Growth and application of wafer-scale hexagonal boron nitride

Abstract

Hexagonal boron nitride (hBN) is a 2D material that has attracted considerable attention in recent years. As a layered material and a wide bandgap semiconductor, hBN has been used in different applications, ranging from deep UV emitters to gate dielectric in field effect devices based on 2D materials. hBN has also been used as a substrate for growing III-V semiconductors via van der Waals epitaxy, for the development of flexible optoelectronic devices. More recently, single photon emission (SPE) from defects in hBN have generated new interests in this material for quantum computing related applications. For many of the applications, hBN is obtained through facile exfoliation of atomically thin flakes, from commercially available bulk crystals. The lateral dimensions of flakes is less than a few tens of um. This severely limits scale-up and large-area applications of hBN. Therefore, this thesis explores wafer-scale growth of hBN, using metal organic vapour phase epitaxy (MOVPE). In this study, hBN was deposited over commercially available 2" sapphire substrates. Triethylboron (TEB) and ammonia were used as B and N growth precursors, respectively. Due to severe parasitic reactions between the precursors, a flow-modulation scheme for precursor injection was adopted. A comprehensive characterization of the deposited films was undertaken using different experimental techniques. hBN films deposited on sapphire had a wrinkled surface morphology, which was studied using AFM. TEM was used to evaluate the orientation of the induvial hBN crystal planes with respect to the substrate. Raman spectroscopy provided a quick and nondestructive method for confirming hBN deposition and analyzing residual strain in the deposited films. With the help of XPS, carbon was identified as the dominant impurity. The optical properties of MOVPE-hBN were studied using photo/cathodo-luminescence spectroscopies. Due to high carbon incorporation, no bandedge luminescence was recorded from MOVPE-hBN. Instead, the emission spectra was dominated by impurity related sub-bandgap luminescence between 300-350 nm, and between 570-750 nm (rPL), with a strong dependence on TEB flux and carbon incorporation. SPEs were reported for the first time from MOVPE-grown hBN films. SPEs were only observed in those hBN films, which had the lowest carbon doping and negligible rPL. Many of the SPEs found in MOVPE-hBN were photo-stable, had a narrow spectral distribution of the emission wavelength and emitter lifetimes of the order of few nanoseconds, consistent with previous reports. hBN films grown on sapphire were transferred onto substrates containing silver and gold nanoparticles (NPs). Being atomically thin and flexible, hBN was able to effectively wrap around Ag nanoparticles to form an impermeable barrier, which was found to be effective in preventing oxidation of Ag NPs even at elevated temperatures. Consequently, the plasmonic activity of the hBN covered Ag NPs remained preserved and was demonstrated through surface enhanced Raman spectroscopy. In a different study, AlN was grown on wafer-scale hBN (and on sapphire for comparison) using MOVPE, under different conditions. Detailed structural and morphological analysis of the AlN films were also undertaken. Using a modified, multi-step high temperature growth process, planar AlN films, with improved crystallinity were grown on hBN. The AlN films easily delaminated from the sapphire, due to underlying hBN and showed a relaxation in compressive strain. Overall, this work provides valuable insights into wafer-scale growth of hBN using MOVPE. The crystallinity, morphology and luminescent properties of MOVPE-hBN films were studied in detail and found to be critically effected by the choice of growth parameters. The versatility of large-area hBN films has also been showcased. Different applications of the MOVPE-hBN films, ranging from passivation of Ag NPs, to SPEs and van der Waals epitaxy of AlN films were demonstrated. Show more