Pulsed laser deposition of BaGa<Subscript>2</Subscript>O<Subscript>4</Subscript>

This paper reports the first results obtained on monobarium gallate thin films grown on silicon and platinum coated substrates by pulsed laser deposition. The influence of oxygen background pressure and substrate (or post-annealing) temperature on the film properties was studied. The films were characterized by XRD, RHEED, AFM, photoelectron and electrical impedance spectroscopy. The structure analysis showed that the films crystallized into a hexagonal phase, most probably into (metastable) α-BaGa₂O₄. Depending on deposition conditions, films with different (from nearly epitaxial to polycrystalline) textures were obtained.     

Structural and electrical properties of 0.7Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>–0.3PbTiO<Subscript>3</Subscript> thin films grown on Ir/MgO buffered Si(100) substrates

Thin films of 0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃ (PMN-PT) have been grown on Ir/MgO buffered Si(100) substrates at different substrate temperatures by pulsed laser deposition. Crystalline phases as well as preferred orientations in the PMN-PT films were investigated by X-ray diffraction analysis (XRD). The microstructure, dielectric and ferroelectric properties of PMN-PT film prepared at 650 °C were studied. The results show that the film prepared at 650 °C exhibits pure perovskite phase and single c-axis orientation. The dielectric constant and dissipation factor of the single c-axis oriented film are 1000 and 0.04 at a frequency of 1 kHz, while the remnant polarization and coercive field are about 13.0 μC/cm² and 100 kV/cm under an electric field of 480 kV/cm, respectively. PACS 81.15.Fg; 77.80.-e; 77. 22.Ej; 77.55.+f; 85.50.Gk     

Pulsed-laser crystallization and epitaxial growth of metal-organic films of Ca-doped LaMnO3 on STO and LSAT substrates

Ca-doped LaMnO₃ (LCMO) thin films have been successfully prepared on SrTiO₃ (STO) and [(LaAlO₃)_0.3-(SrAlTaO₆)_0.7] (LSAT) substrates using the excimer laser assisted metal-organic deposition (ELAMOD) process. The crystallization and the epitaxial growth of the amorphous metal-organic LCMO thin films have been achieved using a KrF excimer laser irradiation while the substrates were kept at constant temperature of 500 °C. Epitaxial films were obtained using laser fluence in the interval of 50-120 mJ/cm². The microstructure of the LCMO films was studied using cross-section transmission electron microscopy. High quality of LCMO films having smooth surfaces and sharp interfaces were obtained on both the STO and the LSAT substrates. The effect of the laser fluence on the temperature coefficient of resistance (TCR) was investigated. The largest values of TCR of the LCMO grown on the LSAT and the STO substrates of 8.3% K⁻¹ and 7.46% K⁻¹ were obtained at different laser fluence of 80 mJ/cm² and 70 mJ/cm², respectively.     

Photoassisted chemical solution deposition method for fabricating uniformly epitaxial VO2 films

Epitaxial VO₂ films were prepared on the TiO₂ (001) substrates by the excimer-laser-assisted metal–organic deposition (ELAMOD). The quality of the epitaxial films obtained by irradiation with a KrF laser was found to be affected by the film structure obtained after preheating at 500 or 300°C. When the films containing crystal domains, which were obtained by preheating at 500°C, were irradiated with the laser at room temperature under a base pressure of 250 Pa, epitaxial and polycrystalline VO₂ phases were simultaneously formed. In contrast, when the amorphous films containing organic components, which were obtained by preheating at 300°C, were irradiated with the laser at room temperature in air, a single phase of epitaxial VO₂ was formed. By using thermal simulations, we determined that the formation of the epitaxial phase was affected both by the temperature distribution within the film during the laser irradiation and by the laser intensity at the interface between the substrate and the film. The latter factor is considered to play a role in the nucleation of crystallization, causing the epitaxial phase to form preferentially compared to the polycrystalline phase in the amorphous matrix of the films. These results indicate that the ELAMOD process is effective for the fabrication of epitaxial VO₂ films at low temperature.     

