Oriented Polycrystalline Films Research Articles

Enhanced magnetic properties and leakage current mechanism in bismuth lanthanum ferrite thin films coupled with impedance and magnetic studies

A highly crystalline and pure phase Bi0.993La0.007FeO3 (BLFO) thin films have been grown using a pulsed laser deposition technique. The structural analysis confirmed the thin film is well crystallized with a rhombohedral crystal structure with an R3c space group. The BLFO phase and composition are also confirmed by transmission electron microscopy and EDAX analysis. The surface chemistry of thin films, analysed by X-ray photon spectroscopy, reveals Fe's presence in Fe3+ and Fe2+ states. At RT, the deposited film exhibited high dielectric permittivity (∼4 × 102) and low dielectric loss (∼10−1). The isothermal magnetization on highly oriented (001)-MgO and LaAlO3 (LAO) substrates is investigated. The response is compared to the randomly oriented polycrystalline films on low-cost substrates (Pt/TiO2/SiO2/Si and n-type Si substrate). The enhanced saturation magnetization of 57 emu/cm3 and 36 emu/cm3 at 10 K and 300 K are observed at 2.5 kOe applied field for the film grown on the LAO substrate. Additionally, the low leakage current density of the film is obtained around ∼1 × 10−7 A/cm2 at a field of 100 kV/cm. Acquiring single-phase BLFO thin films with enhanced electrical and magnetic properties can be relevant in designing multiferroic materials for multiple-state memories, magnetic field sensors, and spintronics applications.

Co-sputtering deposition of high-photoresponsivity and high-mobility polycrystalline BaSi2 films on Si substrates

• Stoichiometric BaSi 2 films were obtained by co-sputtering from BaSi 2 and Ba targets. • Power applied on the targets controlled the Ba and Si species reaching the substrate. • Deposition without Ba target resulted in Ba-deficient films. • Large photoresponsivity was observed in spite of polycrystalline films. Barium disilicide (BaSi 2 ) has shown great promise for solar cell applications. Recent studies have revealed that defects such as Si vacancies in BaSi 2 films caused by the Ba/Si atomic ratios being away from stoichiometry degrade the electrical and photoresponse properties of BaSi 2 films. The formation of BaSi 2 films by sputtering has been performed using basically one BaSi 2 target, which makes it difficult to control the Ba/Si atomic ratio of the grown films. In this work, we investigate the formation of BaSi 2 films on Si(111) substrates by radio-frequency (RF) sputtering of BaSi 2 and Ba targets, and independently control the RF powers set on the targets. It is found that the contribution of sputtering the Ba target to the BaSi 2 growth rate ( GR Ba / GR BaSi2 ) can be used to find the growth conditions for high-photoresponsivity BaSi 2 films. The highest photoresponsivity reached 1.4 A W −1 at room temperature for BaSi 2 films grown at 600°C when a bias voltage of 0.5 V was applied between the front indium tin oxide electrodes and rear Al electrodes. The electron mobility depended on sputtering conditions, but mostly exceeded 1000 cm 2 V −1 s −1 in spite of randomly oriented polycrystalline films. These values are comparable to those obtained for BaSi 2 epitaxial films on Si(111) by molecular beam epitaxy.

Electron transport properties in degenerate magnesium tin oxynitride (Mg1−xSn1+xN2−2yO2y) with average wurtzite structure

