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

Abstract

Synchrotron x-ray absorption spectroscopy (XAS) and electron spin resonance (ESR) experiments were performed to determine, in combination with Raman spectroscopy and x-ray diffraction (XRD) data from previous reports, the structure and paramagnetic defect status of Si-nanoclusters (ncls) at various intermediate formation stages in Si-rich Si oxide films having different Si concentrations (y = 0.36-0.42 in Si y O 1−y ), fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition and isochronally (2 h) annealed at various temperatures (T a = 900-1100 • C) under either Ar or (Ar + 5%H 2 ) atmospheres. The corresponding emission properties were studied by stationary and time dependent photoluminescence (PL) spectroscopy in correlation with the structural and defect properties. To explain the experimental data, we propose crystallization by nucleation within already existing amorphous Si-ncls as the mechanism for the formation of the Si nanocrystals in the oxide matrix. The cluster-size dependent partial crystallization of Si-ncls at intermediate T a can 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 microscopy experiments, is found to have an important impact on light emission. As the crystalline core grows at the expense of a thinning amorphous shell with increasing T a , the PL undergoes a transition from a regime dominated by disorder-induced effects to a situation where quantum confinement of excitons prevails.

(Invited) Growing Si Nanocrystals within a-Si Nanoclusters Embedded in a-SiO2: Evolution of Photoluminescence

“…For T a =900ºC, in contrast, for which the Si-ncls are amorphous, no dispersion is observed and the experimental data points cannot be well fit by the quantum confined exciton model. More effects showing the same trend have been reported in previous papers (2,3).…”

“…An analysis indicates that most of the excess silicon is already precipitated into amorphous nanoclusters in the as grown film (2). The strong chemical ordering tendency deduced by XRD after annealing at T a = 900 °C and 1100ºC is confirmed by the XAS data, as deduced by the reduced contributions from the mixed tetrahedra and the concomitant increase in the SiO 4 peak in the spectrum of Fig.…”

“…3. For T a =1000 and 1100ºC, this trend is accompanied by a reduction of the Si dangling bond density, as determined from the ESR data (2), and an increase of the crystalline Si volume fraction, as deduced from XRD and Raman (3). Figure 3.…”

“…However, the specific origin of the luminescence from real systems is still matter of active discussion in the literature (1,2). Much of the controversy on this respect probably stems from the inherent complexity of the chemically and structurally disordered amorphous oxide matrix and Si-ncl/matrix interfaces, which are seldom characterized or properly controlled in the experiments.…”

Raman scattering from confined acoustic phonons of silicon nanocrystals in silicon oxide matrix

“…It was shown in the previous section that when x increases, the experimental ratio ν 0,0 /ν 0,2 approaches the value predicted by CFM. Now, it is well known that for higher Si content in the film, the crystallization process of Si-nc is enhanced [27,46]. In other words, the formation of Si-nc is always more efficient in the films with higher x.…”