Complete Report - University of New South Wales

ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTEnergy filtered TEM tomography images of Si QDsin a SiO 2matrix formed as multilayers (left), andtop view of one Si QD layer (right).Figure 4.5.8The spectra fittings of the R and T data. The insetshows the corresponding employed optical model inthe fitting of a 20 bilayer structure composted of 5 nmSRN / 3 nm Si 3N 4.Figure 4.5.9Real (ε1) and imaginary (ε2) partsof the complex dielectric functionof Si QDs samples with various sizesobtained from spectral fittings.Figure 4.5.104.5.2.2.3 3D EF-TEM tomography on SiQDs embedded in oxide matrix formedas multilayers (Collaboration withCornell University)Researchers:Xiaojing Hao, Shujuan Huang, Lena FittingKourkoutis (Cornell University), IvanPerez‐Wurfl, Tom PuzzerA significant challenge to the design of nano-scalematerials and devices arises from the difficultyof characterising complex three-dimensionalstructures on small length scales, which isnonetheless critical in understanding the materialperformance. Whilst anumber of techniques, suchas conventional transmissionelectron microscopy,provide sufficient twodimensional(2-D) resolution,they have insufficientdepth sensitivity to obtaininternal three-dimensional(3-D) structure. Electrontomography can be appliedto obtain such informationby using high-angle darkfieldscanning transmissionelectron microscopic(STEM) tomography andenergy filtered transmissionelectron microscopy (EFTEM)tomography. We have appliedthis EFTEM tomography toinvestigate Si QDs embeddedin an oxide matrix formedas multilayers, in order tobetter understand the dotsize, shape and distribution for further optimizationof Si QD device performance. Fig. 4.5.8 illustratesthe ability of electron tomography to reconstructthe morphology and internal structure of a Si QDnanostructure. Fig. 4.5.8 (left) shows the aligned“on-axis” reconstructed EFTEM tomography, andFig. 4.5.8 (right) shows a top-view of the Si QD sizeand shape distribution in one Si QD layer. With suchinformation, we can confirm that inside the Si QDlayer, most of the Si QDs are reasonably sphericaland well separated from each other. This indicatesthat individual QDs have not merged to form largerparticles under these conditions. This is importantin designing the optimum concentrations togive minimum QD spacing whilst maintainingindividual QDs.4.5.2.3. Different materials forQD nanostructuresAs described in section 4.5.2.2.1, different matricesare useful to modify both the tunnelling probabilitybetween adjacent quantum dots and the energylevels in the quantum dots themselves. Thisapproach has been carried out for both Si QDs inSiN xand in SiC. It is also possible to have asymmetricsuperstructures in which quantum confinementand hence enhanced band gap is maintained inthe plane, but in which tunnelling probability ismaximised in the direction normal to the plane suchthat transport of carriers to contacts is maximised.Interlayers use for such asymmetric structurescan also act as diffusion barriers enhancing sizeuniformity of QDs.Alternative group IV materials such as Ge offer thepossibility of lower temperature precipitation ofnanostructures and the potential for band gapslower than that of silicon should these be requiredfor tandem cell elements under a silicon cell. Wehave investigated both QDs and QWs of Ge in eitherSiO 2or SiN xmatrices.66