Controlled synthesis of Mg(OH)2 thin films by chemical solution deposition and their thermal transformation to MgO thin films

The chemical solution deposition of Mg(OH)₂ thin films on glass substrates and their transformation to MgO by annealing in air is presented. The chemical solution deposition consists of a chemical reaction employing an aqueous solution composed of magnesium sulfate, triethanolamine, ammonium hydroxide, and ammonium chloride. The as-deposited films were annealed at different temperatures ranging from 325 to 500?°C to identify the Mg(OH)₂-to-MgO transition temperature, which resulted to be around 375?°C. Annealing the as-deposited Mg(OH)₂ films at 500?°C results in homogeneous MgO thin films. The properties of the Mg(OH)₂ and MgO thin films were analyzed by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, UV–Vis spectroscopy, and by circular transmission line model. Results by X-ray diffraction show that the as-deposited thin films have a brucite structure (Mg(OH)₂), that transforms into the periclase phase (MgO) after annealing at 500?°C. For the as-deposited Mg(OH)₂ thin film, a nanowall surface morphology is found; this morphology is maintained after the annealing to obtain MgO, which occurred with the evident formation of pores on the nanowall surface. The assessed chemical composition from X-ray photoelectron spectroscopy yields Mg_0.36O_0.64 (O/Mg ratio of 1.8) for the as-deposited Mg(OH)₂ film, where the expected stoichiometric composition is Mg_0.33O_0.67 (O/Mg ratio of 2.0); the same assessment yields Mg_0.60O_0.40 (O/Mg ratio of 0.7) for the annealed thin film, which indicates the obtainment of a MgO material with oxygen vacancies, given the deviation from the stoichiometric composition of Mg_0.50O_0.50 (O/Mg ratio of 1.0). These results confirm the deposition of Mg(OH)₂ films and the obtainment of MgO after the heat-treatment. The energy band gap of the films is found to be 4.64 and 5.10?eV for the as-deposited and the film annealed at 500?°C, respectively. The resistivity of both Mg(OH)₂ and MgO thin films lies around 10⁸?Ω·cm.     

Effect of oxygen pressure on the p-type conductivity of Ga,P co-doped ZnO thin film grown by pulsed laser deposition

The effect of the oxygen partial pressure on the conductivity of (Ga, P) co-doped ZnO thin films (ZnO:Ga_0.01P_0.02, ZnO:Ga_0.01P_0.04) was investigated. The thin films were grown by using the pulsed laser deposition (PLD) method. As the oxygen partial pressure increased from 1 mTorr to 200 mTorr, the electron carrier concentration of the ZnO:Ga_0.01P_0.04 thin films decreased. Above 200 mTorr, however, the electron carrier concentration increased and a transition from n-type to p-type conductivity was observed. On the other hand, in the case of the ZnO:Ga_0.01P_0.02 thin films, their electron carrier concentration continuously decreased as the oxygen partial pressure increased from 1 to 500 mTorr, showing the typical n-type semi-conductive characteristics. The X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) analyses were used to characterize the n-type to p-type conductivity transitions with increasing oxygen partial pressure.     

Effects of NIR annealing on the characteristics of al-doped ZnO thin films prepared by RF sputtering

Aluminum-doped zinc oxide (AZO) thin films have been deposited on glass substrates by employing radio frequency (RF) sputtering method for transparent conducting oxide applications. For the RF sputtering process, a ZnO:Al₂O₃ (2 wt.%) target was employed. In this paper, the effects of near infrared ray (NIR) annealing technique on the structural, optical, and electrical properties of the AZO thin films have been researched. Experimental results showed that NIR annealing affected the microstructure, electrical resistance, and optical transmittance of the AZO thin films. X-ray diffraction analysis revealed that all films have a hexagonal wurtzite crystal structure with the preferentially c-axis oriented normal to the substrate surface. Optical transmittance spectra of the AZO thin films exhibited transmittance higher than about 80% within the visible wavelength region, and the optical direct bandgap (E_g) of the AZO films was increased with increasing the NIR energy efficiency.     

