Low temperature plasma assisted atomic layer deposition in nitrogen carrier gas studied by optical emission spectroscopy

A low temperature plasma assisted atomic layer deposition process from tetrakis (dimethylamino)-titanium (TDMAT) and oxygen plasma was investigated using optical emission spectroscopy in a commercial TFS-500 atomic layer deposition reactor in industrial-like conditions with different plasma powers to optimize the plasma-assisted deposition process and to develop a tool for process control. The major emitting species recognized were the nitrogen first followed by the second positive system, carbon monoxide, nitrogen monoxide (γ)-system, atomic carbon and atomic nitrogen. The process measurements were compared to background measurements to see the process induced differences. CH _x appearance in the plasma lowered the intensities of CO, NO and N₂ 1st⁺ system peaks. Also, the nitrogen atom content varied in the process measurements. By monitoring the affected species and vibrational temperature, the effect of TDMAT oxidation on the surface could be seen through the resulting changes in the plasma emission.     

Optical emission spectroscopy during fabrication of indium-tin-oxynitride films by RF-sputtering

Indium-tin-oxide (ITO) and indium-tin-oxynitride (ITON) films have been deposited on glass by rf-sputtering from an ITO target, using Ar plasma and N₂ plasma, respectively, and different rf-power. Optical emission spectroscopy (OES) was employed to identify the species present in the plasma and to correlate them with the properties of the ITO and ITON thin films. Emission lines of ionic In could only be detected in N₂ plasma, whereas in the Ar plasma additional lines corresponding to atomic In and InO, were detected. The deposition rate of thin films was correlated with the In species, rather than the nitrogen species, emission intensity in the plasma. The higher resistivity and lower carrier concentration of the ITON films, as compared to the respective properties of the ITO films, were attributed to the incorporation of nitrogen, instead of oxygen, in the ITON structure.     

Effects of experimental parameters on composition of boron carbon nitride thin films deposited by magnetron sputtering

We have synthesized boron carbon nitride thin films by radio frequency magnetron sputtering. The films structure and composition were characterized by X-ray diffraction, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The results indicate that the three elements of B, C, N are chemically bonded with each other and atomic-level hybrids have been formed in the films. The boron carbon nitride films prepared in the present experiment possess a disordered structure. The influence of PN₂/PN₂+Ar, total pressure and substrate bias voltage on the composition of boron carbon nitride films is investigated. The atomic fraction of C atoms increases and the fractions of B, N decrease with the decrease of PN₂/PN₂+Ar from 75% to 0%. There is an optimum total pressure. That is to say, the atomic fractions of B, N reach a maximum and the fraction of C atoms reaches a minimum at the total pressure of 1.3 Pa. The boron carbon nitride films exhibit lower C content and higher B, N contents at lower bias voltages. And the boron carbon nitride films show higher C content and lower B, N contents at higher bias voltages.     

Electrical and electronic properties of nitrogen doped amorphous carbon (a-CNx) thin films

Nitrogen-doped amorphous carbon thin films (a-CN_x) were prepared on silicon substrate by pulsed laser deposition process using methane (CH₄) and nitrogen (N₂) as source gas. The electrical properties of a-CN_x films changes with nitrogen concentration in the film structure. The intensity ratio of the D and G peak (I_D/I_G) increases with higher nitrogen concentration, which means that sp²-clusters were formed in these films and is responsible for the enhancement of conductivity of the a-CN_x films. We observed that the amorphous carbon (a-C) films becoming more graphitic in nature yielding higher conductivity/lower resistivity with increase of nitrogen concentration. Electron field emission result shows that the emission current density enhances with nitrogen doping that indicates the useful in electron field emission devices application.     

