Direct Current Reactive Magnetron Sputtering Research Articles

Growth of Thin AlN Films on Si Wafers by Reactive Magnetron Sputtering: Role of Processing Pressure, Magnetron Power and Nitrogen/Argon Gas Flow Ratio

AlN is a wide band gap semiconductor that is of growing industrial interest due to its piezoelectric properties, high breakdown voltage and thermal conductivity. Using magnetron sputtering to grow AlN thin films allows for high deposition rates and uniform coverage of large substrates. One can also produce films at low substrate temperatures, which is required for many production processes. However, current models are inadequate in predicting the resulting structure of a thin film when different sputter parameters are varied. In this work, the growth of wurtzite AlN thin films has been carried out on Si(111) substrates using reactive direct current magnetron sputtering. The influence of the processing pressure, magnetron power and N2/Ar ratio on the structure of the grown films has been analyzed by investigating crystallinity, residual film stress and surface morphology using X-ray diffraction, profilometry, atomic force microscopy and scanning electron microscopy. In every case, the films were found to exhibit c-axis orientation and tensile stress. It was found that high-quality AlN films can be achieved at an N2/Ar ratio of 50% and a low pressure of 0.2 Pa. High magnetron powers (900–1200 W) were necessary for achieving high deposition rates, but they led to larger film stress.

Corrosion performance of TiAlVN-Ag nanocomposite coating deposited by reactive direct current magnetron sputtering

Surgical devices and tools are manufactured in AISI 420 martensitic stainless steel, due to its hardenability, adequate hardness and acceptable biocompatibility. Nevertheless, many surface modification strategies are being investigated with a view to improving this material by providing it with self-protection against the colonization of bacteria. One such strategy is the use of a TiAlVN-Ag nanocomposite coating to promote bactericidal effect in surgical devices; however, it is necessary to study the electrochemical response of such coatings to aggressive environments that simulate disinfection and sterilization processes. The aim of this work is to study the relationship between the microstructure, chemical phases and the electrochemical and corrosion response of TiAlVN-Ag coating, varying the amount of silver with the target power increase (0, 50, 70, 80 and 100 W). To study the microstructure, chemical phases, scanning and high resolution transmission electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction were used. Roughness, microhardness, residual stresses, qualitative adhesion of coatings were estimated by atomic force microscopy, Knoop indentation, profilometer and Rockwell C indentation, respectively. Electrochemical behavior and corrosion protection of coatings were studied using electrochemical impedance spectroscopy and potentiodynamic polarization. TiAlVN-Ag 50W coating showed an improvement in electrochemical behavior and in protection of the steel against corrosion and exhibiting higher hardness than that of the AISI 420 stainless steel substrates. This coating exhibited a dense microstructure and high crystallinity, indicating that the low Ag content does not affect the insulating nature of the coating matrix.

Room-temperature DC-sputtered p-type CuO accumulation-mode thin-film transistors gated by HfO2

CuO grown by room-temperature direct current reactive magnetron sputtering is introduced to realize p-type thin-film transistors (TFTs) with a high-k HfO2 gate dielectric fabricated by atomic layer deposition. The devices work in an accumulation mode (AM) with two apparent threshold voltages corresponding to the formation of a buried channel and an accumulation layer, respectively. A CuO AM TFT with a channel length of 25 μm exhibit a competitive on-off ratio (Ion/Ioff) of 1.3 × 102, a subthreshold swing (SS) of 1.04 V dec−1, and a field-effect mobility (μFE) of 1.1 × 10−3 cm2 V−1 s−1 at room temperature. By measuring a CuO metal oxide semiconductor (MOS) capacitor at room temperature, a high acceptor doping density (NA) of ∼5 × 1017 cm−3, a high positive effective fixed surface charge density (Qf) of ∼9 × 1012 cm−2, and a low interfacial trap charge density (Dit) of ∼6 × 1010 eV−1 cm−2 at the HfO2/CuO interface are estimated. The μFE extracted from the accumulation regime appears lower than the Hall mobility measured for a similarly processed CuO layer on glass due to the increased hole concentration in CuO TFTs, compared to a Hall concentration of ∼1014 cm−3, following the MOS process. SS appears limited by the decreased channel to gate capacitance (Ccg) related to the buried channel in AM TFTs, parasitic capacitance to ground, and potentially very high interfacial traps at the non-passivated CuO/air interface.

