Carbon Impurity in Aluminum Nitride Detected Using Optical Absorption Spectroscopy

Aluminum nitride (AlN), a semiconductor showing great promise in optoelectronics, can acquire performance degrading impurities like carbon during growth. Often, defects are identified with optical absorption (OA) spectroscopy, but there is little consensus on which optical signatures belong to which defect. Recently, carbon on a nitrogen site (C_N⁰) was assigned to an electron paramagnetic resonance (EPR) signal in AlN crystals grown by physical vapor transport. We are combining the power of EPR and OA spectroscopy to search for an optical fingerprint for carbon. The EPR C_N⁰ signal increased with 265 nm irradiation, which we attribute to: e_V+hf+C_N^-=C_N⁰. The signal was quenched with 530 nm, attributed to: e_V+hf+C_N⁰=C_N^-. When comparing the same 265 nm excitation in both EPR and OA, one sample had two optical features correlating with the EPR C_N⁰ increase: an OA peak between 2.3–3.2 eV increased, and another peak between 4.3–5 eV, which is often assigned to a C_N transition, decreased. However, in other samples with the C_N⁰ EPR signal, only the OA peak between 2.3-3.2 eV was present. We suggest this peak, which correlates with the EPR across multiple samples, is related to C_N through, e_V+hf+C_N⁰=C_N^-.