Photoluminescence of Si from Si nanocrystal-doped SiO2/Si multilayered sample

A multilayered Si nanocrystal-doped SiO₂/Si (or Si-nc:SiO₂/Si) sample structure is studied to acquire strong photoluminescence (PL) emission of Si via modulating excess Si concentration. The Si-nc:SiO₂ results from SiO thin film after thermal annealing. The total thickness of SiO layer remains 150 nm, and is partitioned equally into a number of sublayers (N = 3, 5, 10, or 30) by Si interlayers. For each N-layered sample, a maximal PL intensity of Si can be obtained via optimizing the thickness of Si interlayer (or d_Si). This maximal PL intensity varies with N, but the ratio of Si to O is nearly a constant. The brightest sample is found to be that of N = 10 and d_Si = 1 nm, whose PL intensity is ∼5 times that of N = 1 without additional Si doping, and ∼2.5 times that of Si-nc:SiO₂ prepared by co-evaporating of SiO and Si at the same optimized ratio of Si to O. Discussions are made based on PL, TEM, EDX and reflectance measurements.     

Film thickness dependence of electro-optic effect in epitaxial BaTiO3 thin films

Based on the phenomenological Landau-Devonshire theory, we investigate the film thickness dependence of ferroelectric and electro-optic properties of epitaxial BaTiO₃ thin films grown on SrTiO₃ and MgO substrates. By using the effective substrate lattice parameter concept, the film thickness dependence of misfit strain is incorporated into the theory. Therefore, the film thickness dependence of ferroelectric and electro-optic properties in epitaxial BaTiO₃ thin films can be explained. Moreover, a large quadratic electro-optic effect was obtained in the BaTiO₃ thin films, which is in good agreement with the experimental result of BaTiO₃ thin films on the MgO substrate.     

Effects of soft layer softness on the magnetic properties of perpendicular exchange-coupled nanocomposite films

The anisotropy of the soft layer in the Co_100−xPt_x/Co₇₁Pt₂₉ (x=0, 7 and 17) perpendicular exchange-coupled composite (ECC) films was varied by changing the Pt content. The effects of soft layer softness (thickness and anisotropy) on the coercivity and magnetization reversal mechanisms of ECC were studied. Results showed that both remanence ratio (M_r/M_s) and coercivity of the ECC films reduced with an increase in soft layer thickness. However, the rate of coercivity reduction reduced when soft layer anisotropy was increased simultaneously. This was confirmed by the following facts. For the ECC with Co soft layer, the magnetization reversal mechanism within the ECC grains changed from coherent rotation to domain wall motion when soft layer thickness was changed from 2 to 15 nm. The impact of soft layer thickness on the magnetization reversals of the ECC grains reduced with an increase in soft layer anisotropy. On the other hand, the change of soft layer easy axis direction could possibly change the reversal mechanism of the ECC grains. The above experimental results showed that the coercivity of ECC film was controlled by the reversal mechanism inside the ECC grains.     

Structural, electrical, optical and magnetic properties of Co0.2AlxZn0.8−xO films

Co_0.2Al_xZn_0.8−xO films prepared with different molar ratio of aluminum nitrate to zinc acetate were deposited on substrates by the sol-gel technique. X-ray diffraction, photoluminescence and ferromagnetism measurements were used to characterize the Co_0.2Al_xZn_0.8−xO diluted magnetic semiconductors. The authors found that the intensity of the acceptor-related photoluminescence increased with increasing aluminum concentration and an increase in the number of the acceptor-like defects (zinc vacancies especially) in the Co_0.2Al_xZn_0.8−xO film might lead to the enhancement of the magnetic properties. This implies that controls of the aluminum concentration and the number of the acceptor-like defects are important factors to produce strong ferromagnetism Co_0.2Al_xZn_0.8−xO films prepared by the sol-gel method.     

