Complete Report - University of New South Wales

ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORT(a) Normalized PL spectra of Si QDs samples (S3-S5)with various sizes. (b) Comparison of band gapexpansion (ΔE g) of Si QD as a function of QD size.Figure 4.5.114.5.2.3.1 Silicon QDs embedded in siliconnitride matrixResearchers:Yong-Heng So, Shujuan Huang(i) Size-dependent optical properties of SiQDs in Si-rich nitride/Si 3N 4superlatticeAn approach similar to spectroscopic ellipsometryanalysis has been proposed to determine the sizedependent optical properties of silicon quantumdots (Si QDs) in Si-rich nitride/silicon nitridesuperlattice structure (SRN/Si 3N 4-SL) [4.5.26]. Theoptical properties of Si QDs are modelled usingthe Tauc-Lorentz (TL) model and Bruggemaneffective medium approximation that can yield theenergy bandgap of the Si QDs based on spectralfitting of the reflection (R) and transmission (T) ofcharacterized samples, as shown in Fig. 4.5.9. A fourphaseoptical model as shown in the inset of Fig.4.5.9 was employed for the spectral fitting.Figure 4.5.10 shows that the dielectric functionsof Si QDs are strongly size dependent. Sample S3,S4 and S5 correspond to SRN/Si 3N 4thicknessesof 3/3 nm, 4/3 nm and 5/3 nm, respectively. Thesuppressed imaginary dielectric function of Si QDsexhibit a broad peak centred between transitionenergies E 1and E 2of bulk crystalline Si and whichblue shift towards E 2as the QD size reduces.Figure 4.5.11(b) shows that the band gapexpansion indicated by the TL model when thesize of Si QD reduces is in good agreement withPL measurements (Fig. 4.5.11(a)). The bandgapexpansion with the reduction of Si QD size is wellsupported by the ab-initio calculations of confinedenergy levels from [4.5.25].(ii) n-type conductivity of nanostructuredthin film composed of antimony-doped Sinanocrystals in silicon nitride matrixHighly conductive thin films composed of antimony(Sb)-doped Si nanocrystals (Si-NCs) embeddedin Si 3N 4matrix were prepared by a co-sputteringtechnique. Results from structural characterizationssuggest that doping with Sb concentration of 0.54at. % has negligible affect on the crystallizationproperties of Si-rich nitride (SRN) films. The X-rayPhotoelectron (XPS) data of Fig. 4.5.12(a) and (b)show the Si 2p and Sb 3d 3/2spectra of the undopedand Sb-doped SRN films. The slight asymmetric Sb3d 3/2peak was analyzed using a curve-fitting routineand could be decomposed into two components,δ 1(537.0 eV) and δ 2(537.8 eV) as shown in Fig.4.5.12(c). The two components δ 1and δ 2(~0.8 eVapart) can be assigned to neutral Sb and positivelycharged donors, respectively.Thus, the appearance of δ 2suggests the presenceof Sb-Si bonds which implies that Sb stoms wereeither incorporated within the Si-NCs or located atthe interface between the NCs and Si 3N 4matrix.XPS spectra of (a) Si 2p and (b) Sb3d 3/2for the undoped and 0.54at.% Sb-doped Si-NC. (c) Fittingof Sb 3d 3/2spectrum by two peaksseparated by ~0.8 eV in energy (δ 1and δ 2).Figure 4.5.1267