Hwa-Mok Kim

Growth and characterization of single-crystal GaN nanorods by hydride vapor phase epitaxy

Single-crystalline GaN nanorods are formed on a sapphire substrate by hydride vapor phase epitaxy (HVPE). Their structural and optical properties are investigated by x-ray diffraction, scanning and transmission electron microscopy, and cathodoluminescence (CL) techniques. The high density of straight and well-aligned nanorods with a diameter of 80–120 nm formed uniformly over the entire 2 in. sapphire substrate. The x-ray diffraction patterns and transmission electron microscopic images indicate that the formed GaN nanorods are a pure single crystal and preferentially oriented in the c-axis direction. We observed a higher CL peak position of individual GaN nanorods than that of bulk GaN as well as a blueshift of CL peak position with decreasing the diameter of GaN nanorods, which are attributed to quantum confinement effect in one-dimensional GaN nanorods. We demonstrate that the well-aligned, single-crystalline GaN nanorods with high density, high crystal quality, and good spatial uniformity are formed b...

High efficiency and brightness of blue light emission from dislocation-free InGaN∕GaN quantum well nanorod arrays

We report on the optical properties of blue-light-emitting, dislocation-free InGaN∕GaN multiple quantum well (MQW) nanorod arrays (NRAs) with high brightness and high efficiency. The InGaN MQW NRAs were grown by metal-organic hydride vapor phase epitaxy and the optical properties were investigated in detail by photoluminescence (PL), PL excitation (PLE) and time resolved PL. We observed a large Stokes-like shift between InGaN PL emission and PLE absorption edge due to the influence of built-in internal electrical field, reflecting the coherent growth of MQW along the NRA growth direction. From the temperature-dependent PL, we extracted a PL intensity ratio at 300to15K of ∼55.4% and large thermal activation energy of ∼171meV from the InGaN∕GaN MQW NRAs. Time-resolved PL results showed almost the same decay time of the InGaN emission at 20 and 300K. From the results, the optical properties are dominated by the radiative recombination process and are insensitive to temperature due to large thermal activation...

Field emission displays of wide-bandgap gallium nitride nanorod arrays grown by hydride vapor phase epitaxy

Abstract We demonstrate the very simple realization of the field emission displays using wide-bandgap gallium nitride nanorod grown by hydride vapor phase epitaxy. For the real device fabrication and mass production, we fabricated high-density nanorod arrays. The electron emission turn-on field (Eto) was about 0.5 V/μm and total current was 4.45 mA at 2.06 V/μm (current density, J=54 μA/cm2). A uniform ‘Q’ character emission image with high stability was obtained from GaN nanorod array electron emitters. The electron emission properties of GaN nanorod arrays were comparable with (or even lower turn-on voltage than) those of carbon nanotubes.

Polarity determination for GaN/AlGaN/GaN heterostructures grown on (0001) sapphire by molecular beam epitaxy

In order to study the formation of polarization-induced two-dimensional electron gases (2DEGs), GaN/AlGaN/GaN heterostructures with different polarity were grown on sapphire (0001) substrates by plasma-assisted molecular-beam epitaxy. The polarity of GaN layers can be changed from the normal N-polarity to a Ga-polarity surface by inserting a thin Al metal layer prior to the growth of AlN buffer layer. The surface stability of each lattice polarity film was studied by in situ reflection high-energy electron diffraction. The polarity of the films and the location of 2DEGs in the heterostructure have been evaluated by investigating the carrier concentration profiles in the GaN/AlGaN/GaN heterostructures. A simple model to explain the polarity change by a thin Al metal layer is proposed.

Formation and characterization of (Zn1-xMnx)O diluted magnetic semiconductors grown on (0001) Al2O3 substrates

(Zn1-xMnx)O diluted magnetic semiconductors (DMSs) grown on (0001) Al2O3 substrates by radio frequency magnetron sputtering were investigated with the aim of producing a material with a high ferromagnetic transition temperature (Tc). X-ray diffraction, photoluminescence, and Hall-effect measurements showed that the grown (Zn1-xMnx)O thin films were p-type crystalline semiconductors with single phases. Magnetization curve as a function of magnetic field at 5 K indicated that ferromagnetism existed in the (Zn1-xMnx)O thin films, and magnetization curve as a function of temperature showed that the Tc of the (Zn0.93Mn0.07)O thin film was 70 K. These observations can improve the understanding of the increase in Tc for (Zn1-xMnx)O DMSs grown on (0001) Al2O3 substrates.

