Effects of Grain Boundary Barrier in ZnO/Si Heterostructure

The influence of ZnO microstructure on electrical barriers is investigated using capacitance-voltage （C - V）, current-voltage （I- V） and deep level transient spectroscopy （DLTS） measurements. A deep level center located at Ec - 0.24 eV obtained by DLTS in the ZnO films is an intrinsic defect related to Zni. The surface states in the ZnO grains that have acceptor behavior of capturing electrons from Zni defects result in the formation of grain barriers. In addition, we find that the current transport is dominated by grain barriers after annealing at 600℃ at 02 ambient. With the increment of the annealing temperature, the current transport mechanism of ZnO/Si heterostructure is mainly dominated by thermo-emission.     

Grain Boundary Channel Formation During Interdiffusion in Alloys

We present results of an experimental study of a new phenomenon accompanying grain boundary (GB) interdiffusion: the hole channel formation along GBs. The objects for study were plates of a homogeneous Cu-5 at.% Sn alloy, which were annealed at 800°C in purified hydrogen. Porous zones were found with GB hole channels perpendicular to the surface and practically equidistant from one another. The porous zone propagation and the average pore size growth at early stages of annealing obey a parabolic law. The observed processes are caused by nucleation and growth of the Cu₃Sn phase at the free surface. The new phase works as diffusion pump pumping out Sn atoms from the alloy towards the growing compound layer. The GB channel formation has been described as a relaxation process accompanying GB interdiffusion of Sn and Cu atoms with unequal partial diffusion coefficients (D _Sn>D _Cu). Excessive vacancies appearing at the GBs due to the inequality between D _Sn and D _Cu are absorbed by bulk and GB sinks, and tensile stresses appear near the GBs stimulating hole channels or groove formation.     

Defect Formation on the Surface of ZnO Using Low-Energy Electrons

Defect formation on the surface of zinc oxide using low-energy electrons is studied by the total-current-spectroscopy method. The formation of cation and anion vacancies on the surface is shown. The energy threshold for the formation of anion vacancies (~20 eV) on the surface of ZnO is determined. It is shown that the negative potential of the surface decreases in the case of irradiation with electrons with energies above 120 eV. The experimentally observed kinetic characteristics of the charging of a ZnO crystal are attributed to modification of the crystal surface and the generation of radiation-induced defects, which are electron traps.     

Defect Formation in a Dynamic Transition

When a system that undergoes a continuous phase transition is swept through its critical point the initial symmetry is broken and domains are formed. Because of critical slowing down it is not possible to sweep adiabatically; the number of domains therefore depends on the rate of increase of the critical parameter. We give a summary of recent theoretical results for the number of defects produced as a function of how rapidly the transition point is passed. They are obtained from a simplified model, using a stochastic partial differential equation that is also solved numerically.     

Defect engineering of ZnO

The defect responsible for the transparent to red color change of nominally undoped ZnO bulk single crystals is investigated. Upon annealing in the presence of metallic Zn as reported by Halliburton et al. and also Ti and Zr a native defect forms with an energy level about 0.7 eV below the conduction band. This change is reversible upon annealing in oxygen. Optical transmission data along with positron depth profiles and annealing studies are combined to identify the defect as oxygen vacancies. Vacancy clustering occurs at about 500 °C if isolated zinc and oxygen vacancies. In the absence of zinc vacancies, clusters form at about 800 °C.     

Grain Boundary Grooving as an Indicator of Grain Boundary Phase Transformations

The atomic force microscopy (AFM) was used to study the grain boundary (GB) groove profiles far away from the melting temperature T _m. It is shown that AFM allows one to measure the temperature dependence of the GB energy in a rather broad temperature interval (from 0.85 T _m to T _m). The GB energy and GB segregation of Bi were measured at 1123 K in the interval of the Bi bulk concentration x ^v _Bi from 5 to 140 ppm Bi. The transition from monolayer to multilayer adsorption is observed for the 19a GB at 1123 K and x ^v _Bi = 60 at. ppm Bi. At the same point (1123 K and x ^v _Bi = 60 at. ppm Bi) a discontinuity of the first derivative of the GB energy is observed. These features were explained using the model of GB prewetting phase transformation developed previously.     

Grain Boundary Motion during Ag and Cu Grain Boundary Diffusion in Cu Polycrystals

Grain boundary (GB) motion in high-purity Cu material (5N8 and 5N Cu) is investigated using the results of radiotracer GB diffusion measurements with tracers exhibiting fundamental differences in the solute-matrix atom interactions. The results on GB solute diffusion of Ag (revealing a miscibility gap in the Ag-Cu phase diagram) and Au (forming intermetallic compounds with Cu) in Cu and on Cu self-diffusion are analyzed.The initial parts of the Ag and Cu penetration profiles turned out to be substantially curved. The profile curvature is explained via the effect of GB motion during ^110m Ag and ⁶⁴Cu GB penetration. The activation enthalpies of GB motion in these two independent measurements occurred to be very close, 95 and 103 kJ/mol, respectively. Moreover, these values turn out to be close, but still somewhat larger than the activation enthalpy of Cu GB self-diffusion in Cu material of the same very high purity, Q ^Cu _gb = 72 kJ/mol. Although tracer diffusion measurements of Au GB diffusion in Cu yielded only limited information on GB motion, the absolute values of GB velocities are consistent with those calculated from the Ag and Cu GB diffusion data.     

