Role of Pressure in the Growth of Hexagonal Boron Nitride Thin Films from Ammonia-Borane

Abstract

We analyze the optical, chemical, and electrical properties of chemical vapor deposition (CVD) grown hexagonal boron nitride (h-BN) using the precursor ammonia-borane (H3N-BH3) as a function of Ar/H2 background pressure (PTOT). Films grown at PTOT ≤ 2.0 Torr are uniform in thickness, highly crystalline, and consist solely of h-BN. At larger PTOT, with constant precursor flow, the growth rate increases, but the resulting h-BN is more amorphous, disordered, and sp 3 bonded. We attribute these changes in h-BN grown at high pressure to incomplete thermolysis of the H3N-BH3 precursor from a passivated Cu catalyst. A similar increase in h-BN growth rate and amorphization is observed even at low PTOT if the H3N-BH3 partial pressure is initially greater than the background pressure PTOT at the beginning of growth. h-BN growth using the H3N-BH3 precursor reproducibly can give large-area, crystalline h-BN thin films, provided that the total pressure is under 2.0 Torr and the precursor flux is well-controlled.* Correspondence should be addressed to lyding@illinois.edu, jkoepkeuiuc@gmail.com, and joshua.wood@northwestern.edu. Films of h-BN have been used as insulating spacers, 1 encapsulants, 2 substrates for electronic devices, 3, 4 corrosion and oxidation-resistant coatings, 5, 6 and surfaces for growth of other 2D nanomaterials such as graphene 7 and WS2. 8 Most of these studies employed small-area (~100 µm 2 ) h-BN pieces exfoliated from sintered h-BN crystals, 9 limiting technological use of h-BN films. Additionally, unlike graphene, h-BN is difficult to prepare in monolayer form by exfoliation. The electronegativity difference between B and N and the reduced resonance stabilization relative to graphene results in electrostatic attractions between layers and in-plane. Consequently, it is more challenging to control h-BN grain size and layer number. Furthermore, partially ionic B-N bonds can form between neighboring BN layers, serving to "spot weld" such layers together. 10 Several groups have sought to overcome these limitations by using chemical vapor deposition (CVD) to grow large-area, monolayer h-BN films. [11][12][13][14][15][16][17][18][19][20][21][22] CVD growth of h-BN has been accomplished using various precursors (e.g., ammonia borane, borazine, and diborane) on transition metal substrates (e.g., Cu, Ni, 23 Fe, 24 Ru, 25, 26 etc.). Of these h-BN growth substrates, we focus on Cu, as Cu has a high catalytic activity, 27 is inexpensive, and is the typical growth substrate 28 for conventional graphene CVD.Regarding h-BN growth precursors, volatile borazine-B3N3H6, isoelectronic with benzene-is far from an ideal choice, as borazine is hazardous and decomposes quickly even at room temperature. While borazine can pyrolyze and dehydrogenate 23, 25,29,30 to generate h-BN films, 13,17,19,20,22,31 partial dehydrogenation is common, [30] resulting in oligomeric BN compounds and aperiodic h-BN grain boundaries. 13,17 Finally, thin films of h-BN can also be grown from mixtures of diborane (B2H6) and ammonia (NH3...

Selective Defect Formation in Hexagonal Boron Nitride

“…The E 2g modes were fit with a Lorentzian, giving full‐width at half maximum of 24.35 cm −1 for the standard grown hBN, which decreases to 17.60 and 15.90 cm −1 for preoxidized and gettering growth conditions, respectively (Figure S7, Supporting Information). This confirms the superior crystallinity of hBN grown films by the gettering method . The increasing crystallinity can be ascribed to modification of the Cu catalyst during gettering growth, which slows nucleation and growth kinetics, and reduces the effects of lattice constant changes, and high defect density growth during cooling by boron precipitation from the catalyst .…”

“…Moreover, the intense B signals generated by the postgrowth depth profile of the nickel support, reveals the preferential B diffusion within nickel, Figure d, confirming nickel as an effective B gettering substrate. Note that N signals were below the detection limit of ToF‐SIMS within Cu for all investigated growth conditions, which is attributed to the very low solubility of N in Cu at this growth temperature . Hence, our gettering approach enables us to mitigate B diffusion into Cu, improving the crystallinity of resulting hBN growth on the topside Cu surface, and offering a controllable way to engineer defect formation during CVD growth.…”

Role of hydrogen carrier gas on the growth of few layer hexagonal boron nitrides by metal-organic chemical vapor deposition

“…This peak may originate from the sp 3 -hybridized boron atoms because their binding energy is typically higher than sp 2 -hydrydized boron atoms. 26 In addition, the FWHM of the B 1s main peak becomes larger from 1.80 eV to 1.88 eV when N 2 is used as the carrier gas instead of H 2 . Since peak broadening is related to the numbers of contributing chemical bonding, 27 more defective chemical bonding states comparing to that grown with H 2 carrier gas.…”

A Review on the Synergistic Features of Hexagonal Boron Nitride (White Graphene) as Adsorbent‐Photo Active Nanomaterial

“…Further, the choice of growth substrate is critical because the h‐BN morphology and growth are dependent on the solubility of the B and N in the same. For instance, B and N dissolve easily in Fe and Cu substrate but the dissolution of N in Cu is sparing and this can affect the synthesis process significantly ,,. The synthesis of h‐BN in electro‐polished Cu enclosures with hydrogen atmosphere results in h‐BN of varied morphology i. e., trapezoidal and triangular shapes were found on diverse Cu grains but on the same Cu foil, while hexagonal shape was found to depend on the electropolishing conditions .…”