Structural Study on Crystallization of a-Ge Using Exafs
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STRUCTURAL STUDY ON CRYSTALLIZATION OF a-Ge USING EXAFS MASATOSHI WAKAGI, T. HIRANO: N. HANAZONO, N. CHIGASAKI• O. ABE** AND A. NAKANO* Hitachi Research Laboratory, Hitachi, Ltd., 4026 Kuji-cho, Hitachi-shi, 319-12 Japan **Production Engineering Research Laboratory, Hitachi, Ltd., 292 Yoshida-cho, Totuka-ku, Yokohama, 244 Japan *
ABSTRACT Extended x-ray absorption fine structure (EXAFS) measurements were made on a-Ge and a-Ge:H films which were prepared by sputtering and were annealed at temperatures between 300 and 450'C. The first nearest neighbor distance and the Debye-¥aller factor were investigated. These values decrease rapidly on the crystallization of the samples. The relationship between them was analyzed in respect of tvo-body potential. The potential of amorphous state is considered to be affected by unharmonic term.
INTRODUCTION The structure of amorphous semiconductors has been studied for many years. Especially, with technological application interests, intensive studies have been made on the structure of tetrahedral amorphous materials such as a-Si and a-Ge. These materials are well known as typical covalently bonded network system. Therefore, structural investigation of these materials may reveal basic information of disordered semiconductors. In order to study disorder system, investigation of disorderto-order transition is very useful. Many studies have been performed to investigate the transition of a-Si and a-Ge by using electron spin resonance [1], magnetic susceptibility [2], conductivity [3J, Raman spectroscopy [4,5] measurement methods, etc. RecentlY, extended x-ray absorption fine structure (EXAFS) method has attracted much attention to investigate the local atomic structure of amorphous materials. Evangelisti et al. [6] analyzed the EXAFS of a-Ge prepared by the thermal evaporation at several substrate temperatures between 130 and 390C. The second and third nearest neighbors of EXAFS radial distribution function (RDF) were analyzed to investigate the crystallization condition. Stern et al. [7] and Paesler et al. [8] also analyzed the second nearest neighbor of EXAFS ROF of a-Gie prepared by sputtering to investigate a prior state to crystallization. Ve have also been studying the structure of a-Ge by using EXAFS [9]. In this paper, we report on the EXAFS of a-Ge ; samples were prepared by sputtering at 50C and annealing was performed between 300 to 450t. The first nearest neighbor distance and the Debye-ialler factor of the samples were analyzed to study amorphous-to-crystal phase transition.
EXPERIMENTAL Amorphous
Ge films were
deposited on the graphite
Mat. Res. Soc. Syrup. Proc. Vol. 118. ' 19689 Materials Research Society
substrates
92
with Imm thickness. The Ar flow rate was 1.2sccm. In the case of a-Ge:H preparation, Hz gas was also introduced at the flow rate of 2.2sccm. The gas pressure was controlled to be 1.3Pa during the sputtering process. The substrate was fixed to water cooled holder. The substrate temperature during the sputtering was 40-60C measured at the back of
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