Jeomoh Kim

Improvement of peak quantum efficiency and efficiency droop in III-nitride visible light-emitting diodes with an InAlN electron-blocking layer

InAlN electron-blocking layers (EBLs) are shown to improve the emission intensity and to mitigate the efficiency droop problem in III-nitride-based visible light-emitting diodes (LEDs). Using an In0.18Al0.82N EBL in blue LEDs, we have achieved a significant improvement in the electroluminescence emission intensity and a mitigated efficiency droop compared to similar LEDs without an EBL or with an Al0.2Ga0.8N EBL. This indicates that an In0.18Al0.82N EBL is more effective in electron confinement and reduces the efficiency droop possibly caused by carrier spill-over than conventional AlGaN EBLs.

Efficiency droop due to electron spill-over and limited hole injection in III-nitride visible light-emitting diodes employing lattice-matched InAlN electron blocking layers

Data and analysis are presented for the study of efficiency droop in visible III-nitride light-emitting diodes (LEDs) considering the effects of both electron spill-over out of active region and hole injection into the active region. Performance characteristics of blue LEDs with lattice-matched In0.18Al0.82N electron-blocking layers (EBLs) with different thicknesses were measured in order to exclude the effects of strain and doping efficiency of the EBL, and the quantum efficiencies were analyzed taking account of the electron spill-over current and the relative hole concentration. The results suggest that the highest efficiency in LEDs with a 15-nm In0.18Al0.82N EBL is due to relatively lower hole-blocking effect, hence higher hole injection than in LEDs with a 20-nm EBL, while providing a higher potential barrier for reduced electron spill-over than in LEDs with thinner EBLs. This study suggests that the EBL hole-blocking and electron-confinement effects should be considered in order to achieve higher l...

Green light-emitting diodes with self-assembled In-rich InGaN quantum dots

A green light-emitting diode (LED) was fabricated using self-assembled In-rich InGaN quantum dots (QDs). The photoluminescence studies showed that the QDs provide thermally stable deeply localized recombination sites for carriers with negligibly small piezoelectric field. The electroluminescence spectra of the LED showed a peak in the green spectral range and the dominant peak was blueshifted with increasing injection current due to the distribution of depth of the potential wells of QDs. The output power of the LED increased with increasing injection current, indicating that the potential wells are thermally stable and deeply localized in the QDs.

Enhanced carrier confinement in AlInGaN-InGaN quantum wells in near ultraviolet light-emitting diodes

To increase carrier confinement, the GaN barrier layer was substituted with an AlInGaN quaternary barrier layer which was lattice-matched to GaN in the GaN-InGaN multiple quantum wells (MQWs). Photoluminescence (PL) and high-resolution X-ray diffraction measurements showed that the AlInGaN barrier layer has a higher bandgap energy than the originally used GaN barrier layer. The PL intensity of the five periods of AlInGaN-InGaN MQWs was increased by three times compared to that of InGaN-GaN MQWs. The electroluminescence (EL) emission peak of AlInGaN-InGaN MQWs ultraviolet light-emitting diode (UV LED) was blue-shifted, compared to a GaN-InGaN MQWs UV LED and the integrated EL intensity of the AlInGaN-InGaN MQWs UV LED increased linearly up to 100 mA. These results indicated that the AlInGaN-InGaN MQWs UV LED has a stronger carrier confinement than a GaN-InGaN MQWs UV LED due to the larger barrier height of the AlInGaN barrier layer compared to a GaN barrier layer

Gradient Doping of Mg in p-Type GaN for High Efficiency InGaN–GaN Ultraviolet Light-Emitting Diode

The performance of a InGaN-GaN multiple quantum-well (MQW) ultraviolet (UV) light-emitting diode (LED) with an emission of 385 nm was enhanced by a gradient doping of Mg in the p-GaN layer. The optical output power was enhanced by 21% at an input current of 20 mA compared to that of a UV LED with a uniformly doped p-GaN layer. The improved performance of the UV LED could be attributed to the decrease in diffusion of Mg into MQW and the suppression of electron transport from the conduction band of the MQW to the acceptor level of the deep donor acceptor pair bands in the p-GaN layer by a gradient doping of Mg in p-GaN layer.

