Structural and dielectric properties of LuFe2O4 thin films grown by pulsed-laser deposition

Epitaxial LuFe₂O₄ thin films are deposited on sapphire substrate by pulsed-laser deposition. Different growth conditions are tackled and it is found that substrate temperature is the most critical condition for the film growth; while below 750 °C the film crystallization is poor. The Lu:Fe ratio is also found to be important in forming the LuFe₂O₄ phase in the films; while higher content of Fe oxide than that of stoichiometric LuFe₂O₄ in the target is favorable for the formation of the LuFe₂O₄ phase. However, impurity phases such as Fe₃O₄ and Fe₂O₃ are induced in the film with a Fe oxide enriched target. A large dielectric tunability under electric field is revealed in the film; while the dielectric tunability decreases as the frequency increases, and eventually the dielectric tunability disappears above 500 MHz.     

Photoluminescence behaviors in ZnGa2O4 thin film phosphors deposited by a pulsed laser ablation

ZnGa₂O₄ thin film phosphors have been deposited using a pulsed laser deposition technique on Si (1 0 0) and Al₂O₃ (0 0 0 1) substrates at a substrate temperature of 550 °C with various oxygen pressures 100, 200 and 300 mTorr, and various substrate temperatures of 450, 550 and 650 °C with a fixed oxygen pressure of 100 mTorr. The films grown under different deposition conditions have been characterized using microstructural and luminescent measurements. Under the different substrate temperatures, ZnGa₂O₄ thin films show the different crystallinity and luminescent intensity. The crystallinity and photoluminescence (PL) of the ZnGa₂O₄ films are highly dependent on the deposition conditions, in particular, oxygen pressure, substrate temperature, a kind of substrates. The luminescent spectra show a broad band extending from 350 to 600 nm peaking at 460 nm. The PL brightness data obtained from the ZnGa₂O₄ films grown under optimized conditions have indicated that the sapphire is one of the most promised substrates for the growth of high quality ZnGa₂O₄ thin film phosphor.     

Influence of Ga2O3 addition on transparent conductive oxide films of In2O3-ZnO

Influence of incorporation of Ga in amorphous In-Zn-O transparent conductive oxide films was investigated as a function of Zn/(Zn + In). For In-Zn-O films with no Ga₂O₃, the range of Zn/(Zn + In) ratio where the amorphous phase appears became narrow at a substrate temperature of 250 °C. With increasing Ga₂O₃ quantity, amorphous films were obtained even at a high substrate temperature of 250 °C in a wider range of Zn/(Zn + In) than that of In-Zn-O films with no Ga₂O₃. This means that the trend of crystallization at higher substrate temperature was disturbed with additional Ga incorporation. For the film deposited from ZnO:Ga (Ga₂O₃: 4.5-7.5 wt%) and In₂O₃ targets, we obtained a resistivity of 2.8 × 10⁻⁴ Ω cm, nearly the same value as that for an In-Zn-O film with no Ga₂O₃. The addition of more than 7.5 wt% Ga₂O₃ induced a widening of the optical band gap.     

Growth and characterization of Bi2Se3 thin films by pulsed laser deposition using alloy target

Bi₂Se₃ thin films were deposited on the (100) oriented Si substrates by pulsed laser deposition technique at different substrate temperatures (room temperature −400 °C). The effects of the substrate temperature on the structural and electrical properties of the Bi₂Se₃ films were studied. The film prepared at room temperature showed a very poor polycrystalline structure with the mainly orthorhombic phase. The crystallinity of the films was improved by heating the substrate during the deposition and the crystal phase of the film changed to the rhombohedral phase as the substrate temperature was higher than 200 °C. The stoichiometry of the films and the chemical state of Bi and Se elements in the films were studied by fitting the Se 3d and the Bi 4d5/2 peaks of the X-ray photoelectron spectra. The hexagonal structure was seen clearly for the film prepared at the substrate temperature of 400 °C. The surface roughness of the film increased as the substrate temperature was increased. The electrical resistivity of the film decreased from 1 × 10⁻³ to 3 × 10⁻⁴ Ω cm as the substrate temperature was increased from room temperature to 400 °C.     