Structural and photoelectrochemical dynamics of in-situ hydrogenated anatase TiO2 thin films grown by DC reactive magnetron sputtering

Hydrogenation has become one of the most used strategies to improve the photoactive properties of titanium dioxide nanomaterials, n-TiO2. To obtain more information about the hydrogenated process on anatase TiO2 thin films and the improvement of the photoactivity of this material, we report a study on the structural changes of hydrogenated anatase thin films produced by direct-current reactive magnetron sputtering. In the first stage of the hydrogenation process, the obtained polycrystalline anatase films present an increment of the {001} facet according to the amount of hydrogen used in the plasma reaction. At higher hydrogen concentrations, amorphous and rutile phases start to appear. The photoactivity of the hydrogenated anatase samples, H:TiO2, presents a redshift of the photoelectrochemical onset and an increase of the reactivity in the UV region. Both results were experimentally and theoretically related to the formation of the {001} facet and the TiH/TiOH bonds at the surface of the hydrogenated thin films.

Sketching novel nanostructured diamond-consisted thin films on AISI 321 stainless steel: Microstructure, mechanical and tribological properties

In a reactive direct current (DC) magnetron sputtering (MS) system, converting the cathode (target) magnetic field configuration from a balanced magnetron (BM) to an unbalanced magnetron (UBM) is an impressive strategy to enforce the energetic ion bombardment, providing sufficient activation energy for deposition of high-quality nanostructured films. In the present work, the nanostructured (grain size ˂ 100 nm) BM and UBM sputtered Niobium-doped amorphous Carbon (Nb: a-Carbon) films' microstructural, mechanical, and tribological characteristics changes were thoroughly investigated and compared. From the XRD and Raman analysis outcomes both graphite-like Carbon (GLC) and Niobium carbide (NbC) phases were detected in the BM sputtered film. However, novel cubic and hexagonal crystalline Diamond (CD) phases were successfully constructed from the UBM sputtered films besides the GLC and NbC phases. Records displayed that the change over the cathode magnetic field configuration from BM to UBM is a responsible factor for the reduction of the surface roughness (from 32 nm to 25 nm) and increment of the crystallite size (from 14 nm to 15.5 nm), hardness (from 18 GPa to 27 GPa), and elastic modulus (from 210 GPa to 425 GPa) values. On the contrary, via changing BM to UBM sputtering the films' tribological properties were negatively affected, leading to the increase of the coefficient of friction (COF) from 0.09 to 0.11 and the wear rate (WR) from 5.25 × 10−5 mm3/N.m to 9 × 10−5 mm3/N.m. The achieved results confirmed that the UBM configuration could be selected as a critical facility over the MS system that gives enough energy to arrange Carbon species in the crystalline Diamond phase at a relatively low temperature. One step further, the Ar/CH4 ratio variation was assessed as an influential factor that leaves considerable changes over the UBM sputtered film characteristics. The results indicated that by doubling the Ar/CH4 ratio, the nanocrystalline Diamond phases are still observable. However, this change led to the increase in surface roughness (from 25 nm to 28 nm) and crystallite sizes (NbC, from 15.5 nm to 18.6 nm, cubic Diamond, from 25.8 nm to 36 nm, and hexagonal Diamond from 18.9 nm to 21.6 nm). While the variations adversely affected the UBM sputtered films' hardness (from 27 GPa to 21 GPa) and modulus of elasticity (from 425 GPa to 330 GPa). Simultaneously, the COF and WR endured a slight increment, from 0.11 to 0.14 and 9 × 10−5 mm3/N.m to 9.8 × 10−5 mm3/N.m, respectively.

