From amorphous to crystalline silicon nanoclusters: structural effects on exciton properties

L.J. BORRERO-GONZALEZ; L.A.O. NUNES; F.E.G. GUIMARAES; J. WOJCIK; P. MASCHER; A. M. GENNARO; M. TIRADO; D. COMEDI

JOURNAL OF PHYSICS CONDENSED MATTER

IOP PUBLISHING LTD

Año: 2011 vol. 23 p. 505202 - 505212

0953-8984

Synchrotron x-ray absorption spectroscopy (XAS) and electron spin resonance (ESR)experiments were performed to determine, in combination with Raman spectroscopy and x-raydiffraction (XRD) data from previous reports, the structure and paramagnetic defect status ofSi-nanoclusters (ncls) at various intermediate formation stages in Si-rich Si oxide films havingdifferent Si concentrations (y D 0:36?0.42 in SiyO1��y), fabricated by electron cyclotronresonance plasma-enhanced chemical vapor deposition and isochronally (2 h) annealed atvarious temperatures (Ta D 900?1100 C) under either Ar or (Ar C 5%H2) atmospheres. Thecorresponding emission properties were studied by stationary and time dependentphotoluminescence (PL) spectroscopy in correlation with the structural and defect properties.To explain the experimental data, we propose crystallization by nucleation within alreadyexisting amorphous Si-ncls as the mechanism for the formation of the Si nanocrystals in theoxide matrix. The cluster-size dependent partial crystallization of Si-ncls at intermediate Tacan be qualitatively understood in terms of a ?crystalline core?amorphous shell? Si-ncl model.The amorphous shell, which is invisible in most diffraction and electron microscopyexperiments, is found to have an important impact on light emission. As the crystalline coregrows at the expense of a thinning amorphous shell with increasing Ta, the PL undergoes atransition from a regime dominated by disorder-induced effects to a situation where quantumconfinement of excitons prevails.