Optical, electrical and structural properties of Cu2Te thin films deposited by magnetron sputtering

Thin films of Cu₂Te were deposited, at room temperature, on glass substrates by magnetron sputtering from independent Cu and Te sources. This work presents the effect of annealing temperature on the optical, structural, and electrical properties of sputtered Cu₂Te films. Annealing above 300 °C resulted in stoichiometric and near stoichiometric Cu₂Te phases, whereas temperatures above 400 °C yielded films with single Cu₂Te phase. In contrast, annealing at temperatures of 250 °C and below resulted in mixed phases of CuTe, Cu₇Te₅, Cu_1.8Te, and Cu₂Te. Analyses of transmittance and reflectance measurements for Cu₂Te indicate that photon absorption occurs via indirect band transitions for incident photons with energy above the band gap energy and free carrier absorption below the band gap energy. The determined indirect band gap was 0.90 eV and its associated phonon energy was 0.065 eV. Optical phonon scattering was identified as the mechanism through which the momentum is conserved during absorption by free carriers. Electrical measurements show p-type conductivity and highly degenerate semiconducting behavior with a hole carrier concentration p = 5.18 × 10²¹ cm^− 3.     

Composition dependence of structural and optical properties of Ba(Zrx,Ti1-x)O3 thin films grown on MgO substrates by pulsed laser deposition

Ba(Zr_x,Ti_1-x)O₃ (BZT) films with Zr concentration ranging from 0 to 40% were grown on MgO single crystal substrates by pulsed laser deposition, and their optical properties in the visible range were systematically characterized. A linear increase in the out-of-plane lattice constant of BZT unit cell with increasing Zr content was detected by X-ray diffraction. The surface morphology was observed by atomic force microscopy and the grain size was shown to increase with Zr concentration. Prism coupling and UV-visible transmission spectroscopy techniques were used to analyze the optical properties of the films. Refractive index between 2.15 and 2.3 was observed at 633 and 1547 nm respectively, which decreased with rising Zr content. The BZT films also possessed large optical band gap energy up to 3.92 eV, which increased with rising Zr content. Quadratic electro-optic effect was observed with electro-optic coefficients between 0.11 and 0.81 × 10^− 18 m²/V², which decreased with Zr concentration. Optical loss was estimated from scattering and absorption, and the absorption coefficient dropped with increasing Zr content at near band gap. The obtained results provide information for the design of BZT thin film-based optical devices.     

Effect of post-annealing on the optoelectronic properties of ZnO:Ga films prepared by pulsed direct current magnetron sputtering

In this study, highly conductive films of ZnO:Ga (GZO) were deposited by pulsed direct current magnetron sputtering to explore the effect of post-annealing on the structural, electrical and optical properties of the films. XRD patterns showed that after annealing, the intensity of c-axis preferentially oriented GZO (002) peak was apparently improved. GZO film annealing at 300 °C for 0.5 h exhibits lowest resistivity of 1.36 × 10^− 4 Ω cm. In addition, the film shows good optical transmittance of 88% with optical band gap, 3.82 eV. Carrier concentration and optical band gap both decreases with the annealing temperature. Besides, the near-infrared transmittance at 1400 nm is below 5%, while the reflectivity at 2400 nm is as high as 70%.     

Half wave rectification of inorganic/organic heterojunction diode at the frequency of 1 kHz

An inorganic/organic vertical heterojunction diode has been demonstrated with p-type Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) deposited by spin coating on n-type Ga-doped ZnO (GZO) thin films. Transparent conducting GZO thin films are deposited on glass substrate by rf-magnetron sputtering. Electrical properties of GZO thin films are investigated depending on the processing temperatures. The resistivity, mobility and carrier concentration of the GZO thin films deposited at processing temperatures of 500 °C are measured to be about 3.6 × 10⁻⁴ Ω cm, 23.8 cm²/Vs and 7.1 × 10²⁰ cm³, respectively. The root mean square surface roughness of the GZO thin films is calculated to be ~ 0.9 nm using atomic force microscopy. Current-voltage characteristics of the n-GZO/p-PEDOT:PSS heterojunction diode present rectifying operation. Half wave rectification is observed with the maximum output voltage of 1.85 V at 1 kHz. Low turn-on voltage of about 1.3 V is obtained and the ideality factor of the n-GZO/p-PEDOT:PSS diode is derived to be about 1.8.     

