Blue/green luminescence based on Zn(S)Se/GaAs heterostructures

Substantial advances have been realized in the aim to achieve blue–green light emitting devices based on Zn(S)Se wide band gap II–VI semi-conductor materials. Two light emitting diodes p on n and n on p heterostructures were grown on GaAs substrate by molecular beam epitaxy. The active layer was a single ZnCdSe quantum well, with ZnSSe guiding layers and ZnSe cladding layers. p-GaInP, p-AlGaAs and p-CdZnSe buffer layers were deposited at the p-ZnSe/GaAs interface to reduce the valence band offset in the case of n on p heterostructures. Electrical and optical properties were investigated using current voltage, capacitance voltage, electroluminescence, photoluminescence and photocurrent measurements at room temperature. Blue–green luminescence centered at 516.7 nm is observed. The highest luminescence intensity is observed under 7 V forward bias. Photoluminescence spectrum shows two wide peaks at 2.2 and 1.9 eV energies. These energies are attributed to the transitions between ZnSe and GaAs conduction bands and the deep level at E_v−0.6 eV. Absorption process from ZnSe and ZnSSe conduction bands to the shallow nitrogen acceptor level (2.6 and 2.8 eV, respectively) have been observed using photocurrent measurements. From these results we present a band alignment diagram which confirms the presence of the two levels at 0.1 and 0.6 eV from the valence band of ZnSe.     

Behavior of chemically deposited PbS thin films subjected to two different routes of post deposition annealing

Lead sulfide (PbS) thin films were prepared on soda lime glass substrates at room temperature by Chemical Bath Deposition (CBD) technique. This paper reports a comparative study of characteristic properties of as-prepared PbS thin films after thermal treatment through two different routes. Studies were carried out for as-prepared as well as rapidly and gradually annealed samples at 100, 200 and 300 °C. The characterizations of the films were carried out using X-ray diffraction, scanning electron microscopy and optical measurement techniques. The structural studies confirmed the polycrystalline nature and the cubic structure of the films. As-deposited films partly transformed to Pb₂O₃ when gradually annealed to 300 °C. The presence of nano crystallites was revealed by structural and optical absorption measurements. The values of average crystallite size were found to be in the range 18–20 nm. The variation in the microstructure, thickness, grain size, micro strain and optical band gap on two types of annealing were compared and analyzed. Data showed that post deposition parameters and thermal treatment strongly influence the optical properties of PbS films. Optical band gap of the film gets modified remarkably on annealing. Direct band gap energy values for rapidly and gradually annealed samples varied in the range of 1.68–2.01 eV and 1.68–2.12 eV respectively. Thus we were succeeded in tailoring direct band gap energies by post deposition annealing method.     

Optical properties of undoped and cobalt doped ZnS nanophosphor

In the present study, the optical properties of ZnS and cobalt (Co) doped ZnS nanoparticles were investigated at room temperature. ZnS and ZnS:Co nanophosphors were prepared through chemical route, namely the chemical precipitation method and the formation of the nanoparticles were confirmed by X-ray diffraction and field emission scanning electron microscope (FESEM). Band gap energy of the prepared samples is determined by using a UV–vis–NIR spectrophotometer. The photoluminescence property of ZnS and ZnS:Co sample is determined by fluorescence spectroscopy. The sizes of as prepared nanoparticles are found to be in the 8–9 nm range. The FESEM morphology shows the formation of nanostructure of ZnS samples. The value of optical band gap has been found to be in the range 4.30–4.03 eV. Room temperature photoluminescence (PL) spectrum of the undoped sample exhibits emission in the blue region with multiple peaks under UV excitation. On the other hand, Co²⁺ doped ZnS samples show enhanced visible light emissions under the same UV excitation wavelength of 310 nm.     

Pulse electrodeposited copper indium sulfide films

Copper indium sulfide (CISu) films were deposited by the pulse galvanostatic deposition technique at different duty cycles. The films are polycrystalline with peaks corresponding to the chalcopyrite phase of CISu. The grain size and surface roughness increased from 10 to 25 nm and 0.85 to 2.50 nm respectively with increase of duty cycle. Optical band gap in the range of 1.30–1.51 eV was observed for the films deposited at different duty cycles. Room temperature resistivity of the films is in the range of 0.1–3.67 Ω cm. Photoconductivity measurements were made at room temperature. Photocurrent spectra exhibited maximum corresponding to the band gap of copper indium sulphide. CdS/CuInS₂ fabricated with CISu films deposited at 50% duty cycle have exhibited a V_oc of 0.62 V, J_sc of 16.30 mA cm^?2, FF of 0.71 and efficiency of 7.16%.     