MgSnN2 with an average wurtzite structure (wz-MgSnN2) has recently emerged as a pseudo-III-nitride semiconductor, studied for applications in tandem solar cells, green light-emitting diodes, and other optoelectronic devices. This compound has only been researched recently, and, therefore, its charge-carrier transport properties are poorly understood. Understanding these properties is essential for optoelectronic applications. In this study, we grew wz-Mg1−xSn1+xN2 biaxially oriented polycrystalline films with x = −0.08 to 0.29 by reactive sputtering and investigated the charge-carrier transport properties using both direct current and optical techniques. We regarded the wz-Mg1−xSn1+xN2 films as magnesium tin oxynitride films (wz-MTNO) because a certain amount of oxygen was unintentionally incorporated into the sputtered wz-Mg1−xSn1+xN2 films. The wz-MTNO layers were n-type degenerate semiconductors with an electron density (ne) of the order of 1020 cm−3. In films with ne > 8 × 1020 cm−3, optically extracted resistivities (ρopt) obtained via a Drude-fit analysis of the infrared transmittance and reflectance spectra were almost identical to the direct-current resistivities (ρdc), indicating that the contribution of grain boundary scattering to the electron transport was negligible. However, the contribution of grain boundary scattering became unignorable with decreasing ne. The Drude-fit analysis also allowed the determination of the conduction-band effective mass (mc*) for the first time. A band edge mass of mc*/m0 ≈ 0.2 (m0 denotes the free-electron mass) was obtained in the wz-MTNO layers with |x| < 0.1. As x was increased from −0.18 to 0.29, mc*/m0 substantially increased from 0.18 to 0.56, indicating that the conduction-band dispersion decreased. That is, the conduction-band dispersion may be affected by the cation composition x. The findings of this study will provide important information to establish this material as a practical nitride semiconductor.

Structure-specific photoluminescence of ZnGa2O4:Eu3+ thin films deposited on sapphire C-plane substrates

1 at. % Eu3+-doped ZnGa2O4 (ZnGa2O4:Eu) thin films were deposited on sapphire C-plane substrates by dual-target sputtering, and the correlation between their crystal structures and luminescent properties was investigated. When deposition was carried out with H2O vapor as the oxygen source gas and subsequently postannealing was done in vacuum, the ZnGa2O4:Eu films consisted of randomly oriented polycrystals. In contrast, ZnGa2O4:Eu films deposited with O2 consisted of (111)-oriented crystallites. The crystallites were epitaxial after postannealing at above 400 °C. At intermediate annealing temperatures between 250 and 400 °C, Ga2O3 and GaO(OH) precipitated as impurity phases with (111)-oriented ZnGa2O4. Upon excitation at 325 nm, the Eu3+ emission intensities from the randomly oriented ZnGa2O4:Eu films were comparable to those of the epitaxial films. The epitaxial films exhibited only the 5D0 → 7F2 transition signal, whereas the randomly oriented polycrystalline films showed considerable emissions from the 5D0 → 7F4 transition as well as 5D0 → 7F2. Furthermore, the emission signal width of the 5D0 → 7F2 transition was narrower for the epitaxial films compared with the polycrystalline films. Such different emission characteristics can be explained from the viewpoint that the single crystal structure allows Eu3+ ions to occupy only a limited number of sites, whereas randomly oriented polycrystalline crystallites formed by incorporating H+ and OH− species have abundant grain boundaries, and defect sites enable larger numbers of structurally and chemically different emission-active sites to hold Eu3+ ions.1 at. % Eu3+-doped ZnGa2O4 (ZnGa2O4:Eu) thin films were deposited on sapphire C-plane substrates by dual-target sputtering, and the correlation between their crystal structures and luminescent properties was investigated. When deposition was carried out with H2O vapor as the oxygen source gas and subsequently postannealing was done in vacuum, the ZnGa2O4:Eu films consisted of randomly oriented polycrystals. In contrast, ZnGa2O4:Eu films deposited with O2 consisted of (111)-oriented crystallites. The crystallites were epitaxial after postannealing at above 400 °C. At intermediate annealing temperatures between 250 and 400 °C, Ga2O3 and GaO(OH) precipitated as impurity phases with (111)-oriented ZnGa2O4. Upon excitation at 325 nm, the Eu3+ emission intensities from the randomly oriented ZnGa2O4:Eu films were comparable to those of the epitaxial films. The epitaxial films exhibited only the 5D0 → 7F2 transition signal, whereas the randomly oriented polycrystalline films showed considerable emissions from the...

