Co-doped Zinc Oxide Thin Films Research Articles

Radio Frequency Sputter Deposition of Ga and F Co-Doped ZnO Thin Films: Effects of Substrate Temperature

Doped zinc oxide (ZnO) thin films can be used as transparent conducting electrodes for various optoelectronic device such as flat panel displays andphotovoltaic cells. The present study investigates the effect of substrate temperature on Ga and F co-doped ZnO (GFZO) thin films.The GFZO thin films were deposited on glass substrates by RF magnetron sputtering in CF4/Ar atmosphere with a ZnO:Ga2O3 (3 wt%) target. The structural, electrical, and optical properties of the samples have been observed and analyzed by x-ray diffraction, field emissionscanning electron microscopy, Hall measurements, and UV-visible spectrophotometry. All the GFZO thin films showed the highly c-axis-oriented phase and their average visible transmittances were over 91%. The electrical resistivity of the samples significantly decreased by an order of magnitude to 6.47×10−4Ω-cm as the substrate temperature raised from room temperature to 200 °C. The maximum carrier concentration of 6.10×1020 cm-3 and Hall mobility of 15.4 cm2/V-s were obtained at the optimum substrate temperature at 200 °C. The optical bandgap of the GFZO films has been widened from 3.388 to 3.620 eV with the increase of the substrate temperature. The findings show the developed GFZO thin filmprepared at the higher substrate temperature haveeffectively improved optoelectronic properties andare potentially able to applications in variousoptoelectronic devices.

Rare earth Eu3+ co-doped AZO thin films prepared by nebulizer spray pyrolysis technique for optoelectronics

Rare earth element (i.e.) europium co-doped aluminum zinc oxide (Eu:AZO) thin films were deposited on microscope glass slides by nebulizer spray pyrolysis with different Eu-doping concentrations (0, 0.5, 1, and 1.5%). The deposited films were investigated using X-ray diffraction, AFM, EDAX, FT-Raman, UV–visible, PL, and Hall effect measurements. X-ray confirmed the incorporation of aluminum and europium ions into the ZnO structure. All films have polycrystalline nature with hexagonal wurtzite structure at (002) direction. Topological depictions exhibited minimum surface roughness and low film thickness for pristine AZO thin film. EDAX study authorizes the existence of Zn, O, Al, and Eu in Eu: AZO thin films. Raman spectra exhibited the characteristic of ZnO-wurtzite structure (E2-high) mode at 447 cm−1. The deposited film showed high optical transmittance of ~90% in visible region, and the direct energy gap was around 3.30 eV for pristine AZO thin film. The PL spectra emitted a powerful UV emission situated at 388 nm, and it indicates that the film has good optical quality. The obtained large carrier concentration and less resistivity values are 4.42 × 1021 cm−3 and 3.95 × 10−4 Ω cm, respectively, for 1.5% Eu-doped AZO thin film. The calculated figure of merit value is 17.29 × 10−3 (Ω/sq)−1, which is more suitable for the optoelectronic device.

Melioration of Optical and Electrical Performance of Ga-N Codoped ZnO Thin Films

Abstract Transparent and conducting p-type zinc oxide (ZnO) thin films doped with gallium (Ga) and nitrogen (N) simultaneously were deposited on glass substrates by spray pyrolysis technique. Phase composition analysis by X-ray diffraction confirmed the polycrystallinity of the films with pure ZnO phase. Energy dispersive X-ray analysis showed excellent incorporation of N in the ZnO matrix by means of codoping. The optical transmittance of N monodoped film was poor but got improved with Ga-N codoping and also resulted in the enhancement of optical energy gap. Hole concentration increased with codoping and consequently, lower resistivity and high stability were obtained.

Effect of acetic acid and water content in the spray solution on structural, morphological, optical and electrical properties of Al and In co-doped zinc oxide thin films

Aluminium and indium co-doped zinc oxide (AIZO) thin films were deposited using ultrasonic spray pyrolysis. Depositions were performed by varying the acetic acid and water content in the spraying solution which resulted in the formation of different nanostructures like hexagons, flowers, chisels, curved nanostructures, hexagonal pyramids, super grown hexagons, and inter-connected nanostructures. Further, the physical properties such as structural, optical, electrical, and surface texture parameters were examined. The structural studies showed that films were of crystalline nature, with different crystallite sizes and grown with a preferential orientation along (002) plane. The optical transmittance assessments proved that films were highly transparent (> 80%) in the visible region. The electrical sheet resistance was found to be in the range 29–1K Ω/□. Surface parameters like average roughness, root mean square roughness, and peak-valley height values helped to understand the homogeneity of the thin films. Finally, the suitability of AIZO films for transparent conductive oxide applications were tested by estimating the figure of merit (FOM). Among the different solution conditions, films fabricated using a starting solution containing 25 ml of acetic acid and 25 ml of water exhibited the lowest resistivity (2.47 ± 0.03 × 10−3 Ω-cm) along with the highest FOM (5.83 ± 0.42 × 10−3/Ω).