Influence of annealing on humidity response of RF sputtered nanocrystalline MgFe2O4 thin films

Humidity response of Radio Frequency sputtered MgFe₂O₄ thin films onto alumina substrate, annealed at 400 °C, 600 °C and 800 °C has been investigated. Crystalline phase formation of thin films annealed at different temperature was analyzed by X-ray Diffraction. A particle/grain like microstructure in the grown thin films was observed by Scanning Electron Microscope and Atomic Force Microscope images. Film thickness for different samples was measured in the range 820-830 nm by stylus profiler. Log R (Ω) response measurement was taken for all thin films for 10-90% relative humidity (% RH) change at 25 °C. Resistance of the film increased from 5.9 × 10¹⁰ to 3 × 10¹² at 10% RH with increase in annealing temperature from 400 °C to 800 °C. A three-order magnitude, 10¹² Ω to 10⁹ Ω drop in resistance was observed for the change of 10 to 90% RH for 800 °C annealed thin film. A good linear humidity response, negligible humidity hysteresis and minimum response/recovery time of 4 s/6 s have been measured for 800 °C annealed thin film.     

Structural order and magnetic properties of Fe3Si/Si(100) heterostructures grown by pulsed-laser deposition

The single-phase Fe₃Si thin films with preferred (220) growth orientation were prepared on Si(100) substrates by pulsed-laser deposition. Different states of order were developed by changing the substrate temperature from room temperature to 500 °C. X-ray diffraction, Mössbauer spectroscopy and macroscopic magnetic measurements were used to analyze changes of structural order and magnetic properties with the substrate temperature. The results show that over the whole range of substrate temperatures considered all films are of Fe₃Si single phase, highly oriented along the (220) plane. With increasing the substrate temperature, the structural order type changes from A2 through B2 to DO₃ and the order degree gradually increases. Meanwhile, the saturation magnetization remarkably decreases with the increase of the substrate temperature, induced by Si segregation from the substrate and embedment into the film probably as the amorphous phase. The room temperature grown film has a high saturation magnetization of 917 kA m^− 1, which nearly equals that of bulk DO₃-Fe₃Si.     

Growth of stoichiometric TiO2 thin films on Au(100) substrates by molecular beam epitaxy

The present study reports on the growth of thin TiO₂ films onto Au(100) single crystals by Ti evaporation in a reactive O₂ atmosphere at two different substrate temperatures: room temperature (RT) and 300 °C. The growth of the oxide films was monitored by means of X-ray photoemission spectroscopy, while the valence and conduction band electronic structure was investigated by UV and inverse photoemission spectroscopy, respectively.The TiO₂ film grows epitaxially on the Au(100) substrate at 300 °C exhibiting the rutile (100) surface. The evolution of the Ti 2p lineshape with the oxide coverage shows the presence of reduced oxide species (characterized by Ti^3 + ions) at the Au(100) interface. A crystalline and stoichiometric TiO₂ oxide is produced at high substrate temperature, while growth at RT gives a measurable concentration of defects. Post growth annealing in ultra-high vacuum of the RT grown film increases this concentration, while subsequent annealing in O₂ atmosphere restores the sample to the as-grown conditions.     

Transparent magnetic oxide thin films of Fe3O4 on glass

Transparent magnetic oxide (TMO) thin films of magnetite (Fe₃O₄) were grown on top of a (Zn,Al)O transparent conducting oxide on a glass substrate. The polycrystalline magnetite thin films show the typical Raman spectrum of Fe₃O₄, a sharp Verwey transition at 120 K and hysteretic behavior. The highest achieved average transmittance in the visible range (400-800 nm) for the entire multilayer was slightly below 80%. TMOs permit the inclusion of magnetic functionalities into transparent electronics. Our results show that TMOs can be successfully used to add magnetic functionalities to transparent conducting oxides.     