Microstructure and optical properties of ultra-thin NiO films grown by atomic layer deposition

High-quality ultrathin films of the p-type wide bandgap semiconductor NiO are potentially interesting candidates for a number of frontier applications. Here we have fabricated a series of NiO thin films with a precise thickness control by atomic layer deposition from Ni-bis(2,2,6,6-tetramethyl-3,5-heptanedionate) and O3 precursors. While randomly oriented polycrystalline films were obtained on glass and Si substrates, epitaxial growth was achieved on sapphire, SrTiO3 and MgO single crystals. Optical energy gaps were determined from UV–vis absorption spectra for the films grown on borosilicate glass, and found to decrease with decreasing film thickness. This observation is in contrast to the expected bandgap increase through quantum confinement, but can be explained by an increased Coulomb interaction. Furthermore, NiO films doped with Co, Cu and Mn were deposited. Electrical measurements showed that the doping has a much higher impact on the two-probe resistance than on the resistivity. A likely explanation is a change of the contact resistance by a change of hole density.

Solution processed nanogap organic diodes based on liquid crystalline materials

Co-planar nanogap organic diodes were fabricated with smectic liquid crystalline materials of the benzothienobenzothiophene (BTBT) derivative by a spin-coating technique. A high rectification ratio of the order of 106 at ±3 V was achieved when a liquid crystalline material of 2,7-didecyl benzothieno[3,2-b][1]benzothiophene (10-BTBT-10) was used in a device configuration of Al/10-BTBT-10/pentafluorobenzenethiol-treated Au on a glass substrate, which was 4 orders higher than that of the device based on non-liquid crystalline materials of 2,7-dibutyl benzothieno[3,2-b][1]benzothiophene (4-BTBT-4) and BTBT. Similar results were also observed when another liquid crystalline material of ω, ω′-dioctylterthiophene (8-TTP-8) and a non-liquid crystalline material of terthiophene (TTP) were used. These improved rectifications can be ascribed to the self-assembly properties and controllable molecular orientation of liquid crystalline materials, which made uniform perpendicular oriented polycrystalline films favorable for superior charge transport in nano-channels.

Nanolayer-transfer method of TiO2 slot layers and its application for fabricating hybrid electro-optic polymer/TiO2 vertical slot waveguide modulators

We have presented a alternative soft process named nanolayer transfer (NLT), to fabricate hybrid electro-optic polymer/TiO2 vertical slot waveguide modulators. We have transfered successfully a 100nm thick TiO2 line pattern layer on sol-gel silica cladding layer, and at the same time we have created a line waveguide in sol-gel silica cladding layer with the widths up to 1μm. The sol-gel silica layer is hydrolyzed and cross-linked, creating a SiO2 network with a highly oriented polycrystalline film of TiO2 in which all crystals grow in the main preferential direction. NLT method also enables to control the high refractive index with better crystallization and lower optical propagation loss of transferred TiO2 slot layer, which allows us to have a higher optical mode confinement of EO polymer ensuring the feasibility of the EO device modulators.

Single crystalline epitaxial platinum film on Al2O3(0001) prepared by oxygen-doped sputtering deposition

We have deposited platinum by inductively coupled plasma-assisted radio frequency sputtering on an Al2O3(0001) substrate annealed at approximately 700 °C with and without oxygen blend. The surface morphology and crystallinity of the obtained Pt films with a thickness of approximately 200 nm were subjected to atomic force microscope (AFM) and X-ray diffraction (XRD) investigations. Without oxygen being introduced during the sputtering process, a (111)-oriented polycrystalline film was obtained. The introduction of oxygen led to the generation of twin single crystalline epitaxial Pt(111) films with a typical fcc(111) surface morphology of hexagonal symmetry.

Physical properties of a non-transparent cadmium oxide thick film deposited at low fluence by pulsed laser deposition

A stable non-transparent CdO film was grown by pulsed laser deposition. The sample was thermally annealed at 500°C in air. A (200) highly oriented polycrystalline film was obtained. The annealed sample has not preferred orientation. Scanning electron micrographs show a grain size reduction for the annealed sample. By Raman spectroscopy, the defects related second order vibrational modes of CdO were observed. Chemical composition analysis shows the presence of CdO together with a substoichiometric CdOx phase for the as-grown sample. For the annealed sample a compensation of oxygen vacancies was observed. Electrical resistivity measurements give a value of 8.602×10−4 (Ωcm) for the as-grown film. For the annealed sample the electrical resistivity increased to a value of 9.996×10−3 (Ωcm). Zero transmission has never been reported for CdO films. The photoluminescence spectra were measured in order to shed some light on the origin of the zero transmission.

