J. H. Ma

Dielectric functions of ferroelectric Bi3.25La0.75Ti3O12 thin films on Si(100) substrates

Ferroelectric Bi3.25La0.75Ti3O12 (BLT) thin films were deposited directly on Si(100) substrates under annealing temperatures of 590 and 700 °C. The optical properties of the BLT films have been investigated using spectroscopic ellipsometry at room temperature in the 0.73–6 eV energy range. To model the dielectric functions of the BLT films, the double Tauc–Lorentz dispersion relation was successfully adopted. A four-layer model was used to fit the measured pseudodielectric function in order to deduce the complex dielectric functions. The results show that the annealing temperature mainly affects the dielectric functions of the BLT films beyond the fundamental band gap energy. The volume fraction of air present in the surface rough layer increases with increasing annealing temperature. The fundamental band gap was observed to shift slightly to a higher energy at a high annealing temperature. The difference of the optical properties due to the structure changes testifies the x-ray diffraction spectral results.

Temperature dependence of ferroelectric and dielectric properties of PbZr0.5Ti0.5O3 thin film based capacitors

The temperature dependence of the ferroelectric and dielectric properties of PbZr0.5Ti0.5O3 thin films deposited on LaNiO3-coated SrTiO3 substrate was investigated. The results showed that both the saturation polarization and remanent polarization increased with decreasing temperature from 300 to ∼50 K, and decreased as the temperature continued to decrease below 40 K. The capacitance of the PbZr0.5Ti0.5O3 ferroelectric thin film capacitor as a function of small ac field and temperature was measured, and the data were processed using Rayleigh law. It was demonstrated that both the reversible and irreversible contributions to the dielectric constant decreased with decreasing temperature; however, they showed an increase when the temperature dropped below 50 K. The anomalous behavior of the temperature dependence may be attributed to a phase transition in the PbZr0.5Ti0.5O3 thin film in the vicinity of 50 K.

Effect of Fe-doping concentration on microstructure, electrical, and magnetic properties of Pb(Zr0.5Ti0.5)O3 thin films prepared by chemical solution deposition

The highly (l00) oriented Pb(Zr0.5Ti0.5)O3 thin films with different Fe3+ doping concentrations were fabricated on LaNiO3-coated silicon substrates by chemical solution deposition. And the microstructure, ferroelectric, leakage, and magnetic properties were investigated. The results indicate that incorporation of Fe3+ into PZT thin films can promote the degree of the lattice distortion and greatly improve the surface roughness. In comparison with the pure PZT sample, the ferroelectric hysteresis loops of Fe-doped PZT samples demonstrate larger and larger polarizations and coercive fields with the increase in Fe3+ doping amount. Moreover, leakage mechanism of present films evolves from the space charge limited conduction to the “modified” space charge limited conduction, and then returns to the space charge limited conduction with increasing Fe3+ doping concentration. The occurrence of exchange bias in these Fe-doped PZT samples implies that the magnetic exchange interaction can be explained by the bound m...

Optical properties of SrTiO3 thin films deposited by radio-frequency magnetron sputtering at various substrate temperatures

SrTiO3 thin films were deposited on vitreous silica substrates at various substrate temperatures (300–700°C) by rf magnetron sputtering technique. The transition from amorphous phase to polycrystalline phase for the films occurred at the substrate temperatures of 300–400°C. Their optical properties were investigated by transmittance measurements. The fitting method was used to calculate the refractive index and the film thickness from the transparent region of the transmittance spectra. The refractive index increased and the film thickness decreased with the substrate temperatures increasing. The dispersion of the refractive index was studied by considering a single electronic oscillator model. The band gaps of the films were estimated from Tauc’s law and showed a decreasing tendency to that of the bulk SrTiO3 with the substrate temperatures increasing. These results provide some useful references for the potential application of SrTiO3 films in integrated optics devices.

