Diffusion Behavior of Ion-Implanted n-type Dopants in Silicon Germanium
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Diffusion Behavior of Ion-Implanted n-type Dopants in Silicon Germanium S. Eguchi, C. W. Leitz, E*. A. Fitzgerald*, and J. L. Hoyt Microsystems Technology Laboratories, Dept. of EECS, Massachusetts Institute of Technology, 60 Vassar St. Cambridge, MA 02139, USA * Dept. of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave. Cambridge, MA 02139, USA ABSTRACT The diffusion behavior of ion implanted arsenic and phosphorus in relaxed-Si0.8Ge0.2 is investigated. Both dopants exhibit enhanced diffusivities in SiGe compared to those in Si under equilibrium conditions. The ratio of the effective diffusivity in SiGe relative to that in Si is found to be approximately seven for arsenic, and roughly two for phosphorus at high concentrations. Under transient diffusion conditions, arsenic diffusion in SiGe is retarded while arsenic diffusion in Si is enhanced by the ion implant damage. The transient retardation of arsenic diffusion in SiGe is observed at temperatures ranging from 900 to 1050oC. These results suggest that using arsenic, it is possible to form n+/p junctions in relaxed-Si0.8Ge0.2 as shallow as those in Si, by optimizing the implant annealing conditions. INTRODUCTION There is recent interest in MOSFETs based on strained-Si channels on relaxed-SiGe layers. These MOSFETs demonstrate enhanced mobility and current drive1. For high performance MOSFETs, it is important to form shallow source/drain junctions. While the diffusion behavior of B in SiGe is relatively well characterized2-4, there is little information about n-type dopant diffusion in SiGe. In this paper, we present experimental observations of ion implanted arsenic (As) and phosphorus (P) diffusion behavior in relaxed-SiGe (20%). Secondary ion mass spectrometry (SIMS) is used to obtain diffusion profiles in Si and SiGe samples. The enhancement factors for the As and P effective diffusivities in SiGe (20%) are extracted by comparing measured SIMS data with simulated profiles from TSUPREM-45. PRINCIPLE The As diffusion model in Si implemented in TSUPREM-4 is discussed in detail in reference [5]. We applied the same type of method for modeling dopant diffusion in SiGe as in Si. As diffusion in Si is assumed to be based on neutral and single-negatively charged defects5. The effective diffusivity of Arsenic in Si under equilibrium conditions (CI/CI*=CV/CV*=1) can be described as: n n 0 − (1) ) + f ( Dv + Dv ) DeffAs ( Si ) = f i ( Di0 + Di− v ni ( Si ) ni ( Si ) where n is the electron concentration and ni(Si) is the intrinsic carrier concentration in Si. Di0 and Di− represent the contributions from neutral and single-negatively charged interstitial defects, A1.7.1
respectively. DV0 and DV− represent the contributions from neutral and single-negatively charged vacancy defects. And the factors f i and f v represent the fractions of interstitial and vacancy mediated diffusion, respectively. The P diffusion model in Si is assumed to be based on neutral, single- and double-negatively charged inte
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