Showing papers by "Y. A. Chang published in 1997"

A thermodynamic description of the Al-Mg-Zn system

Abstract: A thermodynamic description of the Al-Mg-Zn system was developed based on critically evaluated experimental data. All binary intermetallic phases are assumed to have negligible ternary solubility except for MgZn2. Three different thermodynamic models are applied to three different types of phases in this system, i.e., disordered solution phases, stoichiometric compounds, and semistoichiometric phases. The model parameters are optimized based on the thermodynamic descriptions of the constituent binaries and experimental phase equilibrium and thermodynamic data available in the literature. The good agreement obtained between several calculated isopleths and thermodynamic values of the liquid phase and experimental data shows that the current description of this system is reasonable. The calculated phase equilibria in the Al-rich corner are believed to be reliable for practical applications, while those away from the Al-rich region are subjected to large uncertainty. Additional experimental investigations are needed to firmly establish the phase equilibrium of this system over wide ranges of composition and temperature.

A thermodynamic description for the ternary Al-Mg-Cu system

Abstract: A thermodynamic description for the ternary Al-Mg-Cu system was obtained based on the descriptions of its three constituent binaries and ternary phase equilibrium and thermodynamic data available in the literature. All binary intermetallic phases are assumed to have negligible ternary solubility. There are five ternary intermetallic phases; three of them are taken to be line compounds and two are assumed to be semistoichiometric phases. In view of the extensive experimental data available in the Al-rich corner, particularly for the Al-rich ternary eutectic, the objective of this study was to develop a thermodynamic description that allows us to quantitatively calculate the phase equilibria in the Al-rich corner. The calculated phase equilibria in the Mg-rich corner may not be as good as those in the Al-rich corner, and those in the Cu-rich corner should be topologically correct and serve as a guide for materials researchers to carry out key experiments in the future to establish the phase equilibria in that portion of the system. Nevertheless, in the absence of additional data, the calculated phase equilibria in the Cu-rich corner can be used with caution.

Predicting microstructure and microsegregation in multicomponent alloys

Abstract: Accurate predictions of microstructure and microsegregation in metallic alloys are highly important for applications such as process optimization and alloy development. This article gives an overview of the methods developed during the past 30 years for solidification simulation on a microscopic scale, with a special emphasis on multicomponent alloys. Analytical approaches as well as advanced numerical models coupled with greatly simplified phase diagrams and thermodynamically calculated phase diagrams have been critically reviewed. Factors that influence the predictions, such as solid-state diffusion, coarsening of dendrite arms, undercooling effects, and, most importantly, the accuracy of the phase diagram are discussed, and suggestions for further research are given.

Interfacial Reactions and Electrical Properties of Ti/n-GaN Contacts

Abstract: The phase equilibria in the ternary Ti-Ga-N have been investigated. Interfacial reactions in Ti/GaN contacts have been studied by diffusion couple experiments. The ternary phase Ti2GaN was confirmed by x-ray diffraction in bulk samples as well as in massive Ti/GaN diffusion couples and annealed Ti thin films on GaN. The diffusion path in samples, annealed at 85CP C in Ar gas, is GaN/TiN/Ti2GaN/Ti3Ga/Ti. A planar TiN layer forms in direct contact to GaN and governs the electrical properties of annealed Ti/GaN contacts. Thin film contacts were fabricated by sputtering Ti on MOVPE grown n-GaN (5×1017cm-3) and subsequent rapid thermal annealing in an Argon atmosphere. Initially non-linear current-voltage characteristics become ohmic after annealing and a specific contact resistance of approximately 10-2 Ω cm2, measured with the circular transmission line method, was found after annealing at 900°C for 1 min.

Ohmic contacts to n-GaN using PtIn2

Abstract: A new metallization scheme has been developed to form Ohmic contacts to n-GaN. Contacts were fabricated by sputtering the intermetallic compound, PtIn2 on metal–organic vapor phase epitaxy grown n-GaN (n∼5×1017 cm−3) with some of the contacts subjected to rapid thermal annealing. Contacts in the as-deposited state exhibited nearly Ohmic behavior with a specific contact resistance of 1.2×10−2 Ω cm2. Contacts subjected to rapid thermal annealing at 800 °C for 1 min exhibited linear current–voltage characteristics and had specific contact resistances less than 1×10−3 Ω cm2. Auger depth profiling and glancing angle x-ray diffraction were used to examine the interfacial reactions of the PtIn2/n-GaN contacts. Consistent with estimated phase diagram information, the results from Auger depth profiling and glancing angle x-ray diffraction indicated the formation of (InxGa1−x)N at the contact interface, which could be responsible for the Ohmic behavior of PtIn2 contacts.

Equivalence of the generalized bond-energy model, the Wagner-Schottky-type model and the compound-energy model for ordered phases

Abstract: The equivalence of three thermodynamic models used in the literature to describe the compositional and temperature dependencies of the thermodynamic properties of ordered intermetallic phases is presented. The three models are the generalized bond-energy model, the Wagner-Schottky-type model and the compound-energy model. Equations to convert the model parameters of the generalized bond-energy model to those of the other two models are derived. The validity of these equations is demonstrated by showing the successful conversion of the model parameters for the ordered phases in the Ti-Al system from the generalized bond-energy model to those of the Wagner-Schottky-type model and the compound-energy model. However, conversion of the model parameters from these two models to the generalized bond energy model is generally not possible due to additional constraints imposed in developing the later model.

Thermally stable pdin ohmic contacts to n-gaas via exchange mechanism

Abstract: Thermally stable, low-resistance ohmic contacts to GaAs (Si-doped, n=1.6–1.8×1018 cm−3) were formed using PdIn metallization sputter-deposited from an alloy target. Average specific contact resistances (ρc) in the 10−6 Ω cm2 range were reached upon annealing at 600 °C or higher. Contacts annealed under the optimum condition of 850 °C for 15 s exhibited an average ρc of 2.5×10−6 Ω cm2. The 100 h of thermal aging at 400 or 500 °C increased their average ρc to 3.0×10−6 and 1.0×10−5 Ω cm2, respectively. The ohmic behavior of the annealed contacts was ascribed to the exchange of In and Ga atoms between the metallization and the semiconductor and the concomitant formation of InxGa1−xAs, whose presence at the contact interface was confirmed using cross-sectional transmission electron microscopy.

Reply to “Comment on ‘Phase equilibria of the Ga–Ni–As ternary system’ ” [J. Appl. Phys. 82, 493 (1997)]

Abstract: We reply to the comment by Guerin and Guvarc’h concerning the phase equilibria of the Ga–Ni–As ternary system. We refute their position that the quench rate of the phase diagram samples is not important and that the solid-state phase equilibria of the Ga–Ni–As system does not change significantly between the temperatures of 25 and 800 °C. It is also demonstrated that the occurrence of ordered superlattice structures is not inconsistent with our work. We conclude that an adequate number of samples were prepared in our study to justify the modifications proposed to the Ga–Ni–As isothermal section and it is an accurate representation of the phase equilibria at 600 °C.