Optical absorption of defects in aluminum nitride

Abstract

Aluminum nitride (AlN) is an ultrawide bandgap semiconductor material known for its applications in high power microelectronic and optoelectronic devices due to its high thermal conductivity and high electrical resistivity. Point defects are known to hinder the material’s properties and are therefore a subject of interest in semiconductor materials research. The samples used in this work were grown by physical vapor transport (PVT) at Hexatech and they exhibit amber and clear coloration as well as a nonuniformity of charge, which was investigated. Experiments were carried out by electron paramagnetic resonance (EPR) and photo-EPR spectroscopy in order to detect the defect and determine its ionization energy. Simulations of the EPR signal were performed to determine a g-value of g║ = 2.001 and a hyperfine value of 3.8 mT which are consistent with literature reports and hyperfine calculations, which identify the defect as a carbon on a nitrogen site, CN. Photo-EPR results determined that the threshold for photo-excitation occurs at approximately 4 eV and peaking at 4.7 eV, which is consistent with the reported 4.7 eV absorption attributed to carbon. A threshold for photo-quenching occurs at approximately 2 eV. The charge state transition of a negative to neutral CN-/0 then was found to occur at approximately 2 eV above the valence band maximum, Ev, which is in agreement with theoretical predictions for CN. Analysis of EPR results revealed the cause to possibly be a concentration-dependent donor-acceptor-pair recombination between a donor and a compensating carbon. This work reveals the mechanism for the electronic processes involved in this AlN material