In situ formation of tin nanocrystals embedded in silicon nitride matrix
ABSTRACT Tin (Sn) nanocrystals (NCs) embedded in a silicon nitride ( Si 3 N 4) matrix have been fabricated in a cosputtering process employing low temperature (100 ° C ) substrate heating. Transmission electron microscopy (TEM) showed the formation of uniformly sized Sn NCs of 5.2±0.9 nm evenly distributed in the Si 3 N 4 matrix. Both TEM and x-ray diffraction measurements showed that the Sn NCs adopted the semimetallic tetragonal β -Sn structure rather than the cubic semiconducting alpha-Sn structure. X-ray photoelectron spectroscopy revealed that the semimetallic state ( Sn 0) is the major component of Sn in the sample films. Our investigation demonstrates a pronounced effect of the substrate temperature on the formation of Sn NCs. The mechanism of in situ formation of Sn NCs is discussed. We suggest that the formation of uniformly sized Sn NCs is correlated with lowering the surface mobility of the nuclei due to the presence of the cosputtered Si 3 N 4 .
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ABSTRACT: Quantum dot materials, in which Si QDs are embedded in a dielectric matrix, offer the potential to tune the effective band gap, through quantum confinement, and allow fabrication of optimised tandem solar cell devices in one growth run in a thin film process. Such cells can be fabricated by sputtering of thin layers of silicon rich oxide sandwiched between a stoichiometric oxide that on annealing crystallise to form Si QDs of uniform and controllable size. For approx. 2 nm diameter QDs these result in an effective band gap of 1.8 eV. Introduction of phosphorous or boron during the growth of the multilayers results in doping and a rectifying junction, which demonstrates photovoltaic behaviour with an open-circuit voltage of almost 500 mV. However, the doping behaviour of P and B in these QD materials is not well understood. In addition P and B have large but opposite effects on QD crystallisation, with P (B) doped material forming larger (smaller) QDs than for undoped material. Alternative materials for quantum dots are Ge and Sn. These allow lower processing temperatures to be used, more compatible with underlying layers. Alternative matrices to SiO2 such as SiNx or SiC offer higher tunnelling probability and hence lower resistance. These alternative matrix materials can also be used as hetero interlayers to improve the transport in the growth direction whilst maintaining quantum confinement. Group IV alloys can also be used to modify band gap. GeC in particular looks to have useful band gap and sputtering properties. Such alloy materials could be used in hetero-junction or homojunction devices in combination with SiQD based materials to fabricate all thin film tandem cells.Energy Procedia 15:200–205.