Temperature-dependent characteristics of pulse-laser-deposited (Pb,Sr)TiO<Subscript>3</Subscript> films at low temperatures

Pulsed laser deposition (PLD) of (Pb,Sr)TiO₃ (PSrT) film on Pt/SiO₂/Si at low substrate temperatures (T_s), ranging from 300–450 °C, has been investigated. As T_s increases, the films reveal coarsening clusters, improved crystallization of the perovskite phase, distinct capacitance–electric field (C–E) hysteretic loops and a larger dielectric constant. The 350 °C-deposited film shows strong (100) preferred orientation and optimum dielectric properties with the dielectric constant of ∼620. The current density increases as the measurement temperature and the electric field increase. Moreover, PSrT films exhibit a strong negative temperature coefficient of resistance (NTCR) behavior at temperatures ranging from 100 to 390 °C. PACS 81.15.Fg; 77.22.Ch; 68.60.Dv     

Magnetotransport characteristics of strained La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> epitaxial manganite films

The electrical and magnetic characteristics of La_0.7Sr_0.3MnO₃ (LSMO) epitaxial manganite films are investigated by different methods under conditions when the crystal structure is strongly strained as a result of mismatch between the lattice parameters of the LSMO crystal and the substrate. Substrates with lattice parameters larger and smaller than the nominal lattice parameter of the LSMO crystal are used in experiments. It is shown that the behavior of the temperature dependence of the electrical resistance for the films in the low-temperature range does not depend on the strain of the film and agrees well with the results obtained from the calculations with allowance made for the interaction of electrons with magnetic excitations in the framework of the double-exchange model for systems with strongly correlated electronic states. Investigations of the magneto- optical Kerr effect have revealed that an insignificant (0.3%) orthorhombic distortion of the cubic lattice in the plane of the NdGaO₃(110) substrate leads to uniaxial anisotropy of the magnetization of the film, with the easy-magnetization axis lying in the substrate plane. However, LSMO films on substrates (((LaAlO₃)_0.3+(Sr₂AlTaO₆)_0.7)(001)) ensuring minimum strain of the films exhibit a biaxial anisotropy typical of cubic crystals. The study of the ferromagnetic resonance lines at a frequency of 9.76 GHz confirms the results of magnetooptical investigations and indicates that the ferromagnetic phase in the LSMO films is weakly inhomogeneous.     

Electric and magnetic properties of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/Pb(Zr<Subscript>0.52</Subscript>Ti<Subscript>0.48</Subscript>)O<Subscript>3</Subscript> bilayer thin films prepared by pulsed-laser deposition

CoFe₂O₄ (CFO) thin film with highly (111)-preferential orientation was first deposited on the silicon substrate by a pulsed-laser deposition, and then Pb(Zr_0.52Ti_0.48)O₃ (PZT) layers were deposited with different oxygen pressures to form the bilayer CFO/PZT nanocomposite thin films. X-ray diffraction showed that the PZT preferential orientation was strongly dependant on the oxygen pressure. The smooth film surface was obtained after depositing the CFO and PZT layers. The bilayer thin films exhibit good ferromagnetic and ferroelectric properties, and a low leakage current density of 0.004 μA/cm² at 50 kV/cm. The leakage current density curves show loops for the electric polarized field when the electric field reverses. PACS 77.84.Lf; 75.80+q; 81.05.Zx; 81.15.Fg     

An investigation into ultra-thin pseudobinary oxide (TiO<Subscript>2</Subscript>)<Subscript>x</Subscript>(Al<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>1-x</Subscript> films as high-<Emphasis Type="Italic">k</Emphasis> gate dielectrics

As potential gate dielectric materials, pseudobinary oxide (TiO₂)_x(Al₂O₃)_1-x (0.1≤x≤0.6) films (TAO) were deposited on Si (100) substrates by pulsed-laser deposition method and studied systematically via various measurements. By a special deposition process, including two separate steps, the TAO films were deposited in the form of two layers. The first layer was deposited at room temperature and the second layer was completed at the substrate temperature of 400 °C. Detailed data show that the properties of the TAO films are closely related to the ratio between TiO₂ and Al₂O₃. The existence of the first layer deposited at room temperature can effectively restrain the formation of the interfacial layer. And according to the results of X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy performed on the films, no other information belonging to the silicon oxide could be observed. For the (TiO₂)_0.4(Al₂O₃)_0.6 film, the best result has been achieved among all samples and its dielectric constant is evaluated to be about 38. It is valuable for the amorphous TAO film as one of the promising dielectric materials for high-k gate dielectric applications. PACS 77.55.+f; 73.40.Qv; 81.15.Fg     