Effect of organic-buffer-layer on electrical property and environmental reliability of Ga-doped ZnO films prepared by RF plasma assisted DC magnetron sputtering on plastic substrate

Ga-doped ZnO (GZO) transparent conductive films have been prepared by RF plasma assisted DC magnetron sputtering under a reductive atmosphere on organic-buffer-layer (OBL) coated polyethylene telephthalate (PET) substrates without intentionally heating substrates. Electrical and optical properties, crystallinity, and environmental reliability of the GZO films have been investigated. The distributional characteristic of resistivity is observed in the GZO film deposited on the OBL-coated PET substrates. The high resistivity at facing the erosion area in the source target is reduced by providing the RF plasma and H₂ gas near the substrate, resulting in a uniform distribution of the sheet resistance. It has been also found that the increase of resistivity by an accelerated aging test performed under a storage condition at 60 °C and at a relative humidity of 95% is suppressed by employing the OBL. The OBL suppresses the formation of cracks, which are induced by the aging test. These facts are thought to contribute to a high environmental reliability of GZO films on PET substrates. Values of resistivity, Hall mobility and carrier concentration are obtained: 5.0-20 × 10⁻³ Ω cm, 4.0 cm²/Vs, and 3.8 × 10²⁰ cm⁻³, respectively. An average transmittance of the GZO film including OBL and PET substrate is 78% in a visible region. The OBL enables to realize the practical use of GZO films on PET sheets.     

Effect of deposition distance and temperature on electrical, optical and structural properties of radio-frequency magnetron-sputtered gallium-doped zinc oxide

Films of gallium-doped zinc oxide (GZO) were deposited on glass substrates by radio-frequency magnetron sputtering using a ceramic target of Ga:ZnO (4 at.% Ga vs. Zn). Both the substrate temperature (T_s) and the target-substrate distance (d_ts) were varied and the effect on electrical, optical and structural properties of the resulting films were measured. The highest conductivity of 3200 S/cm was obtained at a deposition temperature of 250 °C, at a d_ts of 51 mm. This sample had the highest carrier concentration in this study, 9.6 × 10²⁰/cm³. Optical transmittance of all films was <90% in the visible range. The grain size of the film grown at d_ts = 51 mm was smaller than the grain size for films grown with a shorter d_ts; moreover, the films with d_ts = 51 mm exhibited the smoothest surface, with a root mean square surface roughness of 2.7 nm. Changes in T_s have a more pronounced effect on conductivity compared to changes in d_ts; however, variations in structure do not appear to be well-correlated with conductivity for samples in the 2000-3200 S/cm range. These results suggest that incorporation and activation of Ga is of key importance when attempting to obtain GZO films with conductivities greater than 2000 S/cm.     

Thermally stable, highly conductive, and transparent Ga-doped ZnO thin films

Highly conductive and transparent films of Ga-doped ZnO (GZO) have been prepared by pulsed laser deposition using a ZnO target with Ga₂O₃ dopant of 3 wt.% in content added. Films with resistivity as low as 3.3 × 10^− 4 Ω cm and transmittance above 80% at the wavelength between 400 and 800 nm can be produced on glass substrate at room temperature. It is shown that a stable resistivity for use in oxidation ambient at high temperature can be attained for the films. The electrical and optical properties, as well as the thermal stability of resistivity, of GZO films were comparable to those of undoped ZnO films.     

Effects of sputtering power and pressure on properties of ZnO:Ga thin films prepared by oblique-angle deposition

The effects of power and pressure on radiofrequency (RF) diode sputtering in oblique-angle (80°) deposition arrangement are presented. Oblique-angle sputtering of ZnO:Ga (GZO) thin films resulted in a tilted columnar crystalline structure and inclination of the c-axis by an angle of approximately 9° with respect to the substrate. This improved their structural, electrical and optical properties in comparison with films deposited perpendicularly to the substrate. GZO films sputtered by an RF power of 600 W at room temperature of the substrate in Ar pressure 1.3 Pa showed strong crystalline (002) texture, lowest electrical resistivity 3.4 × 10^− 3 Ωcm, highest electron mobility 10 cm² V^− 1 s^− 1, high electron concentration 1.8 × 10²⁰ cm^− 3 and good optical transparency up to 88%. The small inclination angle of the film structure is caused by the high kinetic energy of sputtered species and additional energetic particle bombardment causes random surface diffusion, which is suppressing the shadow effect produced by oblique-angle sputtering.     

Preparation of AgInS2 chalcopyrite thin films by chemical spray pyrolysis

AgInS₂ thin films were prepared by the spray pyrolysis technique using a water/ethanol solution containing silver acetate, indium chloride and thiourea. We reported our results on the characterization of tetragonal AgInS₂ (chalcopyrite type) films, which were grown from indium deficient spraying solution. The films displayed a n-type conductivity with room temperature resistivities in the range between 10³ and 10⁴ Ω cm. The absorption spectra of sprayed films revealed two direct band-gaps with characteristic energies around 1.87 and 2.01 eV, which are in good agreement with the reported energy values for interband transitions from the split p-like valence band to the s-like conduction band in tetragonal AgInS₂ single crystals.     