Crystal structure deformation and phase transition of AlScN thin films in whole Sc concentration range

This article reports lattice deformation and phase transition of AlScN thin films in the whole composition. AlScN films were deposited on Pt/Ta/SiO2/Si substrates by direct current magnetron reactive sputtering with Al and Sc targets. At Sc concentration up to 30%, AlScN has a wurtzite structure with piezoelectricity. Transition from a wurtzite phase to a two-phase mixture happens between 30% and 35% Sc concentration, and transition from a two-phase mixture to a cubic phase happens between 38% and 43% Sc concentration. The wurtzite structure gradually deforms with the decrease in lattice constant c from 16% to 35% Sc concentration. Lattice constant c at 38% Sc in the two-phase mixture region is larger than that of 35% Sc concentration. These increases mean that distortion of c axis for the wurtzite structure over 35% Sc concentration in the two-phase mixture region is considered to be released and/or eased due to the appearance of the cubic phase, and that Sc concentration of the wurtzite phase to be smaller and that of the cubic phase larger than the film composition measured by energy-dispersive x-ray spectroscopy and Rutherford backscattering spectroscopy. At higher Sc concentration up to 43%, the remained wurtzite phases are replaced by non-piezoelectric cubic phases, and the cubic structure approaches the rock-salt structure of ScN with a further increase in Sc concentration.

Dopant concentration influence on the optical properties of NiO thin films doped by Al and Nb deposited by DC and RF magnetron sputtering

Dopant concentration influence on the optical properties of NiO thin films doped by Al and Nb deposited by DC and RF magnetron sputtering

Tantalum Suboxide Films with Tunable Composition and Electrical Resistivity Deposited by Reactive Magnetron Sputtering

Tantalum-based films with tailored composition, density, and electrical resistivity are of interest for next generation hohlraums for magnetized indirect-drive inertial confinement fusion. Here, we use reactive direct-current magnetron sputtering to deposit tantalum suboxide films with O content in the range of 46–71 at.%. In contrast to a common approach involving varying reactive gas contents, compositional control is achieved kinetically by changing the total chamber pressure and the deposition rate, while keeping the working gas mix of Ar-5%O2 constant. The resultant films are X-ray amorphous with electrical resistivity varying by over seven orders of magnitude. The dominant conduction mechanism changes from metallic to activated tunneling above ∼55 at.% of O, which is characterized by a sharp increase in resistivity and a decrease in the carrier density at low temperatures.

Surface modification of PLLA scaffolds via reactive magnetron sputtering in mixtures of nitrogen with noble gases for higher cell adhesion and proliferation

Despite the widespread use of bioresorbable electrospun poly-L-lactic acid scaffolds in tissue engineering, a significant drawback is the high hydrophobicity of their surface. A proper solution for this problem is the surface modification with titanium oxynitride coatings using reactive magnetron sputtering. However, this method is characterized by a low deposition rate and the difficult selection of suitable working parameters. A feasible solution to this problem is the addition of a noble gas to the working gas nitrogen, which increases the deposition rate and thus the coating thicknesses, even with gentle magnetron sputtering process parameters. In this study, the results of surface modification of poly-L-lactic acid scaffolds using direct current reactive magnetron sputtering of a titanium target in the presence of a mixture of the working gas nitrogen with one of the following noble gases: helium, neon, argon, krypton, and xenon. Due to this modification, depending on the composition of the gas mixture, a thin nitrogen-containing titanium coating forms on the scaffold surface, represented by titanium oxide and titanium oxynitrides of different stoichiometry. At the same time, the modification process does not significantly affect the morphology, crystallinity and molecular conformation of the poly-L-lactic acid scaffold backbones, while an increase in the surface wettability is observed. Another biomedical important effect of this surface modification is an observed increase in the adhesion and proliferative activity of fibroblast culture applied to the investigated poly-L-lactic acid scaffold samples. The poly-L-lactic acid scaffold samples surface-modified via direct current reactive magnetron sputtering of a titanium target in a mixture of the working gas nitrogen and the noble gas argon shows the highest number of adhered fibroblasts.

Effect of Ta Content on Scratching Behavior of Ti-Al-Ta-N Coatings on Titanium Substrate

The effect of Ta alloying on the structure, mechanical properties and scratching behavior of Ti-Al-N-based coatings deposited on Ti substrates by reactive direct-current magnetron sputtering is studied. It was found that increasing the Ta content in the Ti1-x-yAlxTayN coatings from y = 0 to y = 0.65 led to a decrease in hardness and Young’s modulus but an increase in the hardness-to-modulus ratio. This resulted in the reduction of the load-bearing capacity of the Ta-alloyed coatings and enhancement of their toughness. The competition among these trends determined the improved crack resistance and adhesion of the Ti0.31Al0.34Ta0.35N coating under scratching.

