Suk-Min Ko

Electrically Driven Quantum Dot/Wire/Well Hybrid Light‐Emitting Diodes

Electrically driven quantum dot, wire, and well hybrid light-emitting diodes are demonstrated by using nanometer-sized pyramid structures of GaN. InGaN quantum dots, wires, and wells are formed at the tops, edges, and sidewalls of pyramids, respectively. The hybrid light-emitting diodes containing low-dimensional quantum structures are good candidates for broad-band highly efficient visible lighting sources.

Ultrafast single photon emitting quantum photonic structures based on a nano-obelisk

A key issue in a single photon source is fast and efficient generation of a single photon flux with high light extraction efficiency. Significant progress toward high-efficiency single photon sources has been demonstrated by semiconductor quantum dots, especially using narrow bandgap materials. Meanwhile, there are many obstacles, which restrict the use of wide bandgap semiconductor quantum dots as practical single photon sources in ultraviolet-visible region, despite offering free space communication and miniaturized quantum information circuits. Here we demonstrate a single InGaN quantum dot embedded in an obelisk-shaped GaN nanostructure. The nano-obelisk plays an important role in eliminating dislocations, increasing light extraction, and minimizing a built-in electric field. Based on the nano-obelisks, we observed nonconventional narrow quantum dot emission and positive biexciton binding energy, which are signatures of negligible built-in field in single InGaN quantum dots. This results in efficient and ultrafast single photon generation in the violet color region.

Giant Rabi Splitting of Whispering Gallery Polaritons in GaN/InGaN Core–Shell Wire

The hybrid nature of exciton polaritons opens up possibilities for developing a new concept nonlinear photonic device (e.g., polariton condensation, switching, and transistor) with great potential for controllability. Here, we proposed a novel type of polariton system resulting from strong coupling between a two-dimensional exciton and whispering gallery mode photon using a core-shell GaN/InGaN hexagonal wire. High quality, nonpolar InGaN multiple-quantum wells (MQWs) were conformally formed on a GaN core nanowire, which was spatially well matched with whispering gallery modes inside the wire. Both high longitudinal-transverse splitting of nonpolar MQWs and high spatial overlap with whispering gallery modes lead to unprecedented large Rabi splitting energy of ∼180 meV. This structure provides a robust polariton effect with a small footprint; thus, it could be utilized for a wide range of interesting applications.

Nonlinear Photonic Diode Behavior in Energy-Graded Core–Shell Quantum Well Semiconductor Rod

Future technologies require faster data transfer and processing with lower loss. A photonic diode could be an attractive alternative to the present Si-based electronic diode for rapid optical signal processing and communication. Here, we report highly asymmetric photonic diode behavior with low scattering loss, from tapered core-shell quantum well semiconductor rods that were fabricated to have a large gradient in their bandgap energy along their growth direction. Local laser illumination of the core-shell quantum well rods yielded a huge contrast in light output intensities from opposite ends of the rod.

Electro-optic and converse-piezoelectric properties of epitaxial GaN grown on silicon by metal-organic chemical vapor deposition

We report the measurement of the (r13, r33) Pockels electro-optic coefficients in a GaN thin film grown on a Si(111) substrate. The converse piezoelectric (d33) and electro-absorptive coefficients are simultaneously determined. Single crystalline GaN epitaxial layers were grown with a AlGaN buffer layer by metal organic chemical vapor deposition, and their structural and optical properties were systematically investigated. The electro-optic, converse piezoelectric, and electro-absorptive coefficients of the GaN layer are determined using an original method. A semi-transparent gold electrode is deposited on the top of the GaN layer, and an alternating voltage is applied between top and bottom electrodes. The coefficients are simultaneously and analytically determined from the measurement of the electric-field-induced variation ΔR(θ) in the reflectivity of the Au/GaN/buffer/Si stack, versus incident angle and light polarization. The method also enables to determine the GaN layer polarity. The results obtain...

