On the Distribution Mechanism of Voids in Si-Implanted GaAs
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ON THE DISTRIBUTION MECHANISM OF VOIDS IN Si-IMPLANTED GaAs Samuel Chen, S.-Tong Lee, G. Braunstein, and K. Y. Ko
Corporate Research Laboratories, Eastman Kodak Company, Rochester, NY 14650-2132 T. Y. Tan
School of Engineering, Duke University, Durham, NC 27706; and Microelectronics Center of North Carolina, Research Triangle Park, NC 27709 ABSTRACT Voids, formed by the condensation of an excess of implantation-induced vacancies, have been recently identified as the defect directly responsible for dopant diffusion and electrical activation anomalies in Si-implanted and annealed GaAs and GaAs/AIGaAs superlattice materials. Depending on the implanted dose, voids can be distributed either throughout the implanted region or in two bands. We have examined the origin of this void distribution difference. In the asimplanted sample associated with the latter case, a buried continuous band of amorphous GaAs has formed. GaAs formed by the recrystallization of amorphous GaAs does not contain excess vacancies, and therefore cannot form voids. However, on either side of the amorphous layer, the excess vacancies can condense to form the observed banded distribution of voids. In the as-implanted sample associated with the former case, a continuous amorphous GaAs layer did not form and therefore, upon annealing, voids are seen throughout the implanted region. INTRODUCTION Introduction of Si by ion implantation to form n-doped regions in GaAs is currently used as a device fabrication process [1]. However, many anomalies in the diffusion and activation properties of this dopant are present. In n-type GaAs doped by Si during crystal growth or by an external diffusion source, the maximum obtainable electron concentration, n, is limited by the amphoteric nature of Si. This dopant can occupy a Ga lattice site to become a shallow donor. However, at high Si concentrations, it can also occupy an As lattice site to become a shallow acceptor. This self-compensation leads to a saturation, but not a retrograde, in the n value, i.e., n can reach a maximum value which will not decrease as the Si concentration further increases. For implanted Si, an annealing step is needed to electrically activate the dopant. Anomalies in the dopant diffusion and activation profiles [1-3] exist in the annealed materials. Often, in the region where the Si concentration is the highest, the n' value is considerably lower than that of the surrounding regions, where the Si concentrations are lower. This kind of anomaly is most likely induced by the implantation/annealing processes and not by Si self-compensation. The commonly observed defects such as dislocations, dislocation loops and microtwins are not believed to be directly responsible [4,5], since density changes of these defects do not have an effect on the anomalies of n. Recently, voids were found in implanted and annealed GaAs and GaAs/AIGaAs superlattice materials and it was shown that voids are the defects directly responsible for the Si diffusion and electrical activation anomalies [6-8]. The surface st
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