Structure Of Oxide Thin Films Research Articles

Vanadium oxide thin films with high midwave & longwave infrared thermo-optic coefficients and high temperature coefficients of resistance

The development of thermo-optic modulator elements for midwave infrared (MWIR) and longwave infrared (LWIR) electro-optic systems, and the development of resistive thermometer elements for infrared imaging microbolometers require materials with high thermo-optic coefficients (TOC) and high temperature coefficients of resistance (TCR), respectively. In this paper, we synthesize novel vanadium oxide (VxOy) thin film structures and measure their MWIR/LWIR thermo-optic coefficients (TOCs) and their temperature coefficients of resistance (TCRs). The VxOy thin film are synthesized by sputter depositing interchanging, 5 nm-thick, layers of vanadium sesquioxide (V2O3) and vanadium (V) reaching a final thin film thickness of 95 nm. The sputter deposited multilayer structures are then ex-situ annealed in N2 and O2 atmospheres at 300 °C for 30 min. Infrared spectroscopic ellipsometry was used to measure the optical constants of the thin films as a function of temperature across the MWIR and LWIR bands (3000–14000 nm). The synthesized VxOy thin films exhibited high TOCs and high TCRs when operated in their semiconducting phase. A high TOC was measured reaching a maximum of 0.0278 °C−1 and 0.119 °C−1 at λ = 4000 nm for N2 and O2 annealed VxOy thin films respectively; and reaching a maximum of 0.0634 °C−1 and 0.139 °C−1 at λ = 10,000 nm for N2 and O2 annealed VxOy thin films respectively. Moreover, sheet resistance versus temperature measurements were conducted revealing room temperature sheet resistances of 1.495 k Ω/sq. and 1.516 k Ω/sq. and TCRs of −3.54%/°C and −3.46%/°C for N2 and O2 annealed VxOy thin films respectively.

Low-temperature growth and electronic structures of ambipolar Yb-doped zinc tin oxide transparent thin films

The compositional dependence of the crystal structure, optical transmittance, and surface electric properties of the zinc tin oxide (Zn-Sn-O, shortened ZTO) thin films were investigated. ZTO thin films with different compositional ratios were fabricated on glass and p-silicon wafers using radio frequency magnetron sputtering. The binding energy of amorphous ZTO thin films was examined by a X-ray photoelectron spectroscopy. The optical transmittance over 70% in the visible region for all the ZTO films was observed. The optical band gap of the ZTO films was changed as a result of the competition between the Burstein-Moss effect and renormalization. An electron concentration in the films and surface work function distribution were measured by a Hall measurement and Kelvin probe force microscopy, respectively. The mobility of the n- and p-type ZTO thin films have more than 130 cm2/V s and 15 cm2/V s, respectively. We finally constructed the band structure which contains band gap, work function, and band edges such as valence band maximum and conduction band minimum of ZTO thin films. The present study results suggest that the ZTO thin film is competitive compared with the indium tin oxide, which is a representative material of the transparent conducting oxides, regarding optoelectronic devices applications.

EFFECT OF SUBSTRATE TEMPERATURE ON THE GROWTH OF COPPER OXIDE THIN FILMS DEPOSITED BY PULSED LASER DEPOSITION TECHNIQUE

The effect of substrate temperature on growth of pulsed laser deposited copper oxide thin films has been investigated by employing Nd: YAG laser (532[Formula: see text]nm, 6[Formula: see text]ns, 10[Formula: see text]Hz) irradiation at a fluence of 8.2[Formula: see text]J/cm2. XRD analysis reveals that copper oxide films deposited at room temperature are amorphous in nature, whereas films deposited at higher substrate temperatures are polycrystalline in nature. SEM and AFM analyses revealed that films deposited at substrate temperatures, ranging from room temperature to 300[Formula: see text]C are comprised of large sized clusters, islands and particulates, whereas uniform films with an appearance of granular morphology and distinct bump formation are grown at higher substrate temperatures of 400[Formula: see text]C and 500[Formula: see text]C. The optical bandgap of deposited films is evaluated by UV-VIS spectroscopy and shows a decreasing trend with increasing substrate temperature. Four point probe analysis reveals that electrical conductivity of the deposited films increases with increase in the substrate temperature, and is maximum for highest growth temperature of 500[Formula: see text]C. It is revealed that growth temperature plays a significant role for structure, texture, optical and electrical behavior of copper oxide thin films. The surface and structural properties of the deposited films are well correlated with their electrical and optical response.

