(a) 100 µm - Helmholtz-Zentrum Berlin

18 State of the art
Combining this effect of metals on semiconductors and the idea of solid phase
crystallization it was found that metals in contact with amorphous silicon strongly
reduce the crystallization time and temperature. Two microscopic models were
suggested explaining the observed effect. Hiraki [30, 31] deposited monolayers
of gold on amorphous silicon. He found that only upon depositing four monolay-
ers or more crystallization was enhanced. He concluded that some type of bulk
metal must be formed. The bulk is needed because a free electron gas needs to be
formed. The wave function of the free electron gas extends into the amorphous sil-
icon screening the covalent Si bonds. The screened bonds are weakened and thus
crystallization is faster. Tu [32] suggested an interstitial model for near-noble tran-
sition metals. Such metals lack one electron to achieve noble gas configuration. If
such a metal atom would be inserted in an interstitial position of the Si lattice an
electron from the covalent bonds surrounding this atom would be transferred to
the metal. This weakens the bonds around it, enhancing crystallization.
In 1972 S.R. Herd [33] compared the influence of different metals on the crystal-
lization temperature of amorphous silicon and germanium. Metals forming sili-
cides (Ni, Pd, Cu) are distinguished from eutectic metal-silicon systems (Al, Au,
Ag).
Ni in contact with Si forms a NiSi2 phase. The chemical potential for Ni is lower at
the NiSi2/a-Si interface than at the NiSi2/c-Si, thus Ni diffuses to the a-Si side. At
the same time Si diffuses in the opposite direction. This makes the silicide phase
move through the a-Si silicon and leaves crystalline silicon behind. Depending on
the growth parameters the silicide phase either moves anisotropically leading to
needle-like crystallites or isotropically leading to disk-like grains [34]. MIC allows
to grow large and disk-like grains. Fig. 2.8(a) shows a micrograph of disk-like Si
grains from a Pt-induced crystallization process with annual ring-like structures
[35]. The Si grains exhibit a grain size of up to 500 µm. The ring structures corre-
spond to interruptions in the annealing process. Fib. 2.8(b) shows a micrographs
in the case for Ni-induced crystallization of a-Si [36]. The grain size is about
50 µm. Again rings were found due to interruptions during the annealing, but this