Wurtzite Grains Research Articles

Plastic deformation of ZnO thin films through edge and screw dislocation movements

AbstractColumnar wurtzite grains were formed in sputtered ZnO thin films deposited on a plastic polyethylene terephthalate substrate. Selected‐area diffraction patterns reveal that the columnar grains in the sputtered films present two preferred growth planes, namely, the basal (0002) and prismatic (100) growth planes. The diffraction patterns obtained also confirm that the microstructure of sputtered indium tin oxide thin films is amorphous in nature. Tensile tests indicate that the fracture strain of the ZnO thin film occurs between 1.73% and 2.14%, while the fracture strain of the indium tin oxide thin film occurs between 0.24% and 0.67%. Thus, the fracture toughness of the sputtered ZnO thin film is greater than that of the sputtered indium tin oxide thin film. High‐resolution transmission electron microscopic images demonstrate that edge and screw dislocations could be identified in the sputtered ZnO thin films. Moreover, edge and screw dislocation movements may, respectively, be observed in the basal‐ and prismatic‐oriented ZnO columnar grains of the sputtered ZnO thin films. Our results indicate that movements of the edge and screw dislocations in the basal‐ and prismatic‐oriented ZnO columnar grains account for the plastic deformation of the investigated ZnO thin films under tensile stress.

INCREASING THE SELECTIVITY OF USEFUL MINERAL DISCLOSURE WHEN PROCESSING DIFFICULT POLYMINERAL RAW MATERIALS WITH PRELIMINARY WEAKENING

Previous studies at a number of research centers in our country, including the Institute of Mining, SB RAS and the Institute of Nuclear Physics, SB RAS, showed the possibility of improving process parameters of flotation concentration of difficult mineral raw materials due to preliminary electron-beam processing, which allows weakening minerals and rocks. Obtaining the data on selective disintegration and disclosure of minerals during ore pretreatment is important for technological application. This study aims at receiving the experimental data confirming an increase in the selectivity of disintegration of mineral raw materials processed by a high-energy electron beam and an improvement in the disclosure of intergrowths of useful minerals. Complex experimental mineralogical and technological studies by the example of rebellious lead-zinc ore with a grain size of -3 mm provided the data on increasing the selectivity of disintegration of mineral raw materials and improving the disclosure of useful minerals. It is found that if in -0.071 mm size class of ground lead-zinc ore without processing, the fraction of free galena and wurtzite grains (sphalerite modification) is 40.7 and 65.7%, then the fraction of free grains of these minerals increases up to 66, 4 and 71.5% in ore pretreatment. It should be noted that ore processing by accelerated electrons allows obtaining the ground material, in which free mineral grains are less overground. For galena, the size of free grains increases from 2.6 to 4.0 μm, and for wurtzite - from 4.5 to 6.6 μm. Scientific novelty includes increasing the selectivity of disintegration and improving the disclosure of useful minerals through the effect of weakening of mineral raw materials in ore pretreatment.

Zinc blende–wurtzite polytypism in nanocrystalline ZnO films

ZnO is commonly observed and assumed to crystallize fully in a wurtzite structure. Indeed, while polytypism between a wurtzite and a zinc blende phase is often seen in some II-VI compounds, it has only rarely been observed in ZnO. Here, by means of automated crystal orientation mapping and high-resolution imaging in transmission electron microscopy, we identify nanometer-sized regions of zinc blende forming during the growth of compact nanocrystalline metal–organic chemical vapor deposited ZnO films. We analyze a c-fiber-textured wurtzite ZnO film, where the columnar grain growth, typical for polycrystalline films, is interrupted by the formation of these zinc blende regions. These regions in turn provide surfaces for the heteroepitaxial renucleation of newly oriented wurtzite grains. In contrast to the columnar grains, which grow fastest along their c-axis, these new grains are identified to grow fastest along their basal plane. By means of convergent beam electron diffraction, we show that this difference in fast growth direction relates to the polarity of exposed facets. A growth model is proposed, showing how this polytypism affects the microstructure evolution of the film in ways that were not predicted by existing film growth models.

