X-ray-diffraction study of crystalline Si nanocluster formation in annealed silicon-rich silicon oxides

Abstract

The formation and subsequent growth of crystalline silicon nanoclusters ͑Si-ncs͒ in annealed silicon-rich silicon oxides ͑SRSOs͒ were studied by glancing angle x-ray diffraction. SRSO samples with Si concentrations ͑y͒ of 0.40, 0.42, and 0.45 were grown by inductively coupled plasma-enhanced chemical-vapor deposition ͑PECVD͒. Samples with y = 0.42 grown by electron-cyclotron-resonance PECVD were also studied. Annealing treatments were performed at temperatures ͑T͒ of 900, 1000, and 1100°C for times ͑t͒ between 0.5 and 3 h in flowing Ar. As-grown SRSO films did not present signs of Si clusters ͑amorphous or crystalline͒; however, ͑111͒, ͑220͒, and ͑311͒ Bragg peaks corresponding to c-Si were clearly seen after annealing at 900°C for the y = 0.45 sample, but only barely seen for the y = 0.42 and undetected for the y = 0.40 samples. For T = 1000°C, all studied SRSO samples clearly showed the c-Si diffraction peaks, which became narrower with increasing t and T. From the width of the Si ͑111͒ peaks, the mean size of Si-ncs and their dependence on T and t was determined. Activation energies were deduced from the T dependence by fitting the results to two growth models of Si precipitates in an a-SiO 2 matrix reported in the literature. The activation energies qualitatively agree with values deduced from transmission electron microscopy studies of annealed SRSO reported in the literature. However, they are significantly lower than Si diffusion activation energies available in the literature for SiO 2 with low excess Si. A broad feature is also observed in the x-ray diffractograms for as-grown samples with low y, which shifts to the peak position corresponding to a-SiO 2 with increasing T. This behavior is explained by the formation of a well-defined a-SiO 2 phase with increasing T, where mixed Si-O 4−n Si n ͑n = 1,2,3͒ tetrahedra in the as-grown alloy are gradually converted into Si-O 4 and Si-Si 4 as phase separation of Si and SiO 2 proceeds. From the measured Si ͑111͒ peak positions, small Si-ncs are found to be tensilely strained by as much as ϳ0.8%. This effect becomes insignificant as Si-ncs become larger with increasing y or T.

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

“…It can be concluded that the as-grown film contains regions of clustered Si whose overall volume does not change significantly during the annealing processes. Since diffraction peaks are not observed by XRD measurements in these as-grown films [3], it is deduced that the Si clusters are amorphous. From the Raman [21] and XRD data [3] it can be inferred that the amorphous character of the Si clusters for oxide samples with y ≤ 0.42 is maintained for T a = 900 • C, regardless of the gas atmosphere (Ar or Ar + 5%H 2 ) used.…”

“…The substrate temperature during the depositions was 120 • C and the ECR power 500 W. The films were then isochronally annealed under Ar or (Ar + 5%H 2 ) at either 900, 1000 or 1100 • C for 2 h to promote different stages of Si-ncl formation in an amorphous Si oxide matrix, as described elsewhere [3,17]. Some samples on Si substrates were studied by XAS at the spherical grating monochromator (SGM) beamline of the Canadian light source (CLS) synchrotron facility.…”

“…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].…”

“…In this work, we use electron spin resonance (ESR) and synchrotron x-ray absorption (XAS) measurements, in combination with x-ray diffraction (XRD) [3] and Raman spectroscopy [21] data from our previous studies, to determine the structure and paramagnetic defect status of Si-ncls at their various intermediate formation stages in SRSO films having different Si concentrations. 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.…”

“…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.…”