Study on properties of Ga/F-co-doped ZnO thin films prepared using atomic layer deposition

Zinc oxide (ZnO) thin films with co-doped Ga and F were deposited using atomic layer deposition. Structural, electrical, and optical properties of the ZnO thin films were analyzed for different F doping amounts under the condition that the Ga doping amount remained fixed. From the X-ray diffraction analysis results, it was confirmed that the preferred orientation changed from (002) to (100) with changing F amounts. The electrical properties, i.e., carrier concentration and mobility of these co-doped ZnO thin films improved with the F doping amount, resulting in a decrease in the electrical resistivity. The reason for this improved resistivity is that Ga and F atoms replace Zn and O in the lattice sites, respectively, despite a slight decrease in the mobility. An increase in the carrier concentration widened the optical bandgap of ZnO thin films by the Moss-Burstein effect.

Biphase effect on structural, optical, and electrical properties of Al-Sn codoped ZnO thin films deposited by sol-gel spin-coating technique

Aluminum and tin codoped zinc oxide thin films were deposited on glass substrates by sol gel spin coating technique. Aluminum had a constant concentration of 2 at.% whereas tin was incorporated with different concentrations of 1, 2, 3, and 5 at.%. The prepared samples were polycrystalline and the grains growth was preferred along (002) orientation according to x-ray diffraction patterns. Transmittance analysis showed transparency over 80% in visible range for all samples and also the presence of two phases leading to two different optical band gaps in each sample. The amorphous phases had high optical band gaps and high Urbach energies compared to the crystalline phases. The resistivity measurements had revealed that the lowest resistivity value of about 1.65×10-3Ω .cm was obtained for Al 2 at.%, Sn 2 at.% concentration. These Al-Sn codoped ZnO thin films can have big interest in solar cell industry. Sample containing Al 2 at.%, Sn 1 at.% concentration was mainly amorphous and exhibited transparency over 60% in near ultraviolet region. Thereby, it may be promising to fabricate thin films transistors (TFTs) and ultraviolet light emitting diodes (UV-LEDs).

Effect of Mg Doping on Structural and Optical Properties of Al-Mg Co-doped ZnO Thin Films Deposited via Sol-gel Technique

Aluminium doped and (Mg-Al) co-doped zinc oxide thin films were deposited by sol-gel technic from aqueous solution onto glass substrates at optimum conditions. The variations of the structural, electrical and optical properties with the doping and co-doping concentration were investigated. XRD analysis showed typical patterns of the hexagonal ZnO structure for all films and AMZO thin films exhibited preferred orientation growth along the c-axis. No diffraction peaks of any other structure were found. The grain size, texture coefficient and optical band gap values were evaluated for different aluminum concentrations and (Mg-Al) co-doping ratio. The results show that increasing the Mg content, the films improve their crystallinity with the (002) preferred orientation and the transmittance reaches 85%. The optimized results were obtained for co-doped (Mg-Al) concentration at ratio of 2/1.

Microstructure and optoelectronic properties of gallium–magnesium codoped zinc oxide thin films by magnetron sputtering technique

The gallium–magnesium codoped zinc oxide (GMZO) thin films were deposited on glass substrates by radio frequency magnetron sputtering technique in an argon atmosphere. The influence of substrate temperature on the microstructure, morphology and optoelectronic properties of thin films was investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, four-point probe and spectrophotometry. It is found that all the deposited films have a hexagonal structure and a preferred orientation along the c-axis perpendicular to the substrate. As the substrate temperature increases, the dislocation density, lattice strain and electrical resistivity decrease initially and then increase, while the average crystallite size, average visible transmittance and figure of merit exhibit the reverse variation trend. The GMZO thin film deposited at the substrate temperature of 570 K possesses the best optoelectronic properties, with the largest average crystallite size of 52.05 nm, the lowest dislocation density of 3.69 × 1014 lines m− 2, the minimum lattice strain of 1.10 × 10− 3, the lowest electrical resistivity of 1.62 × 10− 3 Ω cm, the highest average visible transmittance of 88.63% and the maximum figure of merit of 5.11 × 103 Ω−1 cm− 1. The optical energy gaps of the films were evaluated by extrapolation method and observed to be in the range of 3.34–3.55 eV. Furthermore, the complex refractive index, the complex dielectric constant and the dissipation factor were determined by optical characterization methods, and the dispersion behaviour of refractive index was studied in terms of the single electronic oscillator model. The results show that the microstructure and optoelectronic properties of the GMZO thin films are dependent on substrate temperature.