Effect of Annealing Heat Treatment on the Composition, Morphology, Structure and Mechanical Properties of the W-S-N Coatings.

Alloyed-transition metal dichalcogenide (TMD) coatings have been under investigation as multi-environment lubricants for the past few decades. These coatings display very low coefficient of friction properties at elevated temperatures. Studies on the annealing of these low-friction coatings are missing in the literature. For the first time, in this study, the annealing of the W-S-N dry lubricant coatings was carried out to study its effects on the composition, morphology, crystal structure and hardness of the coatings. The W-S-N coatings were deposited by direct current (DC) reactive magnetron sputtering. The analysis was carried out for as-deposited, 200 °C and 400 °C annealed coatings. The as-deposited coatings have N content in the range of 0–25.5 at. %. The coatings are compact and the densification increased with the increase in N-alloying. All the coatings are crystalline except the highest N-alloyed coating which is X-ray amorphous. A maximum hardness of 8.0 GPa was measured for the coating alloyed with 23 at. % N. Annealing did not affect the composition and morphology of the coatings, while some variations were observed in their crystal structure and hardness. The maximum hardness increased from 8 GPa to 9.2 GPa after 400 °C annealing of the 23 at. % N-alloyed coating.

Effects of the Deposition Mode and Heat Treatment on the Microstructure and Wettability of Y2O3 Coatings Prepared by Reactive Magnetron Sputtering

A robust hydrophobic Y2O3 coating at high temperatures is important for industrial applications. In this study, Y2O3 thin films on Si substrates were prepared by reactive direct current magnetron sputtering. By changing the deposition power, Y2O3 thin films with different microstructures were obtained in poison mode and metallic mode, respectively. In order to understand the effect of heat treatment on the microstructure and hydrophobicity of Y2O3, the samples were annealed at 400 °C in the air. Compared to metallic mode, no crack was formed on the surface of the Y2O3 film prepared in poison mode. In addition, the water contact angle on the surface of the Y2O3 thin film deposited in poison mode was above 90° before and after annealing at 400 °C. It has been demonstrated that the initial high concentration of physically absorbed oxygen and its slow desorption process in a Y2O3 thin film prepared in poison mode contributes to the hydrophobicity of the thin film at high temperatures. These results can provide insights into the large-scale fabrication of hydrophobic Y2O3 coatings for high-temperature applications.

Study of deposition parameters of reactive-sputtered Si3N4 thin films by optical emission spectroscopy

Deposition parameters of silicon nitride in reactive direct-current magnetron sputtering were studied in two different chambers in order to establish a characterization protocol assisted by optical emission spectroscopy. Different discharge powers, working pressures and gas proportions (N2:Ar) were used to create characterization curves to identify the optimum conditions for the synthesis of silicon nitride. These curves were used to observe changes in the plasma via emission lines and the supplied electrical parameters (voltage and current). Individual thin films were synthesized using different deposition parameters selected from the previous characterization curves, and their optical properties were characterized via spectroscopic ellipsometry. A correlation between deposition parameters, optical properties, and plasma emission lines was established to optimize silicon nitride deposition.

TiO2/CuO/Cu2O Photovoltaic Nanostructures Prepared by DC Reactive Magnetron Sputtering.

In this study, titanium dioxide/copper oxide thin-film solar cells were prepared using the reactive direct-current magnetron sputtering technique. The influence of the deposition time of the top Cu contact layer on the structural and electrical properties of photovoltaic devices was analyzed. The structural and morphological characterization of the TiO2/CuO/Cu2O solar cells was fully studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), and current–voltage (I-V) characteristics. Additionally, using van der Pauw sample geometries, the electrical properties of the titanium dioxide and copper oxide layers were investigated. From the XRD study, solar cells were observed in cubic (Cu2O), monoclinic (CuO), and Ti3O5 phases. In addition, the crystallite size and dislocation density for copper oxide layers were calculated. Basic morphological parameters (thickness, mechanism of growth, and composition of elements) were analyzed via scanning electron microscopy. The thicknesses of the titanium dioxide and copper oxide layers were in the range of 43–55 nm and 806–1223 nm, respectively. Furthermore, the mechanism of growth and the basic composition of the elements of layers were analyzed. The I-V characteristic curve confirms the photovoltaic behavior of two titanium dioxide/copper oxide thin-film structures. The values of short-circuit current density (Jsc) and open-circuit voltage (Voc) of the solar cells were: 4.0 ± 0.8 µA/cm2, 16.0 ± 4.8 mV and 0.43 ± 0.61 µA/cm2, 0.54 ± 0.31 mV, respectively. In addition, the authors presented the values of Isc, Pmax, FF, and Rsh. Finally, the resistivity, carrier concentration, and mobility are reported for selected layers with values reflecting the current literature.

