Thin Films Of Various Thicknesses Research Articles

Unveiling a critical thickness in photocatalytic TiO2 thin films grown by plasma-enhanced chemical vapor deposition using real time in situ spectroscopic ellipsometry

TiO2 thin films of various thicknesses have been deposited on silicon at low-temperature by PECVD operating in continuous mode (T < 130 °C) and pulsed mode (T < 80 °C) using oxygen/titanium isopropoxide low-pressure inductively coupled plasma. The study of the crystallinity and microstructure of the films by atomic force microscopy, scanning electron microscopy and x-ray diffraction allowed showing that the roughness and amount of anatase are closely related to the film thickness. The coalescence of large polycrystalline columns emerging from an assembly of thin columns was found to happen at a critical thickness, at about 150 nm in the continuous mode and 250 nm in the pulsed mode. Real time in situ spectroscopic ellipsometry study allowed to monitor the growth and to determine this critical thickness for both plasma modes. The change of morphology type to large columnar structure is associated with an important increase in the photocatalytic activity. The determination of this coalescence thickness by ellipsometry may provide an interesting method to evaluate the impact of process parameters on TiO2 thin films characteristics.

Roughening and sputtering kinetics of Pt thin films at different angles of ion irradiation

Platinum (Pt) thin films of various thicknesses (tPt = 2–6 nm) deposited on Si(100) surface have been investigated for nanostructures growth under low energy ion irradiation at different angles of incidence. The resultant surface studies show that angle-dependent sputtering and larger tPt favor nanostructures’ formation. Stopping and Range of Ions in Matter (SRIM) simulation, Rutherford backscattering spectrometry (RBS) and kinetic roughening analysis along with scaling components (growth and roughness) predicts that the sputtering and surface diffusion favors surface nanostructuring for thick films at higher angle irradiations. This systematic observation of angle-dependent nanostructuring is important for surface engineering of thin films-based devices.

Optical, electronic, and microstructural properties of mixed phase RF magnetron sputtered vanadium oxide thin films on quartz and aluminized quartz

Amorphous vanadium pentoxide (major phase) thin films of various thicknesses were grown on the substrates that possessed low (aluminized quartz) and high (quartz) infrared (IR) emittance property utilizing radio frequency (RF) magnetron sputtering technique. Microstructural characterizations are carried out by several techniques such as noncontact optical profilometry, energy dispersive X‐ray (EDX) spectroscopy, X‐ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and X‐ray diffraction (XRD). The as‐grown amorphous films show stoichiometric vanadium oxide, where vanadium is in V5+ as a major proportion compared with V4+ states. Thermo‐optical properties, namely, solar transmittance (τs), reflectance (ρs), absorptance (αs) and IR emittance (εir) along with optical constant, for example, optical band gap, are evaluated. A modified Urbach equation was used to simulate the optical absorption data to understand the presence optical absorption below the band edge.

DC electrical conductivity and resistive memory switching in Se36 Sb31Cu33 films

Se36 Sb31Cu33 non - ordered material was synthesized by the melt-quenching procedure. Thin films of various thicknesses (125–520 nm) were obtained by thermal evaporation procedure. The DC conductivity and switching phenomenon were examined in the temperature range (293–373 K) lower than the corresponding glass transition temperature Tg. The conduction activation energy own dual values ∆Eσ1 and ∆Eσ2. The concluded outcomes of DC conductivity are interpreted in accordance with Mott and Davis model. The switching phenomenon for Se36 Sb31Cu33 was of the memory behavior. The mean value of the threshold voltage V¯th rises with raising film thickness in the examined range and reduces with enhances temperature in the corresponding examined range. The outcome results agree with the thermal model for the switching process.

Thickness dependent structural and surface properties of plasma polymerized N-benzylaniline thin films

Thickness dependent structural, surface morphological, topographical and compositional studies have been done on the plasma polymerized N-benzylaniline (PPNBA) thin films, where the plasma is obtained by a parallel plate capacitively coupled glow discharge system under AC of frequency 50 Hz. From the X-ray diffraction studies, it is confirmed that the PPNBA thin films are amorphous in nature. The chemical structure of the PPNBA films is moderately changed from that of the monomer owing to the elimination of some benzyl groups, however, becomes conjugated due to C=C stretching of the quinoid and benzenoid units. The PPNBA films contain mostly carbon and nitrogen, which is confirmed by the energy-dispersive X-ray analysis. However, the presence of oxygen is considered to be incorporated from the reactor atmosphere. The surface morphologies of the PPBNA thin films of various thicknesses are mosaic-like structure, which is formed by the uniform distribution of unequal nanosized clusters over the smooth background of the film surface. The average roughness and root mean square roughness of the surface of the PPNBA thin films are found to be in the order of pm. The increase in the surface roughness is compatible with the increase in the cluster density, which is evidenced in the field-emission scanning electron microscopy observations.