Effects of the annealing process on the structure and valence state of vanadium oxide thin films

V2O5 was used to prepare vanadium oxide films via the sol-gel method. The “n” in V2O5∙nH2O was determined to be approximately 1.67. After annealing at 390 °C for 4 h, a pure VO2 (M) phase was obtained and denoted sample Ⅴ. The crystallite sizes of VO2 (M) in samples Ⅱ, Ⅲ, Ⅳ and Ⅴ were 42, 41, 35 and 24 nm, respectively. The surface morphology of sample Ⅰ was flat; however, many small, dense particles covered the surface of sample Ⅴ. Sample Ⅱ had a layered structure, and samples Ⅲ and Ⅳ consisted of compact grains. The binding energy (BE) differences between the O1s and the V5+2p3/2, V4+2p3/2, and V3+2p3/2 core levels were 12.75, 14.0 and 14.7 eV, respectively. For the sample Ⅴ film, the maximum transmittance change at 2500 nm was 39%, the phase transition temperature (Tc) was 64.3 °C, and the hysteresis loop width was 12.1 °C.

Structural study of NiOx thin films fabricated by radio frequency sputtering at low temperature

Structure and crystal growth of nickel oxide thin films (10–300nm) prepared by low-temperature sputtering have been investigated by scanning electron microscopy (SEM), X-ray diffraction, and spectroscopic ellipsometry. Very thin films are compact and homogeneous and are made of almost randomly oriented crystals. A preferential growth direction is then observed following the (111), (220) and (311) planes to the detriment of the (222) and (200) planes, inducing a growth of the materials in columns perpendicularly to the substrate. An optical model able to account for this particular structure has been created from the spectroscopic ellipsometry measurements, and correlates well with the structure observed by SEM. Moreover, it enables an accurate estimation of the thickness without damage to the substrate.

Effects of gas flow rate on the structure and elemental composition of tin oxide thin films deposited by RF sputtering

Photovoltaic technology is one of the key answers for a better sustainable future. An important layer in the structure of common photovoltaic cells is the transparent conductive oxide. A widely applied transparent conductive oxide is tin oxide (SnO2). The advantage of using tin oxide comes from its high stability and low cost in processing. In our study, we investigate effects of working gas flow rate and oxygen content in radio frequency (RF)-sputtering system on the growth of intrinsic SnO2 (i-SnO2) layers. X-ray diffraction results showed that amorphous-like with nano-crystallite structure, and the surface roughness varied from 1.715 to 3.936 nm. X-Ray photoelectron spectroscopy analysis showed different types of point defects, such as tin interstitials and oxygen vacancies, in deposited i-SnO2 films.

Fabrication of n-ZnO/p-Si (100) and n-ZnO:Al/p-Si (100) Heterostructures and Study of Current–Voltage, Capacitance–Voltage and Room-Temperature Photoluminescence

