The UCSB researchers, in collaboration with researchers from Rutgers University, University of Vienna, KTH Royal Institute of Technology in Sweden and the Centre for Physical Sciences and Technology in Lithuania, have identified that iron, even at concentrations less than parts-per-million, can be detrimental.
The team claims transition metal impurities have long been known to impact electronic and optoelectronic devices based on traditional semiconductors such as silicon and gallium arsenide, leading these impurities to be referred to as ‘killer centres’.
"A naïve application of Shockley-Read-Hall theory, based on an inspection of defect levels within the band gap, would lead one to conclude that iron in GaN would be harmless," explained Dr. Darshana Wickramaratne. "However, our work shows that excited states of the impurity play a key role in turning it into a killer centre."
The scientists identified a recombination cycle involving excited states by which iron can lead to severe efficiency loss.
The results highlight that strict control over growth and processing is required to prevent the unintentional introduction of transition metal impurities. Sources of iron contamination include the stainless steel reactors that are used in some growth techniques for nitride semiconductors.