Zinc Oxide Nanowire Films: Solution Growth, Defect States and Electrical Conductivity - Advanced Materials for Agriculture, Food and Environmental Safety - page 467

Figure 16.3b demonstrates that most accepted three peak i tting is related
to green (P1), yellow (P2) and red emissions (P3). Green emission is the
most commonly observed defect emission in ZnO nanostructures, which is
ot en attributed to singly ionized oxygen vacancies [25]. Other opinions for
green emission as donor-acceptor transitions, recombination at V o centers
(where these centers are generated by surface trapping of photogenerated
holes, followed by recombination with electron in an oxygen vacancy (V o ),
zinc vacancy, and surface defects are also considered [125]. Yellow emis-
sion is typically attributed to doubly charged oxygen vacancies, sometimes
oxygen interstitial or impurity represents another possible candidate [131],
whereas red emission is mainly attributed to oxygen interstitials.

16.4 Role of Defect States in Electrical Conductivity

of ZnO

16.4.1 Defect States Responsible for n-Type Conductivity

h e conductivity of a semiconductor can be given as:

− E

2 3

( )

g

= CT q + exp

(16.6)

2 KT

e e h

where μ e and μ h is the mobility of electron and hole, E g is the band gap of
semiconductor and T is the temperature. At T = 0, there are no electrons
in the conduction band, and the semiconductor does not conduct (lack of
free charge carriers). At T > 0, some fraction of electrons have sui cient
thermal kinetic energy to overcome the gap and jump to the conduction
band [132, 133]. A semiconductor will only have free carriers for conduc-
tion if the impurity atoms/defects are ionized. h e ionization of atoms
leads to electrons jumping across ΔE into the conduction band from donor
or accepter level and govern the electron conduction. Ionization is a pro-
cess by which free charge carriers (electrons or holes) are produced in a
semiconductor [134]. Shallow level states require less ionization energy,
while deep level states need high ionization energy. Impurity atoms or
defect states contribute to electrical conductivity when they form a shal-
low state in band structures. Native defect can also create shallow level
states in semiconductor; in the case of ZnO, oxygen vacancies and/or zinc
interstitial are potential candidates for n-type conductivity, while ZnO is
inherently non-conducting. Native defects can produce n-type doping if