Modeling Boron and Indium Electrical Activities in Silicon in the Presence of Nitrogen
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Modeling Boron and Indium Electrical Activities in Silicon in the Presence of Nitrogen Vladimir Zubkov, Sheldon Aronowitz, Helmut Puchner and Juan P. Senosiain1 LSI Logic Corporation, 3115 Alfred Street, Santa Clara, CA 95054, U.S.A. 1 Department of Materials Science and Engineering, Stanford Unversity Stanford, CA 94305, U.S.A.
ABSTRACT The ab initio pseudopotential code (VASP) was employed to explore indium and boron electrical activities in silicon in the presence of nitrogen. Electrical activities for the combinations B+N, In+N, and In+B+N were explored. Formation energy of a negatively charged supercell, (E-)f , and a band gap, Eg, from calculations with one k point were chosen as indicators of acceptor activity. For separate dopants the calculated (E-)f and Eg values indicate that substitutional B and In are effective acceptors and N is an extremely weak donor. When nitrogen is adjacent to, or separated 3 - 5 bonds from B or In, it suppresses acceptor activity. Binding is greater for In+N than for B+N in agreement with secondary ion mass spectroscopy (SIMS) data that demonstrates a greater retention of N by In. This should lead to a greater drop in activity for In+N combination versus B+N one, in agreement with spreading resistance profiling (SRP) experiments. Loss of activity in In+B+N combination might be due to long range interactions between dopants.
INTRODUCTION Indium has been used to create super-steep retrograde channels in n-channel devices in silicon. This permits undesired effects on device performance such as reverse short channel effects to be significantly reduced. The limitations to the use of In are that it displays a low solid solubility [1] and electrical activation appears to be inefficient. Implanting high doses of nitrogen to modify gate oxide growth further complicates the picture by reducing the activation to a level that excludes In use [2]. ( Implant dose for In: 1x1013 115In+/cm2 at 180 keV, 40 times less than that for N; more details in [2,3]). The threshold voltage was minimally reduced with channels formed with a boron implant and the same pre-gate nitrogen implant dose. The effects of combinations of indium and boron when nitrogen is present or absent have been explored by (SIMS) and SRP [2,3]. According to SRP data boron is much more active than indium and activity of In+B is close to that of B. The presence of nitrogen reduces the concentration of active carriers. This loss is quite substantial in the case of In and In+B: 28 and 54% , respectively, but rather small for B: only 7% [2,3]. In this work, an ab initio method was employed to explore if the calculations could capture the major experimental observations. We assumed that it would be relatively simple, once plausible criteria for electronic activation were established, to study the relative activity of any combination of dopants. That is, we were not interested in diffusion mechanisms but in possible configurations after an anneal was performed. J6.5.1
METHOD The atomic level calculations were performed using the Vien
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