Bandgap engineering by substitution of S by Se in nanostructured ZnS1−xSex thin films grown by soft chemical route for nontoxic optoelectronic device applications

Thin films of nanostructured ZnS_1−xSe_x with optimized growth parameters were prepared by soft chemical route on glass substrates. Ammonia free precursors were used at 80 °C constant bath temperature. The ratio of sulphur to selenium was changed continuously by changing the composition x (0-1), while atomic concentration of zinc was kept constant. Structure, composition and surface morphology of as-deposited films were characterized by X-ray diffraction (XRD), energy dispersive X-ray analysis (EDAX) and scanning electron microscopy (SEM), atomic force microscopy (AFM) respectively. XRD studies revealed that as-deposited films were nanostructured in nature with cubic zinc blended structure. It was further observed that the preferred orientations are along (1 1 1) plane and crystallite size decreased with increase in the value of x. SEM and AFM images revealed that films were uniform and pinhole free. The optical band gap (E_g) was calculated from the observed transmittance spectra by Urbach method. It was found that the band gap varied linearly from 3.71 to 2.70 eV, as composition x varies 0-1. The electrical properties’ study revealed that the decrease in resistivity and increase in photosensitivity, as composition x varied 0-1. The observed interesting properties of ZnS_1−xSe_x thin films justified their significance in optoelectronic device fabrication and applications, and as an environment friendly alternative to the commonly used toxic material such as CdS.     

Effect of Ag-doping on crystal structure and high temperature thermoelectric properties of c-axis oriented Ca3Co4O9 thin films by pulsed laser deposition

Ag-doped Ca₃Co₄O₉ thin films with nominal composition of Ca_3−xAg_xCo₄O₉ (x = 0∼0.4) have been prepared on sapphire (0 0 0 1) substrates by pulsed laser deposition (PLD). Structural characterizations and surface chemical states analysis have shown that Ag substitution for Ca in the thin films can be achieved with doping amount of x ≤ 0.15; while x > 0.15, excessive Ag was found as isolated and metallic species, resulting in composite structure. Based on the perfect c-axis orientation of the thin films, Ag-doping has been found to facilitate a remarkable decrease in the in-plane electrical resistivity. However, if doped beyond the substitution limit, excessive Ag was observed to severely reduce the Seebeck coefficient. Through carrier concentration adjustment by Ag-substitution, power factor of the Ag-Ca₃Co₄O₉ thin films could reach 0.73 mW m⁻¹ K⁻² at around 700 K, which was about 16% higher than that of the pure Ca₃Co₄O₉ thin film.     

Structural, morphological and optical properties of Bi2−xSbxSe3 thin films grown by electrodeposition

Ternary single-phase Bi_2−xSb_xSe₃ alloy thin films were synthesized onto Au(1 1 1) substrates from an aqueous solution containing Bi(NO₃)₃, SbCl₃, and SeO₂ at room temperature for the first time via the electrodeposition technique. The electrodeposition of the thin films was studied using cyclic voltammetry, compositional, structural, optical measurements and surface morphology. It was found that the thin films with different stoichiometry can be obtained by controlling the electrolyte composition. The as-deposited films were crystallized in the preferential orientation along the (0 1 5) plane. The SEM investigations show that the film growth proceeds via nucleation, growth of film layer and formation of spherical particles on the film layer. The particle size and shape of Bi_2−xSb_xSe₃ films could be changed by tuning the electrolyte composition. The optical absorption spectra suggest that the band gap of this alloy varied from 0.24 to 0.38 eV with increasing Sb content from x = 0 to x = 0.2.     

Structural, optical and electrical properties of CuIn5S8 thin films grown by thermal evaporation method

Stoichiometric compound of copper indium sulfur (CuIn₅S₈) was synthesized by direct reaction of high purity elemental copper, indium and sulfur in an evacuated quartz tube. The phase structure of the synthesized material revealed the cubic spinel structure. The lattice parameter (a) of single crystals was calculated to be 10.667 Å. Thin films of CuIn₅S₈ were deposited onto glass substrates under the pressure of 10⁻⁶ Torr using thermal evaporation technique. CuIn₅S₈ thin films were then thermally annealed in air from 100 to 300 °C for 2 h. The effects of thermal annealing on their physico-chemical properties were investigated using X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDX), scanning electron microscope (SEM), optical transmission and hot probe method. XRD studies of CuIn₅S₈ thin films showed that as-deposited films were amorphous in nature and transformed into polycrystalline spinel structure with strong preferred orientation along the (3 1 1) plane after the annealing at 200 °C. The composition is greatly affected by thermal treatment. From the optical transmission and reflection, an important absorption coefficient exceeds 10⁴ cm⁻¹ was found. As increasing the annealing temperature, the optical energy band gap decreases from 1.83 eV for the as-deposited films to 1.43 eV for the annealed films at 300 °C. It was found that CuIn₅S₈ thin film is an n-type semiconductor at 300 °C.     