Characterization of as-deposited and sintered yttrium iron garnet thick films formed by aerosol deposition

We have employed the aerosol deposition method (ADM) to direct-write 39-µm-thick polycrystalline films of yttrium iron garnet at room temperature onto sapphire at a rate of 1–3 µm/min as a first step toward integration into microwave magnetic circuits. The resulting randomly oriented polycrystalline films are composed of fractured compact nanosized crystallites. Upon postdeposition sintering at 1280 °C the density and magnetic properties become closer to those of the bulk. These results suggest that the ADM shows promise for depositing very thick films at high deposition rates to produce structurally and magnetically uniform films that retain the structural properties of the starting powder.

Growth characteristics and properties of ZnO:Ga thin films prepared by pulsed DC magnetron sputtering

Transparent conductive ZnO:Ga thin films were deposited on Corning 1737 glass substrate by pulsed direct current (DC) magnetron sputtering. The effects of process parameters, namely pulse frequency and film thickness on the structural and optoelectronic properties of ZnO:Ga thin films are evaluated. It shows that highly c-axis (0 0 2) oriented polycrystalline films with good visible transparency and electrical conductivity were prepared at a pulsed frequency of 10 kHz. Increasing the film thickness also enlarged the grain size and carrier mobility which will subsequently lead to the decrease in resistivity. In summary, ZnO:Ga thin film with the lowest electrical resistivity of 2.01 × 10 −4 Ω cm was obtained at a pulse frequency of 10 kHz with 500 nm in thickness. The surface RMS (root mean square) roughness of the film is 2.9 nm with visible transmittance around 86% and optical band gap of 3.83 eV.

Thermal conductivity of Ni, Co, and Fe-doped La 5Ca 9Cu 24O 41 thin films measured by the 3ω method

The effect of Ni-, Co- and Fe-substitution for Cu on the thermal conductivity of La 5Ca 9Cu 24O 41 thin films is investigated. Highly b-axis oriented polycrystalline films were grown onto MgO (100) substrates using the pulsed laser deposition technique. Thermal conductivity measurements were made between 90 K and 300 K using the dynamic 3ω method. The room-temperature thermal conductivity of 1% Ni:La 5Ca 9Cu 24O 41 and 1% Co:La 5Ca 9Cu 24O 41 is approximately 15% smaller than the thermal conductivity of pristine La 5Ca 9Cu 24O 41 thin films of similar thickness. On the other hand, the thermal conductivity of 1% Fe:La 5Ca 9Cu 24O 41 is approximately the same as the thermal conductivity of pristine La 5Ca 9Cu 24O 41 thin films of similar thickness. These results enable us to indirectly conclude that Ni 2+ and Co 2+ ions substitute Cu 2+ ions in the crystal structure while Fe ions are not incorporated in the lattice.

Microstructure and electrical conductivity of YSZ thin films prepared by pulsed laser deposition

Yttria-stabilized zirconia (YSZ) is the most common solid electrolyte material used e.g. in ceramic fuel cells. Thin films of YSZ were deposited on c-cut sapphire single crystals by pulsed laser deposition using a KrF excimer laser focused on a polycrystalline 8 mol% Y2O3-stabilized ZrO2 target. Depending on the substrate temperature and the oxygen background pressure during deposition, different microstructures are obtained. XRD and high-resolution SEM revealed the formation of dense amorphous films at room temperature. At 600°C preferentially (111) oriented polycrystalline films consisting of densely agglomerated nm-sized grains of the cubic phase resulted. Grain size and surface roughness could be controlled by varying the oxygen background pressure. RBS and PIXE evidenced congruent transfer only for a low number of pulses, indicating a dynamical change of the target stoichiometry during laser irradiation. The in-plane ionic conductivity of the as-deposited crystalline films was comparable to bulk YSZ. The conductivity of initially amorphous YSZ passes a maximum during the crystallization process. However, the relative changes remain small, i.e. no significant enhancement of ionic conductivity related to the formation of a nanocrystalline microstructure is found.

