STEM Analysis of Interfacial Defects in Cubic Boron Nitride on Diamond

Ultrawide-band gap materials promise greater performance benefits over state-of-the-art wide-band gap materials in high-power RF amplifier and power electronics applications. Among the group-III nitrides, cubic boron nitride (c-BN) is one of the most promising candidate materials due to its large band gap (Eg ∼ 6.4 eV), high thermal conductivity, and high predicted saturated carrier velocity. In spite of many efforts, c-BN is still the least mature of the ultrawide-band gap materials [1]. This is primarily due to the difficulties in synthesizing high-quality phase pure c-BN crystals and the large number of unintentional defects present in grown crystals. Aberration-corrected scanning transmission electron microscopy (STEM) imaging, electron ptychography, and electron energy-loss spectroscopy (EELS) are ideally suited to examine the local structural, chemical, defect, and phase/growth relationships, in epitaxially grown c-BN. Here, we build off of our previous work [2] examining c-BN films grown on diamond with post-growth cross-sectional STEM analysis. In this work, we show the results of growth improvements obtained via a cyclic growth scheme, where c-BN growth is done using a boron e-beam evaporator, RF plasma-based atomic nitrogen source (N), and RF plasma-based argon ion source (Ar+), followed by a cycle with the boron shutter closed. This process is then repeated until the desired film thickness is achieved. The cyclic approach has reduced the concentration of N-vacancies in our films, which are identifiable in EELS via a pre-peak on the B K edge at 191eV, and enables a more systematic study of the role of the ion beams during c-BN growth. Selected samples were analyzed in a Nion UltraSTEM 200X, with a convergence angle of 27 mrad equipped with a MerlinEELS direct electron counting detector on a Gatan Enfinium EELS spectrometer. EELS analysis was initially performed at 60kV to preserve the fine-structure, and Electron Ptychography was later done at 200kV, using a unique spectrometer configuration based on the work of Sahib et al [3]. Results are presented in Figure 1, with EELS showing the elimination of the N-vacancy pre-peak on the B K edge, and a single reconstructed phase image frame from a Multislice Electron Ptychography dataset, reconstructed using PtyRAD [4], directly showing the c-BN/diamond interface, revealing interfacial defects, nanoscale twins, and large amounts of nanoscale disorder. Continuing work using Multislice Electron Ptychography will explore the 3-dimensional structure of the interface and examine the high through-thickness disorder present in the grown c-BN films. Left, EELS spectra showing the B K edge from different c-BN growths showing the lack of a pre-peak at 191eV in the cyclic c-BN growths indicating suppression of N-vacancies and a higher quality film. Right, a reconstructed phase image consisting of a single frame of a Multislice Electron Ptychography dataset of the c-BN/diamond interface. The image was acquired [011] and the scale bar in represents 1 nm.