Rotationally aligned hexagonal boron nitride on sapphire by high-temperature molecular beam epitaxy

Hexagonal boron nitride (hBN) has been grown on sapphire substrates by ultrahigh-temperature molecular beam epitaxy (MBE). A wide range of substrate temperatures and boron fluxes have been explored, revealing that high crystalline quality hBN layers are grown at high substrate temperatures, $>1600{}^{\circ }\mathrm{C}$, and low boron fluxes, $\sim 1\times {10}^{-8}$ Torr beam equivalent pressure. In situ reflection high-energy electron diffraction revealed the growth of hBN layers with ${60}^{\circ }$ rotational symmetry and the $\left[11\overline{2}0\right]$ axis of hBN parallel to the $\left[1\overline{1}00\right]$ axis of the sapphire substrate. Unlike the rough, polycrystalline films previously reported, atomic force microscopy and transmission electron microscopy characterization of these films demonstrate smooth, layered, few-nanometer hBN films on a nitridated sapphire substrate. This demonstration of high-quality hBN growth by MBE is a step toward its integration into existing epitaxial growth platforms, applications, and technologies.

Figure 1

(a), (b) RHEED patterns of the sapphire substrate taken on the same azimuth (a) before surface reconstruction (b) and after $\sqrt{31}\times \sqrt{31}R\pm 9^{\circ }$ surface reconstruction. (c), (d) Characteristic rough AFM morphology and RHEED pattern of polycrystalline hBN on sapphire grown at low substrate temperatures ($T_S<1600{}^{\circ }\mathrm{C}$). (c) $10\times 10\mu \mathrm{m}$ AFM image and (d) the corresponding diffuse, ringed RHEED pattern demonstrate polycrystalline growth. (e)–(h) AFM and RHEED patterns of smooth, rotationally aligned hBN on sapphire grown at high substrate temperature ($T_s>1700{}^{\circ }\mathrm{C}$) and low boron flux ($\sim 1\times {10}^{-8}$ Torr BEP): (e) $10\times 10\mu \mathrm{m}$ AFM image and (f) the corresponding RHEED pattern shows sharp streaks attributed to AlN and hBN (see text). (g) $2\times 2\mu \mathrm{m}$ AFM image of layered hBN on sapphire, exhibiting a dense network of wrinkles. (h) A line height profile of the white bar in (g) shows the height of the wrinkles to be 1–2 nm.

Figure 2

Raman spectra of (a) few-layer hBN and (b) polycrystalline hBN on sapphire substrates. The Raman-active $E_{2g}$ mode in each spectrum is fitted to a Voigt function (red line). The $E_{2g}$ peak in polycrystalline hBN is broader and more intense, relative to the sapphire substrate peaks, implying a thicker film of lower crystalline quality. The few-layer hBN demonstrates a less intense, but much narrower peak, suggesting a thinner film of higher crystalline quality.

Figure 3

XPS spectra of (a) the B 1s and (b) Al 2p peaks of few-layer hBN on sapphire. The position of the B 1s peak, centered at 190.2 eV, is consistent with B-N bonding. A less intense peak is also observed 9 eV to higher energy, attributed to a $\pi$ plasmon resonance, a fingerprint of $s{p}^2$ bonding. The Al 2p peak can be deconvoluted into the sum of two Gaussian peaks centered at 74.2 eV and 73.1 eV, attributed to Al-O bonding and Al-N bonding, respectively, demonstrating that the sapphire substrate was nitridated during the initial growth.

Figure 4

Absorbance spectrum of few-layer hBN shows the characteristic peak centered at $\sim 6.1$ eV. A Tauc plot (inset) was used to estimate the band gap of the material. The Tauc method for an indirect band-gap semiconductor was used to estimate the optical band gap to be $\sim 5.86$ eV, assuming a three-dimensional density of states.

Figure 5

(a) Cross-sectional STEM image of hBN grown on sapphire by MBE shows smooth, layered growth of few-layer films. Evidence of surface nitridation is directly corroborated by the presence of an interfacial layer between the bulk sapphire substrate and the hBN layers with a crystal structure consistent with $\left[11\overline{2}0\right]$ AlN. (b) A proposed schematic of this sapphire/AlN/hBN heterostructure, demonstrating the ${30}^{\circ }$ rotation of the AlN and hBN with respect to the sapphire. The Al-polarity of the AlN shown in the schematic is not experimentally verified.