Substitutional carbon in silicon

Details Substitutional carbon in silicon Substitutional carbon in silicon

1991-11-06

1993-05-18

Fourson, George

Fishing, trapping, and vermin destroying

437 27, 437905, 437247, 148DIG4, 148DIG155, H01K 21265

Patent

active

052121011

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a method of incorporating substitutional carbon in silicon, and more specifically to incorporating very high concentrations of substitutional carbon by a method compatible with microcircuit fabrication.
Substitutional carbon, when present within a luminescent complex such as the C.sub.S --Si.sub.I --C.sub.S (where C.sub.s =a substitutional carbon atom and Si.sub.I =a interstitial silicon atom) centre has been shown to greatly enhance the efficiency of silicon light emitting diodes (LEDs) (patent application number 8711373). Dopants such as carbon are needed to obtain relatively efficient luminescence from silicon, an indirect band gap material. The incorporation of substitutional carbon at concentrations of about 10.sup.18 atoms cm.sup.-3 gives rise to an increase of 3 orders of magnitude in silicon luminescence.
Under equilibrium conditions an isolated carbon atom resides on a substitutional site within the silicon lattice. The equilibrium solubility of carbon in Float Zone (FZ) silicon varies with temperature, reaching a maximum of .about.3.times.10.sup.17 cm.sup.-3 at the melting point of silicon (1415.degree. C.), whilst the solubility of carbon in liquid silicon is .about.5.times.10.sup.18 cm.sup.-3. (K G Barraclough in `Properties in Silicon` Chapter 12, pp 285-8, 296-8, 304-6 (1988) Inspec).
To incorporate higher levels of carbon in silicon and avoid precipitation requires introduction of the impurity under conditions far removed from equilibrium. It is widely known that the maximum equilibrium solubility of substitutional impurities in silicon can be greatly exceeded by ion implantation with subsequent removal of damage by annealing. Two very different annealing techniques have proved successful in this regard: annealing (J S Williams, R G Elliman, Nuclear Instrumental Methods, 182/3 p389 1981) damaged layer (C W White, B R Appleton, S R Wilson in `Laser Annealing of Semiconductors` Chapter 5 pp112-145, 1982).
As recrystallisation occurs, both SPEG and LPEG can be characterised by a planar interface (separating the crystal-amorphous or the crystal-liquid phases respectively) moving towards the surface. In the case of SPEG the interface velocity can be made very low (e.g. .about.1 .ANG. s.sup.-1 at 550.degree. C.) whilst for LPEG, extremely high (e.g. 100 cm s.sup.-1). However, in both cases supersaturation of an impurity in the recrystallised layer relies on the same mechanism, namely solute trapping at the moving interface when the residence time is larger than the one monolayer regrowth time (S U Campisano et al, Applied Physics Letters 37 p719 1980). This mechanism is generally found to be effective for slowly diffusing substitutional impurities, but ineffective for fast interstitial diffusers.
Although a vast amount of information exists for electrically active substitutional dopants such as boron, indium, phosphorus, arsenic and antimony, very little is known about the behaviour of ion implanted carbon in silicon.
However, one extremely important characteristic of the behaviour of carbon in silicon at non-equilibrium conditions, which is known, is that the substitutional impurity can be rendered interstitial by the well-established Watkins' replacement mechanism (G D Watkins in `Radiation Damage in Semiconductors` (Paris:Dunod) pp 97-113 1965): carbon atom and C.sub.I is an interstitial carbon atom. Interstitial carbon is considerably more mobile than substitutional carbon e.g. the diffusion coefficient of interstitial carbon .about.10 orders of magnitude larger than substitutional carbon at .perspectiveto.750.degree. C. Extrapolation of diffusion data measured at high temperatures to lower temperatures reveals that whereas the substitutional impurity is effectively immobile at temperatures below 600.degree. C., interstitial carbon only becomes `frozen-in` below room temperature.
Clustering (the gathering together of atoms in a non-stoichiometric manner) and subsequent precipitation of mobile carbon can give rise to a number of different chemical phases in silicon, de

REFERENCES:
patent: 3622382 (1971-11-01), Brack et al.
patent: 4891332 (1990-01-01), Bloem et al.
Wolf, S., et al., Silicon Processing for the VLSI Era, vol. 1: Process Technology, .COPYRGT.1986 pp. 303-308.
Journal of Applied Physics, vol. 58, No. 12, "Ion Implantation of Si . . . ," Wang et al., pp. 4553-4564 (Dec. 15, 1985).
Thin Solid Films, vol. 163, No. 1+index, "High-dose carbon ion implantation studies in silicon," Srikanth et al., pp. 323-329 (Sep. 1988).
Japanese Journal of Applied Physics, vol. 24, No. 10, "The Raman spectrum of carbon in silicon," Forman et al., pp. L848-L850, L848, right-hand column, paragraph 2 (Oct. 1985).

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