Effects of the annealing temperature on the structure and up-conversion photoluminescence of ZnO film

The up-conversion film is being tried to increase the photoelectric conversion efficiency of the silicon solar cell. To improve the efficiency of the photoluminescence film, the effects of the annealing temperature were investigated on the structure and photoluminescence of the ZnO up-conversion film, which was prepared using the sol-gel method and the spin-coating technique. The results show that the organic compounds and water in the ZnO film were completely eliminated when the annealing temperature reached 500?°C. The crystallinity of film is improved and the average grain size continuously increases as increasing the annealing temperature. The transmittance in the wavelength range of 400–2000?nm continuously increases as the annealing temperature increases from 500?°C to 700?°C, whilst it decreases first and then increases as the annealing temperature increases from 800?°C to 1000?°C. When the film is excited with a laser of 980?nm, there are two intense emission bands in the up-conversion emission spectra, 542-nm green light and 660-nm red light, corresponding to Ho³⁺: ⁵S₂/⁵F₄?→?⁵I₈?and ⁵F₅?→?⁵I₈ transitions, respectively. In addition, the intensity of up-conversion luminescence for the film increases first and then decreases with the increase of the annealing temperature. When the annealing temperature is at 900?°C, the film consists of small round compact particles with a high degree of crystallization, reaching maximum up-conversion intensity of the film.     

Low temperature annealing effects on the structure and optical properties of ZnO films grown by pulsed laser deposition

ZnO thin films were deposited on glass substrates at room temperature (RT) ∼500 °C by pulsed laser deposition (PLD) technique and then were annealed at 150-450 °C in air. The effects of annealing temperature on the microstructure and optical properties of the thin films deposited at each substrate temperature were investigated by XRD, SEM, transmittance spectra, and photoluminescence (PL). The results showed that the c-axis orientation of ZnO thin films was not destroyed by annealing treatments; the grain size increased and stress relaxed for the films deposited at 200-500 °C, and thin films densified for the films deposited at RT with increasing annealing temperature. The transmittance spectra indicated that E_g of thin films showed a decreased trend with annealing temperature. From the PL measurements, there was a general trend, that is UV emission enhanced with lower annealing temperature and disappeared at higher annealing temperature for the films deposited at 200-500 °C; no UV emission was observed for the films deposited at RT regardless of annealing treatment. Improvement of grain size and stoichiometric ratio with annealing temperature can be attributed to the enhancement of UV emission, but the adsorbed oxygen species on the surface and grain boundary of films are thought to contribute the annihilation of UV emission. It seems that annealing at lower temperature in air is an effective method to improve the UV emission for thin films deposited on glass substrate at substrate temperature above RT.     

Highly luminescent columnar ZnO films grown directly on n-Si and p-Si substrates by low-temperature electrochemical deposition

In this study, nanocolumnar zinc oxide thin films were catalyst-free electrodeposited directly on n-Si and p-Si substrates, what makes an important junction for optoelectronic devices. We demonstrate that ZnO thin films can be grown on Si at low cathodic potential by electrochemical synthesis. The scanning electron microscopy SEM showed that the ZnO thin films consist of nanocolumns with radius of about 150 nm on n-Si and 200 nm on p-Si substrates, possess uniform size distribution and fully covers surfaces. X-ray diffraction (XRD) measurements show that the films are crystalline material and are preferably grown along (0 0 2) direction. The impact of thermal annealing in the temperature range of 150-800 °C on ZnO film properties has been carried out. Low-temperature photoluminescence (PL) spectra of the as-prepared ZnO/Si samples show the extremely high intensity of the near bandgap luminescence along with the absence of visible emission. The optical quality of ZnO thin films was improved after post-deposition thermal treatment at 150 °C and 400 °C in our experiments, however, the luminescence intensity was found to decrease at higher annealing temperatures (800 °C). The obtained results indicate that electrodeposition is an efficient low-temperature technique for the growth of high-quality and crystallographically oriented ZnO thin films on n-Si and p-Si substrates for device applications.     

