Optical and electrical properties of nickel oxide thin films synthesized by sol–gel spin coating

Nickel oxide thin films were prepared by the sol–gel technique combined with spin coating onto glass substrates. The as-deposited films were pre-heated at 275 °C for 15 min and then annealed in air at different temperatures. The effects of the annealing temperature on the structural and optical properties of the films are studied. The results show that 600 °C is the optimum annealing temperature for preparation of NiO films with p-type conductivity and high optical transparency. Then, by using these optimized deposition parameters, NiO thin films of various thicknesses were deposited at the same experimental conditions and annealed under different atmospheres. Surface morphology of the films was investigated by atomic force microscopy. The surface morphology of the films varies with the annealing atmosphere. Optical transmission was studied by UV–vis spectrophotometer. The transmittance of films decreased as the thickness of films increased. The electrical resistivity, obtained by four-point probe measurements, was improved when NiO layers were annealed in N₂ atmosphere at 600 °C.     

Effect of precursor solutions on the structural and optical properties of sprayed NiO thin films

Nickel oxide thin films were deposited by a simple and low-cost spray pyrolysis technique using three different precursors: nickel nitrate, nickel chloride, and nickel acetate on corning glass substrates. X-ray diffraction show that the NiO films are polycrystalline and have a cubic crystal structure, although predominantly with a preferred 111-orientation in the growth direction and a random in-plane orientation. The deconvolution of the Ni 2p and O 1s core level X-ray photoelectron-spectra of nickel oxides produced by using different precursors indicates a shift of the binding energies. The sprayed NiO deposited from nickel nitrate has an optical transmittance in the range of 60–65% in the visible region. The optical band gap energies of the sprayed NiO films deposited from nickel nitrate, nickel chloride and nickel acetate are 3.5, 3.2 and 3.43 eV respectively. Also, the extinction coefficient and refractive index of NiO films have been calculated from transmittance and reflectance measurements. The average value of refractive index for sprayed films by nickel nitrate, nickel chloride and nickel acetate are 2.1, 1.6 and 1.85 respectively. It is revealed that the band gap and refractive index of NiO films by using nickel nitrate corresponds to the commonly reported values. We attribute the observed behavior in the optical band gap and optical constants as due to the change of the Ni/O ratio.     

Nickel oxide nanoparticles: Synthesis and spectral studies of interactions with glucose

Nickel oxide (NiO) nanoparticles were successfully synthesized by the reaction of nickel chloride with hydrazine at room temperature and thermal decomposition of the precursor nickel hydroxide (Ni(OH)₂) nanoparticles. The products were characterized by X-ray diffraction, Transmission electron microscopy, Fourier transform infrared spectroscopy, and UV–vis absorption spectroscopy. The result of thermogravimetric analysis showed that the Ni(OH)₂ nanoparticles are calcinated at ～400 °C. The interactions between NiO nanoparticles and glucose have been studied using UV–vis absorption and fluorescence spectroscopy. The zeta-potential of NiO nanoparticles was used to gain insight about the interaction mode between NiO nanoparticles and glucose.     

Single-domain nickel films for production of graphene

A technique for depositing single-domain heteroepitaxial nickel films onto sapphire substrates is presented. It is demonstrated that high-temperature annealing of these substrates in oxygen alters their near-surface layer in a way that enables the growth of single-domain heteroepitaxial (111) Ni films on a (0001) Al₂O₃ substrate. Single-domain heteroepitaxial (111) Ni films on a (0001) Al₂O₃ substrate, which can be taken as the basis for a technique for fabrication of large-area single-crystalline graphene films, were synthesized for the first time.     

Growth of AgGaTe2 Layers by a Closed-Space Sublimation Method

AgGaTe₂ layers were deposited on Si substrates by the closed-space sublimation method. Multiple samples were deposited with various source temperatures and holding times, and constant temperature differential. Variation of the source temperature was used primarily to improve the stoichiometry of the film. Deposited films were evaluated by the θ–2θ method of x-ray diffraction (XRD) and transmission electron microscopy. These results confirmed that the deposited films were stoichiometric (after optimizing the above parameters). From XRD, it was also clear that films deposited on Si (111) have strong preference for (112) orientation.     

