Experimental evidence for semiconducting behavior of Si-XII

The conventional diamond cubic phase of silicon, so-called Si-I, exhibits the desirable semiconducting properties on which the silicon chip industry relies and it is by far the most stable of all silicon phases. However, other phases of silicon can be formed under pressure and some of them are metastable at room temperature and pressure. Two such phases, Si-III (BC8) and Si-XII (R8), can be produced by indentation with a diamond tip but, despite an understanding of their structure, little is known about their electrical properties. As we demonstrate experimentally, such phases can have entirely different (electrical) properties to normal (diamond cubic) silicon, consistent with recent theoretical studies that predict Si-XII to be a narrow-band-gap semiconductor and Si-III to be a semimetal. We report here electrical measurements on the Si-XII phase and demonstrate that it is indeed a semiconductor. Furthermore, and somewhat surprisingly, both boron and phosphorus can be electrically activated in the Si-XII structure during its formation by indentation at room temperature.