Characterization of non-stoichiometric Ga2O3-x thin films grown by radio-frequency powder sputtering

We report the synthesis and characterization of non-stoichiometric Ga₂O_3-x thin films deposited on sapphire (0001) substrates by radio-frequency powder sputtering. The chemical and electronic states of the non-stoichiometric Ga₂O_3-x thin films were investigated. By sputtering in an Ar atmosphere, the as-grown thin films become non-stoichiometric Ga₂O_2.7, due to the difference in sputtering yield between Ga and O species of the Ga₂O₃ target. The electronic states of the thin films consist of ~85% Ga³⁺ and ~15% Ga¹⁺, corresponding to Ga₂O₃ and Ga₂O, respectively. The films have the electrical characteristics of a semiconductor, with electrical conductivity of approximately 5.0 × 10^-4 S cm^-1 and a carrier concentration of 4.5 × 10¹⁴ cm^-3 at 300 K.     

Effects of tungsten thickness and annealing temperature on the electrical properties of W-TiO2 thin films

TiO₂ thin films were prepared by RF magnetron sputtering onto glass substrates and tungsten was deposited onto these thin films (deposition time 15-60 s) to form W-TiO₂ bi-layer thin films. The crystal structure, morphology, and transmittance of these TiO₂ and W-TiO₂ bi-layer thin films were investigated. Amorphous, rutile, and anatase TiO₂ phases were observed in the TiO₂ and W-TiO₂ bi-layer thin films. Tungsten thickness and annealing temperature had large effects on the transmittance of the W-TiO₂ thin films. The W-TiO₂ bi-layer thin films with a tungsten deposition time of 60 s were annealed at 200 °C-400 °C. The band gap energies of the TiO₂ and the non-annealed and annealed W-TiO₂ bi-layer thin films were evaluated using (αhν)^1/2 versus energy plots, showing that tungsten thickness and annealing temperature had major effects on the transmittance and band gap energy of W-TiO₂ bi-layer thin films.     

Influence of N2/Ar gas mixture ratio and annealing on optical properties of SiCBN thin films prepared by rf sputtering

Optical properties of amorphous thin films of silicon carbon boron nitride (Si–C–B–N) obtained by reactive sputtering has been studied. Compositional variations were obtained by changing the nitrogen and argon gas mixture ratio in the sputtering ambient. The effect of gas ratios and annealing on the optical properties was investigated. It was found that the transmittance of the films increases with nitrogen incorporation. Annealing at higher temperatures leads to considerable increase in transmittance. Optical energy gap (Tauc gap) calculated from absorption data is influenced by annealing temperatures and reactive process gas mixture. Changes in optical properties were correlated to the chemical modifications in the films due to annealing, through X-ray photoelectron spectroscopy. Studies reveal that the carbon and nitrogen concentrations in the films are highly sensitive to temperature. Annealing at higher temperatures leads to broken C–N bonds which results in the loss of C and N in the films. This is believed to be the primary cause for variations in optical properties of the films.     

Electro-optical performances of nanostructured SrTiOx films: The effect of plasma power,Ar/O2 ratio and annealing

