Crystal Quality Of ZnO Films Research Articles

Influence of nitrogen annealing on structural and photoluminescent properties of ZnO thin film grown on c-Al 2O 3 by atmospheric pressure MOCVD

ZnO thin films on (0 0 0 1) sapphire large area substrates (Φ50×0.43 mm) have been deposited by atmospheric pressure MOCVD. To investigate the effects of post-annealing treatment on the structural and luminescent properties of ZnO thin films, films have been annealed in nitrogen at various annealing temperatures from 400 to 800 °C. The best crystal quality of ZnO films was obtained with annealing temperatures up to 800 °C by measuring the FWHM of (1 0 2) and (0 0 2) peak. At annealing temperature above 700 °C, the intensity of UV (377 nm) peak is greatly decreased, and the deep-level emission peak (500 or 500–525 nm) is enhanced. The study of structural and luminescent properties of ZnO thin films annealed in nitrogen shows that the crystallinity of ZnO thin films is not easily degraded by oxygen vacancies, or say, the deficient oxygen nonstoichiometric ZnO single crystal films may be of better crystallinity as that of the deficient nitrogen nonstoichiometric single crystal GaN films reported.

Effects of Annealing on Luminescence of ZnO Films Deposited on Si Substrates by RF Magnetron Sputtering

Effects of growth ambience, annealing ambience and temperature on the photoluminescence (PL) emission properties of ZnO films deposited on Si (100) substrates by RF magnetron sputtering have been investigated. After annealing, the crystal quality of ZnO films was markedly improved, and the intensity of UV emission peak increased obviously. By varying the flow rate ratio of O2/Ar, annealing atmosphere in oxygen-deficient or oxygen-rich ambience and heating temperature during deposition, the evolution of peak intensities and positions for blue and green emission is formed. This is attributed to the deposition and annealing parameters that control the desorptions and adsorptions of oxygen atoms on the films, and leads to the changes of concentrations of Zinc and oxygen vacancies in the films.

Fabrication and characterization of high frequency SAW device with IDT/ZnO/AlN/Si configuration: role of AlN buffer

Surface acoustic wave (SAW) devices with a ZnO/buffer/Si configuration have been fabricated, employing poly-AlN (deposited by RF magnetron sputtering) and epi-AlN (deposited by plasma-assisted MBE) as the buffer layer. In particular, the effects of the AlN buffers on the orientation of ZnO films as well as the performance of SAW devices are studied. Piezoelectric ZnO films are deposited using RF sputtering at a nominal condition of RF power of 300 W, O 2/(Ar+O 2) ratio of 10%, and substrate temperature of 300 °C. For all the deposited AlN and ZnO films, the X-ray diffraction (XRD) spectra and full width at half maximum ( ω FWHM) of rocking curves are measured in terms of the deposition conditions, to characterize the c-axis preferred orientation and crystal quality. The frequency response characteristics (including S 21) of the fabricated SAW devices are also measured. The experimental results indicate that the c-axis orientation and crystal quality of ZnO films are determined mainly by the properties of AlN buffer. Furthermore, the insertion loss of ZnO-SAW devices is found to be dependent on the crystal quality and surface morphology of AlN buffer. The correlations between the properties of AlN buffers and the characteristics of ZnO-SAW devices are also discussed in detail.

Interface Engineering in Heteroepitaxy

We report the importance of interface engineering in heteroepitaxy with examples of plasma-assisted molecular beam epitaxial ZnO growths on (0001) sapphire substrates and on (0001) GaN/sapphire templates, whose interfacesare engineered to improve and to control properties of ZnO films. The growth of rocksalt structure MgO buffer on Al 2 O 3 (0001) is developed for ZnO epitaxy. By employing the MgO buffer layer, the formation of 30° rotated mixed domains is prohibited and two-dimensional layer-by-layer growth of ZnO on sapphire substrate is achieved. High-resolution X-ray diffraction reveals the superior improvement in a crystal quality of ZnO films with an MgO buffer. Polarity of wurtzite structure ZnO films on Ga-polar GaN/sapphire templates is controlled by changing interface structures. By forming a single crystalline, monoclinic Ga 2 O 3 interfacial layer between GaN and ZnO through O-plasma pre-exposure on the Ga-polar GaN surface, O-polar ZnO films are grown. By forming the ZnO/GaN heterointerface without an interfacial layer through the Zn pre-exposure on the Ga-polar GaN surface, Zn-polar ZnO films are grown.

Hetero-epitaxial growth of ZnO thin films by atmospheric pressure CVD method

High-quality ZnO films were successfully prepared by atmospheric pressure CVD using zinc acetylacetonate (Zn(C 5H 7O 2) 2) and oxygen as sources. Epitaxial growth was achieved on ( 0 1 1 ̄ 2 )-oriented single-crystal sapphire substrates. The crystal quality of ZnO films was highly dependent on the film thickness, and crystallinity was improved remarkably at thicknesses over 0.32 μm. The cathodoluminescence spectra of the films had a strong peak of band-edge emission at 379 nm and the so-called green emission at visible region was very weak.

ZnO/GaN heterointerfaces and ZnO films grown by plasma-assisted molecular beam epitaxy on (0001) GaN/Al2O3

We report the structures of ZnO/GaN heterointerfaces and properties of ZnO films grown by plasma-assisted molecular beam epitaxy (P-MBE) on (0001) GaN/Al2O3 substrates. At first, different preparations (zinc or oxygen-plasma pre-exposures) of the GaN surface are examined and the resultant heterointerface structures and properties of ZnO films are studied. The oxygen-plasma pre-exposure of the GaN surface results in the formation of an interface layer, while the formation of interface layers is prevented by zinc pre-exposure. The interface layer is identified as single-crystalline monoclinic Ga2O3 through extensive transmission electron microscopy (TEM) study. The orientation relationship between monoclinic Ga2O3 of the interface layer and GaN and ZnO epilayers is investigated as (0001) GaN∥(001) Ga2O3∥(0001) ZnO and [2-1-10] GaN∥[010] Ga2O3∥[2-1-10] ZnO. Based on structural investigations and stability of bondings, we propose interface-bonding sequences at the ZnO/GaN heterointerfaces in zinc or oxygen-plasma pre-exposed samples: “N–Ga–O–Zn” bonding sequence for zinc pre-exposed samples and “N–Ga–(O-terminated Ga2O3 layer)–Zn–O” for oxygen-plasma pre-exposed ones. Lattice misfit of Ga2O3 against underlying GaN (against upper ZnO) is estimated as −4.7% (−6.5%) and 10.7% (8.6%) along the [010] and [100] directions of the Ga2O3, respectively. The structural and optical properties of ZnO films evaluated by TEM, high-resolution x-ray diffraction and photoluminescence spectroscopy consistently reveal better crystal quality of zinc pre-exposed ZnO films than oxygen-plasma pre-exposed ones. Second, the effects of high temperature annealing on ZnO films grown at low temperature on zinc pre-exposed GaN surface are studied. Surface morphology is dramatically changed by high temperature annealing; three-dimensional island morphology on as-grown ZnO layers changes into atomically flat morphology with atomic level steps and terraces through high temperature annealing. Optical properties are also improved by high temperature annealing: (i) The intensity of band edge emissions from the ZnO films increases with their full width at half maximum values being decreased. (ii) Free exciton emission from annealed films is considerably enhanced in intensity compared to as grown films. These results indicate that high temperature annealing greatly improve the crystal quality of ZnO films grown at low temperature.