Surface Conductivity and Preferred Orientation of TiN Film for Ti Bipolar Plate

The properties of thin films are often influenced by the crystal’s preferred orientation. In the present study, we report the strong dependence of surface conductivity on the preferred orientation of TiN film that acts as the coating material for Ti bipolar plate. The preferred orientation of TiN film is successfully controlled along the (111) or (200) planes by adjusting the N2 flow rate or Ti substrate temperature during the deposition process via DC (direct current) reactive magnetron sputtering. Small N2 flow rate of 3 to 6 sccm or low substrate temperature (e.g., 25 °C) facilitates the growth of TiN films along the (111). The (111) preferred orientated TiN films show much lower interfacial contact resistance (ICR) than the (200) preferred orientated films. A considerably low ICR value of 1.9 mΩ·cm2 at 140 N/cm2 is achieved at the N2 flow of 4 sccm and the substrate temperature of 25 °C.

High-quality transparent conductive indium oxide film deposition by reactive pulsed magnetron sputtering: Determining the limits of substrate heating

A transparent conductive oxide (TCO) should have a combination of high electrical conductivity and optical transmission property to fulfill the challenging demands of industrial applications. So far, doped TCOs have been mostly considered to fulfill the technological challenges. In this study, we demonstrate that indium oxide (In2O3) thin films without intentional doping can be deposited with high crystallinity under specific film growth conditions leading to thin films with high mobility, high electrical conductivity, and high transmittance. In2O3 thin films have been deposited by reactive pulsed direct current magnetron sputtering (pulsed DCMS) from an indium target on substrates in a temperature range from room temperature (RT) to 600 °C. Detailed investigations on In2O3 thin films are performed and the film properties such as crystallinity, microstructure, chemical bonding states, electrical and optical properties are revealed. Enhanced plasma density and ionization degree provided by the employed reactive pulsed DCMS and the intentional substrate heating during the thin film deposition give possibility to deposit highly crystalline thin films which are preferentially oriented in (222) direction. The substrate heating enhances the crystallinity of the grown films up to a certain optimum temperature: 400 °C. When the substrate temperature is above 400 °C, the carrier concentration and mobility decrease due to the grain refinement effect caused by growth competition of grains in different orientations. The films grown at 400 °C indicate the presence of high oxygen vacancy concentrations, which can directly be associated with a high charge carrier concentration despite the lack of intentional doping. The undoped In2O3 films in this study grown at 400 °C show highly promising and competitive electrical properties such as low resistivity of 1.28 × 10-4 Ω⋅cm and the high carrier mobility of 69 cm2/Vs. The average transmittance of the In2O3 films with the highest conductivity is found to be greater than 80% in the visible to the near-infrared spectral region owing to an enhancement in the carrier mobility and an optical bandgap in the range from 3.61 eV to 3.77 eV.

Effect of sodium doping on characteristics of p-SnOx films prepared by reactive direct current magnetron sputtering

Conventionally, p-type semiconducting oxide films of high optical transmission and good electric conduction are materials of great demand in electronic and optoelectric devices. They can conveniently be integrated with their n-type counterparts for formation of homo or heterojunctions. In this report, Na-doped SnOx films were fabricated by reactively sputtering the Sn + Na alloy targets at substrate temperature of 200 °C. Effect of Na dopant concentration on crystal structure, electric and optical properties of Na-doped SnOx films was investigated with X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy, Hall and UV–Visible transmission spectroscopy. The obtained results revealed that when Na + substituted Sn2+, the hole concentration increased but the crystallite size reduced, leading to increased grain boundary and reduced carrier mobility. However, electric resistivity still reduced thanks to the increase of carrier concentration. Averange optical transmission of Na-doped SnOx films in visible range was approximately 65%. In addition, optical bandgap value reduced as Na doping amount in those films increased. The Na-doped SnOx films sputtered from Sn + 3 at% Na alloy targets, corresponding to 1.56 at% Na existing in films, presented the best electric property (nH = 2.75 × 1019cm−3, μ = 2.4 cm2/VS, ρ = 9.35 × 10−2Ωcm). This film proceeded with diode structure of glass/FTO/n-nc-Si:H/p-SnOx:Na/Cu, resulted in the best characteristics (VOC = 0.59 V, n = 3.57, rectifying ratio = 22, Rs = 38.1 Ω).