Heterojunction diodes of n-ZnO/p-Si (100) and n-ZnO:Al/p-Si (100) were fabricated by spray pyrolysis technique. X-ray diffraction (XRD), energy dispersive x-ray spectroscopy (EDX), and field emission scanning electron microscopy (FESEM) were used to characterize the as-prepared samples. The XRD pattern indicates the hexagonal wurzite structure of zinc oxide (ZnO) and Al-doped ZnO (AZO) thin films grown on Si (100) substrate. The compositional analysis by EDX indicates the presence of Al in the AZO structure. The FESEM image indicates the smooth and compact surface of the heterostructures. The current–voltage characteristics of the heterojunction confirm the rectifying diode behavior at different temperatures and illumination intensities. For low forward bias voltage, the ideality factors were determined to be 1.24 and 1.38 for un-doped and Al-doped heterostructures at room temperature (RT), respectively, which indicates the good diode characteristics. The capacitance–voltage response of the heterojunctions was studied for different oscillation frequencies. From the 1/C 2–V plot, the junction built-in potentials were found 0.30 V and 0.40 V for un-doped and Al-doped junctions at RT, respectively. The differences in built-in potential for different heterojunctions indicate the different interface state densities of the junctions. From the RT photoluminescence (PL) spectrum of the n-ZnO/p-Si (100) heterostructure, an intense main peak at near band edge (NBE) 378 nm (3.28 eV) and weak deep-level emissions (DLE) centered at 436 nm (2.84 eV) and 412 nm (3.00 eV) were observed. The NBE emission is attributed to the radiative recombination of the free and bound excitons and the DLE results from the radiative recombination through deep level defects.

Band structure of amorphous zinc tin oxide thin films deposited by atomic layer deposition

Recently, zinc tin oxide (ZTO) has attracted attention as an alternative buffer layer to replace CdS for photovoltaic cells. ZTO thin films were grown by atomic layer deposition from diethylzinc, tetrakis(dimethylamido)tin, and water. Compositional, structural and optical properties were characterized to construct band diagram of the ZTO films depending on Sn content. The ZTO films exhibit optical bandgaps of 2.95–3.07eV which are wider than that of CdS. Furthermore, their work function is also observed to vary in a wide range of 4.32–5.16eV. It is attributed to incorporation of Sn into ZTO which strongly influences formation of oxygen vacancies.

Studying the composition and structure of films obtained by thermal oxidation of copper

The composition and structure of copper oxide thin films obtained by thermal oxidation of copper layers with a thickness of 100 nm, deposited onto quartz glass by high-frequency magnetron sputtering, have been studied. The dependences of the film thickness and content of CuO and Cu2O oxides in it on the temperature–time conditions of thermal treatment are obtained. The studies have shown that depending on the preparation modes, the films can be multiphase and contain different copper oxides, predominantly, CuO and Cu2O, which possess different bandgaps and p-type conductivity.

Cathodic arc sputtering of functional titanium oxide thin films, demonstrating resistive switching

The formation of thin films of the different stable and metastable titanium oxide phases is demonstrated by cathode arc sputtering of a titanium target in an oxygen atmosphere. We also show that sputtering of titanium in vacuum yields the formation of titanium silicides on the silicon substrate. The crystal structure of the produced samples was investigated using Raman spectroscopy and X-ray diffraction. We conclude that cathode arc sputtering is a flexible method suitable for producing the functional films for electronic applications. The functionality is verified by the memory effect demonstration, based on the resistive switching in the titanium oxide thin film structure.

High mobility hydrogenated zinc oxide thin films

In this work we present a comprehensive study of the role of hydrogen in the structure, morphology, composition and electro-optical properties of zinc oxide thin films deposited by rf reactive magnetron sputtering at room temperature.The transparency and conductivity of the in situ hydrogenated zinc oxide films (ZnO:H) are improved with the addition of H2 to the sputtering atmosphere, where the resistivity dropped one order of magnitude (from 3×10−2 to 2.8×10−3Ωcm) for an optimal dilution of 1.5% in argon. The effect of hydrogen in the conductivity of undoped ZnO thin films has been attributed to the shallow donor behaviour. However, we observed that the increase in the conductivity came essentially from the improvement of the mobility that reached 47.1cm2/Vs for a carrier concentration of 4.4×1019cm−3. This improvement in the mobility happens with an effective hydrogen incorporation into the bulk of the sputtered ZnO:H films, being attributed to the passivation of grain boundaries trap states while carrier concentration is dominated by the formation of Zn interstitials.

