Temperature dependent Raman scattering in polycrystalline Bi4Ti3O12 thin films

Polycrystalline Bi₄Ti₃O₁₂ thin films were prepared on quartz substrates by pulsed laser deposition. The films were crystallized in the orthorhombic layer perovskite structure confirmed by X-ray diffraction and Raman spectroscopy. The Raman spectra are strongly dependent on temperature. A subtle phase transition in the temperature range 473-573 K exists in polycrystalline BTO thin films, which is evidenced by the disappearance of the Raman band at 116 cm⁻¹ and appearance of a new Raman band at 151 cm⁻¹. The two broad Raman bands centered at the 57 and 93 cm⁻¹ at 300 K break up into clusters of several sharp Raman peaks at 77 K, due to monoclinic distortion of orthorhombic structure at low temperature in the as-prepared Bi₄Ti₃O₁₂ thin films.     

Investigation of structural phase transition in polycrystalline SrTiO3 thin films by Raman spectroscopy

Polycrystalline SrTiO₃ thin films were prepared by pulsed laser deposition technique. The phonon properties and structural phase transition were studied by Raman spectroscopy. The first-order Raman scattering, which is forbidden in SrTiO₃ single crystal, has been observed in the films, due to the structural distortion caused by strain effect and oxygen vacancies. The Fano-type line shape of TO₂ phonon reveals the existence of polar microregions in the STO thin films. The evolution of TO₂ and TO₃ phonons with temperature shows the occurrence of a structural phase transition at 120 K related to the formation of polar macroregions in the films.     

Piezoelectric evaluation of ion beam etched Pb(Zr,Ti)O3 thin films by piezoresponse force microscopy

The evolution of piezoelectric properties of Pb(Zr,Ti)O₃ (PZT) thin films after ion beam etching have been investigated at the nanoscale level by piezoelectric force microscopy. A comparison of the piezoelectric properties on etched and unetched films is realized. Piezoelectric contrasts imaging evidences a modification of the domain architecture at the film surface. Local piezoelectric hysteresis loops measurements on grains indicate that the coercive voltage for switching is much higher for the etched films (2.3 V) compared to the unetched ones (1.0 V) while the average piezoelectric activity is slightly lower. The results are explained in terms of grain-damaging during etching and domain-wall pinning.     

Infrared optical properties of Ba(Zr0.20Ti0.80)O3 and Ba(Zr0.30Ti0.70)O3 thin films prepared by sol-gel method

Ba(Zr_xTi_1−x)O₃ (BZT) (x = 0.20 and 0.30) thin films are deposited on Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrate by sol-gel method. X-ray diffraction patterns show that the thin films have a good crystallinity. Optical properties of the films in the wavelength range of 2.5-12 μm are studied by infrared spectroscopic ellipsometry (IRSE). The optical constants of the BZT thin films are determined by fitting the IRSE data using a classical dispersion formula. As the wavelength increases, the refractive index decreases, while the extinction coefficients increase. The effective static ionic charges are derived, which are smaller than that in a purely ionic material for the BZT thin films.     

Electrical behavior of BaZr0.1Ti0.9O3 and BaZr0.2Ti0.8O3 thin films

BaZr_0.1Ti_0.9O₃ and BaZr_0.2Ti_0.8O₃ (BZT) thin films were deposited on Pt/Ti/LaAlO₃ (1 0 0) substrates by radio-frequency magnetron sputtering, respectively. The films were further annealed at 800 °C for 30 min in oxygen. X-ray diffraction θ-2θ and Φ-scans showed that BaZr_0.1Ti_0.9O₃ films displayed a highly (h 0 0) preferred orientation and a good cube-on-cube epitaxial growth on the LaAlO₃ (1 0 0) substrate, while there are no obvious preferential orientation in BaZr_0.2Ti_0.8O₃ thin films. The BaZr_0.1Ti_0.9O₃ films possess larger grain size, higher dielectric constant, larger tunability, larger remanent polarization and coercive electric field than that of BaZr_0.2Ti_0.8O₃ films. Whereas, BaZr_0.1Ti_0.9O₃ films have larger dielectric losses and leakage current density. The results suggest that Zr⁴⁺ ion can decrease dielectric constant and restrain non-linearity. Moreover, the enhancement in dielectric properties of BaZr_0.1Ti_0.9O₃ films may be attributed to (1 0 0) preferred orientation.     

