S. Aksu

Bio: S. Aksu is an academic researcher. The author has contributed to research in topics: Thin film & Wurtzite crystal structure. The author has an hindex of 9, co-authored 11 publications receiving 326 citations.

Structural, optical and electrical properties of Al-doped ZnO microrods prepared by spray pyrolysis

Abstract: Al-doped ZnO thin films were obtained on glass substrates by spray pyrolysis in air atmosphere. The molar ratio of Al in the spray solution was changed in the range of 0–20 at.% in steps of 5 at.%. X-ray diffraction patterns of the films showed that the undoped and Al-doped ZnO films exhibited hexagonal wurtzite crystal structure with a preferred orientation along (002) direction. Surface morphology of the films obtained by scanning electron microscopy revealed that pure ZnO film grew as quasi-aligned hexagonal shaped microrods with diameters varying between 0.7 and 1.3 μm. However, Al doping resulted in pronounced changes in the morphology of the films such as the reduction in the rod diameter and deterioration in the surface quality of the rods. Nevertheless, the morphology of Al-doped samples still remained rod-like with a hexagonal cross-section. Flower-like structures in the films were observed due to rods slanting to each other when spray solution contained 20 at.% Al. Optical studies indicated that films had a low transmittance and the band gap decreased from 3.15 to 3.10 eV with the increasing Al molar ratio in the spray solution from 0 to 20 at.%.

The influence of substrate temperature on the morphology, optical and electrical properties of thermal-evaporated ZnSe thin films

Abstract: The structural, morphological, optical and electrical properties of ZnTe films deposited by evaporation were investigated as a function of substrate temperature (at −123 and 27 °C) and post-deposition annealing temperature (at 200, 300 and 400 °C). It was determined that films deposited at both substrate temperatures were polycrystalline in nature with zinc-blende structure and a strong (1 1 1) texture. A small Te peak was detected in XRD spectra for both substrate temperatures, indicating that as-deposited ZnTe films were slightly rich in Te. Larger grains and a tighter grain size distribution were obtained with increased substrate temperature. Scanning electron microscopy (SEM) studies showed that the microstructures of the as-deposited films agreed well with the expectations from structure zone model. Post-deposition annealing induced further grain growth and tightened the grain size distribution. Annealing at 400 °C resulted in randomization in the texture of films deposited at both substrate temperatures. Optical spectroscopy results of the films indicated that the optical band gap value increased from 2.13 to 2.16 eV with increased substrate temperature. Increasing the annealing temperature sharpened the band-edge. Resistivity measurements showed that the resistivity of films deposited at substrate temperatures of −123 and 27 °C were 32 Ω cm, and 1.0 × 104 Ω cm, respectively with corresponding carrier concentrations of 8.9 × 1015 cm−3 and 1.5 × 1014 cm−3. Annealing caused opposite changes in the film resistivity between the samples prepared at substrate temperatures of −123 and 27 °C.

Structural, optical and magnetic properties of Mn diffusion-doped CdS thin films prepared by vacuum evaporation

Abstract: The effect of Mn-doping on the vacuum deposited CdS thin films has been investigated by studying the changes in the structural, optical and magnetic properties of the films. A thin Mn layer evaporated on the CdS film surface served as the source layer for the diffusion doping. Doping was accomplished by annealing the CdS/Mn stack layers at the temperature range from 300 °C to 400 °C in step of 50 °C for 30 min under vacuum. The X-ray diffraction results showed that the undoped CdS film had a zinc-blende structure with a strong preferred orientation along the (1 1 1) direction. The incorporation of Mn did not cause any change in the texture but reduced the peak intensity and lead to a smaller crystallite size. Investigation of surface morphology using atomic force microscopy confirmed the decrease in the grain size with the Mn diffusion. In addition, a more uniform grain size distribution was observed in the doped films. X-ray photoelectron spectroscopy analysis showed that Mn atoms on the surface of the films were bounded to either sulphur or oxygen atoms. Auger electron spectroscopy of the diffusion-doped CdS sample at 350 °C indicated that the atomic Mn concentration was higher close to the surface region and Mn was distributed with a steadily decreasing profile through the bulk of the film with an average atomic concentration value around few percent. Band gap energy of the undoped sample decreased from 2.42 to 2.36 eV upon Mn diffusion at 400 °C. The magnetization of films as a function of magnetic field and temperature were measured. Clear ferromagnetic loops were observed for the Mn diffusion-doped CdS films prepared by annealing above 350 °C.

