The composition and structure of SIPOS: A high spatial resolution electron microscopy study
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The nanostructure and chemical distribution in semi-insulating polycrystalline oxygen-doped silicon (SIPOS) deposited on (001) Si and its isothermal transformation behavior at 900 °C were investigated by high resolution electron microscopy (HREM) and electron energy loss nanospectroscopy (EELS). The structure of the as-deposited film, which contained 15 at. % oxygen, was amorphous. No evidence for nanocrystalline second phases was found. It was similar in appearance to amorphous silicon. After annealing for 30 min at 900 °C in an inert environment (N 2 ), a dispersion of small nanocrystals, identified as silicon by imaging, diffraction and EELS, formed in the amorphous SIPOS matrix, with a thin precipitate free zone (PFZ) adjacent to the Si substrate. The SIPOS matrix oxygen concentration increased to 36 at. % and the matrix remained amorphous after annealing. No other phases were observed in annealed specimens. Changes in S i - L near edge fine structure and low loss peaks in EELS spectra from SIPOS with increasing oxygen concentration indicated that it is a solid solution supersaturated with silicon. Microstructures indicated that the Si nanocrystals formed during a homogeneous precipitation reaction.
I. INTRODUCTION Semi-insulating polycrystalline oxygen-doped silicon (SIPOS) films are technologically important as surface passivation layers for high voltage power devices1'2 and in solar cell technology.3 Other applications include use as resistors in CMOS memory circuits4'5 and as emitters in heterojunction transistors.6 SIPOS films are grown by low pressure chemical vapor deposition (LPCVD) from mixtures of SiH4 and N 2 O at deposition temperatures of 600 to ~700 °C, usually on Si or SiO 2 substrates. The SIPOS oxygen concentration can be varied over a wide range by varying the N 2 O/SiH 4 ratio in the reactant stream, with higher ratios producing higher oxygen concentration.7 Early on, SIPOS was considered to be polycrystalline Si containing a large concentration of oxygen, up to —50 at. %, because of its relatively high deposition temperature. An early diffraction contrast/selected area diffraction transmission electron microscopy (TEM) and x-ray diffraction investigation of the structure of SIPOS films containing 15 to 40 at. % oxygen produced evidence for Si nanocrystals in some of the as-deposited films and in all of them after annealing between 800 and 1100 °C in high purity N 2 . 8 In the same investigation electron spectroscopy for chemical analysis (ESCA) showed that the Si-2p peak from una)
Permanent address: C.N.R.-I.M.E., Dipartimento Scienza dei Materiali, Universita di Lecce, Via Arnesano, 73100 Lecce, Italy. ^Lawrence Livermore National Laboratory, University of California, MSL-369, P.O. Box 808, Livermore, California 94551. J. Mater. Res., Vol. 8, No. 11, Nov 1993
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annealed SIPOS film was shifted slightly (+0.4 eV) to higher energy than the corresponding peak for pure Si (97.9 eV) and contained fine structure similar to the Si2p peak from
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