Composite Metallic Nano Emitters Coated with a Layer of Insulator Covered by Au Layer

Abstract: In this work, the differences in the behavior and properties of the emitted electron beam from tungsten (W) tips were studied before and after coating these tips with a thin layer of dielectric material followed by a thin layer of gold, to improve the emission current density, stability and emission current pattern concentration. The core of the composite cathode is made of high-purity tungsten (W). Measurements have been made with clean W emitters before and after coating these tips with two types of epoxy resins (epoxy 478 resins or epoxy UPR- 4 resins) followed by a thin layer of gold. For critical comparison, several tungsten tips with various apex-radii have been prepared using electrochemical etching techniques. The emitters have been coated by dielectric thin films of various thicknesses and the layer of Au used for coating the Epoxy layer has the same thicknesses. Their behavior has been recorded before and after the process of coating. These measurements include the current-voltage (I-V) characteristics and Fowler-Nordheim (F-N) plots. Imaging has also been done using a visible light microscope (VLM), along with a scanning electron microscope (SEM) to help in characterizing the epoxy layer thickness on the tip surface after coating. Besides, the emission patterns have been recorded from the phosphorescent screen of a field electron emission microscope (FEM). Having two types of composite systems tested under similar conditions provided several advantages. These measurements helped in producing a new type of emitters that have more suitable features with each of the two resins. Keywords: Cold field emission, Nano emitter, Dielectric coating, Au layer.

Analysis of the Various Effects of Coating W Tips with Dielectric Epoxylite 478 Resin or UPR-4 Resin Coatings under Similar Operational Conditions

Abstract: The objective of this work is to study the differences that occur in behavior and properties of the emitted electron beam from tungsten (W) tips before and after coating these tips with a thin layer of a good proven dielectric material. Core metallic tips have been prepared from a polycrystalline (99.995% purity) tungsten (W) wire. Analysis has been carried out for clean W emitters before and after coating these tips with two differences types of epoxy resins; namely: (Epoxylite 478 and UPR-4). For critical comparison and analysis, several tungsten tips with various apex- radii (very sharp) have been prepared with the use of electrochemical etching techniques. The tips have been coated by dielectric thin films of various thicknesses. Their characteristics have been recorded before and after the process of coating. These measurements have included the current-voltage (I-V) characteristics, Fowler-Nordheim (F-N) plots, visible light microscope (VLM) image and scanning electron microscope (SEM) micrographs to measure the influence of the Epoxylite resin coating’s thickness on the tips after coating. Special distributions have been recorded from the phosphorescent screen of a field electron emission microscope as well. Comparing the two sets of composite systems tested under similar conditions has provided several advantages. Recording highly interesting phenomena has produced a wide opportunity to develop a new type of emitter that includes the most beneficial features of both types. Keywords: Cold field emission, Epoxylite 478, Epoxylite UPR-4.

Surface engineering of Pt thin films by low energy heavy ion irradiation

A systematic study of nanostructuring of platinum (Pt) thin films of different thicknesses deposited on silicon substrates has been performed under the irradiation of low energy Kr+ ions. First, Pt thin films of various thicknesses (~2–6 nm) were grown on the Si substrates, which were subsequently irradiated with 100 keV Kr+ ions as a function of ion fluence. The resultant surface modifications after irradiation are studied by atomic force microscopy (AFM), which reveals that higher ion fluence and larger thickness of Pt thin films are more favorable for nanostructures formation. Surface growth features have been analysed using kinetic roughening of surfaces. Scaling components (roughness and growth components) reveals that the sputtering is the principal factor causing surface nanostructuring in platinum (Pt) thin films. For higher thicknesses ion beam induced adatoms diffusion is favoring the surface roughning. The mechanism of nanostructuring on different thicknesses of Pt films has been analyzed using Rutherford backscattering spectrometry (RBS) study and SRIM-TRIM calculations. For higher thickness of Pt film, irradiating projectile ions results in high energy deposition in the film. Thus, thickness-dependent sputtering yield and adatoms surface diffusions play an important role in nanostructuring of Pt thin films under ion irradiation.