Schottky diode properties of CuInSe2 films prepared by a two-step growth technique

Abstract: CuInSe 2 films were synthesized by a thermal evaporation method on both glass and Mo coated glass substrates. To obtain Al/CuInSe 2 /Mo Schottky diode, Al metal was evaporated on the upper surface of CuInSe 2 as a front contact and electrical properties of the structure were analyzed. The structural and optical behaviors of the CuInSe 2 thin films were also investigated. The X-ray diffraction studies showed that the chalcopyrite CuInSe 2 was obtained with a preferential orientation in the (1 1 2) plane with lattice parameters a and c as 0.577 and 1.161 nm, respectively. Mo back contact layer had a preferential orientation in the (1 1 0) plane. Scanning electron microscopy equipped with energy dispersive spectroscopy revealed an irregular and rough surface morphology with Cu-rich protruding regions. Optical studies showed the existence of three different band gaps, which were determined as 1.06, 1.17 and 1.39 eV, respectively. From the Hall Effect measurements, we determined the carrier concentration of CuInSe 2 films as 4.0 × 10 17 cm −3 . The electrical properties of the CuInSe 2 films were further studied by fabricating Al/p-CuInSe 2 /Mo structures and obtaining their forward and reverse bias current–voltage characteristics in a wide temperature range of 100–300 K, in steps of 25 K. A thorough analysis of the forward bias current–voltage characteristics based on thermionic emission theory showed that the zero bias barrier height increases while series resistance and ideality factor decreases with an increase in temperature. After a barrier height inhomogeneity correction, the Richardson constant and mean barrier height were found to be 34.71 A/cm 2 K 2 and 0.72 eV, respectively.

Structural, optical and magnetic properties of Zn1-xCoxO thin films prepared by spray pyrolysis

Abstract: A series of Co-doped ZnO films were fabricated by a spray pyrolysis method. The X-ray diffraction patterns of the films showed that the undoped and Co-doped ZnO films exhibit wurtzite crystal structure. Surface morphology of the films obtained by scanning electron microscopy reveals that pure ZnO film has the hexagonal shaped microrods. The introduction of Co content in the structure did not affect the surface morphology of the films significantly. Magnetic hysteresis loops were observed at room temperature, indicating that ferromagnetism can be realized with Co doping into ZnO. The values of remnant magnetization and coercive field slightly decreased when the concentration of Co was increased beyond 1 at.%. The calculated band gap value of undoped and 1 at.% Co doped samples was found to be around 3.08 eV. The band gap energy increases upon 3 at.% Co concentration followed by a decrease for further increase in Co concentration toward 5 at.%.

Copper(I) Thiocyanate (CuSCN) Hole-Transport Layers Processed from Aqueous Precursor Solutions and Their Application in Thin-Film Transistors and Highly Efficient Organic and Organometal Halide Perovskite Solar Cells

Abstract: This study reports the development of copper(I) thiocyanate (CuSCN) hole-transport layers (HTLs) processed from aqueous ammonia as a novel alternative to conventional n-alkyl sulfide solvents. Wide bandgap (3.4–3.9 eV) and ultrathin (3–5 nm) layers of CuSCN are formed when the aqueous CuSCN–ammine complex solution is spin-cast in air and annealed at 100 °C. X-ray photoelectron spectroscopy confirms the high compositional purity of the formed CuSCN layers, while the high-resolution valence band spectra agree with first-principles calculations. Study of the hole-transport properties using field-effect transistor measurements reveals that the aqueous-processed CuSCN layers exhibit a fivefold higher hole mobility than films processed from diethyl sulfide solutions with the maximum values approaching 0.1 cm2 V−1 s−1. A further interesting characteristic is the low surface roughness of the resulting CuSCN layers, which in the case of solar cells helps to planarize the indium tin oxide anode. Organic bulk heterojunction and planar organometal halide perovskite solar cells based on aqueous-processed CuSCN HTLs yield power conversion efficiency of 10.7% and 17.5%, respectively. Importantly, aqueous-processed CuSCN-based cells consistently outperform devices based on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate HTLs. This is the first report on CuSCN films and devices processed via an aqueous-based synthetic route that is compatible with high-throughput manufacturing and paves the way for further developments.

