Hexagonal Boron Nitride is the perfect substrate for extreme optoelectronics in graphene, but what are its electronic properties exactly and how far wan we push it? LPENS analyzed state-of-the-art hBN from NIMS and LMI/INSA Lyon to find out.
Two pre-requisites are mandatory to reach the electroluminescence of graphene : extremely large mobilies, and the possibility to apply very large in-plane electric fields, up to 20kV/cm, i.e. about the breakdown field of air in ambient conditions. If graphene doping is electrostatically controlled, the backgate nearby contacts produce even larger local electric fields of 2 000kV/cm! At such voltage, even solids can electrically break.
Usually, this breakdown occurs because charges localized around defects start to migrate at sufficiently large electric field (Frenkel-Poole mechanism). When doing so, strong local heating modifies the crystalline structure of the crystal definitely modifying its electronic properties by creating conductive channels if not by vaporizing the whole sample. Interestingly, the small leakage current at high fields depends critically on the density of defects and on their nature, therefore it can be an excellent probe to size differences between hBN crystals grown differently despite their otherwise similar response to usual characterization methods.