Photoluminescence studies of ZnO thin films on R-plane sapphire substrates grown by sol–gel method

Zinc oxide (ZnO) thin films on R-plane sapphire substrates were grown by the sol–gel spin-coating method. The optical properties of the ZnO thin films were investigated using photoluminescence. In the UV range, the asymmetric near-band-edge emission was observed at 300 K, which consisted of two emissions at 3.338 and 3.279 eV. Eight peaks at 3.418, 3.402, 3.360, 3.288, 3.216, 3.145, 3.074, and 3.004 eV, which respectively correspond to the free exciton (FX), bound exciton, transverse optical (TO) phonon replica of FX recombination, and first-order longitudinal optical phonon replica of FX and the TO (1LO+TO), 2LO+TO, 3LO+TO, 4LO+TO, and 5LO+TO, were obtained at 12 K. From the temperature-dependent PL, it was found that the emission peaks at 3.338 and 3.279 eV corresponded to the FX and TO, respectively. The activation energy of the FX and TO emission peaks was found to be about 39.3 and 28.9 meV, respectively. The values of the fitting parameters of Varshni's empirical equation were α=4×10^?3 eV/K and β=4.9×10³ K, and the S factor of the ZnO thin films was 0.658. With increasing temperature, the exciton radiative lifetime of the FX and TO emissions increased. The temperature-dependent variation of the exciton radiative lifetime for the TO emission was slightly higher than that for the FX emission.     

Epitaxial ZnO thin films grown by pulsed electron beam deposition

In this work, the pulsed electron beam deposition method (PED) is evaluated by studying the properties of ZnO thin films grown on c-cut sapphire substrates. The film composition, structure and surface morphology were investigated by means of Rutherford backscattering spectrometry, X-ray diffraction and atomic force microscopy. Optical absorption, resistivity and Hall effect measurements were performed in order to obtain the optical and electronic properties of the ZnO films. By a fine tuning of the deposition conditions, smooth, dense, stoichiometric and textured hexagonal ZnO films were epitaxially grown on (0001) sapphire at 700 °C with a 30° rotation of the ZnO basal plane with respect to the sapphire substrate. The average transmittance of the films reaches 90% in the visible range with an optical band gap of 3.28 eV. Electrical characterization reveals a high density of charge carrier of 3.4 × 10¹⁹ cm^?3 along with a mobility of 11.53 cm²/Vs. The electrical and optical properties are discussed and compared to ZnO thin films prepared by the similar and most well-known pulsed laser deposition method.     

Structural and optical properties of Mn-doped ZnO nanocrystalline thin films with the different dopant concentrations

The transparent nanocrystalline thin films of undoped zinc oxide and Mn-doped (Zn_1−xMn_xO) have been deposited on glass substrates via the sol–gel technique using zinc acetate dehydrate and manganese chloride as precursor. The as-deposited films with the different manganese compositions in the range of 2.5–20 at% were pre-heated at 100 °C for 1 h and 200 °C for 2 h, respectively, and then crystallized in air at 560 °C for 2 h. The structural properties and morphologies of the undoped and doped ZnO thin films have been investigated. X-ray diffraction (XRD) spectra, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were used to examine the morphology and microstructure of the thin films. Optical properties of the thin films were determined by photoluminescence (PL) and UV/Vis spectroscopy. The analyzed results indicates that the obtained films are of good crystal quality and have smooth surfaces, which have a pure hexagonal wurtzite ZnO structure without any Mn related phases. Room temperature photoluminescence is observed for the ZnO and Mn-doped ZnO thin films.     

Temperature dependent photoluminescence processes in ZnO thin films grown on sapphire by pulsed laser deposition

Temperature dependence of the photoluminescence (PL) transitions in the range of 10–300 K was studied for ZnO thin films grown on sapphire by pulsed laser deposition. The low temperature PL spectra were dominated by recombination of donor bound excitons (B_X) and their phonon replicas. With increasing temperature, free exciton (F_X) PL and the associated LO phonon replicas increased in intensity at the expense of their bound counterparts. The B_X peak with line width of ∼6 meV at 10 K exhibited thermal activation energy of ∼17 meV, consistent with the exciton-defect binding energy. The separation between the F_X and B_X peak positions was found to reduce with increasing temperature, which was attributed to the transformation of B_X into the shallower donor bound exciton complexes at consecutive lower energy states with increasing temperature, which are possible in ZnO. The energy separation between F_X peak and its corresponding 1-LO phonon replica showed stronger dependence on temperature than that of 2-LO phonon replica. However, their bound counterparts did not exhibit this behavior. The observed temperature dependence of the energy separation between the free exciton and it is LO phonon replicas are explained by considering the kinetic energy of free exciton. The observed PL transitions and their temperature dependence are consistent with observations made with bulk ZnO crystals implying high crystalline and optical quality of the grown films.     

