Substrate Modification during Chemical Vapor Deposition of hBN on Sapphire

Abstract

A comparison of hexagonal boron nitride (hBN) layers grown by chemical vapor deposition on C-plane (0001) versus A-plane (112̅ 0) sapphire (α-Al 2 O 3 ) substrate is reported. The high deposition temperature (>1200 °C) and hydrogen ambient used for hBN deposition on sapphire substantially alters the C-plane sapphire surface chemistry and leaves the top layer(s) oxygen deficient. The resulting surface morphology due to H 2 etching of C-plane sapphire is inhomogeneous with increased surface roughness which causes non-uniform residual stress in the deposited hBN film. In contrast to C-plane, the A-plane of sapphire does not alter substantially under a similar high temperature H 2 environment, thus providing a more stable alternative substrate for high quality hBN growth. The E 2g Raman mode full width at half-maximum (FWHM) for hBN deposited on C-plane sapphire is 24.5 ± 2.1 cm −1 while for hBN on A-plane sapphire is 24.5 ± 0.7 cm −1 . The lesser FWHM standard deviation on A-plane sapphire indicates uniform stress distribution across the film due to reduced undulations on the surface. The photoluminescence spectra of the hBN films at 300 and 3 K, obtained on C-plane and A-plane sapphire exhibit similar characteristics with peaks at 4.1 and 5.3 eV reported to be signature peaks associated with defects for hBN films deposited under lower V/III ratios. The dielectric breakdown field of hBN deposited on A-plane sapphire was measured to be 5 MV cm −1 , agreeing well with reports on mechanically exfoliated hBN flakes. Thus, under the typical growth conditions required for high crystalline quality hBN growth, A-plane sapphire provides a more chemically stable substrate.

Chemical vapor deposition of sp2-boron nitride films on Al2O3 (0001), (112¯), (11¯02), and (101¯) substrates

“…Notably, a recent study on the surface properties of the substrate comparing h-BN growth using diborane (B₂H₆) on a-cut and c-cut Al 2 O 3 revealed the former to be less likely to modify under growth conditions in the absence of an AlN buffer layer. 21 It is noteworthy from this study that when B₂H₆, a carbon-free boron precursor, was used to deposit BN without nitridizing the surface, h-BN films were observed.…”

“…These results agree with suggestions from a recent surface study of BN growth on a-cut Al 2 O 3 , which described it to be more suitable for the growth of h-BN. 21 The presence of AlN 0002 and AlN 0004 peaks on both c-cut and a-cut Al 2 O 3 evidently dictates the epitaxial r-BN film growth on these substrates. This is supported by previous studies that have shown that h-BN initially nucleates epitaxially followed by r-BN epitaxial film growth on AlN (0001) grown on c-cut sapphire.…”

“…The a-cut sample shows a lower FWHM of 0.98°c ompared to 1.15°of the c-cut sample, which is a difference of about 15%, further confirming suggestions of a more suitable substrate in a-cut Al 2 O 3 for the growth of sp 2 -BN. 21 Broadness of the peaks suggests a less ordered film with respect to the substrate. Samples grown using TMB precursor were analyzed using ɸ-scans and ω-scans due to the difference in the amount of r-BN present as described above.…”

“…Hitherto, substrates for epitaxially grown h-BN and r-BN phases include 3C-SiC(111), 4H-and 6H-SiC(0001) on-axis substrates, ZrBr 2 and α-Al 2 O 3 . 15,17,[19][20][21] These substrates resulted in epitaxially grown [0001]-oriented sp 2 -BN films, here it is challenging to distinguish between the h-BN and r-BN phases. 22 Therefore, it is of interest to study epitaxial sp 2 -BN film growth in other crystal orientations.…”

ReaxFF Force Field Development for Gas-Phase hBN Nanostructure Synthesis

“…These simulations are performed with 700 HBNH molecules at 2000 K at different stages along with the simulation with a corresponding initial real gas pressure of 73 bar. A recent experimental study by Bansal et al 35 employs similar temperatures to synthesize hBN on a sapphire substrate with B 2 H 6 /NH 3 as precursors. Their work suggests that the precursor species might undergo chemical reactions before reaching the substrate surface.…”