Structural, morphological and electrical properties of spray deposited CdIn2Se4 thin films

Semiconducting n-CdIn₂Se₄ thin films have been deposited on to the amorphous and fluorine doped tin oxide (FTO) coated glass substrates using spray pyrolysis technique. The influence of solution concentration on to the photoelectrochemical, structural, morphological, compositional, thermal and electrical properties has been investigated. The PEC characterization shows that the short circuit current (I_sc) and open circuit voltage (V_oc) are at their optimum values (I_sc = 1.04 mA and V_oc = 409 mV) at the optimized precursor concentration (12.5 mM). The structural analysis shows the films are polycrystalline in nature having cubic crystal structure. The average crystallite size determined was in the range of 50-66 nm. Surface morphology and film composition have been analyzed using scanning electron microscopy and energy dispersive analysis by X-rays, respectively. The addition of solution concentration induces a decrease in the electrical resistivity of CdIn₂Se₄ films up to 12.5 mM solution concentration. The type of semiconductor was examined from thermoelectric power measurement.     

Structural, optical and electrical properties of In-doped Cd2SnO4 thin films by spray pyrolysis method

Preparation of highly conducting and transparent In-doped Cd₂SnO₄ thin film by spray pyrolysis method at a substrate temperature of 525 °C is reported. In-doping concentration is varied between 1 and 5 wt.%. The effect of In-doping on structural, optical and electrical properties was investigated using different techniques such as X-ray diffraction, atomic force microscopy, optical transmittance and Hall measurement. X-ray diffraction studies revealed that the films are polycrystalline with cubic crystal structure. The undoped and In-doped Cd₂SnO₄ films exhibit excellent optical transparency. The average optical transmittance is ∼87% in the visible range for 3 wt.% In-doping. Further In-doping widens the optical band gap from 2.98 ± 0.1 eV to 3.04 ± 0.1 eV. A minimum resistivity of 1.76 ± 0.2 × 10⁻³ Ω cm and maximum carrier concentration of 9.812 ± 0.4 × 10¹⁹ cm⁻³ have been achieved for 1 wt.% In-doping in Cd₂SnO₄ thin films.     

Analysis of current-voltage characteristics of Al/p-ZnGa2Se4/n-Si nanocrystalline heterojunction diode

The polycrystalline ZnGa₂Se₄ thin film was prepared by thermal evaporation technique on n-Si wafer followed by annealing at 700 K. Then, the Al/p- ZnGa₂Se₄/n-Si/Al heterojunction diode was fabricated. XRD pattern shows that the annealed ZnGa₂Se₄ film has a polycrystalline structure. AFM images indicate that the ZnGa₂Se₄ film is formed of nanoparticles. The dark current-voltage characteristics of the heterojunction diode at various temperatures have been investigated to determine the electrical parameters and conduction mechanism. The Al/p-ZnGa₂Se₄/n-Si/Al diode shows a rectification ratio of 2.644 × 10² at ±2 V at room temperature. It was found that at forward bias voltages ≤0.5 V, the conduction mechanism of the diode is controlled by the thermionic emission mechanism, while at bias voltages higher than 0.5 V, it is controlled by the space charge limited current mechanism. The series resistance R_s, the ideality factor n and the barrier height ?_b values of the diode are determined by performing different plots from the forward current-voltage characteristics. The reverse current mechanism of the diode is controlled by the carrier generation-recombination process in the depletion region. The obtained results show that the Al/p-ZnGa₂Se₄/n-Si/Al heterojunction is a good candidate for the electronic device applications.     