Effect of microwave remote plasma and radiofrequency plasma on the photoluminescence of (0001) epitaxial ZnO films

Photoluminescence of (0001) epitaxial ZnO films with thicknesses of 10, 30 and 100 nm on C-sapphire substrates have been studied at room temperature and after exposure to Ar, Ar–O₂, Ar–N₂ and Ar–H by remote microwave and radiofrequency plasmas. The photoluminescence are not modified by remote plasma treatments where only neutral species were involved. On the contrary, the photoluminescence signal is enhanced or quenched after radiofrequency plasma treatments when energetic ion species are involved in the surface treatment processes. Little changes of electric properties are observed, however, the optical transmission indicates that the absorption edge and probably also the index of refraction are affected. Photoluminescence peak shifts, widths and intensities changes show very strong similarities with polarized emission of ZnO single crystal where it exists a strong dichroism. The photoluminescence emission properties may then result from this optical modification. However, the plasma treatments on the different samples show very low stability in time, except, for the treatment in argon plasma alone. In this later case, in-situ monitoring of photoluminescence as a function of temperature revealed a partial recovery of the photoluminescence properties after a heat treatment at 400 °C for few minutes. These results indicate that photoluminescence of (0001) ZnO thin film, related to σ-emission polarized emission from c-axis polar surfaces, is highly affected by surface and implanted charged species.     

Excitonic luminescence in ZnO nanopowders and ceramics

Fast photoluminescence spectra in the spectral region of 3.1–3.45 eV in ZnO and ZnO:Al ceramics were studied at 14 and 300 K. Ceramics with grains smaller than 100 nm were sintered from nanopowders by high pressure (8 GPa) and low temperature (350 °C). Ceramics with grain sizes 1–5 μm were sintered at 1400 °C. It is shown that excitonic luminescence spectra depend on the ceramics grain size, post preparing annealing and doping. The excitonic luminescence decay time was faster than 2 ns and the afterglow at 30 ns was 0.05%.     

Enhanced conductivity of aluminum doped ZnO films by hydrogen plasma treatment

Aluminum doped zinc oxide (AZO) thin films prepared by radio-frequency (RF) magnetron sputtering at various RF power were treated by hydrogen plasma to enhance the characteristics for transparent electrode applications. The hydrogen plasma treatment was carried out at 300 °C in a plasma enhanced chemical vapor deposition system. X-ray diffraction analysis shows that all AZO films have a (002) preferred orientation and film crystallinity seems no significant change after plasma treatment. The plasma treatment not only significantly decreases film resistivity but enhances electrical stability as aging in air ambient. The improved electrical properties are due to desorption of weakly bonded oxygen species, formation of Zn-H type species and passivation of deep-level defects during plasma treatment.     

Properties of N-doped ZnO grown by DBD-PLD

N-doped ZnO thin films have been grown on sapphire substrates by dielectric barrier discharged pulsed laser deposition (DBD-PLD). Low temperature photoluminescence spectra of N-doped ZnO film verified the p-type doping status to find the acceptor-bound exciton peaks with the high resolution detection. At low temperature growth, the major defects in the N-doped ZnO film were the oxygen interstitials that can combine with N, so that the N played the role as an acceptor. On the other hand, the major defects in the samples processed at high temperature were oxygen vacancies with which N doesn't play the role as an acceptor. The acceptor binding energy of N acceptor was estimated to be about 105 meV.     

Effect of annealing treatment on structural, electrical, and optical properties of Ga-doped ZnO thin films deposited by RF magnetron sputtering

The effect of annealing on structural, electrical, and optical properties of Ga-doped ZnO (GZO) films prepared by RF magnetron sputtering was investigated in air and nitrogen. GZO films are polycrystalline with a preferred 002 orientation. The resistivities of annealed films are larger than the as-deposited. The transmittance in the near IR region increases greatly and the optical band gap decreases after annealing. The photoluminescence spectra is composed of a near band edge emission and several deep level emissions (DLE) which are dominated by a blue emission. After annealing, these DLEs are enhanced evidently.     

