  Deep level transient spectroscopy (DLTS) is used extensively to study defects
in semiconductors. We demonstrate that great care should be exercised in
interpreting activation energies extracted from DLTS as ionization energies. We
show how first-principles calculations of thermodynamic transition levels,
temperature effects of ionization energies, and nonradiative capture
coefficients can be used to accurately determine actual activation energies
that can be directly compared with DLTS. Our analysis is illustrated with
hybrid functional calculations for two important defects in GaN that have
similar thermodynamic transition levels, and shows that the activation energy
extracted from DLTS includes a capture barrier that is temperature dependent,
unique to each defect, and in some cases large in comparison to the ionization
energy. By calculating quantities that can be directly compared with
experiment, first-principles calculations thus offer powerful leverage in
identifying the microscopic origin of defects detected in DLTS.
