Textured Polycrystalline Films Research Articles

Disorder driven variations in magnetoresistance and planar Hall effect in Bi2Te3 thin films

The degenerately doped sesquichalcogenide Bi2Te3, a well-known thermoelectric material, has attracted much attention in recent years as a topological insulator in which conducting surface states lead to exotic electronic transport. Here we report the observation of anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) in polycrystalline thin films of Bi2Te3 over a broad range of temperature and magnetic field strength. The values of AMR, PHE and the Hall mobility (μH) are compared with results of similar measurements on single crystals and epitaxial films of Bi2Te3, where these properties are understood in the framework of electronic transport in the topologically protected conducting surface states. Our textured polycrystalline films show a robust metallic character with a carrier density (ns) and mobility μH of 9×1020 cm−3 and 16.7 cm2V−1s−1 respectively. This carrier density is higher and the mobility lower by an order of magnitude compared to the reported values of ns and μH in epitaxial films and single crystals. However, we see a comparable AMR and PHE in our films for which the concept of conducting surface states is ill-defined due to their polycrystallinity and disordered structure. We suggest that an anisotropic backscattering of charge carriers in the strongly spin – orbit coupled electronic states of this heavy-metal-based non-magnetic compound may lead to the measured resistivity anisotropy. The observation of AMR and PHE in polycrystalline films may be important for scalable production of Bi2Te3 based AMR and PHE sensors of magnetic fields.

Physical origins of the magnetoresistance of platinum in contact with polycrystalline antiferromagnetic NiO

Abstract We report measurements of the magnetoresistance (MR) of platinum in Pt/NiO bilayers made with well textured NiO polycrystalline films over a wide temperature range. We show that the MR arises from two sources, the spin Hall magnetoresistance (SMR) and the anisotropic magnetoresistance (AMR). The latter results from the magnetic proximity effect of Pt in atomic contact with one of the antiferromagnetic NiO sublattices. The SMR is produced by the charge current generated in the Pt layer by the spin current back reflected from the antiferromagnetic insulating (AFI) NiO film through the inverse spin Hall effect (ISHE). As previously observed in Pt with NiO in the form of bulk crystals or epitaxial films, here the SMR exhibits oscillations in the field angle variation shifted by 90° relative to that observed in the insulating ferrimagnet yttrium iron garnet (YIG). Measurements in the temperature range 10–300 K show that the SMR increases with temperature while the AMR remains constant. The total magnetoresistance of Pt/NiO measured at room temperature with well textured NiO polycrystalline films is substantially larger than the values reported for crystalline or epitaxial NiO.

Production and potential applications of needle-like diamonds

Sharp apexes and narrow cross-sections of the needle-like crystallites provide unique combination of properties. Mass production of the needle-like diamond crystallites is realized by thermal oxidation of textured polycrystalline films obtained by chemical vapor deposition. Individual crystallites have shape of rectangular pyramids with length determined mostly by duration of the film deposition process and apex angle determined by activation of the gas mixture and substrate temperature. Proposed empirical models and computer simulations reveal mechanism of the diamond needles formation. Developed methodologies allow manipulation of individual needles and assembling them with massive holders suitable for further application

Ultrathin films of polycrystalline MnGa alloy with perpendicular magnetic anisotropy

Room temperature growth of textured polycrystalline films of MnGa alloys using a CoGa buffer layer on a thermally oxidized Si substrate is demonstrated. MnGa thin films with a thickness of 2 nm exhibit out-of-plane rectangular hysteresis loops. A small saturation magnetization of about 200 emu/cm3 and a large perpendicular magnetic anisotropy of up to 3–5 Merg/cm3 were achieved for 2- and 3-nm-thick MnGa ultrathin films; such values have never been reported before, and they provide a pathway for integration with conventional Si technology.

High-Hall-Mobility Al-Doped ZnO Films Having Textured Polycrystalline Structure with a Well-Defined (0001) Orientation.