The effects of substrate temperature on refractive index dispersion and optical constants of CdZn(S0.8Se0.2)2 alloy thin films

The effect of substrate temperature on optical properties of CdZn(S_0.8Se_0.2)₂ thin films deposited onto glass substrates by the spray pyrolysis method has been investigated. The average optical transmittance of the films was over 74% in the visible range. The optical absorption studies reveal that the transition is direct with band gap energy values between 2.86 and 2.92 eV. The optical constants such as refractive index and dielectric constant of the films were determined. According to variation of the substrate temperature, the important changes in absorption edge, refractive index and the dielectric constant were observed. The refractive index dispersion curves of the films obey the single oscillator model and oscillator parameters changed with substrate temperature. The most significant result of the present study is to indicate that substrate temperature of the film can be used to modify in the optical band gaps and optical constants of CdZn(S_0.8Se_0.2)₂ thin films.     

Synthesis, structural and electrical characterization of Sb substituted spinel nickel ferrite (NiSbxFe2−xO4) nanoparticles by reverse micelle technique

Spinel nickel ferrite with nominal composition NiFe_2−xSb_xO₄ (where x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1) has been synthesized by the reverse microemulsion method. The samples synthesized were characterized by XRD, FTIR, TGA/DTGA, SEM, AFM, Mossbauer Spectroscopy and DC electrical resistivity measurements. The XRD analysis confirmed the formation of single spinel phase and the crystallite size was found to be in the range of 8-38 nm. The particle size of the synthesized samples was also confirmed by the AFM and SEM which was found in the range of 5-45 nm and 10-45 nm, respectively and this size is small enough for obtaining the suitable signal-to-noise ratio in high density recording media. The Mossbauer spectra of samples showed two well-resolved Zeeman patterns corresponding to A and B sites and also observed a doublet at higher substitution. The DC electrical resistivity showed an interesting behavior with temperature and observed a metal-to-semiconductor transition temperature (T_M-S) which suggests that the material can be applied for switching applications. The resistivity increases with the increase in Sb-content and it suggests that the material can be fruitfully used for applications in microwave devices.     

Synthesis of nanocrystalline nickel-zinc ferrite (Ni0.8Zn0.2Fe2O4) thin films by chemical bath deposition method

The nickel-zinc ferrite (Ni_0.8Zn_0.2Fe₂O₄) thin films have been successfully deposited on stainless steel substrates using a chemical bath deposition method from alkaline bath. The films were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), static water contact angle and cyclic voltammetry measurements. The X-ray diffraction pattern shows that deposited Ni_0.8Zn_0.2Fe₂O₄ thin films were oriented along (3 1 1) plane. The FTIR spectra showed strong absorption peaks around 600 cm⁻¹ which are typical for cubic spinel crystal structure. SEM study revealed compact flakes like morphology having thickness ∼1.8 μm after air annealing. The annealed films were super hydrophilic in nature having a static water contact angle (θ) of 5°.The electrochemical supercapacitor study of Ni_0.8Zn_0.2Fe₂O₄ thin films has been carried out in 6 M KOH electrolyte.The values of interfacial and specific capacitances obtained were 0.0285 F cm⁻² and 19 F g⁻¹, respectively.     

Synthesis of nanostructured CuxS thin films by chemical route at room temperature and investigation of their size dependent physical properties

Nanostructured semiconductors show very interesting physical properties than bulk crystal due to size effects that arises because of quantum confinement of the electronic states. Using cupric acetate and sodium thiosulphate as cationic and anionic precursor, nanostructured Cu₂S thin films were successfully prepared at room temperature by chemical bath deposition technique. By varying the deposition time from 9 to 24 h, the Cu₂S films of thickness 70-233 nm were prepared. The different characterization methods such as X-ray diffraction (XRD), scanning electron microscopy (SEM), optical absorption and electrical resistivity measurement techniques were used to investigate size dependent properties of Cu₂S thin films. As thickness increases, the hexagonal covellite phase of CuS observed at thickness 70 nm gets converted to monoclinic chalcosite phase of Cu₂S. The resistivity and activation energy is found to be thickness dependent. The optical band-gap energy increases from 2.48 to 2.90 eV as thickness decreases from 233 to 70 nm. The influence of film thickness on carrier concentration, mobility and thermo-emf is reported.     

In doped ZnO thin films

ZnO thin films were deposited by ultrasonic spray technique, zinc acetate was used as starting solution with a molarity of 0.1 M. A set of indium (In) doped ZnO (between 2 and 8 wt%) thin films were grown on glass substrate at 350 °C. The present work is focused on the influence of the doping level on the structural, optical and electrical films properties. Optical film characterization was carried by using UV-visible transmission spectroscopy, the optical gap was deduced from absorption. From X ray diffraction (XRD) analysis, we have deduced that ZnO films are formed with nanocrystalline structure with preferential (0 0 2) orientation. The grain size is increased with In doping from 28 to 37 nm. Electrical characterization was achieved using two-probes coplanar structure, the measured conductivity varies from 2.3 to 5.9 Ω cm⁻¹ when increasing the doping level. However the optical gap is reduced from 3.4 to 3.1 eV.     

