Sang Ho Oh

4 Gbps direct modulation of 450 nm GaN laser for high-speed visible light communication

We demonstrate high-speed data transmission with a commercial high power GaN laser diode at 450 nm. 2.6 GHz bandwidth was achieved at an injection current of 500 mA using a high-speed visible light communication setup. Record high 4 Gbps free-space data transmission rate was achieved at room temperature.

Green Semipolar (202̄1̄) InGaN Light-Emitting Diodes with Small Wavelength Shift and Narrow Spectral Linewidth

We study the optical spectral properties for green semipolar (20) and (201) light-emitting diode (LED) with same indium compositions. Compared to (201) devices, the fabricated (20) micro-LED (~0.005 mm2) showed negligible blue shift and smaller full width at half maximum (FWHM) up to extremely high current densities (10,000 A/cm2). Theoretical simulation indicates that the (20) InGaN quantum well (QW) has reduced polarization-related effects due to combined effects of electric field cancelling and Coulomb screening effect. In addition, the packaged device performance for small-area (~0.144 mm2) semipolar green (20) and (201) LEDs were also discussed. The green (20) LED showed smaller wavelength shift and narrower FWHM than green LEDs fabricated on other planes.

High-power low-droop violet semipolar (303¯1¯) InGaN/GaN light-emitting diodes with thick active layer design

Devices grown on nonpolar and semipolar planes of GaN offer key performance advantages over devices grown on the conventional c-plane, including reduced polarization fields. This allows for a wider design space on semipolar planes for light emitting diodes (LEDs) to address the problem of efficiency droop at high current densities. LED structures with very thick (10–100 nm) InGaN single-quantum-well/double heterostructure active regions were grown using conventional metal organic chemical vapor deposition on semipolar (303¯1¯) free-standing GaN substrates and processed and packaged using conventional techniques. Simulated band diagrams showed reduced polarization fields on the (303¯1¯) plane. The calculated critical thickness for misfit dislocation formation is higher on the (303¯1¯) plane than on other semipolar planes, such as (202¯1¯), allowing for thicker active regions than our previous work to further reduce droop. The higher critical thickness was confirmed with defect characterization via cathodol...

Thermally enhanced blue light-emitting diode

We investigate thermoelectric pumping in wide-bandgap GaN based light-emitting diodes (LEDs) to take advantage of high junction temperature rather than avoiding the problem of temperature-induced efficiency droop through external cooling. We experimentally demonstrate a thermally enhanced 450 nm GaN LED, in which nearly fourfold light output power is achieved at 615 K (compared to 295 K room temperature operation), with nearly no reduction in the wall-plug efficiency (i.e., electrical-optical energy conversion efficiency) at bias V<ℏω/q. The LED is shown to work in a mode similar to a thermodynamic heat engine operating with charged carriers pumped into the active region by a combination of electrical work and Peltier heat (phonons) drawn from the lattice. In this optimal operating regime at 615 K, the LED injection current (3.26 A/cm2) is of similar magnitude to the operating point of common high power GaN based LEDs (5–35 A/cm2). This result suggests the possibility of removing bulky heat sinks in curre...

Hybrid tunnel junction contacts to III–nitride light-emitting diodes

In this work, we demonstrate highly doped GaN p–n tunnel junction (TJ) contacts on III–nitride heterostructures where the active region of the device and the top p-GaN layers were grown by metal organic chemical vapor deposition and highly doped n-GaN was grown by NH3 molecular beam epitaxy to form the TJ. The regrowth interface in these hybrid devices was found to have a high concentration of oxygen, which likely enhanced tunneling through the diode. For optimized regrowth, the best tunnel junction device had a total differential resistivity of 1.5 × 10−4 Ω cm2, including contact resistance. As a demonstration, a blue-light-emitting diode on a () GaN substrate with a hybrid tunnel junction and an n-GaN current spreading layer was fabricated and compared with a reference sample with a transparent conducting oxide (TCO) layer. The tunnel junction LED showed a lower forward operating voltage and a higher efficiency at a low current density than the TCO LED.

Semipolar III-nitride light-emitting diodes with negligible efficiency droop up to -1 W

We demonstrate 1 mm2 blue light-emitting diodes with a negligible efficiency droop up to ~1 W. LEDs with 12- to 14-nm-thick single quantum wells were grown by metalorganic chemical vapor deposition on a free-standing semipolar GaN substrate. Packaged devices showed an external quantum efficiency of 42.3% at 20 A/cm2 with a negligible efficiency droop up to 991 mW at 900 mA. At 900 mA, the thermal droop and hot/cold factor were 8.2% and 0.92, respectively. The adoption of a thick active region resulted in excellent optical and thermal performance characteristics that are suitable for high-power lighting applications.

Dynamic characteristics of 410 nm semipolar (202¯1¯) III-nitride laser diodes with a modulation bandwidth of over 5 GHz

The dynamic characteristics of III-nitride multi-quantum well laser diodes (LDs) emitting at 410 nm were investigated. LDs were grown on semipolar (202¯1¯) bulk GaN substrates and fabricated into devices with cavity lengths ranging from 900 nm to 1800 nm. A 3-dB bandwidth of 5 GHz and 5 Gbit/s direct modulation with on-off keying were demonstrated, which were limited by the bandwidth of the photodetector used for the measurements. The differential gain of the LDs was determined to be 2.5 ± 0.5 × 10−16 cm2 by comparing the slope efficiency for different cavity lengths. Analysis of the frequency response showed that the K-factor, the gain compression factor, and the intrinsic maximum bandwidth were 0.33 ns, 7.4 × 10−17 cm3, and 27 GHz, respectively.

