Degree Of Misorientation Research Articles

Studies of GaAs–oxide interfaces with and without Si interlayer

We have studied the properties of metal–oxide–semiconductor structures fabricated by plasma enhanced chemical vapor deposition of SiO2 upon GaAs substrates. We have characterized the dependence of these properties upon the presence of a silicon interlayer, the type and the degree of misorientation of the substrate, and the type of plasma enhancement. We conclude that the presence of a silicon interlayer is beneficial for n-type GaAs if the oxide is deposited by a remote plasma technique. For such oxides on n-type GaAs coated with silicon, integration of the quasi-static capacitance curve suggests a band-bending range of 0.6–0.9 V. Also for these samples, we observe a hysteresis of order 0.6 V, and shifts of only 0.2 V in the midpoint of the rise from minimum to maximum capacitance upon changing frequency from 10 to 200 kHz. The benefits of using a silicon interlayer with a direct plasma deposited SiO2 film are less evident. It is assumed that much of the improvement of these results arises not from a large reduction of the interface density but rather from a shift of the spectrum towards the conduction band edge. This appears evident from the improvement on n-type samples coincident with a degradation on p-type samples. The total band bending change derived from the integral from the quasi-static capcitance–voltage curves is less sensitive to this interlayer than is the frequency dispersion.

Surface diffusion and related phenomena in MBE growth of III–V compounds

Surface diffusion in MBE is studied taking GaAs and In z Ga 1- z As as examples. First, the mode transition between 2D nucleation and step flow in MBE on vicinal surfaces is studied theoretically and experimentally. The principal idea of the theory is to assume that the transition occurs when the surface supersaturation on the step terrace becomes identical with the critical supersaturation for 2D nucleation. This allows the diffusion length of Ga to be calculated at the experimental critical temperature for the mode transition. It is found that the diffusion length increases as the temperature is decreased, due to the increased residence time, and that the diffusion length on (111)B is longer than that on (001) using the assumption of equal formation energy of 2D nuclei for both surfaces. Then the theory is applied to help understand the dependence of the In z Ga 1- z As composition on the growth temperature, the substrate orientation and the degree of misorientation. The theory is in good agreement with the present experiments and it is concluded that surface diffusion is one of the most important and essential processes controlling MBE growth and impurity incorporation.

Hrtem Observations of Sol-Gel Derived Yag and Alumina/Yag Thin Films

AbstractThin films of YAG and YAG/alumina mixtures were prepared by gelling a sol across a TEM grid. The grids were heat-treated to temperatures as high as 1550°C. The resulting ceramic thin films were observed by standard and high resolution TEM techniques. Ion-milling, carbon coating, or other specimen preparation was not necessary. The phase and microstructure evolution, as well as pore structure evolution and spheroidization of film edges could be easily observed. Abnormal grain growth was observed in yttrium-aluminum garnet (YAG) films. Lattice images were taken of the matrix, abnormal grains, and structures that evolved from the abnormal rains at higher temperatures. The grain size preceding abnormal grain growth was 20–50 nm. The abnormal grains were composed of 20–50 nm subgrains with up to several degrees of misorientation. High resolution observations were also made of small (< 0.2 μm) YAG inclusions in alumina. An orientation relationship of (111)[011]alumina//(112)[021]YAGwas observed. This orientation relationship was not observed in large inclusions.

Effects of Substrate Misorientation and the Growth Mechanism of Si Doped GaAs Grown on (111)A Substrate by MBE

AbstractThe MBE growth of Si doped GaAs on slightly misoriented (111)A substrates was examined. The conduction types and the carrier concentrations were greatly affected by the degree of substrate misorientation. In the growth on an exactly (111)A oriented substrate, substantially all Si atoms acted as acceptors. With increase of the degree of misorientation and/or with increase of the flux ratio (JAs4/JGa), the number of donor-site Si atoms increased, that is, the hole concentration first decreased by autocompensation of Si and then the conductivity reversed from p-type to n-type. The photoluminescence of highly compensated (111)A films showed characteristics of donor-acceptor pair recombination. A doping mechanism including preferential decomposition of As4 molecules at the steps was proposed.

A Tem Investigation of Secondary Dislocations in Grain Boundaries in Germanium

AbstractGermanium bicrystals containing dissociated low energy grain boundaries with a common [011] tilt axis were studied. The dislocation structure of a grain boundary with a misorientation of about 28° has been investigated in detail and could be interpreted as a near-coincidence Σ 27 - tilt grain boundary. Both conventional and high-resolution techniques were applied to analyze the Burgers vectors of the secondary dislocation network. In all cases non - primitive Burgers vectors were observed. The comparison with previous results indicates that the Burgers vectors of the dislocation network may depend on the degree of misorientation of the bicrystal.

