Growth of hexagonal boron nitride on electrochemically prepared polycrystalline copper substrates
Gaskill, D. Kurt
Fuhrer, Michael S
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The growth of two dimensional materials such as graphene and hexagonal boron nitride (h-BN) have been demonstrated by chemical vapor deposition (CVD) on polycrystalline copper catalytic substrates. Polycrystalline copper foil substrates most often utilized in CVD approaches are produced by metallurgical rolling, a technique which forms irregular ridges on the foil surface and can introduce contaminants. The Cu foil supplied by the vendor, such as Sigma Aldrich or Alfa Aesar, also has a film of native oxide on the surface. These processing artifacts, irregular ridges, along with the native oxide are a limiting factor for controlled and reproducible large area (several cm2) growth of these low-dimensional materials envisaged to be used for microelectronics. Greater control of growth can be achieved by controlling the density of nucleation sites and improving the catalytic activity of Cu by removing the native oxide on the Cu surface. Previous attempts to pre-treat the Cu substrate either by using wet chemistry and thermal annealing to control growth has been weakly addressed. In this work, electrochemical polishing (electropolishing) combined with prior thermal annealing at 1030°C for 5hrs under H2 is used to control the degree of roughness of cold rolled polycrystalline Cu foils, and subsequently, to explore the influence of electropolishing on the growth of h-BN. Electropolishing dissolves a thin surface layer of copper, which contains surface defects and contaminants. This helps in decreasing the density of spontaneous nucleation sites by producing a morphologically uniform and contaminant-free surface. Secondary effects during electropolishing, such as etch pits, arise from the standard electropolishing recipe of concentrated H3PO4. These secondary effects, ascribed to oxygen bubbling at random nucleation sites on Cu surface, can be overcome by using additives, such as acetic acid and ethylene glycol, as oxygen suppressors in the electrolyte to counter formation of etch pits and to prepare a planarized substrate. The additives are added to the standard electrolyte with a volume ratio of 25% Ethylene Glycol, 25% Acetic Acid, 50% concentrated H3PO4 forming a compound electrolyte and a potential of 2.3 V is applied for 30 minutes for electropolishing to occur. Atomic Force Microscopy (AFM) reveals that a roughness of ~1.2 nm (Rq) and a greatly planarized Cu foil without the waviness from metallurgical rolling can be achieved with the two step: thermal annealing followed by two-additive electropolishing process. Fourier Transform Infrared (FTIR) spectroscopy is used to confirm the existence of h-BN on electropolished substrates. Subsequent growth studies of h-BN on the high quality copper substrates demonstrate improved growth, as demonstrated by the metrics of size and count of h-BN crystals from Scanning Electron Microscopy (SEM) micrographs. The work demonstrates that thermal annealing followed by electropolishing leads to optimization of copper foil surface resulting in the larger crystal size of h-BN and reduction in nucleation sites that induce h-BN crystal growth. AFM is also used to establish the Cu substrate morphology and its relationship to the growth of h-BN crystals.