How vacancies in silicon affect its electronic propertySolut

How vacancies in silicon affect its electronic property?

Solution

The simplest point defect is a vacancy. This refers to an empty (unoccupied) site of a crystal lattice, i.e. a missing atom or vacant atomic site [Fig. 2 (a)] such defects may arise either from imperfect packing during original crystallisation or from thermal vibrations of the atoms at higher temperatures. In the latter case, when the thermal energy due to vibration is increased, there is always an increased probability that individual atoms will jump out of their positions of lowest energy. Each temperature has a

corresponding equilibrium concentration of vacancies and interstitial atoms (an interstitial atom is an atom transferred from a site into an interstitial position). For instance, copper can contain 10^-13 atomic percentage of vacancies at a temperature of 20-25°C and as many as 0.01 % at near the melting point (one vacancy per 10⁴ atoms). For most crystals the-said thermal energy is of the order of I eV per vacancy. The thermal vibrations of atoms increases with the rise in temperature. The vacancies may be single or two or more of them may condense into a di-vacancy or trivacancy. We must note that the atoms surrounding a vacancy tend to be closer together, thereby distorting the lattice planes. At thermal equilibrium, vacancies exist in a certain proportion in a crystal and thereby leading to an increase in randomness of the structure. At higher temperatures, vacancies have a higher concentration and can move from one site to another more frequently. Vacancies are the most important kind of point defects; they accelerate all processes associated with displacements of atoms: diffusion, powder sintering, etc.