The Wonderbra Mechanism--
Deep to Shallow Donor Conversion

  The N_iO_2i defect exhibits the Wonderbra Mechanism [223], a method of converting a deep level to a shallow one through electrostatic compression. In isolated N_i, there are two lone pair orbitals, a filled p- orbital located on the N, and a single occupied p- orbital on its Si neighbour. This Si orbital normally leads to a deep state. In the presence of a single O_i, forming N_iO_i, the level remains deep, since the single electron is able to distribute most of its wavefunction to the lobe furthest from the O_i. However in N_i-O_2i this orbital is squeezed between the two highly negative oxygen atoms. This pushes the energy of the level up to become a shallow donor level.

There are many attractive forces holding the defect together, including the dilated bonds which allow tensile strain relief, and large local polarisation of the lattice due to the electronegative N_i and O_i atoms. The N_i draws charge away from the central Si core, leaving it overall positively charged, despite the electron in the p- orbital. This attracts the oxygen atoms and forms a weak dative bond, inadvertently causing local electrostatic repulsion between the single electron on the Si p- orbital and the oxygen atoms. The drawing of charge to N_i from its Si neighbours also acts as an extra initial attraction for O_i attack, in addition to the dilated bonds.

Figure 8.3 shows the pseudo-wavefunction of the partially filled shallow level of the neutral N_i-O_2i defect. The occupation of this orbital is anti-bonding in nature, suggesting that there is repulsion from the oxygen lone-pairs, which will also contribute to the elevation of the Si dangling bond state to a shallow level. There must then be a bonding orbital lying below this state and this shows that the O atoms are to some extent overcoordinated. Therefore the transformation of the deep to shallow level occurs through a combination of electrostatic repulsion together with some dative bonding of the Si radical with the oxygen atoms.