Impact ionization and transport properties of hexagonal boron nitride in a constant-voltage measurement

The electrical evaluation of the crystallinity of hexagonal boron nitride (h-BN) is still limited to the measurement of dielectric breakdown strength, in spite of its importance as the substrate for two-dimensional van der Waals heterostructure devices. In this study, physical phenomena for degradation and failure in exfoliated single-crystal h-BN films were investigated using the constant-voltage stress test. At low electrical fields, the current gradually reduced and saturated with time, while the current increased at electrical fields higher than ~80.16em0exMV/cm and finally resulted in the catastrophic dielectric breakdown. These transient behaviors may be due to carrier trapping to the defect sites in h-BN because trapped carriers lower or enhance the electrical fields in h-BN depending on their polarities. The key finding is the current enhancement with time at the high electrical field, suggesting the accumulation of electrons generated by the impact ionization process. Therefore, a theoretical model including the electron generation rate by an impact ionization process was developed. The experimental data support the expected degradation mechanism of h-BN. Moreover, the impact ionization coefficient was successfully extracted, which is comparable to that of SiO₂, even though the fundamental band gap for h-BN is smaller than that for SiO₂. Therefore, the dominant impact ionization in h-BN could be band-to-band excitation, not defect-assisted impact ionization.