Surface chemical composition and optical properties of nitrogen-doped Ba0.6Sr0.4TiO3 thin films

The effect of nitrogen (N) doping on surface chemical composition and optical properties of Ba0.6Sr0.4TiO3 (BST) thin films have been investigated using x-ray photoemission spectroscopy (XPS) and transmittance measurement. It was found that the nitrogen doping induced Ti3+∕4+ states and formed Ti–N bonds in BST films. With lower implantation energy, the oxygen vacancies can be effectively eliminated because of N dopants substitution. Unfortunately, the oxygen vacancies possibly increased when enhance implantation energy. In addition, the N-doped BST films indicated larger amount of perovskite phase and lower content additional phase at surface. The chemical shifts in XPS confirmed the forming of Ba–N and Sr–N bonds in N-doped BST films. The optical properties for as-grown and N-doped BST films were considered at wavelength range from 190to1100nm. The optical constants, including refractive index n and extinction coefficient k, were calculated by fitting transmittance spectra with single Tauc-Lorenz and Lo...

Buried homojunction in CdS/Sb2Se3 thin film photovoltaics generated by interfacial diffusion

Antimony selenide (Sb2Se3) emerges as a very promising non-toxic absorber material for thin film photovoltaics, and most of the devices, either in the superstrate or substrate configuration, employed CdS as the buffer layer. Due to the peculiar one-dimensional crystal structure of Sb2Se3, severe interfacial diffusion would be expected. In this letter, the interfacial diffusion in CdS/Sb2Se3 photovoltaics was carefully characterized from a combined material and device physics characterization. The results indicated that a buried homojunction located deep inside the Sb2Se3 absorber layer due to Cd diffusion, instead of the apparent CdS/Sb2Se3 heterojunction, dictated charge separation and device performance in Sb2Se3 thin film solar cells. Cd diffusion converted p-type Sb2Se3 into n-type by introducing a donor level with an activation energy of 0.22 eV. Our studies deepen the understanding of Sb2Se3 photovoltaics and shed light on their further performance optimization.

Optical characterization of ferroelectric Bi3.25La0.75Ti3O12 thin films

Abstract.Amorphous and crystalline Bi3.25La0.75Ti3O12 (BLT) thin films on vitreous silica and sapphire substrates are prepared from chemical solutions. Their optical properties are investigated by transmittance measurements at energies from 1.1 to 5.0 eV. A four-phase model consisting of air, surface rough layer, BLT, and substrate is used to simulate the measured transmittance spectra. The inverse synthesis method with a double Tauc-Lorentz (DTL) dispersion function is used to calculate the optical constants and film thicknesses. The dispersion of the refractive index in the transparent region agrees with Sellmeier’s dispersion relation. The absorption edges of the BLT films are different in the amorphous and crystalline cases.

Characterization of Mg and Fe doped Sb2Se3 thin films for photovoltaic application

Sb2Se3 holds a great potential for low-cost thin film photovoltaics because of its very attractive material and optoelectronic properties, and the demonstrated 5.6% certified efficiency and decent device stability. A full understanding of the influence of external impurities on the properties of Sb2Se3 films would help the further improvement of Sb2Se3 solar cells. In this work, we carefully characterized the Mg and Fe doping in Sb2Se3 films. Both Kelvin probe force microscope and Hall measurements revealed that Mg was largely inert while Fe introduced the n-type doping. Temperature-dependent conductivity and admittance further demonstrated that Fe doping introduced two defect levels within the bandgap with their positions ∼0.3 eV and ∼0.4 eV below the conduction band. We caution that iron contamination should be minimized for high efficiency Sb2Se3 solar cells.

Electron injection of SrTiO3∕Si interfacial layer

The electrical properties of the SrTiO3(STO)∕Si interfacial layer were studied by measuring STO metal-insulator-semiconductor (MIS) structure. The C-V measurements showed that there existed electron injection at the STO/Si interface under the higher sweep voltages. The electron injection behavior was analyzed and discussed in detail. By analyzing the voltage distribution of the semiconductor Si, the insulator STO, and the STO/Si interfacial layer in MIS structure, the electron injection electric field of interfacial layer was estimated to be about 5.5MV∕cm.