Hyeong-Yong Hwang

III-nitride core–shell nanorod array on quartz substrates

We report the fabrication of near-vertically elongated GaN nanorods on quartz substrates. To control the preferred orientation and length of individual GaN nanorods, we combined molecular beam epitaxy (MBE) with pulsed-mode metal–organic chemical vapor deposition (MOCVD). The MBE-grown buffer layer was composed of GaN nanograins exhibiting an ordered surface and preferred orientation along the surface normal direction. Position-controlled growth of the GaN nanorods was achieved by selective-area growth using MOCVD. Simultaneously, the GaN nanorods were elongated by the pulsed-mode growth. The microstructural and optical properties of both GaN nanorods and InGaN/GaN core–shell nanorods were then investigated. The nanorods were highly crystalline and the core–shell structures exhibited optical emission properties, indicating the feasibility of fabricating III-nitride nano-optoelectronic devices on amorphous substrates.

Variability and feasibility of CVD graphene interconnect

Graphene and its derivatives (graphite, CNT) have very high conductivity and critical current density higher than 108 A/cm2, which can be utilized in interconnect applications. Theoretically, a doped graphene is predicted to have better performance than Cu as an interconnect conductor. However, the feasibility of graphene interconnect has not been experimentally examined systematically. In this paper, the critical current density of single layer and multilayer graphene are studied to provide insights about the feasibility of graphene interconnect technology.

Optical and structural properties of microcrystalline GaN on an amorphous substrate prepared by a combination of molecular beam epitaxy and metal–organic chemical vapor deposition

Microscale platelet-shaped GaN grains were grown on amorphous substrates by a combined epitaxial growth method of molecular beam epitaxy (MBE) and metal–organic chemical vapor deposition (MOCVD). First, MBE GaN was grown on an amorphous substrate as a pre-orienting layer and its structural properties were investigated. Second, MOCVD grown GaN samples using the different growth techniques of planar and selective area growth (SAG) were comparatively investigated by transmission electron microscopy (TEM), cathodoluminescence (CL), and photoluminescence (PL). In MOCVD planar GaN, strong bound exciton peaks dominated despite the high density of the threading dislocations (TDs). In MOCVD SAG GaN, on the other hand, TDs were clearly reduced with bending, but basal stacking fault (BSF) PL peaks were observed at 3.42 eV. The combined epitaxial method not only provides a deep understanding of the growth behavior but also suggests an alternative approach for the growth of GaN on amorphous substances.

Distinctive mapping of strain and quantum size effects using depth-resolved photoluminescence in ZnO nanoneedles

In order to locate the spatially resolved influence of the strain, carrier localization, and quantum size effect (QSE) in tapered ZnO nanoneedles (NNs), the photoluminescence(PL) was measured as a function of the incident laser angle θ from 0∘ (normal to a surface) to 85∘. With increasing θ, the excitation point is spatially restricted along the axis of the NNs and varies from the ZnO buffer/sapphire interface to the tips of the NNs. In this way, we identified a strain-induced blue-shift of 25.3 meV at the ZnO buffer/sapphire interface, which corresponds to a tensile strain of 0.319%. The influence of strain and the concomitant indications of carrier localization decreased as the excitation point moved to a higher location along the NNs with increasing θ whereas the QSE revealed an abrupt blue-shift near the tips of the NNs. Furthermore, time-resolved PLmeasurement as a function of the excitation angle was used to distinguish the strain effect from the QSE. We observed two spatially competing tendencies: (1) the decay times are influenced by the increase in the interfacial strain and (2) the decay times are influenced by the decrease in the diameter-dependent QSE near the tips of the tapered ZnO NNs.

Mechanisms of ambient dependent mobility degradation in the graphene MOSFETs on SiO 2 substrate

Two different mechanisms affecting the device instability and mobility degradation at graphene MOSFET on SiO2 substrate and their time constant, 40μsec and ~ 370μsec, have been identified. Oxygen/H2O reaction at the surface of graphene was identified as a major source of device hysteresis causing mobility degradation and device instability.

Characteristics of Metal/Ferroelectric (PVDF-TrFE)/Graphene (MFG) device

Characteristics of new reconfigurable graphene device with Metal/ Ferroelectric (PVDF-TrFE)/Graphene (MFG) stack is presented. Key features include programming speed <; 100nsec, retention up to 1000sec, endurance upto 1000 cycles and more than 775% on/off ratio. While memory like functionalities are primarily presented in this paper, MFG device has many versatile applications such as reconfigurable interconnect resistor or logic device, pressure sensitive touch sensor and so on.