The present work evaluates the effects of plasma power and oxygen mixing ratios (OMRs) on structural, morphological, optical, and electrical properties of strontium titanate SrTiO_x (STO) thin films. STO thin films were grown by magnetron sputtering, and later thermal annealing at 700°C for 1 h was applied to improve film properties. X-ray diffraction analysis indicated that as-deposited films have amorphous microstructure independent of deposition conditions. The films deposited at higher OMR values and later annealed also showed amorphous structure while the films deposited at lower OMR value and annealed have nanocrystallinity. In addition, all as-deposited films were highly transparent (~80%–85%) in the visible spectrum and exhibited well-defined main absorption edge, while the annealing improved transparency (90%) within the same spectrum. The calculated direct and indirect optical band gaps for films were in the range of 3.60-4.30 eV as a function of deposition conditions. The refractive index of the films increased with OMRs and the postdeposition annealing. The frequency dependent capacitance measurements at 100 kHz were performed to obtain film dielectric constant values. High dielectric constant values reaching up to 100 were obtained. All STO samples exhibited more than 2.5 μC/cm² charge storage capacity and low dielectric loss (less than 0.07 at 100 kHz). The leakage current density was relatively low (3 × 10⁻⁸Acm⁻² at +0.8 V) indicating that STO films are promising for future dynamic random access memory applications.     

CIGS absorbing layers prepared by RF magnetron sputtering from a single quaternary target

Cu(In_1−xGa_x)Se₂ (CIGS) thin films were prepared by RF magnetron sputtering from a single quaternary target at multiple processing parameters. The structural, compositional, and electrical properties of the as-deposited films were systematically investigated by XRD, Raman, SEM, and Hall effects analysis. The results demonstrate that by adjusting the processing parameters, the CIGS thin films with a preferential orientation along the (112) direction which exhibited single chalcopyrite phase were obtained. The films deposited at relatively higher substrate temperature, sputtering power, and Ar pressure exhibited favorable stoichiometric ratio (Cu/(In+Ga):0.8–0.9 and Ga/(In+Ga):0.25–0.36) with grain size of about 1–1.5 µm, and desirable electrical properties with p-type carrier concentration of 10¹⁶−10¹⁷ cm⁻³ and carrier mobility of 10–60 cm²/Vs. The CIGS layers are expected to fabricate high efficiency thin film solar cells.     

Recrystallization behavior,oxygen vacancy and photoluminescence performance of sputter-deposited Ga2O3 films via high-vacuum in situ annealing

Ga₂O₃ films were deposited on Si substrates through radio-frequency magnetron sputtering at room temperature and were annealed in situ in a high-vacuum environment. The as-deposited Ga₂O₃ film exhibited an island-like surface morphology and had an amorphous microstructure, with a few nanocrystalline grains embedded in it. After high-temperature in situ annealing, the films recrystallized and exhibited coalesced surfaces. Because of the thermally driven diffusion of Ga, the interfacial layer between Si and Ga₂O₃ was composed of SiGaO_x. Compared with ex situ annealing in air, in situ annealing in high vacuum is more advantageous because it enhances surface mobility and improves the crystallinity of the Ga₂O₃ films. The higher oxygen vacancy concentration of in situ annealed films revealed that oxygen atoms were easily released from the Ga₂O₃ lattice during high-vacuum annealing. Photoluminescence (PL) spectra exhibited four emission peaks centered in ultraviolet, blue, and green regions, and the peak intensities were significantly enhanced by thermal annealing at >600 °C. This work elucidates the effect of the in situ annealing treatment on the recrystallization behavior, interfacial microstructure, oxygen vacancy concentration, and PL performance of the Ga₂O₃ films, making it significant and instructional for the further development of Ga₂O₃-based devices.     

Structural,optical and morphological evolution of Ga2O3/Al2O3 (0001) films grown at various temperatures by pulsed laser deposition

This study systematically investigated the structural, optical, and morphological evolution of Gallium oxide (Ga₂O₃) films deposited at different substrate temperatures on Al₂O₃(0001) using pulsed laser deposition (PLD). The thickness of the Ga₂O₃ films was standardized in order to eliminate its effect on the film properties. The effect of substrate temperature from room temperature to 600 °C on the film's transmittance, crystalline structure, chemical composition and surface morphology, was explored. The plasma species generated during the deposition of the PLD process were monitored and analyzed employing in situ optical emission spectroscopy. The deposition rate of the films decreased with increasing substrate temperature. X-ray photoelectron spectroscopy was used to detect both Ga³⁺ and Ga ⁺ oxidation states in all prepared films, which indicated substoichiometric Ga₂O₃ films deficient in oxygen. The percentage of non-lattice oxygen decreased with increasing substrate temperature. At optimal condition, mono-crystaline β-Ga₂O₃ was produced with a high visible and near-infrared transmittance, large grain size and smooth surface, which is suitable for the application in high-performance power electric devices and photoelectronic devices.     