Structural, morphological and electrical properties of spray deposited nano-crystalline CeO2 thin films

Nanocrystalline, uniform, dense, and adherent cerium oxide (CeO₂) thin films have been successfully deposited by a simple and cost effective spray pyrolysis technique. CeO₂ films were deposited at low substrate and annealing temperatures of 350 °C and 500 °C, respectively. Films were characterized by differential thermal analysis, X-ray diffraction, scanning electron microscopy, atomic force microscopy; two probe resistivity method and impedance spectroscopy. X-ray diffraction analysis revealed the formation of single phase, well crystalline thin films with cubic fluorite structure. Crystallite size was found to be in the range of 10-15 nm. AFM showed formation of smooth films with morphological grain size 27 nm. Films were found to be highly resistive with room temperature resistivity of the order of 10⁷ Ω cm. Activation energy was calculated and found to be 0.78 eV. The deposited film showed high oxygen ion conductivity of 5.94 × 10⁻³ S cm⁻¹ at 350 °C. Thus, the deposited material shows a potential application in intermediate temperature solid oxide fuel cells (IT-SOFC) and might be useful for μ-SOFC and industrial catalyst applications.     

Effect of annealing temperature on optical band-gap of amorphous indium zinc oxide film

The effect of annealing temperature on the electrical and optical properties of indium zinc oxide (IZO) (In₂O₃:ZnO = 90:10 wt.%) thin films has been investigated. The IZO thin films were deposited on glass substrates by radio frequency magnetron sputtering and then subjected to annealing in a mixed ambient of air and oxygen at 100, 200 and 300 °C. All the IZO films were found to have amorphous structure. With the increase of the annealing temperature, the carrier concentration decreased and the resistivity increased. The average transmittance of IZO thin films decreased slightly with annealing temperature. Interestingly, a systematic reduction of the optical band-gap from 3.79 eV to 3.67 eV was observed with annealing temperature. The change in optical band-gap was observed to be caused predominantly by Burstein-Moss band-gap widening effect suggesting unusual absence of band narrowing effect. The effects on optical and electrical properties of IZO films have been discussed in detail.     

Optical and electrical properties of zinc oxide thin films with low resistivity via Li-N dual-acceptor doping

Zinc oxide thin films with low resistivity have been deposited on glass substrates by Li-N dual-acceptor doping method via a modified successive ionic layer adsorption and reaction process. The thin films were systematically characterized via scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction, ultraviolet-visible spectrophotometry and fluorescence spectrophotometry. The resistivity of zinc oxide film was found to be 1.04 Ω cm with a Hall mobility of 0.749 cm² V⁻¹ s⁻¹ and carrier concentration of 8.02 × 10¹⁸ cm⁻³. The Li-N dual-acceptor doped zinc oxide films showed good crystallinity with prior c-axis orientation, and high transmittance of about 80% in visible range. Moreover, the effects of Li doping level and other parameters on crystallinity, electrical and ultraviolet emission of zinc oxide films were investigated.     

Effects of Mn substitution on ferro- and piezoelectric properties of Bi0.86Sm0.14FeO3 thin films

The Bi_0.86Sm_0.14FeO₃ (BSFO) and Bi_0.86Sm_0.14Fe_1 − xMn_xO₃ (BSFMO) (x = 0.01, 0.03, 0.05) thin films were deposited on indium tin oxide/glass substrates via a metal organic deposition method. 1 at.% Mn doping leads to an evident reduction of the leakage current in BSFO film. More importantly, the Bi_0.86Sm_0.14Fe_0.99Mn_0.01O₃ film exhibits the lowest coercive field (E_c = 272 kV/cm), the largest remanent polarization (P_r = 53.6 μc/cm²) and the remanent out-of-plane piezoelectric coefficient (d₃₃ = 146 pm/V). However, further increase of Mn doping content results in the deterioration of the charge retaining capability and the piezoelectric properties of the films. The negative influence of high Mn doping contents was discussed based on the structure change and the contribution of irreversible movement of non-180° domain walls in the aged films.     

