Kinetics of solid phase crystallization in amorphous silicon

Amorphous layers, approximately 4000 A thick, were formed on single‐crystal Si samples by implantation of 28Si ions at LN2 substrate temperature. Channeling‐effect measurements with MeV 4He ions were used to measure the thickness of the amorphous layers and to measure the subsequent epitaxial regrowth on the underlying crystalline substrates. For annealing temperatures between 450 and 575 °C, the growth rate showed a strong dependence on the substrate orientation with 〈100〉‐oriented samples exhibiting about a 25 times higher growth rate than 〈111〉‐oriented samples. Measurements of the growth rate on a series of samples cut in 5° angular increments show that there is a monotonic decrease from the 〈100〉 to the 〈111〉 orientation. A simple model is proposed to explain the observed orientation dependence.

Substrate‐orientation dependence of the epitaxial regrowth rate from Si‐implanted amorphous Si

The nucleation and growth of isolated nickel disilicide precipitates in Ni‐implanted amorphous Si thin films and the subsequent low‐temperature silicide‐mediated crystallization of Si was studied using in situ transmission electron microscopy. Analysis of the spatial distribution of the NiSi2 precipitates strongly suggested the occurrence of site saturation during nucleation. NiSi2 precipitates were observed in situ to migrate through the amorphous Si thin films leaving a trail of crystalline Si at temperatures as low as ∼484 °C. Initially, a thin region of epitaxial Si formed on {111} faces of the octahedral NiSi2 precipitates with a coherent interface which was shown by high‐resolution electron microscopy to be Type A. Migration of the NiSi2 precipitates led to the growth of needles of Si which were parallel to 〈111〉 directions. The growth rate of the crystalline Si was limited by diffusion through the NiSi2 precipitates, and an effective diffusivity was determined at 507 and 660 °C. A mechanism for the enhanced growth rate of crystalline Si is proposed.

Silicide formation and silicide‐mediated crystallization of nickel‐implanted amorphous silicon thin films

The outstanding feature of materials research in the eighties has been the convergence of basic research and practical application, leading to ever shorter cycles of innovation. This is especially true of materials which form the basis of key technologies. The mass storage units of the next generations of computers will be based on optical processes having a storage density which exceeds that of all hitherto known storage techniques that are practicable from the technical standpoint. In view of the fact that since 1982 read-only memories in the form of compact discs (CD-ROM) have become very successful in the field of audio electronics, research and development are now concentrated on materials for write-once (WORM) and erasable (EDRAW) storage systems. Suitable materials for optical data storage are substances in which data markings can be recorded and deleted respectively using semiconductor lasers. Materials development is centered on the synthesis of infrared-absorbing dyes for WORM memories and the production of rare earth/transition metal alloys for magneto-optical data recording. An introduction to CD-ROM technology will be followed by an overview of the state of development of the most important storage materials which are currently available commercially, and then the properties of these materials will be discussed with reference to selected examples.

Materials for Optical Data Storage

Aluminum-induced crystallization of amorphous silicon is studied as a promising low-temperature alternative to solid-phase and laser crystallization. Its advantages for the formation of polycrystalline silicon on foreign substrates are the possible usage of simple techniques, such as thermal evaporation and dc magnetron sputtering deposition, and relatively short processing times in the range of 1 h. The overall process of the Al and Si layer exchange during annealing at temperatures below the eutectic temperature of 577 °C is investigated by various microscopy techniques. It is shown that the ratio of the Al and a-Si layer thicknesses is vitally important for the formation of continuous polycrystalline silicon films on glass substrates. The grain size of these films is dependent on the annealing temperature and evidence is given that grain sizes of 20 μm and more can be achieved. The poly-Si films are described as solid solutions containing 3×1019 cm−3 Al atoms as solute. Only a fraction of the solute is...

Elucidation of the layer exchange mechanism in the formation of polycrystalline silicon by aluminum-induced crystallization

The crystallization and compound formation temperature of vacuum deposited amorphous Si (and Ge) while in contact with various crystalline metals as a thin film sandwich are investigated by electron microscopy and electron diffraction. The results show that in simple eutectic systems the Si crystallizes at 0.72 (and Ge at approximately 0.65) of the eutectic temperature expressed in degrees Kelvin. Compounds are formed generally by the more rapid diffusion of Si into the metal.

Metal contact induced crystallization in films of amorphous silicon and germanium

The structure and crystallization behavior of Te85Ge15, Te81Ge15Sb4, and Te81Ge15As4 glasses has been investigated by transmission electron microscopy. The vapor deposited films showed coherently scattering regions of 15–25 A. On subsequent heating, crystalline tellurium appeared followed by cubic or distorted cubic GeTe. An intermediate phase was detected in the Te81Ge15Sb4 phase. These results are discussed in terms of the thermodynamics of amorphous to crystalline transformation.

Crystallization of certain chalcogenide glasses

Transmission electron microscopy is used to characterize the hole‐formation process due to laser writing in evaporated and sputtered Te thin films and in a Se‐Te bilayer. Bilayers, with Se as the top layer, combine the effect of an antireflective coating with protection against oxidation and provide the advantage of alloy formation during spot writing. Solidification of the melt in the Se‐Te spot takes place as an amorphous alloy resulting in smooth‐rimmed, clean holes. In contrast, Te crystallizes from the melt, giving rise to uneven rims and debris inside the spot.

Structural changes in Se‐Te bilayers by laser writing

The structure and morphology of laser written lines has been examined in a Te 81 Ge 15 As 4 glass by transmission and scanning electron microscopes. It is found that the difference in optical reflection and transmission properties between the laser exposed and unexposed regions in the specimen can be ascribed only in part to an amorphous to crystalline transformation. The remaining difference is attributed to a smoothening of the external surface and formation of voids in the film.

S.R. Herd

Papers

Metal contact induced crystallization in films of amorphous silicon and germanium

Crystallization of certain chalcogenide glasses

Structural changes in Se‐Te bilayers by laser writing

The structure of laser written lines in a Te81Ge15As4 glass

Structural changes in Se-Te bilayers by laser writing