Brush electrodeposited silver indium selenide films and their optical characteristics

Silver indium selenide films were brush electrodeposited on tin oxide coated glass substrates at different substrate temperatures. The films were single phase with chalcopyrite structure. Optical absorption measurements indicated a band gap in the range of 1.20–1.30 eV with decrease of substrate temperature. Transmission spectra exhibited interference fringes. Using the envelope method, calculated values of refractive index at 850 nm decreased from 3.53 to 2.62 with decrease of substrate temperature. From the refractive index data, the value of N/m^⁎ was estimated to be in the range of 0.89–1.22. Optical data were analyzed by the single-effective oscillator model, and the single oscillator energy as well as the dispersion energy was estimated. The single oscillator energy decreased from 1.83 eV to 1.68 eV with the increase of substrate temperature. The dispersion energy increased from 5.42 eV to 12.25 eV with the increase of substrate temperature.     

Effects of annealing temperature on the photoluminescence of RF sputtered Barium titanate thin films

BaTiO₃ thin films were deposited onto quartz substrates by an RF magnetron sputtering method. The films deposited at room temperature and annealed at 773–1173 K were characterized using X-ray diffraction (XRD)Scanning electron microscopy (SEM), UV–vis spectroscopy and Photoluminescence spectroscopy (PL). X-ray diffraction studies revealed that the film is amorphous in nature at 773 K and that the crystallinity increases with increase in annealing temperature. The average crystallite size of the films increased from 13–18 nm and the optical band gap decreased in the range of 4.33–3.43 eV, with increase in annealing temperature. The films exhibited good adherence to the substrates and the SEM images showed smooth surface morphology. Energy dispersive X-ray (EDX) analysis confirmed the presence of barium, titanium and oxygen in the film. The red-shifts of excitonic UV emission peaks were observed in all samples which can be attributed to the stress produced due to lattice distortions. The visible PL emission intensity showed appreciable enhancement with post-deposition annealing.     

Valence band anticrossing model for GaSb1−xBix and GaP1−xBix using k.p method

The reduction in band gap as well as the increase in spin–orbit splitting energy in GaSb_1−xBi_x and GaP_1−xBi_x are explained by the Valence Band Anticrossing (VBAC) model. This restructuring of the valence band is due to the interaction of the Bi related impurity levels with the extended states of the valence band of the host semiconductor. The band gap reduction in GaSb_1−xBi_x and GaP_1−xBi_x calculated using VBAC model are respectively 40.2 meV and 206 meV/at% Bi. A comparison of the theoretical and experimentally obtained values of band gap in GaSbBi shows good agreement. Valence band structure for GaPBi is obtained by the extrapolation of the parameters used for modeling of the GaSbBi system. The upward movement of the spin–orbit split-off E₊ energy level in GaSbBi by 19.2 meV/at% Bi is also responsible for the suppression of Auger recombination processes making it a potential candidate for near and mid-infrared optoelectronic applications.     

Intragap-trapped-carriers enhancement of the low-temperature delayed phosphorescence in Alq3

Time resolved electroluminescence of Alq₃ based organic light emitting devices was measured on a millisecond time scale. The millisecond delayed electroluminescence (DEL) was detected at 80 K and 26 K, while no long lived electroluminescence was observed at room temperature. The DEL spectrum consists of two components centered at ∼2.3 and ∼1.9 eV. The shape of the high energy band is similar to the prompt fluorescence emission peak, although a systematic red shift of about 30 meV is reproducibly found. The low-energy band position corresponds to the phosphorescence of Alq₃. The time decay of both bands is nonexponential, with the low energy band decaying slower. To explain dependencies of intensities of both components on the delay and the injection-current density a mechanism based on enhancement of the triplet-exciton radiative decay by carriers trapped in deep intragap traps is proposed.     