Effect of bilayer structure and a SrRuO3 buffer layer on ferroelectric properties of BiFeO3 thin films

Effects of the BiFe_0.95Mn_0.05O₃ thickness and a SrRuO₃ (SRO) buffer layer on the microstructure and electrical properties of BiFeO₃/BiFe_0.95Mn_0.05O₃ (BFO/BFMO) bilayered thin films were investigated, where BFO/BFMO bilayered thin films were fabricated on the SRO/Pt/Ti/SiO₂/Si(100) substrate by a radio frequency sputtering. All thin films are of a pure perovskite structure with a mixture of (110) and (111) orientations regardless of the BFMO layer thickness. Dense microstructure is demonstrated in all thin films because of the introduction of BFMO layers. The SRO buffer layer can also further improve the ferroelectric properties of BFO/BFMO bilayered thin films as compared with those of these thin films without a SRO buffer layer. The BFO/BFMO bilayered thin film with a thickness ratio of 220/120 has an enhanced ferroelectric behavior of 2P _r??165.23???C/cm² and 2E _c??518.56?kV/cm, together with a good fatigue endurance. Therefore, it is an effective way to enhance the ferroelectric and fatigue properties of bismuth ferrite thin films by constructing such a bilayered structure and using a SRO buffer layer.     

Nanoscale imaging of ferroelectric domain and resistance switching in hybrid improper ferroelectric Ca3Ti2O7 thin films

Hybrid improper ferroelectrics have their electric polarization generated by two or more combined non-ferroelectric structural distortions such as the rotation and tilting of Ti-O octahedral in Ca₃Ti₂O₇ (CTO) family. In this work, we prepared different thickness CTO thin films on Pt substrates by pulsed laser deposition, and investigated their ferroelectric polarization reversal and the current transport properties by using the piezoresponse force microscopy and conducting atomic force microscopy, respectively. It is found that the CTO films exhibit clear ferroelectric domain switching and ferroelectric resistance switching behaviors, and the maximum resistive ratios of CTO film reaches ∼1750. These results demonstrate that hybrid improper ferroelectrics CTO films are promising materials for being employed in non-volatile memory and logic devices.     

Electrical and optical properties of GexFexSe100−2x chalcogenide thin films

Optical absorption at room temperature and electrical conductivity at temperatures between 283 and 333 K of vacuum evaporated Ge_xFe_xSe_100−2x (0≤x≤15) amorphous thin films have been studied as a function of composition and film thickness. It was found that the optical absorption is due to indirect transition and the energy gap increases with increasing both Ge and Fe content; on the other hand, the width of the band tail exhibits the opposite behavior. The optical band gap E_opt was found to be almost thickness independent. The electrical conductivity show two types of conduction, at higher temperature the conduction is due to extended states, while the conduction at low temperature is due to variable range hopping in the localized states near Fermi level. Increasing Ge and Fe contents were found to decrease the localized state density N(E_F), electrical conductivity and increase the activation energy for conduction, which is nearly thickness independent. Variation of the atomic densities ρ, molar volume V, glass transition temperature T_g cohesive energy C.E and number of constraints N_Co with average coordination number Z was investigated. The relationship between the optical gap and chemical composition is discussed in terms of the cohesive energy C.E, average heat of atomization and coordination numbers.     

Preparation and characterization of ZrO2:Sm amorphous thin films by solid state photochemical deposition method

Thin films of ZrO₂ loaded with 10, 30 and 50 mol% Sm were prepared by a photochemical method using thin films of metal acetylacetonate complexes as precursors. The photolysis of these films induces the fragmentation of the acetylacetonate ligand and the partial reduction of metal ion together with volatile organic compounds. When the metallic complex is exposed to air, the product of the reaction is metal oxide. The photoreactivity of these films was monitored by FT-IR spectroscopy, followed by a post-annealing treatment process. The obtained films were characterized by X-ray photoelectron spectroscopy and atomic force microscopy.Photoluminescense studies of the films employed 400 nm radiation for excitation of the Sm ions present. The emission spectra showed signals arising from the ⁴G_5/2→⁶H_J (J=3/2, 7/2, 9/2) transitions, where the ⁴G_5/2→⁶H_3/2 transition has the highest intensity. The concentration dependence of the PL intensity was also studied. A maximum PL intensity was observed with 10 mol% Sm content but then diminished with higher Sm concentrations.     

