J. A. Sekhar

ULTRAHIGH SI+ IMPLANT ACTIVATION EFFICIENCY IN GAN USING A HIGH-TEMPERATURE RAPID THERMAL PROCESS SYSTEM

Si+ implant activation efficiencies above 90%, even at doses of 5×1015 cm−2, have been achieved in GaN by rapid thermal processing at 1400–1500 °C for 10 s. The annealing system utilizes molybdenum intermetallic heating elements capable of operation up to 1900 °C, producing high heating and cooling rates (up to 100 °C s−1). Unencapsulated GaN shows severe surface pitting at 1300 °C and complete loss of the film by evaporation at 1400 °C. Dissociation of nitrogen from the surface is found to occur with an approximate activation energy of 3.8 eV for GaN (compared to 4.4 eV for AlN and 3.4 eV for InN). Encapsulation with either rf magnetron reactively sputtered or metal organic molecular beam epitaxy-grown AlN thin films provides protection against GaN surface degradation up to 1400 °C, where peak electron concentrations of ∼5×1020 cm−3 can be achieved in Si-implanted GaN. Secondary ion mass spectrometry profiling showed little measurable redistribution of Si, suggesting DSi⩽10−13 cm2 s−1 at 1400 °C. The imp...

Solidification microstructure evolution in the presence of inert particles

Abstract The development of solidification microstructure can be significantly influenced by the presence of inert particles in the liquid during the processing of particulate composites by the solidification techniques. The effect of particles on the microstructural development is characterized through directional solidification experiments in a transparent organic system, succinonitrile, in which the interactions between the particles and the interface can be examined in situ . These particle-interface interactions have been examined in an alloy system in which the long-range solute field interactions between the particles and the interface are dominant. These interactions have been found to be significantly different for different interface morphologies, i.e. planar, cellular or dendritic interfaces. The effect of particles on the morphological changes and on the nature of the particle trapping processes are characterized for different particle distributions, particle sizes and interface morphologies. It is shown that the presence of particles in binary alloys can either cause the instability of the interface, such as dendrite tip splitting, or lead to morphological transitions, such as a dendritic-to-cellular transition. When the particle density is large, the presence of particles significantly influences the interface shape and thereby alters the interface growth characteristics such that an oscillatory behavior of the interface can be obtained under constant externally imposed velocity. Appropriate theoretical ideas are developed to explain the effects of particles on the morphological development of the interface.

Numerical modeling of solidification combustion synthesis

A numerical study of self-propagating combustion synthesis is carried out to determine the effect of the heat of reaction(Q), the activation energy (E), the frequency factor (K0), thermal conductivity(K*), and initial temperature(T0 on the combustion velocity and combustion temperature in the presence of cooling at one end. The numerical procedure allows for the formation and solidification of nonstoichiometric combustion products. This includes the phase change of the product through its solidification and eutectic range. Calculations are carried out for the Ti-C system with an objective of predicting solutions which are comparable to previously reported, experimentally determined values. Calculations are also compared with the thin zone analytical solution to the combustion problem. The use of lowK0 values to bring the solutions close to the experimentally determined numbers is discussed. Solutions are presented to elucidate the effect of relevant parameters on the thickness of the combustion and preheat zones. Conditions where extinction is expected to occur are identified. The effect of the thermal conductivity on the velocity may be to increase or decrease the velocity, depending on the value ofK0. At lowK0 values, an increase in the thermal conductivity may lead to a decrease in the combustion velocity. The effect of the initial temperature on the combustion velocity and temperature is to increase both; however, the increase in the combustion temperature may not be proportional to the increase in the initial temperature. The activation energyE has a pronounced effect on reducing the combustion velocity while not influencing the combustion temperature. The time rate of the solidification process which determines the final microstructure is discussed.

An investigation of the effect of porosity and diluents on micropyretic synthesis

A numerical model of combustion/micropyretic synthesis of a composite system is developed. The new features of the model include the consideration of melting of each constituent of the reactants and products and the inclusion of considerations involving dilution and porosity. The effect of porosity is to change both the thermal conductivity and density. The model for the porosity which is considered in this article shows the significance of its effect on the velocity and the type of the combustion front. Different values of porosities are considered for the reactants and products. For an illustrative analysis, a systematic study of the variation of porosity of reactants and products is carried out for the combustion synthesis of TiB2 and TiC. The numerical results indicate that as the reactant porosity values are decreased, the combustion velocity first increases because of an increase in the thermal conductivity. The combustion velocity, after reaching a maximum, decreases with a further decrease in the porosity. As the porosity is varied, there is a considerable effect on the nature of propagation of the combustion front, which may change from a steady state to an oscillatory mode. Results indicate that as the reactant porosity is decreased, the frequency of oscillations of the combustion front first increase and then decrease with a further decrease in the reactant porosity. The differences in the mechanism for the decrease in velocities at very low values of reactant porosities for the two systems TiC and TiB2 are identified. The results of considering different values of porosities in the reactants and the product are also presented. For the study of the effect of diluents, the product itself is considered as the diluent. The effect of adding the diluent to the initial reactants is to decrease the combustion temperature and the combustion velocity. The diluent also changes the mode of combustion. An increase in the amount of the diluent results in the decrease of the frequency of oscillations.