Influence of annealing and substrate orientation on metalorganic chemical vapor deposition GaAs on silicon heteroepitaxy

GaAs layers grown on misoriented silicon substrates are examined for defect reduction as a function of thermal annealing and degree of misorientation. These GaAs layers (3–4 μm) are grown by a two-step metalorganic chemical vapor deposition process on Si substrates misoriented 1°, 1.5°, 2°, 3°, 4°, and 6° from (100) toward [011]. Annealing takes place in an open tube furnace under an arsine ambient at 850 °C for one or two 30-min cycles. Double-crystal x-ray rocking measurements and plan-view and cross-section transmission electron microscopy are used to evaluate the resulting crystal quality. Prior to annealing, all cases exhibit approximately equal defect densities with the average size of the microtwins being a function of misorientation. There also exists an anisotropy in the microtwin variant distribution in the layers. After annealing, however, the defect density is found to be dependent on the misorientation. The 1° and 2° layers, which have smaller microtwins, exhibit a greater reduction in defect density after thermal cycling than the 4° and 6° layers. The annealed 4° and 6° layers instead exhibit larger microtwins on average than the as-grown layers. These data indicate that smaller microtwins are more likely to be annihilated through thermal cycling. In addition to the change in the microtwin structures, thermal cycling produces elongated dislocations in the 1° and 2° layers and dislocation tangles associated with the remaining microtwins in the 4° and 6° layers.

The effect of off-axis Si (100) substrates on the defect structure and electrical properties of β-SiC thin films

Antiphase domain-free, monocrystalline β-SiC thin films have been epitaxially grown on off-axis Si (100) substrates. The effects of degree of misorientation and substrate preannealing on the antiphase domain boundaries (APB's) have been investigated. Wet oxidation, optical microscopy, and transmission electron microscopy were used to characterize the defect structures in both β-SiC thin films grown on exact Si (100) and on off-axis Si (100) substrates. The results revealed that many dislocations were included in APB's and that APB's were eliminated in the β-SiC thin films grown on Si (100) substrates that were oriented 2°–4° from [100] toward the [011] direction. Some APB's were observed near the edge of the β-SiC film on the 2° off-axis Si (100) substrates; however, they were eliminated by substrate preannealing. The carrier concentration and electron mobility of these β-SiC films were similar to those of β-SiC films grown on exact Si (100) substrates as determined by differential capacitance-voltage and Hall effect measurements. Au-β-SiC Schottky diodes were also fabricated on the β-SiC thin films on 4° off-axis substrates and had an ideality factor of 1.4. Differential capacitance-voltage measurements and current-voltage characteristics of Au-β-SiC Schottky diodes indicated that APB's caused significant leakage current in the β-SiC thin films.

Lattice imaging analysis of Σ3 coincidence-site-lattice boundaries in gold

Abstract High-resolution transmission electron microscope images of Σ3 coincidence-site-lattice boundaries in gold have been observed, and analysed in terms of structure models. In particular (112) A/B and (111) A/B boundaries, having different degrees of misorientation and amounts of relaxation, have been observed. The maximum amount of misorientation correlates well with Brandon's θ=15/Σ1/2 criterion. Relaxation of the atoms near the boundary is explained in terms of simple atomic repulsions.

Discontinuous γ′ coarsening in a Ni-Al-Mo base superalloy

Discontinuous coarsening in a Ni-AI-Mo base superalloy was studied emphasizing the influence of grain boundary misorientation. Nucleation and growth rates were determined as a function of temperature, time, and degree of misorientation for (001) twist boundaries. Both nucleation and growth were controlled by grain boundary diffusion and increased with twist angle and temperature. Grain boundary diffusivities (calculated assuming reduced γ′ surface energy as the driving force) were more than one order of magnitude greater than those reported for static boundaries of similar misorientation. Consideration of atomic jump frequencies in static and dynamic boundaries indicated that the high diffusivities cannot be attributed to motion enhanced grain boundary diffusion. Rather, the high calculated diffusivities are interpreted to indicate the presence of additional driving forces for discontinuous coarsening in the Ni-AI-Mo base superalloys. Potential sources of additional driving force are presented.

Order and disorder in molecular crystals:trans-stilbene

Recent refinements of the structure of trans-stilbene suggested the presence of disorder at only one of the two crystallographically independent sites of the unit cell. Calculations based on atom-atom intermolecular potentials have shown that this disorder is adequately described by a model in which some of the molecules at that site are misoriented in a manner which results in large positional changes for only a few of the atoms in each molecule. A technique has been developed for including the degree of misorientation into the lattice energy calculations and the results are compatible both with the preference for disorder at one site and with the degree of misorientation at that site as estimated from difference maps. The system was analysed at two temperatures (113 and 298 K). The calculations confirm that the probability of orientational disordering at low temperature is smaller than at high temperature and show that it is limited to about 20% in the latter case.

Electron microscopy of cuprous oxide island growth

Abstract The growth of Cu2O islands on (111) surfaces of Cu foils at a pressure of 8 × 10−4 torr and in the temperature range 250°–400°C has been followed up to 55% coverage by in situ transmission electron microscopy and high resolution electron microscopy. The oxide islands grow into the copper in both parallel and twin orientations which are different in shape, degree of misorientation, internal stress and growth rate. For islands grown at 250°C it appears that the oxide-metal interface is determined by crystallographic planes in the oxide rather than in the copper. No evidence of any strong influence of dislocations or other lattice defects in the copper was observed in the nucleation.