Observation of optical properties related to room-temperature ferromagnetism in co-sputtered Zn1−xCoxO thin films

We report on the analysis of optical transmittance spectra and the resulting ferromagnetic characteristics of sputtered Zn_1−xCo_xO films. Zn_1−xCo_xO films were prepared on (0001)-oriented Al₂O₃ substrates by the radio-frequency (rf) magnetron co-sputtering method. The XRD results showed that the crystallinity of films was properly maintained up to x=0.30 and no second phase peaks were detected up to x=0.40. The transmittance spectra showed both the increase of the absorption band intensity and the red shift of the absorption peak as well as the band edge with increasing x. We have proved experimentally that these changes depend on Co concentration. These optical properties suggest that sp-d exchange interactions and typical d-d transitions become activated with increasing x, which leads to the enhancement of ferromagnetic properties in Zn_1−xCo_xO films as shown in the AGM results. Therefore, it is concluded that the ferromagnetism derives from the substitution of Co²⁺ for Zn²⁺ without changing the wurtzite structure.     

Carbon nitride films by RF plasma assisted PLD: Spectroscopic and electronic analysis

Carbon nitride (CNx) thin films have been grown on Si 〈1 0 0〉 by 193 nm ArF ns pulsed laser ablation of a pure graphite target in a low pressure atmosphere of a RF generated N₂ plasma and compared with samples grown by PLD in pure nitrogen atmosphere. Composition, structure and bonding of the deposited materials have been evaluated by X-ray photoelectron spectroscopy (XPS), and Raman scattering. Significant chemical and micro-structural changes have been registered, associated to different nitrogen incorporation in the two types of films analyzed. The intensity of the reactive activated species is, indeed, increased by the presence of the bias confined RF plasma, as compared to the bare nitrogen atmosphere, thus resulting in a different nitrogen uptake in the growing films. The process has been also investigated by some preliminary optical emission studies of the carbon plume expanding in the nitrogen atmosphere. Optical emission spectroscopy reveals the presence of many excited species like C⁺ ions, C atoms, C₂, N₂; and CN radicals, and N₂⁺ molecular ions, whose relative intensity appears to be increased in the presence of the RF plasma. The films were also characterised for electrical properties by the “four-probe-test method” determining sheet resistivity and correlating surface conductivity with chemical composition.     

Effects on measurement of photon-atom scattering of applied pressures on sample at different thickness

N_coh/N_Comp and NK_β/NK_α intensity ratios of scattered photons by Zn in the linear region and the infinite mass thickness region are measured as functions of the pressure used for compressing the pellets with a Si(Li) detector using Am-241 and Fe-55 annular source. Besides, the effect on the experimental intensity ratio of relation between two different thicknesses of sample with applied different pressures on the sample is investigated in this study. Harmony between the linear and infinitive (critical) thickness region and the applied pressure on the sample has directly affected the experimental intensity ratios. Experimental results were not compared with various theoretical values in the literature, for present results constitute the first experimental measurements.     

Effects of interface on the dielectric properties of Ba0.6Sr0.4TiO3 thin film capacitors

Ba_0.6Sr_0.4TiO₃ thin films were deposited on Pt/SiO₂/Si substrate by radio frequency magnetron sputtering. High-resolution transmission electron microscopy (HRTEM) observation shows that there is a transition layer at BST/Pt interface, and the layer is about 7-8 nm thickness. It is found that the transition layer was diminished to about 2-3 nm thickness by reducing the initial RF sputtering power. X-ray photoelectron spectroscopy (XPS) depth profiles show that high Ti atomic concentration results in a thick interfacial transition layer. Moreover, the symmetry ν of ?_r-V curve of BST thin film is enhanced from 52.37 to 95.98%. Meanwhile, the tunability, difference of negative and positive remanent polarization (P_r), and that of coercive field (E_C) are remarkably improved.     