Photoluminescence of chemical solution-derived amorphous ZnO layers prepared by low-temperature process

Growth characteristic and optical properties of the amorphous ZnO thin films prepared on soda-lime–silica glass substrates by chemical solution process at 100 and 200 °C were investigated by using X-ray diffraction analysis, scanning probe microscope, ultraviolet spectrophotometer, and photoluminescence. The films exhibited an amorphous pattern even when finally heat treated at 100–200 °C for 60 min. The photoluminescence spectrum of amorphous ZnO films shows a strong near-band-edge emission, while the visible emission is nearly quenched.     

Near-infrared photoluminescence and thermally stimulated current in Cu3Ga5Se9 layered crystals: A comparative study

Near-infrared photoluminescence (PL) and thermally stimulated current (TSC) spectra of Cu₃Ga₅Se₉ layered crystals grown by Bridgman method have been studied in the photon energy region of 1.35–1.46 eV and the temperature range of 15–115 K (PL) and 10–170 K (TSC). An infrared PL band centered at 1.42 eV was revealed at T = 15 K. Radiative transitions from shallow donor level placed at 20 meV to moderately deep acceptor level at 310 meV were suggested to be the reason of the observed PL band. TSC curve of Cu₃Ga₅Se₉ crystal exhibited one broad peak at nearly 88 K. The thermal activation energy of traps was found to be 22 meV. An energy level diagram demonstrating the transitions in the crystal band gap was plotted taking account of results of PL and TSC experiments conducted below room temperature.     

Comparative study on structural and optical properties of ZnO thin films prepared by PLD using ZnO powder target and ceramic target

Zinc oxide thin films have been obtained in O₂ ambient at a pressure of 1.3 Pa by pulsed laser deposition (PLD) using ZnO powder target and ceramic target. The effect of temperature on structural and optical properties of ZnO thin films was investigated systematically by XRD, SEM, FTIR and PL spectra. The results show that the best structural and optical properties can be achieved for ZnO thin film fabricated at 700 °C using powder target and at 400 °C using ceramic target, respectively. The PL spectrum reveals that the efficiency of UV emission of ZnO thin film fabricated by using powder target is low, and the defect emission of ZnO thin film derived from Zn_i and O_i is high.     

Investigation of reduced graphene oxide effects on ultra-violet detection of ZnO thin film

The reduced graphene oxide (rGO) incorporated ZnO thin films were fabricated by dip-coating method. The Raman and FT-IR spectra of 0.075 wt% incorporated composite film showed reduction of GO in composite film. The transmittanceProd. Type: FTP spectra have shown that rGO incorporation increase the visible light absorption of ZnO thin film while the calculated band gaps of samples were decreased from 3.28 to 3.25 eV by increasing the rGO content. The linear trend of I–V curve suggests an ohmic contact between ZnO and rGO. Besides, it was found that by increasing the rGO content, the electrical resistivity was decreased from 4.32×10² Ω cm for pure ZnO film to 2.4×10¹ Ω cm for 0.225 wt% rGO incorporated composite film. The composite photodetectors not only possessed a desirable UV photosensitivity, but also the response time of optimum sample containing 0.075 wt% rGO was reduced to about one-half of pure ZnO thin film. Also, the calculated signal to noise (SNR) showed that highly conductive rGO in composite thin films facilitate the carrier transportation by removing the trapping centers. The mechanism of photoresponsivity improvement of composite thin films was proposed by carrier transportation process.     

Low-temperature MOVPE growth of ZnO thin films by using a buffer layer

ZnO thin films have been grown on a-plane (1,1,−2,0) sapphire substrates by metalorganic vapor phase epitaxy (MOVPE) at low substrate temperature of 350 °C. It is showed that the crystal and electrical quality of the thin films was improved by using a ZnO buffer layer. The photoluminescence (PL) measurements indicate that the ZnO thin films grown at such a low substrate temperature have a strong UV emission.     

