the Cu–O bond length, the peak positions of the A1g and Ag phonon modes shifted toward a lower frequency with increasing x. The optical transmittance of the films approached about 60–80% in the visible region and the values of the direct band gap linearly decrease from 3.56 to 3.09 eV with increasing x. The Cr-introduction effects on the electronic band transition have been investigated in detail. The new energy state located at 0.17 eV above the top of the valence band is observed in the CuGa0.8Cr0.2O2 film, which can be derived from the defect energy level. It can induce the increment of the hole in the valence band, contribute to the electrical conductivity, and lower the thermal activation energy. Moreover, the CuGa0.8Cr0.2O2 film is found to be of the larger electrical conductivity of 0.071 S cm−1 at room temperature, which shows the promising application values, as compared to other CuGa1−xCrxO2 films.

Oxygen-vacancy-related dielectric relaxation in BiFeO3 films grown by pulsed laser deposition

Two kinds of BiFeO3 (BFO) thin films with different oxygen stoichiometry are fabricated on (La, Sr)CoO3 coated silicon substrates. A Debye-like dielectric relaxation was observed in the samples thermally treated at 3 Pa oxygen. The frequency dependence of permittivity of the samples treated at 3 Pa oxygen can be fitted by a model containing the Debye-like dielectric response and the universal dielectric response. According to the model, the dielectric relaxation can be ascribed to the oxygen vacancy, and the possible influences from the interfacial polarization between BFO and electrodes have been excluded by the measurement of the dielectric responses of BFO films at different dc biased voltages. The calculated value of dc electric conductivity in BFO films from this model has the same order of magnitude as the published results. These results indicate that the existence of oxygen vacancy not only influences the leakage performance of BFO films but also affects the dielectric properties of BFO. The electrical performance of BFO films and devices can be improved by decreasing the density of oxygen vacancy.

Optoelectronic and Ferroelectric Properties of Cerium-Doped (Na0.5Bi0.5)(Ti0.99Fe0.01)O3 Nanocrystalline Films on (111) Pt/TiO2/SiO2/Si: A Composition-Dependent Study

The optical and ferroelectric properties of (Na0.5Bi0.5)1-xCex(Ti0.99Fe0.01)O3 (NBCTFx; 0 ≤ x ≤ 0.10) nanocrystalline films deposited on platinized silicon (Pt/TiO2/SiO2/Si) substrates using a sol-gel method were investigated. The microstructure, surface, and cross-sectional morphology and compositions of the films were analyzed by X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy, respectively. The X-ray diffraction patterns indicate that all films are polycrystalline and show the single perovskite structure. The dielectric functions of the NBCTFx films can be uniquely extracted by fitting the measured ellipsometric spectra with a four-phase-layered model (air/surface rough layer/NBCTFx/Pt) in the photon energy range of 0.6-6.4 eV. The Tauc-Lorentz model was successfully applied and reasonably describes the spectral response behavior of ferroelectric NBCTFx films in the light-frequency region. It was found that the optical band gap and grain size decrease with increasing cerium composition because of the introduction of disorder and defects. The electrical results show that the leakage current density of the films was decreased with increasing cerium composition by reducing the density of oxygen vacancies and forming the defect complexes. The optimal ferroelectric properties were obtained in the film doped with x = 0.10, whose remnant polarization and coercive field values are 14.9 μC/cm(2) and 217.3 kV/cm, respectively. The present results could be crucial for future applications of lead-free ferroelectric and optoelectronic devices.

Modulated photoluminescence of shallow levels in arsenic-doped Hg1−xCdxTe (x≈0.3) grown by molecular beam epitaxy

Shallow levels in arsenic-doped Hg1−xCdxTe grown by molecular beam epitaxy have been investigated by temperature- and excitation power-dependent modulated photoluminescence spectroscopy. The ionization energies of the shallow levels of AsTe, AsHg, and the AsHg–VHg complex are preliminarily determined to be about 11.0, 8.5, and 33.5meV, respectively. Correspondingly, the forming energy of the AsHg–VHg complex has been deduced to be approximately 10.5meV. The results could be used as guidelines for the material growth or the fabrication of related devices.

