Thermal Annealed ZnO Research Articles

Energy level gradient trapping based on different work functions of ZnO enhancing response and stablity for lateral photodetectors

Abstract The slow response speed and instability of the lateral photodetectors (L-PDs), often caused by the persistence of the excess minority carriers in the long channel after the termination of the illumination (called Persistent Photoconductivity effects) are the main factors that hinder the practical application of this type of device. In this research, two types of ZnO with different work functions, including ZnO nanoparticles (ZnO-NPs) doped in bulk heterojunction (BHJ) and thermal annealed ZnO nanocrystalline film (ZnO–NF), are incorporated into the L-PDs to construct an energy gradient, which could facilitate the quick transfer of the minority carriers and restrict their release into the active layer. The structure of L-PDs are optimized as quartz/ZnO–NF/PffBT4T-2OD:PC61BM:ZnO-NPs (component ratio = 5:6:1)/Au–Au electrodes. Compared with the control device, the test results show that the optimized L-PDs exhibit higher stability with much lower fall time (3.7 ms, boosted 200-fold) and gentler rise slope of dark current versus time (Kdark) (8.3 × 10−6 nA/s, reduced four orders of magnitude). Meanwhile, the very low dark current (

Comparative surfacing studies of sputtered and thermal annealed ZnO films wet etched with NH4Cl and acidic etchants

The wet etchings of as-grown and thermal annealed ZnO films were carried out by using NH4Cl and acidic etchants respectively. The lateral profiles and side faces of the etched ZnO samples were exhibited by profiler and SEM measurements. NH4Cl etchant can avoid W-like lateral profile and eave-like slope at the side face of the etched ZnO patterns for both as-grown and post-annealed ZnO samples. The etching rate has been compared between NH4Cl etchant and acidic etchants for both as-grown and post-annealed ZnO samples. NH4Cl etchant can maintain slower etching rate with higher NH4Cl concentration. The thermal annealing procedure can be helpful to achieving uniform etching surface, and the etching rates of the annealed ZnO samples are even slower, which can be explained by the better crystallinity and lower density of oxygen vacancy.

Improved luminescence intensity and stability of thermal annealed ZnO incorporated Alq3 composite films.

The 30 wt% of ZnO (weight percentage of ZnO has been optimised) incorporated tris- (8-hydroxyquinoline)aluminum (Alq3) has been synthesised and coated on to glass substrates using dip coating method. The structural and optical properties of the Alq3/ZnO composite film after thermal annealing from 50 to 300 °C insteps 50° has been studied and reported. XRD pattern reveals the presence of crystalline ZnO in all the annealed films. The films annealed above 150 °C reveal the presence of crystalline Alq3 along with crystalline ZnO. The FTIR spectra confirm the presence of hydroxyquinoline and ZnO vibration in all the annealed composite films. The composite films annealed above 150 °C show a partial sublimation and degradation of hydroxyquinoline compounds. The ZnO incorporated composite films (Alq3/ZnO) exhibit two emission peaks, one corresponding to ZnO at 487 nm and another at 513 nm due to Alq3. The films annealed at 200 °C exhibit maximum photoluminescence (PL) intensity than pristine film at 513 nm when excited at 390 nm.

Catalyst free growth of ZnO nanowires on graphene and graphene oxide and its enhanced photoluminescence and photoresponse

We demonstrate the graphene assisted catalyst free growth of ZnO nanowires (NWs) on chemical vapor deposited (CVD) and chemically processed graphene buffer layers at a relatively low growth temperature (580 °C) in the presence and absence of ZnO seed layers. In the case of CVD graphene covered with rapid thermal annealed ZnO buffer layer, the growth of vertically aligned ZnO NWs takes place, while the direct growth on CVD graphene, chemically derived graphene (graphene oxide and graphene quantum dots) without ZnO seed layer resulted in randomly oriented sparse ZnO NWs. Growth mechanism was studied from high resolution transmission electron microscopy and Raman spectroscopy of the hybrid structure. Further, we demonstrate strong UV, visible photoluminescence (PL) and enhanced photoconductivity (PC) from the CVD graphene–ZnO NWs hybrids as compared to the ZnO NWs grown without the graphene buffer layer. The evolution of crystalinity in ZnO NWs grown with ZnO seed layer and graphene buffer layer is correlated with the Gaussian line shape of UV and visible PL. This is further supported by the strong Raman mode at 438 cm−1 significant for the wurtzite phase of the ZnO NWs grown on different graphene substrates. The effect of the thickness of ZnO seed layers and the role of graphene buffer layers on the aligned growth of ZnO NWs and its enhanced PC are investigated systematically. Our results demonstrate the catalyst free growth and superior performance of graphene–ZnO NW hybrid UV photodetectors as compared to the bare ZnO NW based photodetectors.