Exciton-polariton state in nanocrystalline SiC films

We studied the features of optical absorption in the films of nanocrystalline SiC (nc-SiC) obtained on the sapphire substrates by the method of direct ion deposition. The optical absorption spectra of the films with a thickness less than ~500 nm contain a maximum which position and intensity depend on the structure and thickness of the nc-SiC films. The most intense peak at 2.36 eV is observed in the nc-SiC film with predominant 3C-SiC polytype structure and a thickness of 392 nm. Proposed is a resonance absorption model based on excitation of exciton polaritons in a microcavity. In the latter, under the conditions of resonance, there occurs strong interaction between photon modes of light with λ_ph=521 nm and exciton of the 3С polytype with an excitation energy of 2.36 eV that results in the formation of polariton. A mismatch of the frequencies of photon modes of the cavity and exciton explains the dependence of the maximum of the optical absorption on the film thickness.     

High quality nitrogen-doped zinc oxide thin films grown on ITO by sol–gel method

Highly transparent N-doped ZnO thin films were deposited on ITO coated corning glass substrate by sol–gel method. Ammonium nitrate was used as a dopant source of N with varying the doping concentration 0, 0.5, 1.0, 2.0 and 3.0 at%. The DSC analysis of prepared NZO sols is observed a phase transition at 150 °C. X-ray diffraction pattern showed the preferred (002) peak of ZnO, which was deteriorated with increased N concentrations. The transmittance of NZO thin films was observed to be ~88%. The bandgap of NZO thin films increased from 3.28 to 3.70 eV with increased N concentration from 0 to 3 at%. The maximum carrier concentration 8.36×10¹⁷ cm⁻³ and minimum resistivity 1.64 Ω cm was observed for 3 at% N doped ZnO thin films deposited on glass substrate. These highly transparent ZnO thin films can be used as a window layer in solar cells and optoelectronic devices.     

Effect of phosphorus irradiation on the structural,electrical, and optical characteristics of ZnO thin films

Phosphorus irradiation at a low energy (50 keV) and at a dosage of 8×10¹⁴ ions/cm² was carried out on 〈002〉 ZnO films grown by using a pulsed laser deposition technique (Sample A). Subsequent rapid thermal annealing at 650 °C and 750 °C was performed to remove defects resulting from the irradiation (samples B and C, respectively). Atomic force microscopy was used to determine the root mean square roughness, which was 10.07, 8.66, and 9.31 nm for samples A, B, and C, respectively. Low-temperature photoluminescence measurements revealed increased deep-level defect peaks following irradiation; however, the subsequent annealing minimized the defects. Although the dominant donor-bound exciton peak verifies the n-type conductivity of the films, the free–electron–to–acceptor and donor-to-acceptor pair peaks in the irradiated samples confirm an increase in acceptor concentration.     

Strong violet emission from zinc oxide dumbbell-like microrods and nanowires

ZnO of different morphologies with controlled size and aspect ratio (l/d) such as dumbbell-like microrods, thick nanowires and thin nanowires were prepared by a hydrothermal method. Possible mechanisms for the formation of ZnO crystals with the different morphologies were discussed. Strong violet photoluminescence bands at～413 nm (3.0 eV) without band edge emission were observed from the dumbbell-like ZnO microrods and thick nanowires. Correspondingly, the thin nanowires showed a weak shoulder UV photoluminescence band at～390 nm. Such result indicates that the photoluminescence properties could be improved by the morphologies or aspect ratio (l/d) and the potential fabrication violet-light-emitting devices.     

Room temperature near-field photoluminescence of zinc-blend and wurtzite ZnO structures

Near-field photoluminescence (PL) was measured from ZnO film, composed of nanocrystallites with zinc-blend (ZB) and wurtzite (W) structures, on a sapphire (0 0 0 1) substrate at room temperature (RT). The size of nanocrystallites was in the range of 30–50 nm. Using a fiber probe with aperture size of 80 nm, two near-field emission peaks attributed to one ZB and one W structures were observed. The difference in the emission energies was 0.10 eV close to the calculated bandgap difference between ZB and W structures. The intensity of emission peak from ZB structure with lower energy was stronger than that from W structure, which is supposed to be resulted from the quenched excitonic effect of W structure.     

