1. 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.

2. 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.

3. 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

4. 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 = nm
3. Si content in oxide: vol.%
4. Annealing: 1140оС, 40 min, N2
NC size: 3 – 5 nm Si

5. 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

6. 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

7. HREM cross sectional images for the Si NCs in irradiated layer
Kr, 90 MeV, 1012 cm 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

8. 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 x1012 cm-2 8x1012 cm-2

9. 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.

10. 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

11. PL intensity of initial Si NCs as a function of Si concentration1 2 3 4 5 6
Max effect for points 3 and 4 5%
Extra Si concentration
90%

12. 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.

13. Thank you for your attention!