Effect of cadmium oxide incorporation on the microstructural and optical properties of pulsed laser deposited nanostructured zinc oxide thin films

CdO doped (doping concentration 0, 1, 3 and 16 wt%) ZnO nanostructured thin films are grown on quartz substrate by pulsed laser deposition and the films are annealed at temperature 500 °C. The structural, morphological and optical properties of the annealed films are systematically studied using grazing incidence X-ray diffraction (GIXRD), energy dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), atomic force microscopy (AFM), Micro-Raman spectra, UV–vis spectroscopy, photoluminescence spectra and open aperture z-scan. 1 wt% CdO doped ZnO films are annealed at different temperatures viz., 300, 400, 500, 600, 700 and 800 °C and the structural and optical properties of these films are also investigated. The XRD patterns suggest a hexagonal wurtzite structure for the films. The crystallite size, lattice constants, stress and lattice strain in the films are calculated. The presence of high-frequency E₂ mode and the longitudinal optical A₁ (LO) modes in the Raman spectra confirms the hexagonal wurtzite structure for the films. The presence of CdO in the doped films is confirmed from the EDX spectrum. SEM and AFM micrographs show that the films are uniform and the crystallites are in the nano-dimension. AFM picture suggests a porous network structure for 3% CdO doped film. The porosity and refractive indices of the films are calculated from the transmittance and reflectance spectra. Optical band gap energy is found to decrease in the CdO doped films as the CdO doping concentration increases. The PL spectra show emissions corresponding to the near band edge (NBE) ultra violet emission and deep level emission in the visible region. The 16CdZnO film shows an intense deep green PL emission. Non-linear optical measurements using the z-scan technique indicate that the saturable absorption (SA) behavior exhibited by undoped ZnO under green light excitation (532 nm) can be changed to reverse saturable absorption (RSA) with CdO doping. From numerical simulations the saturation intensity (I_s) and the effective two-photon absorption coefficient (β) are calculated for the undoped and CdO doped ZnO films.     

Transparent conductive ZnO:B films deposited by magnetron sputtering

This paper focuses on the preparation of boron doped ZnO (ZnO:B) films prepared by nonreactive mid-frequency magnetron sputtering from ceramic target with 2 wt.% doping source. Adjusting power density, ZnO:B film with low resistivity (1.54 × 10^− 3 Ω cm) and high transparency (average transparency from 400 to 1100 nm over 85%) was obtained. Different deposition conditions were introduced as substrate fixed in the target center and hydrogen mediation. Hall mobility increased from 11 to above 26 cm²/V·s, while carrier concentration maintained almost the same, leading to low resistivity of 6.45 × 10^− 4 Ω cm. Transmission spectra of ZnO:B films grown at various growth conditions were determined using a UV-visible-NIR spectrophotometer. An obvious blue-shift of absorption edge was obtained while transmittances between 600 nm and 1100 nm remained almost the same. Optical band baps extracted from transmission spectra showed irregular enhancement due to the Burstein-Moss effect and band gap renormalization. Photoluminescence spectra also showed a gradual increase at UV emission peak due to free exciton transition near band gap. We contributed this enhancement in both optical band gap and UV photoluminescence emission to the lattice structure quality melioration.     

Effect of Thickness on the Properties of Ga-doped Nano-ZnO Thin Films Prepared by RF Magnetron Sputtering

Nano transparent conductive oxide (TCO) Ga-doped ZnO (GZO) thin films with thickness from 260 nm to 620 nm were prepared on glass substrates by RF magnetron sputtering from a powder target with 3 at.% Ga₂O₃. The substrate temperature was kept at 300 °C. The effect of thickness on the structural, electrical, and optical properties of GZO thin films was investigated. It shows that the nano-GZO films are dense and flat, and have polycrystalline structure with preferentially in the (002) orientation. With the increase of thickness, the crystallinity and the grain sizes of the films are improved, meanwhile the carrier concentration increases and the lowest resistivity of 3.685×10⁻³ Ω cm occurs in the 620 nm thick GZO film. The average optical transmittance of all the films is over 80% in the visible range. Decreasing the thickness, the optical transmission of the films increase, and the absorption edge shifts to shorter wavelength, which means the optical band gap is broadened.     

