Magnetic resonance studies of defects in GaN with reduced dislocation densities

Magnetic resonance experiments, including optically detected magnetic resonance (ODMR) and electron paramagnetic resonance (EPR), have been performed on Si-doped homoepitaxial GaN layers grown by MOCVD and on high quality, free-standing (B200mm-thick) GaN grown by HVPE. This allowed us to obtain information on the properties of native defects and dopants in GaN with a significantly reduced density of dislocations (o10 cm ) compared to that typically observed (Bmid 10–10 cm ) in conventional heteroepitaxial GaN layers. The high structural and optical quality of the layers was revealed by cross-sectional TEM and detailed low-temperature photoluminescence (PL) studies, respectively. ODMR at 24GHz on strong shallow donor–shallow acceptor recombination from the Si-doped homoepitaxial layer reveals evidence for Si or C shallow acceptors on the N sites. EPR of the new free-standing HVPE GaN confirms the low concentration of residual donors (B10 cm ) as determined by Hall effect measurements. In addition, new deep centers are found from ODMR on the 2.4 eV ‘‘green’’ PL band and on the broad emission less than 1.8 eV from the HVPE GaN template. However, contrary to expectations, the reduction of random strain fields (associated with dislocations) has not led to significant changes in the character of the magnetic resonance (such as resolved electron-nuclear hyperfine structure) compared to that typically found for heteroepitaxial GaN layers. r 2001 Elsevier Science B.V. All rights reserved.