Ferroelectric polarization induced memristive behavior in bismuth ferrite (BiFeO3) based memory devices

Bismuth ferrite (BiFeO 3 , BFO) thin films of various thickness were deposited using spray pyrolysis technique at 673 K. The structural studies of the prepared BFO thin films show a rhombohedral structure with R3c space group. The morphological analysis reveals that prepared BFO thin film samples exhibit a uniform and less porous grains. Different thickness of BFO thin films (174–808 nm) was stacked as an active layer between aluminum (Al) and indium doped tin oxide (ITO) electrodes . With the optimum thickness of the BFO intermediate layer, the polarization-electric field (P-E) loop analysis of the ITO/BFO/Al device showed a hysteresis loop with remnant polarization (P r ) of 0.21 μC/cm 2 and coercive field (E c ) of 33.4 kV/cm respectively. The impact of the BFO switching layers with varying thickness towards control of oxygen vacancy (V O ) defects and the effective role of polarization induced resistive switching (RS) characteristics were explored. • Realization of BFO resistive switching layers by cost-effective spray pyrolysis technique. • XPS analysis of different thickness of BFO layers to study the control of oxygen vacancy (V O ) defects. • The effective role of polarization in BFO layers towards resistive switching (RS) characteristics were explored.

Effect of Acid Treatment in Dye Loading Capacity of Spin Coated ZnO Film

In recent times, the research on ZnO is growing tremendously, because of its potential use in many applications such as optical devices, dye sensitized solar cell (DSSC), gas sensors and biomedical. ZnO thin films of various thicknesses were prepared by the spin coating method using the zinc acetate precursor with diethanolamine and ethanol. The structural and optical characterizations of as-prepared ZnO films carried out using XRD and UV-Vis spectrophotometer, respectively. The XRD results showed polycrystalline wurtzite structure of ZnO. The average crystallite size of ZnO as calculated using Debye Scherrer’s formula was 26 nm. The optical band gap of ZnO was found to decrease with film thickness. The dye extracted from the leaves of Tectona Grandis (Sagun) which possesses a high and wide absorbance was used in this experiment. The study on the effect of acid treatment on ZnO films in dye loading showed enhanced absorbance in acid treated ZnO as compared to untreated ZnO.

Dispersity effects on phase behavior and structural evolution in ultrathin films of a deuterated polystyrene-block-poly(methyl methacrylate) diblock copolymer

We report the unique phase behavior and structural transitions in thin films of a distorted network (DNW) nanostructure, which is an equilibrium phase contained in the bulk of a nearly-symmetric deuterated polystyrene-block-poly (methyl methacrylate), dPS-b-PMMA, block copolymer. The DNW is due to a broad dispersity for the PMMA block. Grazing incidence small-angle X-ray scattering, neutron reflectometry, atomic force microscopy and transmission electron microscopy techniques were used to characterize thermally-annealed dPS-b-PMMA ultrathin films with neutral boundaries at both the free surface and substrate interface. Brief thermal annealing imposed on the ultrathin films leads to perpendicular lamellae (L⊥). Upon increasing the duration of thermal annealing, the dPS-b-PMMA ultrathin films exhibit thickness dependence of phase transition kinetics and different irreversible routes of phase transition. As a result, the protracted thermal annealing of the thin films of various thicknesses produces either hexagonal arrays of perpendicular PMMA cylinders (HEX-C⊥) or disordered PMMA nanostructures (DIS). The phase transition routes and kinetics in ultrathin films suggest various possibilities to obtain diverse nanoscale patterns via thin film self-assembly.

