The Thermal Stability of Heavily Carbon-Doped GaAs Grown by Metalorganic Molecular Beam Epitaxy
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THE THERMAL STABILITY OF HEAVILY CARBON-DOPED GaAs GROWN BY METALORGANIC MOLECULAR BEAM EPITAXY
Hyunchul Sohn and E. R. Weber, S. Nozaki*, M. Konagai* and K. Takahashi*
Department of Materials Science and Mineral Engineering, University of California, Berkeley CA94720, *Department of Physical Electronics, Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, Meguro-ku, Tokyo 152, Japan
ABSTRACT
cm" 3 , Heavily carbon-doped GaAs thin films with a hole concentration of 5.8 x 1020 0 for 30 at 900 C were annealed beam epitaxy(MOMBE), grown by metalorganic molecular minutes. The microstructural changes due to annealing were investigated by transmission electron microscopy. Electron diffraction study showed some evidence of carbon clustering on {1111 in as-grown samples. A high density of precipitates was found in the annealed sample, together with a decrease of the lattice contraction and hole concentration. In the as-grown layer, misfit dislocations with only one type of Burgers vector were observed, while misfit dislocations with the several Burgers vectors were observed in [ITO] direction after annealing. The density of misfit dislocations in both directions increased significantly even though the lattice contraction in carbon-doped GaAs decreased after annealing. A model is proposed to explain the change of misfit dislocation density in heavily carbon-doped GaAs layer. INTRODUCTION
Heavily carbon-doped GaAs has become increasingly important in devices such as heterojunction bipolar transistors (HBTs)[1] and non-alloyed ohmic contacts.[2] In particular, the small diffusivity of carbon has improved the stability of an AlGaAs/GaAs HBT with p+ doped base, and the carbon doped HBT showed no significant change in its dc characteristic even after 12 hours of bias application at 200'C unlike HBT with a Bedoped base.[3] However, several researchers have reported changes in the hole concentration and in lattice constants of heavily carbon-doped GaAs epitaxial layers due to annealing,[4,5] which has recently increased the concern over thermal stability of the heavily carbon-doped GaAs. Hanna et al.[5] suggested two possible mechanisms to explain changes in material properties: movement of CAs into interstitial sites, creating VAs, or formation of misfit dislocations. They, however, admitted that further study would be necessary to understand the mechanism for annealing-induced changes in heavily carbon- doped GaAs. In this letter, we investigate microstructures of as-grown and annealed heavily carbondoped GaAs thin films by transmission electron microscopy (TEM) and propose a mechanism to explain the observed changes in material properties due to annealing. EXPERIMENTAL
Carbon-doped GaAs layers were grown on (100) semi-insulating GaAs substrates by MOMBE. Details of the growth procedure is described elsewhere.[6,7] Heavily carbondoped GaAs layers were annealed at 900°C for 30 minutes in N2 flow in the proximity configuration with another GaAs wafer. Hole concentration and lattice contraction
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