  We present a comprehensive study of vacancy and vacancy-impurity complexes in
InN combining positron annihilation spectroscopy and ab-initio calculations.
Positron densities and annihilation characteristics of common vacancy-type
defects are calculated using density functional theory and the feasibility of
their experimental detection and distinction with positron annihilation methods
is discussed. The computational results are compared to positron lifetime and
conventional as well as coincidence Doppler broadening measurements of several
representative InN samples. The particular dominant vacancy-type positron traps
are identified and their characteristic positron lifetimes, Doppler ratio
curves and lineshape parameters determined. We find that In vacancies and their
complexes with N vacancies or impurities act as efficient positron traps,
inducing distinct changes in the annihilation parameters compared to the InN
lattice. Neutral or positively charged N vacancies and pure N vacancy complexes
on the other hand do not trap positrons. The predominantly introduced positron
trap in irradiated InN is identified as the isolated In vacancy, while in
as-grown InN layers In vacancies do not occur isolated but complexed with one
or more N vacancies. The number of N vacancies per In vacancy in these
complexes is found to increase from the near surface region towards the
layer-substrate interface.