The effect of strain on abnormal grain growth in Cu thin films

To understand a grain growth mechanism in Cu thin films that were deposited on rigid substrates by sputter deposition and subsequently annealed at various temperatures, microstructures of the Cu films with or without the rigid substrates were analyzed by x-ray diffraction (XRD), transmission electron microscopy (TEM), and electrical resistivity measurements. Significant grain growth (with bimodal grain size distribution) was observed during room-temperature storage in the Cu films deposited on the Si₃N₄ and rock salt substrates. However, in the free-standing Cu films, no grain growth was observed during room temperature storage. The present result suggested that the grain growth rates in the Cu thin films were strongly influenced by the existence of the rigid substrates, indicating stress (or strain) introduced in the films was a primary factor to induce the grain growth in the Cu films.     

Effects of oxygen content on the structural,optical, and electrical properties of NiO films fabricated by radio-frequency magnetron sputtering

Nickel oxide (NiO) films were deposited on Corning glass substrate with variable (0–100%) oxygen content by radio-frequency sputtering. Effects of different oxygen content on the structural, optical, and electrical properties of NiO films were studied. X-ray diffraction showed that the NiO film deposited on substrate with 0% oxygen content resulted in a random polycrystalline structure and small grain size. The introduction of oxygen gas leaded to a (200) preferential orientation and larger grain size. The transmittance decreases with oxygen content due to the increase of oxygen interstitials in NiO films. The 0%-O₂ deposited NiO film has a tensile strain and a small band gap. Upon introducing 33%-O₂ content, the NiO film exhibits a compressive strain, increasing the bandgap. However, the compressive strain is released and gradually turns into tensile strain, which leads to the narrowing of bandgap with the increase of oxygen content. Hall measurement shows the obtained NiO is p-type and the resistivity decreases from 4.3 × 10⁵ Ω-cm to 5.02 Ω-cm with increasing oxygen content from 0% to 100%. The carrier concentration increases from 6.3 × 10¹⁴ cm⁻³ to 4.6 × 10¹⁸ cm⁻³ and the mobility decreases from 26 cm²/V-s to 0.26 cm²/V-s for the NiO films deposited with oxygen content increasing from 50% to 100%. X-ray photoelectron spectroscopy showed that the Ni⁺³/Ni⁺² ratio is the origin of p-type NiO and the ratio increases from 1.32 to 2.63 by increasing the oxygen content from 0% to 100%, which caused more defects, oxygen interstitials and nickel vacancies.     

Photoelectrochemical (PEC) studies on Cd0.5Fe0.5Se nanocrystalline thin films deposited by a spray pyrolysis technique

Recently, nanostructured thin films have attracted the attention of researchers from several disciplines, due to their outstanding electronic and optical properties and potential applications in various optoelectronic devices. The ternary Cd_0.5Fe_0.5Se nanocrystalline thin films were deposited by a spray pyrolysis method onto glass, aluminium, copper and stainless steel substrates. The structural and morphological properties were studied by X-ray diffraction and scanning electron microscopy analysis. The XRD study revealed that, Cd_0.5Fe_0.5Se films are nanocrystalline in nature with hexagonal lattice. The optical absorption study showed that, the semiconductor Cd_0.5Fe_0.5Se thin film deposited on glass exhibits direct band gap energy of the order of 1.95 eV. The PEC study revealed that, Cd_0.5Fe_0.5Se thin films deposited on aluminium substrate exhibited maximum fill factor (FF) and efficiency (η) as compared to the films deposited on stainless steel and copper substrates.     

The effects of carrier gas and substrate temperature on ZnO films prepared by ultrasonic spray pyrolysis