Five hundred-nanometer-thick ZnO-based textured polycrystalline films consisting of 490-nm-thick Al-doped ZnO (AZO) films deposited on 10-nm-thick Ga-doped ZnO (GZO) films exhibited a high Hall mobility (μH) of 50.1 cm2/Vs with a carrier concentration (N) of 2.55 × 1020 cm−3. Firstly, the GZO films were prepared on glass substrates by ion plating with dc arc discharge, and the AZO films were then deposited on the GZO films by direct current magnetron sputtering (DC-MS). The GZO interface layers with a preferential c-axis orientation play a critical role in producing AZO films with texture development of a well-defined (0001) orientation, whereas 500-nm-thick AZO films deposited by only DC-MS showed a mixture of the c-plane and the other plane orientation, to exhibit a μH of 38.7 cm2/Vs with an N of 2.22 × 1020 cm−3.

Magneto-optical studies of BaFe_12O_19 films grown by metallo-organic decomposition

M-type barium hexagonal ferrites BaFe12O19 (BaM) films considered for new devices that operate in the 40-70 GHz range with small or zero applied magnetic fields were characterized by magneto-optical (MO) complex polar “Kerr” effect (PKE) spectroscopy, MO magnetometry, and spectral ellipsometry (SE). The textured polycrystalline films were grown on Pt(111)/TiO2 template on Si wafer using metallo-organic decomposition technique (MOD) followed by rapid thermal annealing. In the films grown in one, two and three MOD iterations, the thickness was evaluated by SE and transmission electron microscopy. The film thickness ranged from 30 nm to 50 nm per MOD iteration. The best films display out-of-plane effective magnetic anisotropy field of 13 kOe, high perpendicular remanent magnetization and ferromagnetic resonance linewidth of 340 Oe at 60 GHz. The coercivity deduced from the MO hysteresis loops ranged between 0.25 kOe and 0.52 kOe. The SE and PKE spectra were taken at photon energies from 0.7 eV to 6.4 eV and from 1.2 eV to 4.8 eV, respectively. The PKE spectra display the structure observed on BaM single crystal natural faces normal to the c-axis. They are consistent with magnetoplumbite structure, with high degree of grain c-axis ordering, absence of foreign phases and Fe valence-exchange mechanism. Single phase nature of the films was further confirmed by grazing incidence X-ray diffraction and 57Fe nuclear magnetic resonance at 4.2 K.

Giant Self-Polarization in FeRAM Element Based on Sol-Gel PZT Films

ABSTRACTIntegrated ferroelectric capacitors Pt/PZT/Pt/Ti/SiO2/Si with sol-gel deposited PZT films are studied. The (111) textured polycrystalline films are shown to have nonconductive PZT grain boundaries. The short-circuited photocurrents measured under illumination of the films by light with the quantum energy of 2.7 eV indicate the polarization inside the film directed from the top to the bottom electrode. Using the modified method of depolarization hysteresis loops, we found a non-switchable part of polarization which was measured to be -16 μC/cm2 and directed from the top to the bottom electrode. We consider this result to be a giant self-polarization and explain it in terms of flexoelectricity caused by lattice mismatch between the PZT and bottom Pt layers. The strain gradient across the PZT film thickness is estimated from the in-plane lattice constants measured in Pt and PZT films to be ∼103cm-1, which can produce the downward flexoelectric polarization of ∼14 μC/cm2, coinciding well with the measured one. Nonsymmetrical depolarization loops are found in the films when the polarization switching itself becomes more difficult under the negative or positive driving voltage. We show experimentally how depolarization with compensating bias or film illumination can affect the film polarization switching.

