High Resolution In Situ TEM Studies of Silicide-Mediated Crystallization of Amorphous Silicon

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1994 Materials Research Society

RESULTS

Initiation of Crystallization Annealing of the Ni implanted a-Si samples in a furnace at 4000 C for 3hrs led to NiSi2 precipitation close to the peak of the Ni concentration profile. Cross sectional TEM revealed the isolated precipitates to be completely buried within the a-Si film as demonstrated previously at higher implantation doses.I The NiSi2 precipitates were regular octahedra bounded by eight ( 111 ) faces and exhibited edge lengths (which run along directions) of -30-35nm. Previous studies have revealed a suppression of the initiation of Si crystallization (though not of subsequent silicideto knock-on mediated growth) at accelerating voltages of 300keV, and this has been attributed 3 displacement damage of c-Si by electrons above the threshold energy of 185keV. Using an accelerating voltage of 150keV, a sample was heated in situ to 5050C to initiate of the same area of a furnace-annealed crystallization. Fig. 1 shows plan-view TEM images sample at different tilt angles after cooling to 452 0C. Fig. 1(a) shows a region of silicide-mediated crystallization of a-Si together with two isolated precipitates. One of the precipitates, which is in a [110] orientation, has a step clearly visible on one of the (111) faces (arrowed). In the center of Fig. 1(a), a precipitate has given rise to silicide-mediated crystallization of a-Si, in which initiation of c-Si on a [110J-oriented NiSi 2 precipitate, has led to a series of growth fronts and to the breakup of the precipitate. At the leading edge of each of the growth fronts is a narrow band of NiSi 2. Fig. 1(b) shows the same region as Fig. 1(a) following tilting through an angle of -42'. The triangular faces of the migrating NiSi 2 prisms appear dark against the Si single crystal and provide direct evidence of the morphology of the migrating prisms of NiSi 2.

Fig. 1. TEM images of silicide-mediated crystallization of a-Si recorded in situ at 452rC. a) Showing dark bands of NiSi2 at the growth fronts of the Si single crystal. A step appears on the [110]oriented precipitate. b) Showing the triangular faces of the NiSi 2 prisms after tilting -42*. 580

We have demonstrated previously that the initiation of silicide-mediated crystallization of a-Si occurs via the low-temperature formation of epitaxial Type A c-Si on one or more of the (111 ) NiSi2 faces and that subsequent growth occurs via the migration of NiSi2 precipitates in directions trailing epitaxial c-Si. 3' 5 Of the major zone axes, , and , only the -oriented precipitates provide ( 11) planes having normals within the plane of the 5 thin film and are therefore suitably oriented for extensive in-plane growth. In Fig. 2(a), formation of c-Si is shown schematically on one face of a [1 10J-oriented precipitate. In Fig. 2(b), silicide-mediated growth of c-Si is shown by the subsequent migration of NiSi2 away from this face, trailing epitaxial c-Si with a Type A interface behind it. The prism of NiSi 2 and trailing needle of c-Si are bounded by three (112) planes.

Fig. 2.

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