Electrical transport properties and laser-induced voltage effect in La0.8Ca0.2MnO3 epitaxial thin films

La_0.8Ca_0.2MnO₃ (LCMO) thin films about 200 nm thickness were grown on untilted and tilted (5°, 10° and 15°) LaAlO₃ (100) single crystal substrates by pulsed laser deposition technique. Electrical properties of the epitaxial thin films were studied by conventional four-probe technique and the anisotropic thermoelectric properties of the films grown on the tilted substrates have been investigated by laser-induced voltage (LIV) measurements. X-ray diffraction analysis and atomic force microscopy results show that the prepared LCMO thin films have a single phase and high crystalline quality. The remarkably large temperature coefficient of resistance (TCR) values (above 11 %/K) are observed in the all films. TCR value reaches 18 %/K on the film grown on 10° tilted substrate. The intensity of LIV signals monotonously increases with the tilting angles, and the largest signal is 148 mV with the fast time response 229 ns for the film grown on 15° tilted substrate.     

Preparation and characterization of La0.8Sr0.2MnO3 thin films by an excimer laser MOD process for a bolometer

La_0.8Sr_0.2MnO₃ (LSMO) films were prepared on LaAlO₃ substrates by excimer laser metal organic deposition (ELMOD) at 500 °C. The temperature dependence of resistance of the LSMO films was investigated by changing the laser fluence, irradiation time, and film thickness. It was found that the resistance of the LSMO films 80 nm in thickness that were irradiated by an ArF laser at a fluence of 100 mJ/cm² for 60 min showed a metallic temperature dependence, and the maximum temperature coefficient of resistance of the films (defined as 1/R×dR/dT) was 3.4% at 265 K. PACS 81.15.-z; 81.15.Fg; 81.15.Np; 73.61.-r; 71.30.+h     

Electro-optic properties of epitaxial Sr<Subscript>0.6</Subscript>Ba<Subscript>0.4</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> films grown on MgO substrates using Li<Subscript>x</Subscript>Ni<Subscript>2-x</Subscript>O buffer layer

Textured Li_xNi_2-xO (LNO) thin films have been fabricated on (001)MgO substrates by pulsed laser deposition technique. The as-deposited LNO films shows a conductivity of 2.5×10^-3 Ω m and possess a transmittance of about 35% in the visible region. Subsequent deposition of Sr_0.6Ba_0.4Nb₂O₆ (SBN60) thin film on these LNO-coated MgO substrates resulted in a textured SBN layer with a 〈001〉 orientation perpendicular to the substrate plane. Phi scans on the (221) plane of the SBN layer indicated that the films have two in-plane orientations with respect to the substrate. The SBN unit cells were rotated in the plane of the film by ± 8.2° as well as ± 45° with respect to the LNO/MgO substrate. Besides the highly (00l)-orientation, the SBN films also exhibited a dense microstructure as shown by scanning electron microscopy. The electro-optic coefficient (r₃₃) of the SBN film was measured to be 186 pm/V. On the basis of our results, we have demonstrated that the LNO film can be used as a buffer layer as well as a transparent bottom electrode for waveguide applications. The SBN/LNO heterostructure is also a suitable candidate for integrated electro-optics devices. PACS  42.79.Gn; 42.82.Et; 78.20.Ci     

Preparation of the La0.8Sr0.2MnO3 films on STO and LAO substrates by excimer laser-assisted metal organic deposition using the KrF laser

La_0.8Sr_0.2MnO₃ films were prepared on SrTiO₃ (STO) and LaAlO₃ (LAO) substrates using excimer laser-assisted metal organic deposition (ELAMOD). For the LAO substrate, no epitaxial La_0.8Sr_0.2MnO₃ film was obtained by laser irradiation in the fluence range from 60 to 110 mJ/cm² with heating at 500 °C. On the other hand, an epitaxial La_0.8Sr_0.2MnO₃ film on the STO substrate was formed by laser irradiation in the fluence range from 60 to 100 mJ/cm² with heating at 500 °C. To optimize the electrical properties for an IR sensor, the effects of the laser fluence, the irradiation time and the film thickness on the temperature dependence of the resistance and temperature coefficient of resistance (TCR: defined as 1/R·(dR/dT)) of the LSMO films were investigated. An LSMO film on the STO substrate that showed the maximum TCR of 3.9% at 265 K was obtained by the ELAMOD process using the KrF laser.     