Effect of precursor type and doping concentration on the physical properties of ultrasonically sprayed aluminium and indium co-doped zinc oxide thin films

In this work, we have studied the effect of aluminium dopant precursor type and doping concentration on the structural, morphological, optical and electrical properties of Al and In co-doped zinc oxide (AIZO) thin films deposited by ultrasonic spray pyrolysis. Zinc acetate dihydrate and indium acetate were used as zinc and indium precursors, respectively. Aluminium chloride and aluminium sulphate were used as aluminium precursors. The doping concentrations of Al (1 to 3at%) and In (1 to 3at%), were varied equally and the physical properties were analyzed. X-ray diffraction examinations confirmed that AIZO films were poly crystalline and grown as a hexagonal wurtzite structure. Scanning electron microscopy observations revealed that thin films were grown with different types of hexagonal nanostructures. From the optical and electrical measurements, the figure of merit was estimated. The values were 4.09×10−3/Ω and 0.75×10−3/Ω for the AIZO thin films deposited using aluminium chloride and aluminium sulphate respectively.

Single oscillator model of undoped and co-doped ZnO thin films

Undoped and co-doped zinc oxide thin films were deposited on glass substrates using a chemical spray technique. The experimental Lorentz profiles of co-doped ZnO thin films were measured in the spectral region 720–1750nm. A single oscillator model of the resulted materials is constructed by calculation of: zero-frequency refractive index (no), average interband oscillator wavelength (λo), oscillator strength (So), dispersion energy (Ed), single-effective oscillator energy (Eo), plasma oscillation frequency (ωp) and interband transition strength moments (M−1, M−3).

Low temperature synthesis of radio frequency magnetron sputtered gallium and aluminium co-doped zinc oxide thin films for transparent electrode fabrication

Gallium and aluminium co-doped zinc oxide (GAZO) thin films were prepared on glass substrates at low temperatures by radio frequency (rf) magnetron sputtering and their physical properties were investigated. All films possessed a hexagonal wurtzite crystal structure with a strong growth orientation along the (002) c-axis. The (002) peak intensity and mean crystallite size increased with substrate temperature from room temperature (RT) to 75°C and then decreased at 100°C, indicating an improvement in crystallinity up to 75°C and its deterioration at 100°C. Scanning electron microscopy (SEM) micrographs revealed the strong dependency of surface morphology on substrate temperature and energy dispersive spectroscopy (EDS) confirmed the incorporation of Ga and Al into the ZnO films. All films exhibited excellent transmittances between 85 and 90% in the visible region and their optical band gap increased from 3.22eV to 3.28eV with substrate temperature. The Urbach energy decreased from 194meV to 168meV with increasing substrate temperature, indicating a decrease in structural disorders which was consistent with X-ray Diffraction (XRD) analysis. Films deposited at 75°C exhibited the lowest electrical resistivity (2.4Ωcm) and highest figure of merit (7.5×10−5Ω−1), proving their potential as candidates for transparent electrode fabrication.

Physical properties of gallium and aluminium co-doped zinc oxide thin films deposited at different radio frequency magnetron sputtering power

Gallium and aluminium co-doped zinc oxide (GAZO) thin films were deposited by radio frequency (rf) magnetron sputtering onto glass substrates and the effect of rf power on their structural, optical and electrical properties was respectively investigated by X-ray Diffraction (XRD), Spectrophotometry and Four–Point Probe Resistivity measurements. All films had a hexagonal wurtzite crystal structure with a preferred (002) grain orientation. The films’ crystallinity deteriorated with increasing rf power from 150W to 250W as revealed by the increase in full width at half maximum (FWHM), decrease in mean crystallite size and increase in dislocation density. A further increase in rf power to 300W caused slight improvements in crystallinity due to enhanced surface diffusion of the ad-atoms. High optical transmittances, around 80–90% were observed for all films in the visible region and their optical band gap red shifted from 3.32eV to 3.20eV with increasing rf power. The electrical resistivity firstly increased as the rf power changed from 150W to 250W and then decreased at 300W. The lowest electrical resistivity of 5.0×10−1Ωcm and maximum figure of merit of 4.8×10−4Ω−1 were obtained for films deposited at 150W, indicating their better performance in optoelectronic applications.

