Ho Young Ahn

GaN light-emitting diodes on glass substrates with enhanced electroluminescence

We report the enhanced electroluminescence (EL) of GaN light-emitting diodes (LEDs) on glass substrates by controlling GaN crystal morphology, crystallinity, and device fabrication. Depending on the degree of epitaxy, we studied three different GaN morphologies; randomly oriented GaN polycrystals, nearly single-crystalline pyramid arrays, and fully single-crystalline pyramid arrays, which were fabricated by controlling the epitaxial relationship with the substrates. At proper growth temperature, GaN crystallinity was improved with increasing GaN crystal size irrespective of the GaN crystallographic orientation, as determined by spatially resolved cathodoluminescent spectroscopy. All the different GaN morphologies were further fabricated into LEDs to investigate their EL characteristics. The optimized GaN LEDs on glass composed of the nearly single-crystalline GaN pyramid arrays exhibited excellent microscopic EL uniformity and luminance values of about 2700 and 1150 cd m−2 at peak wavelengths of 537 and 480 nm, respectively.

Frequency shifts in two-level ultra-deep reactive ion etched slow-wave structures for 0.1 THz backward-wave oscillations

We present microfabricated slow-wave structures for millimeter- or terahertz-wave vacuum electronic sources. A two-level ultra-deep reactive ion etching (u-DRIE) on highly doped silicon wafers has been employed and allowed for complicated 3-dimensional structures with high aspect ratio. The measured spectra of return loss, however, show 1.2% and 6.8% upshifts in both cutoff and resonant frequencies, respectively. We found the suppression of two-level u-DRIE at the narrow channel between resonant cavities has caused the change of aspect ratios, i.e., saddle-shaped bottom surfaces, which is proved to be associated with the difference in frequency shifts as well as RF attenuation by comparison with theoretical prediction.

Local Crystallization of

Local microheating of amorphous LaB6 film could control the degree of crystallization as determined by spatially resolved Raman spectroscopy and transmission electron microscopy. With full crystallization of the LaB6, we achieved micrometer-sized thermionic electron emission source with the maximum current density of 1.2 A/cm2. The advantage of fabricating a micrometer-sized emitter with high current density enables versatile applications such as compact X-ray or vacuum type terahertz radiation sources. A new structure of micrometer-sized, lateral-type vacuum channel transistor is proposed based on the simulation. The calculated electron travelling time from emitter-to-collector was 8.3 ps at the traveling distance of ~10 μm, meaning that the maximum operation frequency is 120 GHz.

{\rm LaB}_{6}

We report on an electrically driven diffraction grating designed for visible light, where the refractive index of a liquid crystal (LC) was modulated periodically at an interval of 700 nm by applying an external dc bias to a metallic nanograting (NG). The LC-NG structure exhibited a maximum refractive index variation (Δn) of 0.088 and a diffraction efficiency (η) change of 0%-16% with a large diffraction angle of 64° for incident light of 633-nm wavelength. This approach, with the help of faster electronics, provides an opportunity of developing active holograms for real 3-D displays.

Yielding Compact, Strong Thermionic Electron Emission Source

Precise measurement on the RF return loss was performed for a microfabricated slow-wave structure. The interaction circuit was designed to operate at 100 GHz of W-band frequency. A deep reactive ion etching (DRIE) showed a good side-wall profile but inaccurately curved bottom surface. The result represents that the etch rate was strongly dependent on the mask-opening area, which caused a frequency shift of about 5%.

Electrically Driven Diffraction Grating Designed for Visible-Wavelength Region

In our earlier paper dealing with dispersion retrieval from ultra-deep, reactive-ion-etched, slow-wave circuits on silicon substrates, it was proposed that splitting high-aspect-ratio circuits into multilevels enabled precise characterization in sub-terahertz frequency regime. This achievement prompted us to investigate beam-wave interaction through a vacuum-sealed integration with a 15-kV, 85-mA, thermionic, electron gun. Our experimental study demonstrates sub-terahertz, backward-wave amplification driven by an external oscillator. The measured output shows a frequency downshift, as well as power amplification, from beam loading even with low beam perveance. This offers a promising opportunity for the development of terahertz radiation sources, based on silicon technologies.

Return loss measurement of a microfabricated slow-wave structure for backward-wave oscillation

A multi-level microstructure is proposed for terahertz slow-wave circuits, with dispersion relation retrieved by scattering parameter measurements. The measured return loss shows strong resonances above the cutoff with negligible phase shifts compared with finite element analysis. Splitting the circuit into multi levels enables a low aspect ratio configuration that alleviates the loading effect of deep-reactive-ion etching on silicon wafers. This makes it easier to achieve flat-etched bottom and smooth sidewall profiles. The dispersion retrieved from the measurement, therefore, corresponds well to the theoretical estimation. The result provides a straightforward way to the precise determination of dispersions in terahertz vacuum electronics.

Experimental observation of sub-terahertz backward-wave amplification in a multi-level microfabricated slow-wave circuit

The experimental implementation of W-band backward-wave oscillator is achieved by using a multilevel microfabrication of interaction circuit including beam tunnel, slow-wave structure, and output transition, on deep reactive ion etched (DRIE) and metal deposited silicon wafers. The interaction circuit shows precise accuracy in full 3 dimensions, and the return loss measurement agrees well with HFSS simulation. Here we describe the experimental observation of W-band backward-wave oscillation and amplification after successful vacuum sealed integration of interaction circuit, electron gun, beam collector, and output window.

Dispersion retrieval from multi-level ultra-deep reactive-ion-etched microstructures for terahertz slow-wave circuits

In this letter, we propose sub-terahertz (sub-THz) slow-wave circuits for coherent radiation sources through beam-wave interaction mechanism. The circuits are prepared using microfabrication in advanced silicon (Si) technologies. Our approach is to split the circuit into multi levels allowing a low aspect ratio configuration and alleviating the loading effect of deep-reactive-ion etching on silicon wafers. This makes it easier to achieve flat-etched bottom and smooth sidewall profiles in nanoscale accuracy for high frequency operation. The dispersion relation retrieved from the measurement, therefore, corresponds well to the theoretical estimation. In particular, the sub-THz radiation is successfully measured in pulsed operation through the vacuum-sealed integration of the slow-wave circuit with a 15-kV, 90-mA thermionic electron gun. This observation offers a promising opportunity for the development of terahertz radiation sources based on silicon micro- and nanofabrication technologies.

Experimental measurement of W-band backward-wave amplification driven by external pulsed signals

We propose an efficient beam-wave interaction circuit employing a multi-tunnel, slow-wave structure for W-band backward-wave oscillators. The tunnel is disposed one of above and below the beam tunnel, which enhances RF characteristics. The interaction circuit is prepared using a deep-reactive ion etched (DRIE), multi-level microfabrication on silicon wafers. The return loss shows strong resonances predicted by finite-element method (FEM) simulations. The multi-tunnel interaction circuit demonstrates almost similar aspect in return loss to the circuit without beam tunnel. A 1.6 times increase in RF output power is estimated from the particle-in-cell calculation, when compared to the case without multi-tunnel structure. Therefore, we conclude that the multi-tunnel, slow-wave structure successfully improves RF performance.