Influence of the laser wavelength on the epitaxial growth and electrical properties of La0.8Sr0.2MnO3 films grown by excimer laser-assisted MOD

Epitaxial La_1−xSr_xMnO₃ (LSMO) films were prepared by excimer laser-assisted metal organic deposition (ELAMOD) at a low temperature using ArF, KrF, and XeCl excimer lasers. Cross-section transmission electron microscopy (XTEM) observations confirmed the epitaxial growth and homogeneity of the LSMO film on a SrTiO₃ (STO) substrate, which was prepared using ArF, KrF, and XeCl excimer lasers. It was found that uniform epitaxial films could be grown at 500 °C by laser irradiation. When an XeCl laser was used, an epitaxial film was formed on the STO substrate at a fluence range from 80 to 140 mJ/cm² of the laser fluence for the epitaxial growth of LSMO film on STO substrate was changed. When the LaAlO₃ (LAO) substrate was used, an epitaxial film was only obtained by ArF laser irradiation, and no epitaxial film was obtained using the KrF and XeCl lasers. When the back of the amorphous LSMO film on an LAO substrate was irradiated using a KrF laser, no epitaxial film formed. Based on the effect of the wavelength and substrate material on the epitaxial growth, formation of the epitaxial film would be found to be photo thermal reaction and photochemical reaction. The maximum temperature coefficient of resistance (TCR) of the epitaxial La_0.8Sr_0.2MnO₃ film on an STO substrate grown using an XeCl laser is 4.0%/K at 275 K. XeCl lasers that deliver stabilized pulse energies can be used to prepare LSMO films with good a TCR.     

Synchrotron X-ray diffraction study of SrRuO<Subscript>3</Subscript>/SrTiO<Subscript>3</Subscript>/SrRuO<Subscript>3</Subscript> nano-sized heterostructures grown by laser MBE

Structural investigation using X-ray synchrotron radiation has been performed on SrRuO₃/SrTiO₃/SrRuO₃ epitaxial heterostructures, with each constituent layer a few unit cell thick grown on (001) SrTiO₃ substrate. Detailed information on the evolution of the in-plane lattice structure has been obtained, in these heterostructures, by grazing incidence diffraction measurements. The samples have been grown by low-pressure pulsed laser deposition. Under our deposition conditions, SrRuO₃ layers grow with an elongated cell perpendicular to the substrate surface. The in-plane pseudocubic lattice parameters do not match the in-plane square SrTiO₃ structure even in the case of very thin SrRuO₃ layers. Such a distortion was found to decrease with increasing the thickness of the SrTiO₃ barrier layer.     

Two-dimensional electron gas at the interface of Ba<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>TiO<Subscript>3</Subscript> ferroelectric and LaMnO<Subscript>3</Subscript> antiferomagnet

The temperature dependence of the electrical resistance has been studied for heterostructures formed by antiferromagnetic LaMnO₃ single crystals of different orientations with epitaxial films of ferroelectric Ba_0.8Sr_0.2TiO₃ deposited onto them. The measured electrical resistance is compared to that exhibited by LaMnO₃ single crystals without the films. It is found that, in the samples with the film, for which the axis of polarization in the ferroelectric is directed along the perpendicular to the surface of the single crystal, the electrical resistance decreases significantly with temperature, exhibiting metallic behavior below 160 K. The numerical simulations of the structural and electronic characteristics of the BaTiO₃/LaMnO₃ ferroelectric?antiferromagnet heterostructure has been performed. The transition to the state with two-dimensional electron gas at the interface is demonstrated.     

Effect of substrate temperature on the properties of LiNbO<Subscript>3</Subscript> nanocrystalline films during pulsed laser deposition

The results are presented from studying the effect substrate temperature has on the morphological, electrophysical, and optical parameters of LiNbO₃ films obtained via pulsed laser deposition. It is established that films formed at temperatures of 300 to 600°C have a nanocrystalline structure with crystallites predominantly oriented in planes (110), (1010), and (220). The refractive index of the obtained films in the wavelength range of 500–800 nm varies from 2.13 to 2.26.     

