Zinc Oxide Nanowire Films: Solution Growth, Defect States and Electrical Conductivity - Advanced Materials for Agriculture, Food and Environmental Safety - page 468

they are creating a shallow level in band structure (defect states should be
near to conduction band minima), donor defects must have low formation
energy (defect states should be abundant) and electron killer center must
have high formation energy [26]. h ough many studies have proposed that
V O and Zn i are responsible for n-type conductivity, some theoretical pro-
posals have said that oxygen vacancies are deep rather than shallow and
cannot contribute to conductivity [135]. It has also been proposed that
zinc interstitial is thermally unstable and has high formation energy, thus
it cannot help in electrical conduction, though it is a shallow donor. Janotti
et al. have suggested that two forms of hydrogen act as electrically active
impurities, interstitial hydrogen which attaches to an oxygen host atom,
and substitutional hydrogen on an oxygen site, both of which are strong
candidates for n-type conductivity [27].
h e experimental studies have also given mixed results about n-type
doping in ZnO. In 1941, Miller showed experimental result about the elec-
trical conductivity of ZnO and proposed that the conductivity in the lower
temperature is due to the ionization of interstitial zinc atom pairs, whose
ionization energy is 20 mV or less. In the higher temperature range the
conductivity is caused by the ionization of single interstitial zinc atoms,
whose ionization energy is found to be 700 mV. He also observed free
electron density of ZnO about 10 15 cm -3 at room temperature by hall mea-
surement [134]. In 1957, h omas found that pairs of positive (zinc) plus
negative (oxygen) ion vacancies with excess oxygen ion vacancies are capa-
ble of reproducing diversii ed electrical properties in ZnO [136]. Heating
of ZnO crystal in zinc vapor followed by rapid quenching results in zinc
interstitial and facilitates n-type conductivity [132]. A zinc self-dif usion
process study indicated that oxygen vacancies are probably the intrinsic
ionic defects responsible for n-type conductivity in reducing atmospheres
[137]. High-energy electron irradiation in ZnO produces shallow donors
at about 30 meV below the conduction band and the donor is identii ed
as a Zn-sublattice defect, most likely the interstitial Zn i or a Zn i -related
complex [138]. Characterization of ZnO by admittance spectroscopy has
shown that residual donors such as Zn interstitials and oxygen vacancies
are responsible for n-type conductivity.[139] Impurity atoms such as Al, In
and Ga are shown as n-type dopant by secondary ion mass spectroscopy
(SIMS) analysis [140]. Dissociated in water, O -2 and H + are also considered
as sources of electrons in ZnO crystal. Some experimental reports also
support hydrogen as a cause for n-type conductivity [141]. However, Kim
and his group have given strong justii cation against the hydrogen assump-
tion that a high concentration of electron carriers is observed, even when
H contamination is minimized or when H is removed. h ey illustrated