  Although zinc oxide is a promising material for the fabrication of short
wavelength optoelectronic devices, p-type doping is a step that remains
challenging for the realization of diodes. Out of equilibrium methods such as
ion implantation are expected to dope ZnO successfully provided that the
non-radiative defects introduced by implantation can be annealed out. In this
study, ZnO substrates are implanted with nitrogen ions, and the extended
defects induced by implantation are studied by transmission electron microscopy
and X-ray diffraction (XRD), before and after annealing at 900^{\circ}C. Before
annealing, these defects are identified to be dislocation loops lying either in
basal planes in high N concentration regions, or in prismatic planes in low N
concentration regions, together with linear dislocations. An uniaxial
deformation of 0.4% along the c axis, caused by the predominant basal loops, is
measured by XRD in the implanted layer. After annealing, prismatic loops
disappear while the density of basal loops decreases and their diameter
increases. Moreover, dislocation loops disappear completely from the
sub-surface region. XRD measurements show a residual deformation of only 0.05%
in the implanted and annealed layer. The fact that basal loops are favoured
against prismatic ones at high N concentration or high temperature is
attributed to a lower stacking fault energy in these conditions. The
coalescence of loops and their disappearance in the sub-surface region are
ascribed to point defect diffusion. Finally, the electrical and optical
properties of nitrogen-implanted ZnO are correlated with the observed
structural features.