Reactive Magnetron Plasma Modification of Electrospun PLLA Scaffolds with Incorporated Chloramphenicol for Controlled Drug Release.

Surface modification with the plasma of the direct current reactive magnetron sputtering has demonstrated its efficacy as a tool for enhancing the biocompatibility of polymeric electrospun scaffolds. Improvement of the surface wettability of materials with water, as well as the formation of active chemical bonds in the near-surface layers, are the main reasons for the described effect. These surface effects are also known to increase the release rate of drugs incorporated in fibers. Herein, we investigated the effect of plasma modification on the chloramphenicol release from electrospun poly (lactic acid) fibrous scaffolds. Scaffolds with high—50 wt./wt.%—drug content were obtained. It was shown that plasma modification leads to an increase in the drug release rate and drug diffusion coefficient, while not deteriorating surface morphology and mechanical properties of scaffolds. The materials’ antibacterial activity was observed to increase in the first day of the experiment, while remaining on the same level as the unmodified group during the next six days. The proposed technique for modifying the surface of scaffolds will be useful for obtaining drug delivery systems with controlled accelerated release, which can expand the possibilities of local applications of antibiotics and other drugs.

Epitaxial Growth of CaMnO3–y Films on LaAlO3(112¯0) by Pulsed Direct Current Reactive Magnetron Sputtering

CaMnO3 is a perovskite with attractive magnetic and thermoelectric properties. CaMnO3 films are usually grown by pulsed laser deposition or radio frequency magnetron sputtering from ceramic targets. Herein, epitaxial growth of CaMnO3–y (002) films on a ‐oriented LaAlO3 substrate using pulsed direct current reactive magnetron sputtering is demonstrated, which is more suitable for industrial scale depositions. The CaMnO3–y shows growth with a small in‐plane tilt of <≈0.2° toward the (200) plane of CaMnO3–y and the () with respect to the LaAlO3 substrate. X‐ray photoelectron spectroscopy of the electronic core levels shows an oxygen deficiency described by CaMnO2.58 that yields a lower Seebeck coefficient and a higher electrical resistivity when compared to stoichiometric CaMnO3. The LaAlO3 substrate promotes tensile‐strained growth of single crystals. Scanning transmission electron microscopy and electron energy loss spectroscopy reveal antiphase boundaries composed of Ca on Mn sites along <101> and <002>, forming stacking faults.

Effect of the sputtering time on the characteristics of p-SnOx:Na films prepared by the reactive direct current magnetron sputtering method

The fabrication of n- and p-type transparent oxide semiconductor films is critical in the production of transparent electrical and optoelectronic components on the p‒n junction, particularly the p-type, because the maximum of the valence band formed by the 2p orbitals of oxygen and their local (narrow) dispersions have formed deep levels, making difficult in doping of acceptor impurities in p-type oxide semiconductors. Among the p-type transparent oxide semiconductor films that have been studied so far, the p-type SnOx film which had outstanding electrical conductivity and optical transparency, is a good candidate for constructing a p‒n junction with another n-type semiconductor in homogeneous or heterogeneous form. In this paper, a Na-doped SnOx film was produced from Sn + 3 at% Na alloy target using the reactive sputtering process at a temperature of 200 0C. XRD, XPS, AFM, Hall, and UV-Visible techniques were used to evaluate the effect of sputtering duration on the structural and electro-optical properties of SnOx:Na films. The results of the survey showed that when the sputtering time increased from 7 minutes to 14 minutes, 21 minutes, and 35 minutes, the film thickness increased, resulting in an increase in the crystal size, a decrease in the grain boundary, and an increase in carrier mobility, all of which lead to a reduction in the film's resistivity. However, as the film thickness increased, the transmittance dropped and the optical bandgap shrinked.