Atomic layer deposition of TiO2 from tetrakis-dimethylamido-titanium and ozone

Ozone (O₃) was employed as an oxygen source for the atomic layer deposition (ALD) of titanium dioxide (TiO₂) based on tetrakis-dimethyl-amido titanium (TDMAT). The effects of deposition temperature and O₃ feeding time on the film growth kinetics and physical/chemical properties of the TiO₂ films were investigated. Film growth was possible at as low as 75 °C, and the growth rate (thickness/cycles) of TiO₂ was minimally affected by varying the temperatures at 150–225 °C. Moreover, saturated growth behavior on the O₃ feeding time was observed at longer than 0.5 s. Higher temperatures tend to provide films with lower levels of carbon impurities. The film thickness increased linearly as the number of cycles increased. With thicker films and at higher deposition temperatures, surface roughening tended to increase. The as-deposited films were amorphous regardless of the substrate temperatures and there was no change of crystal phase even after annealing at temperatures of 400–600 °C. The films deposited in 0.5 mm holes with an aspect ratio of 3: 1 showed an excellent conformality.     

Annealing temperature controlled crystallization mechanism and properties of gallium oxide film in forming gas atmosphere

Gallium oxide (Ga₂O₃) films had been fabricated on Al₂O₃(0001) substrate by employing pulsed laser deposition (PLD) and annealed at different temperatures under forming gas (FG) atmosphere (95% N₂ + 5% H₂). The influence of annealing temperature on the structural, optical, chemical composition, and surface morphological properties of the Ga₂O₃ thin films was investigated comprehensively. The annealing processes with hydrogen gas play a crucial role in the characteristics of Ga₂O₃ thin films. A crystallization mechanism of Ga₂O₃ films controlled by annealing temperature has been proposed firstly and analyzed systematically, which contains three kinds of competitive mechanism, namely the thermal enhanced crystallization, the enhanced H₂ dissociative adsorption on Ga₂O₃ surfaces, and the high-temperature decomposition of Ga₂O₃. Both Ga⁺ and Ga³⁺ oxidation valence states were presented in all samples, which indicated lattice oxygen deficiency in Ga₂O₃ films. The variation of the non-lattice oxygen proportion of Ga₂O₃ films related to the crystallization mechanism firstly increased and then decreased with the increase of annealing temperature. The detailed crystallization mechanism of PLD-Ga₂O₃ films annealed in FG offers a guideline and references for the further fabrication of high-quality Ga₂O₃ films and their applications in high-performance devices.     

Synthesis of polycrystalline gallium oxide solar-blind ultraviolet photodetector by Aerosol Deposition

An aerosol deposition method was used to fabricate a solar-blind photodetector (for UV-C) using thin films of β-Ga₂O₃, which is a wide-bandgap oxide material. The Ga₂O₃ films deposited at room temperature presented a polycrystalline structure and a thickness of approximately 4 µm and showed a high transmittance of approximately 70–80 % in the visible region; the transmittance was approximately 60–80 % even after heat treatment up to a 800 °C. The Ga₂O₃ films that were post-annealed at a temperature of 800 °C showed an I_photo/I_dark ratio of approximately 40,000 in the solar-blind region with a light source of 254 nm, together with very good light detection characteristics (initial rising and decay times of 0.45 s and 0.13 s, respectively). Because of the good performances observed for the Ga₂O₃ thin films even at extreme conditions, they exhibit a high potential for use as photodetectors in several applications.     

