Effect of V/III ratio on the growth of hexagonal boron nitride by MOCVD
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Effect of V/III ratio on the growth of hexagonal boron nitride by MOCVD Qing S. Paduano,1 Michael Snure,1 and Jodie Shoaf2 1 2
Air Force Research Laboratory, Sensors Directorate, Wright-Patterson AFB, OH Wyle Laboratories, Inc., Dayton, OH
ABSTRACT In this report, we describe a process for achieving atomically smooth, few-layer thick, hexagonal boron nitride (h-BN) films on sapphire substrates by MOCVD, using Triethylboron (TEB) and NH3 as precursors. Two different growth modes have been observed depending on the V/III ratio. Three-dimensional (3D) island growth is dominant in the low V/III range; in this range growth rate decreases with increasing deposition temperature. This island growth mode transitions to a self-terminating growth mode when V/III > 2000, over the entire deposition temperature range studied (i.e. 1000-1080oC). Raman spectroscopy verifies the h-BN phase of these films, and atomic force microscopy measurements confirm that the surfaces are smooth and continuous, even over atomic steps on the surface of the substrate. Using X-ray reflectance measurements, the thickness of each film grown under a range of conditions and times was determined to consistently terminate at 1.6nm, with a variation of less than 0.2 nm. Thus we have identified a self-terminating growth mode that enables robust synthesis of h-BN with highly uniform and reliable thickness on non-metal catalyzed substrates. Furthermore, this selfterminating growth behavior has shown signs of transitioning to continuous growth as deposition temperature increases. INTRODUCTION Hexagonal boron nitride (h-BN) is an insulating isomorph of graphite. It has a number of properties analogous to graphite, such as excellent mechanical properties, but with a wide band gap [1,2], low dielectric constant, piezoelectricity and good stability at high temperature [3]. It is thus a promising material for many applications. The potential application of mono-layer or fewlayer h-BN as a dielectric building block for graphene-inspired nano-electronics and photonics has motivated extensive research focused on synthesis of h-BN with controlled atomic layer thickness. The synthesis of h-BN on non-metal catalyzed substrates, such as sapphire, has been reported using a high temperature CVD process above 1500oC [4,5]. Kobaryashi showed that growth of h-BN on sapphire, Ni, or SiC substrates is possible using metalorganic chemical vapor deposition (MOCVD) in the conventional III-N processing temperature range (~1100oC) [6]. However, these early studies were focused on producing thick h-BN layers, and little attention was paid to achieving controlled atomic layer growth of h-BN film, which is more desirable for the fabrication of graphene-based 2-D nano-device structures. In a previous report [7], we described self-terminating growth of ultra-thin (few-layer) BN films on sapphire substrates by MOCVD using Triethylboron (TEB) and NH3 as precursors. The conditions that result in self-terminating growth of BN also produce high uniformity and reproducible thickness, in a way
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