The common point between the works discussed so far is the lack of justification of the existence of the new oxidation state Si2+. All of them assume its presence from theoretical computations. Besides, the amorphous nature of SiO implies that diffraction techniques can be misleading, since they provide information about modelled crystalline structures. Characterization techniques such as Transmission Electron Microscopy (TEM) and electron diffraction have given a boost to the comprehension of SiO structure, helping at developing models including even the presence of Si+ and Si3+ valence states.
The concept of suboxide SiOx must be introduced when debating the nature of SiO. The discussion should start from what happens at the atomic scale, by comparing the configuration of the Si-O and Si-Si bonds in Si, SiOx (x:
0<x<2) and SiO2. Three models have been computed so far, one continuously improving the other after constructive criticisms. They are called random-bonding (RB), random-mixing (RM) and interface-cluster mixture (ICM) model.
Philipp [45] proposed the random-bonding (RB) model after studying optical properties of non-crystalline SiOx -films. The main assumption by the author is that each Si atom has a tetrahedral coordination to n oxygen atoms and (4-n) silicon atoms. Each value of n has its own statistical probability of existence p(n), which the model computes for different oxides and suboxides. This results into a random distribution of Si-O and Si-Si bonds in the material. Philipp uses absorption data to justify his model. Amorphous SiO films have 20 times lower absorptivity compared to amorphous silicon films. By assuming that absorption and Si coordination are related, the results match well with the model, where only one Si atom out of 16 (6.25%) is bond to four Si neighbors in solid SiO. The experimental results agree with the theory proposed. Table 2 resumes the computed values of p(n) for the RB-model.
Table 2: p(n) computed by the RB-model [45]
n SiO2 (%) SiO1.5 (%) SiO (%) SiO0.5 (%) Si (%)
The RB-model cannot justify the missing characteristic peaks of SiO2 and other sub-oxides, which appear in crystalline state, since each value of n has also its own probability of existence. Besides, the optical absorption varies with the annealing temperature of the film. Shifting of the absorption edges occurs if the extent of coordination between Si and O varies, i.e. based on the value of x in the SiOx phases formed. Tempkin [41]
includes these criticisms, formulating the random-mixture model.
The RM-model is based on calculation of peak areas for radial distribution functions (RDF) of SiOx. The theory is compared with the calculations from Yasaitis and Kaplow [46]. Tempkin computes p(n) by assuming tetrahedral Si coordination and absence of O-O bonds. The bond length and angles are based on properties of amorphous silicon and SiO2. Figure 12 resumes the five configurations Ti available for the Si-O-Si bonds. In this work, p(n) is referred as Ci. i varies from 1 to 5 and has the same meaning as n. For example, i=1 equals n=0 (Si), and i=5 equals n=4 (SiO2). Tempkin compares the RDF of commercial silicon monoxide and a microscopic Si+SiO2 mixture, finding consistency between them.
The model is inconsistent with an RB-model and a large-scale phase mixture. Yasaitis and Kaplow [46] showed that there should be two resolved peaks for Si and SiO2, if the material was a microscopic mixture into large clusters. A single peak is detected instead, justifying the mixture at the atomic level. Hydrofluoric acid leaching experiments on SiO presented by Holland [42] confirmed the nature of the mixture at atomic level. In fact, bulk SiO and SiO2 are soluble in the acid, whereas Si is not. No criticism was moved to this model. However, it needs to be improved, since it does not justify why SiO does not react as a simple mixture of Si and SiO2, even if this is present at atomic scale.
Figure 12: The five possible configurations assumed in the RM-model and their probability of existence as a function of x [41]
Ching [50] compares the random-network and random-mixture theories, by assessing atomic modelling calculation. The difference between the RM and RB-models is explained very simply in this paper. Starting from an amorphous-Si configuration, one can insert O atoms between each pair of Si atoms (Figure 13a). By rescaling the system to a cubic cell, the average density of the amorphous SiO2 generated in this way should correspond to 2.20 g/cm3. Most likely, not all the available O sites can be occupied. In this way, SiOx can be modelled. If the O sites are randomly taken, we are looking at an RB-model (Figure 13b). If they are not, we are computing a RM-model (Figure 13c).
45 The bond distortion creates a configuration, whose energy must be reduced by a potential function, which can minimize the energy. By changing the stoichiometry of Si and O, i.e. the value of x, it was seen that an RB-model gives closer results to the experimental, when x=1. This openly contrasts with Temkin’s RM-model. Ching observes flaws in Temkin’s theories. For example, Temkin assumes a constant ideal value of the O-O and Si-O-Si angle (109.5° and 144°C respectively). Besides, the Ci values are significantly different from a statistical prediction. Lastly, the radial distribution function computed by Ching correspond to other experimental results from other references, especially when the sample contains defect centers such as O-O bonds.
Figure 13: Schematic illustrations of obtaining initial configurations from the periodic model of amorphous Si:
a) amorphous SiO2; b) RB-model of SiOx; c) RM-model of SiOx [50]
Hohl et al. [47] investigated amorphous SiO utilizing diffraction, microscopy, spectroscopy and magnetometry methods. This allowed them to form a third model called interface cluster mixture model (ICM). The authors base their model mainly on the TEM analysis and pair distribution functions computed by Schulmeister and Mader [48], as well as from their own X-ray diffraction, TEM and XPS analysis. The interface cluster mixture model (ICM) states that SiO is a non-equilibrium state with three phases at different stoichiometry. Nanoclusters of amorphous SiO2 (a-SiO2) and amorphous Si (a-Si) are surrounded and connected by thin layers of SiOx (Figure 14) The size of the nanoclusters ranges between 0.5 to 2.5 nm. Due to the small cluster size, the amount of SiOx
cannot be neglected. The interface layers can gather up to approximately 10 at. % of the total silicon involved.
Sin+ atoms in the lattice form the SiOx. The authors tried to model the possible atomic interactions at the interface of two nanoclusters containing SiO2-Si, SiO2-SiO2 and Si-Si. The generation of suboxides will give the atomic coordination schemes shown in Figure 14.
Since the existence of Si+ and Si3+ ions have not been widely discussed in the literature, no certain conclusions can be drawn about the efficacy of the proposed ICM model. So far, the ICM model looks like the most complete and exhaustive model for justifying the properties of solid SiO, but further confirmation in the experimental results is necessary.
Figure 14: ICM model (left) and atomic bond configurations of SiOx at different interfaces (right) [47].
Sugiyama et al. [51] studied the brown rims formed after exposing vitreous silica to silicon melts. They used Electron Spin Resonance (ESR) to analyze the bond types of the system. The spectra revealed the presence of dangling bonds •Si≡Si3, •Si≡Si2O and •Si≡SiO2. The work suggests that the dangling bonds belong to the detected paramagnetic suboxide species in SiO1.49. A sample of commercial silicon monoxide was analyzed. Its O/Si ratio is 1.04. The same signals were found for silicon monoxide. It was concluded that both the considered materials contain the same kind of dangling bonds, despite their different composition.