Structural engineering toward enhanced luminescence from hexagonal boron nitride grown by chemical vapor deposition

DoctorHexagonal boron nitride (h-BN) is a two-dimensional (2-D) van der Waals material featuring a high chemical stability and a wide energy band gap of ~ 6 eV, which makes it attractive not only as an ultrathin dielectric insulator but also as a promising luminous material for deep ultraviolet (DUV) optoelectronic devices and stable single photon sources. While many efforts to understand radiative transition mechanism in h-BN and utilize it as an emitter source have been made by using photoluminescence (PL) and cathodoluminescence (CL) techniques, realization of an electrically-driven emission from h-BN remains challenging due to difficulties in doping, formation of a stable p-n junction and low resistance ohmic contacts owing to wide bandgap nature of h-BN. The low mobility-life time products for electrons and holes in h-BN decline transport of charges and formation of excitons, which limits the electroluminescence (EL) from h-BN as well. Realization of electrically-driven h-BN light emitters, that would be practically much more advantageous than PL- and CL-based sources in terms of efficiency, integrability and scalability requires significant achievements in both high quality large scale h-BN films and a device structure facilitating an efficient confinement of the injected carriers to enhance probability of radiative recombination within h-BN active layers. In this dissertation, two strategies and corresponding experimental results on (1) improvement of wafer-scale h-BN crystallinity by high temperature post-growth annealing for enhancing luminescence from h-BN and (2) fabrication of Al2O3/h-BN multiple heterostructure to utilize 2-D h-BN as an active layer in electrically-driven light emitters are presented. Firstly, high temperature post-growth annealing of h-BN layers grown by metal-organic chemical vapor deposition (MOCVD) at over 1500 ℃ was performed in order to produce wafer-scale high quality h-BN films. The effects of the post-growth annealing on the structural properties of the h-BN were investigated by using various spectroscopic and microscopic characterization methods such as Raman spectroscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption fine-structure (NEXAFS), and transmission electron microscopy (TEM) analyses. Sharper Raman scattering of in-plane E2g mode and XPS core level spectra, reduced defect-related shoulder peaks in NEXAFS spectra, and well-oriented domains constituting the h-BN film in TEM analysis were observed after the post-annealing of the h-BN film at 1600 ℃ in N2 ambient. The improved crystallinity of the h-BN films after the post-annealing is attributed to the rearrangement of the atoms around imperfection of polycrystalline h-BN by obtaining sufficient kinetic energy during the annealing process. As a result, enhanced PL from the post-annealed h-BN was achieved due to reduced non-radiative recombination of excitons at structural defect states in comparison with as-grown h-BN films. Secondly, electrically-driven light emission from 2-D h-BN active layer was demonstrated by employing a five-period Al2O3/h-BN multiple heterostructure. Based on the investigation of the electrical properties of the devices, the Al2O3 layers play an important role as potential barriers for an effective confinement of injected carriers within the h-BN wells, enhancing the probability of radiative recombination as well as suppressing the trap-assisted tunneling process, and hence, the electrostatic breakdown in the h-BN layers. The broad visible EL spectrum with peak intensity at ~ 1.9 eV is ascribed to the ensemble of optical transitions via structural defects in the h-BN layers. In addition, the red-shift in the EL spectra by 0.102 eV under increased bias from 11.92 to 12.54 MV/cm was observed, which is possibly owing to the combination of multiple effects including Stark effect, change of active defect states by electrical stress, and bandgap shrinkage at high current injection. This study demonstrates (1) a promising strategy to achieve wafer-scale high quality h-BN film from a defective h-BN film grown by MOCVD at relatively low temperature simply by high temperature post-growth annealing, and (2) a promising potential of Al2O3/h-BN multiple heterostructure as a practical and efficient platform for electrically-driven light emitters based on wide bandgap two-dimensional materials