Photoluminescence study of ZnO structures grown by aqueous chemical growth

ZnO micro-structures were deposited by aqueous chemical growth on Si (100) substrates, their morphology and size depending on the growth period. Characterization of the structures was performed using X-ray diffraction, scanning electron microscopy and Raman spectroscopy. Photoluminescence spectra recorded at 18 and 295 K for 325 nm CW excitation indicated that these are strongly affected by the morphology of the structures. Rods and tubes emit stronger UV radiation, in contrast to stronger yellow-green emission observed for flower-like structures. A red shift of the UV emission was found for increasing input power, while, thermal annealing of the samples induced stimulated emission for quite high excitation intensities.     

Structural and optical properties of ZnO nanostructures grown on silicon substrate by thermal evaporation process

Two types of one-dimensional ZnO nanostructures have been synthesized on silicon substrate by the thermal evaporation of metallic zinc powder in the presence of oxygen without the use of any catalyst or additives. Detailed structural analysis revealed that the formed ZnO nanostructures are single crystalline with wurtzite hexagonal phase and grow along the [0001] direction in preference. Presence of a sharp and strong, optical phonon Raman-active E₂ (high) mode and suppressed E₁ (LO) mode in the Raman spectra, in both the cases, confirmed the good crystallinity with the wurtzite hexagonal phase for the as-grown products. A sharp and dominant near band edge emission with a suppressed green emission is observed from the as-synthesized nanostructures which affirmed the good optical properties with very less structural defects for the grown nanostructures.     

Effect of ZnO seed layer on the catalytic growth of vertically aligned ZnO nanorod arrays

We have grown vertically aligned ZnO nanorods and multipods by a seeded layer assisted vapor–liquid–solid (VLS) growth process using a muffle furnace. The effect of seed layer, substrate temperature and substrate material has been studied systematically for the growth of high quality aligned nanorods. The structural analysis on the aligned nanorods shows c-axis oriented aligned growth by homoepitaxy. High crystallinity and highly aligned ZnO nanorods are obtained for growth temperature of 850–900 °C. Depending on the thickness of the ZnO seed layer and local temperature on the substrate, some region of a substrate show ZnO tetrapod, hexapods and multipods, in addition to the vertically aligned nanorods. Raman scattering studies on the aligned nanorods show distinct mode at ∼438 cm⁻¹, confirming the hexagonal wurtzite phase of the nanorods. Room temperature photoluminescence studies show strong near band edge emission at ∼378 nm for aligned nanorods, while the non-aligned nanorods show only defect-emission band at ∼500 nm. ZnO nanorods grown without the seed layer were found to be non-aligned and are of much inferior quality. Possible growth mechanism for the seeded layer grown aligned nanorods is discussed.     

Annealing Effect of ZnO on the Performance of Inverted Organic Photovoltaic Devices

ZnO nanoparticles films were prepared via sol-gel process and incorporated into inverted organic photovoltaic devices with a structure of ITO/ZnO/P3HT:PCBM/MoO_3/Ag,in which ZnO film served as an electron selective layer.The effects of annealing temperature of ZnO film on the device performance were investigated.When the annealing temperature was 300 ℃,a well-arranged ZnO thin film was obtained,and the optimized device had doubled short circuit current density(J_(SC)) and seven-fold higher power conversion efficiency(PCE)compared to the devices without ZnO film.This improvement could be attributed to the enlarged interfacial area of ZnO/active layer and better energy band matching which causes an efficient electron extraction and a decreased interface energy barrier.At particularly high annealing temperature,dramatically increased sheet resistance of indium tin oxide(ITO) was found to cause PCE deterioration.Our finding indicates that it is highly important to investigate both morphology and electrical effects for understanding and optimizing organic photovoltaic(OPV) performance.     

Effect of annealing on the spectroscopy performance of YVO4:Ce3+ single crystals

In this study, 1.0 at.% YVO₄:Ce³⁺ single crystals were grown in the protective atmosphere by using the Czochralski method. The crystals were annealed in Ar and H₂ atmospheres at different temperatures. The absorption and fluorescence spectra of the samples before and after annealing were measured. Results showed that the luminescent efficiency of the crystals was significantly enhanced after annealing in H₂ than after annealing in Ar. This phenomenon can be attributed to the existence of some Ce⁴⁺ ions in the crystal lattice. These Ce⁴⁺ ions can be effectively reduced to Ce³⁺ via annealing in H₂. With a fixed annealing time in H₂, the luminescent intensity significantly increased with increasing annealing temperature. The possibility of the crystal as white light material was also discussed according to the luminescence properties.     