Spectroscopic ellipsometry studies of GaN films deposited by reactive rf sputtering of GaAs target

GaN films have been deposited by reactive rf sputtering of GaAs target in 100% nitrogen ambient on quartz substrates at different substrate temperatures ranging from room temperature to 700°C. A series of films, from arsenic-rich amorphous to nearly arsenic-free polycrystalline hexagonal GaN, has been obtained. The films have been characterized by phase modulated spectroscopic ellipsometry to obtain the optical parameters, viz., fundamental band gap, refractive index, and extinction coefficient, and to understand their dependence on composition and microstructure. A generalized optical dispersion model has been used to carry out the ellipsometric analysis for amorphous and polycrystalline GaN films and the variation of the optical parameters of the films has been studied as a function of substrate temperature. The refractive index values of polycrystalline films with preferred orientation of crystallites are slightly higher (2.2) compared to those for amorphous and randomly oriented films. The dominantly amorphous GaN film shows a band gap of 3.47eV, which decreases to 3.37eV for the strongly c-axis oriented polycrystalline film due to the reduction in amorphous phase content with increase in substrate temperature.

Growth of copper metal by atomic layer deposition using copper(I) chloride, water and hydrogen as precursors

Films of copper metal have been grown on two different oxide substrate materials by atomic layer deposition. The film depositions were carried out in the temperature range 375–475 °C. Copper(I) chloride, water and hydrogen were used as precursors. Two deposition schemes were used and compared, one including water (CuCl/H 2 O/H 2 ) and another without water (CuCl/H 2 ). It was found that the addition of water enhanced growth rate on alumina substrates up to four times compared to the water-free deposition process. Hence, the extra water step leads to faster copper formation kinetics than from the direct reaction between CuCl and H 2 . On the contrary, film growth rate increased when water was excluded from the deposition process on fused silica substrates, indicating that the additional water did not lower the activation energy for the total reaction. The film growth rate was also more sensitive to changes in hydrogen pulse length and partial pressure on SiO 2 substrates (i.e. hydrogen dosing). Randomly oriented polycrystalline films were obtained on amorphous SiO 2 , whereas on single crystalline α-Al 2 O 3 substrates with orientations (001), (110) and (102), films showed a strong Cu(111) texture. Finally, the film morphology was rough due to island growth behaviour.

Layer arrangement in the structure of octakis-(trimethylsiloxy)octasilsesquioxane and dodecakis-(trimethylsiloxy)cyclohexasiloxane

Layers of [(CH3)3SiO]8(SiO1.5)8 and [(CH3)3SiO]12(SiO)6 organosilicon compounds obtained by chemical vapor deposition were investigated by X-ray diffraction (DRON-RM4, R = 192 mm, CuKα radiation) and Raman spectroscopy (Triplemate, SPEX). The layers were found to be ideally oriented polycrystalline films. The octakis-(trimethylsiloxy)octasilsesquioxane polycrystals are oriented in one crystallographic direction — [001], while the dodecakis-(trimethylsiloxy)cyclohexa-siloxane polycrystals are oriented in the \(\left[ {\bar 120} \right]\) and \(\left[ {\bar 111} \right]\) directions. Crystal structure analysis in these directions yielded the type of the planar lattice followed by the molecules and their orientation relative to the support.

Growth of copper metal by atomic layer deposition using copper(I) chloride, water and hydrogen as precursors

Films of copper metal have been grown on two different oxide substrate materials by atomic layer deposition. The film depositions were carried out in the temperature range 375–475 °C. Copper(I) chloride, water and hydrogen were used as precursors. Two deposition schemes were used and compared, one including water (CuCl/H 2O/H 2) and another without water (CuCl/H 2). It was found that the addition of water enhanced growth rate on alumina substrates up to four times compared to the water-free deposition process. Hence, the extra water step leads to faster copper formation kinetics than from the direct reaction between CuCl and H 2. On the contrary, film growth rate increased when water was excluded from the deposition process on fused silica substrates, indicating that the additional water did not lower the activation energy for the total reaction. The film growth rate was also more sensitive to changes in hydrogen pulse length and partial pressure on SiO 2 substrates (i.e. hydrogen dosing). Randomly oriented polycrystalline films were obtained on amorphous SiO 2, whereas on single crystalline α-Al 2O 3 substrates with orientations (001), (110) and (102), films showed a strong Cu(111) texture. Finally, the film morphology was rough due to island growth behaviour.

