Eun-Hyun Park

The effect of silicon doping in the selected barrier on the electroluminescence of InGaN/GaN multiquantum well light emitting diode

The effect of silicon doping in the selected barrier on the electroluminescence of InGaN∕GaN multiquantum well light emitting diode (LED) was studied using dual wavelength LEDs. The result verified that the hole carrier transport is easily blocked by the silicon doped barrier, and the dominant electron and hole recombination occurs at the wells between p-GaN and the silicon doped barrier. The electroluminescence spectrum and the wavelength blueshift of the silicon doped LEDs were compared with undoped LEDs. The numerical simulation was done to clearly explain the hole blocking effect by the silicon doped barrier.

InGaN-light emitting diode with high density truncated hexagonal pyramid shaped p-GaN hillocks on the emission surface

To increase the light extraction efficiency, high density truncated hexagonal pyramid shaped submicron p-GaN hillocks were formed on the emission surface of an InGaN∕GaN multiple quantum well light emitting dicode (LED) using an in situ silicon carbon nitride self-masking layer. The self-assembled hillock density was raised up to a low 109cm−2 using several nanometers of a Si0.4C0.6N1 self-masking layer. The self-assembled hillock LED resulted in the optical power improvement up to 80% with similar electrical properties as a normal LED. This device showed a higher electrostatic discharge pass yield at over 1000V reverse stress voltage.

Via-hole-based vertical GaN light emitting diodes

A vertical GaN-light emitting diode (LED) has been fabricated on a sapphire substrate with periodic via holes formed by a laser drilling technique. n-contact metal which was deposited on the backside of sapphire substrate was directly connected with an Ohmic metal of n-GaN layer through the via holes. The via-hole-based vertical GaN-LED demonstrated an optical power improvement of up to 12.5% with lower forward operating voltage compared with a conventional GaN-LED. In addition, this vertical LED showed just 0.8% and 1.5% variations of optical power and operation voltage at the 500h reliability test.

Optical gain and luminescence of a ZnO-MgZnO quantum well

The optical gain and the luminescence of a ZnO quantum well with MgZnO barriers is studied theoretically. We calculated the non-Markovian optical gain and the luminescence for the strained-layer wurtzite quantum well taking into account the excitonic effects. It is predicted that both optical gain and luminescence are enhanced for the ZnO quantum well when compared with those of the InGaN-AlGaN quantum well structure due to the significant reduction of the piezoelectric effects in the ZnO-MgZnO systems.

Many-body optical gain and intraband relaxation time of wurtzite InGaN∕GaN quantum-well lasers and comparison with experiment

The optical gain and the intraband relaxation time of wurtzite InGaN∕GaN QW lasers are investigated theoretically considering the non-Markovian gain model with many-body effects. Gain spectra are compared with those obtained from the experiment. The calculated intraband relaxation time is about 24fs at the subband edge and gradually increases with the energy. The intraband relaxation time is shown to be nearly independent of the sheet carrier density in an investigated range. The calculated gain spectra is in good agreement with the experiment. The intraband relaxation time τin obtained from fitting with experiment is 25fs. This value agrees well with the calculated value (∼24fs) at the subband edge, for which most of optical transitions occur.

Growth of GaN on ZnO for Solid State Lighting Applications

In this work, ZnO has been investigated as a substrate technology for GaN-based devices due to its close lattice match, stacking order match, and similar thermal expansion coefficient. Since MOCVD is the dominant growth technology for GaN-based materials and devices, there is a need to more fully explore this technique for ZnO substrates. Our aim is to grow low defect density GaN for efficient phosphor free white emitters. However, there are a number of issues that need to be addressed for the MOCVD growth of GaN on ZnO. The thermal stability of the ZnO substrate, out-diffusion of Zn from the ZnO into the GaN, and H2 back etching into the substrate can cause growth of poor quality GaN. Cracks and pinholes were seen in the epilayers, leading to the epi-layer peeling off in some instances. These issues were addressed by the use of H2 free growth and multiple buffer layers to remove the cracking and reduce the pinholes allowing for a high quality GaN growth on ZnO substrate.

Metalorganic chemical vapor deposition of InGaN layers on ZnO substrates

InGaN layers have been grown on (0001) ZnO substrates by metalorganic chemical vapor deposition utilizing a low temperature grown thin GaN buffer. Good quality InGaN films with a wide range of In composition were confirmed by high-resolution x-ray diffraction. Even at high indium concentrations no In droplets and phase separation appeared, possibly due to coherent growth of InGaN on ZnO. Photoluminescence showed broad InGaN-related emissions with peak energy lower than the calculated InGaN band gap, possibly due to Zn/O impurities diffused into InGaN from the ZnO substrate. An activation energy of 59 meV for the InGaN epilayer is determined.

Non-Markovian gain and luminescence of an InGaN-AlInGaN quantum-well with many-body effects

The optical gain and the luminescence of an InGaN quantum well with quaternary AlInGaN barriers is studied theoretically. We calculated the non-Markovian optical gain and the luminescence for the strained-layer wurtzite quantum well taking into account of many-body effects. It is predicted that both optical gain and luminescence are enhanced significantly when aluminum and indium are introduced into the quaternary barrier composition. Adding the aluminum to the barrier will increase of the confinement potentials for electrons and holes, while the indium will reduce the biaxial strain, which in turn reduces the internal field caused by spontaneous polarization and piezoelectric effects.

Bell-shaped light emitting diodes (BS-LED) with a 45 degree corner reflector, deep side-wall, and microlens

A new type Bell Shaped Light Emitting Diode(BS-LED) with a circular 45 degree(s) corner reflector, deep side-wall and microlens is proposed and fabricated. Because the light of in-plane radiation in the active layer of Surface Emitting LED(SE-LED) can be extracted to emission surface by a circular 45 degree(s) corner reflector, the output power saturation phenomena that occur due to the in-plane superluminescence can be considerably improved. So, the light output power and the linearity of light-current curve can be improved efficiently by the corner reflector. The deeply etched side-wall can dramatically improve the external quantum efficiency of LED by side-wall reflection and photon recycling mechanism. Microlens is formed on light emission surface to improve the beam pattern. The fabricated BS-LED shows the dramatically improved external quantum efficiency up to about 8 times than that of conventional LED. The output power improvement is simulated as device design parameters. The BS-LED is characterized using spectrum, near-field pattern and light-current measurement.

InGaN Light-Emitting Diode With Quasi-Quantum-Dot-Shaped Active Layer Using SiCN Interfacial Layer

A quasi-quantum-dot (QQD)-shaped InGaN-GaN multilple-quantum-well light-emitting diode (LED) was achieved using a silicon carbon nitride (SiCN) interfacial layer. QQDs with ~100-nm diameter and ~4-nm height were uniformly formed inside the InGaN active layer due to strain and affinity difference between the InGaN and SiCN layer. The surface morphology and structural properties of QQD-LED were measured with atomic force microscopy, secondary ion mass spectrometry, and X-ray diffraction. Device performance of QQD-LEDs were evaluated and compared with normal LEDs. The QQD-LED showed ~15% higher photoluminescence intensity and ~10% higher optical output power