Low-defect-density ZnO homoepitaxial films grown by low-temperature ALD (original) (raw)
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Growth by atomic layer epitaxy and characterization of thin films of ZnO
physica status solidi (c), 2005
Atomic layer epitaxy (ALE) was applied to grow thin films of monocrystalline and polycrystalline ZnO. Monocrystalline films were obtained only for GaN/Al 2 O 3 substrates, whereas use of sapphire, silicon or soda lime glass resulted in either 3D growth mode or in polycrystalline films showing preferential orientation along the c axis. Successful Mn doping of ZnO films is reported, when using organic Mn precursors.
Physica B: Condensed Matter, 2007
High energy electron irradiation (HEEI) effects on the as-grown and annealed ZnO thin films grown by electrochemical deposition were investigated. Both samples were exposed to the sequential electron irradiations of 6, 12 and 15 MeV energies at a fluence of 1 Â 10 12 e À /cm 2 . The results of X-ray diffraction suggest that a highly strong crystallographic structure can be produced by annealing process. Photoluminescence (PL) studies show that the EI produces violet emission which results from the zinc interstitial. Recombination lifetime (RL) values of the both films reveal that the high quality crystals are obtained. The decreasing trends of RL values with increasing electron energy have been explained by the formation of crystal defects due to the HEEI.
Properties of homoepitaxial ZnO and ZnO:P thin films grown by pulsed-laser deposition
Zinc Oxide Materials and Devices III, 2008
We have investigated the morphology, crystalline quality, the transport and electronic properties of homoepitaxial ZnO and ZnO:P thin films grown by pulsed-laser deposition. Atomic surface steps are visible for growth temperatures of 650°C and higher. The unit cell volume of undoped thin films is smaller than that of the hydrothermal substrates. Phosphorous doping increases the unit cell volume such that a perfect lattice match is achieved for a nominal phosphorous content of 0.01 wt.%. Undoped thin films have a net doping concentration below 10 15 cm -3 , whereas the phosphorous doping increases the free electron concentration at room temperature to 10 17 cm -3 and above. Temperature dependent Hall effect measurements show that interstitial zinc with a thermal activation energy of 34 meV is a dominant donor in homoepitaxial ZnO:P thin films. The Hall mobility of such samples is similar to ZnO single crystals grown by seeded chemical vapor transport. Low temperature photoluminescence measurements reveal recombination of free excitons and excitons bound to interstitial zinc and excitons bound to neutral and ionized aluminum donors. Defect related deep luminescence is not observed for undoped homoepitaxial thin films. In contrast phosphorous doping introduces two broad recombination bands centered at 2.9 eV and 1.9 eV.
Journal of Electronic Materials, 2016
Aluminum-doped zinc oxide (ZnO:Al) thin films were synthesized by atomic layer deposition on silicon, quartz and sapphire substrates and characterized by x-ray diffraction (XRD), high-resolution scanning electron microscopy, optical spectroscopy, conductivity mapping, Hall effect measurements and positron annihilation spectroscopy. XRD showed that the as-grown films are of single-phase ZnO wurtzite structure and do not contain any secondary or impurity phases. The type of substrate was found to affect the orientation and degree of crystallinity of the films but had no effect on the defect structure or the transport properties of the films. High conductivity of 10 À3 X cm, electron mobility of 20 cm 2 /Vs and carrier density of 10 20 cm À3 were measured in most films. Thermal treatments in various atmospheres induced a large effect on the thickness, structure and electrical properties of the films. Annealing in a Zn and nitrogen environment at 400°C for 1 h led to a 16% increase in the thickness of the film; this indicates that Zn extracts oxygen atoms from the matrix and forms new layers of ZnO. On the other hand, annealing in a hydrogen atmosphere led to the emergence of an Al 2 O 3 peak in the XRD pattern, which implies that hydrogen and Al atoms compete to occupy Zn sites in the ZnO lattice. Only ambient air annealing had an effect on film defect density and electrical properties, generating reductions in conductivity and electron mobility. Depth-resolved measurements of positron annihilation spectroscopy revealed short positron diffusion lengths and high concentrations of defects in all as-grown films. However, these defects did not diminish the electrical conductivity in the films.
