Zinc Oxide Nanowire Films: Solution Growth, Defect States and Electrical Conductivity - Advanced Materials for Agriculture, Food and Environmental Safety - page 469

that annealing at high temperature at 1100 C will dif use out H and will

not make any contribution in conductivity, but experimental results have
shown high electrical conduction [28]. h ey supported the premise that
O-dei cient ZnO can easily become n-type even without the introduction
of any intentional dopants and a metastable shallow-donor state of V O was
suggested as an alternative source of the n-type doping. h e cause of n-type
intrinsic doping is very ambiguous; no clear correlation is observed among
various theoretical and experimental studies and in-depth investigations
are still needed.

°

16.4.2 Defect States Responsible for p-Type Conductivity

P-type doping of ZnO is still challenging. If we are looking at doping with
respect to native defects, the following condition must be satisi ed: Acceptor
defects (O i , V zn ) must have shallow levels (defect states should be near to
valence band maxima) and formation energy for acceptor defect must be
lower than donor defects. Even ZnO has p-type acceptor defects (O i , V Zn )
which can supply holes at room temperature. Despite the fact that they are
shallow, p-type doping is not possible because hole-killing defects have low
formation energy and form readily in both Zn-rich and O-rich conditions
and can easily compensate p-type doping (V O , Zni) [26]. h us, this asym-
metry of intrinsic n-type versus p-type doping leads to n-type conductiv-
ity in ZnO and kills the possibility of p-type doping. Many other external
dopants like impurities of P and As or nitrogen and formation of N Zn -V O
complex or Zr, and N co-doping are theoretically suggested for p-type dop-
ing [142-145]. While in experimental works nitrogen, lithium, sodium,
potassium, phosphorous, arsenic and bismuth, are shown as p-type dopants
in ZnO [146-156]. Control p-type conductivity in sputter ZnO i lm is
also demonstrated by adjusting the oxygen partial pressure in the sputter-
ing plasma [146]. h us, various theoretical and experimental results dem-
onstrate that p-type doping is possible in ZnO but needs more systematic
experimental work and deep investigation to achieve a good control. Stability
and reproducibility are the existing challenges with p-type doping in ZnO.

16.5 Defects and Electrical Conductivity of ZnO

Nanowire Films

Nanowire (NW) i lms have all favorable properties for various optoelec-
tronic applications, but poor electrical conductivity of these nanowire i lms