Optical and magneto-optical properties of Co-doped CeO2−δ films in the 0.5 to 4 eV range

Magnetically doped Ce1−xCoxO2−δ (nominal x = 0.05 and 0.10) films were systematically studied by spectroscopic ellipsometry and magneto-optical spectroscopy. The samples were prepared by pulsed laser deposition on MgO(100) substrates and grew as textured polycrystalline films with thickness between 200 and 750 nm. They exhibited room temperature ferromagnetism and an out-of-plane easy axis attributed to magnetoelastic effects from the in-plane compressive strain. The dispersion of dielectric function of Ce1−xCoxO2−δ films was parametrized by the sum of Tauc-Lorentz and damped Lorentz oscillators and adjusted numerically. Deduced optical band gaps were similar to those of pure CeO2, but the Co doping increased the optical absorption. The magneto-optical spectroscopy was carried out in both Faraday and Kerr configurations in the photon energy range from 0.5 to 4 eV, showing a strong dependence of the magneto-optical effect on the Co content near the optical band edge.

Single-crystal diamond microneedles shaped at growth stage

Abstract Single-crystal diamond microneedles were extracted from (001) textured polycrystalline films. The films were produced using a plasma enhanced chemical vapor deposition (CVD) from a CH 4 /H 2 gas mixture activated by a direct current discharge. The as-grown textured polycrystalline CVD films consist of pyramid-shaped micrometer size diamond crystallites embedded into a nanodiamond ballas-like material. The less ordered fraction of the CVD film material was removed selectively using thermal oxidation. A dependence of the diamond needle shape on the CVD and the oxidation process parameters was revealed via a computer simulation and experimental studies. Ability for mass production of the diamond microneedles of different shapes was demonstrated. The needles are suitable for various applications from microcutting tools to quantum information processing.

Corrosion resistance of TiN films in 3% NaCl solution

The paper examines the corrosion behavior of textured polycrystalline and amorphous TiN films in 3% NaCl solution. It is shown that TiN films are oxidized by pitting corrosion. The main cause of corrosion instability of the textured polycrystalline films is interaction with impurities (in particular, oxygen and carbon) that form “clouds” around the grain boundaries. In case of the amorphous–crystalline films, the clouds are observed around associated interfaces. The corrosion resistance of textured polycrystalline films decreases with higher thermal stresses resulting from different coefficients of thermal expansion of the film and substrate. The corrosion resistance of amorphous–crystalline TiN films is about 700 times higher than that of bulk TiN powder material and 2 to 10 times higher than that of textured polycrystalline films.

Al doped Ba hexaferrite (BaAlxFe12-xO19) thin films on Pt using metallo-organic decomposition

We grew a series of aluminum-substituted M-type barium hexaferrite (BaAlxFe12-xO19) thin films on a Pt (111) template and Si wafer using metallo-organic decomposition technique. We varied the composition from x = 0 to x = 2 with 0.25 step increments. X-ray diffraction patterns confirm highly textured c-axis polycrystalline films while atomic force microscope measurements allow us to estimate the lateral grain sizes which range from 0.2-1 micron depending on Al content. The microwave properties of these films were studied using a broadband ferromagnetic resonance spectrometer from 35 to 70 GHz. The measured out of plane effective anisotropy field increases in a nearly linear fashion with increasing Al concentration, between 12.8 kOe for x = 0 and 25 kOe for x = 2. The measured ferromagnetic resonance linewidths were relatively low, on the order of 150–300 Oe for compositions below x = 1, increasing significantly up to 800 Oe for x = 2. The easy axis magnetic hysteresis loops exhibit high squareness.

Structure, magnetic and magnetostrictive properties of as-deposited Fe–Ga thin films

Polycrystalline Fe 100− x Ga x (19⩽ x⩽23) films were grown on Si(1 0 0) substrates at different partial pressures of sputtering gas ranging from 3 to 7 μbar. Microstructural, magnetic and magnetostrictive properties were studied using X-ray diffraction (XRD), atomic force microscopy (AFM), energy dispersive spectroscopy (EDS) and magneto-optic Kerr effect (MOKE) magnetometry respectively. X-ray diffraction showed that all films have the body-centered cubic (bcc) Fe–Ga phase with the 〈1 1 0〉 direction out of the film plane. Magnetic characterization of the films showed that the films prepared at 3 μbar had weak uniaxial anisotropy whereas films grown at Ar pressures in the range 4–7 μbar were magnetically isotropic. The effective saturation magnetostriction constants ( λ eff ) of the films were measured using the Villari effect. It was found that effective saturation magnetostriction constants were almost constant over the Ga composition range achieved by varying the sputtering pressure. The measured effective magnetostriction constants fit closely to the calculated saturation magnetostriction constants of 〈1 1 0〉 textured polycrystalline films with the 〈1 1 0〉 directions slightly canted with respect to the normal to the sample surface. It was found that a high pressure of the sputtering gas effected the magnetic softness of the films. The saturation field increased and remanence ratio decreased with increase in pressure.