Densification Behavior, Phase Transition, and Preferred Orientation of Hot‐Pressed ZnS Ceramics from Precipitated Nanopowders

In this study, transparent ZnS ceramics were hot pressed from precipitated wurtzite nanopowders. Influences of sintering temperature on wurtzite‐to‐sphalerite phase transition and densification behavior have been investigated. Maximum sphalerite phase content and highest densification were simultaneously obtained in the sample hot pressed at 900°C with uniaxial pressure of 250 MPa for 2 h, which accounts for the highest transmittance above 55% and 70% in the range 2–5 μm and 5–13 μm, respectively. Preferred orientation of wurtzite grains in [002] direction paralleled to the press direction was also observed, which is supposed to be benefit to transmittance by reducing birefraction and second‐phase scattering. Furthermore, second‐phase scattering caused by wurtzite grains has been investigated. It is found that fine grains are conducive to hot‐pressed ZnS ceramics with high transmittance, especially in the short‐wavelength range.

Amorphous interlayers between crystalline grains in ferromagnetic ZnO films

The nanograined thin films of undoped ZnO were synthesized by the wet chemistry method. Films consist of the equiaxial nanograins, and possess ferromagnetic properties. Structural investigations by the XRD and HREM reveal that the crystalline wurtzite grains do not contact each other and are completely surrounded by a layer of amorphous phase. It forms a kind of continuous foam-like network, where the amorphous intergranular phase amount could be increased by the synthesis parameters. Simultaneously, the saturation magnetization increases as well.

Atomic structures of supersaturated ZnO–Al2O3 solid solutions

Supersaturated ZnO–Al2O3 (>20at.% Al) thin films are grown by pulsed laser deposition technique on silica glass substrates at 600°C. They are characterized by combining x-ray diffraction, Al-K edge x-ray absorption near edge structures (XANESs), high resolution transmission electron microscope (TEM) imaging, TEM analysis, and a series of first principles calculations. The films are composed of textured wurtzite grains with c planes parallel to the substrate. The distance between c planes expands significantly when the Al concentration is greater than 10at.%. The expansion disappears after annealing the films at above 800°C. High density of dislocationlike defects is found in the as deposited film. Any segregation of Al cannot be detected either at the grain boundaries or inside the grains. The lattice expansion toward c axis and the experimental XANES can be satisfactorily explained by taking a hypothetical homologous model with the composition of (ZnO)3(Al2O3) as the local environment of Al in the supersaturated solid solution. Simplified substitutional models with Al at the Zn site in wurtzite ZnO cannot explain these experimental results. First principles calculations show that the homologous phase is energetically more favorable than the simplified substitutional models, although decomposition into ZnO and ZnAl2O4 is more favorable than the homologous phase. The local atomic structures of the supersaturated solid solution are therefore concluded to be analogous to the metastable homologous phase.

Structural study of GaN grown on (001) GaAs by organometallic vapor phase epitaxy

Detailed transmission electron microscope (TEM) and transmission electron diffraction (TED) examination has been performed on organometallic vapor phase epitaxial GaN layers grown on (001) GaAs substrate to investigate microstructures and phase stability. TED and TEM results exhibit the occurrence of a mixed phase of GaN. The wurtzite (α) phase grains are embedded in the zinc-blende (β) phase matrix. It is shown that there are two types of the wurtzite GaN phase, namely, the epitaxial wurtzite and the tilted wurtzite. The tilted wurtzite grains are rotated some degrees ranging from ∼5° to ∼35° regarding the GaAs substrate. A simple model is presented to describe the occurrence of the mixed phases and the two types of the wurtzite phase.

CRYSTAL STRUCTURE AND DEFECTS IN NITROGEN-DEFICIENT GaN

We have studied crystal structure and associated defects in GaN films grown on sapphire under nitrogen-deficient conditions by metalorganic chemical vapor deposition (MOCVD) and pulsed laser deposition (PLD). The structural quality of the PLD films grown at 750 °C was comparable with those grown by MOCVD at 1050 °C having threading dislocations density of about 1010 cm−2 at a film thickness 150-200 nm. Microstructure of the PLD films grown at temperatures above 780°C was found to be similar to that of nitrogen-deficient MOCVD films indicating the loss of nitrogen due to thermal decomposition of the nitride layers. Nitrogen-deficient MOCVD and PLD films exhibit polycrystalline structure with a mixture of cubic zinc-blende and wurtzite hexagonal GaN grains retaining tetragonal bonding across the boundaries and hence the epitaxial orientations and polarity. Renucleation of the wurtzite phase at different {111} planes of cubic GaN results in a rough and faceted surface of the film. Most of the stoichiometric films displayed (0001) Ga-face polarity, but the renucleated inclined wurtzite grains grew in the opposite N-face polarity. The major defects related to the cubic structural metastability are stacking faults and microtwins which being nuclei of the metastable cubic phase have an extremely low energy. We elucidate that the cubic phase is more stable under the nitrogen deficiency and, therefore, can exist without decomposition at higher nitrogen vacancy concentrations in the material.