Study of polarisation ratios in mixed molecular crystals

The perturbation theory of Craig and Thirunamachandran is used to obtain the polarisation ratio (b/a) of guest transitions in an anthracene host crystal as a function of trap depth. Results from several substituted anthracenes as dopant are compared with the calculated curve. Substantial agreement is obtained over a wide range of trap depths confirming the applicability of the simple theory in this energy region. Degree of misorientation and possible deviation of transition moment from the short molecular axis (M) are briefly discussed.

The influence of substrate characteristics on the contact angles between liquid gallium and gallium arsenide crystals

Parameters influencing the interfacial interaction between liquid gallium and single crystalline gallium arsenide were investigated under vacuum by means of the sessile-drop technique over the temperature range ∼ 30 to 200° C. In addition to their dependence on the anisotropy of the {111} planes, contact angles in the Ga(I)/GaAs(s) system were found to be sensitive to the degree of misorientation and the direction of tilt of these planes. Furthermore, contact angles were found to be dependent on the size of the liquid drop and on the surface roughness of the substrate. In agreement with theoretical expectations the measured angles increased with increasing roughness of the GaAs surfaceS. However, these angles were found to be unaffected by the presence of N2, Ar, and He atmospheres, and by the nature and concentration of charge carriers in the substrate.

Critical size of magnetic particles with high uniaxial anisotropy

The critical sizes of particles of magnetic material with high uniaxial anisotropy are calcu­lated by equating the energies in the single and multi-domain states. Exact calculations are presented of the variation of critical size with shape for a rectangular particle, and the variation of critical size in an applied field is analysed. The critical size of an individual grain in a polycrystalline material is calculated, using two models, ( a ) in which the internal demagnetizing effects arise at non magnetic gaps at grain boundaries, and ( b ) in which the grains are misoriented from the common easy axis. Curves of critical size against grain boundary gap and degree of misorientation are presented, and it is concluded that the latter is the more satisfactory explanation of observed domain structures.

Meßverfahren zur Bestimmung der Kristallitverkippung in dünnen Aufdampfschichten

In the present paper a method is described, which permits the measurement of the crystal misorientation in thin films by means of electron diffraction. In some special cases it was possible to make use of other techniques to determine the degree of misorientation and to compare the values obtained by the different methods. The result of this comparison is a consistency of the measuring values within the limits of about 10%.

The Solidification of Bi2Te3 and Its Solid Solutions with Bi2Se3 and Sb2Te3 under the Influence of Ultrasonic Agitation

The effect of ultrasonic agitation during solidification was studied for three systems: , , and . Reduction in grain size was most pronounced in ; the other compositions showed relatively little refinement. A definite tendency for alignment of acicular grains with low intensities of ultrasonic irradiation was noted in all three systems. Compressiye fracture stress was increased by agitation for all three systems with considerable anisotropy of strength, relative to the growth direction, occurring in agitated and unagitated material. Highest fracture strength was found in , 7000 vs. 5000 psi for unagitated material. Reduction in length was about equal at 3%. Seebeck coefficient and resistivity were measured as functions of temperature and found to exhibit a difference between agitated and unagitated material which correlated with the amount of grain refinement observed. This is a result of the crystallographic anisotropy of electrical properties in alloys, reflected by the degree of misorientation caused by ultrasonic agitation.

The Effects of Substrate Orientation on Epitaxial Growth

The effect of substrate orientation on the quality of silicon epitaxial layers was studied in terms of deposition rate and surface topography. The layer was deposited by the hydrogen reduction of silicon tetrachloride. Deposition rate was determined from the time of deposition and from the layer thickness obtained by angle lapping and staining. Deposition rate was lowest on accurately oriented 〈111〉 substrates and increased as the degree of misorientation from the 〈111〉 direction increased. The frequently observed surface pyramid defect was found to be a function of (a) deposition rate and (b) substrate orientation. These pyramidal defects were not observed for low deposition rates, and when the substrate was misoriented from the 〈111〉 direction. These observations are consistent with the proposal that the deposition rate used in practice is determined by two fundamental rate processes, namely, (a) reaction rate and (b) growth rate.

Recrystallization and Grain Boundary Sliding in a Magnesium Alloy

DURING tensile deformation of magnesium and its dilute alloys in the temperature range 100°–350° C, cells form preferentially close to the grain boundaries1. Afterwards irregular boundary migration occurs and evidence has been presented for the grain boundaries being pinned at their intersections with the cell walls2. This migration results in an increase in grain boundary area and thus the driving force must arise from the difference in stored energy between neighbouring grains. It is unlikely that sliding can occur to any appreciable extent at these irregular boundaries and in these circumstances it is probable that the shearing forces in the vicinity of the grain boundaries are accommodated by increases in the degree of misorientation across the cell walls. Eventually the irregular migration leads to primary recrystallization2–6 along the old grain boundaries and an example is illustrated in Fig. 1.