Multi-target reactive sputtering—A promising technology for large-area Pb(Zr,Ti)O3 thin film deposition

Beyond target diameters of 100 mm, multi-target reactive sputtering becomes a promising technology for ferroelectric thin film deposition. The main advantages of multi-target sputtering technology are: (i) thin films with precise composition control, (ii) stoichiometric variations on the target surface during repeated use are prevented by target preconditioning and operation in the metallic mode, and (iii) higher deposition rate due to sputtering from metals in the metallic mode. The latter requires a much greater precision in control of the partial pressure of oxygen, e.g., by a plasma emission monitor. In this work, Pb(Zr,Ti)O₃ thin film deposition on 150 mm silicon wafers by an industrial system is demonstrated. This technology can be easily scaled-up for larger silicon wafers and is compatible with standard semiconductor technology. Films deposited onto ZrO₂ buffer layers were polarized in-plane and they are suitable for piezoelectric MEMS application.     

Enhanced photocatalytic activity of TiO2-ZnO thin films deposited by dc reactive magnetron sputtering

The effect of deposition conditions on the photocatalytic activity of TiO₂-ZnO thin films was studied. By using a (Ti)₉₀-(Zn)₁₀ alloy target, the samples were deposited at room temperature on glass substrates by dc reactive magnetron sputtering and post-annealed in air at 500 °C. The dependence of the physical properties of the films on the O₂/Ar gas ratio and the deposition working pressure was investigated. XRD patterns showed mainly the formation of the anatase phase of TiO₂. Optical absorption measurements exhibited a blue shift of the band-gap energy with increasing working pressure. XPS spectra indicated the presence of the Ti⁴⁺ and Zn²⁺ oxidation states, which correspond to TiO₂ and ZnO, respectively. The chemical state of Ti was further analyzed by means of the modified Auger parameter, α’, which gave a value of ca. 873 eV. The photocatalytic property of the films was assessed by the degradation of a methylene blue aqueous solution. The maximum photocatalytic performance was observed for the samples deposited at 3.0 mTorr and O₂/Ar gas ratio of 10/90. These results are explained in terms of the structural, optical, and morphological properties of the films.     

ZnO:Al thin films from (Al2O3)x(ZnO)(1-x) powder targets by magnetron sputtering

In the present work we prepared Aluminum doped Zinc Oxide (AZO) thin films from powder targets. Various concentrations (W/W percentages) of Al₂O₃ such as1%, 2%, 3%, 4%, 5%, 6%, 7% and 8% were mixed in ZnO powder and made in the form of a 3 inch disc target. These ceramic targets are sputtered in RF magnetron sputtering unit for the deposition of AZO thin films. Optical and electrical properties are analyzed to get an optimized percentage of mixing for achieving high transparency and low resistivity. At Al₂O₃ percentage of 3% there is a considerable decrement in the resistivity, and at 7% there is a considerable decrease in the optical transmittance. Mobility and carrier concentration are increasing with Al₂O₃ percentage. Bandgap of the films is observed to be decreasing with increasing the Al₂O₃ percentage.     

Transparent and conductive indium tin oxide/polyimide films prepared by high‐temperature radio‐frequency magnetron sputtering

A highly transparent and thermally stable polyimide (PI) substrate was prepared and used for the fabrication of indium tin oxide (ITO)/PI films via radio‐frequency magnetron sputtering at an elevated substrate temperature. The effect of the deposition conditions, that is, the oxygen flow rate, substrate temperature, sputtering power, and working pressure, on the optical and electrical properties of the ITO/PI films were investigated from the microstructural aspects. The results indicate that the optical and electrical properties of ITO were sensitive to the oxygen. Moreover, it was beneficial to the improvement of the ITO conductivity through the adoption of a high substrate temperature and sputtering power and a low working pressure in the deposition process. A two‐step deposition method was developed in which a thick bulk ITO layer was overlapped by deposition on a thin seed ITO layer with a dense surface to prepare the highly transparent and conductive ITO/PI films. The ITO/PI film after annealing at 240°C gave a transmittance of 83% and a sheet resistance of 19.7 Ω/square. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42753.     