Fabrication and characterization of HfC coated Si field emitter arrays

We fabricated hafnium carbide (HfC) coated Si field emitter arrays (HfC FEAs) with an extraction-gate electrode to improve the emission characteristics of Si FEAs. Hafnium carbide thin film was deposited by inductively coupled plasma-assisted magnetron sputtering. The HfC film was characterized by x-ray photoelectron spectroscopy and x-ray diffraction measurement, and was found to be (111)-oriented polycrystalline film. The HfC FEAs exhibited superior performance. An emission of more than 10 mA could be obtained from the 16 000 tip array, which is 20 times higher than that for Si FEAs. The operational voltage for emission of 1 μA decreased from 61 to 45 V due to the HfC coating. The long-term emission characteristics were also measured. Si FEAs degraded rapidly even in an ultrahigh vacuum chamber. However, the emission degradation in the HfC FEAs was much slower. The number of active tips was counted using an electrostatic-lens projector, and the results revealed that the HfC FEAs had six times as many tips as the Si FEAs had.

Ferroelectric properties of YMnO3 epitaxial films for ferroelectric-gate field-effect transistors

Ferroelectric properties of YMnO3 epitaxial films were studied. The ferroelectric properties of epitaxially grown (0001) YMnO3 films on (111)Pt/(0001)sapphire (epi-YMO/Pt) with an excellent crystallinity were compared to (0001)-oriented poly crystalline films on (111)Pt/ZrO2/SiO2/Si. The epi-YMO/Pt had saturated polarization–electric-field (P–E) hysteresis loops, with a remanent polarization (Pr) of 1.7 μC/cm2 and a coercive field (Ec) of 80 kV/cm. The fatigue property showed no degradation up to 1010 measured cycles. These results suggested that the YMnO3 epitaxial films were suitable ferroelectric material for the ferroelectric-gate field-effect transistors. Consequently, epitaxially grown (0001)YMnO3 films on epitaxial Y2O3/Si (epi-YMO/Si) were fabricated. The epi-YMO/Si capacitor had almost equivalent crystallinity compared to epi-YMO/Pt. It was recognized that the epi-YMO/Si capacitor exhibited the ferroelectric type C–V hysteresis loop with the width of the memory window of 4.8 V, which was almost identical to the value of twice coercive voltage of the P–E hysteresis loops of the epi-YMO/Pt. A retention time exceeding 104 s was obtained in the epi-YMO/Si capacitor.

Role of Grain Boundaries in Exceptionally H 2 Sensitive Highly Oriented Laser Ablated Thin Films of SnO2

A comparative study on growth, electrical conductivity, and sensor characteristics of highly oriented (transparent) and randomly oriented thin films of grown by pulsed laser (KrF; ablation technique have been carried out. Sensors made of randomly oriented polycrystalline films (deposited at 725°C on alumina) exhibited a sensitivity of about 90% for 50 ppm of at sensor operating temperatures above 240°C with a good response (∼30 s) and retracing times (180 s). Sensors made of a axis orientated films [deposited on at 525°C] exhibited an exceptionally high sensitivity of 30 to 40% even for 1 ppm of at 310°C with a shorter response time of about 15 s. However, the retrace time was very long (about 20 min). Sensors made of predominantly (101) orientated films [grown at 525°C on sapphire (1102)] exhibited an exceptional sensitivity of 90% even for 5 ppm at 300°C also had remarkably short response times of a axis oriented films as well as the quick retracing times of polycrystalline films. Thin films, which exhibited exceptionally high sensitivity showed large changes in electrical conductivity and activation energy as function of oxygen partial pressure. Atomic force microscopy investigation reveals that the films are highly granular with average size of about 150-200 nm which is ten times larger than the critical size of 8 nm as being 8 nm for Analysis of results based on the model for carrier transport across the grain boundaries in polycrystalline semiconductors reveals that the surface barrier height of the grain boundaries is responsible for the large variation in activation energy and sensitivity. © 2001 The Electrochemical Society. All rights reserved.