Annealing effects on physical properties of doped CdTe thin films for photovoltaic applications

Polycrystalline Cadmium Telluride (CdTe) thin films were prepared on glass substrates by thermal evaporation at the chamber ambient temperature and then annealed for an hour in vacuum ~1×10⁻⁵ mbar at 400 °C. These annealed thin films were doped with copper (Cu) via ion exchange by immersing these films in Cu (NO₃)₂ solution (1 g/1000 ml) for 20 min. Further these films were again annealed at different temperatures for better diffusion of dopant species. The physical properties of an as doped sample and samples annealed at different temperatures after doping were determined by using energy dispersive x-ray analysis (EDX), x-ray diffraction (XRD), Raman spectroscopy, transmission spectra analysis, photoconductivity response and hot probe for conductivity type. The optical band gap of these thermally evaporated Cu doped CdTe thin films was determined from the transmission spectra and was found to be in the range 1.42–1.75 eV. The direct energy band gap was found annealing temperatures dependent. The absorption coefficient was >10⁴ cm⁻¹ for incident photons having energy greater than the band gap energy. Optical density was observed also dependent on postdoping annealing temperature. All samples were found having p-type conductivity. These films are strong potential candidates for photovoltaic applications like solar cells.     

Model based precise analysis of the injection currents in Al/ZrO2/Al2O3/ZrO2/SiO2/Si structures for use in charge trapping non-volatile memory devices

Metal/insulator/Silicon (MIS) capacitors containing multilayered ZrO₂/Al₂O₃/ZrO₂/SiO₂ dielectric were investigated in order to evaluate the possibility of their application in charge trapping non-volatile memory devices. The ZrO₂/Al₂O₃/ZrO₂ stacks were deposited by reactive rf magnetron sputtering on 2.4 nm thick SiO₂ thermally grown on p-type Si substrate. C–V characteristics at room temperature and I–V characteristics recorded at temperatures ranging from 297 K to 393 K were analyzed by a comprehensive model previously developed. It has been found that Poole-Frenkel conduction in ZrO₂ layers occurs via traps energetically located at 0.86 eV and 1.39 eV below the bottom of the conduction band. These levels are identified as the first two oxygen vacancies related levels in ZrO₂, closest to its conduction band edge, whose theoretical values reported in literature are: 0.80 eV, for fourfold, and 1.23 eV, for threefold coordinated oxygen vacancies.     

Deposition and characterization of pure and Cd doped SnO2 thin films by the nebulizer spray pyrolysis (NSP) technique

Pure and cadmium doped tin oxide thin films were deposited on glass substrates from aqueous solution of cadmium acetate, tin (IV) chloride and sodium hydroxide by the nebulizer spray pyrolysis (NSP) technique. X-ray diffraction reveals that all films have tetragonal crystalline structure with preferential orientation along (200) plane. On application of the Scherrer formula, it is found that the maximum size of grains is 67 nm. Scanning electron microscopy shows that the grains are of rod and spherical in shape. Energy dispersive X-ray analysis reveals the average ratio of the atomic percentage of pure and Cd doped SnO₂ films. The electrical resistivity is found to be 10² Ω cm at higher temperature (170 °C) and 10³ Ω cm at lower temperature (30 °C). Optical band gap energy was determined from transmittance and absorbance data obtained from UV–vis spectra. Optical studies reveal that the band gap energy decreases from 3.90 eV to 3.52 eV due to the addition of Cd as dopant with different concentrations.     

Effect of cobalt substitution on the optical properties of bismuth ferrite thin films

Effect of cobalt substitution on the band gap and absorption coefficient of the BiFeO₃ thin films formed on quartz substrate by low cost spin coating method have been investigated. BiFe_1−xCo_xO₃ (x=0, 0.03, 0.06 and 0.10) thin films are polycrystalline and it retains the rhombohedral distorted perovskite structure up to 10 mole % of Co substitution. Smooth and compact surface morphology with uniform size particles are observed in SEM micrographs. Narrowing and broadening of band gap is observed as a function of Co content. Two strong emission peaks at ~2.51 eV and ~2.38 eV are recorded for all films with noticeable change in intensity. Results obtained from the optical absorption and photoluminescence spectroscopy experiments have shown that there exists an inverse correlation between the variation in the band gap and the concentration of oxygen vacancies. Band gap decreased by ~100 meV and absorption coefficient increased by 28% at the wavelength of 375 nm in 6 mole % Co substituted thin film and these observations are necessary requirements to improve the efficiency of photovoltaic devices.     

