Heavy reliance on extensively studied AlGaN based light emitting diodes (LEDs) to replace environmentally hazardous mercury based ultraviolet (UV) lamps is inevitable. However, external quantum efficiency (EQE) for AlGaN based deep UV emitters remains poor. Dislocation induced nonradiative recombination centers and poor electron-hole wavefunction overlap due to the large polarization field induced quantum confined stark effect (QCSE) in “Al” rich AlGaN are some of the key factors responsible for poor EQE. In addition, the transverse electric polarized light is extremely suppressed in “Al”-rich AlGaN quantum wells (QWs) because of the undesired crossing over among the light hole (LH), heavy hole (HH) and crystal-field split-off (SH) states. Here, optical and structural integrities of dislocation-free ultrathin GaN quantum disk (QDisk) (~ 1.2 nm) embedded in AlN barrier (~ 3 nm) grown employing plasma-assisted molecular beam epitaxy (PAMBE) are investigated considering it as a novel nanostructure to realize highly efficient TE polarized deep UV emitters. The structural and chemical integrities of thus grown QDisks are investigated by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). We, particularly, emphasize the polarization dependent photoluminescence (PL) study of the GaN Disks to accomplish almost purely TE polarized UV (~ 260 nm) light. In addition, we observed significantly high internal quantum efficiency (IQE) of ~ 80 %, which is attributed to the enhanced overlap of the electron-hole wavefunction in extremely quantum confined ultrathin GaN QDisks, thereby presenting GaN QDisks embedded in AlN nanowires as a practical pathway towards the efficient deep UV emitters.
Growing III-nitride materials on unconventional substrates is attractive since it gives the possibility to fabricate novel devices at a potentially reduced cost. Spontaneously grown III-nitride nanowire structure is capable of achieving this goal, as they provide the capability to epitaxially grow high-quality single crystalline structures without global lattice and thermal matching requirement between the material and the substrate. In this work, we present the growth and characterization of GaN nanowire using plasma-assisted molecular beam epitaxy (PA-MBE) using an indium tin oxide (ITO)-coated silica substrate. The nanowires are shown to grow in the [0001] direction perpendicular to the substrate plane, with n-polar polarity. We found that the lateral size of the nanowires closely follows the grain size of the underlying ITO layer. Temperature-dependent photoluminescence measurement indicates a high-quality GaN material as indicated by the high internal quantum efficiency value. Conductive AFM measurement was performed to evaluate the feasibility of utilizing the GaN nanowire on ITO for device fabrication. By growing GaN nanowires on top of ITO-coated silica substrate, we open up the possibility of fabricating transparent nitride-based devices using transparent scalable substrate.
Group III-nitride semiconductor materials especially AlGaN are key-emerging candidates for the advancement of ultraviolet (UV) photonic devices. Numerous nanophotonics approaches using nanostructures (e.g., nanowires, nanorods, and quantum dots/disks) and nanofabrication (e.g., substrate patterning, photonic crystals, nanogratings, and surface-plasmons) have been demonstrated to address the material growth challenges and to enhance the device efficiencies of photonic devices operating at UV wavelengths. Here, we review the progress of nanophotonics implementations using nanostructured interfaces and nanofabrication approaches for the group III-nitride semiconductors to realize efficient UV-based photonic devices. The existing challenges of nanophotonics applications are presented. This review aims to provide analysis of state-of-the-art nanophotonic approaches in advancing the UV-photonic devices based on group III-nitride semiconductors.
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