2.6 GHz high-speed visible light communication of 450 nm GaN laser diode by direct modulation

Gallium Nitride (GaN) based light emitting diodes (LEDs) has been considered as a next generation lighting source due to its high efficiency, long lifetime, and high brightness. In addition to the lighting purpose, visible light communication (VLC) on LEDs has been studied since it is easily available in the existing light infrastructure. Exponentially increasing wireless data traffic in radio frequency (RF) also motivated the need for VLC. However, the modulation bandwidth of LEDs with a long carrier recombination lifetime (~ns) is limited up to ~ 400 MHz even though orthogonal frequency division multiplexing (OFDM) rather than direct modulation could improve data rate up to 3 Gbit/s [1]. It can be seen that laser based VLC is necessary because the modulation bandwidth of laser diodes is limited by photon lifetime (~ps). In addition, since Denault et al. reported 76 lm/W efficacy for 442 nm laser based white lighting, the blue laser is promising lighting source readily available for white lighting communication, which is also called light fidelity (Li-Fi) [2]. Recent progress on high-speed VLC using a 422 nm laser diode showed 1.4 GHz bandwidth and 2.5 Gbps. Even if this is about three times larger bandwidth than blue LED VLC, the system was limited by the bandwidth of the photo detector (PD) due to the absence of commercially available high speed PD covering in the blue region with enough responsivity [3]. Our study demonstrated the first novel VLC system limited by the bandwidth of 450 nm laser diode with a high-speed UV-extended PD giving 2.6 GHz modulation bandwidth and 4 Gbit/s data transmission rate.

On the optical polarization properties of semipolar ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN quantum wells

In the framework of k · p-theory, semipolar ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN quantum wells (QWs) have equivalent band structures and are expected to have identical optical polarization properties. However, ( 20 2 ¯ 1 ) QWs consistently exhibit a lower degree of linear polarization (DLP) than ( 20 2 ¯ 1 ¯ ) QWs. To understand this peculiarity, we investigate the optical properties of ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN single QW light-emitting diodes (LEDs) via resonant polarization-resolved photoluminescence microscopy. LEDs were grown on bulk substrates by metal organic vapor phase epitaxy with different indium concentrations resulting in emission wavelengths between 442 nm and 491 nm. We discuss the origin of their DLP via k · p band structure calculations. An analytical expression to estimate the DLP in the Boltzmann-regime is proposed. Measurements of the DLP at 10 K and 300 K are compared to m-plane LEDs and highlight several discrepancies with calculations. We observe a strong correlation between DLPs and spectral widths, which indicates that inhomogeneous broadening affects the optical polarization properties. Considering indium content fluctuations, QW thickness fluctuations, and the localization length of charge carriers, we argue that different broadenings apply to each subband and introduce a formalism using effective masses to account for inhomogeneous broadening in the calculation of the DLP. We conclude that the different DLP of ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) QWs might be related to different effective broadenings of their valence subbands induced by the rougher upper QW interface in ( 20 2 ¯ 1 ), by the larger sensitivity of holes to this upper interface due to the polarization field in ( 20 2 ¯ 1 ), and/or by the different degrees of localization of holes.In the framework of k · p-theory, semipolar ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN quantum wells (QWs) have equivalent band structures and are expected to have identical optical polarization properties. However, ( 20 2 ¯ 1 ) QWs consistently exhibit a lower degree of linear polarization (DLP) than ( 20 2 ¯ 1 ¯ ) QWs. To understand this peculiarity, we investigate the optical properties of ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN single QW light-emitting diodes (LEDs) via resonant polarization-resolved photoluminescence microscopy. LEDs were grown on bulk substrates by metal organic vapor phase epitaxy with different indium concentrations resulting in emission wavelengths between 442 nm and 491 nm. We discuss the origin of their DLP via k · p band structure calculations. An analytical expression to estimate the DLP in the Boltzmann-regime is proposed. Measurements of the DLP at 10 K and 300 K are compared to m-plane LEDs and highlight several discrepancies with...

Enhancing light extraction from III-nitride devices using moth-eye nanostructures formed by colloidal lithography

Summary form only given. We report a novel, reliable, and scalable technique for the surface nanostructuring of GaN by colloidal lithography. Moth-eye protuberances of varying dimensions have been formed on bulk GaN substrates and on samples containing an InGaN multiple-quantum-well active layer, grown by metal organic chemical vapor deposition. Angle-resolved optical characterization and photoluminescence (PL) indicates enhanced transmission through these samples compared to flat, unstructured surfaces. We analyze the potential mechanisms causing this enhancement, such as recycling of PL pump light, improved first-pass extraction, and improved diffuse scattering. The impact of these effects is evaluated using finite-difference time-domain and effective medium theory simulations. Light extraction with moth-eye nanostructures is compared to state-of-the-art methods for c-plane and semipolar GaN surface roughening.