Optoelectronic properties and polar nano-domain behavior of sol–gel derived K0.5Na0.5Nb1−xMnxO3−δ nanocrystalline films with enhanced ferroelectricity

High-quality lead-free piezoelectric K0.5Na0.5Nb1−xMnxO3−δ (KNNMx, 0 ≤ x ≤ 0.10) films have been successfully deposited on Pt(111)/Ti/SiO2/Si(100) substrates by a modified sol–gel method. The effects of Mn substitution on the microstructure, morphology, lattice vibrations, and optical and ferroelectric properties of the KNNMx films have been investigated in detail. All films are polycrystalline, crack-free and show a pseudo-cubic (pc) structure with a thickness of about 215 nm. Raman analysis indicates that the characteristic frequency of ν1, ν5 and ν1 + ν5 modes shifts towards lower wavenumbers with increasing Mn concentration. The optimal ferroelectric properties were obtained in the film doped with x = 0.06, whose remnant polarization (2Pr) and coercive field (2Ec) values at the applied electric field of 1000 kV cm−1 are 51 μC cm−2 and 265 kV cm−1, respectively. The increased valence of Mn2+, which is substituted at the Nb5+ site as Mn3+, plays an important role in reducing the amount of both oxygen vacancies and holes. In addition, the dielectric functions of the KNNMx films have been uniquely extracted by fitting ellipsometric spectra with the Adachi dielectric function model and a four-phase layered model (air/surface rough layer/KNNMx/Pt) in the photon energy range of 1.5–5.5 eV. The optical band gap (Eg) slightly decreases, while the high-frequency dielectric constant (e∞) linearly increases with increasing Mn concentration. Moreover, temperature dependent optical dispersion behavior of the KNNM0.06 film has been investigated from 300 K to 800 K. The analysis of Eg and the extinction coefficient (κ) reveals the correlation between optical properties and structural phase transition. Furthermore, a distinct in-plane (180°) polar nano-domain pattern with a well-defined rectangular phase hysteresis loop has been observed in the KNNM0.06 film from piezoresponse force microscopy (PFM) experiments. The present results could be crucial for potential multifunctional KNN-based device applications.

Temperature-dependent dielectric functions and interband critical points of relaxor lead hafnate-modified PbSc1/2Ta1/2O3 ferroelectric ceramics by spectroscopic ellipsometry

The electronic band structures and dielectric functions of (1−x)PbSc1/2 Ta 1/2O3−xPbHfO3 ceramics with different composition have been investigated by variable-temperature spectroscopic ellipsometry. Using the standard critical-point (SCP) model, three typical interband transitions can be observed from the second derivative of dielectric functions. The CP transitions, which are sensitive to B-site order degree, show a redshift trend with the temperature due to the electron-phonon interactions and lattice thermal expansion. The linear temperature coefficients are varied with oxygen vacancy, B-atom (Sc, Ta, Hf) arrangement, and Pb-O bonds owing to addition of PbHfO3.

Evolution of orientation degree, lattice dynamics and electronic band structure properties in nanocrystalline lanthanum-doped bismuth titanate ferroelectric films by chemical solution deposition

Ferroelectric lanthanum (La)-substituted bismuth titanate (Bi(4-x)La(x)Ti(3)O(12), BLT) nanocrystalline films with the composition range of 0 ≤x≤ 1 have been directly deposited on n-type Si (100) substrates by chemical solution deposition. The La substitution effects on the preferred orientation, surface morphology, phonon modes, emission bands and electronic band structures of the BLT films have been investigated by microscopy, Raman scattering, photoluminescence and spectroscopic ellipsometry at room temperature. X-Ray diffraction analysis shows that the films are polycrystalline and exhibit the pure perovskite phase structure. With increasing La composition, the (100)-orientation degree can be enhanced and the root-mean-square roughnesses slightly increase from 6.5 to 8.3 nm. It was found that the Raman-active mode A(1g)[Bi] at about 59 cm(-1) is unchanged while the B(1g) and A(1g)[Ti] phonon modes at about 648 and 853 cm(-1) are shifted towards higher frequency by about 36.6 and 8.4 cm(-1), respectively. Photoluminescence spectra show that the intensity of the peak located at about 2.3 eV increases with the La composition, except for the Bi(3)LaTi(3)O(12) film, due to the smallest grain size and oxygen vacancy defects. The optical constants of the BLT films have been uniquely extracted by fitting the measured ellipsometric spectra with a four-phase layered model (air/surface rough layer/BLT/Si) in the photon energy range of 0.73-4.77 eV. The Adachi dielectric function model has been successfully applied and reasonably describes the optical response behavior of the ferroelectric BLT films. Moreover, the film packing density decreases while the optical band gap linearly increases from 3.610 ± 0.066 to 3.758 ± 0.068 eV with increasing La composition. It is surmised that the phenomena are mainly ascribed to the variations of the electronic structure, especially for the conduction band, which is perturbed by the La doping.

