Thermodynamic Analysis for the Solid-State Amorphization and Subsequent Crystallization of GaAs/Co
- PDF / 377,194 Bytes
- 7 Pages / 420.48 x 639 pts Page_size
- 11 Downloads / 206 Views
THERMODYNAMIC ANALYSIS FOR THE SOLID-STATE AMORPHIZATION AND SUBSEQUENT CRYSTALLIZATION OF GaAs/Co
F.-Y. SHIAU*, S.-L. CHEN, M. LOOMANS AND Y. A. CHANG Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706, USA. *Currently with Tze-Chang Foundation of Science and Technology, Semiconductor Research Center, Hsin-Chu, Taiwan, ROC ABSTRACT Phase equilibria along the CoGa-CoAs join were determined by DTA and metallography. On the basis of these data and the phase diagram of Co-Ga-As at 600 0 C, thermodynamic values for the various phases along the GaAs-Co join were estimated. The Gibbs energy of the amorphous phase is approximated to be that of the supercooled liquid phase. These data were used to rationalize the amorphization process. 1.0
INTRODUCTION
When GaAs/Co thin-film couples are annealed above ambient temperature, amorphization occurs first, followed by crystallization and eventually decomposition to the 3 thermodynamically stable phasesI- . In order to provide a thermodynamic rationale for the observed phase transformations, we need to have thermodynamic data for the various competing phases, i.e. the amorphous phase, the GaAs phase (B3), the Co phase (A3), the CoGa phase (B2) and the CoAs phase (B31). In the present study, the Gibbs energy of the amorphous phase is taken to be that of the liquid phase since the difference in the Gibbs energy between these two states is not known. However, this approximation underestimates the stability of the amorphous phase. Since obtaining accurate thermodynamic values of the phases which occur in GaAs/Co couples requires too much experimental effort, we resorted to determining the phase equilibria of the psuedobinary CoGa-CoAs. Using these data and the 600 0 C isotherm of Co-Ga-As determined by Shiau et al. 4 , thermodynamic values of the various phases are estimated. Using these values, phase transformations of GaAs/Co to the various phases are discussed. 2.0
EXPERIMENTAL METHODS AND RESULTS
2.1
Phase Ecuilibrium Determination
Phase equilibrium samples with an average weight of 0.5 g were prepared from GaAs, Co, Ga and As of purity of 99.99%. The component elements in the appropriate proportions were weighed and sealed in evacuated quartz capsules. The samples were annealed at 1200 0 C for 2 weeks. Their melting temperatures were determined by DTA using a procedure described elsewhere 5 . 2.2
Experimental Results
The melting points for all nine samples were plotted in Fig. 1. These data suggest an eutectic invariant reaction between CoGa and CoAs. The eutectic composition of 47 ± 2 mol% Mat. Res. Soc. Symp. Proc. Vol. 230. c1992 Materials Research Society
Downloaded from https://www.cambridge.org/core. The Librarian-Seeley Historical Library, on 11 Jan 2020 at 19:00:41, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-230-39
40
CoAs was obtained by extrapolating the liquidus data, and the solubility limits at 600 0 C are taken from Shiau et al. 4 .
Data Loading...