Solidification Microstructures near the Limit of Absolute Stability

A theoretical model of microstructural transitions in a binary alloy is examined to establish the conditions under which dendritic to cellular to planar interface transitions occur at high imposed growth rates. Critical experimental studies then are carried out in a transparent carbon-tetrabromide system to study the changes in microstructures which occur in the velocity regime where the planar interface is unstable. Low velocity transitions from a planar to cellular to dendritic structure and the high velocity transition from dendritic to microcellular structure are observedin situ. It is shown that these microstructural transitions occur continuously as the growth rate is increased. A reverse transition, from microcellular to dendritic structure with an increase in composition at a given velocity, also is observed. These results then are compared with the theoretical model.

Rapid solidification by unstable combustion synthesis

During combustion/micropyretic synthesis, conditions that give rise to rapid solidification and rapidly solidified microstructures may be encountered. In this article, many such conditions are identified for the first time in a Ni–Al system. In addition, the banded structures and aligned dendrites that are encountered in this system are also examined. The various techniques of rapid solidification that may be initiated with combustion synthesis are examined and discussed.

Metal-ceramic composites based on the Ti-B-Cu porosity system

A systematic study of the microstructure/fracture toughness/processing correlation of metal-ceramic composites in the Ti-B-Cu porosity system is presented. The composites are produced by the combustion synthesis process. Fracture surfaces indicate both ductile and brittle regions. The composites are made up of Ti as the only ductile phase and TiB, TiB2, Ti2Cu, and Ti3Cu4 as brittle phases. Density measurements and scanning electron microscopy (SEM) indicate that the samples contain distributed porosity. Ductile phase toughening is responsible for the increase in fracture toughness to a maximum value of 9.9 MPa(m)1/2. Samples with large amounts of porosity do not benefit from this toughening process even though they containin situ formed whiskers. The fracture toughness of the composite is modeled by considering the additive influence of the ductile phase reinforcement (Ashby model) and the residual porosity (exponential model). Microstructural constants required for the model are evaluated from the comparison. A correlation between the mechanical properties and the combustion temperature is established.

Development of solidification microstructures in the presence of fibers or channels of finite width

Abstract Directional solidification studies in a transparent succinonitrile system have been carried out in the presence of single or multiple fibers. The solute-rich region and the solute bands which form as the interface approaches the fibers are discussed. The microstracture formation in small interfiber spacings has been examined and it is shown that the development of microstructure depends on (a) the interfiber spacing, (b) the angle between the two fibers and (c) the orientation of fibers with respect to the growth direction. Experimental studies have also been carried out to examine systematically the microstructural development in well-characterized rectangular channels of controlled widths. Under given experimental conditions of growth rate, temperature gradient and composition, it is shown that the microstructure changes from dendritic to cellular to a nearly planar interface as the width of the channel is reduced. Furthermore, for certain channel widths, half-dendritic or half-cellular structures have also been observed. The various implications of these observations for the fundamental understanding of microstructural evolutions and for the applied aspects of solidification of composite materials are discussed.

Micropyretic synthesis of NiAl containing Ti and B

The effect of alloying additions of Ti and B on the process of micropyretic synthesis on NiAl and on the microstructure of the synthesized alloy was examined. It was observed that the combustibility of the quaternary alloy is good despite the presence of the alloying elements because of an additional combustion reaction between Ti and B. The microstructure of the quaternary alloy was found to consist primarily of the NiAl and Ti boride phases. The effect of preheating of the specimen prior to synthesis on the process of synthesis was also examined. It was observed that preheating not only can change the morphology of the phases but also influence the nature of the phases present in the alloy. The mechanism of the formation of the two phase microstructure during the synthesis from the elemental powders was established by stopping the combustion front and by carrying out a detailed microstructural characterization of regions around the stopped combustion front.

Numerical analysis for micropyretic synthesis of NiAl intermetallic compound

A numerical investigation of the micropyretic synthesis response parameters of the Ni-Al stoichiometric compound was undertaken. The influence of the enthalpy of the combustion reaction,Q, activation ienergy,E, amount of diluent, pre-exponential factor,K0, and initial temperatureT0, on the combustion velocity, temperature, and mode was studied. The porosity of the unreacted compact, which is related to the initial compaction pressure, was considered in the calculation. It was found that the change in porosity significantly affects the thermal conductivity and the length of the pre-heat zone as also do the temperature patterns and propagation velocities. The combustion front was noted to be extinguished if the temperature in the reaction zone became lower than the melting point of the aluminium phase. This result was obtained simply by considering the changes in the thermal conductivity after the melting of aluminium without having to invoke any changes in the rate of reaction after the melting. A comparison of the numerical data with the experimental and analytical results was also made.