Growth and applications of HVPE-GaN nanorods

We demonstrate the properties of gallium nitride nanorods by hydride vapor phase epitaxy (HVPE). Single crystalline gallium nitride nanorods are formed on a sapphire substrate by HVPE. Single crystalline p-type and n-type gallium nitride nanorods have been grown and characterized by electrical transport measurements. HVPE was used to controllably introduce either magnesium or silicon dopants during the growth of the gallium nitride nanorods. The electron emission properties of gallium nitride nanorod array electron emitters were comparable with (or displayed even lower turn-on voltage than) those of carbon nanotubes. Wide-bandgap current rectifiers with high breakdown voltage (over −10 V) and near-ultraviolet p-n junction LEDs with emission wavelength of 390 nm, based on the single-rod gallium nitride p-n junction array, were obtained.

New Technique for the Thermal Resistance Measurement of Power Field Effect Transistors Using Cathodoluminescence

A new approach for measuring the thermal resistance of power field-effect transistors (FETs) with extremely high spatial resolution below 40 nm has been demonstrated using cathodoluminescence generated by a low-energy electron beam. The energy shift of the fundamental bandgap caused by the current heating in the channel region is spatially probed, and is converted to the channel temperature using the temperature dependence of the material bandgap. The obtained thermal resistances of the GaAs metal semiconductor field effect transistor (MESFET) and the GaN MESFET are compared with those measured by the conventional electrical technique.

Electrical and Magnetic Properties of Doped ZnO Nanowires

ZnO nanowires doped with Mn, Fe, Sn, and Li during the thermal growth following direct chemical synthesis were investigated using electric and magnetic measurements. Currentvoltage characteristics of individual nanowires configured as a two-terminal device with Al electrodes show apparent rectify behavior indicating the Schottky-like barrier formation and resistivity being less 3 Ω·cm. Reproducible resistance modulation by a dc voltage at room temperature is observed. Magnetic susceptibility of the doped nanowires as a function of temperature demonstrates Curie–Weiss behavior. Magnetization versus field curves show hysteresis with the coercive field of about 200 Oe. The spatially-resolved magnetic force measurements of individual nanowires revealed the magnetic domain structure. The domains align perpendicular to c-axis and can be polarized in the external magnetic field.

Preparation of 30×30 mm 2 Free-Standing GaN Wafer by Mechanical Liftoff and Optical Properties in The Backside of The Free GaN by Cathodoluminescence

In this study, free-standing GaN was produced from 350- to 400-μm-thick GaN films grown on sapphire by using hydride vapor phase epitaxy. The thick films were separated from the substrate by using the mechanical polishing method with a diamond slurry. After liftoff, the bow is slight or absent in the resulting free-standing GaN. Double-crystal X-ray diffraction, hall measurements, and cathodoluminescence were used for characterizing the free-standing GaN wafer. To investigate spatially the backside of the free-standing GaN substrate, we controlled the electron beam energy from 5 keV to 30 keV. As the beam energy increased, dark regions, i.e., nonradiative regions, become smaller than bright regions. We think this means that nonradiative centers, i.e., threading dislocations, merge during film growth.

Correlation of surface morphologies with Mn compositions of Ga1−xMnxAs epilayers grown by liquid phase epitaxy

Abstract In this letter, we investigated the correlation between as-grown surface morphologies and Mn compositions of Ga 1− x Mn x As epilayers—a III–V-diluted magnetic semiconductor—grown by liquid phase epitaxy (LPE). Ga 1− x Mn x As epilayers were grown at 595 °C from 50% Ga+50% Bi mixed solvent. The grown layers were characterized by energy dispersive X-ray analysis (EDS) and atomic forced microscopy (AFM). The Mn composition measured by EDS after growth process was varied from 1% to 7%. As increasing Mn composition surface morphologies of as-grown Ga 1− x Mn x As epilayers were varied. At higher Mn compositions, the morphology of the surface layers degrades strongly, preventing removal of the solution-melt from it.