Spin Pumping and Magnetic Anisotropy in La2/3Sr1/3MnO3/Pt Systems

La2/3Sr1/3MnO3 (LSMO) thin films of various thicknesses (6, 8, 10, 20, and 30 nm), capped by 7 nm‐thick Pt layer, are grown by pulsed laser deposition on SrTiO3 (001) substrates. X‐ray diffraction revealed that LSMO films are (001) oriented. Vibrating sample magnetometer is used to determine the magnetization at saturation and the magnetic dead layer thickness. This latter is around 3.4 nm, significantly thicker compared with the one induced at interfaces of Pt with ferromagnetic transition metals. Microstrip line ferromagnetic resonance (MS‐FMR) is used to extract the gyromagnetic ratio, which is found to increase with LSMO thickness. MS‐FMR revealed that the in‐plane magnetic anisotropy is dominated by a uniaxial contribution for the Pt capped film, whereas the noncapped 10 nm‐thick LSMO layer shows a fourfold anisotropy. Furthermore, the thickness dependence of the effective magnetization reveals the existence of a second‐order perpendicular anisotropy term, which is thickness‐dependent, and of a weak uniaxial interface anisotropy. The Gilbert damping coefficient is found to vary linearly with the inverse of the effective LSMO thickness due to spin pumping leading to relatively low spin mixing conductance of LSMO/Pt interface.

A Clean and Tunable Mussel-Inspired Coating Technology by Enzymatic Deposition of Pseudo-Polydopamine (ψ-PDA) Thin Films from Tyramine.

The tyrosinase-catalyzed oxidation of tyramine, leading to the deposition of pseudo-polydopamine (ψ-PDA) thin films, is disclosed herein as a superior technology for surface functionalization and coating at a neutral pH and at a low substrate concentration, compared to the standard autoxidative PDA coating protocols. Smooth ψ-PDA thin films of variable thickness up to 87 nm were obtained from 1 mM tyramine by varying tyrosinase concentrations (5–100 U/mL). Compared to the PDA films obtained by the similar enzymatic oxidation of 1 mM dopamine with tyrosinase (T-PDA), ψ-PDA displayed slower deposition kinetics, lower water contact angles in the range of 11°–28°, denoting higher hydrophilicity but similar UV-vis absorption profiles, as well as electrochemical properties and antioxidant activity. MALDI-MS analysis indicated for ψ-PDA a well defined pattern of peaks compatible with dopamine tetrameric structures degraded to a variable extent. The exposure to a tyramine solution of tyrosinase-loaded alginate spheres, or films deposited on glass or polyethylene, resulted in a rapid gel-confined ψ-PDA formation with no leakage or darkening of the solution, allowing the complete recovery and re-utilization of the unreacted tyramine. In contrast, an abundant PDA precipitation outside the gel was observed with dopamine under the same conditions. The ψ-PDA deposition by tyrosinase-catalyzed tyramine oxidation is thus proposed as a controllable and low-waste technology for selective surface functionalization and coating or for clean eumelanin particle production.

Domain Size Dependence of the Oxygen Reduction Reaction on (100)-Oriented Au Thin Films

Due to its importance for energy conversion and storage, the oxygen reduction reaction (ORR) is one of the most studied electrochemical reactions. It has been reported that, in alkaline media, Au(100) single crystal is the most active electrocatalyst for the ORR1,2. Different reasons have been proposed to explain this high activity3,4. Understanding this remarkable activity can guide researchers in the design of more active electrocatalysts for practical applications using nanomaterials with specific shape and size. Since the ORR is a structure-sensitive reaction5–7 the manipulation of the structural properties of the materials is a powerful tool to elucidate the reaction mechanism responsible for the ORR and increase the activity of materials towards the reaction.In this work, (100)-oriented Au thin films were deposited on (100) MgO. Thin films with different thicknesses were prepared by pulsed laser deposition and were used as model surfaces to study the ORR activity. Pulsed Laser Deposition (PLD) is a versatile method to prepare thin films with different surface orientations (epitaxial thin films)8,9 , 10. The structure and microstructure of the films were assessed by X-Ray Reciprocal Space Mapping (RSM) and Atomic Force Microscopy (AFM). There results were correlated with those obtained from Cyclic Voltammetry (CV) and Sampled Current Voltammetry (SCV).The ORR activity was observed to be dependent on the film thickness, which is reflected by the shift of the onset and half-wave potentials of the ORR towards more positive potentials for the thicker films. When correlating the ORR activity with the structural and microstructural properties, Fig.1, the evidences suggest that larger lateral coherent lengths are responsible for the enhancement of the ORR catalysis on (100)-oriented Au thin films. The reasons underlying this behavior will be presented and discussed. Figure 1: Plot showing the variation of the onset and half-wave potential, and the variation of the coherence length for thin films of various thicknesses. References (1) Stamenkovic, V. R.; Strmcnik, D.; Lopes, P. P.; Markovic, N. M. Nature Materials. 2016, pp 57–69.(2) Markovic, N. M.; Tidswell, I. M.; Ross, P. N. Langmuir 1994, 10 (1), 1–4.(3) Duan, Z.; Henkelman, G. ACS Catal. 2019, 9, 5567–5573.(4) Schneider, O. Size Dependent Electrocatalysis of Gold Nanoparticles; Elsevier, 2018.(5) Adžić, R. R.; Marković, N. M.; Vešović, V. B.; Adzic, R. R.; Markovic, N. M.; Vesovic, V. B. J. Electroanal. Chem. 1984, 165 (1–2), 105–120.(6) Markovic, N. M.; Gasteiger, H. A.; Ross, P. N. J. Phys. Chem. 1995, 99 (11), 3411–3415.(7) Schmidt, T. J.; Stamenkovic, V.; Arenz, M.; Markovic, N. M.; Ross, P. N. Electrochim. Acta 2002, 47 (22–23), 3765–3776.(8) Sacré, N.; Hufnagel, G.; Galipaud, J.; Bertin, E.; Hassan, S. A.; Duca, M.; Roué, L.; Ruediger, A.; Garbarino, S.; Guay, D. J. Phys. Chem. C 2017, 121 (22), 12188–12198.(9) Garbarino, S.; Imbeault, R.; Sacré, N.; Guay, D. In Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry; 2017.(10) Imbeault, R.; Reyter, D.; Garbarino, S.; Roué, L.; Guay, D. J. Phys. Chem. C 2012, 116 (8), 5262–5269. Figure 1