Anisotropic in-plane misfit strains dependence of phase diagrams and dielectric behavior in epitaxial Pb(Zr1−xTix)O3 thin films

A phenomenological Landau–Devonshire thermodynamic theory is used to describe the effects of anisotropic in-plane misfit strains on equilibrium polarization states and dielectric properties of single domain epitaxial Pb(Zr_1−xTi_x)O₃ thin films grown on dissimilar orthorhombic substrates. Compared with the “isotropic in-plane misfit strains-temperature” phase diagrams, the characteristic features of “misfit strain-misfit strain” and “misfit strain-temperature” phase diagrams under the circumstance of strain anisotropy are the presence of four different phases (a^′$a^{\prime }$, a^″$a^{″}$, a^′c$a^{\prime }c$, and a^″c$a^{″}c$) and the direct 90° polarization switching between c   phase and a^′$a^{\prime }$ phase (or a^″$a^{″}$ phase), between a^′$a^{\prime }$ phase and a^″$a^{″}$ phase. The misfit strain dependence of polarization components, the small-signal dielectric responses and the tunabilities at room temperature are also calculated. We find that the phase diagrams and dielectric properties largely depend on anisotropic in-plane misfit strains as well. Moreover, the strain anisotropy will lead to the polarization and dielectric anisotropy.     

Tetragonal distortion and the domain structure of thin Pb(Zr,Ti)O3/MgO(100) films

3 /MgO(100) films was studied in synchrotron X-ray scattering experiments. In the thin epitaxial films, the tetragonal distortion of the ferroelectric phase and the transition temperature were significantly reduced. In sharp contrast to the reported mixture of the a-type and the c-type domains in thicker films, the 250-Å-thick film was purely composed of the c-type domains in the tetragonal phase. We attribute the suppression of the transition to the substrate effect, which prefers the c-type domains near the interface, and reduces the tetragonal distortion to minimize the strain energy caused by the lattice mismatch. Received: 1 November 1997/Accepted: 5 January 1998     

Growth and thermal stability of epitaxial BiFeO3 thin films

We have investigated the oxygen pressure and the temperature dependence on BiFeO₃ thin films deposited on SrTiO₃ substrates by pulsed laser deposition. Reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM) and X-ray diffraction measurements indicate that high-quality epitaxial thin films are obtained for and T=650 °C. Outside of this pressure-temperature window, parasitic peaks attributed to β-Bi₂O₃ appear. We find an increase of the out-of-plane lattice parameter with oxygen pressure that we ascribe to Bi-deficiency due to its high volatility at low pressure. Ex-situ anneals have been performed and results show that as-grown single-phase BiFeO₃ thin films degrade after annealing, whereas as-grown BiFeO₃ containing impurity phases evolve toward a single-phase structure. These experiments demonstrate that parasitic phases can stabilize compounds which are usually unstable in air at elevated temperatures.     

Structural, dielectric and pyroelectric studies of Pb1−XCaXTiO3 thin films

Thin films of Pb_1−xCa_xTiO₃ [x=0.20, 0.24 and 0.28] have been prepared on ITO coated Corning glass substrates by sol gel technique. The perovskite phase of PCT films is formed at 650 °C with a polycrystalline tetragonal structure. The tetragonal factor (c/a) decreases with increasing Ca concentration. Dielectric, pyroelectric and ferroelectric studies have been carried out on these films. The effects of introduction of Ca ion in PbTiO₃ have also been discussed.     

Large blue shift in the optical band-gap of sol-gel derived Ba0.5Sr0.5TiO3 thin films

In this paper, we report and analyze the large blue shift in the optical band-gap of sol-gel derived Ba_0.5Sr_0.5TiO₃ (BST) thin films. BST films of different thickness (150 nm, 320 nm and 480 nm respectively) were deposited layer by layer onto fused quartz substrates by a spin coating technique. The drying temperature for individual layers (pre-sintering temperature) was varied as 400, 500 and 600 ^°C. A large blue shift in the band-gap was observed (with a value 4.70 eV compared to the bulk value of 3.60 eV) for films pre-sintered at 400 ^°C, which decreased with increase in the pre-sintering temperature. To date such blue shifts have been attributed to grain size reduction, stress and the amorphous nature of the films. Here, the blue shift has been correlated with the presence of charge carriers generated by oxygen vacancies and explained on the basis of the Burstein-Moss effect.     