Growth morphology study of cathodically electrodeposited Fe3O4 thin films at elevated temperatures

We investigated the electrodeposition of Fe₃O₄ thin films in the Fe³⁺-triethanolamine system by galvanostatic deposition at elevated temperatures. It was found that with a fixed current density, the concentration of Fe³⁺ ions had a significant impact on the electrodeposition rate, while the deposition temperature and time strongly affected the morphology of the Fe₃O₄ thin films. Fe₃O₄ thin films deposited in electrolyte with high Fe³⁺ concentration and at high temperature (>80 °C) exhibited a dense and uniform morphology and were composed of globular or polyhedral crystallites, while the Fe₃O₄ thin films deposited at low temperatures (<70 °C) were loose and flake-like. Based on the empirical observations, a hypothetical growth model for the electrodeposition of Fe₃O₄ at elevated temperatures was proposed.     

Effect of working pressure on the properties of BaTiO3-CoFe2O4 composite films deposited on STO (100) by PLD

The BaTiO₃-CoFe₂O₄ (BTO-CFO) composite films were grown on SrTiO₃ (STO) (100) substrates at 750 °C under various working pressures by pulsed laser deposition. The composite film grew into a supersaturated single phase at the working pressure of 10 mTorr, BTO and CFO (00 l) oriented hetero-epitaxial films on STO (100) at 100 mTorr, and a polycrystalline composite film at 500 mTorr. The slow growth rate at high working pressure led to the phase separation in the composite film. The CFO was compressively strained along out-of-plane due to the lattice mismatch with the BTO matrix phase. The BTO-CFO composite film grown at 100 mTorr showed reversible switching of ferroelectric polarization and magnetic hysteresis with strong magnetic anisotropy.     

Rapid growth of Fe3O4 nanowires with oxide assisted vapor-solid process

Oxide assisted vapor-solid (VS) process has been used for rapid crystal growth of Fe₃O₄ nanowires (NWs) on Fe:Ni (1:1) alloy substrate. Oxide layers have been created initially on the substrate by heating it inside the quartz tube of a single tube furnace at 600 °C in air. On rising the temperature of the chamber to 700 °C in vacuum and flowing argon (Ar) at a flux 10 sccm for 20 min NWs of lengths up to 10 μm and average width 120 nm can be grown on the oxidized substrate. Capillarity model and resident time theorem have been combined to analyze the nanowire growth process. Energy dispersive X-ray (EDX), X-ray diffraction (XRD) and vibrating sample magnetometry (VSM) studies show that the product formed are Fe₃O₄ NWs which are magnetic materials.     

Fe3Si nanodots epitaxially grown on Si(111) substrates using ultrathin SiO2 film technique

Ultrahigh density (> 10¹² cm⁻²) Fe₃Si nanodots (NDs) are epitaxially grown on Si(111) substrates by codeposition of Fe and Si on the ultrathin SiO₂ films with ultrahigh density nanovoids. We used two kinds of methods for epitaxial growth: molecular beam epitaxy (MBE) and solid phase epitaxy. For MBE, low temperature (< 300 °C) growth of the Fe₃Si NDs is needed to suppress the interdiffusion between Fe atoms deposited on the surfaces and Si atoms in the substrate. These epitaxial NDs exhibited the ferromagnetism at low temperatures, which were expected in terms of the application to the magnetic memory device materials.     

Effect of growth temperature on the structural and transport properties of magnetite thin films prepared by pulse laser deposition on single crystal Si substrate

Magnetite (Fe₃O₄) thin films are prepared by pulsed laser deposition using an α-Fe₂O₃ target on silicon (111) substrate in the substrate temperature range of 350 °C to 550 °C. X-ray diffraction (XRD) measurement shows that the film deposited at 450 °C is a single phase Fe₃O₄ film oriented along [111] direction. However, the film grown at 350 °C reveals mixed oxide phases (FeO and Fe₃O₄), while the film deposited at 550 °C is a polycrystalline Fe₃O₄. X-ray photoelectron spectroscopy study confirms the XRD findings. Raman measurements reveal identical spectra for all the films deposited at different substrate temperatures. We observe abrupt increase in the resistivity behavior of all the films around Verwey transition temperature (T_V) (125 K-120 K) though the transition is broader in the film deposited at 350 °C. We observe that the optimized temperature for the growth of Fe₃O₄ film on Si is 450 °C. The electrical transport behavior follows Shklovskii and Efros variable range hopping type conduction mechanism below T_V for the film deposited at 450 °C possibly due to the granular growth of the film.     