GaN/InGaN avalanche phototransistors

We report on III–nitride (III–N) avalanche phototransistor (APT) action by illuminating ultraviolet (UV) photons onto a GaN/InGaN npn heterojunction bipolar transistor in an open-base configuration. A high responsivity of >1 A/W was measured for the device operating at a collector-to-emitter voltage (VCE) of 68 A/W at VCE = 95 V. The InGaN APT demonstrates the feasibility of using III–N bipolar transistor structures for high-sensitivity UV photodetection applications.

Comparison of AlGaN p–i–n ultraviolet avalanche photodiodes grown on free-standing GaN and sapphire substrates

We compare the performance characteristics of Al0.05Ga0.95N UV avalanche photodiodes (APDs) grown on different substrates. UV-APDs grown on a free-standing GaN substrate show lower dark-current densities for all fabricated mesa sizes than similar UV-APDs grown on a GaN/sapphire template. In addition, a stable avalanche gain higher than 5 × 105 and a significant increase in the responsivity of UV-APDs grown on a free-standing GaN substrate are observed. We believe that the high crystalline quality of Al0.05Ga0.95N UV-APDs grown on a free-standing GaN substrate with low dislocation density is responsible for the observed low leakage currents, high performance characteirstics, and reliability of the devices.

Direct periodic patterning of GaN-based light-emitting diodes by three-beam interference laser ablation

We report on the direct patterning of two-dimensional periodic structures in GaN-based light-emitting diodes (LEDs) through laser interference ablation for the fast and reliable fabrication of periodic micro- and nano-structures aimed at enhancing light output. Holes arranged in a two-dimensional hexagonal lattice array having an opening size of 500 nm, depth of 50 nm, and a periodicity of 1 μm were directly formed by three-beam laser interference without photolithography or electron-beam lithography processes. The laser-patterned LEDs exhibit an enhancement in light output power of 20% compared to conventional LEDs having a flat top surface without degradation of electrical and optical properties of the top p-GaN layer and the active region, respectively.

Heterostructures in GaInP grown using a change in V/III ratio

A natural monolayer {111} superlattice (the CuPt ordered structure) is formed spontaneously during organometallic vapor phase epitaxial (OMVPE) growth of Ga0.52In0.48P. The extent of this ordering process is found to be a strong function of the input partial pressure of the phosphorus precursor during growth due to the effect of this parameter on the surface reconstruction and step structure. Thus, heterostructures can be produced by simply changing the flow rate of the P precursor during growth. It is found, by examination of transmission electron microscope (TEM) and atomic force microscope (AFM) images, and the photoluminescence (PL) and PL excitation (PLE) spectra, that order/disorder (O/D) (really less ordered on more ordered) heterostructures formed by decreasing the partial pressure of the P precursor during the OMVPE growth cycle at a temperature of 620 °C are graded over several thousands of A when PH3 is the precursor. The ordered structure from the lower layer persists into the upper layer. Sim...

Temperature-Dependent Resonance Energy Transfer from Semiconductor Quantum Wells to Graphene

Resonance energy transfer (RET) has been employed for interpreting the energy interaction of graphene combined with semiconductor materials such as nanoparticles and quantum-well (QW) heterostructures. Especially, for the application of graphene as a transparent electrode for semiconductor light emitting diodes, the mechanism of exciton recombination processes such as RET in graphene-semiconductor QW heterojunctions should be understood clearly. Here, we characterized the temperature-dependent RET behaviors in graphene/semiconductor QW heterostructures. We then observed the tuning of the RET efficiency from 5% to 30% in graphene/QW heterostructures with ∼60 nm dipole-dipole coupled distance at temperatures of 300 to 10 K. This survey allows us to identify the roles of localized and free excitons in the RET process from the QWs to graphene as a function of temperature.