Thickness determination of porous Pt cathode thin film capped by atomic layer-deposited alumina for low-temperature solid oxide fuel cells

This study investigated the effects of the apparent thickness of porous Pt cathode thin films thermo-mechanically stabilized by an atomic layer-deposited (ALD) alumina capping agent on effective electrical conductivity and electrochemical performance. Four-point probe resistance measurements revealed that a 120 nm-thick porous Pt film exhibits a relatively high electrical conductivity of 9.2 × 103 S/cm. Cathode reaction resistance measurements by means of alternating current impedance spectroscopy (ACIS) analysis show that ALD alumina can act as a thermo-mechanical stabilizer for a 120 nm-thick porous Pt cathode film. The ACIS analysis results for solid oxide fuel cells (SOFCs) with porous Pt cathode thin films of various thicknesses showed that a 120 nm-thick porous Pt cathode film is thermo-mechanically stabilized by the conformal coating of an ALD alumina capping agent. The iR-free polarization curves show that an ALD alumina-capped 120 nm porous Pt cathode film-embedded SOFC delivers ~65% higher power density than an ALD alumina-capped 480 nm-thick porous Pt cathode film-embedded SOFC.

Microstructured Biodegradable Fibers for Advanced Control Delivery

AbstractBiodegradable polymers are increasingly employed at the heart of therapeutic devices. Particularly in the form of thin and elongated fibers, they offer an effective strategy for controlled release in a variety of biomedical configurations such as sutures, scaffolds, wound dressings, surgical or imaging probes, and smart textiles. So far however, the fabrication of fiber‐based drug delivery systems has been unable to fulfill significant requirements of medicated fibers such as multifunctionality, adequate mechanical strength, drug loading capability, and complex release profiles of multiple substances. Here, a novel paradigm in the design and fabrication of microstructured biodegradable fibers with tailored mechanical properties and capable of predefined release patterns from multiple reservoirs is proposed. Different biodegradable polymers compatible with the scalable thermal drawing process are identified, and their release properties as thin films of various thicknesses in the fiber form are experimentally investigated and modeled. Multimaterial microstructured fibers with predictable complex release profiles of potentially different substances are then designed and fabricated. Moreover, the tunability of the mechanical properties via tailoring the drawing process parameters is demonstrated, as well as the ability to weave such fibers. This work establishes a novel platform for biodegradable microstructured fibers for applications in implants, sutures, wound dressing, or tissue scaffolds.

