Acceptor ionization energy

Table 5.3 Donor and acceptor ionization energies in silieon and germanium...

Xiu et al. [23] reported in Bi-doped ZnO films by molecular-beam epitaxy that Bi-induced acceptor ionization energy was estimated to be 0.185-0.245 eV by photoluminescence measurements based on the donor-acceptor pair peak position in the Bi-doped ZnO films Bi in ZnO films had positive charge state determined by X-ray photoelectron spectroscopy measurements, indicating that BiZn at Zn sites, rather than Bio at O sites, was formed in the films. BiZn itself, however, is a donor. The origin of the shallow acceptor states was, therefore, identified as a donor-acceptor pair such as BiZn-VZn-0, or BiZn-2VZn complexes. 

We roughly attempted to compare the acceptor ionization energies between the experimental and calculated values, for the monovalent substitutional impurities of group-VA elements at the host O site. The comparison was made for group-VA elements because of the existence of the most reliable data for them. The experimental values [20,22,29-31] of the acceptor activation energies for group-VA elements were already described in Section 4.1 to be 0.170-0.200 eV for N,... 

The calculated transition energies of the last column are the difference between the ground and excited states energies (in parentheses) calculated by [61]. The acceptor ionization energies are obtained by adding 15.4 meV to the energy of the 2Pb/2 (Ts) transition... 

The activation energy Ei is the normal acceptor ionization energy, associated with transitions from the acceptor ground state to the valence band, and is observed in all samples provided the acceptor concentration is not too high. The activation energy E2 can be observed only in the intermediate concentration range and is associated with conduction in an impurity band. When the acceptor concentration is small 2 is close to Ei, but when the acceptor concentration is increased, so that there is an appreciable overlap between the wave functions of neighboring... 

When a lightly boron-doped layer is grown on top of an oriented textured film with 100 faces, mobility values up to 165 cm V s may be obtained (38) and the acceptor ionization energies for such films are around 0.35 eV, close to the value for natural type lib diamond. 

Although nitrogen has been considered as the best candidate for p-type doping for ZnO, the point should be remade that the solubility of N in ZnO is low and it introduces a relatively deep acceptor level. Therefore, it is necessary to find methods that can enhance the solubility of N in ZnO and lower the acceptor ionization energy. 

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