Synthesis and characterization of sol-gel derived gallium-doped zinc oxide thin films

Gallium-doped ZnO (GZO) semiconductor thin films were prepared by a sol-gel spin coating process. The effects of Ga dopant concentrations on the microstructure, electrical resistivity, optical properties, and photoluminescence (PL) were studied. XRD results showed that all the as-prepared GZO films had a wurtzite phase and a preferred orientation along the [0 0 2] direction. ZnO thin films doped with Ga had lower electrical resistivity, lower RMS roughness, and improved optical transmittance in the visible region. The lowest average electrical resistivity value, 2.8 × 10² Ω cm, was achieved in the ZnO thin films doped with 2% Ga, which exhibited an average transmittance of 91.5%. This study also found that the optical band gap of Ga-doped films was 3.25 eV, slightly higher than that of undoped samples (3.23 eV), and the PL spectra of GZO films showed strong violet-light emission centers at about 2.86 eV (the corresponding wavelength of which is about 434 nm).     

Structural, optical and electrical properties of Mg-doped CuCrO2 thin films by sol-gel processing

CuCr_1−xMg_xO₂ (x = 0, 0.03, 0.05, 0.07) thin films were prepared on sapphire substrates by sol-gel processing. The effect of Mg concentrations on the structural, morphological, electrical and optical properties was investigated. Highly transparent ≧70% Mg-doped CuCrO₂ thin films with p-type conduction and semiconductor behavior were obtained. The microstructure of the systems was characterized by scanning electron microscopy and the roughness increased as the content of Mg increased. The photoluminescence spectra results indicated that it had a green luminescent emission peak at the 530 nm. In this paper, CuCr_0.95Mg_0.05O₂ film has the lowest resistivity of 7.34 Ω cm with direct band gap of 3.11 eV. In order to investigate the conduction mechanism, the energy band of the CuCrO₂ films is constructed based on the grain-boundary scattering.     

Dual codoping for the fabrication of low resistive p-ZnO

A dual codoping method has been proposed to fabricate low resistive and stable p-ZnO thin films. Both nitrogen (N) and arsenic (As) have been used as acceptors while aluminum (Al) as donor in our dual codoping process. The As-Al-N dual codoped ZnO films have been prepared by RF magnetron sputtering on GaAs substrate using AlN doped ZnO targets (0.5, 1 and 2 mol%). In our dual codoping approach, Al and N from target and As from GaAs substrate (back diffusion) take part. X-ray diffraction (XRD), room temperature and low temperature photoluminescence (PL), electron probe micro analysis (EPMA), energy dispersive spectroscopy (EDS), atomic force microscopy (AFM) and Hall effect measurement have been performed to investigate the effect of AlN concentration on the dual codoped ZnO films. All the films (0, 0.5 and 1 mol%) showed p-type conductivity except 2 mol% AlN doped film. The lowest room temperature resistivity, 8.6 × 10⁻² Ω cm has been achieved with a hole concentration of the order, 10²⁰ cm⁻³ for the optimum 1 mol% AlN concentration. The observed resistivity is much lower than that of monodoped (As or N) and codoped (AlN or AlAs) ZnO films. The p-type conductivity has been explained by the new complex formation mechanism.     

Realization of stable p-type ZnO thin films using Li-N dual acceptors

Lithium and nitrogen dual acceptors-doped p-type ZnO thin films have been prepared using spray pyrolysis technique. The influence of dual acceptor (Li, N) doping on the structural, electrical, and optical properties of (Li, N):ZnO films are investigated in detail. The (Li, N):ZnO films exhibit good crystallinity with a preferred c-axis orientation. From AFM studies, it is found that the surface roughness of the thin films increases with the increase of doping percentage. The Hall Effect measurements showed p-type conductivity. The Hall measurements have been performed periodically up to seven months and it is observed that the films show p-type conductivity throughout the period of observation. The samples with Li:N ratio of 8:8 mol% showed the lowest resistivity of 35.78 Ω cm, while sample with Li:N ratio of 6:6 mol% showed highest carrier concentration. The PL spectra of (Li, N):ZnO films show a strong UV emission at room temperature. Furthermore, PL spectra show low intensity in deep level transition, indicating a low density of native defects. This indicates that the formation of intrinsic defects is effectively suppressed by dual acceptor (Li, N) doping in ZnO thin films. The chemical bonding states of N and Li in the films were examined by XPS analysis.