Channeling contrast microscopy of epitaxial lateral overgrowth of ZnO/GaN films (original) (raw)
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Impurity distribution and microstructure of Ga-doped ZnO films grown by molecular beam epitaxy
Journal of Applied Physics, 2012
We report microstructural characterization of heavily Ga-doped ZnO (GZO) thin films on GaN and sapphire by aberration-corrected scanning transmission electron microscopy. Growth under oxygen-rich and metal-rich growth conditions leads to changes in the GZO polarity and different extended defects. For GZO layers on sapphire, the primary extended defects are voids, inversion domain boundaries, and low-angle grain boundaries. Ga doping of ZnO grown under metal-rich conditions causes a switch from pure oxygen polarity to mixed oxygen and zinc polarity in small domains. Electron energy loss spectroscopy and energy dispersive spectroscopy spectrum imaging show that Ga is homogeneous, but other residual impurities tend to accumulate at the GZO surface and at extended defects. GZO grown on GaN on c-plane sapphire has Zn polarity and no voids. There are misfit dislocations at the interfaces between GZO and an undoped ZnO buffer layer and at the buffer/GaN interface. Low-angle grain boundaries are the only threading microstructural defects. The potential effects of different extended defects and impurity distributions on free carrier scattering are discussed. V
ZnO nanostructures grown on epitaxial GaN
Thin Solid Films, 2009
Presented is the growth of zinc oxide nanorod/nanowire arrays on gallium nitride epitaxial layers. A hierarchical zinc oxide morphology comprising of different scale zinc oxide nanostructures was observed. The first tier of the surface comprised of typical zinc oxide nanorods, with most bridging to adjacent nanorods. While the second tier comprised of smaller zinc oxide nanowires approximately 30 nm in width often growing atop the aforementioned bridges. Samples were analysed via scanning electron microscopy, as well as, cross-sectional and high resolution transmission electron microscopy to elucidate the detailed growth and structural elements of the heterostructure.
Journal of Crystal Growth, 2010
Single crystalline ZnO film was grown on (1 1 1) Si substrate through employing an oxidized CrN buffer layer by plasma-assisted molecular beam epitaxy. Single crystalline characteristics were confirmed from in-situ reflection high energy electron diffraction, X-ray pole figure measurement, and transmission electron diffraction pattern, consistently. Epitaxial relationship between ZnO film and Si substrate is determined to be (0 0 0 1) ZnO :(1 1 1) Si and [1 1 2 0] ZnO :[0 1 1] Si . Full-width at halfmaximums (FWHMs) of (0 0 0 2) and (1 0 1 1) X-ray rocking curves (XRCs) were 1.3791 and 3.6341, respectively, which were significantly smaller than the FWHMs (4.5321 and 32.81, respectively) of the ZnO film grown directly on Si (1 1 1) substrate without any buffer. Total dislocation density in the top region of film was estimated to be $ 5 Â 10 9 cm À 2 . Most of dislocations have a screw type component, which is different from the general cases of ZnO films with the major threading dislocations with an edge component.
Tem Characterization of ZnO and AIN/ZnO Thin Films Grown on Sapphire
MRS Proceedings, 1998
Thin (~ 250 nm) films of ZnO grown by pulsed laser deposition on basal plane of sapphire were studied by transmission electron microscopy (TEM). Plan-view TEM study proved the films to be single crystal with the following epitaxial relationship with the substrate: (0001)znO || (0001)sap with the 30 30° in-plane rotation - [0110]ZnO || [1210]sap. Dislocations lying mostly in basal plane of ZnO and aligned along both and <1120> directions having b=1/3[1120] were found. ZnO films were found to have layered growth morphology contrary to columnar morphology of III-nitrides. Consequently, the threading dislocation density in ZnO films (opposing to the AIN and GaN) drops very fast with the thickness: down to 107cm-2 at ~ 250 nm. The effect of post-annealing (which caused significant improvement in electrical and optical properties) on the microstructure of ZnO films was also studied. Contrary to the atomically sharp and clean interface in the as-deposited films, the post-annealed ZnO...
