Silicon metastable

As described earlier, the early studies of metastable silicon [2-7,18] probed the possibility of a liquid amorphous transition. More recent work has attempted to find evidence that the transition is one between two liquid phases. In this section, we present a brief discussion of such recent work. 

Figure 4. Schematic phase diagram of metastable silicon in the pressure-temperature (P, T) plane discussed in [20,113]. The thick solid line represents the liquid-crystal (cubic diamond) transition line, extended into the j6-Tin phase. The dotted lines represent the liquid-/S-Tin and the Cubic diamond-yS-Tin transition lines. The thin line is the liquid-liquid phase transition line ending at a critical point represented by a filled circle. The dashed lines represent the spinodals associated with the liquid-liquid transition. The oval symbol represents the amorphous-liquid transition as predicted by some of the earlier experiments. [With permission from McMillan [20,113].]...

Modifications to Precipitates. Silicon is sometimes added to Al—Cu—Mg alloys to help nucleate S precipitates without the need for cold work prior to the elevated temperature aging treatments. Additions of elements such as tin [7440-31-5] Sn, cadmium [7440-43-9] Cd, and indium [7440-74-6] In, to Al—Cu alloys serve a similar purpose for 9 precipitates. Copper is often added to Al—Mg—Si alloys in the range of about 0.25% to 1.0% Cu to modify the metastable precursor to Mg2Si. The copper additions provide a substantial strength increase. When the copper addition is high, the quaternary Al CuMg Si Q-phase must be considered and dissolved during solution heat treatment. 

Metal dusting usually occurs in high carbon activity environments combined with a low oxygen partial pressure where carburisation and graphi-tisation occur. Usually pits develop which contain a mixture of carbon, carbides, oxide and metal (Fig. 7.52). Hochmann" proposed that dusting occurs as the result of metastable carbide formation in the high carbon activity gas mixture which subsequently breaks down into metal plus free carbon. The dependence of the corrosion resistance of these nickel alloys on the protective oxide him has been described accelerated or internal oxidation occurs only under conditions that either prevent the formation, or lead to the disruption, of this him. In many petrochemical applications the pO is too low to permit chromia formation (ethylene furnaces for example) so that additions of silicon" or aluminium are commonly made to alloys to improve carburisation resistance (Fig. 7.53). 

In addition to the thermal CVD reactions listed above, tungsten can be deposited by plasma CVD using Reaction(l)at350°C.[ ll P At this temperature, a metastable alpha structure (aW) is formed instead of the stable be.c. Tungsten is also deposited by an excimer laser by Reaction (1) at < 1 Torr to produce stripes on silicon substrate.P l... 

In addition, silicon adopts a number of metastable structures that can be obtained, depending on pressure, by rapid release of the pressure from Si-II, Si-XII is formed, and from this Si-III upon heating, Si-III transforms to the hexagonal diamond structure (Si-IV). Si-III has a peculiar structure with a distorted tetrahedral coordination of its atoms. The atoms are arranged to interconnected right- and left-handed helices (Fig. 12.7). The structure being cubic, the helices run in the directions a, b as well as c. Si-VIII and Si-IX... 

The Si=P bond is significantly less polar than the Si=N bond, thereby providing greater kinetic stability and, at the same time, lower Lewis acidity on silicon. However, the isolation of moderately metastable phos-... 

The familiar diamond structure, with each atom covalently bonded in a perfect tetrahedral fashion to its four neighbors, is adopted not only by C but also by Si and Ge. Silicon can also adopt a wurtzite structure (see below), an example of a polytype (one of several crystal structures possible for a substance having an identical chemical composition but differing in the stacking of layers, and which may exist in a metastable state after its formation at some different temperature or pressure). 

Table III summarizes the reported relative energies appropriate to the BC, Si—AB, and B—AB sites for the H—B pair in silicon. In many calculations, the B—AB site has been predicted from calculations to be not only higher in energy than the BC site but also to be a saddle point for reorientation between equivalent BC sites. Some calculations predict that the Si—AB site may be metastable, consistent with evidence from channeling and PAC studies. The computed total-energy differences, however, between the stable (BC) and metastable (Si—AB) configurations might be too large to explain the simultaneous observation of both species at the reported temperatures.

One of the earliest studies was an MNDO-cluster treatment by Corbett et al. (1983) of the isolated interstitial hydrogen and the corresponding molecule. In this early study, the isolated H was found to be stable at the M-site in silicon. This is directly between two adjacent C-sites, where the C-site is directly between next-near-neighbor silicons. (We note that in these calculations, the C- and M-site energies are very similar.) It was not known at that time, however, that the BC site is the stable location for neutral isolated interstitial hydrogen (see Chapter 16). In the Corbett study, an H2 molecule was found to be stable (or at least, metastable) in the tetrahedral interstitial site when oriented along a (111) direction. The... 

The section on silicon is followed by a relatively brief discussion of muonium in other semiconductors. The /xLCR study of Mu in GaAs is noteworthy because again it permits a detailed model to be inferred. The important observation of the Mu— Mu transition in diamond and the unusual metastable centers in CuCl and CuBr also will be discussed. The main emphasis of this chapter will be on developments in the field since the extensive review by Patterson (1988), which covered the field up to December 1986. Other reviews on muonium in semicondutors that may be of... 

In high purity silicon below T = 140 K the Mu center is stable on the time scale of the muon lifetime (2.2 ps). However, in electron irradiated silicon Westhauser et al. (1986) have reported that Mu is metastable and makes a thermally induced transition to Mu at a temperature of 15 K. A similar transition between Mu and Mu was first discovered in diamond (Holz-schuh et al., 1982, Odermatt et al., 1988) and will be discussed in Sec-... 

The close correspondence between the properties of Mu in Si as determined by /u,SR and pLCR and those for the AA9 center produced by implanting hydrogen in silicon shows that Mu in silicon and the AA9 center are isostructural and in fact almost identical. They are neutral isolated bond-centered interstitials. Numerous theoretical studies support this conclusion. The observation of such similar centers for muonium and hydrogen supports the generalization that hydrogen analogs of many of the muonium centers exist. Of course, this assumes that the effects of the larger zero-point vibration of the muon relative to the proton do not make a major contribution to structural differences. The p-SR experiments, reinforced by theory, demonstrate that another structure also exists for muonium in silicon, called normal muonium or Mu. This structure is metastable and almost certainly is isolated neutral muonium at a tetrahedral interstitial site. 

---