Growth and melting behaviour of thin in films on Ge(100)

Growth and melting behaviour of thin indium films on Ge(100) have been investigated by Auger-electron spectroscopy (AES), atomic force microscopy (AFM) and perturbed angular correlation (PAC) spectroscopy, respectively. At room temperature inidium is found to grow in three-dimensional islands even at submonolayer coverages. A very rough film surface is observed for thicknesses up to 230 ML. The melting behaviour of such films has been studied by PAC. A reduction of the melting temperature T _m as well as a strong supercooling of the films is observed. The electric field gradient for ¹¹¹In(¹¹¹Cd) in the indium islands is determined as a function of temperature and is used to monitor the local crystalline order of the films up to temperatures just below the melting point.     

Characteristics of Si-doped Sb2Te3 thin films for phase-change random access memory

The characteristics of Si-doped Sb₂Te₃ thin films were investigated using differential scanning calorimetry (DSC), four-point probe technique, X-ray diffraction (XRD) analysis and high resolution transmission electron microscopy (HRTEM). It is found that the as-deposited Sb₂Te₃ film in our study is partly crystallized. Silicon doping increases the crystallization temperature and resistivity of Sb₂Te₃ film significantly. XRD and HRTEM analyses indicated that some of the doped Si atoms substitute for Sb or Te in the lattice, while others exists at the grain boundaries in the form of amorphous phase, which may be responsible for grain size reduction and high crystalline resistivity of Si-doped specimens. Compared with the conventional Ge₂Sb₂Te₅ film, Si-doped Sb₂Te₃ films exhibit lower melting temperature and higher crystalline resistivity, which is beneficial to RESET current reduction of phase-change random access memory (PRAM). These results show the feasibility of Si-doped Sb₂Te₃ films in PRAM application.     

Effects of Ge doping on the properties of Sb2Te3 phase-change thin films

Ge-doped Sb₂Te₃ films were prepared by magnetron sputtering of Ge and Sb₂Te₃ targets on SiO₂/Si (1 0 0) substrates. The effect of Ge doping on the structure was studied in details by X-ray diffraction, differential scanning calorimetry, and X-ray photoelectron spectroscopy measurements. It is indicated that Ge atoms substitute for Sb/Te in lattice sites and form Ge-Te bonds, moreover, a metastable phase was observed in Ge-doped specimens. Both crystallization temperature and resistivity of amorphous Sb₂Te₃ increase after Ge doping, which are beneficial for improving room temperature stability of the amorphous state and reducing the SET current of chalcogenide random access memory.     

Epitaxial growth and structural characterization of Pb(Fe1/2Nb1/2)O3 thin films

We have grown lead iron niobate thin films with composition Pb(Fe_1/2Nb_1/2)O₃ (PFN) on (0 0 1) SrTiO₃ substrates by pulsed laser deposition. The influence of the deposition conditions on the phase purity was studied. Due to similar thermodynamic stability spaces, a pyrochlore phase often coexists with the PFN perovskite phase. By optimizing the kinetic parameters, we succeeded in identifying a deposition window which resulted in epitaxial perovskite-phase PFN thin films with no identifiable trace of impurity phases appearing in the X-ray diffractograms. PFN films having thicknesses between 20 and 200 nm were smooth and epitaxially oriented with the substrate and as demonstrated by RHEED streaks which were aligned with the substrate axes. X-ray diffraction showed that the films were completely c-axis oriented and of excellent crystalline quality with low mosaicity (X-ray rocking curve FWHM?0.09°). The surface roughness of thin films was also investigated by atomic force microscopy. The root-mean-square roughness varies between 0.9 nm for 50-nm-thick films to 16 nm for 100-nm-thick films. We also observe a correlation between grain size, surface roughness and film thickness.     

Effect of Ti doping on Ta2O5 stacks with Ru and Al gates

The Ti-doped Ta₂O₅ thin films (<10 nm) obtained by rf sputtering are studied with respect to their composition, dielectric and electrical properties. The incorporation of Ti is performed by two methods - a surface doping, where a thin Ti layer is deposited on the top of Ta₂O₅ and a bulk doping where the Ti layer is sandwiched between two layers of Ta₂O₅. The effect of the process parameters (the method and level of doping) on the elemental distribution in-depth of the films is investigated by the time of flight secondary ion mass spectroscopy (ToF-SIMS). The Ti and Ta₂O₅ are intermixed throughout the whole thickness but the layers are very inhomogeneous. Two sub-layers exist in all the samples — a near interfacial region which is a mixture of Ta-, Ti-, Si-oxides as well as TaSiO, and an upper Ti-doped Ta₂O₅ sub-layer. For both methods of doping, Ti tends to pile-up at the Si interface. The electrical characterisation is performed on capacitors with Al- and Ru-gate electrodes. The two types of MIS structures exhibit distinctly different electrical behavior: the Ru gate provides higher dielectric permittivity while the stacks with Al electrode are better in terms of leakage currents. The specific metal-dielectric reactions and metal-induced electrically active defects for each metal electrode/high-k dielectric stack define its particular electrical behavior. It is demonstrated that the Ti doping of Ta₂O₅ is a way of remarkable improvement of leakage characteristics (the current reduction with more than four orders of magnitude as compared with undoped Ta₂O₅) of Ru-gated capacitors which originates from Ti induced suppression of the oxygen vacancy related defects.     