We report on the polarity control of ZnO grown by plasma assisted molecular beam epitaxy on Ga polar (0001) GaN/sapphire templates simply via the oxygen-to-Zn (VI/II) ratio during the growth of a thin nucleation layer at 300 °C. Following Zn pre-exposure, the ZnO layers nucleated with low VI/II ratios (<1.5) exhibited Zn-polarity. Those nucleated with VI/II ratios above 1.5, exhibited O-polarity. Supported by scanning transmission electron microscopic imaging, we have unequivocally demonstrated that polarity inversion takes place without formation of any vertical inversion domains and within one monolayer of presumably non-stoichiometric GaOx formed at the ZnO/GaN interface. A direct correlation between polarity and strain sign of ZnO layers has been found. The Zn-polar ZnO layers were under tensile biaxial strain, whereas the O-polar material exhibited compressive strain. Moreover, the amount of residual strain varied linearly with VI/II ratio used during the low-temperature nucleation layer growth. Strain control with VI/II ratio has been explained by the potential formation of Zn interstitials.
Growth of epitaxial p-type ZnO thin films by codoping of Ga and N
Applied Physics Letters, 2006
Codoping of Ga and N was utilized to realize p-type conduction in ZnO films using rf magnetron sputtering. The films obtained at 550°C on sapphire showed resistivity and hole concentrations of 38 ⍀ cm and 3.9ϫ 10 17 cm −3 , respectively. ZnO films also showed a p-type behavior on p-Si with better electrical properties. ZnO homojunctions synthesized by in situ deposition of Ga-N codoped p-ZnO layer on Ga doped n-ZnO layer showed clear p-n diode characteristics. Low temperature photoluminescence spectra of codoped films also revealed a dominant peak at 3.12 eV. The codoped films showed a dense columnar structure with a c-axis preferred orientation.
Surface modification of sapphire (0001) by Ga can eliminate multiple rotation domains in ZnO films. The existence of Ga at ZnO/sapphire interface was confirmed by x-ray energy dispersive spectroscopy in a transmission electron microscope. Atomic detail of mismatch dislocations at interface was imaged by high resolution transmission electron microscopy. Inside the ZnO film, there is a high density of stacking fault. Both pure gliding of ZnO ͑0001͒ plane and condensation of vacancies or interstatials are possible mechanisms to generate the stacking fault.
Journal of Crystal Growth, 2010
Microstructural analyses of of ZnO films on (1 1 1) Si substrates grown by plasma-assited molecualr beam epitaxy were performed in this study. Zn pre-deposition and its subsequent oxidation, in which either oxygen gas or oxygen-plasma was used as the oxygen source, were employed before ZnO growth. Both reflection high energy electron diffraction and x-ray pole figure showed the single crystalline features in the ZnO films with both post-oxidation of deposited Zn. Detailed transmission electron microscopy (TEM), however, revealed a locally multi-crystalline feature with 30 degrees-rotated domians at the near-interface regions in the ZnO film with oxidation by oxygen gas. ZnO film with oxidation of pre-deposited Zn by oxygen-plasma was observed to be single crystalline through the whole thickness by TEM. We observed a new epitaxial relationship, (0 0 0 1)ZnO//(1 1 1)Si and [0 1 1 0]ZnO//[1 1 0]Si, with a crystallographic rotation of ZnO with respect to Si by 30 degrees, which is energitically more favorable because of a lower lattice misfit (2.2%). No cracks were observed from the ZnO film with a thickness of 1.5 mm, supporting the mechanical integrity of the film prepared in this study.
ECS Journal of Solid State Science and Technology, 2016
Undoped and Ga-doped ZnO films are grown on GaN templates by plasma-assisted molecular beam epitaxy for systematic investigation of the influence of native defects on their electrical properties. The distinct electrical properties of the undoped and Ga-doped ZnO films are observed after thermal treatment in a nitrogen and an oxygen ambient. It is found that the undoped ZnO films show improved characteristics when annealed in an oxygen ambient, and the Ga-doped ZnO films exhibit stable characteristics when annealed in a nitrogen ambient. The variation in electrical properties revealed by the photoluminescence measurements can be attributed to the generation and annihilation of native defects, which are dependent on the ambient of treatments. The properties of the annealed undoped ZnO films grown in an O-rich environment are affected mainly by the oxygen vacancies, antisite oxygen and oxygen interstitials, while the annealed Ga-doped ZnO films are dominated by oxygen interstitials.