Simulating X-ray diffraction of textured films

Computationally efficient simulations of grazing-incidence X-ray diffraction (GIXD) are discussed, with particular attention given to textured thin polycrystalline films on supporting substrates. A computer program has been developed for simulating scattering from thin films exhibiting varying degrees of preferred orientation. One emphasized common case is that of a `fibre' symmetry axis perpendicular to the sample plane, resulting from crystallites having one well defined crystal facet towards the substrate, but no preferred in-plane orientation. Peak splitting caused by additional scattering from the totally substrate-reflected beam (two-beam approximation) and refraction effects are also included in the program, together with the geometrical intensity corrections associated with GIXD measurements. To achieve `user friendliness' for scientists less familiar with diffraction, the mathematically simplest possible descriptions are sought whenever feasible. The practical use of the program is demonstrated for a selected thin-film example, perylene, which is of relevance for organic electronics.

Stresses in textured and polycrystalline cubic films by Raman spectroscopy: Application to diamond

The determination of the stress/strain level in cubic films is carried out here by polarized Raman spectroscopy. The Voigt-Reuss-Hill averaging technique has been used for calculating the phonon deformation potentials in textured cubic films, especially here in diamond. With the corresponding elastic constants known, the resolution of the secular equation has allowed us to calculate the strain/stress configuration from the shifts and splitting of textured diamond polycrystalline films. For isotropic polycrystalline films, the results obtained by averaging over the different textures compare very well to the results obtained with the complete averaging procedure of Anastassakis [J. Appl. Phys. 86, 249 (1999)]. With both averaging methods, the stress/strain level in polycrystalline films are evaluated with a fairly good accuracy, either for biaxial isotropic in-plane stresses due to thermal expansion mismatch between the film and the titanium alloy substrates or for anisotropic stresses induced by permanent deformations of titanium tensile specimens. The difference between these averaging procedures and a method using the addition of the contributions of different textures is shown for evaluating the strains/stresses in diamond films. A slightly more accurate method is obtained because of the different influences of the various textures on the Raman shifts. However, the textured polycrystalline state being averaged with a method that results in an effective hexagonal symmetry, this approach is still insufficient to take into account the influence of the in-plane orientation of each crystallite relative to strong anisotropic biaxial stresses, an influence which is shown experimentally and theoretically for one texture, along the [110] direction of growth.

Tin oxide thin films deposited by ultrasonic spray pyrolysis

This study investigated microstructure of SnO 2 thin films deposited by ultrasonic spray pyrolysis technique using 0.2 M of SnCl 4·5H 2O in absolute ethanol as a precursor. The deposition temperature (350–450 °C) and time (20–90 min) were varied. The influence of film-deposition conditions on grain size and orientation were discussed. The deposited SnO 2 films were textured polycrystalline films. The preferred orientation of SnO 2 films were quantitatively evaluated by texture coefficient (TC). The mean grain size and film thickness determined by SEM could be controlled over a range of 50–325 nm and 80–2690 nm, respectively.