Activation of phosphorous doping in high quality ZnO thin film grown on Yttria-stabilized zirconia (1 1 1) by thermal treatment

Phosphorous doped (P-doped) ZnO thin films are grown on c-sapphire and Yttria-stabilized zirconia (YSZ) (111) substrates by pulsed laser deposition. Post growth annealing is carried out to activate phosphorous to act as acceptor. The rocking curve of annealed P-doped ZnO films grown on YSZ (111) has full width at half maximum of 0.08°, more than two times narrower than that of the as-grown one. Neutral acceptor bound exciton (A⁰X) is observed from low temperature photoluminescence with estimated activation energy of 11.3 meV. Low carrier concentration of as-grown P-doped ZnO films indicated phosphorous doping creates acceptor states and/or reduced the oxygen vacancies. The carrier concentration of annealed samples is reduced by five order magnitudes from 3.21 × 10¹⁸ cm^− 3 to 1.21-8.19 × 10¹³ cm^− 3. At annealing temperature of 850 °C, the sample has the lowest carrier concentration and highest resistivity. This is an indication that the phosphorous in P-doped ZnO has been activated.     

Effect of hydrogen plasma treatment on the surface morphology, microstructure and electronic transport properties of nc-Si:H

Hydrogenated nanocrystalline silicon (nc-Si:H) films, deposited by reactive radio-frequency sputtering with 33% hydrogen dilution in argon at 200 °C, were treated with low-power hydrogen plasma at room temperature at various power densities (0.1-0.5 W/cm²) and durations (10 s-10 min). Plasma treatment reduced the surface root mean square roughness and increased the average grain size. This was attributed to the mass transport of Si atoms on the surface by surface and grain boundary diffusion. Plasma treatment under low power density (0.1 W/cm²) for short duration (10 s) caused a significant enhancement of crystalline volume fraction and electrical conductivity, compared to as-deposited film. While higher power (0.5 W/cm²) hydrogen plasma treatment for longer durations (up to 10 min) caused moderate improvement in crystalline fraction and electrical properties; however, the magnitude of improvement is not significant compared to low-power (0.1 W/cm²)/short-duration (10 s) plasma exposure. The results indicate that low-power hydrogen plasma treatment at room temperature can be an effective tool to improve the structural and electrical properties of nc-Si:H.     

Evolution of silicon nanoclusters and hydrogen in SiNx:H films: Influence of high hydrostatic pressure under annealing

Hydrogenated silicon-rich nitride (SRN) films of various stoichiometry (SiN_x:H, 0.7 < x ≤ 1.3) were deposited on single-crystalline Si substrates with the use of plasma enhanced chemical vapor deposition at a temperature of 100 °С. Furnace annealing for 5 h in ambient Ar at 1130 °С under atmospheric and enhanced hydrostatic pressure (HP — 11 kbar, 1.1 GPa) was applied to modify the structure of the films. The properties of as-deposited and annealed films were studied using ellipsometry, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, photoluminescence (PL), as well as high-resolution transmittance electron microscopy (HRTEM). According to the Raman data, the as-deposited film, in which the stoichiometry parameter x is below 1.0, contains amorphous silicon nanoclusters. Furnace annealing leads to crystallization of the nanoclusters. From the HRTEM and Raman data, the average size of the Si nanocrystals in the annealed films was 6-7 nm. No silicon nanoclusters were observed in the as-deposited films with relatively low concentration of excessive silicon atoms (the case of SiN_x:H, x > 1); furnace annealing leads to segregation of the Si and Si₃N₄ phases, so, the amorphous Si clusters were observed in annealed films according to Raman data. Surprisingly, after annealing with such high thermal budget, according to the FTIR data, the SRN film with parameter x close to that of the stoichiometric silicon nitride contains hydrogen in the form of SiH bonds. From analysis of the FTIR data of the SiN bond vibrations one can conclude that silicon nitride is partly crystallized in the films with x > 1 after annealing for 5 h. No influence of HP on the structure of Si nanoclusters was observed in the case of SRN films with x ≤ 1.1. Dramatic changes in the PL spectra of the SRN films with the x parameter close to that of the stoichiometric silicon nitride (x ≈ 1.3), annealed under atmospheric pressure and HP, were observed. HP stimulates the formation of very small hydrogenated amorphous nanoclusters. The size of amorphous Si nanoclusters determined from the quantum size effect model describing the PL spectra, should be 2-4 nm in this case.     