Piezoelectric coefficients of multilayer Pb(Zr,Ti)O<Subscript>3</Subscript> thin films

Sol–gel techniques were used to prepare thin films of Pb(Zr _x ,Ti_1−x )O₃ (PZT) with three different Zr/Ti ratios and a graded PZT film with three different compositional layers. A Michelson interferometer was used to measure the thickness strains due to an applied ac electric field. Effective d ₃₃ piezoelectric strain coefficients were computed from the experimental data. Interfacial pinning caused these coefficients to differ from the true ones. They were corrected for the pinning using both an analytical model and finite-element analysis. The corrected coefficients of the PZT (52/48) sample were in excellent agreement with values of bulk materials. The coefficients of the multilayer sample were very low, probably due to insufficient poling or domain switching.     

Growth and characterization of Y2O3 thin films

Y₂O₃ thin films were grown on silicon (1 0 0) substrates by pulsed-laser deposition at different substrate temperatures and O₂ pressures. The structure and composition of films are studied by using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The Y₂O₃ thin films deposited in vacuum strongly oriented their [1 1 1] axis of the cubic structure and the film quality depended on the substrate temperature. The magnitude of O₂ pressure obviously influences the film structure and quality. Due to the silicon diffusion and interface reaction during the deposition, yttrium silicate and SiO₂ were formed. The strong relationship between composition and growth condition was discussed.     

Band bending measurement of HfO2/SiO2/Si capacitor with ultra-thin La2O3 insertion by XPS

The flat band voltage shifts of HfO₂/SiO₂/nSi capacitors with ultra-thin La₂O₃ insertion at HfO₂/SiO₂ interface have been confirmed using hard X-ray photoelectron spectroscopy (HX-PES). By increasing the amount of La₂O₃ insertion, the binding energy of Si 1s core spectra increases, which means that the surface potential of Si substrate also increases. A voltage drop difference of HfO₂ and La₂O₃ at SiO₂ interface can be estimated to be 0.40 V.     

Phase diagram and dielectric behavior of Ba0.6Sr0.4TiO3 thin films grown on orthorhombic substrates

Using Landau-Devonshire (LD)-type phenomenological model, we investigate the phase diagrams and dielectric behaviors of single-domain single-crystal Ba_0.6Sr_0.4TiO₃ films deposited on orthorhombic substrates. An anisotropic strain factor is introduced to quantitatively calculate the effects of anisotropic in-plane misfit strains. Investigation indicates that anisotropic strains play a crucial role on formation of stable ferroelectric phases and dielectric properties. The anisotropic strains induce tetragonal phases which only contain one in-plane spontaneous polarization component. These phases do not exist in BST films of the same composition under isotropic strains. Moreover, permittivity and tunability of films can reach to maximum when the corresponding spontaneous polarization component disappears at the boundaries of structural phase transition.     

Effects of interface on the dielectric properties of Ba0.6Sr0.4TiO3 thin film capacitors

Ba_0.6Sr_0.4TiO₃ thin films were deposited on Pt/SiO₂/Si substrate by radio frequency magnetron sputtering. High-resolution transmission electron microscopy (HRTEM) observation shows that there is a transition layer at BST/Pt interface, and the layer is about 7-8 nm thickness. It is found that the transition layer was diminished to about 2-3 nm thickness by reducing the initial RF sputtering power. X-ray photoelectron spectroscopy (XPS) depth profiles show that high Ti atomic concentration results in a thick interfacial transition layer. Moreover, the symmetry ν of ?_r-V curve of BST thin film is enhanced from 52.37 to 95.98%. Meanwhile, the tunability, difference of negative and positive remanent polarization (P_r), and that of coercive field (E_C) are remarkably improved.