Grain Boundary Migration in Ceramics

During ceramic fabrication, densification processes compete with coarsening processes to determine the path of microstructural evolution. Grain growth is a key coarsening process. This paper examines grain boundary migration in ceramics, and discusses the effects of solutes, pores, and liquid phases on grain boundary migration rates. An effort is made to highlight work in the past decade that has contributed to and advanced our understanding of solute drag effects, pore-boundary interactions, and the role of liquid phases in grain growth and microstructural evolution. Anisotropy of the grain boundary mobility, and its role in the development of anisotropic (anisometric) microstructures is discussed as it is a central issue in recent efforts to produce ceramic materials with new combinations of properties and functionality.     

Segregation Effects at a High-Angle Twist Boundary in ZnO

Ab initio density functional plane-wave pseudopotential calculations were performed for a = 7 ( = 21.79°) [0001] twist boundary in ZnO with and without the presence of Sb impurities. The segregation energies revealed a significant driving force for segregation and it was shown that the formation of an Sb monolayer was favoured. Decreased coordination in the boundary core suggested a trend towards the formation of an intergranular phase. The impurity states caused by the monolayer were located within the band gap and higher in energy relative to the state produced by a single impurity. Charge transfer to the Sb monolayer was observed indicating a possible enhancement of the grain boundary potential barrier.     

The Effect of Triple Junctions on Grain Boundary Motion and Grain Microstructure Evolution

The theory of steady state motion of grain boundary sytems with triple junctions and the main features of such systems are considered. A special technique of in-situ observations and recording of triple junction motion is introduced, and the results of experimental measurements on Zn tricrystals are discussed. It is shown, in particular, that the described method makes it possible to measure the triple junction mobility. It was found that the measured shape of a moving half-loop with a triple junction agrees with theoretical predictions. A transition from triple junction kinetics to grain boundary kinetics was observed. This means that triple junctions can drag boundary motion. It is demonstrated that the microstructural (granular) evolution is slowed down by triple junction drag for any n-sided grain. The second consequence pertains to six-sided grains. For a boundary system with dragging triple junctions there is no unique dividing line between vanishing and growing grains with respect to their topological class anymore, like n = 6 in the Von Neumann-Mullins relation.     

The Geometric and Thermodynamic Properties of Grain Boundary Junctions

We provide an overview of the properties of triple junctions and quadruple points. It is shown that these junctions may exhibit distinct behaviors that imply that they have and thermodynamically distinct properties in the same way that grain boundaries can be considered as thermodynamically distinct phases, separate from the material that they inhabit. It is shown that the treatment of triple junctions as thermodynamically distinct defects is a natural extension of the treatment of grain boundaries, and that it can be further extended to other junctions such as quadruple nodes. Equilibrium dihedral angles under conditions of anisotropic interfacial energy are explored, and it is found that the dihedral angles may be variable under a range of different conditions.     

Compensation Effect for Grain Boundary Diffusion in a Bicrystalline Experiment

The paper is devoted to the problem of the compensation effect for grain boundary (GB) diffusion, i.e. the linear dependence of the pre-exponential factor of the GB diffusion coefficient on the activation energy. Specific features of GB diffusion as a thermally activated process namely, the influence of segregation factor, K, and variation of the GB width, d, on the diffusion rate are discussed. A special diffusion experiment was designed to estimate the contribution of the separate component parts of the triple product, KdD_GB (D_GB is the GB diffusion coefficient). The experiment was performed with Al bicrystals. The variation of the GB width d, and a value of the segregation factor K, due to GB structure change are estimated. It is concluded that D_GB is the main GB structure-sensitive parameter in the triple product. This circumstance allows us to consider the GBs in Al bicrystals as a series of uniform objects and to describe the kinetics of GB diffusion in terms of the compensation temperature T_c and a “barrier” phase. The value of T_c for GB diffusion of Zn and Ge in Al bicrystals is practically the same and equals 709 and 706 K, respectively. The character of the “barrier” phase is discussed.     

Grain Boundary Resistivity and Electrically Induced Grain Boundary Migration (EIGM) in Metallic Bamboo Microstructures

As VLSI conductor line dimensions continue to decrease, electrotransport properties increasingly effect device lifetimes. Grain boundaries are intimately linked to these processes, providing paths of varying diffusivity, and as mobile defects themselves. Haessner et al. [6] make a challenging finding in experiments with thin gold films: based on calorimetric data, in order to account for the velocity of grain boundaries migrating in high electric current densities, the force on the atoms of a grain boundary would have to be two orders of magnitude larger than what the accepted theory for bulk ions predicts. The failure is attributed to the simplicity of the model which does not account for possible variations of the resistivity and effective valance charge that could occur in the vicinity of a grain boundary. In this paper, expressions are developed for the electron wind force on the atoms near grain boundaries, and they are written in terms of thermodynamic variables: the boundary specific volume expansion and specific resistivity. The enhancement of the wind force of the boundary atoms over the bulk wind force is calculated using published data. This model allows for more than an order of magnitude enhancement in gold, and Haessner's observation is rationalized.     