Metal organic chemical vapor deposition and investigation of ZnO thin films grown on sapphire

A new type of large area metal organic chemical vapor deposition (MOCVD) system for the growth of high quality and large size ZnO materials is introduced. Materials properties of the un-doped, n- and p-doped ZnO epi-films grown on sapphire substrates by this MOCVD system are studied by various techniques, including high resolution X-ray diffraction (XRD), UV-Visible optical transmission (OT), photoluminescence (PL) and photoluminescence excitation (PLE), synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS). The wurtzite (w) ZnO crystal structures grown with primary (0002) orientation were identified. Results have shown the high crystalline quality of MOCVD-grown ZnO films, indicated by the narrow XRD, PL and Raman line widths, strong PL signals, sharp OT edge and smooth surface. In particular, high p-type carrier concentration of > 10¹⁷ cm^− 3 have been achieved besides the good n-type doping in ZnO.     

Effect of Zn2+ source concentration on hydrothermally grown ZnO nanorods

We studied the effect of Zn2+ source concentration on the structural and optical properties of hydrothermally grown ZnO nanorods. The nanorods were grown on ZnO/p-Si(111) substrate using by a hydrothermal process in various concentrations of reagent at a low temperature (approximately 95 degrees C) and the structural and optical characteristics of ZnO nanorods were subsequently investigated by X-ray diffraction, field-emission scanning electron microscopy, and room temperature photoluminescence. The results demonstrate that the morphology and crystallinity of ZnO nanorods are influenced by the overall concentration of the precursor. The density and diameter of ZnO nanorods with a hexagonal structure are especially sensitivite to concentration of reactants. Furthermore, the structural transition is shown by increasing concentration. At the lowest concentration of Zn2+, the ZnO nanorods grow as single crystals with a low density and variable orientations. On the contrary, at the highest concentration, the nanorods grow as polycrystas due to the supersaturated Zn2+ source.     

Effect of heat treatment on characteristics of nanocrystalline ZnO films by electron beam evaporation

Zinc oxide thin films have been grown on glass substrate at room temperature by electron beam evaporation and then were annealed in annealing pressure 600 mbar at different temperatures ranging from 250 to 550 °C for 30 min. Electrical, optical and structural properties of thin films such as electrical resistivity, optical transmittance, band gap and grain size have been obtained as a function of annealing temperature. X-ray diffraction has shown that the maximum intensity peak corresponds to the (002) predominant orientation for ZnO films annealed at various temperatures. The full width at half maximum, decreases after annealing treatment which proves the crystal quality improvement. Scanning electron microscopy images show that the grain size becomes larger by increasing annealing temperature and this result agrees with the X-ray diffraction analysis.     

Effect of annealing temperature and composition on photoluminescence properties of magnetron sputtered SiCN films

Silicon carbonitride (SiCN) films were prepared by means of reactive magnetron sputtering of a sintered SiC target on n-type Si (1 0 0) substrates in the reactant gas of nitrogen, and then the films were respectively annealed at 600, 800 and 1100 °C for 5 min in nitrogen ambient. The films were characterized by energy dispersive spectrometer, X-ray diffraction, Fourier transform infrared spectroscopy and photoluminescence (PL) spectrophotometry. Intense PL peaks at 370, 400 and 440 nm were observed at room temperature. The results show that annealing temperature and composition play an important role in the structures and PL properties of the films. The annealing temperature of 600 °C favors the formation of the SiC (1 0 9) crystal in the SiCN films, and results in a maximal PL peak. The intensity of the 440 nm PL peak can be improved by increasing the abundance of the Si-C bond.