Sputtered Al–N codoped p-type transparent ZnO thin films suitable for optoelectronic devices

Aluminum–nitrogen (Al–N) codoped zinc oxide (ZnO) thin films were grown on glass substrate by radio frequency (RF) reactive magnetron sputtering using aluminum doped zinc oxide (AZO, 2.5wt% Al2O3) target and N2 as reactive gas. The structural, morphology, optical and electrical properties were also investigated with different flow rate of N2 gas. X-ray diffraction results shows that codoped ZnO thin films have similar wurtzite structure like undoped ZnO film. Al–N thin films have high transparency 78% in visible region as the nitrogen flow rate increases the transparency and band gap decreases. The best p-type codoped sample shows a resistivity and hole concentration of 0.554Ωcm and 8.3×1019cm−3 at room temperature, respectively. Current–voltage (I–V) characteristics of p-type codoped ZnO thin films are also discussed.

Effect of hydrogen doping on the properties of Al and F co-doped ZnO films for thin film silicon solar cell applications

Aluminum and fluorine co-doped zinc oxide (AFZO) thin films were prepared in Ar+H2 atmospheres by rf magnetron sputtering at room temperature. The structural, electrical, and optical properties of the prepared films were investigated using X-ray diffraction, scanning electron microscopy, atomic force microscopy, Hall-effect measurement, X-ray photoelectron spectroscopy, and ultraviolet–visible spectrometry, and their dependence on deposition atmosphere (i.e. H2/(H2+Ar) ratio) was studied. The resulting films showed a (0 0 2) diffraction peak, indicating a typical wurtzite structure, and the optimal film crystallinity was obtained with the H2/(H2+Ar) ratio of 3%. The electrical resistivity of AFZO films decreased to 9.16×10−4Ω-cm, which was lower than ZnO:Al and ZnO:F films due to double doping effect of Al and F. The resistivity further decreased to below 5×10−4Ω-cm for the AFZO film with the H2/(H2+Ar) ratio of 3%–5%. All the films regardless of hydrogen content displayed high transmittances (>92%) in the visible wavelength range. Applying the developed AFZO films as front transparent electrodes, amorphous Si thin film solar cells were fabricated and the open-circuit voltage, fill factor, and efficiency of the cell with the hydrogenated AFZO film were improved in contrast to those without the hydrogenated film.

Effects of post anneal for the INZO films prepared by ultrasonic spray pyrolysis

Indium-nitrogen co-doped zinc oxide thin films (INZO) were prepared on glass substrates in the atmosphere by ultrasonic spray pyrolysis. The aqueous solution of zinc acetate, ammonium acetate and different indium sources: indium (III) chloride and indium (III) nitrate were used as the precursors. After film deposition, different anneal temperature treatment as 350, 450, <TEX>$550^{\circ}C$</TEX> were applied. Electrical properties as concentration and mobility were characterized by Hall measurement. The surface morphology and crystalline quality were characterized by SEM and XRD. With the activation energy analysis for both films, the concentration variation of the films at different heat treatment temperature was realized. Donors correspond to zinc related states dominate the conduction mechanism for these INZO films after <TEX>$550^{\circ}C$</TEX> high temperature heat treatment process.

Yellowish-White Emission from Sol-Gel Mediated Lithium and Aluminium Co-Doped Zinc Oxide Thin Films

Li and Al codoped ZnO (LAZO) thin films have been prepared by sol-gel method. There are two emission bands were observed in the spectra of pure ZnO thin films. One band possessed a sharp peak at round 380 nm locating in the UV range; the other band consists of green luminescence. Yellowish-white emission were observed from the LAZO films, which can be fitted into three bands: a strong yellow emission, a moderate green emission and a weak violet emission centered at 598, 538, and 430 nm, respectively. The luminescence color was seen to be yellowish white due to the recombination for the dominant green and yellow emission.

