Core-Level Shift of Si Nanocrystals Embedded in a SiO₂ Matrix

Abstract

It is expected from existing theories that the core level of Si nanocrystals (nc-Si) embedded in a SiO2 matrix should shift toward a higher binding energy as compared to that of bulk crystalline Si due to quantum size effect. Indeed, it is observed in X-ray photoemission experiments that the Si 2p core level shifts to a higher apparent binding energy by 1−2 eV for all five oxidation states of Si n + (n = 0, 1, 2, 3, and 4) in the material system of SiO2 containing nc-Si. However, it is found that the core-level shift is due to a charging effect in the material system. After correction for the charging effect by using C 1s binding energy due to contamination on the SiO2 surface, the core level of the oxidation state Si4+ is the same as that of pure SiO2, whereas the core level of the isolated nc-Si with an average size of about 3 nm shifts by ∼ 0.6 eV to a lower binding energy as compared to that of bulk crystalline Si. It is suspected that the core-level shift of the nc-Si toward a lower binding energy is due to the influence of the differential charging between the SiO2 surface layer and the nc-Si underneath.

An experimental study of charge distribution in crystalline and amorphous Si nanoclusters in thin silica films

“…The reduction in intensity of the Si 0 peak may be attributed to oxidation due to residual oxygen in the nitrogen atmosphere. 8 Comparison of the as-grown sample with the sample annealed at 1100°C for 30 min showed that annealing shifted the Si 2p 0 peak position to a lower binding energy at 0.2 eV ͑Ϯ0.1͒. This decrease in binding energy between the as-grown sample and the heat-treated sample with a silicon fraction of 70% is also observed in samples with 50 and 60% silicon.…”

“…6,7 The main expectations include longer retention, lower gate voltage, and lower power consumption. 8 In addition, the discontinuity between the nanocrystals can prevent lateral charge loss and can also result in short writing times at lower voltages, as well as improved reliability. 9,10 Both injection and retention of electrons in these devices are very sensitive to the size, distribution, interfaces, and electronic structure of the nanocrystals.…”

“…The majority of them was focused on the position and energy shift of the Si 2p 0 peak ͑where Si 0 is Si in the elemental state͒. 8,[12][13][14][15] Studies of the planar SiO 2 / Si͑100͒ interface attributed the shift either to final state relaxation effects 13 or to enhanced differential charging by application of a negative bias to the substrate. 9 Sun et al 14 concluded that the shift is a result of an enlargement of the band gap due to surface imperfections at the Si nanocrystal-oxide interface.…”

Quantum Confinement, Surface Roughness, and the Conduction Band Structure of Ultrathin Silicon Membranes

“…Moreover, in contrast to the assumption in our calculation, the Si 2p core levels may shift somewhat with changing thickness, relative to the vacuum level. 26 Strain can also shift and split levels. 27 There is evidence that the Si template layer contains a small amount of innate strain, perhaps as much as 0.3%, 28 because of the Si/oxide interface.…”

Characterization of Si Nanocrystals

“…This decrease suggests that Si-nc shell is mainly formed from suboxides and all the detectable SiO 2 bonds are replaced to form the shell of Si nanocrystals and Al 2 O 3 matrix [47]. In a similar study, Liu et al Chen et al measured a decrease of 0.6 eV in the binding energy of the core electrons of nanocrystalline Si with respect to crystalline Si , which is in the opposite direction than expected from the quantum confinement effect [57]. They used the C 1s signal exists due to the carbon contamination on the surface of the films as reference.…”