Influence of ZnO buffer layer on AZO film properties by radio frequency magnetron sputtering

Transparent conductive films of Al-doped zinc oxide (AZO) were deposited on glass substrates under various ZnO buffer layer deposition conditions (radio frequency (r.f.) power, sputtering pressure, thickness, and annealing) using r.f. magnetron sputtering at room temperature. This work investigates the influence of ZnO buffer layer on structural, electrical, and optical properties of AZO films. The use of grey-based Taguchi method to determine the ZnO buffer layer deposition processing parameters by considering multiple performance characteristics has been reported. Findings show that the ZnO buffer layer improves the optoelectronic performances of AZO films. The AZO films deposited on the 150-nm thick ZnO buffer layer exhibit a very smooth surface with excellent optical properties. Highly c-axis-orientated AZO/ZnO/glass films were grown. Under the optimized ZnO buffer layer deposition conditions, the AZO films show lowest electrical resistivity of 6.75 × 10⁻⁴ Ω cm, about 85% optical transmittance in the visible region, and the best surface roughness of R_a = 0.933 nm.     

Influence of deposition parameters and annealing treatment on the properties of GZO films grown using rf magnetron sputtering

In this study, transparent conductive films of gallium-doped zinc oxide (GZO) are deposited on soda-lime glass substrates, under varied coating conditions (rf power, sputtering pressure, substrate-to-target distance and deposition time), using radio frequency (rf) magnetron sputtering, at room temperature. The effect of the coating parameters on the structural, morphological, electrical and optical properties of GZO films was studied. This study uses a grey-based Taguchi method, to determine the parameters of the coating process for GZO films, by considering multiple performance characteristics. In the confirmation runs, with grey relational analysis, improvements of 14.1% in the deposition rate, 39.81% in electrical resistivity and 1.38% in visible range transmittance were noted. The influence of annealing treatment, in a vacuum, oxygen, and nitrogen gas atmospheres, at temperatures ranging from 130 to 190 °C, for a period of 1 h, was also investigated. GZO films annealed at 190 °C, in a vacuum, showed the lowest electrical resistivity, at 1.07 × 10⁻³ Ω-cm, with about 85% optical transmittance, in the visible region. It is likely that films grown at lower temperatures (190 °C) could be coated onto polymeric substrates, to produce flexible optoelectronic devices.     

Microstructure Evolution of In Situ Pulsed‐Laser Crystallized Pb(Zr0.52Ti0.48)O3 Thin Films

Integration of lead zirconate titanate (PZT) films with temperature‐sensitive substrates (CMOS, polymers) would benefit from growth at substrate temperatures below 400°C. In this work, in situ pulsed‐laser annealing [Rajashekhar et al. (2013) Appl. Phys. Lett., 103 [3] 032908] was used to grow crystalline lead zirconate titanate (PbZr_0.52Ti_0.48O₃) thin films at a substrate temperature of ~370°C on PbZr_0.30Ti_0.70O₃‐buffered platinized silicon substrates. Transmission electron microscopy analysis indicated that the films were well crystallized into columnar grains, but with pores segregated at the grain boundaries. Lateral densification of the grain columns was significantly improved by reducing the partial pressure of oxygen from 120 to 50 mTorr, presumably due to enhanced adatom mobility at the surface accompanying increased bombardment. It was found that varying the fractional annealing duration with respect to the deposition duration produced little effect on lateral grain growth. However, increasing the fractional annealing duration led to shift of 111 PZT X‐ray diffraction peaks to higher 2θ values, suggesting residual in‐plane tensile stresses in the films. Thermal simulations were used to understand the annealing process. Evolution of the film microstructure is described in terms of transient heating from the pulsed laser determining the nucleation events, while the energy of the arriving species dictates grain growth/coarsening.