Nucleation of cubic boron nitride on boron-doped diamond via plasma enhanced chemical vapor deposition

Abstract

Cubic boron nitride (c-BN), with a small 1.4% lattice mismatch with diamond, presents a heterostructure with multiple opportunities for electronic device applications. However, the formation of c-BN/diamond heterostructures has been limited by the tendency to form hexagonal BN at the interface. In this study, c-BN has been deposited on free standing polycrystalline and single crystal boron-doped diamond substrates via electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD), employing fluorine chemistry. In situ x-ray photoelectron spectroscopy (XPS) is used to characterize the nucleation and growth of boron nitride (BN) films as a function of hydrogen gas flow rates during deposition. The PECVD growth rate of BN was found to increase with increased hydrogen gas flow. In the absence of hydrogen gas flow, the BN layer was reduced in thickness or etched. The XPS results show that an excess of hydrogen gas significantly increases the percent of sp2 bonding, characteristic of hexagonal BN (h-BN), particularly during initial layer growth. Reducing the hydrogen flow, such that hydrogen gas is the limiting reactant, minimizes the sp2 bonding during the nucleation of BN. TEM results indicate the partial coverage of the diamond with thin epitaxial islands of c-BN. The limited hydrogen reaction is found to be a favorable growth environment for c-BN on boron-doped diamond.

Hydrogen-driven boron nitride phase differentiation during the epitaxial nucleation on the diamond (001) surface

“…The steps and kinks on the substrate surface are known to serve as the nucleation site, significantly lowering the nucleation barrier, as reported in experiments. 7,22 While both single-and double-layer steps are observed on this surface, we focus on the double-layer steps due to their prevalence on the vicinal substrate surface and their ability to avoid antiphase disorder in principle, which is a key concern in the epitaxial binary films. Steps on the (001) surface are separated into A and B types, corresponding to the steps running perpendicularly and parallel to the dimer row, respectively.…”

“…It indicates that the adsorption barrier on the surface is significantly higher (~0.8 eV for D A I and ~0.4 eV for T 1 diamond sites, compared to 0-0.1 eV on top of "BN layer"), explaining the difficulty of the initial nucleation process on the diamond surface observed in experiments. 7,11,14 Crystal growth is practically realized under conditions where Δµ BN > 0, ensuring sufficient speed of growth. Furthermore, the environment is often tuned to modify chemical potential, allowing preferential formation of specific target products.…”

“…6 However, a reliable and scalable synthesis of wafer-sized, high-quality crystals prerequisite for use in electronics is yet to be achieved for cBN. The concurrent formation of multiple phases, mostly amorphous BN (aBN), sp 2 -bonded hexagonal BN (hBN) and turbostratic BN (tBN), and sp 3 -bonded cBN, poses a significant obstacle, [7][8][9][10] calling for the investigation of the underlying mechanisms.…”

“…The successful synthesis of single-crystal cBN was demonstrated through several methods using diamond as a convenient heteroepitaxial substrate. 7,[10][11][12][13][14][15][16] Molecular beam epitaxy (MBE) and physical vapor deposition (PVD) approach in the absence of hydrogen typically produce aBN followed by cBN formation. Commonly employed etching of aBN through ion bombardment leaves behind a pure cBN phase, which, unfortunately, is plagued by poor crystallinity, high defect density, and internal stress caused by high-energy ions.…”

“…The addition of fluorine in relatively mild growth conditions (fraction of ambient pressure, 1000-1300 K) leads to the formation of pure cBN or mixed with the hBN phase by adjusting the H/F ratio. 7,10,14 Notably, PECVD also serves as a mainstream method of diamond growth for electronics applications. Significant efforts have been devoted to understanding the surface CVD diamond growth mechanism, resulting in a theoretical model of carbon addition, either by insertion into the surface dimer bond (a dimer-opening mechanism) or across the trough site between rows of dimer bonds (a trough-briding mechanism, see dimer and trough site in Fig.…”