Carbon-acceptor pair centers (X centers) in silicon

Abstract

An electrically active complex called the X center has long been known in indium-doped silicon. Similar defects have now been associated with the other Group IIIA elements in silicon. These X centers are acceptors with energies approximately 80% of the corresponding IIIA substitutional atoms energy and with concentrations 10−2 to 10−5 of the corresponding Group IIIA concentration. Doping studies show that the X-center concentrations depend on carbon and fit the reaction As +Cs⇄K X, where As is the substitutional Group IIIA acceptor, Cs is the substitutional carbon, and K is the equilibrium constant. The equilibrium constants as a function of temperature and the complex binding energies have been determined for all of the Group IIIA X centers except Tl. The binding energy for the complex increases from 0.03 eV for the boron–X center to 0.70 eV for the indium–X center. The complexes do not anneal out up to the melting point of silicon and they can be cyclically annealed. Stress studies on the aluminum–X center show that the complex has [111] trigonal orientational symmetry. The dependence on carbon, the stress symmetry, and the high-temperature stability of these centers all suggest a model of these complexes of a nearest-neighbor substitutional carbon-acceptor pair. Mathematical modeling using extended Hückel calculations confirms that this defect should have a single energy level slightly shallower than the substitutional acceptor.