Effect of Proton Irradiation on Breakdown Field of Hexagonal Boron Nitride
Authors: Lee Dongryul , Park Hyunik , Bae Jinho , Kim Jihyun 
Published: February 2020

Hexagonal boron nitride (h-BN) is widely used as a two-dimentional (2D) insulating material. Owing to its high breakdown field (8-12 MV/cm), wide band gap (5.2-5.9 eV), and thermal⋅chemical stability, h-BN is employed as a dielectric layer of 2D integrated electronic device. Unlike to other insulating materials, h-BN has an atomically flat surface free from the dangling bond at the interface. Therefore, it is less affected by trap charges and mismatch of the lattice structure. Without any complicated process, it is possible to obtain an atomically thin flake by mechanical exfoliation and is easy to apply to extremely thin electronic device application such as field effect transistor and complementary metal-oxide semiconductor.

Dielectric layer of semiconductor device must maintain its insulating property in extreme condition. It is of great importance to find out the breakdown field to ensure the stability of the device. However, the atoms constituting the lattice of the insulator can be damaged by protons, heavy ions and cosmic rays. These rays make an irreversible degradation on the insulating materials, and the destruction of the lattice structure can be accompanied. It poses a critical hazard on the performance of the device in that degradation of insulator lead to lower the breakdown field of dielectric layer. Measurement of changes of the breakdown field by radiation can define the safe operating voltage range of the device. H-BN is a next-generation insulating material, but the effect of radiation on h-BN has not been fully studied. In this work, we quantitatively analyzed the damage on h-BN and effect on breakdown field from varying proton-ray irradiation conditions.

We fabricated pre-patterned source/drain electrode of Ti/Au on a Si/SiO2 wafer. A thin h-BN flake was mechanically exfoliated from the high-crystalline h-BN bulk crystal. Prepared flakes were transferred to the substrate by dry transfer method. We employed atomic force microscope to measure the thickness of h-BN. H-BN flakes were proton-irradiated under varying condition of power (5MeV, 10MeV) and fluence (1×1013, 5×1013). To measure breakdown field, graphene was used as a top electrode on h-BN to creat a vertical structure. We proceeded a electrical measurement to find out the hard breakdown field of pristine and irradiated h-BN flakes respectively.

Mean value of the breakdown field of pristine h-BN flakes is 10.532 MV/cm, comparable with value of reported exfoliated h-BN. Whereas the breakdown field of proton-irradiated h-BN flakes ranged from 1 to 7 MV/cm. We used density functional theory and molecular dynamics simulation to estimate the mechanism of damaging on the lattice structure of h-BN. Proton irradiation causes irreversible degradation on atomic structure of h-BN, such as interstitial or vacant defects. This triggered a sudden current jump and failure at a lower voltage. We can enable more stable operation by considering the expected breakdown field of the device under the condition of radiation. Details of our work will be discussed in the presentation.