Crystal Structure and Defects in Nitrogen-Deficient GaN

AbstractWe have studied crystal structure and associated defects in GaN films grown on sapphire under nitrogen-deficient conditions by metalorganic chemical vapor deposition (MOCVD) and pulsed laser deposition (PLD). The structural quality of the PLD films grown at 750 °C was comparable with those grown by MOCVD at 1050 °C having threading dislocations density of about 1010 cm-2 at a film thickness 150-200 nm. Microstructure of the PLD films grown at temperatures above 780°C was found to be similar to that of nitrogen-deficient MOCVD films indicating the loss of nitrogen due to thermal decomposition of the nitride layers. Nitrogen-deficient MOCVD and PLD films exhibit polycrystalline structure with a mixture of cubic zinc-blende and wurtzite hexagonal GaN grains retaining tetragonal bonding across the boundaries and hence the epitaxial orientations and polarity. Renucleation of the wurtzite phase at different {111} planes of cubic GaN results in a rough and faceted surface of the film. Most of the stoichiometric films displayed (0001) Ga-face polarity, but the renucleated inclined wurtzite grains grew in the opposite N-face polarity. The major defects related to the cubic structural metastability are stacking faults and microtwins which being nuclei of the metastable cubic phase have an extremely low energy. We elucidate that the cubic phase is more stable under the nitrogen deficiency and, therefore, can exist without decomposition at higher nitrogen vacancy concentrations in the material.

Microstructures of GaN films deposited on (001) and (111) Si substrates using electron cyclotron resonance assisted-molecular beam epitaxy

The microstructures of GaN films, grown on (001) and (111) Si substrates by a two-step method using Electron Cyclotron Resonance assisted-Molecular Beam Epitaxy (ECR-MBE), were studied by electron microscopy techniques. Films grown on (001) Si had a predominantly zinc-blende structure. The GaN buffer layer, grown in the first deposition step, accommodated the 17% lattice mismatch between the film and substrate by a combination of misoriented domains and misfit dislocations. Beyond the buffer layer, the film consisted of highly oriented domains separated by inversion domain boundaries, with a substantial decrease in the defect density away from the interface. The majority of defects in the film were stacking faults, microtwins, and localized regions having the wurtzite structure. The structure of the GaN films grown on (111) Si was found to be primarily wurtzite, with a substantial fraction of twinned zinc-blende phase. Occasional wurtzite grains, misoriented by a 30°twist along the [0001] axis, were also observed. A substantial diffusion of Si was seen in films grown on both substrates.

The structure of epitaxial films of ZnS evaporated onto NaCl in a vacuum

The defect structures of epitaxial films of ZnS evaporated onto NaCl in vacuum were studied by transmission electron microscopy. Included grains of doubly-positioned wurtzite-structure material were found in many of the monocrystalline sphalerite-structure films. No twins or wurtzite grains in other orientations were ever observed.

Epitaxial growth of films of ZnS evaporated on to NaCl

Films of ZnS were deposited on to NaCl by vacuum evaporation. The influence of the growth conditions on the structure of the films was studied by transmission electron microscopy. The most important single factor in producing the widest range of epitaxial substrate temperatures and the most perfect films was the use of focused electron-beam evaporation. Contamination raised the minimum substrate temperature for epitaxial growth. It also resulted in the incorporation of doubly positioned wurtzite-structure grains in the sphalerite-structure films. Co-evaporation of an excess of either Zn or S with ZnS produced sphalerite-structure films containing singly positioned wurtzite grains. The use of conventional hot tantalum evaporators resulted in the incorporation of 0·2 wt.% of tantalum in the films and in poor structural quality. Reproducible current-voltage characteristics were obtained in the best structural quality films. These current-voltage characteristics could be understood in terms of the effect of Schottky barriers but not of space-charge-limited currents.