Nonlinear dielectric response of epitaxial Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript> TiO<Subscript> 3</Subscript> thin films

Based on Landau-Devonshire (LD)-type phenomenological thermodynamic theory, the electric field dependence of the dielectric properties of tetragonal single-domain barium strontium titanate(Ba_1-xSr_xTiO₃) films on cubic substrates is theoretically investigated by taking into account the high order terms of the polarization. At room temperature, the nonlinear dielectric responses of epitaxial Ba_0.6Sr_0.4TiO₃ films are provided by adjusting the film thickness and growth temperature. The strong nonlinearity of relative dielectric constant and pyroelectric coefficient are attained around critical film thickness on MgO (69 nm) and LaAlO₃ (132 nm) substrates or critical growth temperature on MgO (337 °C) substrate with respect to epitaxy-induced lattice misfit and thermal stresses during deposition. This can be explained that small compressive stresses are effective to support high nonlinearity of dielectric constant and pyroelectric coefficient for Ba_0.6Sr_0.4TiO₃ films irrespective of whether they are on compressive substrate or tensile substrate. It is also predicted that a large tunability may be achieved by altering processing conditions, such as the film thickness and growth temperature for different substrates. Our theoretical results are in good agreement with the experimental data reported in literature.     

Giant magnetorefractive effect in La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> films

Complex experimental investigations of the structural, optical, and magneto-optical properties (magnetotransmission, magnetoreflection, and transversal Kerr effect, as well as the magnetoresistance, of La_0.7Ca_0.3MnO₃ epitaxial films indicate that magnetoreflection and magnetotransmission in manganite films can reach giant values and depend strongly on the magnetic and charge homogeneity of the films, their thickness, and spectral range under investigation. It has been shown that the optical enhancement of the magnetorefractive effect occurs in thin films as compared to manganite crystals. In the region of the minimum of the reflectance near the first phonon band, the resonance-like magnetorefractive effect has been observed, which is accompanied by change of the sign of the magnetoreflection. A model based on the theory of the magnetorefractive effect has been proposed to qualitatively explain this behavior.     

Response of the electrical resistivity of La<Subscript>0.67</Subscript>Ba<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>(20 nm)/LaAlO<Subscript>3</Subscript>(001) films to magnetic field and variation in temperature

The structure, electrical resistivity, and magnetotransport parameters of 20-nm-thick epitaxial La_0.67Ba_0.33MnO₃ films grown by laser ablation on LaAlO₃(001) substrates are studied. The unit cell volume V _eff = 58.80 ų of the as-grown manganite films is found to be less than that for bulk La_0.67Ba_0.33MnO₃ crystals. Maximum values of the negative magnetoresistance MR(μ₀ H = 1 T) = ?0.27 for La_0.67Ba_0.33MnO₃ films are observed at a temperature of about 225 K. For 5 < T < 100 K, the film magnetoresistance depends only weakly on temperature and is on the order of ?0.1. At temperatures below 100 K and for 3 < μ₀ H < 5 T, the electrical resistivity of the as-grown films decreases linearly with increasing magnetic field.     

Fabrication of transparent p-n junction composed of heteroepitaxially grown p-Li<Subscript>0.15</Subscript>Ni<Subscript>0.85</Subscript>O and n-ZnO films for UV-detector applications

Thin films of high-quality p-type Li_0.15Ni_0.85O (LNO) and n-type ZnO were heteroepitaxially grown on MgO(111) substrate by pulsed laser deposition technique to form transparent wide bandgap heterojunctions. The epitaxial nature of this p-LNO/n-ZnO/MgO heterojunction was confirmed to be (111)LNO||(0001)ZnO||(111)MgO (out-of-plane) and (002)LNO||(1002)ZnO||(002)MgO (in-plane) by X-ray diffraction. Optical transmittance spectrum and I–V characteristics were obtained at room temperature. The heterojunction exhibits reasonable optical transmittance of 50–60% on average in the whole infrared and visible region, and highly asymmetric electrical rectification with a turn-on voltage of about 1.0 V and a small leakage current. The highest photoresponsivity for a wavelength of 350 nm is 3.4×10³ V/W when the junction is irradiated under 5 μW UV illumination. The spectral response peak is obtained in the UV region and a reasonable large responsivity is shown for this p-LNO/n-ZnO/MgO heterojunction, which suggests the possibility of an inexpensive transparent oxide UV detector in a wide variety of electronics applications. PACS 68.55.Jk; 81.05.Dz; 81.15.Fg