Correlation of Void Formation with the Reduction of Carrier Activation and Anomalous Dopant Diffusion in Si-Implanted Ga
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CORRELATION OF VOID FORMATION WITH THE REDUCTION OF CARRIER ACTIVATION AND ANOMALOUS DOPANT DIFFUSION IN Si-IMPLANTED GaAs Kei-Yu Ko*, Samuel Chen*, S.-Tong Lee*, Longru Zheng*, and T.Y. Tan** *Corporate Research Laboratories, Eastman Kodak Company, Rochester, NY 14650 **Dept. of Mechanical Eng. and Materials Sci., Duke Univ., Durham, NC 22706, and Microelectronics Center of North Carolina, Research Triangle Park, NC 27709
ABSTRACT We report the study of high-dose Si-implanted GaAs containing doses 14 15 2 0 ranging from 1xl0 to Ix10 cmand with subsequent anneals at 850 C for 1 14 2 hour. At doses > 3x10 cm- , a severe reduction of carrier concentration and anomalous Si diffusion are observed in the near-surface region. In the same region, small, near-spherical voids are found by transmission electron 14 2 microscopy. In contrast, for samples implanted with doses < lxl0 cm- , voids are not found, and both normal carrier activation and Si diffusion profiles are observed. The concurrent onset of these three phenomena in the same region in high-dose samples leads us to conclude that the severe reduction of carrier concentration and anomalous Si diffusion are attributable to the formation of voids.
INTRODUCTION Ion implantation of Si has been used extensively in GaAs devices and integrated circuit processing, such as the fabrication of n-type channel layers and n+ regions for good ohmic contacts in MESFET devices. Implantation is usually followed by a high temperature annealing step to activate the implanted species and to remove lattice damage caused by the implantation process. However, it is known that high electrical activation (close to 90%) 2 of implanted Si is obtained only at low dose implants (• 1013 cm" ) [1-3]. At 2
higher implant doses ( > 1014 cm- ), activation < 20% is typical [4,5]. Previous studies showed that this low activation of Si in GaAs in high dose implants could be linked to phenomena such as (1) self compensation by silicon acceptors, SiAs [6], (2) formation of SiGa-SiAs neutral pairs [4,5,7], and (3) compensation by defects related to residual implantation damage after annealing [3,8]. In a recent study Chen et al. [91 reported the formation of voids in high dose Si-implanted GaAs and GaAs/AlGaAs superlattices. They postulated that the internal surfaces of voids could trap the Si dopant and they became electrically inactive. Furthermore, surface states associated with the surfaces of voids could also compensate free carriers. In this paper, we present experimental evidence to examine these postulates and show that the occurrence of carrier concentration reductions and Si diffusion anomalies in the near-surface region in high dose implants are related to the formation of voids.
EXPERIMENTAL -oriented, undoped, semi-insulating GaAs was used in this study. Room temperature Si implantation was performed in a non-channeling direction 1 4 1 5 2 at an energy of 220 keV and doses ranging between Ixl0 and 1x10 cm . The samples were then encapsulated with 1000 A-thick plasma-deposited silicon nitri
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