Optical and electrical measurement of FeSe2 thin films obtained at low temperature

FeSe₂ thin films were prepared at low temperature by thermal annealing at 350 °C during 6 h of sequentially evaporated iron and selenium films under selenium atmosphere. The structural, optical and electrical characteristics were investigated. The roughness of films (~76 nm) was confirmed by AFM images. Moreover, optical band gap of FeSe₂, which was evaluated as nearly 1.11 eV and confirmed by the electrical study which yielded a value in the order of 1.08 eV. The electrical conductivity, conduction mechanism, dielectric properties and relaxation model of theses thin films were studied using impedance spectroscopy technique in the frequency range 5 Hz–13 MHz under various temperatures (180–300 °C). Besides, complex impedance and AC conductivity have been investigated on the basis of frequency and temperature dependence.     

Defect studies in Cu2ZnSnSe4 and Cu2ZnSn(Se0.75S0.25)4 by admittance and photoluminescence spectroscopy

To achieve higher record efficiencies for solar cells containing Cu₂ZnSnSe₄ (CZTSe), Cu₂ZnSnS₄ (CZTS) or their solid solution Cu₂ZnSn(Se_xS_1?x)₄ (CZTSSe) as an absorber, it is necessary to obtain more knowledge about defect structure of these materials. In this work, admittance spectroscopy (AS) and low temperature photoluminescence spectroscopy (PL) were used for defect studies. Admittance spectroscopy in the frequency range from 20 Hz to 10 MHz was used for studies of CZTSe/CdS and CZTSSe/CdS monograin layer heterojunctions. The measurement temperature varied from 140 K to 245 K. Two defect states (labelled E_A1 and E_A2) were found in Cu₂ZnSnSe₄ and Cu₂ZnSn(Se_0.75S_0.25)₄. In different CZTSe/CdS heterojunctions the E_A2 state was present at 74 meV, but the second E_A1 defect state changed from 87 meV to 100 meV during time and had varying properties. In Cu₂ZnSn(Se_0.75S_0.25)₄ the E_A2 state was found at 25 meV. The E_A1 state at 154 meV showed the same properties as the two defect levels in CZTSe. In both cases the E_A2 defect state was attributed to an acceptor defect and the E_A1 state with changing properties to interface states. The detected PL bands were at 0.946 eV in CZTSe and at 1.028 eV in Cu₂ZnSn(Se_0.75S_0.25)₄. Obtained by PL measurements, defect states at 69 meV in CZTSe and at 39 meV in Cu₂ZnSn(Se_0.75S_0.25)₄ were attributed to the same acceptor defect that was found from the AS measurements.     

Characteristics of pulse plated copper indium telluride films

Copper Indium Telluride films were deposited for the first time by the pulse electrodeposition technique at different duty cycles in the range of 6–50% at room temperature and at a constant potential of −0.66 V(SCE). The films exhibited single phase copper indium telluride. The grain size increased with increase of duty cycle. Optical band gap of the films varied in the range of 0.98–1.02 eV. Atomic force microscopy studies indicated that the grain size and surface roughness vary from 15 nm to 30 nm and 1.0 nm to 1.5 nm, respectively with increase of duty cycle. Capacitance voltage measurements indicated the films to exhibit n-type behavior. The flat band potential was −0.76 V(SCE) and carrier density was in the range of 10¹⁶ cm⁻³.     

Structural,morphological, optical,and magnetic properties of Ni-doped CuO nanostructures prepared by a rapid microwave combustion method

In the present paper, we report a facile and rapid microwave-assisted combustion synthesis method for the preparation of pure and Ni-doped CuO nanostructures with different weight ratios (0.5, 1.0, 1.5, and 2.0 at wt% of Ni). The structure and morphology of the pure and Ni-doped CuO samples were investigated by X-ray diffraction (XRD), high resolution scanning electron microscopy (HR-SEM), energy dispersive x-ray analysis (EDX), diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy and vibrating sample magnetometry (VSM). XRD patterns refined by the Rietveld method indicated the formation of single-phase monoclinic structure and also confirmed that Ni ions are successfully doped into CuO crystal lattice by occupying Cu ionic sites. Interestingly, the morphology was found to transform substantially from nanoflowers to nanoparticles with close-packed periodic array, and then into nanocrystals with the variation of Ni content. The optical band gap estimated using DRS was found to be 3.9 eV for pure CuO and then increases up to 4.3 eV with increasing Ni content. PL spectra at room temperature showed a strong green emission band, and thereby confirmed the above results. Magnetic measurements reveal a room temperature ferromagnetism (RTFM) with an optimum value of saturation magnetization of 1.3140×10⁻³ emu/g for 2.0 wt% of Ni.     

