Amorphous silicon is one of the key materials in the manufacturing of next-generation solar panels and flat-screen televisions. A recent study carried out by researchers from the University of Girona, with the support of laboratories operated by the University of Barcelona and the French National Centre for Scientific Research (CNRS) has revealed that the energy of amorphous silicon – the state in which it exhibits the greatest stability – is 50% lower than the value commonly accepted until now. According to the researchers, this information is important for understanding the structure of the material and improving its properties.
Unlike crystalline materials, in which atoms are found in ordered arrangements, amorphous solids do not have a clearly defined structure. While each atom in an ordered configuration has only a single possible position, the atoms in an amorphous structure can shift to different positions to adopt multiple arrangements with different energy levels. According to a theory published at the end of the 1980s, amorphous silicon could only exist above a minimum degree of disorder. The most ordered configuration, known as the relaxed state, gives the material greater stability and reduces the variability of its properties over time. Consequently, deposition techniques for amorphous silicon thin films are designed get as close as possible to the relaxed state.
The study has revealed that the energy of amorphous silicon is 50% lower than the value commonly accepted until now.
Despite the importance of theoretical prediction, until now the energy of silicon in the relaxed state had not been experimentally determined. In the study published in the specialist journal Physica Status Solidi–Rapid Research Letters, differential scanning calorimetry was used to measure the energy of 20 samples grown by several deposition techniques. It was found that although different values were obtained for samples deposited in the same way, the minimum value coincided for all deposition techniques. This fact, together with observations based on previous studies, has led to the conclusion that the value of the minimum energy corresponds to the relaxed state. The value obtained is 50% lower than the standard figure accepted until now and is a crucial finding for specialists in amorphous silicon structure, since theoretical models will be more or less realistic depending on their proximity to this value.
Finally, the results corroborate the experimental findings up to this point, which indicate that the best films are those obtained from the vapour phase and whose structure includes hydrogen atoms.
Kail, F.; Farjas, J.; Roura, P.; Secouard, C.; Nos, O.; Bertomeu, J.; Roca i Cabarrocas, P. “The configurational energy gap between amorphous and crystalline silicon”. Physica Status Solidi – Rapid Research Letters, September 2011, vol. 5, p. 361-363. DOI: 10.1002/pssr.201105333.