Laser-induced multicrystallization events of thin germanium films
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S. Ajuria Materials Science and Engineering Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 25 May 1988; accepted 28 June 1988) The crystallization kinetics of thin (35 and 60 nm) amorphous as-deposited Ge films were studied using diffraction limited laser beam irradiation and laser pulses between 30 ns and 1 ms. The recrystallization of crystalline as-deposited films was also studied for similar laser conditions. Crystallization was observed for pulses as short as 50 ns. It was concluded that the irradiation of amorphous thin films with small beam spots ( ~ 1 /nm) gives a very different crystallization morphology from that observed previously for larger beam diameter and same laser pulse length. In the present case for short irradiation times, the nucleation process dominates over crystal growth. Temperature calculations allow the understanding of these results by showing that only the small spot irradiation sustains the material at high temperature for times comparable to the pulse width. Laser irradiation of as-deposited crystalline films produced grains with significantly fewer defects than grains crystallized from as-deposited amorphous films.
I. INTRODUCTION
II. EXPERIMENTAL
The amorphous to crystalline phase transformation has received much attention on the basis of its scientific and technical importance. Much of the previous work on laser crystallization has been motivated by applications in electronics1 to obtain large areas of uniformly crystallized material and in optical recording.2 When the amorphous to crystalline transition is accompanied by the release of a large amount of heat, phenomena such as explosive crystallization are observed. For example, in Ge and other materials, scanning the focal spot of a continuous wave (cw) laser at uniform speed can produce a crystallized region having the form of periodically overlapping crescents. In micrometer thick Ge films the minimum scan speed to produce such an effect is ~ 1 m/s. This phenomenon has been well studied and adequately explained.3 A similar phenomenon has been observed with static, nanosecond pulse laser irradiation in Si or Ge films.4'5 Transmission electron microscope (TEM) examination of the crystallized areas show a spherulite morphology with a radical dendritic growth surrounded by periodic rings. When the laser energy density is increased above a certain threshold, the central area develops a polycrystalline texture.6 We report the crystallization kinetics of thin Ge films using diffraction limited laser beam irradiation and laser pulses between 30 ns and 1 mS. Both amorphous and fine-grain polycrystalline films were studied. The experiments presented here used thin (35 and 60 nm) films of germanium on Si3N4 membranes.
In our study, the substrates were customized for TEM analysis: they were prepared from a (100) Si wafer covered on both sides with 150 nm Si3N4. Using photolithography and reaction ion etching a large number of square windows, each about 1 mm across, were etched in the Si3N4
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