Semiconducting properties of Tm doped Yb-ZnO films by spray pyrolysis

In this work, we have investigated the structural, optical and electrical properties of rare earth co-doped zinc oxide thin films prepared by spray pyrolysis technique. X-ray diffraction has shown that the films are polycrystalline and textured with the c -axis of the wurtzite structure along the growth direction. Scanning electronic microscopy and transmission electronic microscopy were used to study the films composition and morphology. Photoluminescence measurements showed that all the films have a strong emission band at around 380 nm. Layers with electrical resistivity values as low as 5.7 × 10 −2 Ω cm were obtained.

Investigation of structural and optical properties in Cobalt–Chromium co-doped ZnO thin films within the Lattice Compatibility Theory scope

(Co,Cr)-codoped zinc oxide thin films (ZnO:Cr:Co) at different percentages (0%, 1–1%, 1–2%, 2–1%) were deposited on glass substrates using a chemical low-cost spray technique. The effect of Cr and Co concentration on the structural, morphological and optical properties of the ZnO:Cr:Co thin films were investigated by means of X-ray diffraction, optical measurement, contact Atomic Force Microscopy (AFM), and Photoluminescence spectroscopy. The results revealed that all films consist of single phase ZnO and were well crystallized in würtzite phase with the crystallites preferentially oriented towards (002) direction parallel to c-axis. Also, the co-doping has effective role in the enhancement of the crystallinity and leads to an improvement of roughness of the ZnO films. Doping by chrome and cobalt resulted in a slight decrease in the optical band gap energy of the films. The optical band gap of these films is calculated. The optical absorption spectra show that the absorption mechanism is a direct transition. The UV peak positions for ZnO:Cr:Co samples slightly red shift to the longer wavelength in comparison with the pure ZnO which can be attributed to the change in the acceptor level induced by the substitutional Co2+ and Cr3+ and the band-gap narrowing of ZnO with the Cr and Co dopants. The Lattice Compatibility Theory analyses have been applied in order to give original, plausible and founded explanation to the recorded preferential incorporation of cobalt ions within ZnO lattice over chromium.

Structural, Optical, and Magnetic Properties of Cobalt-Doped Dip Coated ZnO Films

Co-doped zinc oxide thin films (ZnCoO) have been deposited on glass substrates by sol-gel dip coating deposition method. X-ray diffractometer patterns for these thin films prove crystalline behavior with wurtzite structure. The films have lattice parameters similar to that of ZnO. When Zn is replaced by Co, more impurity levels are produced within the bandgap, and more defects are developed. However, Co-doping does not alter the wurzite structure. The bandgap of the films tends to decrease with increasing withdrawal speed. These transparent thin films possess bandgap in the ultra-violet (3.86-3.45 eV) and high transparency throughout the visible spectral region. Ellipsometry also show the same trend for bandgaps (3.83-3.45). Morphological analysis by scanning electron microscope shows surface with porous structure. Magnetic property studied shows the ferromagnetic behavior. Chemical composition of these Co-doped ZnO thin films was investigated through Fourier transform infrared analysis.

Structure and characterization of Sn, Al co-doped zinc oxide thin films prepared by sol–gel dip-coating process

Transparent conductive zinc oxide co-doped with tin and aluminum (TAZO) thin films were prepared via sol–gel dip-coating process. Non-toxic ethanol was used in this study instead of 2-methoxyethanol used in conventional work. Dip-coating was repeated several times to obtain relatively thick films consisting of six layers. The films were then annealed at 500°C for 1h in air or in vacuum and not subsequently as employed in other studies. The X-ray diffraction patterns indicated that all the samples revealed a single phase of hexagonal ZnO polycrystalline structure with a main peak of (002). The optical band gap and resistivity of the TAZO films were in the ranges of 3.28 to 3.32eV and 0.52 to 575.25Ωcm, respectively. The 1.0at.% Sn, 1.0at.% Al co-doped ZnO thin film annealed in vacuum was found to have a better photoelectrochemical performance with photocurrent density of about 0.28mA/cm2 at a bias of 0.5V vs. SCE under a 300W Xe lamp illumination with the intensity of 100mW/cm2. Compared to the same dopant concentration but annealed in air (~0.05mA/cm2 bias 0.5V vs. SCE), the photocurrent density of the film annealed in vacuum was 5 times higher than the film annealed in air. Through electrochemical measurements, we found that the dopant concentration of Sn plays an important role in TAZO that affected photocurrent density, stability of water splitting and anti-corrosion.