Molecular Beam Epitaxy of Wide Bandgap Al(Ga)N and h-BN for Deep-Ultraviolet Optoelectronics

Abstract

Al-rich AlGaN is required for light-emitting diodes (LEDs) and lasers operating in the deep-ultraviolet (UV) spectral range, solar-blind photodetectors, integrated UV photonics, and future high-power electronic devices. For many of these applications, it is essential that AlGaN with atomically smooth surface and with a minimal level of defects and dislocations can be epitaxially grown on foreign lattice-mismatched substrates, which are of lower cost, larger size and more widely available than bulk GaN or AlN substrates. In this work, with the use of molecular beam epitaxy (MBE), superior quality AlN and Al-rich AlGaN grown on sapphire are demonstrated. For AlN epilayers grown directly on sapphire, the X-ray diffraction (XRD) (002) rocking curve peak is significantly narrower than that previously reported for samples of comparable thicknesses. By employing a careful sequence of cycled in situ high-temperature annealing, many of the dislocations and stacking faults generated at the AlN/sapphire interface are reduced within the first 50 nm of growth. The photoluminescence (PL) emission is twice as strong as commercial AlN epitaxial templates that are over 10 times thicker, without the presence of defect-related emissions. With increasing thicknesses, the (002) and (102) rocking curve peak widths are among the best reported for AlN epitaxially grown on sapphire. Furthermore, a detailed study of Al-rich AlGaN epilayers was conducted. A method was developed to precisely control the alloy composition by tuning the Al flux and N flow rate. Under optimized conditions, Al0.6Ga0.4N epilayers exhibit a surface roughness < 0.4 nm, and strong PL emission at room temperature. Despite the lattice mismatch between AlGaN, AlN and sapphire, the formation and propagation of dislocations is significantly suppressed. This work presents important insights into obtaining superior-quality wide-bandgap Al(Ga)N epilayers on lattice-mismatched substrate without the limitations of thick buffer layers, in order to break the efficiency bottleneck of deep-UV optoelectronics.

Hexagonal boron nitride (h-BN) has shown tremendous promise when used alongside other two-dimensional (2D) materials such as graphene, and as a wide-bandgap semiconductor for deep-ultraviolet optoelectronics and quantum photonics. Owing to its large bandgap energy comparable or higher than Al(Ga)N, h-BN can be used to form heterostructures to address some of the critical challenges of Al(Ga)N-based systems. In this context, dislocation-free AlN/BN nanowire heterostructures were grown directly on Si substrate. AlN/BN deep-UV LEDs, exhibiting a relatively low turn-on voltage (< 7 V) and strong electroluminescence (EL) emission at ~210 nm at room temperature were demonstrated for the first time. The epitaxy of h-BN was then studied. Using high-temperature MBE, domains of exceptional crystalline quality were obtained on Ni substrate, with strong excitonic PL emission. It was theoretically and experimentally demonstrated that, even though the energy gap of h-BN is indirect, it luminesces as strongly as direct-gap materials, because of unusually strong phonon coupling. The luminescence intensity (~220 nm) of such a h-BN sample was 10 to 100 times stronger than that of commercially grown direct-bandgap AlN, demonstrating the extraordinary potential of epitaxial h-BN for deep-UV optoelectronics and quantum photonics. By forming a p-i-n structure using this high-quality h-BN as the active region, the current-voltage (I-V) and EL characteristics of a h-BN deep-UV LED is reported for the first time.