Separation of Nucleation and Growth Processes of Nanocrystalline Silicon by Hydrogen Radical Treatment of Hydrogenated A
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SEPARATION OF NUCLEATION AND GROWTH PROCESSES OF NANOCRYSTALLINE SILICON BY HYDROGEN RADICAL TREATMENT OF HYDROGENATED AMORPHOUS SILICON MASANORI OTOBE AND SHUNRI ODA Department of Physical Electronics, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152, Japan. ABSTRACT We have investigated nucleation and growth mechanism of nanocrystalline silicon (nc-Si) based on the experimental observation of plan-view transmission electron microscopy Nanocrystalline Si has been prepared by hydrogen radical annealing of hydrogenated amorphous silicon (a-Si:H) layer, which is deposited on hydrogen radical treated a-Si:H buffer layer. Nanocrystallization depends critically upon hydrogen radical annealing time and the thickness of a-Si:H layer. Hydrogen radicals play important roles in both nucleation and growth processes in a different way Growth of nc-Si can be explained by "hydrogen diffusion model", in which hydrogen radicals diffusing through a-Si:H layer to interface cause nanocrystallization. Our results imply that nuclei for nc-Si are generated at the interface between a-Si:H and under layer when treated by hydrogen radicals.
INTRODUCTION Nanocrystalline silicon (nc-Si) with grain size of less than lOnm is expected to exhibit a zero dimensional quantum size effect. Recently, several groups reported the quantum effect in nc-Si; i. e., widening of the optical bandgap', visible photoluminescence 2 and resonant tunneling3 . 4 . These properties are expected for the application to optoelectronics and the next generation ultra large scale integrated circuits. However, it is necessar3y to establish the technology to control grain size, nucleation sites, and grain boundary states, precisely In order to achieve these controls, we are proposing "digital chemical vapor deposition (CVD)" 5 , which means the separation of nc-Si formation process into nucleation, deposition of Si, crystallization, and grain boundary passivation. Thanks to this separation, each process will be controlled precisely. In order to realize this concept, it is necessary to clarify the growth mechanism of nc-Si. In the last several years, the growth mechanism of nano- or micro crystalline Si (pc-Si) was studied by using layer by layer deposition technique5-10. Asano 6 found that pc-Si film was obtained by alternating deposition of hydrogenated amorphous silicon (a-Si:H) (thickness of -I nm) and exposure to H-12 plasma. Nomoto et al. , have pointed out that sufficient hydrogen coverage of the growing surface causes the formation of pc-Si, by the experiment varied substrate temperature and waiting time (the time between the every growth of a-Si:H several monolayers and hydrogen radical treatment). H. Shirai et al.8 found that the hole transport and stability of a-Si:H under light illumination were improved by the preparation technique, in which the deposition of several nanometer thick a-Si:H and exposure to atomic hydrogen were alternately repeated. In our recent studies, we showed that pc-Si fabricated by alternating deposition of a-Si:H (thickness
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