Band Gap Energy Of ZnO Research Articles

Structure and photoluminescence of Mn-passivated nanocrystalline ZnO:S thin films

Mn-passivated nanocrystalline ZnO:S thin films were fabricated by thermally oxidizing Mn-doped ZnS (ZnS:Mn) films prepared by electron beam evaporation. Mn was introduced to passivate the surface defects of ZnO and to improve the optical properties. X-ray diffraction (XRD) and photoluminescence (PL) spectra at 81.9 K indicated the S content in ZnO thin film gradually decreased with increasing annealing temperature. The fitted result of the temperature-dependent PL spectra in the range from 81.9 to 302.2 K showed that S dopant could broaden the optical band gap energy of ZnO. Room temperature PL spectra confirmed that the ultraviolet peak shifted to lower energy with the decrease of S content in the thin film because of the Burstein–Moss effect.

Luminescence properties of ZnO and Eu 3+-doped ZnO nanorods

We have studied crystal structure, shape, and photoluminescence (PL) properties of undoped and Eu-doped ZnO nanorods. The transmission electron microscope studies show that the undoped and Eu-doped ZnO nanocrystals have the rod-like shape. Under the high energy excitation above the band-gap energy of ZnO, broad PL band as well as bound-exciton emission were observed in Eu-doped ZnO nanorods. On the contrary, several sharp PL lines were observed when the excitation energy almost coincides with the intra-4f transition energy of Eu 3+ ions. The broad PL band is thought to be due to the defects in ZnO nanorods while the sharp PL peaks are attributed to intra-4f transitions of Eu 3+ ions in ZnO nanorods.

Textured Zinc Oxide by Expanding Thermal Plasma CVD: the Effect of Aluminum Doping

AbstractAluminum doped ZnO films are deposited on glass substrates at a temperature of 200°C by expanding thermal plasma CVD. Surface texture, morphology and crystal structure have been studied by AFM, SEM and XRD. A rough surface texture, which is essential for application as front electrode in thin film solar cells, is obtained during deposition. The addition of aluminum as a dopant results in distinct differences in film morphology, a transition from large, rounded crystallites to a more pyramid-like structure is observed. The structure of films is hexagonal with a preferred crystal orientation in the faces (002) and (004), indicating that films are oriented with their c-axes perpendicular to the substrate plane. In addition, spectroscopic ellipsometry is used to evaluate optical and electronic film properties. The presence of aluminum donors in doped films is confirmed by a shift in the ZnO band gap energy from 3.32 to 3.65 eV. In combination with reflection and transmission measurements in the visible and NIR ranges, film resistivities have been obtained from the free-carrier absorption. These results are consistent with direct measurements. Resistivities as low as 6.0 10-4 Ωcm have been obtained.

In-plane anisotropic strain of ZnO closely packed microcrystallites grown on tilted (0001) sapphire

We have found that ZnO films consisting of epitaxially ordered arrays of closely packed hexagonal microcrystallites grown on vicinal (1000) sapphire exhibit biaxial in-plane optical anisotropy. The optical anisotropy resonance occurs near the band gap energy of ZnO. The line shape of the resonance is consistent with that induced by an in-plane anisotropic strain. The direction of the anisotropy coincides with the projection of the miscut direction of the (0001) sapphire substrates onto the sample surface plane. The magnitude of the anisotropy is generally larger for films with high crystalline quality, and on substrates with larger miscut angles. A possible origin of the strain anisotropy due to the miscut angle and the difference in thermal expansion rate of sapphire along its c and a axes is proposed.

Chemisorption, photodesorption and conductivity measurements on ZnO surfaces

Experimental results of a mass-spectro metric analysis of photodesorption from ZnO single crystals at different temperatures are reported. They provide direct evidence that CO 2 is the only photodesorbed species from both the single crystal and powder samples studied. The CO 2 photodesorption occurs only when the incident photon energy exceeds the ZnO band gap energy. Excellent agreement between the illumination time dependence of the CO 2 photodesorption and surface conductivity data in both single crystals and powder samples strongly suggests a substrate dependent mechanism in which photodesorption occurs by the neutralization of chemisorbed CO 2 − “ion-molecules” by photogenerated holes. In addition, measurements of the chemisorption kinetics of oxygen on ZnO single crystal and powder surfaces are reported. The results are compared with CO 2 and CO chemisorptiun experiments to show that, of these gases, only oxygen chemisorbs from the gas phase. Auger analysis of oxygen saturated and photodesorbed surfaces of ZnO show a significant relation between the carbon content and the photo-desorptive and conductive activities of those surfaces. These observations indicate that impurity carbon atoms on ZnO surfaces can be oxidized by electron capture to produce chemisorbed CO 2 − “ion-molecules’ which will then readily photodesorb by bandgap radiation. This proposed process is discussed together with further supporting evidence.

Photodesorption from powdered zinc oxide

Mass spectrometric measurements of photodesorption from powdered ZnO under controlled radiation are reported. Neutral carbon dioxide and atomic oxygen prevail as the desorbing species. From photodesorption rate decay curves we obtain cross sections for photodesorption of ~ 2 × 10 −17 cm 2 for O and ~4 × 10 −19 cm 2 for CO 2. Both species desorb only when the incoming photon energy exceeds the ZnO band gap energy (3.2 eV). The desorption rate is linear with incoming photon flux. All the data seem to support a substrate dependent process of photodesorption by neutralization of chemisorbed species by photogenerated holes. The temperature dependence of the photodesorption rate for CO 2 yields a value of 0.26 eV for the apparent binding energy of the physisorbed CO 2 molecule.