Point Defects in High Purity AlN Substrates

The class of semiconductors with bandgap greater than ~ 4 eV, conventionally referred to as ultrawide bandgap semiconductors, are excellent candidates for material applications under extreme conditions. In particular, AlN with one of the largest bandgaps (6.2 eV) is predicted to withstand electric fields in excess of 15 MV/cm, making it one of the top choices for the next generation of high power electronics [1]. Future power devices will also benefit from the radiation hard piezoelectric properties [2]. To understand why these remarkable properties are realizable, it is helpful to look at the atomic level and investigate the point defects which can sometimes trigger, and other times minimize, degradation incurred by extreme conditions. Point defects in the as-grown material may be responsible for the premature electrical breakdown under large electric fields, and additional defects may be generated during ionizing radiation. The talk begins with a review of four different types of point defects found in as-grown AlN bulk crystals, and then summarizes radiation-induced centers.<br/>Two different AlN crystals were studied. Both were grown by physical vapor transport, but the concentration of the typical impurities, C, Si, and O, differ by two orders of magnitude – 10¹⁹ cm^-3 in one and 10¹⁷ cm^-3 in the other. The point defects were studied by electron paramagnetic resonance (EPR) and photo-EPR. Whereas the former leads to chemical and structural identification of the defect, the latter can enable measurement of the defect level. Basically, photo-EPR is an optical absorption measurement performed on a specific, known defect.<br/>The sole EPR center observed in the crystals with high impurity concentration is D5 [3], commonly reported as a deep donor. However, comparison of the EPR parameters with those recently calculated by theory indicate conclusively that the EPR spectrum is due to a neutral carbon atom sitting on a nitrogen site, C_N⁰. Photo-EPR measurement yield an acceptor level, C_N^-/0, about 2 eV above the valence band edge, which corresponds well with the calculated level. The high purity samples revealed three additional spectra. One is thought to be a shallow donor, often attributed to O_N. Again, recent calculations rule this out, but Si substituting for Al is a reasonable alternative candidate. The other two EPR spectra have not previously been identified in AlN. Both have an electron spin (S) greater than ½ and must be induced by either 430 nm or 265 nm light. One may be simulated assuming S=3/2 and a weak hyperfine interaction with two or three Al, suggesting that the defect may be a simple aluminum vacancy or one coupled to Si or O as has been suggested by theoretical studies. At this time, there is not sufficient information on the second EPR spectrum in the high purity material to comment on its origin. Nevertheless, either defect could be a significant factor in achieving successful application of AlN under extreme electric fields. Furthermore, their presence indicates that lowering the impurity content reveals new defects which may present unforeseen challenges to production of high quality AlN.<br/><br/>This work was supported as part of the Ultra Materials for a Resilient Energy Grid, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0021230.<br/><br/>1. R. J. Kaplar et al, ECS J Solid State Science and Technology, 6 (2) Q3061-Q3066 (2017)<br/>2. D. A Parks and B.R. Tittmann, IEEE Trans on Ultrasonics, Ferroelectrics ad Frequency Control <b>61</b>, 1216 (2014).<br/>3. P. M. Mason, et al, Phys. Rev. B <b>59</b>, 3 (1999).