  Activation of shallow acceptor state has been observed in ion irradiated and
subsequently air annealed polycrystalline ZnO material. Low temperature
photoluminescence (PL) spectrum of the sample exhibits clear signature of
acceptor bound exciton (ABX) emission at 3.360 eV. The other two samples,
pristine and ion irradiated (without annealing), however, do not show acceptor
related PL emission in the nearby energy region. Electron transition from
shallow donor (most probable site is interstitial zinc for undoped ZnO) to such
newly formed shallow acceptor state creates new donor-acceptor pair (DAP)
luminescence peak ~ 3.229 eV. ABX and DAP peak energy positions confirm that
the acceptor is N related. The acceptor exciton binding energy has been
estimated to be 180 +/- 15 meV which is in conformity with earlier reports. The
activation of shallow acceptors without any source of atomic nitrogen can only
be possible through diffusion of molecular nitrogen inside the sample during
annealing. The N2 molecules get trapped at bulk defect sites incorporated by
ion irradiation and subsequent annealing. X-ray diffraction (XRD) and Raman
spectroscopic (RS) investigation have been employed to probe the changing
defective nature of the ZnO samples. Irradiation induced increased disorder has
been detected (both by XRD and RS) which is partially removed/modified by
annealing at 300 oC. Simultaneous activation of molecular nitrogen acceptor in
purposefully defective ZnO is the key finding of this work. Results presented
here provide a simple but controlled way of producing shallow acceptor state in
ZnO. If optimized through suitable choice of ion, its energy and fluence as
well as the annealing temperature, this methodology can trigger further scope
to fabricate devices using ZnO epitaxial thin films or nanowires.