Comparison and semiconductor properties of nitrogen doped carbon thin films grown by different techniques

Amorphous carbon nitride (a-CN_x) thin films have been synthesised by three different deposition techniques in an Ar/N₂ gas mixture and have been deposited by varying the percentage of nitrogen gas in the mixture (i.e. the N₂/Ar + N₂ ratio) from 0 to 10%. The variation of the electrical conductivity and the gap values of the deposited films versus the N₂/Ar + N₂ ratio were investigated in relation with their local microstructure. Film composition was analysed using Raman spectroscopy and optical transmission experiments. The observed variation of electrical conductivity and optical properties are attributed to the changes in the atomic bonding structures, which were induced by N incorporation, increasing both the sp² carbon content and their relative disorder. The low N content samples seem to be an interesting material to produce films with interesting properties for optoelectronic applications considering the facility to control the gas composition as a key parameter.     

Exchange coupling and remanence enhancement in nanocomposite Nd–Fe–B/FeCo multilayer films

Nd–Fe–B-type hard phase single layer films and nanocomposite Nd₂₈Fe₆₆B₆/Fe₅₀Co₅₀ multilayer films with Mo underlayers and overlayers have been fabricated on Si substrates by rf sputtering. The hysteresis loops of all films indicated simple single loops for fixed Nd–Fe–B layer thickness (10 nm) and different FeCo layer thickness (d_FeCo=1–50 nm). The remanence of these films is found to increase with increasing d_FeCo and the coercivity decrease with increasing d_FeCo. It is shown that high remanence is achieved in the nanocomposite multilayer films consisting of the hard magnetic Nd–Fe–B-type phase and soft magnetic phase FeCo with 20 nm?d_FeCo?3 nm. The sample of maximum energy product is 27 MG Oe for d_FeCo=5 nm at room temperature. The enhancement of the remanence and energy products in nanocomposite multilayer films is attributed to the exchange coupling between the magnetically soft and hard phases.     

Orientation dependence of the electrocaloric effect of ferroelectric bilayer thin films

An analytical thermodynamic theory is applied to investigate the electrocaloric effect of ferroelectric BaTiO₃/SrTiO₃ bilayer thin films with different orientations at room temperature. Theoretical analysis indicates that the strong electrostatic coupling between the layers results in the suppression of ferroelectricity at a critical relative thickness which occurs approximately at 50%, 23%, and 12% of SrTiO₃ fraction in the (001), (110), and (111) bilayer thin films, respectively. The ferroelectric bilayer thin films are respected to have the largest electrocaloric effect at this critical relative thickness. Moreover, the electrocaloric effect strongly depends on the orientation and the (110) oriented bilayer thin films have the largest electrocaloric effect. Consequently, control of the orientation and the relative thickness of SrTiO₃ layer can be used to adjust the electrocaloric effect of ferroelectric bilayer thin films, which may provide the potential for practical application in refrigeration devices.     

Structural and optical properties of ZnO thin films prepared by sol-gel method with different thickness

In this work, ZnO thin films with different thickness were prepared by sol-gel method on glass substrates and the structural and optical properties of these films were studied by X-ray diffractometer, atomic force microscope, UV-visible spectrophotometer, ellipsometer and fluorophotometer, respectively. The structural analyses show that all the samples have a wurtzite structure and are preferentially oriented along the c-axis perpendicular to the substrate surface. The growth process of highly c-axis oriented ZnO thin films derived from sol-gel method is a self-template process. With the increase of film thickness, the structural disorder decreases and the crystalline quality of the films is gradually improved. A transition of crystal growth mode from vertical growth to lateral growth is observed and the transition point is found between 270 and 360 nm thickness. The optical analyses show that with the increase of film thickness, both the refractive index and ultraviolet emission intensity are improved. However, the transmittance in the visible range is hardly influenced by the film thickness, and the averages are all above 80%.