Hrem In Situ Annealing of the CdTe/GaAs Heterojunction
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HREM IN SITU ANNEALING OF THE CdTe/GaAs HETEROJUNCTION ALAN SCHWARTZMAN AND ROBERT SINCLAIR Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305 ABSTRACT This paper describes the application of high-resolution electron microscopy (HREM) to the determination of phase diagrams. Using a heating holder in a high-resolution transmission electron microscope (TEM), the chemical stability of a CdTe/GaAs interface is confirmed, as predicted by the Ga-As-Cd-Te quaternary phase diagram. Crisp, near-atomic-resolution video imaging is obtained at an annealing temperature of 500 TC. Not only are the above results in complete agreement with ex situ annealing of the CdTe/GaAs heterojunction, but also direct observations of dynamic events occuring at the atomic level are recorded in real-time. Sublimation at the edge of a CdTe ( 111 ) thin film occurs via a ledge mechanism. Instead of a steady progression of this solid-vapor reaction, one obtains a real-time impression of dynamic equilibrium between vaporization and recondensation as atoms from the surface disappear and reappear several times before their ultimate removal. INTRODUCTION Isobaric, isothermal phase diagrams are molar representations of condensed phases in equilibrium with each other at a fixed temperature, pressure and composition. Since four elements are often present at a fabricated interface in a compound semiconductor device, knowledge of the quaternary phase diagram is critical in the optimization of the processing steps and in the design of stable devices. One example is the CdTe/GaAs heterojunction, which is important for the production of certain opto-electronic devices, where CdTe is used as a buffer layer before growing ZnTe/CdTe or HgTe/CdTe multilayers as well as ZnCdTe and HgCdTe ternary alloys. This paper will demonstrate the chemical stability of the CdTe/GaAs interface. First, construction of the Ga-As-Cd-Te quaternary phase diagram is explained. After this theoretical discussion, which predicts that CdTe is in equilibrium with GaAs, verification by the TEM, an accepted experimental technique for testing phase diagrams, is presented. Affirmation of the stable CdTe/GaAs interface is based on both in situ heating in a high-resolution TEM and ex situ heating with subsequent thinning for examination by HREM. THE Ga-As-Cd-Te QUATERNARY PHASE DIAGRAM One possible isobaric, isothermal molar representation of phase equilibrium in a ternary system is the two-dimensional Gibbs equilateral triangle. A natural extension for the quaternary system is the three-dimensional regular tetrahedron. In this case, the four elements are placed at the corners, binary phases on the edges, and ternary phases on the faces of the tetrahedron. Quaternary phases are located within the tetrahedron. Phases present in the Ga-As-Cd-Te system, which has no quaternary compounds, are shown in Figure 1. Similar to ternary phase diagrams, quaternary diagrams consist of tie-lines connecting pairs of mutually stable phases. Now there are internal tie-lines
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