Epitaxial growth and exchange coupling of spinel ferrimagnet Ni0.3Zn0.7Fe2O4 on multiferroic BiFeO3

Thin films of the zinc nickel ferrite, Zn_0.7Ni_0.3Fe₂O₄ (ZNFO), were deposited by the RF magnetron sputtering on a number of substrates, including (001) oriented single crystals of LaAlO₃ (LAO) and SrTiO₃ (STO), polycrystalline Pt/Si, and epitaxial films of BiFeO₃ (BFO) and LaNiO₃ (LNO). Except for the films on Pt/Si, the ZNFO films grown on other substrates were epitaxial and their magnetic properties were affected by the heteroepitaxy induced strains. Typically, the coercivity (H_c) was increased with the strain, i.e. H_c varied from 31 Oe for the 150 nm thick polycrystalline films grown on Pt/Si, to 55 Oe and 155 Oe for the 20 nm thick epitaxial films grown on BFO and LAO, respectively. The saturation magnetization of the epitaxial films was reduced accordingly to about 470 emu/cm³ from 986 emu/cm³ in the polycrystalline films. The all-oxide architecture allowed field-annealing to perform at the temperature above the Neel temperature of BFO (~ 370 °C), after which clear exchange bias was observed.     

The preparation of Zn-ferrite epitaxial thin film from epitaxial Fe3O4:ZnO multilayers by ion beam sputtering deposition

A new method to grow a well-ordered epitaxial ZnFe₂O₄ thin film on Al₂O₃(0001) substrate is described in this work. The samples were made by annealing the ZnO/Fe₃O₄ multilayer which was grown with low energy ion beam sputtering deposition. Both the Fe₃O₄ and ZnO layers were found grown epitaxially at low temperature and an epitaxial ZnFe₂O₄ thin film was formed after annealing at 1000 °C. X-ray diffraction shows the ZnFe₂O₄ film is grown with an orientation of ZnFe₂O₄(111)//Al₂O₃(0001) and ZnFe₂O₄(1-10)//Al₂O₃(11-20). X-ray absorption spectroscopy studies show that Zn²⁺ atoms replace the tetrahedral Fe²⁺ atoms in Fe₃O₄ during the annealing. The magnetic properties measured by vibrating sample magnetometer show that the saturation magnetization of ZnFe₂O₄ grown from ZnO/Fe₃O₄ multilayer reaches the bulk value after the annealing process.     

Material characteristics of sputter-deposited Zr-doped In2O3/ZnO heterostructures on sapphire (0001) substrates

5 wt.% Zr-doped In₂O₃ (Zr-In₂O₃) films with thicknesses from 95 to 220 nm were grown on 90 nm-thick ZnO-buffered sapphire (0001) substrates by radio-frequency magnetron sputtering in an oxygen-deficient atmosphere. The dependence on thickness of the structural information and electrical properties of the Zr-In₂O₃ films on the ZnO-fuffered sapphire substrates was studied. The X-ray diffraction patterns show that the (002)-textured ZnO buffer-layer is a good template for the growth of the highly (222)-textured In₂O₃ films on the sapphire substrate. The surface of the Zr-In₂O₃ film becomes rougher as the film thickness increases, perhaps because of the formation of larger mounds on the film surface as the thickness of Zr-In₂O₃ increases. The carrier concentration increased markedly from 5.8 × 10²⁰ to 1.83 × 10²¹ cm^− 3 with film thickness from 95 to 220 nm, because more growth-induced defects are formed in the thick Zr-In₂O₃ film. The large increase in the number of charge carriers and the improvement in the crystalline quality in the film reduce the resistivity of the thicker Zr-In₂O₃ film.     

