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.
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ABSTRACT: The α phase of tin is a zero-gap semiconductor with an inverted band structure with respect to other group-IV elements like Ge. The Γ6c states lie energetically below the Γ8v levels. How these unique electronic properties transform in nanostructures with spatial confinement has not been studied. We apply density-functional theory within the local density approximation to investigate the energetic, structural, and electronic properties of bulk α-Sn and its nanocrystals (NCs) up to a size of 363 Sn atoms. For NCs with larger diameters up to 14 nm the tight-binding method is applied for the electronic states. Spin-orbit coupling is taken into account. The clusters are modeled in such a way that the Td symmetry of the bulk system is conserved. Their surfaces are passivated with hydrogen. We show that the spatial confinement causes not only a decrease of the fundamental gap for increased NC size but also a topological transition where the ordering of s- and p-like highest-occupied molecular orbital and lowest-unoccupied molecular orbital states is interchanged. The influence of quasiparticle and excitonic effects on the lowest pair excitation energies is investigated within approximations based on the hybrid exchange-correlation functional by J. Heyd, G. E. Scuseria, and M. Ernzerhof [J. Chem. Phys.JCPSA60021-960610.1063/1.1564060 118, 8207 (2003)] (HSE) and the ΔSCF method.Physical review. B, Condensed matter 06/2013; · 3.66 Impact Factor
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ABSTRACT: The effect of ultrathin silicon nitride (Si3N4) barrier layers on the formation and photoluminescence (PL) of Si nanocrystals (NCs) in Si-rich nitride (SRN)/Si3N4 multilayer structure was investigated. The layered structures composed of alternating layers of SRN and Si3N4 were prepared using magnetron sputtering followed by a different high temperature annealing. The formation of uniformly sized Si NCs was confirmed by the transmission electron microscopy and X-ray diffraction measurements. In particular, the 1 nm thick Si3N4 barrier layers was found to be sufficient in restraining the growth of Si NCs within the SRN layers upon high annealing processes. Moreover, X-ray photoelectron spectroscopy spectra shown that films subjected to post-anneal processes were not oxidized during the annealing. X-ray reflection measurements revealed that high annealing process induced low variation in the multilayer structure where the 1 nm Si3N4 layers act as good diffusion barriers to inhibit inter-diffusion between SRN layers. The PL emission observed was shown to be originated from the quantum confinement of Si NCs in the SRN. Furthermore, the blue shift of PL peaks accompanied by improved PL intensity after annealing process could be attributed to the effect of improved crystallization as well as nitride passivation in the films. Such multilayer structure should be advantageous for photovoltaic applications as the ultrathin barrier layer allow better electrical conductivity while still able to confine the growth of desired Si NC size for bandgap engineering.Thin Solid Films 06/2011; 519(16):5408-5412. · 1.87 Impact Factor