Lietuvos mokslas ir pramonė

Vytauto Didžiojo universitetasŽemės ūkio akademij

Study of p-Fe₃O₄/n-(GaAs,Si) Heterostructures: Fabrication and Physical Properties

We report preparation and investigation of p -n heterostructures based on Fe₃O₄ thin films grown on semiconductor Si and GaAs substrates. Fe₃O₄ films with thickness ranging from 60 to 300 nm were grown at 350÷450°C using dc magnetron sputtering technique. The measurement of X-ray diffraction and reflection high energy electron diffraction revealed polycrystalline microstructure of thin Fe₃O₄ films deposited on both Si and GaAs substrate. Investigation of surface composition by X-ray photoelectron spectroscopy showed that Fe 2p peak consists of three main peaks, namely, metallic iron Fe(0), Fe(II), and Fe(III). Transport measurements of Fe₃O₄/n-(Si, GaAs) heterostructures demonstrated nonlinear current-voltage (I -V) dependences in the temperature range from 300 K to 78 K

Response of Grained La 0.67

It was found that the response of grained La$\text{}_{0.67}$Ca$\text{}_{0.33}$MnO$\text{}_{3}$ films to microwave radiation is of thermal nature. A nonresonant method for measuring of the electric resistance of grains was developed. It allows one to avoid the influence of the magnetic field of the wave on the measurement results. The measurements of the temperature dependences of the grain electric resistance indicate that in the vicinity of the maximum-resistance temperature T$\text{}_{m}$, the intrinsic electric resistance of the grains is more than by two orders of magnitude lower than the film resistance measured by applying the dc current. The obtained experimental results agree well with those given by resonant techniques

Synthesis and Electrical Properties of La-Pr-Mn-O Thin Films and Heterostructures

In this paper we report the results of synthesis and study of both ceramic samples and thin films of electronically doped La$\text{}_{0.7}$Pr$\text{}_{0.3}$MnO$\text{}_{3}$ and related heterostructures composed of La$\text{}_{0.7}$Pr$\text{}_{0.3}$MnO$\text{}_{3}$ and p-type La$\text{}_{0.67}$Ca$\text{}_{0.33}$MnO$\text{}_{3}$. The ceramic La$\text{}_{0.7}$Pr$\text{}_{0.3}$MnO$\text{}_{3}$ samples were prepared by a conventional solid state reaction technique. Single phase La$\text{}_{0.7}$Pr$\text{}_{0.3}$MnO$\text{}_{3}$ thin films and La$\text{}_{0.7}$Pr$\text{}_{0.3}$MnO$\text{}_{3}$/La$\text{}_{0.67}$ Ca$\text{}_{0.33}$MnO$\text{}_{3}$ heterostructures were grown on lattice-matched perovskite NdGaO$\text{}_{3}$ substrates by pulsed laser deposition. Electron doping was indicated both for ceramic La$\text{}_{0.7}$Pr$\text{}_{0.3}$MnO$\text{}_{3}$ samples and thin films from thermopower data. Both ceramic samples and thin films of La$\text{}_{0.7}$Pr$\text{}_{0.3}$MnO$\text{}_{3}$ demonstrated resistivity of about 10 mΩ cm at 300 K and semiconductor-like resistance vs. temperature behavior with cooling down to 78 K. Meanwhile, the resistance of the La$\text{}_{0.7}$Pr$\text{}_{0.3}$MnO$\text{}_{3}$/La$\text{}_{0.67}$Ca$\text{}_{0.33}$MnO$\text{}_{3}$ interface showed an anomalous peak at 185 K. A series of post-deposition annealing experiments demonstrated a crucial role of annealing temperature and ambience on both electrical and magnetic properties of La$\text{}_{0.7}$Pr$\text{}_{0.3}$MnO$\text{}_{3}$ material and the heterostructures

Response of Grained La0.67\text{}_{0.67}Ca0.33\text{}_{0.33}MnO3\text{}_{3} Films to Microwave Radiation and the Method for Measuring Electrical Resistance of Grains

It was found that the response of grained La$\text{}_{0.67}$Ca$\text{}_{0.33}$MnO$\text{}_{3}$ films to microwave radiation is of thermal nature. A nonresonant method for measuring of the electric resistance of grains was developed. It allows one to avoid the influence of the magnetic field of the wave on the measurement results. The measurements of the temperature dependences of the grain electric resistance indicate that in the vicinity of the maximum-resistance temperature T$\text{}_{m}$, the intrinsic electric resistance of the grains is more than by two orders of magnitude lower than the film resistance measured by applying the dc current. The obtained experimental results agree well with those given by resonant techniques

Growth and Investigation of p−La2//3Ca1//3MnO3p-La_{2//3}Ca_{1//3}MnO_3/n-Si Heterostructures

We report the fabrication and investigation of p-n diode structures based on thin hole-doped $La_{2//3}Ca_{1//3}MnO_3$ films grown on n-type silicon substrates. $La_{2//3}Ca_{1//3}MnO_3$ films with typical thickness of about 400 nm were prepared using pulsed laser deposition. Reflection high-energy electron diffraction measurements revealed polycrystalline quality of $La_{2//3}Ca_{1//3}MnO_3$ thin films on Si substrates. The surface roughness of $La_{2//3}Ca_{1//3}MnO_3$ films investigated by atomic force microscopy was found to be in the range of 25÷30 nm. Studies of electrical properties showed that $La_{2//3}Ca_{1//3}MnO_3$/Si heterostructures exhibit nonlinear asymmetric I-V characteristics both at room temperature and at 78 K. Furthemore, it was shown that these I-V dependences are sensitive to magnetic field, especially at lower voltages

Electroresistance of Electrically Nonhomogeneous La0.67\text{}_{0.67}Ca0.33\text{}_{0.33}MnO3\text{}_{3}/MgO Thin Films

Current and electrical field-induced electroresistive effects were investigated for La$\text{}_{0.67}$Ca$\text{}_{0.33}$MnO$\text{}_{3}$/MgO thin films demonstrating nanosized electrical inhomogeneities. Two different models based on enhanced conductivity of intergrain boundaries by injecting spin-polarized carriers from ferromagnetic grains and electrical field-enhanced hopping of carriers in high resistance intergrain media were carried out to explain nonlinear electrical properties of the films

Electroresistance of Electrically Nonhomogeneous La 0.67

Current and electrical field-induced electroresistive effects were investigated for La$\text{}_{0.67}$Ca$\text{}_{0.33}$MnO$\text{}_{3}$/MgO thin films demonstrating nanosized electrical inhomogeneities. Two different models based on enhanced conductivity of intergrain boundaries by injecting spin-polarized carriers from ferromagnetic grains and electrical field-enhanced hopping of carriers in high resistance intergrain media were carried out to explain nonlinear electrical properties of the films

Growth and Investigation of Heterostructures Based on Multiferroic BiFeO3BiFeO_3

We report heteroepitaxial growth of multiferroic $BiFeO_3$ thin films by RF magnetron sputtering on lattice-matched $SrTiO_3$ substrates, as well as preparation and electrical properties of the heterostructures formed by growing $BiFeO_3$ thin films on highly conductive $LaNiO_3$ films and n-Si substrates. Nonlinear and rectifying current-voltage (I-U) characteristics were revealed for the heterojunctions in a wide temperature range (T=78-300 K)