Electronic properties of deep defects in n-type GaN

The recurrent presence of deep levels in n-type hexagonal GaN films grown by metal organic chemical vapour deposition (MOCVD) is testified by Fourier transform deep level transient spectroscopy (FTDLTS) studies performed in various samples. Some of these electron traps are systematically detected with emission rates showing activation energies in the range 0.94–1.30 eV and apparent capture cross sections from 10−15 to 10−12 cm2. These properties are deduced from an Arrhenius diagram where data coming from samples of different origins are close to the same straight line, assessing a common physical identity for the corresponding defects. However, isothermal spectroscopy and trap filling kinetics characteristics contradict these capture cross section values and even show nonmonotonic variations which must be ascribed to the emission and capture rates of interactive multiple deep levels. Fine structure of the energy levels, although it is never resolved by standard FTDLTS, is indeed revealed by a high resolution DLTS method (HRDLTS) which relies on a time domain treatment of the transients recorded over a large range of time. Multiple levels are well evidenced in this way and both their ionisation energy and capture cross sections turn out to be much less than those measured by standard FTDLTS, respectively in the ranges 0.26–0.90 eV and near 10−17 cm2 on the one hand or less than 10−20 cm2 on the other hand, suggesting that these defects can capture two electrons, leading to three charge states and negative charges at least when filled with 2 electrons. Correlation of the concentration of these defects both with the shallow donor concentration and dislocation density suggest that they are due to the doping impurities in dislocation sites.