Surface plasmon modulation induced by a direct-current electric field into gallium nitride thin film grown on Si(111) substrate

We report here the experimental results on a field effect refractive index change into gallium nitride (GaN) structures. This effect is characterized through the common prism-coupling technique with the application of a vertical direct-current electric field. Surface plasmon propagation was used to increase the sensitivity of the electro-optic measurements. We have obtained a large refractive index variation for GaN epilayer, around 1.4×10−2 at 1.55μm wavelength. In order to understand the origin of the index modulation, we have conducted a scanning transmission electron microscopy analysis and discussed the influence of threading dislocations density acting as traps and thermo-optic effect. According to recent works, we observed experimentally the optical response of a non-linear electro-optic effect on GaN on Si(111) substrate and estimated a Kerr coefficient of about 2.14×10−16m2 V−2.

Novel photonic devices using core-shell nanostructures

Recent research into group III-nitride semiconductors and their heterostructures has enabled the development of highly efficient optoelectronic devices, such as LEDs, lasers, transistors, sensors, and solar cells. These advances have been made possible by the superior chemical resistance, stable luminescence, and tunable direct bandgap inherent to this class of semiconductor. There are, however, several difficulties that must be overcome on the route toward wider adoption. Among these challenges, high-density threading dislocations (crystal line defects generated by the strain that arises due to mismatches between the lattice constant and the thermal expansion coefficient) in conventional gallium nitride (GaN) epilayers and heterogeneous substrates must be reduced. These defects can act as non-radiative and scattering centers, adversely affecting electron transport and thereby proving detrimental to the performance of GaN-based devices. Additionally, total internal reflection occurs at the interface between the GaN film and air, resulting in only 4% of the light generated inside the GaN film escaping. Moreover, due to large piezoelectric polarization, indium GaN/GaN (InGaN/GaN) heterostructures grown along the polar axis (c-axis) suffer from high built-in electric fields (several MV/cm). Employing nanostructure geometry offers a way to reduce these problems. GaN nanostructures formed on heterogeneous substrates have a reduced dislocation density, due to the freesurface-induced strain relaxation which occurs as a result of the large surface-to-volume ratio. The geometry also gives rise to efficient light scattering, thereby enhancing light extraction efficiencies. Furthermore, because the GaN/InGaN core-shell heterostructures are grown on the non-polar and semi-polar facets of the GaN nanostructures, the built-in field is reduced. Due to the high surface-to-volume ratio of the core-shell structure, the active region area of its heterostructure is much Figure 1. Scanning electron microscope (SEM) images of gallium nitride (GaN) nanostructures: (a) nano-obelisks (NOs) and (b) nanorods (NRs). Optical properties of GaN nanostructures: (c) photoluminescence (PL) spectra of ensemble NOs and a reference GaN epilayer and (d) cathodoluminescence (CL) spectra of a single NR at various positions (from bottom to top: P1 to P12). Reprinted from S. M. Ko, et al.1 with permission. Copyright American Chemical Society 2014. a.u.: Arbitrary units. DBE: Donor-bound exciton.

Group III-Nitride Semiconductor Nanostructures for Novel Photonic and Quantum Photonic Applications

We present group III-nitride semiconductor nanostructures grown on various types of GaN-based pyramid, annular, columnar, and tapered structures, together with their applications in broadband light emitting devices, unidirectional photonic diodes, and single photon sources.

Investigation of structural, morphological and optical properties of GaN/AlGaN heterostructures on Si

A good justification for gallium nitride on silicone is the potential for integrated optoelectronic circuits and for the low cost bring by growth of GaN on a large size wafers. Actually, the application interest for GaN/Si is power electronics. This work focused on the optimization of the growth process for GaN/Si and the relation between the structure and the optical properties. Using the guided wave prism coupling technique, we have fully established the index dispersion of GaN at room temperature and its temperature dependence in the wavelength range 0.4 to 1.5μm. We report a slightly low temperature dependence. Results demonstrated excellent waveguide properties of GaN on silicon with optical propagation loss below 1dB/cm. We compared trhe results on Si with those on sapphire. This opens a real opportunity of future device using this technology.