Optical and electrical properties of ZnO films prepared by URT-IP method

Optical and electrical properties were studied on thin polycrystalline ZnO films (200-nm thick) deposited on glass substrates at 200 °C by a DC-arc ion plating method (URamoto-Tanaka-type ion plating method). Effects of the oxygen flow rate (OFR) on film properties were examined. The resistivity of undoped films changed from 4.2×10⁻³ to 9.6×10⁻¹ Ω cm, corresponding to the carrier concentration of 1.0×10²⁰-1.2×10¹⁸ cm⁻³, depending on the increase in OFR from 0 to 40 sccm. The Hall mobility tends to be the maximum value of 28 cm² (V s)⁻¹ at OFR of 10 sccm. Photoluminescence (PL) spectra exhibited a dominant near-band-edge (NBE) emission together with weak PL bands at 2.2 and 3.2 eV. Intensity of NBE was maximum at OFR of 10 sccm. Intensity of the PL band at 2.2 eV increased with increase in OFR. As a result of Ga-doping, the resistivity decreased and the carrier concentration increased by one order of magnitude. The optical transmittance was more than 90% in 400-1200 nm. The ZnO:Ga (3 and 4 wt.% Ga-doped) thin films with the lowest resistivity of 2.6×10⁻⁴ Ω cm, the highest mobility of 25 cm² (V s)⁻¹, and the highest PL intensity were obtained at OFR of 10 sccm. Further increase of OFR led to the decrease in both mobility and PL intensity.     

Influence of laser-irradiation on the optical constants Se75S25 − xCdx thin films

Amorphous thin films of glassy alloys of Se₇₅S_25 − xCd_x (x = 2, 4 and 6) were prepared by thermal evaporation onto chemically cleaned glass substrates. Optical absorption and reflection measurements were carried out on as-deposited and laser-irradiated thin films in the wavelength region of 500-1000 nm. Analysis of the optical absorption data shows that the rule of no-direct transitions predominates. The laser-irradiated Se₇₅S_25 − xCd_x films showed an increase in the optical band gap and absorption coefficient with increasing the time of laser-irradiation. The results are interpreted in terms of the change in concentration of localized states due to the shift in Fermi level. The value of refractive index increases decreases with increasing photon energy and also by increasing the time of laser-irradiation. With the large absorption coefficient and change in the optical band gap and refractive index by the influence of laser-irradiation, these materials may be suitable for optical disc application.     

Study of structural, electrical, optical, thermoelectric and photoconductive properties of S and Al co-doped SnO2 semiconductor thin films prepared by spray pyrolysis

In this paper, the effect of S and Al concentrations on the structural, electrical, optical, thermoelectric and photoconductive properties of the films was studied. The [Al]/[Sn] and [S]/[Sn] atomic ratios in the spray solutions were varied from 10 at.% to 40 at.% and 0 to 50 at.%, respectively. X-ray diffraction analysis showed the formation of SnO₂ cassiterite phase as a main phase and the numerous sulfur phases including S, SnS, SnS₂ and Sn₂S₃ in SnO₂:Al films. Scanning electron microscopy studies showed that in the absence of S, increasing the Al content results in a smaller grain size and with the addition of S, the films appear to contain small cracks and nodules. The minimum resistance of 0.175 (kΩ/□) was obtained for S-doped SnO₂:Al (40 at.%) film with 20 at.% S-doping. From the Hall effect measurements, the majority carrier concentration was obtained in order of 10¹⁷-10¹⁸ cm^− 3. The thermoelectric measurements showed that majority carriers change from electrons to holes for S-doping in SnO₂:Al (40 at.%) thin films. The maximum Seebeck coefficient of + 774 μV/K (at T = 370 K) was obtained for S-doped SnO₂:Al (10 at.%) film with 50 at.% S-doping. The band gap values were obtained in the range of 3.8-4.2 eV. The S-doped SnO₂:Al (40 at.%) films have shown considerably photoconductivity more than S-doped SnO₂:Al (10 at.%) with increasing S-doping. The best photoconductive property was obtained for co-doped SnO₂ thin film with 40 at.% Al and 5 at.% S concentration in solution.     