The influence of post annealing on the structure and optical properties of vanadium oxide thin films

Vanadium and vanadium oxide films were deposited onto glass and silicon substrates at room temperature by DC magnetron sputtering and reactive DC magnetron sputtering, respectively. The effect of post annealing in air atmosphere were explored. The as-deposited vanadium and vanadium oxide films were annealed under air atmosphere at different temperatures (100-500°C) and annealing times (6, 12 hours). The structural properties of the films were studied by X-ray diffraction and scanning electron microscopy. The optical properties of the films were studied by UV-Vis spectrophotometer. The results showed that the annealed vanadium films have a formation of polycrystalline V2O5 structure while the optical transmittance is low due to the imperfect thermal oxidation. For the annealed vanadium oxide films, the vanadium oxide with the phase of V2O5 is also presented while the transmittance decreased when the annealing temperature is increased. This is due to the light loss by the scattering at the rough surface. A comparison of the results with the annealed vanadium and annealed vanadium oxide films will be discussed. Moreover, the effect of the post annealing temperature and the annealing time on the structural and optical properties will also be discussed.

Influence of Precursor Solution Concentration on Structure and Magnetic Properties of Zinc Oxide Thin Films

Zinc oxide thin films were deposited on glass substrate at a substrate temperature of 673K by spray pyrolysis method using different concentration of 0.0125M, 0.025M and 0.05M of Zinc acetate solutions. The effect of molar concentrations on structure, surface morphology and magnetic properties of ZnO films were investigated using x-ray diffraction, scanning electron microscopy and vibrating sample magnetometer. All deposited films were polycrystalline in nature with hexagonal wurtzite structure having a preferential growth orientation along (101) plane. An improvement of crystallinity in the deposits with increasing concentration of sprayed solution was noticed. All deposit exhibit fibrous structure which increases with increase of precursor concentration solutions. At room temperature, all deposited films were shown diamagnetic character but when cooled to 5K, they have shown paramagnetic characteristics.

Structure evolution in reactively sputtered molybdenum oxide thin films

The current work investigates structure and property relations of molybdenum oxide thin films deposited by reactive dc magnetron sputtering from a molybdenum target with varying oxygen/argon gas flow ratio during deposition. With increasing oxygen partial pressure, the film growth rate decreased from 90 to 7 nm/min, whereas the oxygen content in the films increased up to 75 at.%. The dominating phases found by X-ray diffraction and Raman spectroscopy were MoO2, different polymorphs of Mo4O11, and MoO3. The optical appearance changed from metallic to yellowish transparent with increasing oxygen content in the thin film, while the electrical properties varied from electrically conductive to insulating. In general, structure and properties of the deposited molybdenum oxide thin films are tuneable by adjusting the oxygen/argon gas flow ratio during deposition, which enables the use of these films in a wide range of optical and electronic applications.

A light-stimulated synaptic transistor with synaptic plasticity and memory functions based on InGaZnOx–Al2O3 thin film structure

In this work, a synaptic transistor based on the indium gallium zinc oxide (IGZO)–aluminum oxide (Al2O3) thin film structure, which uses ultraviolet (UV) light pulses as the pre-synaptic stimulus, has been demonstrated. The synaptic transistor exhibits the behavior of synaptic plasticity like the paired-pulse facilitation. In addition, it also shows the brain's memory behaviors including the transition from short-term memory to long-term memory and the Ebbinghaus forgetting curve. The synapse-like behavior and memory behaviors of the transistor are due to the trapping and detrapping processes of the holes, which are generated by the UV pulses, at the IGZO/Al2O3 interface and/or in the Al2O3 layer.

