Characterization of nitrides by electron paramagnetic resonance (EPR) and optically detected magnetic resonance (ODMR)

We will highlight our recent work on the properties of residual defects and dopants in GaN heteroepitaxial layers and on the nature of recombination from InGaN single quantum well (SQW) light emitting diodes (LEDs) through magnetic resonance techniques. Electron paramagnetic resonance (EPR) and optically detected magnetic resonance (ODMR) were performed on undoped (highly resistive and n-type) and intentionally doped (Si, Mg, or Be) GaN films grown by a variety of techniques (MOCVD, MBE, and HVPE) in order to obtain general trends and behavior. Through the spin-Hamiltonian parameters, these methods can reveal symmetry information, the character of the wave function and (ideally) the chemical identity of the defect. In addition, low temperature EPR intensities can be used to determine the neutral acceptor or donor concentrations without the need for contacts or the high temperatures required for Hall effect measurements. The ODMR was performed on both bandedge (mainly shallow donor–shallow acceptor recombination) and deep (visible and near-IR) PL bands. In spite of the radically different (non-equilibrium) growth techniques, many of the same defects were found in the various samples. Finally, earlier ODMR studies of recombination from Nichia InGaN ‘green’ and ‘blue’ LEDs were extended to include shorter (‘violet’) and longer (‘amber’) wavelength LEDs and an undoped 30 Å In0.3Ga0.7N/GaN heterostructure. The results provide evidence for spatially separated electrons and holes in the optically-active 30 Å InGaN layers under low photoexcitation conditions, likely due to localization at different potential minima in the x–y planes and/or the large strain-induced piezoelectric fields parallel to the growth direction.