Impact of material migration and radiation damage on the reflectivity of molybdenum mirrors: laboratory test for DEMO

Molybdenum mirrors were irradiated with high-Z (Mo) and low-Z (He) ions to simulate the effect of plasma exposure on diagnostic mirrors to be applied in D-T fusion devices. Selected mirrors before irradiation were additionally coated with tungsten thin films of various thicknesses to simulate the real conditions in the reactor: tungsten erosion and migration. Irradiations were followed by reflectivity measurements and detailed microscopy observations. The threshold dose for the degradation of optical performance by 4He+ irradiation is below 0.1 × 1017 cm−2. The presence of tungsten films (4 and 10 nm in thickness) decreases total reflectivity by 30%–40% in the short wavelength and 5% (4 nm-thick film) and 12% (10 nm-thick film) in the infrared. Ion irradiation further worsens reflectivity by 5%–10%. The size of blisters visible on the surface of mirrors after irradiation depends on the thickness of tungsten films. Helium content after irradiation with helium ions of a dose 6 × 1017 cm−2 decreased during two years by 70% from the value measured directly after the implantation.

Probing the outermost layer of thin gold films by XPS and density functional theory

This work examines gold thin films of various thicknesses varying from 5 to over 100 nm along a 7 cm distance to investigate the effect on the surface sensitivity of the XPS Au4f signal. Increasing the relative intensity of the surface XPS signal can be useful for studying properties of the surface atoms in case of functionalized gold, and for shedding light on reactions occurring at the interface with the metal surface as well as adsorption geometry.The surface-to-bulk signal ratio increased with decreasing thickness for thicknesses greater than 15 nm. Below this value, the intensity of the surface signal decreased. The binding-energy shift of the surface component with respect to the bulk component was found to be in agreement with that calculated by density functional theory (DFT). The calculated shift was found to be essentially unaffected, whether or not final-state effects were included. KPFM was also performed to investigate correlations between the measured work function and the surface/bulk BE shift, revealing that the contact potential difference changes with thickness because of the influence of the substrate.Gradients were prepared by a novel evaporation method, by template stripping, and by photolithography, yielding stripes of increasing thickness in the same sample.

Fabrication and estimation of optical constants of hydrophobic Ta2O5 thin films

Present study investigates the optical properties and film thickness estimation of tantalum oxide (Ta2O5) thin films of various thicknesses, deposited by physical evaporation technique. The optical constants and thickness of deposited films were estimated by using envelope method. The thickness value obtained from this method has been compared with in-situ (quartz crystal thickness monitor) as well as Ex-situ (Coherence Correlation Interferometry) thickness estimation techniques. Refractive index calculated by envelope method was also validated using Reflectometer. These thickness values were used for the estimation of other optical constants such as refractive index, absorption and extinction coefficients. Contact angle measurement studies are also performed on uncoated as well as Ta2O5 coated glass substrates, which indicate the hydrophobic nature of these films.

Investigating the effect of thickness on the structural, morphological, optical and electrical properties of AgBiSe2 thin films

Herein, a one-step thermal evaporation was used for fabrication of silver bismuth selenide (AgBiSe2) thin films of various thickness. The influence of thickness on the structural, optical and electrical properties of AgBiSe2 thin films has been studied. The X-ray diffraction confirmed single hexagonal structure of deposited AgBiSe2 thin films. The average grain sizes D and microstrain ε of deposited AgBiSe2 films were estimated by Williamson-Hall (W-H) relation. The grain size increases from 11.52 to 29.12 nm as the thickness was increased from 150 to 550 nm, whereas strain dislocation density (ξ) and the number of crystallites N tends to decrease with thickness. The selected area electron diffraction pattern (SAED) obtained by high resolution transmission electron microscopy (HRTEM) verified the polycrystalline nature of samples and it is found to nearly agree with X-ray diffraction data. The elemental composition measurement of the as-deposited AgBiSe2 thin film showed that the as-deposited film is near stoichiometric in compound. The influence of film thickness on the optical parameters of the AgBiSe2 thin films, such as refractive index n, extinction coefficient k, real dielectric ε1, imaginary dielectric constant ε2, skin depth and the optical band gap, Eg has been studied. It is found that the values of energy gap decrease from 1.88 to 1.68 eV with increasing thickness, while the values of Urbach energy follow an opposite trend. The electrical conductivity was measured by four-probe experiment in the range from 300 to 500 K. The analysis of dc conductivity data revealed the presence of two distinct regions with two various slopes confirming the transformation from hexagonal phase to rhombohedral phase at about 460 K.