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Unformatted text preview: T. Y. Tan 1 17. POINT DEFECTS IN SEMICONDUCTORS As in metals and ionic crystals, the native atomic point defects in semiconductors are lattice vacancies and lattice atom self-interstitials. In Chapter 15 it has been mentioned that for metals whether the point defect species are charged or not is unimportant, because in metals the electron concentration is at least as high as the lattice atom density and there is little, if not at all, possibil- ity that in an experiment the electron concentration is noticeably altered. It has also been men- tioned that a point defect species in an ionic crystal is always charged, because of space-charge neutrality requirement. In contrast, in semiconductors a point defect species such as vacancies in Si can adopt more than one charge states, i.e., neutral, singly-negatively charged, etc. The intrin- sic carrier concentration n i in a semiconductor can be fairly low at even high temperatures, and it can be doped to become extrinsic with a majority carrier concentration exceeding n i by many or- ders of magnitude. Thus, while the concentrations of the neutral point defects are not influenced by doping in thermal equilibrium, in response to the semiconductor majority carrier type and concentration, the thermal equilibrium concentrations of the charged point defect species can be greatly altered in accordance with their charge type and number. The formation of charged point defects consumes charge carriers. Charged point defects are apparently deep level defects in group IV and III-V semiconductors, e.g., Ge, Si, and GaAs, respectively. However, they can also be shallow level defects in certain II-VI compound semiconductors. A compound semiconductor, e.g., GaAs, has a thermal equilibrium composition range, in equilibrium coexistence with a num- ber of vapor phases, each with a thermal equilibrium pressure range, and possibly also one of two liquid (or solid) phase materials. The composition deviation of a compound semiconducting crystal from the A 0.5 B 0.5 stoichiometry is consisting of the sum of the thermal equilibrium con- centrations of primarily its (six kinds) single point defect species. Clearly, each point defect spe- cies must also have a concentration range, in equilibrium with all other A-B materials present, including the vapor phases and possibly also a liquid or solid material that is not the AB com- pound. In this chapter the situation associated with the III-V compound GaAs with be used as an example for some fairly detailed discussions. 17.1 Neutral Point Defects in Elemental Semiconductors For the elemental semiconductors Ge and Si, the expression describing the fractional thermal equilibrium neutral point defect concentration of the species x o is given by T. Y. Tan 2 c x o eq = exp - g x o f k B T , (17.1) where x o denotes vacancies V o or self-interstitials I o . The definition of the Gibbs free energy of formation of x o , g x o f , is the same as that for metals. For V o it is the crystal Gibbs free energy...
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