The Effects of Dopants on Surface-Energy-Driven Secondary Grain Growth in Ultrathin Si Films
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Mat. Res. Soc. Symp. Proc. Vol. 54. 11986 Materials Research Society
730
TABLE I Sample labels, implantation doses, implantation energies, approximate film compositions, and the temperature at which films transform to 50% (by area) secondary grains after 20 minute anneals.
Sample
Film Thickness
Dopant(s)
(A)
Dose2 (cm" )
1140A
P
1.45 x 10 cm-
B
1140
P
C
1140
D
1140
F
G
1140
1140
1140
Calculated Composition
Tx=0.5
3
(Cm- ) 16
A
E
Energy
2
(OC) 21
3
90
1 x 10 cm-
1071
1.16 x 1016
90
8x
1020
1097
P
7.22 x 1015
90
5 x 1020
1149
P and( B
1.45 x 1016
90
1 x 1021
1091
P and{ B P and( B undoped
20
15
3.01 x 10
30
2 x 10
1.16 x 1016
90
8x
1020
3.01 x 101"
30
2 x
1020
7.22 x 10'5
90
5x
1020
15
7.52 x 10
30
1129
>1350
20
5 x 10
>1350
0.2 torr of ultrapure Ar. Normal and secondary grain sizes were determined via transmission electron microscopy. Any grain larger than the average normal grain size by more than twice the standard deviation was considered a secondary grain. The fraction of a film transformed to secondary grains, x, was also determined from TEM micrographs. RESULTS Detailed results and analyses of secondary grain growth in these films is given elsewhere. 5,8-9 Here we summarize the evidence that grain growth is enhanced by P doping and that the effect of P doping can be compensated by codoping with B. Shown in figure 1 are the temperatures at which films that have been subjected to 20 minute anneals have undergone secondary grain growth to the point at which 50% of the film is occupied by secondary grains. These temperatures are plotted versus the number of donors minus the number of acceptors. Undoped films and films doped with 5 x 3 1020 cm-3P and 5 x 1020 cm' B did not undergo secondary grain growth even at temperatures as high as 1350'C.
731
(-)
~-1110Figure
0 ,..
1190
1
Temperature
at
which 50% of the area of a
1170 1050
SI
I
I
IlX02 5x 1020 6x20 7x1020 8X1020 9x 1020 3 - *acceptors (cm- ) *donors
film has been consumed by secondary grains after a 20minute anneal. Plotted versus the number of donor atoms minus the number of acceptor atoms. Squares represent films doped with P alone and circles represent films codoped with P and B (see Table I).
DISCUSSION AND CONCLUSIONS
The above results suggest that grain boundary mobility in secondary grain of the Fermi energy.13 This effect is analogous to doping effects on growth is a function self-diffusion' 0 '12 and dislocation motion in silicon. In these cases it has been postulated that increases in the Fermi energy lead to increased numbers or mobilities of charged defects which in turn lead to enhanced kinetics. The same general mechanism is likely to apply here. There is evidence 9 that the rate limiting step for secondary grain growth in the heavily doped films is P diffusion in bulk silicon, (presumably due to solute drag). Since P diffusivity in Si is a function of the Fermi energy, this might explain the observed Fermi energy dependence of grain boundary mobility.
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