The effects of ZnO buffer layers on the properties of phosphorus doped ZnO thin films grown on sapphire by pulsed laser deposition

The properties of phosphorus doped ZnO thin films grown on sapphire by pulsed laser deposition were examined, specifically focusing on the effects of undoped ZnO buffer layers. In particular, buffer layers were grown under different conditions; the transport properties of as-deposited and rapid thermal annealed ZnO : P films were then examined. As-deposited films showed n-type conductivity. After rapid thermal annealing, the film on buffer layer grown at a low temperature showed the conversion of carrier type to p-type for specific growth conditions while the films deposited on buffer layer grown at a high temperature remained n-type regardless of growth condition. The films deposited on buffer layer grown at a low temperature showed higher resistivity and more significant change of the transport properties upon rapid thermal annealing. These results suggest that more dopants are incorporated in films with higher defect density. This is consistent with high resolution x-ray diffraction results for phosphorus doped ZnO films on different buffer layers. In addition, the microstructure of phosphorus doped ZnO films is substantially affected by the buffer layer.

Rapid thermal annealing effects on the structural and optical properties of Na–N codoped ZnO films

Polycrystalline ZnO thin films codoped with Na and N were obtained by chemical bath deposition. The structural characteristic and the optical properties of the rapid thermal annealed ZnO:(Na,N) films were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive spectrometer (EDS), Raman spectrum and room-temperature photoluminescence. After RTA treatment, the XRD spectra showed a continuous decrease of the full- width at half-maximum (FWHM) of the (0 0 2) diffraction peak of the ZnO:(Na,N) film. The Raman spectra revealed that the intensity of the mode around 582 cm −1 increased with the increase of the RTA temperature. The PL spectra showed different trends in the UV luminescence of ZnO:(Na,N) films after RTA treatments.

Improvement in crystallinity and optical properties of ZnO epitaxial layers by thermal annealed ZnO buffer layers with oxygen plasma

ZnO epitaxial layers with treated low-temperature (LT) ZnO buffer layers were grown by plasma-assisted molecular beam epitaxy (PA-MBE) on p-type Si (1 0 0) substrates. The LT-ZnO buffer layers were treated by thermal annealing in O 2 plasma with various radio frequency (RF) power ranging from 100 to 300 W before the ZnO epilayers growth. Atomic force microscopy (AFM), high-resolution X-ray diffraction (HR-XRD), and room-temperature (RT) photoluminescence (PL) were carried out to investigate their structural and optical properties. The surface roughness measured by AFM was improved from 2.71 to 0.59 nm. The full-width at half-maximum (FWHM) of the rocking curve observed for ZnO (0 0 2) XRD and photoluminescence of the ZnO epilayers was decreased from 0.24° to 0.18° and from 232 to 133 meV, respectively. The intensity of the XRD rocking curve and the PL emission peak were increased. The XRD intensity ratio of the ZnO (0 0 2) to Si substrates and PL intensity ratio of the near-band edge emissions (NBEE) to the deep-level emissions (DLE) as a function of the RF power was increased from 0.166 to 0.467 and from 2.54 to 4.01, respectively. These results imply that the structural and optical properties of ZnO epilayers were improved by the treatment process.

Behavior of rapid thermal annealed ZnO:P films grown by pulsed laser deposition

The transport properties of as-deposited and rapid thermal annealed phosphorus-doped ZnO films grown by pulsed laser deposition are reported. As-grown ZnO:P samples showed n-type characteristics, presumably due to the formation of antisite PZn defects. Rapid thermal annealing yielded a carrier-type conversion from n- to p-type for the ZnO:P films grown at ∼700 °C; samples grown at substantially lower or higher temperatures tended to remain n-type even after the thermal annealing process. The properties and behavior of the n-to-p conversion are most consistent with the formation of PZn-2VZn as the active acceptor state. Variable magnetic field Hall measurements confirmed the p-type behavior. Phosphorus doping concentrations in the range of 0.5−1.0 at. % were considered, with evidence for P segregation in the higher phosphorus concentrations.