Characteristics of highly orientated BiFeO3 thin films on a LaNiO3-coated Si substrate by RF sputtering

BiFeO₃ (BFO) films were grown on LaNiO₃-coated Si substrate by a RF magnetron sputtering system at temperatures in the range of 300-700 °C. X-ray reflectivity and high-resolution diffraction measurements were employed to characterize the microstructure of these films. For a substrate temperature below 300 °C and at 700 °C only partially crystalline films and completely randomly polycrystalline films were grown, whereas highly (001)-orientated BFO film was obtained for a substrate temperature in the range of 400-600 °C. The crystalline quality of BFO thin films increase as the deposition temperature increase except for the film deposited at 700 °C. The fitted result from X-ray reflectivity curves show that the densities of the BFO films are slightly less than their bulk values. For the BFO films deposited at 300-600 °C, the higher the deposition temperature, the larger the remnant polarization and surface roughness of the films present.     

Effect of annealing on the microstructure of NiFe1.925Dy0.075O4 thin films

Dysprosium-doped nickel-ferrite (NiFe_1.925Dy_0.075O₄) thin films were fabricated using RF sputter-deposition. Structural studies indicate that the effect of post-deposition annealing is significant on structural evolution in NiFe_1.925Dy_0.075O₄ films. As-grown NiFe_1.925Dy_0.075O₄ films were amorphous. Annealing (T_a) in air at 450-1000 °C results in the formation of nanocrystalline NiFe_1.925Dy_0.075O₄ films, which crystallize in the inverse spinel structure. The average grain size (L) increases from 5 to 40 nm with increasing T_a from 450 to 1000 °C. Lattice constant of NiFe_1.925Dy_0.075O₄ films is higher compared to that of NiFe₂O₄ due to partial substitution of Dy³⁺ ions for Fe³⁺ ions. The lattice parameter increases from 8.353 to 8.362 Å with increasing T_a from 450 to 1000 °C which is attributed to the lattice-strain developed in the NiFe_1.925Dy_0.075O₄ films with increasing T_a. The corresponding density of NiFe_1.925Dy_0.075O₄ films increases from 3.2 to 3.9 g/cm³ with increasing annealing temperature. Magnetization measurements indicate the ferromagnetic behavior of all the films while the coercive field values at 300 K are found to be 0.0134 T and 0.0162 T for as grown and T_a = 1000 °C films, respectively.     

Growth, structure and carrier transport properties of Ga2O3 epitaxial film examined for transparent field-effect transistor

Growth conditions of heteroepitaxial thin films of tin-doped Ga₂O₃ were surveyed from the viewpoint of visible application to field-effect transistors (FETs). Films were deposited by pulsed laser deposition, and post-annealing was examined to improve film structures. Atomically flat surfaces were obtained for films grown on yttria-stabilized zirconia (111) plane and post-annealed at 1400 °C, but they were insulating. Conductive heteroepitaxial films applicable to FETs were obtained on α-Al₂O₃ (0001) at specific deposition conditions, i.e. substrate temperatures from 500 to 550 °C and oxygen pressures from 5 × 10^− 4 to 1 × 10^− 3 Pa. It was found that the resulting epitaxial films have a crystal structure different from that of β-Ga₂O₃. The crystal lattice for the films is determined to be orthorhombic with a large possibility of a higher-symmetry hexagonal or rhombohedral system. The films exhibited high transparency in the near infrared-deep ultraviolet region and had bandgap of ∼ 4.9 eV. The operation of top-gate MISFETs using the Ga₂O₃ film for the n channel was demonstrated.     

Temperature effects on the growth and electrical properties of Er2O3 films on Ge substrates

Er₂O₃ films were grown on Ge (001) substrates at different temperatures by molecular beam epitaxy using metallic Er and molecular oxygen sources with otherwise identical conditions. High-resolution transmission electron microscopy and X-ray photoelectron spectroscopy were used to characterize the microstructures and compositions of the films. The film deposited at room temperature is found to be composed of an Er₂O₃ layer and an ErGe_xO_y interface layer with a thickness of 5.5 nm; the film grown at 300 °C has a mixed structure of Er₂O₃ and ErGe_xO_y and the thickness was found to be reduced to 2.2 nm; the film grown at 450 °C becomes much rougher with voids formed underneath the film, having a mixed structure of three compounds of Er₂O₃, GeO and ErGe_xO_y. The growth mechanisms of the films at different temperatures are suggested. Current images obtained by tunneling atomic force microscopy show that the film grown at 450 °C has much more leaky spots than those grown at RT and 300 °C, which may arise from the formation of volatile GeO in the film.