Growth, Microstructure, and Infrared-Ultraviolet Optical Conductivity of La0.5Sr0.5CoO3 Nanocrystalline Films on Silicon Substrates by Pulsed Laser Deposition

La(0.5)Sr(0.5)CoO(3) (LSCO) nanocrystalline (nc) films have been directly grown on silicon wafers under different substrate temperatures by pulsed laser deposition. The X-ray diffraction analysis indicate that the films are polycrystalline with the pure perovskite phase at higher substrate temperatures. The columnar growth formation with the nanocrystalline structure in the films has been confirmed by microscopy experiments. Infrared-ultraviolet optical properties of the LSCO films have been investigated with the aid of spectroscopic ellipsometry (SE). Dielectric function in the photon energy range of 1.1-3.1 eV (400-1100 nm) has been extracted by reproducing the experimental data with a Lorentz oscillator model. It is found that the real part is decreased from 4.7 to -0.7 at the near-infrared region with increasing substrate temperature. The optical conductivity shows a different variation trend for the lower and higher growth temperatures, respectively. Note that the films deposited above 650 degrees C exhibit the well-defined metallic phase behavior. The discrepancies could be mainly ascribed to different crystalline structure and surface morphology. The present results may be crucial for future applications of ferromagnetic-based optoelectronic and spin-electronic devices.

Improved electric behaviors of the Pt/Bi1−xLaxFe0.92Mn0.08O3/n+-Si heterostructure for nonvolatile ferroelectric random-access memory

Multiferroic BiFeO3 (BFO) and lanthanum-substituted Bi1−xLaxFe0.92Mn0.08O3 (BLFMx, 0 ≤ x ≤ 0.2) films have been directly deposited on heavily doped Si(100) with an electric resistivity of about 0.001 Ω cm. The La substitution effects on the microstructure and lattice dynamics of the BLFMx films have been investigated by X-ray diffraction (XRD), scanning electron microscopy, far-infrared reflectance and Raman scattering studies. XRD analysis shows that the rhombohedral structure of BFO films reduced toward the orthorhombic or tetragonal structure by Mn and La substitution. It was found that the leakage current density tended to decrease with increasing La composition in a low electric field because the La dopant can suppress the formation of oxygen vacancies by stabilizing the oxygen octahedron and controlling the volatility of Bi atoms. On the other hand, the leakage current density in the high electric field can be suppressed by Mn substitution due to compensation of the charge of Fe2+ ions. The well-saturated polarization hysteresis can be obtained in the Pt/BLFMx/n+-Si prototype devices. As an example, the electric remanent polarization (2Pr) and coercive field (2Ec) at the electric field of 1600 kV cm−1 for the BLFM0.15 film are 150 μC cm−2 and 870 kV cm−1, respectively. Moreover, its relative dielectric constant at the frequency of 3.5 × 105 Hz is about 125. These results could be crucial for future applications of silicon-based nonvolatile ferroelectric random-access memory.

Spin-manipulated phonon dynamics during magnetic phase transitions in triangular lattice antiferromagnet CuCr1−xMgxO2 semiconductor films

Raman scattering and infrared reflectance spectra of CuCr1−xMgxO2 films (x = 0.03, 0.06 and 0.09) in the temperature range of 5–300 K have been studied, combined with first-principles calculations. The abnormal redshift of the Eg Raman mode center with decreasing temperature below 100 K for the lightly doped film (x = 0.03) is ascribed to the Cr 3d–O 2p–Cu 3d interaction. Strong disturbance of the local spin fluctuation at Cr sites in heavily Mg-doped films (x = 0.06 and 0.09) drives the Eg mode center to a normal blueshift with decreasing temperature. With further decreasing temperature, the out-of-plane structure increases the internal spin-charge coupling. It enhances the spin-flip splitting and brings back the abnormal redshift of the Eg Raman mode center. A similar but more obvious trend can be found from temperature-dependent Eu infrared mode center shifts. Two successive magnetic transitions were observed at the corresponding Neel temperatures TN1 around 24.7 K and TN2 around 23.0 K, as manifested by magnetoresistance measurements. The interesting phenomena of phonon dynamics are suggested to be manipulated by the spin structures during the magnetic transitions.