Formation and evolution of luminescent Si nanoclusters produced by thermal annealing of SiOx films

Abstract

Si nanoclusters embedded in SiO2 have been produced by thermal annealing of SiOx films prepared by plasma enhanced chemical vapor deposition. The structural properties of the system have been investigated by energy filtered transmission electron microscopy (EFTEM). EFTEM has evidenced the presence of a relevant contribution of amorphous nanostructures, not detectable by using the more conventional dark field transmission electron microscopy technique. By also taking into account this contribution, an accurate quantitative description of the evolution of the samples upon thermal annealing has been accomplished. In particular, the temperatures at which the nucleation of amorphous and crystalline Si nanoclusters starts have been determined. Furthermore, the nanocluster mean radius and density have been determined as a function of the annealing temperature. Finally, the optical and the structural properties of the system have been compared, to demonstrate that the photoluminescence properties of the system depend on both the amorphous and crystalline clusters.

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

“…This is interesting, as the crystallization temperature of amorphous Si is known to be significantly lower (about 500 • C) [28]. High crystallization temperatures have been observed before in Si-ncls embedded in amorphous oxides [2,3] and it is tempting to attribute this effect to the influence of the surrounding amorphous oxide network. However, studies of isolated Si-ncls prepared by PECVD have also shown crystallization temperatures well above those typical in bulk a-Si [57].…”

“…Previous [2,12,36,[39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] and our own [21] studies have shown that the integral PL time decay in Si-based nanostructures can be fitted by the well-known stretched exponential function…”

“…This system is regarded as a basis for an important class of materials for the development of lasers [13], optical waveguides [14], photonic [15] and third generation solar cell devices [16] based on Si. The formation of Si-ncls within amorphous silicon oxides at different annealing stages has been studied by various techniques, and surprisingly high crystallization temperatures [2,3] and low activation energies of nanocrystal size growth [2,4] have been reported. Luminescence in the visible, however, is often observed to appear and to evolve in correlation with Si-ncls formation and growth, as is expected from quantum confinement (QC) of excitons in crystalline Si-ncls [17,18], albeit its specific origin is still a matter of active discussion in the literature [19].…”

“…A great deal of experimental and theoretical work has been devoted to the understanding of the thermally driven formation and structure of luminescent silicon nanoclusters (Si-ncls) in metastable Si-rich Si oxides (SRSO) [1][2][3][4] and their optical properties [5][6][7][8][9][10][11][12]. This system is regarded as a basis for an important class of materials for the development of lasers [13], optical waveguides [14], photonic [15] and third generation solar cell devices [16] based on Si.…”

“…The materials were fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR-PECVD) and annealed at various temperatures under either Ar or (Ar + 5%H 2 ) atmospheres. We then analyze the corresponding emission properties of the materials by stationary and time dependent photoluminescence (PL) spectroscopy in correlation with the structural and defect properties.…”