Pulsed laser deposition and characterization of La1−xSrxMnO3

La_1−xSr_xMnO₃ manganite films with x=0.15, 0.33 and 0.4 were deposited onto silicon substrates by pulsed laser deposition in an 80/20 Ar/O₂ atmosphere at room temperature. After being deposited, the films were annealed at 900 °C in air in order to obtain the desired crystalline phase. Structural characterizations using X-Ray diffraction showed polycrystalline compounds having the predominant peaks corresponding to the perovskite structure. Morphological studies carried out by scanning electron microscopy revealed a very rough surface and led to deducing the nature of the growth process. Spectral ellipsometry was performed between 1.5 and 5 eV range at room temperature, showing electronic transitions near to 2.2 and 4.7 eV in a range of thicknesses between 94 and 107 nm.     

Local electronic structure,optical bandgap and photoluminescence (PL) properties of Ba(Zr0.75Ti0.25)O3 powders

Ba(Zr_0.75Ti_0.25)O₃ (BZT-75/25) powders were synthesized by the polymeric precursor method. Samples were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques. Their electronic structures were evaluated by first-principle quantum mechanical calculations based on density functional theory at the B3LYP level. Their optical properties were investigated by ultraviolet-visible (UV-Vis) spectroscopy and photoluminescence (PL) measurements at room temperature. XRD patterns and Rietveld refinement data indicate that the samples have a cubic structure. XANES spectra confirm the presence of pyramidal [TiO₅] clusters and octahedral [TiO₆] clusters in the disordered BZT-75/25 powders. EXAFS spectra indicate distortion of Ti–O and Ti–O–Ti bonds the first and second coordination shells, respectively. UV-Vis absorption spectra confirm the presence of different optical bandgap values and the band structure indicates an indirect bandgap for this material. The density of states demonstrates that intermediate energy levels occur between the valence band (VB) and the conduction band (CB). These electronic levels are due to the predominance of 4d orbitals of Zr atoms in relation to 3d orbitals of Ti atoms in the CB, while the VB is dominated by 2p orbitals related to O atoms. There was good correlation between the experimental and theoretical optical bandgap values. When excited at 482 nm at room temperature, BZT-75/25 powder treated at 500 °C for 2 h exhibited broad and intense PL emission with a maximum at 578 nm in the yellow region.     

Properties of CdTe films brush plated on high temperature substrates

CdTe thin films were brush plated on substrates maintained at temperatures in the range 30–90 °C from the precursors. The films exhibited cubic structure. Optical band gap of 1.45 eV was obtained. XPS measurements indicated the formation of CdTe. AFM studies indicated the formation of fine grains of the order of 50 nm, for the films deposited on room temperature substrates. Hot probe measurements indicated films to be n-type. A mobility in the range of 5–60 cm² V⁻¹ s⁻¹ and a carrier density of 10¹⁵ cm⁻³ was obtained.     

Physical investigations on Sb2S3 sprayed thin film for optoelectronic applications

Sb₂S₃ thin films have been obtained at 250 °C on glass substrates using the spray pyrolysis techniques. The structural study by means of XRD analysis shows that Sb₂S₃ thin film crystallized in the orthorhombic phase. The discussion of some structural constants has been made by means of both XRD and AFM investigations. Moreover, the optical analysis via the transmittance and the reflectance measurements reveals that Sb₂S₃ sprayed thin film has a direct transition with the band gap energy E_g equal to 1.72 eV. The analysis in 300–2500 nm domain of the refractive index data through Wemple–DiDomenico model leads to the single oscillator energy (E₀=2.32 eV), and the dispersion energy (E_d=10.03 eV). The electrical study leads to the dc activation energy is of the order of 0.72 eV and the maximum barrier high is W_M=0.87 eV. From the power exponent variation in terms of the heated temperature, it is found that the mechanism of conduction matches well the correlated barrier hopping CBH model.