Thickness dependence of optoelectronic properties in ALD grown ZnO thin films
Applied Surface Science, 2014
ZnO thin films with high conductivity and high transparency were grown on Si (1 0 0) substrates by atomic layer deposition. Thickness dependent (43-225 nm) changes in crystallographic, optical and electrical properties are reported and discussed. Increase in film thickness caused a decrease in the bandgap by relaxation of stress in the plane of the film and led to an improvement in crystallinity and conductivity. The optical studies showed a noticeable change towards the contribution of excitonic and phonon replica to the UV-emission band.
Energy Shift of Native 2.45 eV Related Defects in Annealed ZnO Films
IOP Conference Series: Materials Science and Engineering, 2010
We study the influence of annealing temperature on the structural and optical properties of ZnO. Zinc oxide films were prepared by thermal oxidation of metallic Zn films. First, high quality (5N) Zn was evaporated onto sapphire substrate. Then zinc films were annealed in the oxygen atmosphere at several temperatures from 500 o C to 1100 o C. The surface was analyzed by Scanning Electron Microscopy (SEM) and structural properties were studied by XRD. Photoluminescence (PL) was measured from 350 nm to 800 nm, at two temperatures -80 K and 300 K. Ordinary PL spectra could be divided into two parts. One band observed at ~3.3 eV (UV PL) and second at 2.2 -2.5 (VIS PL as "visible") -the maximum energy depends on annealing temperature. The origin of UV PL is in recombination of free excitons and bound excitons to donors and acceptors [1]. However, for VIS PL exact mechanism of emission recombination is still not fully understood. The main discussion is related to the native defects such as zinc vacancies or oxygen vacancies or oxygen antisite [2] but impurities like Cu [3] and/or hydrogen donors [4] are also candidates. Sample annealing temperature influenced the peak position of VIS PL and intensity of both part of the PL spectrum
International Journal of Nanotechnology, 2013
ZnO thin films have been successfully deposited by Atomic Layer Deposition (ALD) using Diethylzinc (DEZn) and water (H 2 O) as precursors. The preferred orientations of the thin films were found to be strongly dependent on the deposition temperature. (100)-oriented ZnO thin films were grown in the temperature range of 155 to 220 o C, whereas (002)-oriented ZnO thin films were formed between 220 to 300 o C. It is worth mentioning that ALD technique allowed ZnO thin films with preferred orientation, i.e. (002), to be deposited on both Si and glass substrates which have a large lattice mismatch to ZnO. This process capability could be attributed to the unique characteristics of a slow growth rate due to the self-limiting growth and a relatively high deposition temperature (220-300°C) which provided sufficient energy for Zn and O atoms to migrate towards <0001> in the ALD process. Besides, the (002)-oriented ZnO thin films have the best crystal quality and lowest resistivity. The thickness of as-deposited films could be controlled at nanoscale as the growth rate was proportional to the ALD process cycles.
Photoluminescence of Atomic Layer Deposition Grown ZnO Nanostructures
Materials Today: Proceedings, 2018
Atomic Layer Deposition (ALD) technique was used to grow crystalline, nanostructured Zinc Oxide (ZnO) thin films, having very low surface roughness, on silicon (Si) and fused quartz (SiO 2) substrates. The film thickness and structural properties were characterized by using X-ray reflectivity and X-ray diffraction techniques. The excitonic light emission of ZnO thin films wasstudied by means of photoluminescence (PL) measurements in the ultraviolet-visible spectral region. Characteristic near band edge emission (NBE) of the ZnO thin films corresponding to free excitonic recombination shows strong signal in the UV range at room temperature. Visible region of the PL spectra of ZnO nanostructures shows that there is no defect related deep level emission (DLE), suggesting the absence of defect states. The results confirm the growth of ZnO thin films of a high quality crystalline structure by ALD technique.
Evolution of microstructure and related optical properties of ZnO grown by atomic layer deposition
Beilstein Journal of Nanotechnology, 2013
A study of transmittance and photoluminescence spectra on the growth of oxygen-rich ultra-thin ZnO films prepared by atomic layer deposition is reported. The structural transition from an amorphous to a polycrystalline state is observed upon increasing the thickness. The unusual behavior of the energy gap with thickness reflected by optical properties is attributed to the improvement of the crystalline structure resulting from a decreasing concentration of point defects at the growth of grains. The spectra of UV and visible photoluminescence emissions correspond to transitions near the band-edge and defect-related transitions. Additional emissions were observed from band-tail states near the edge. A high oxygen ratio and variable optical properties could be attractive for an application of atomic layer deposition (ALD) deposited ultrathin ZnO films in optical sensors and biosensors.