Diffusion of Ion-Implanted Tin in Gallium Arsenide

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DIFFUSION OF ION-IMPLANTED TIN IN GALLIUM ARSENIDE E.L. Allen*, M.D. Deal** and J.D. Plummer** *Department of Materials Science and Engineering, Stanford University ** Integrated Circuits Laboratory, Stanford University, Stanford, CA 94305.

ABSTRACT The diffusion of ion-implanted tin in gallium arsenide was investigated as a function of temperature, dose and background doping. The chemical depth profiles were determined using SIMS and the carrier profiles were determined by CV Etch Profiling. The data was fit using a numerical process simulator, SUPREM 3.5. Sn diffusivity was found to depend on the square of the electron concentration. Sn and Ge were found to have relatively high diffusivities when implanted, while Si diffused very little.

INTRODUCTION Tin has not assumed an important role as a dopant in digital GaAs technology primarily because of its high diffusivity. Silicon, another n-type Group IV dopant used extensively for channel implants, has a much lower diffusivity. However, its diffusion behavior is not well understood. Hopefully an understanding of Sn and Ge diffusion mechanisms will lead to a better understanding of Si diffusion behavior. In this work we studied the one-dimensional behavior of implanted Sn as a function of temperature, dose and background doping, and compared it to Si and Ge. Implanted n-type dopants in GaAs usually have suppressed diffusivities compared to solid-source or grown-in dopants in GaAs [1]. This fact has generally been attributed to implant damage. The work presented here shows that high dose implants of Sn and Ge exhibit significant diffusion outside the implanted region, whle Si diffuses very little. Both

Sn and Ge diffusivities depend on the square of the electron concentration. Results of CV Etch Profiling suggest that the mechanism by which damage influences diffusion may be in the activation process.

EXPERIMENTAL PROCEDURE Two sets of experiments were done.

In the first set we determined the one

-dimensional diffusivity of implanted Sn as a function of background doping. For these experiments, GaAs substrates with various background doping levels were solvent-cleaned, 2 3 2 4 etched, and implanted with ll8Sn at 185 keV and a dose of either lx101 cm- , lxO10 cmor lx1015 cm- 2 . The semi-insulating wafers were undoped LEC(100), while the doped wafers were Horizontal Bridgman-grown(100), doped with either 3x1018 cm- 3 Si for n-type 19 or Wxl0 cm- 3 Zn for p-type. The samples were then capped on both sides with 900 A of

SiNx deposited by PECVD. The implanted specimens were annealed for various times and temperatures from 700 to 1000oC in an open tube furnace under flowing forming gas. The caps were then stripped and the dopant profiles analyzed using Secondary Ion Mass

Spectrometry (SIMS). Some samples were also analyzed by Polaron CV Etch Profiling to obtain carrier profiles. The diffusion profiles were fit using the numerical process simulator SUPREM 3.5 to extract values of the tin diffusivity as a function of concentration. In the second set of experiments we