ZnO films were deposited on glass substrates in the temperature range of 350–470 °C under an atmosphere of compressed air or nitrogen (N₂) by using ultrasonic spray pyrolysis technique. Structural, electrical and optical properties of the ZnO films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical two-probe and optical transmittance measurements. The ZnO films deposited in the range of 350–430 °C were polycrystalline with the wurtzite hexagonal structure having preferred orientation depending on the substrate temperature. The ZnO films deposited below 400 °C had a preferred (100) orientation while those deposited above 400 °C mostly had a preferred (002) orientation. The resistivity values of ZnO films depended on the types of carrier gas. The ZnO thin films deposited under N₂ atmosphere in the range of 370–410 °C showed dense surface morphologies and resistivity values of 0.6–1.1 Ω-cm, a few orders of magnitude lower than those deposited under compressed air. Hydrogen substition in ZnO possibly contributed to decreasing resistivity in ZnO thin films deposited under N₂ gas. The Hall measurements showed that the behavior of ZnO films deposited at 410 °C under the N₂ atmosphere was n-type with a carrier density of 8.9–9.2×10¹⁶ cm^-3 and mobility of ～70 cm²/Vs. ZnO thin films showed transmission values at 550 nm wavelength in a range of 70–80%. The values of band gaps extrapolated from the transmission results showed bandgap shrinkage in an order of milli electron volts in ZnO films deposited under N₂ compared to those deposited under compressed air. The calculation showed that the bandgap reduction was possibly a result of carrier–carrier interactions.     

The effects of doping impurities and substrate crystallin on the formation of nickel suicides on silicon at 200–280° C

Transmission electron microscopy and Auger electron spectroscopy have been applied to investigate the effects of doping impurities and substrate crystallinity on the formation of nickel suicides at 200–280° C in nickel thin films on silicon. The systems investigated included samples with as-implanted BF₂, B, F, As, and P and recrystallized (001) Si as well as P-doped low pressure chemical vapor deposited (LP-P) and B-doped plasma enhanced chemical vapor deposited (PE-B) amorphous silicon substrates. In samples annealed at 220–280° C, substantial amounts of epitaxial NiSi₂ were found to form on crystalline structure of BF₂, B and F implanted samples to various extents at different temperatures. High resolution lattice imagings of cross-sectional samples showed that the epitaxial NiSi₂/Si interfaces are coherent. No NiSi₂ was detected in all nickel thin films deposited on implantation-amorphous specimens. NiSi₂ epitaxy was found to be a sensitive function of annealing temperature. Good correlation was found between the atomic size factor and resulting stress and NiSi₂ epitaxy at low temperature. The formation of Ni₂Si and NiSi was observed to be influenced by the dopant species and crystallinity of the substrates. The vast difference in inducing the formation of nickel suicides in implantation-amorphous and recrystallized samples is likely due to variations in initial structure and/or dopant distribution. The finding that bothn-type andp-type dopants influenced the formation of Ni₂Si and NiSi suggested that they may be related to the electrical activity of the doping species in recrystallized samples. NiSi, possessing one of the lowest resistivity among all metal silicides, was found to be the only phase formed in all implantation-amorphous as well as LP-P and PE-B amorphous silicon samples annealed at 280° C. Nickel thin film appears to be an attractive candidate for the metallization of amorphous silicon devices.     

Structural and electrical investigation of BLT films deposited at different times using electron beam evaporation technique

Thin films Bi4Ti3O12 (BLT) were deposited using electron beam evaporation on silicon substrate at several times, also on AlN/Si and SiO2/Si substrates. Thin films morphology and thickness were measured via scanning electron microscopy (SEM). The crystallography was studied using X-ray diffraction (XRD) technique for films which have a (0010) preferred orientation in all substrate types. The capacitance values were contingent on frequency value in C-V measurement. The ferroelectric characterization was investigated for BLT film deposited on isolator layer (SiO2 or AlN) for Al/Bi4Ti3O12/SiO2/Si devices. Memory effect value varied from 1 V to 3 V depending on the thin films isolator on substrate.     

ZnO films deposited on GaAs substrates with a SiO2 thin buffer layer

Sputter deposition of ZnO films on GaAs substrates has been investigated. ZnO films were radio frequency (rf)-magnetron sputter deposited on GaAs substrates with or without SiO₂ thin buffer layers. Deposition parameters such as rf power, substrate-target distance, and gas composition/pressure were optimized to obtain highly c-axis oriented and highly resistive films. Deposited films were characterized by x-ray diffraction, scanning electron microscopy (SEM), capacitance, and resistivity measurements. Thermal stability of sputter-deposited ZnO films (0.5–2.0 μm thick) was tested with a post-deposition heat treatment at 430°C for 10 min, which is similar to a standard ohmic contact alloying condition for GaAs. The ZnO/SiO₂/GaAs films tolerated the heat treatment well while the ZnO/GaAs films disintegrated. The resistivity (10¹¹ Ω-cm) of the ZnO films on SiO₂-buffered GaAs substrates remained high during the heat treatment. The post-deposition anneal treatment also enhances c-axis orientation of the ZnO films dramatically and relieves intrinsic stress almost completely. These improvements are attributed to a reduction of grain boundaries and voids with the anneal treatment as supported by SEM and x-ray diffraction measurement results.     