Temperature effects on the optoelectronic properties of AgIn5S8 thin films

Polycrystalline AgIn₅S₈ thin films are obtained by the thermal evaporation of AgIn₅S₈ crystals onto ultrasonically cleaned glass substrates under a pressure of ~ 1.3 × 10⁻³ Pa. The temperature dependence of the optical band gap and photoconductivity of these films was studied in the temperature regions of 300-450 K and 40-300 K, respectively. The heat treatment effect at annealing temperatures of 350, 450 and 550 K on the temperature dependent photoconductivity is also investigated. The absorption coefficient, which was studied in the incidence photon energy range of 1.65-2.55 eV, increased with increasing temperature. Consistently, the absorption edge shifts to lower energy values as temperature increases. The fundamental absorption edge which corresponds to a direct allowed transition energy band gap of 1.78 eV exhibited a temperature coefficient of −3.56 × 10⁻⁴ eV/K. The 0 K energy band gap is estimated as 1.89 eV. AgIn₅S₈ films are observed to be photoconductive. The highest and most stable temperature invariant photocurrent was obtained at an annealing temperature of 550 K. The photoconductivity kinetics was attributed to the structural modifications caused by annealing and due to the trapping-recombination centers' exchange.     

Conductive Ga doped ZnO/Cu/Ga doped ZnO thin films prepared by magnetron sputtering at room temperature for flexible electronics

Ga doped ZnO(GZO)/Cu/GZO multilayers were deposited by magnetron sputtering on polycarbonate substrates at room temperature. We investigated the structural, electrical, and optical properties of multilayers at various thicknesses of Cu and GZO layers. The lowest resistivity value of 3.3 × 10^− 5 Ω cm with a carrier concentration of 2.9 × 10²² cm^− 3 was obtained at the optimum Cu (10 nm) and GZO (10 nm) layer thickness. The highest value of figure of merit φ_TC is 2.68 × 10^− 3 Ω^− 1 for the GZO (10 nm)/Cu(10 nm)/GZO(10 nm) multilayer. The highest average near infrared reflectivity in the wavelength range 1000-2500 nm is as high as 70% for the GZO(10 nm)/Cu(10 nm)/GZO(10 nm) multilayer.     

Effect of Ga doping on micro/structural, electrical and optical properties of pulsed laser deposited ZnO thin films

Undoped and Ga doped ZnO thin films (1% GZO, 3% GZO and 5% GZO) were grown on c-Al₂O₃ substrates using the 1, 3 and 5 at. wt.% Ga doped ZnO targets by pulsed laser deposition. X-ray diffraction studies revealed that highly c-axis oriented, single phase, undoped and Ga doped ZnO thin films with wurtzite structure were deposited. Micro-Raman scattering analysis showed that Ga doping introduces defects in the host lattice. The E₂^High mode of ZnO in Ga doped ZnO thin film was observed to shift to higher wavenumber indicating the presence of residual compressive stress. Appearance of the normally Raman inactive B₁ modes (B₁^Low, 2B₁^Low and B₁^High) due to breaking of local translational symmetry, also indicated that defects were introduced into the host lattice due to Ga incorporation. Band gap of the Ga doped ZnO thin films was observed to shift to higher energy with the increase in doping concentration and is explicated by the Burstein-Moss effect. Electrical resistivity measurements of the undoped and GZO thin films in the temperature range 50 to 300 K revealed the metal to semiconductor transition for 3 and 5% GZO thin films.     

Properties of cu-doped low resistive ZnSe films deposited by two-sourced evaporation

Two-sourced evaporation technique is used to prepare hard ZnSe films by controlling the evaporation rates of both Zn and Se at substrate temperature of 400 °C. The films are doped with Cu by immersion in the Cu(NO₃)₂-H₂O solution for different periods of time. The XRD has not shown a drastic change in the film structure while the electrical resistivity of the deposited film dropped from 10⁹ Ω-cm to about 1.6 Ω-cm for solution immersed films after heat treatment. Optical properties of deposited and doped films, such as film thickness, absorption coefficient and optical band gap have been calculated from the normal transmission spectra in the range of 300-2200 nm.The optical results show a decrease of the transmission and an increase of the refractive index and a slight shift in the optical band gap. Chemical composition of the Cu is determined by using absorption of immersed films. The composition of Cu is also compared with the composition detected by electron microprobe analyzer (EMPA).     

Anode material properties of Ga-doped ZnO thin films by pulsed DC magnetron sputtering method for organic light emitting diodes

This study examined the anode material properties of Ga-doped zinc oxide (GZO) thin films deposited by pulsed DC magnetron sputtering along with the device performance of organic light emitting diodes (OLEDs) using GZO as the anode. The structure and electrical properties of the deposited films were examined as a function of the substrate temperature. The electrical properties of the GZO film deposited at 200 °C showed the best properties, such as a low resistivity, high mobility and high work function of 5.3 × 10^− 4Ω cm, 9.9 cm²/Vs and 4.37 eV, respectively. The OLED characteristics with the GZO film deposited under the optimum conditions showed good brightness > 10,000 cd/m². These results suggest that GZO films can be used as the anode in OLEDs, and a lower deposition temperature of 200 °C is suitable for flexible devices.