Physical Properties of Undoped and Doped Microcrystalline SiC:H Deposited by PECVD
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PHYSICAL PROPERTIES OF UNDOPED AND DOPED MICROCRYSTALLINE SiC:H DEPOSITED BY PECVD F.DEMICHELIS, C.F.PIRRI, E.TRESSO Dip. Fisica Politecnico - C.so Duca degli Abruzzi 24 - 10129 Torino (Italy) G.DELLAMEA and V.RIGATO Laboratori INFN - Via Romea 4 Legnaro e Unita' INFM Padova (Italy) P.RAVA Elettrorava S.p.A - Via Don Sapino 120 Savonera Torino (Italy)
ABSTRACT Experimental results on a systematic investigation on the elemental composition, structural, optical and electrical properties of undoped and doped microcrystalline silicon carbide films deposited by Plasma Enhanced Chemical Vapor Deposition. The doped samples show high values of dark conductivity accompanied by good optical properties so to satisfy the requirements for heterojunction window material.
INTRODUCTION In the last years a great deal of efforts has been payed to develop high quality amorphous silicon carbide a-SiCH since its use as a window layer in a-SiC/a-Si heterojunction [1] and as LED's material, due its wide range of photoluminescence [2]. However increasing the band gap of a-SiC:H, through the introduction of carbon, decreases the conductivity. Recently investigations on wide band gap a-Si alloys lead to remarkable results on high conductivity of heavily doped amorphous silicon carbide [3]. On the other hand the conductivity of microcrystalline phase is much higher than that of amorphous phase [4,5]. Then improvement in the conductivity can be obtained both by doping amorphous films and by growing microcrystalline samples. In order to investigate the physical properties of undoped and doped microcrystalline SiC:H alloy, samples have been realized by PECVD of SiH4+CH 4+H2 mixtures both pure and with the addition of B 2H 6 or PH 3 gases. In this paper we report the results on a systematic investigation on the elemental composition, optical and electrical properties, structure of the samples determined by infrared (I.R.) and Raman spectroscopy, Transmission Electron Microscopy and X-ray diffraction analysis. The doped samples show high values of dark conductivity accompanied by good optical properties so to satisfy the requirements for heterojunction window material.
EXPERIMENTAL The films of tc -SiC:H were deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) of SiH 4+CH 4+H 2 mixtures with controlled addition of B 2H6 or PH 3 gases. Typical discharge conditions include: flow rate of Sill4 4 sccm, of CH4 0.5 - 1 sccm, of H2 200 sccm and of B 2H6 or PH3 varying in the range 5 10 - 7 102 volume part per million (vppm). The name of samples is B for boron-doped and P for phosphine-doped followed by a number indicating the vppm of the dopant gases. The pressure was 40 Pa, the substrate temperature 200"C and the power density 0.15 W cm 2 . Measurements performed by Rutherford back-scattering (RBS), Elastic Recoil Detection (ERD) combined with Secondary Ion Mass Spectroscopy (SIMS) have provided the elemental composition of the samples. The nature of chemical bonding was studied by I.R. spectroscopy using a Bruker FS85 spectrometer betwee
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