Interfacial microstructure and intermetallics developed in the interface between in solders and Au/Ni/Ti thin films during reflow process

Intermetallic phases and microstructures formed between In solder and Au/Ni/Ti thin films during reflow were characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Two types of two-step heat treatment were performed in a rapid thermal annealing (RTA) system or in a furnace to simulate the flip-chip solder-joining process. The AuIn₂ and In₂₇Ni₁₀ intermetallic phases were observed after the two-step heat treatment at the lower temperature. Additional In-Ni intermetallic layers formed between the In₂₇Ni₁₀ and Ni layer, which was two-step heat treated at the higher temperature. This phase was identified as metastable InNi of CsCl type with a=～3.1 Å by convergent-beam electron diffraction (CBED).     

The reaction characteristics of ultra-thin Ni films on undoped and doped Si (100)

Reaction characteristics of ultra-thin Ni films (5 nm and 10 nm) on undoped and highly doped (As-doped and B-doped) Si (100) substrates are investigated in this work. The sheet resistance (Rs) measurements confirm the existence of a NiSi salicidation process window with low Rs values within a certain annealing temperature range for all the samples except the one of Ni(5 nm) on P⁺-Si(100) substrate (abnormal sample). The experimental results also show that the transition reaction to low resistivity phase NiSi is retarded on highly doped Si substrates regardless of the initial Ni film thickness. Micro-Raman and x-ray diffraction (XRD) measurement show that NiSi forms in the process window and NiSi₂ forms in a higher temperature annealing process for all normal substrates. Auger electron spectroscopy (AES) results for the abnormal sample show that the high resistivity of the formation film is due to the formation of NiSi₂.     

Characterization of ZnO and ZnO:Al films deposited by MOCVD on oriented and amorphous substrates

Zinc oxide (ZnO) and ZnO:Al-doped films were deposited by metal organic chemical vapour deposition (MOCVD) using the Zn(tta)₂·tmeda (H-tta=2-thenoyltrifluoroacetone, tmeda=N,N,N′,N′-tetramethylethylendiamine) and Al(acac)₃ (H-acac=acetylacetone) precursors on different substrates. The deposited layers were characterised by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM). Film structure is strongly dependent on the substrate nature and deposition conditions. AFM and XRD measurements show a good film texture and a preferential orientation along the c-axis. The Al concentration of ZnO:Al film has been confirmed by energy dispersive X-ray (EDX) analysis. Optical transparency of these ZnO layers has been studied in order to evaluate their applications as a transparent conducting oxide (TCO) material.     

Microstructures and electrical properties of SrRuO3 thin films on LaAIO3 substrates

Conductive SrRuO₃ thin films have been deposited using pulsed laser deposition on LaA10₃ substrates at different substrate temperatures. Structural and microstructural properties of the SrRuO₃/LaAlO₃ system have been studied using x-ray diffraction, scanning electron microscopy, and scanning tunneling microscopy. Electrical properties of SrRuO₃ thin films have been measured. It was found that the film deposited at 250°C is amorphous, showing semiconductor-like temperature dependence of electrical conductivity. The film deposited at 425°C is crystalline with very fine grain size (100∼200?), showing both metallic and semiconductor-like temperature dependence of electrical conductivity in different temperature regions. The film deposited at 775°C shows a resistivity of 280 μΩ.cm at room temperature and a residual resistivity ratio of 8.4. Optimized deposition conditions to grow SrRuO₃ thin films on LaA10₃ substrates have been found. Possible engineering applications of SrRuO₃ thin films deposited at different temperatures are discussed. Bulk and surface electronic structures of SrRuO₃ are calculated using a semi-empirical valence electron linear combination of atomic orbitals approach. The theoretical calculation results are employed to understand the electrical properties of SrRuO₃ thin films.     