Characteristic of Bismuth-Doped Tin Oxide Thin-Film Transistors

The influences of bismuth doping contents on the structure, morphology and electrical properties of amorphous bismuth-doped tin oxide (a-TBO) thin films have been investigated. With the increase of Bi content, the resistivity of TBO films monotonously increases due to the decreased oxygen vacancies. The thin-film transistors (TFTs) with a-TBO films as channel layers have been prepared on SiO2–Si substrate. Through the comparison of the transfer characteristic curves of the TFTs with different Bi contents, it is observed that Bi doping is useful to suppress the excess carrier concentration and improve the performances. TBO-TFTs with Bi content 2.7 at.% show the optimum performances, with a field effect mobility of 4.7 cm $^{2}\text{V}^{-1}\text{s}^{-1}$ , a threshold voltage of −3.9 V, a subthreshold swing value of 1.3 V/decade, and an ON–OFF current ratio of $2.3 \,\, \times \,\, 10^{6}$ . In addition, the bias stress stabilities of intrinsic SnO2 TFTs and a-TBO TFTs were compared and analyzed.

Effect of oxygen deficiency on electronic properties and local structure of amorphous tantalum oxide thin films

The dependence of electronic properties and local structure of tantalum oxide thin film on oxygen deficiency have been investigated by means of X-ray photoelectron spectroscopy (XPS), Reflection Electron Energy Loss Spectroscopy (REELS), and X-ray absorption spectroscopy (XAS). The XPS results showed that the oxygen flow rate change results in the appearance of features in the Ta 4f at the binding energies of 23.2eV, 24.4eV, 25.8, and 27.3eV whose peaks are attributed to Ta1+, Ta2+, Ta3+/Ta4+, and Ta5+, respectively. The presence of nonstoichiometric state from tantalum oxide (TaOx) thin films could be generated by the oxygen vacancies. In addition, XAS spectra manifested both the increase of coordination number of the first Ta-O shell and a considerable reduction of the Ta-O bond distance with the decrease of oxygen deficiency.

Electrical characterization of ZnO/4H‐SiC n–p heterojunction diode

The structure and electrical properties of zinc oxide (ZnO) thin films fabricated by atomic layer deposition (ALD) as well as electrical properties of ZnO/4H‐SiC n–p heterojunction diodes were investigated. Hall measurements of ZnO films show their n‐type high intrinsic conductivity. Structural characterization of the ZnO layers performed using X‐ray diffraction show their polycrystalline morphology. Aluminum (Al) ohmic contacts fabricated to n‐ZnO demonstrate linear characteristics and low resistivities. The I–V measurements of ZnO/4H‐SiC n–p heterojunction showed strong diode‐like behavior with the low leakage current, turn‐on voltage of around 1.7 V, ideality factor of 1.17, and high rectification ratio. Extracting the built‐in potential (1.71 V) from the C–V measurements allowed to determine band offsets and thus the flat‐band energy diagram of produced heterostructure.

Influence of Post-Deposition Annealing on Structure and Optical Characteristics of Niobium Oxide Thin Films

In this work, niobium oxide thin films were deposited on silicon wafer (100) and glass slide substrate by reactive magnetron sputtering. The niobium oxide films were further annealed and effect of post annealing on the crystallinity, microstructure and optical properties was studies. In order to identify the crystalline structure and microstructure, X-ray diffraction (XRD) and scanning electron microscope (SEM) measurement carried out. The optical property was determined by UV-Vis spectrophotometer. It has been observed that with increase in annealing temperature films become microcrystalline films. In particularly, the optical property of niobium oxide thin film also improves with annealing temperature.

Correlation between Microstructure of Copper Oxide Thin Films and its Gas Sensing Performance at Room Temperature

Radio-frequency magnetron sputtering using a Cu target was used to deposit cuprous oxide and cupric oxide thin films on silicon wafer. The substrate bias voltage and the O2 flow ratio were varied during the deposition. The deposited thin films were characterized using scanning electron microscope. We found that the spherical and pyramid shapes structure of copper oxide thin films were deposited at critical O2 flow ratio between 7 and 14%. The influence of substrate bias voltage was small and negligible. The deposited thin films were used for sensing characterization using ethanol vapor. Experimental results reveal that the pyramid shape of copper oxide thin film contribute to high respond rate when exposed to ethanol vapor. The respond and recovery rates which were measured at room temperature were very fast. This work had successfully demonstrated the formation of optimized copper oxide thin films and their usage for gas sensing application.