From h-BN to Graphene: Understanding Electrical Conductivity of Single-Crystal Carbon-Doped h-BN through Its Structure

Hexagonal boron nitride (h-BN) is a promising ultrawide-bandgap semiconductor for emerging optoelectronic and high-power electronic devices. Nevertheless, ohmic current injection into h-BN remains a significant challenge in developing these current-driven devices. Interestingly, the hybridized two-dimensional structure containing boron, carbon, and nitrogen, which was reported to have modifiable electrical properties by controlling C concentration [1], is a potential material to overcome this challenge. It is thus possible that a small C concentration introduced to hybridize with h-BN could improve the electrical conductivity of h-BN.<br/><br/>In this study, the effect of C on the electrical conductivity of h-BN was investigated. To minimize the influences from other C-unrelated defects, the single-crystal h-BN synthesized under high pressure and high temperature (HPHT) was selected. C was introduced by post-growth diffusion with two different conditions: at 2.5 GPa and 3100°C for 30 minutes and at 1 atm and 2000°C for 4 hours. The former condition results in gray-colored h-BN and the latter one gives yellow-colored h-BN. According to SIMS profiles, the gray C-doped h-BN (h-BN:C) shows ~10 at.% C, which is ~100 times higher in C concentration than the yellow h-BN:C. The transverse dielectric breakdown of both h-BN:C is comparable to that of undoped h-BN. Raman spectra show the apparent differences between these two types of h-BN:C. At 3 K, the E_2g vibrational mode of h-BN is observed at ~1368 cm^-1 for both h-BN:C, while the peaks corresponding to G and 2D modes in graphene are detected only in the gray h-BN:C, indicating the existence of C–C bonds. It can be expected that C and h-BN domains coexist in the gray h-BN:C. The electrical conductivity was investigated by forming metal contact following the method described in Ref. 2. The I-V characteristic of the gray h-BN:C initially exhibited nonohmic conduction within an applied voltage of ±100 V at 298 K and transformed to ohmic conduction after the breakdown-like behavior at 598 K. The surface topology change of the h-BN channel can be seen in most of the devices with ohmic conduction after the high-T measurements. On the other hand, the yellow h-BN:C requires at least ±50 V to inject current with nonohmic conduction for all T, and no breakdown-like behavior nor topology change in the channel is observed. After the T-dependent I-V measurements, the devices were characterized by electron-beam-induced current (EBIC) mapping, cross-sectional TEM, and EELS. The EBIC results indicate the formation of a conductive path in the gray h-BN:C channel. The EELS mapping and its energy loss spectra reveal the existence of discrete graphite/graphene layers and randomly distributed C domains in the gray h-BN:C, which is consistent with the Raman spectra. These results suggest the possibility that C structures existing in h-BN induced the local breakdown of h-BN, which functions as a conductive path. Therefore, to enable the development of h-BN:C-based high-power electronics, the formation of randomly distributed C domains and graphite/graphene layers must be controlled.<br/> <br/>[1] L. Ci, et al., Nat. Mater 9, 430 (2010).<br/>[2] S. Ngamprapawat, et al., ACS Appl. Mater. Interfaces 14, 25731 (2022).