Thermal stability, work function and Fermi level analysis of 2D multi-layered hexagonal boron nitride films

Thermal stability and Fermi level analysis of commercial multilayer (ML) and as-grown 10 nm thick hexagonal boron nitride (hBN) films on Cu foil by atmospheric chemical vapor deposition (APCVD) by heating in N₂ and air at temperatures ≤900 $°C$ are studied. For all samples, progressive surface oxidation was observed correlating with higher hBN nucleation density. Thermal stability was measured in air from 200 $°C$ to 400 $°C$ (relative humidity $\sim$65%) where commercial ML and APCVD material degraded; BN peaks begin to be replaced by BO peaks at temperatures $>$300 $°C$. It is shown that hBN thermal stability is sensitive to T_growth and Cu foil surface preparation methods such as electropolishing (EP). We evaluate as-grown material for different T_growth conditions (1000 $°C$, 1030 $°C$, 1050 $°C$ and 1060 $°C$) by ultraviolet photoelectron spectroscopy (UPS) to locate and understand subsequent Fermi level and work function variation, confirming our material is in general n-type, which becomes increasingly so as thermal stability regresses. To this effect, we observe a work function increase of 0.45 eV as BO increases with in-situ anneal temperature. An average electron affinity of 2.13 eV and 1.7 eV is determined for commercial hBN and APCVD material, respectively. Further work is required by material scientists to optimize the material properties of CVD hBN.