Study of the interfaces in resistive switching NiO thin films deposited by both ALD and e-beam coupled with different electrodes (Si, Ni, Pt, W, TiN)

Thin NiO films, included in a metal/resistive oxide/metal (MRM) stack, are receiving great interest, as they exhibit resistive switching when subjected to an external applied field, and can thus be implemented in a resistive random access memory (ReRAM). The electrical switching characteristic is seen to depend on the NiO/metal coupling. Therefore a characterization of the interface between NiO and the electrode is vital to optimize and get insights on the switching phenomena. In this work we deposited NiO thin films by atomic layer deposition (ALD) at 300 °C and electron beam deposition (e-beam) at 40 °C on Si, Ni, Pt, W and TiN substrates and we characterized them with X-ray reflectivity (XRR), grazing incidence X-ray diffraction (GIXRD) and time of flight secondary ion mass spectrometry (ToF-SIMS). Depending on the growth process, we found an influence of the substrate on the NiO film roughness, which exhibits values in the 1.2-6.2 nm range. NiO electron density was 1.35-1.96 e⁻Å⁻³ spread around the nominal value of 1.83 e⁻ Å⁻³ for bulk cubic polycrystalline NiO. X-ray diffraction showed that NiO is polycrystalline in the cubic phase. ToF-SIMS profiles confirm NiO/Metal interface sharpness and the optimal uniformity of NiO layers. Intermixing phenomena are limited or absent and the presence of contaminants, such as C, F, and Cl is very low.     

Laser processing of TiSi2 and CoSi2 thin films on silicon (100) substrates

We have investigated the formation of TiSi₂ and CoSi₂ thin films on Si(100) substrates using laser (wave length 248 nm, pulse duration 40 ns and repetition rate 5 Hz) physical vapor deposition (LPVD). The films were deposited from solid targets of TiSi₂ and CoSi₂ in vacuum with the substrate temperature optimized at 600° C. The films were characterized using x-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and four point probe ac resistivity. The films were found to be polycrystalline with a texture. The room temperature resistivity was found to be 16 μΩ-@#@ cm and 23 μΩ-cm for TiSi₂ and CoSi₂ films, respectively. We optimized the processing parameters so as to get particulate free surface. TEM results show that the silicide/silicon interface is quite smooth and there is no perceptible interdiffusion across the interface.     

Structural and chemical characterization of as-deposited microcrystalline indium oxide films prepared by dc reactive magnetron sputtering

Microcrystalline indium oxide (InO_x) films with thickness of 120–1600 nm were prepared by dc reactive magnetron sputtering in various mixtures of oxygen in argon at room temperature. The depositions were carried out onto Corning 7059 glass and silicon substrates. The conductivity of the as-deposited films can change in a controllable and fully reversible manner by about six orders of magnitude by alternately exposing the films to ultraviolet (UV) light (hv≥3.5eV) in vacuum and reoxidizing them in ozone. The microstructure of the films was investigated using transmission electron microscopy (TEM) and electron diffraction. For this purpose, films with a thickness of about 100 nm were deposited onto NaCl substrates. The surface and depth composition of the films were examined using Auger electron spectroscopy (AES) combined with depth profiling analysis. The depth profiles showed that all the films exhibit an extremely good in-depth uniformity, all the way to the interface with the glass substrate, regardless of their thickness. Quantitative Auger and energy dispersive x-ray (EDX) analyses were employed to determine the stoichiometry of the films. An oxygen deficiency of 2–5% has been observed with respect to the stoichiometric composition. The effects of film thickness and oxygen content in the sputtering gas on the stoichiometry were examined. Both AES and EDX analyses confirmed that the stoichiometry is invariant for these parameters.     

Intermetallic reactions in vacuum-deposited nickel and gold films on (111) silicon single crystals

Pure nickel and gold thin films were vacuum-deposited on (111) silicon single crystals. When Ni/Au/Si or Au/Ni/Si samples were heated to about 550° insitu, hexagonal or deformed hexagonal shaped crystallites were formed on the silicon substrates. The composition of these crystallites was determined by using x-ray diffraction, scanning elec-tron microscopy and scanning Auger microprobe methods. The crystallites were identified as NiSi₂. The crystal-lites on the (111) silicon plane parallel to the surface appeared as regular hexagons while the inclined crystal-lites resembled trapezia. The results of Auger spectra and in-depth composition profiles for Ni, Au, and Si showed that the NiSi₂ crystallites are islands in a matrix of Au-Si eutectic. Work supported by the Materials Research Division of the National Science Foundation