Investigating the elastic properties of β-SiC films

Direct elastic properties measurements of β-SiC films have been made using the interferometric strain gage displacement (ISDG) technique and compared with data acquired by the Brillouin light scattering (BLS) technique. BLS permits to selectively determine the three independent elastic constants ( c 11 = 395 GPa; ( c 11 − c 12 )/2 = 136 GPa and c 44 = 236 GPa) of a β-SiC single crystal epitaxial film from the analysis of a number of different surface acoustic modes. The calculated Voigt average values of the elastic constants for the 〈1 1 1〉 textured polycrystalline films ( C 11 = 500 GPa, C 33 = 534 GPa, C 44 = 166 GPa, C 66 = 201 GPa, C 13 = 62 GPa) using the single crystal constants provides good agreement with experimental results on Young’s modulus measured by the ISDG technique. Nevertheless, BLS gave more accurate values of Poisson’s ratios.

Electrical characterization of aln mis and mim structures

Aluminum nitride (AlN) thin films have been deposited on p-Si[100] and Mo-Si[100] substrates. The sputter deposited Mo was polycrystalline, predominantly showing a [110] orientation. Thin AlN films were grown under different process conditions in a physical vapor deposition (PVD) system to attain highly textured polycrystalline films as well as close to amorphous films. MIS and MIM structures were fabricated and electrical properties such as the dielectric constant, leakage current, and high-frequency behavior were investigated. It is found that the dielectric constant is 10 and does not change with the crystallinity of the films. High-frequency measurements up to 10 GHz show no frequency dispersion of the capacitance. The leakage current stays relatively constant between films and is believed to be Poole-Frenkel controlled. Capacitance-voltage (C-V) measurements for MIS structures revealed the presence of charges in the interface layer between the substrate and the dielectric film. The temperature dependence of the capacitance has also been studied.

Epitaxial ferroelectric Pb(Zr, Ti)O3 thin films on Si using SrTiO3 template layers

In this letter, we report on the integration of epitaxial ferroelectric Pb(Zr, Ti)O3 (PZT) thin films on Si [100] substrates using a SrTiO3 (STO) template layer and a conducting perovskite (La0.5Sr0.5)CoO3 electrode. X-ray diffraction studies reveal both in-plane and out-of-plane alignment of the heterostructure. The epitaxial films show extremely high remnant polarization as well as piezoelectric d33 coefficients compared to textured and untextured polycrystalline films.

MgB 2 superconducting thin films on Si and Al 2O 3 substrates

Thin films of the superconductor MgB 2 were prepared by three different procedures on sapphire and silicon substrates. Boron thin films, ex situ annealed in magnesium vapour, resulted in textured polycrystalline films with crystal dimensions below about 1 μm, onset critical temperature T c on near 39 K and width of phase transition ΔT⩽1 K. Both, ex situ and in situ annealed co-deposited boron and magnesium thin films on sapphire and silicon substrates give smooth nanocrystalline films. DC properties of nanocrystalline MgB 2 films co-deposited on silicon substrate reached T c on =33 K and zero resistance T c0=27 K, the highest values received until now on Si substrates. In addition, microwave analyses prove the existence of unconnected superconducting parts of the film below 39 K. This result confirms the possibility to synthesize nanocrystalline superconducting MgB 2 thin films on silicon substrate with critical temperature near 39 K, prepared by vacuum co-deposition of boron and magnesium films.

Structural characterization of SnO 2 gas sensing films deposited by spray pyrolysis

The results of SnO 2 thin film structure characterization using X-ray diffraction, scanning electron microscopy, and Auger electron spectroscopy are presented in this report. We discuss the influence of film deposition conditions and modes of heat treatment in the range of temperatures (400–900°C) on crystallite size and predominant orientation. It was determined that SnO 2 films, deposited by spray pyrolysis, are textured polycrystalline films. The crystallite sizes in the films could be controlled over a range from 9 to 25 nm by varying the film thickness, deposition method, and post-deposition annealing temperature. The structure of these SnO 2 films was very stable in an oxygen-containing atmosphere. It was found that crystallite orientation, in addition to crystallite size, plays a major role in determining the gas sensitivity of SnO 2 films. Crystallites with crystallographic (110) and (200) planes parallel to substrate are predominant in the films. The relative amounts of crystallites with a (110) and (200) orientation depend on film thickness and deposition mode.