Читать книгу Green Energy - Группа авторов - Страница 27

When comparisons are done on consideration of the price between crystalline wafers and thin-film technology, the latter does not demand the expensive sawing and crystal traction. This technology as well restricts the losses due to kerf, which is normally about 100 mm per cut of wafer and is an added benefit as compared to c-Si technology. In its manufacturing process, layers of silicon are allowed to get deposited on the glass or on other very low cost material at the temperature which should be very low, even below the melting temperature of c-Si. It is quite possible to have other types of silicon films that are electronically very active and are deposited with the help of plasma on the surface of the Si and H and most of them contain silane. The commonly available scheme is plasma-enhanced chemical vapour deposition (PECVD) in which reactants the kept over the electrodes of a structure made of parallel plate and that too within a reactor chamber with the vacuum. Thereafter, precursor gases having controlled flows are allowed to enter into the area where RF power, of frequency 13.56 MHz, is introduced to the electrodes. Sometimes, frequencies of greater discharge are also used in PECVD technique. Generally, thin-film type silicon is amorphous as compared to FCC c-Si lattice and the earlier contains a large amount of H, which gets deposited in due course. The atoms of silicon try to retain its tetrahedron shape due to covalent bonds with four other atoms of silicon. Because of the distortions in the bond angle and also due to the dangling nature of bonds, distortions are normally observed in the long and medium range. Uniformity of structures is often found in tiny volumes and because of the principle of uncertainty, momentum of the charge could not be determined at a fixed spatial location [9]. The family of Si thin-film also involves mc-Si:H, nitrogen and oxygen compounds, and alloys of carbon or germanium. To make it more precise, it should be understood that it’s not only material, yet it forms mixed phase class of materials with the wider scale of very small-scale structures of micron level where phases are mixed and the conditions are mainly dependent on the rate of deposition and the substrate. The process of deposition begins with a phase of amorphous and a minimum thickness is needed before the nucleation process starts. At the occurrence of nucleation phase, the crystal begins to grow around the deposits. When the alloying of silicon is done with germanium, the band gap becomes narrower as the content of germanium is increased. It is found that lower values of the content of germanium is not feasible as it is very poor electronically while when oxygen or carbon is introduced, the band gaps above 1.7 eV becomes feasible.