Fabrication of transition metal selenides and their applications in energy storage

Abstract: Electrochemical energy storage devices (lithium ion batteries, sodium ion batteries, magnesium ion batteries, and super capacitors) with high power and energy densities are considered the most promising equipment for large-scale applications of portable electronic devices and electric vehicles. These devices can be realized by exploring nanostructured materials with high capacity, favorable cycling stability, and superior rate capability. Transition metal selenides (TMSs) are potential materials for electrochemical energy storage systems. In this paper, we summarized the nanostructured transition metal selenides and indicated their properties, preparation methods, and applications in electrochemical energy storage systems. We discussed the electronic properties of TMSs and showed that these materials have tunable electronic properties. We enumerated the 10 most-used preparation methods of TMSs as well as their composites with other functional materials. Subsequently, we systematically reviewed their applications in lithium ion batteries, sodium ion batteries, magnesium ion batteries, and super capacitors. Finally, we proposed the challenges and opportunities of their applications in energy storage.

Synthesis and characterization of zinc oxide thin films for optoelectronic applications

Abstract: Micro-ring structured zinc oxide (ZnO) thin films were prepared on glass substrates by spray pyrolysis and their structural, morphological, optical and electrical properties were investigated. X-ray Diffraction (XRD) analysis revealed the films’ hexagonal wurtzite phase with a preferred (002) grain orientation. The mean crystallite size calculated on the basis of the Debye-Scherrer model was 24 nm and a small dislocation density of 1.7 × 10 − 3 n m − 2 was obtained, indicating the existence of few lattice defects and good crystallinity. Scanning Electron Microscopy (SEM) micrographs revealed the film’s granular nature composed of rod-shaped and spherical nanoparticles which agglomerated to form micro-ring like film clusters on the film surface. The average transmittance in the visible region, optical band gap and Urbach energy were approximately 75–80%, 3.28 eV and 57 meV, respectively. The refractive index and extinction coefficient were determined using Swanepoel’s envelope method. Raman spectroscopy revealed the presence of small amounts of residual tensile stress and low density of defects in the ZnO thin films. This was consistent with XRD analysis. A low sheet resistivity ( 6.03 × 10 1 Ω c m ) and high figure of merit ( 4.35 × 10 − 6 Ω − 1 ) were obtained for our films indicating their suitability in optoelectronic applications.

A highly selective and wide range ammonia sensor—Nanostructured ZnO:Co thin film

Abstract: Ammonia sensing characteristics of undoped and cobalt (Co)-doped nanostructured ZnO thin films were investigated. Polycrystalline nature with hexagonal wurtzite structure and high crystalline quality with dominant (0 0 2) plane orientation of Co-doped ZnO film were confirmed by the X-ray diffractogram. Scanning electron micrographs of the undoped film demonstrated the uniform deposition of sphere-shaped grains. But, smaller particles with no clear grain boundaries were observed for Co-doped ZnO thin film. Band gap values were found to be 3.26 eV and 3.22 eV for undoped and Co-doped ZnO thin films. Ammonia sensing characteristics of Co-doped ZnO film at room temperature were investigated in the concentration range of 15–1000 ppm. Variation in the sensing performances of Co-doped and pure ZnO thin films has been analyzed and compared.

Effect of annealing on structural and optoelectronic properties of nanostructured ZnSe thin films

Abstract: Thin films of ZnSe were deposited on soda lime glass substrates by thermal evaporation and annealed in vacuum at various temperatures in the range of 100–300 °C. Structural and optoelectronic properties of these films were investigated and compared with the available data. XRD studies revealed that as-deposited films were polycrystalline in nature with cubic structure. It was further observed that the grain size and crystallinity increased, whereas dislocations and strains decreased with the increase of annealing temperature. The optical energy band gap estimated from the transmittance data was in the range of 2.60–2.67 eV. The observed increase in band gap energy with annealing temperature may be due to the quantum confinement effects. Similarly, refractive index of the films was found to increase with the annealing temperature. The AFM images revealed that films were uniform and pinhole free. The RMS roughness of the films increased from 1.5 nm to 2.5 nm with the increase of annealing temperature. Resistivity of the films decreased linearly with the increase of temperature.