Thermal annealing is an important postgrowth treatment necessary to be performed before proceeding to further processing. For some crystals, whether optical or electronic, strains develop during growth and can be reduced by thermal annealing. During thermal annealing where the crystals are heated at a particular rate to a particular temperature, kept there for a particular time duration and then slowly cooled down to room temperature. Thumb rule for thermal annealing is to anneal at a temperature 2/3 of the melting point (taken in degrees Kelvin) of the material. At this temperatures the defects (dislocations) become mobile and the crystal "heals" itself Annealing can be done either in high vacuum or in the presence of inert atmosphere,i.e., in presence of argon or nitrogen atmosphere.
Many of the useful crystals consist of more than one component, ie. compound crystals, for example, CdS, CdSe, GaAs, InP etc. In the cases where the vapor pressure of one of the component is higher compared to the other, a deficit of the more volatile component occurs. To prevent such deviations from the stoichiometric composition we anneal such crystals in an atmosphere containing an excess of the component having the higher vapor pressure. For example CdS can be annealed in sulfur atmosphere, GaAs can be annealed in As atmosphere.