Si and Ge nanocluster formation in silica matrix

Abstract

High resolution transmission electron microscopy, scanning transmission electron microscopy and cathodoluminescence have been used to investigate Si and Ge cluster formation in amorphous silicon dioxide layers. Commonly, cathodoluminescence emission spectra of pure SiO 2 are identified with particular defect centers within the atomic network of silica including the nonbridging oxygen hole center associated with the red luminescence at 650 nm (1.9 eV) and the oxygen deficient centers with the blue (460 nm; 2.7 eV) and ultraviolet band (295 nm; 4.2 eV). In Ge + ion implanted SiO 2 an additional violet emission band appears at 410 nm (3.1 eV). The strong increase of this violet luminescence after thermal annealing is associated with formation of low-dimension Ge aggregates like dimers, trimers and higher formations, further growing to Ge nanoclusters. On the other hand, pure silica layers were modified by heavy electron beam irradiation (5 keV; 2.7 A/cm 2 ) leading to electronic as well as thermal dissociation of oxygen and appearance of under-stoichiometric SiOx . This SiOx will undergo a phase separation and we observe Si cluster formation with a most probable cluster diameter of 4 nm. Such largely extended Si clusters will diminish the SiO 2 related luminescence and Si crystal related luminescence in the near IR appears.

How to make silica luminescent?

“…These findings should affect quantum-confined luminescence and may explain the unusual broad IR luminescence bands of these samples as quoted already in Ref. [24]. In this context it should be mentioned that the strong stable violet (410 nm) luminescence of SiO 2 :Ge is not related to quantum confinement of these higher Ge clusters but related to lowdimensional aggregates like the Ge dimers (ODC) or trimers up to hexamer rings [14].…”

“…With further thermal annealing at higher temperatures T a 41000 1C , Si and Ge nanocrystals are formed embedded in the amorphous SiO 2 matrix. The nanocluster sizes grow with annealing temperature from 1-7 nm at T a ¼ 900 1C to 2-12 nm at T a ¼ 1100 1C producing then new luminescence bands in the near-IR region, then partly related to quantum confinement effects [24].…”

“…4 and Ref. [24]. The nanocluster sizes are growing with annealing temperature and distributed from 1 to 7 nm with a maximum of 3 nm at T a ¼ 900 1C and 2-12 nm with a maximum of 6 nm at T a ¼ 1100 1C.…”

Cathodoluminescence of non-stoichiometric silica: The role of oxygen

“…The initial values with x = 2.04 are somewhat higher than that of stoichiometric silica (x = 2.00) but we suppose an excess of oxygen on interstitial sites within the silica network still remaining from the oxidation process and the thermal diffusion of oxygen through SiO 2 towards the interface to the Si substrate. In order to demonstrate the formation of Si nano-clusters in the SiO 2 matrix under very intensive electron bombardment we used direct transmission electron microscopy (TEM) imaging in cross section technique [4,13] as well as IR-CL spectra [13] shown in Fig. 5.…”

Formation of Si‐nc by Reactive Magnetron Sputtering

“…Thus, a considerable effort was put in to produce and study Si nanostructures consisting of Si nanograins embedded in a silica matrix in order to understand the physical processes underlying the visible emission and to define future applications. Several methods have been used for fabricating such materials and consist in (i) implantation of Si þ ions in thermally grown SiO 2 [2][3][4][5][6][7][8][9][10], (ii) laser-induced decomposition of gas precursors [11][12][13][14], (iii) plasma-enhanced chemical vapor deposition [15][16][17][18][19][20], (iv) magnetron cosputtering [21,22], (v) porous silicon [1,[23][24][25], and (vi) evaporation [26,27]. The visible emission observed has been attributed to a quantum confinement (QC) effect of photogenerated carriers in the nanoscale silicon particles [28] and has been thoroughly analyzed theoretically [29][30][31][32].…”