Modification of Si nanocrystallites in SiO2 matrix by swift heavy ion irradiation By: V. E. Thelelo & A. Rossouw In co-operation with, University of Fort Hare & Stellenbosch University Supervised By: N.S.Kirilkin, V.A.Skuratov Joint Institute for Nuclear Research, Dubna, Russia Good morning everyone, My name is V.E Thelelo and I will be presenting our work done under the supervision of Dr. V.A Skuratov over the past weeks. Just a little background information about us, Vuyo: University of Fort Hare, Direction of study, etc. And my colleague, Arnoux: Stellenbosch University, Masters – Hybrid photocatalytic membrane reactors, etc.

Brief project overview The irradiation testing of nuclear ceramics and oxides with heavy ions of fission fragment energy Our irradiation testing experiments will be focused on study of: – correlation between surface and material bulk radiation damage induced by heavy ions with energies above 1 MeV/atomic mass unit (amu). – temperature dependence of swift heavy ion-induced phase transformations and dense ionization effect on pre-existing defect structure in irradiating materials. The project fulfillment allows us to acquire new knowledge concerning evolution of defect structure in nuclear ceramics under dense electronic excitations simulating the fission fragment impact. Studies of Si nanostructures are currently an area of intense investigations. The ongoing drive for smaller Si device dimensions coupled with the discovery of strong luminescence from quantum-sized Si nanocrystals (Si-ncs) [1] stimulates research into the processes of Si nanostructures formation and modification. Si-ncs have been conventionally fabricated by either ion implantation of Si ions into SiO2 layers or by deposition of SiOx films followed by annealing.

Irradiation with swift heavy ions (SHI) The source of structural changes in tracks of high energy (E > 1 MeV/amu) heavy ions is a huge energy deposition (Se  tens keV/nm) in electron subsystem of irradiating material SHI cause exotic effects in different classes of materials which cannot be generated by any other means. Studies of Si nanostructures are currently an area of intense investigations. The on going drive for smaller Si device dimensions coupled with the discovery of strong luminescence from quantum-sized Si nanocrystals (Si-ncs) stimulates research into the processes of Si nanostructures formation and modification. Si-ncs have been conventionally fabricated by either ion implantation of Si ions into SiO2 layers or by deposition of SiOx films followed by annealing. In both cases the optimum conditions have involved a Si excess of about 10–15 at.%, and anneal temperatures above 1000◦C. The Si-ncs formed reveal strong luminescence at the wavelengths of 750–850 nm. Along with furnace treatments, the formation of light-emitting Si-ncs employing pulsed annealing with pulse duration of 1 s and less has also proven feasible. Pulsed anneals are of high practical importance as they enable one to perform high-temperature heat treatment locally while insignificantly affecting the adjacent regions. This is particularly advantageous for the processing of ultra-large-scale integrated circuits. Irradiation with swift heavy ions (SHI) may be considered, to a certain extent, as a kind of pulse treatment. G. Schiwietz et al. NIMB 226 (2004) 683

SiO2 layers with variable NC concentration Deposition of SiO2 and Si on Si substrate from two sources separated by distance of 100 mm 2. dSiO2 = 400 - 1000 nm 3. Si content in oxide: 5 - 90 vol.% 4. Annealing: 1140оС, 40 min, N2 NC size: 3 – 5 nm Si

Ordered arrays of Si nanocrystals in SiO2: structural, optical and properties The ordering of composites under high-energy ion irradiation and formation of elongated particles (up to 250 nm length with diameter 3-5 nm) is known for metal nanoparticales in SiO2 Pt in SiO2 185 MeV Au NIM B 266 (2008), 3158

Formation of the ordered NC distribution surface Irradiated NC-SiO2 layer (37 % Si) Kr, 90 MeV, 1012 cm-2 Ion pass Formation of the ordered NC distribution 30 nm nano9_9_095205 substrate 20 nm

HREM cross sectional images for the Si NCs in irradiated layer Kr, 90 MeV, 1012 cm-2 non-irradiated layer In nanostructures, whenever the electron wavelength exceeds the appropriate dimensions of the device structure, the quantum nature dictates the physical properties in them. To observe quantum-size effects in an array of quantum dots (QDs) embedded in a dielectric matrix, one needs the semiconductor nanocrystallites to have small size and uniform distribution as well as an uniformly ordered dispersion in the corresponding matrix. In order to find the radiation-induced structure modifications of the NC arrays, we have applied cross-sectional high-resolution electron transmission microscopy (HRTEM). Comparison of the images obtained prior to irradiation and under the present irradiation demonstrates, as seen in figure (a) and (b) respectively, that the irradiation creates an ordered distribution of nanocrystallites along the ion tracks. The alignment of these atomic planes of the NCs is directed along the ion tracks, with a deviation of ±20◦. Whereas in the case of no irradiation, NCs are randomly distributed in the SiO2 matrix. Atomic planes of NCs in the irradiated samples are oriented along the ion tracks

Photoluminescence of irradiated layer Bi, 670 MeV , ex. = 488 nm, Ar laser The PL spectra for the reference and the irradiated samples for some x values are presented in the above figures. The PL peak of the reference sample SiO-1 that was found at ∼785 nm shows a slight shift with x here. After irradiation, the main spectral features of the spectra remain the same, but the irradiated samples exhibited a very strong increase (SiO-2) or a very strong decrease (SiO-3) of the PL intensity, and a much more pronounced shift of the spectral PL peak on the one hand, and its x-position shift to lower Si contents, on the other hand. Initial 1x1012 cm-2 8x1012 cm-2

Photoluminescence of 670 MeV Bi ion irradiated layer The PL intensity as a function of the Si content IPL(x) is summarized in this figure. These results suggest that there is a bimodal distribution of the crystallites size in our samples in accordance with our previous analysis of the green and red PL, such that the larger crystallites dominate the spectra of the reference sample and the weakly radiated sample, while the few smaller crystallites that remain after the irradiation dominate the spectra of the strongly irradiated sample.

Experimental Setup 1. Diod with 380nm wavelength produces excitation of Nc Si 2. Nc Si specimen emit light 3. Spectrometer register this light and cuts off signal less than 550nm wavelength Spectrometer Spectrometer ANDOR iDUS Diod 380nm Nc Si specimen

PL intensity of initial Si NCs as a function of Si concentration 1 2 3 4 5 6 Max effect for points 3 and 4 5% Extra Si concentration 90%

Acknowledgments Dr. N. M. Jacobs Prof. M. L. Lekala Dr. V. A. Skuratov Mr. N. S. Kirilkin JINR UFH US And everyone else involved in making this practice possible.

Thank you for your attention!