Quantum size effect in low energy electron diffraction of thin films

Low energy electron microscopy (LEEM) is used to study the quantum size effect (QSE) in electron reflectivity from thin films. Strong QSE interference peaks are seen below 20 eV for Cu and Ag films on the W(1 1 0) surface and Sb films on the Mo(0 0 1) surface. Simple inspection of QSE interference peaks reveals that all three metals grow atomic layer-by-atomic layer. Layer-specific I(V) spectra obtained with LEEM permit structural analysis by full dynamical multiple scattering LEED calculations for a layer-by-layer view of thin film structure.     

Fabrication of PbS thin films composed of highly (200)-oriented nano-/micro-rods in deep eutectic solvent

PbS thin films composed of highly (200)-oriented shuttle-like nano-/micro-rods were successfully fabricated on glass substrates by the environment friendly ionothermal method at 140 °C in deep eutectic solvent (DES). The as-prepared products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), ultraviolet-visible (UV–vis) and photoluminescence (PL) spectra, respectively. The possible mechanism of the oriented growth of PbS nano-/micro-rods was discussed. The PbS thin films composed of shuttle-like nano-/micro-rods exhibited a large absorbance property in the wavelength range of 350–1100 nm, and moreover, the PL spectrum had a broad emission band centered at 490 nm. The shuttle-like PbS nano-/micro-rods-based thin films might have potential application in solar cells.     

Room temperature photoluminescence property of Mo-doped In2O3 thin films

Mo-doped In₂O₃ thin films have been prepared on glass substrates using an activated reactive evaporation method and systematically studied the effect of oxygen partial pressure on the structural, optical, electrical and photoluminescence properties of the films. The obtained films are highly transparent and conductive. The films exhibited the lowest electrical resistivity of 5.2 × 10⁻⁴ Ω cm, with an average optical transmittance of 90% in the visible region. An intensive photoluminescence emission peaks were observed at 415 and 440 nm.     

ZnO薄膜近带边紫外发光的研究

用ZnO陶瓷靶,采用脉冲激光沉积(PLD)技术在c-Al₂O₃衬底上制备了ZnO薄膜。通过不同温度下光致发光(PL)光谱的测量,对样品的紫外发光机理进行研究。 在较低温度(10 K)下的PL光谱中,观测到一个位于3.354 eV处的束缚激子(D⁰X)发射,随着温度的升高(~50 K),在D⁰X的高能侧观测到了自由激子的发射峰。在10 K温度下,3.309 eV处出现了一个较强的发光带A,此发光带强度随着温度升高先增大然后减小,并且一直延续到室温。重点讨论了此发光带的起源,并认为A带可归属于自由电子-受主之间的复合发射。     

Characteristics of ZnO:In thin films prepared by RF magnetron sputtering

In-doped ZnO (ZnO:In) transparent conductive thin films were deposited on glass substrates by RF magnetron sputtering. The effect of substrate temperature on the structural, electrical and optical properties of the ZnO:In thin films was investigated. It was found that higher temperature improves the crystallinity of the films and promotes In substitution easily. ZnO:In thin films with the best crystal quality were fabricated at 300 °C, which exhibit a larger grain size of 29 nm and small tensile strain of 0.9%. The transmittance of all the films was revealed to be over 85% in the visible range independence of the substrate temperatures and the lowest resistivity of ZnO:In thin films is 2.4×10⁻³ Ω cm.     

The role of torsional modes in the electronic absorption spectrum of acetone

The electronic absorption spectrum of acetone is revisited to evaluate the role of hot bands due to low lying torsional modes in the assignment of vibronic transitions. The UV–VUV photoabsorption spectrum of acetone is recorded in the energy region 3.5–11.8 eV at a resolution of ～4 meV at 4 eV and ～10 meV at 10 eV using synchrotron radiation. The absorption spectrum is dominated by richly structured Rydberg series (ns, np and nd) converging to the first ionization potential of acetone at 9.708 eV. Careful consideration of hot band contributions from torsional modes and symmetry selection rules have resulted in an improved set of vibronic assignments as compared to earlier room temperature work. Revised quantum defect values for some of the Rydberg transitions and a few new assignments in the nd series are also reported in this paper.