Defect induced local moment in ZnO as a consequence of Stoner mechanism

We have examined the formation of a local moment by considering various defects in ZnO. The localization of the defect induced state is found to determine the presence/absence of a local moment. A lot of attention on the probable origin of magnetism in wide band gap oxides has focused on cation vacancies. Here we show that oxygen interstitial atoms give rise to a large magnetic moment which results in a spin polarization of both the conduction and valence bands, in addition to spin polarized gap states. A Stoner mechanism is invoked and the relevant Stoner parameters are determined to be 0.7 eV for an oxygen atom in the presence of an oxygen interstitial but reduced to 0.2 eV on oxygen in the presence of a Zn vacancy.     

Quantitative HRTEM Investigations of a Symmetrical Tilt Σ11 Grain Boundary with Two Different Grain Boundary Planes in α-Al2O3

High resolution transmission electron microscopy was used for the determination of the Σ11 grain boundary structure in α-Al₂O₃. Two orientations of the Σ11 boundary were investigated, the (101¯1) and the (2¯116) interface planes, respectively. The atomistic structures of the grain boundary core were quantitatively determined by comparing the experimental micrographs with simulated images. Each GB was imaged along two perpendicular directions. 3D models are proposed for each of the grain boundary orientations. For the Σ11 (101¯1) grain boundary an empirical shell model was used as the starting model for the structure refinement, whereas a CSL type model was used for the Σ11 (2¯116) grain boundary. After the quantitative evaluation both Σ11 grain boundaries revealed that the best matching structures were non CSL model structures. Non-mirror symmetry was observed for both configurations for at least one investigated zone axes. For the Σ11 (101¯1) interface a relative shift of one crystal halve with respect to the other was observed perpendicular to the grain boundary plane, where the (101¯1) lattice spacing is reduced by Δy = 0.04 ± 0.01 nm. For the Σ11 (2¯116) interface the resulting atomic structure is shifted by Δx = 0.322 ± 0.035 nm in the [21¯1¯1] direction from the CSL model structure. This corresponds to half of the repeat length of the [21¯1¯1] direction. No relaxation perpendicular to the interface plane was detected.     

First-Principles Study on Native Defect Complexes in InN

We present first-principles calculations of the formation energy of different native defects and their complexes in wurtzite InN using density-functional theory and the pseudopotential plane-wave method. Our calculations are aimed in the three cases： N/In = 1, N/In 〉 1 （N-rich）, and N/In 〈 1 （In-rich）. Our results indicate that the antisite defect has the lowest formation energy under N/In = 1. The formation energy of nitrogen interstitial （nitrogen vacancy） defect is significantly low under the N-rich （In-rich） condition. Thus the antisite defect is an important defect if N/In = 1, and the nitrogen interstitial （nitrogen vacancy） defect is a vital defect under the N-rich （In-rich） condition. The atomic site relaxation around the nitrogen interstitial and vacancy is investigated. Our calculations show that the nitrogen vacancy cannot be observed although it is one of the most important defects in InN. Our results are confirmed by experiments.     

Dynamics of Faceted Grain Boundary Grooves

The dynamics of dislocation-free crystal facets is examined in the context of grain boundary grooves at the junction between two crystallites of a solid and the liquid phase. The geometry and thermal conditions of grain boundary grooves allow a detailed analysis of facet morphology during solidification in terms of the nucleation and spreading rates of elementary crystal planes. Observations on the freezing of water in a two-dimensional cell reveal several dynamical features which are treated by the theory. Additional observations provide indications for the stiffness and premelting of grain boundaries.     

Defect induced ferromagnetism in carbon-doped ZnO thin films

We report on room temperature ferromagnetism in C-doped ZnO thin films prepared by electron beam evaporation. Magnetization, Hall effect, X-ray photoemission spectroscopy (XPS) and X-ray diffraction studies have been conducted to investigate the source and nature of ferromagnetism in C-doped ZnO. The samples were observed to have n-type conduction with the carrier concentration increasing with C doping. XPS does not give any evidence for C substituted at the O site, and is more consistent with the formation of C-O bonds and with the presence of C primarily in the +4 state. It is suggested that the ferromagnetism originates in the development of Zn vacancies that are stabilized due to the incorporation of C in a high valence state (C⁴⁺).     

Kinetics of Grain Boundary Recovery in Deformed Polycrystals

Annealing kinetics are studied for nonequilibrium ensembles of dislocations occurring in grain boundaries during plastic deformation. Two types of dislocation ensembles are considered: 1) walls of sessile extrinsic grain boundary dislocations (EGBDs), which cause a change of the GB misorientation angle, and 2) arrays of glissile EGBDs having a Burgers vector tangential to the grain boundary plane. For both types similar exponential relationships are obtained for the relaxation of the average EGBD density, with approximately the same characteristic time proportional to the cube of grain size.