Radiation induced crystallization of amorphous Si : H alloy
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I.
INTRODUCTION
THE microcrystalline-phase
of hydrogenated silicon (/xc-Si: H) has recently drawn interest as a new phase material for applications in thin film solar cells. The/xc-Si: H can be called "an intermediate range order material between crystalline and amorphous silicon (a-Si:H)". Ill The main advantages of using/zc-Si: H are: (1) the higher electrical conductivity and lower optical absorption coefficient of /xc-Si:H compared to a-Si:H are attractive properties for solar cell applications, t2'31 The px--Si:H exhibits a higher doping efficiency than a-Si :H. It is possible to use heavily doped /xc-Si:H as a contact material, t4-71 and (2) /zc-Si: H is a more stable material than a-Si : H, because it is a relaxed structure of the a-Si:H alloy. N It has been found that the conductivity of the Izc-amixture material is directly proportional to the volume fraction of the/~w-phase present in the material, r9'1~ The reason for the high conductivity of the/xc-phase is not totally understood as yet. It has been speculated that in addition to improved doping efficiency, the limited long range order in the tzc material eliminates some of the tail states present in the amorphous material.l~ ~J The /~c-Si:H has been produced by the following methods: (1) DC or RF glow discharge decomposition of a SiH4/H2 gas mixture, i5,10,12 16]The volume fraction of the/xc-phase in the film produced by this method varies from 25 pct to 90 pct, depending upon the deposition parameters. (2) Plasma enhanced CVD film deposition followed by annealing in a hydrogen atmosphereJ 16'171The film produced by using this method contains 10 pct to 30 pct of the /zc-phase. It is believed that the presence of hydrogen in the film can induce/zc-phase formation.[7] A large fraction of the H in the film is in molecular form. The unbonded H increases the instability of the film. 1171 (3) Inert ion sputtering enhanced CVD to produce /zcSi:H.[3'~8-2~ Inert ions, such as Ar + or Ne +, induce crystallinity in the material. However, some inert ions may be trapped inside the film as impurities. The tzc-phase material
Y.C. KOO, Graduate Student, R. PERRIN, Assistant Professor, and K. T. AUST, Professor, all with the Department of Metallurgy and Materials Science, and S. ZUKOTYNSKI, Professor, Department of Electrical Engineering, are with The University of Toronto, 184 College Street, Toronto, ON, Canada, M5S 1A4. Manuscript submitted May 26, 1987. METALLURGICALTRANSACTIONS A
produced by this method usually exhibits poor electrical properties.t2~ Notable among all the methods of producing the tzc-phase is the use of ion bombardment. The objective of this paper is to study a radiation induced transition of silicon from the amorphous phase to the/xc-phase. II.
EXPERIMENTAL
An a-Si:H film was produced in a capacitively coupled RF glow discharge reactor using a SiH4/H2 gas mixture. The RF glow discharge deposition conditions are summarized.in Table I. The SiH4/H2 gas ratio was 10/90. The film was deposited on a high purity fused quartz substrate (0.25 pct A
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