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Liquid Alloy Source for Dopants

IP.com Disclosure Number: IPCOM000039665D
Original Publication Date: 1987-Jul-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 4 page(s) / 75K

Publishing Venue

IBM

Related People

Delage, SL: AUTHOR [+4]

Abstract

In silicon molecular beam epitaxy (Si MBE), a dopant beam is used in neutral or ionized form to simultaneously dope Si films during epitaxial growth. Thermally-generated neutral beams are preferred for this purpose because the system is simple to implement and the growth is better understood and easy to accomplish. Unfortunately, the choice of N type dopants is severely restricted due to practical limitations. All the N type dopants have extremely high vapor pressures. This has several drawbacks. First, there is appreciable flux at temperatures as low as 250oC, typically used during bake-out and these species deposited on vacuum parts will re-evaporate when these parts warm up during growth.

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Liquid Alloy Source for Dopants

In silicon molecular beam epitaxy (Si MBE), a dopant beam is used in neutral or ionized form to simultaneously dope Si films during epitaxial growth. Thermally-generated neutral beams are preferred for this purpose because the system is simple to implement and the growth is better understood and easy to accomplish. Unfortunately, the choice of N type dopants is severely restricted due to practical limitations. All the N type dopants have extremely high vapor pressures. This has several drawbacks. First, there is appreciable flux at temperatures as low as 250oC, typically used during bake-out and these species deposited on vacuum parts will re-evaporate when these parts warm up during growth. Second, most of the evaporation is via sublimation and is thus very dependent on the area of the dopant lumps and flux is very dependent on retarding oxide skins that easily form on the Group V species. Finally, the knudsen (effusion) cells are not easy to control at low temperatures.

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For these reasons, there is yet no satisfactory source for the N type dopants. One alternative that has been suggested is the use of compound sources, such as GaAs for As. There are severe problems in this approach. In this case, it is seen that at high vapor pressures, corresponding to high temperatures, the total As partial pressure is significantly higher than that of Ga. However, at the lower pressures, which are more useful for doping applications, the two partial vapor pressures are comparable. This means that there will be significant p type doping as well, which is clearly undesirable. While the vapor pressures for other compounds are not available, we expect similar conditions to be operative. In addition, as the source depletes, the system moves to the metal rich side of the phase diagram. It no longer remains a single phase system and instabilities may develop. Thus, these kinds of sources have not found use in Si MBE. This article describes an alloy source to effect evaporation of Group V elements at more convenient temperatures. Since alloys are not chemically bound, the chemistry is simpler. In a solid solution, at high temperatures, there is evaporation of the higher vapor pressure constituent from the surface. The effusing flux is proportional to the surface concentration of the species.

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Fd = aNds The surface concentration Nds is quickly depleted, and a gradient is set up in the vicinity of the surface, due to which there is a diffusion flux. The surface concentration is given by an expression of the type

exp( - u2)erfc(u) where

(Image Omitted)

1

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This is plotted in Fig. 1. Clearly, the system is not stable and there is a rapid fall in evaporation rate. Physically, this is because the solid-state diffusion is too low. In order to achieve a useful source life of 10 to 100 hours, a diffusivity of 100cm2sec-1 is required. Such diffusivities are not possible in the solid state....