Seong-Ran Jeon

Lateral current spreading in GaN-based light-emitting diodes utilizing tunnel contact junctions

InGaN/GaN multiple-quantum-well light-emitting-diode structures utilizing tunnel contact junctions grown by metalorganic chemical vapor deposition have been demonstrated. The p+/n+ GaN tunnel junctions are located in the upper cladding layers of conventional devices, allowing n-type GaN instead of p-type GaN as a top contact layer. Thus, metal ohmic contacts are done at the same time on the top and the lower contact layers. The reverse-biased tunnel contact junction provides lateral current spreading without semitransparent electrode and spatially uniform luminescence exhibiting an improved radiative efficiency. The tunnel contact junction is shown to be an effective method to make possible hole injection via a lateral electron current, with only a small penalty in voltage drop compared to conventional devices.

Doping selective lateral electrochemical etching of GaN for chemical lift-off

An electrochemical etching based on oxalic acid was developed for use in the chemical lift-off of GaN epitaxial structures. It was shown that only the Si-doped n-GaN layer was etched away, while the p-type and undoped GaN layers were not etched at all. The etch rate and the remaining structure were analyzed for various doping concentrations and etching voltages. A lateral etch rate of 12 μm/min was achieved under 60 V for n-type doping concentration of 8×1018 cm−3. This doping selective etching was used to lift-off a GaN epitaxial layer patterned into 300×300 μm2 squares.

GaN-based light-emitting diodes using tunnel junctions

We demonstrate GaN-based light-emitting diodes (LEDs) with tunnel junction (TJ) structure and surface-emitting light-emitting diodes with TJ current aperture for lateral current confinement. The p/sup +//n/sup +/ GaN TJs are located in the upper cladding layers of conventional devices, allowing n-type GaN instead of p-type GaN as a top contact layer. The reverse-biased tunnel contact junction provides lateral current spreading without a semitransparent electrode and spatially uniform luminescence exhibiting an improved radiative efficiency. Also, the current confinement aperture for the lateral injection current in the LEDs was defined by mesa etching of a TJ structure and regrowth of the current blocking layer surrounding the TJ mesa. The very uniform light emission just through a buried TJ aperture confirms that the buried TJ structure acts very effectively as a confinement aperture of lateral current injection, particularly in GaN-based vertical-cavity surface-emitting lasers.

Improved Efficiency by Using Transparent Contact Layers in InGaN-Based p-i-n Solar Cells

InGaN/GaN p-i-n solar cells were fabricated either without a current spreading layer or with ITO or Ni/Au spreading layers. A 10.8% indium composition was confirmed within an i-InGaN layer using X-ray diffraction. I-V characteristics were measured at AM1.5 conditions, with solar cell parameters being obtained based on I-V curves in all cases. Current spreading layers produced strong effects on efficiency. The solar cell with the ITO current spreading layer showed the best results, i.e., a short circuit current density of 0.644 mA/cm2, an open circuit voltage of 2.0 V, a fill factor of 79.5%, a peak external quantum efficiency of 74.1%, and a conversion efficiency of 1.0%.

Enhanced light output power of near UV light emitting diodes with graphene / indium tin oxide nanodot nodes for transparent and current spreading electrode.

We report GaN-based near ultraviolet (UV) light emitting diode (LED) that combines indium tin oxide (ITO) nanodot nodes with two-dimensional graphene film as a UV-transparent current spreading electrode (TCSE) to give rise to excellent UV emission efficiency. The light output power of 380 nm emitting UV-LEDs with graphene film on ITO nanodot nodes as TCSE was enhanced remarkably compared to conventional TCSE. The increase of the light output power is attributed to high UV transmittance of graphene, effective current spreading and injection, and texturing effect by ITO nanodots.

InGaN-Based p–i–n Solar Cells with Graphene Electrodes

InGaN-based p–i–n solar cells with graphene electrodes were fabricated and compared with solar cells using indium tin oxide (ITO) electrodes. In particular, we analyzed the properties of graphene film by means of high-resolution transmission electron microscopic (HRTEM) and Raman spectroscopy, also comparing optical properties with those of ITO, conventionally used as transparent electrodes. The solar cells using graphene revealed a short circuit current density of 0.83 mA/cm2, an open circuit voltage of 2.0 V, a fill factor of 75.2%, and conversion efficiency of 1.2%, comparable to the performance of solar cells using ITO.

GaN tunnel junction as a current aperture in a blue surface-emitting light-emitting diode

We have demonstrated surface-emitting GaN-based diodes with a buried tunnel junction (TJ) current aperture. The current confinement aperture for lateral injection current was defined by mesa etch of a TJ structure and regrowth of current blocking layer surrounding the TJ mesa. Lateral electron current drives a tunnel contact junction providing hole injection into the active region. The very uniform light emission just through a buried TJ aperture represents that the buried TJ structure acts very effectively as a confinement aperture of lateral current injection, particularly in GaN-based vertical-cavity surface-emitting lasers.

Free-Standing GaN-Based Photonic Crystal Band-Edge Laser

We report the fabrication of a GaN-based membrane-type photonic crystal (PC) band-edge laser (BEL) that requires a smaller PC active area than previous designs due to strong field confinement. A honeycomb-lattice PC was designed such that the Γ1 monopole band-edge mode fell within the emission band of InGaN quantum wells. The BEL exhibited pulsed lasing at room-temperature when optically pumped above its threshold pump energy density of ~ 15.5 mJ/cm2. Based on polarization angle analysis, we confirmed that the BEL indeed lased at the Γ1 monopole band-edge mode.

An improvement of light extraction efficiency for GaN-based light emitting diodes by selective etched nanorods in periodic microholes.

We have demonstrated the enhancement of a GaN-based light emitting diode (LED) by means of a selective etching technique. A conventional LED structure was periodically etched, to form periodic microholes. It showed an improvement of the light extraction efficiency (LEE) of approximately 15%, compared to that of a conventional LED. Furthermore, nano-sized rods inside the microholes were randomly formed by using a powder mask, resulting in an LEE of 43%. From the result of confocal scanning electroluminescence measurement, the light emission arises mainly from the vicinity of the nanorods in the periodic microholes. Therefore, we found that nanorods randomly distributed in periodic microholes in a LED structure play a significant role in the reduction of total internal reflection, by acting as photon wave-guides and scattering centers. This method would be valuable for the fabrication of high efficiency GaN-based LED, in terms of technical simplification and cost.

Influence of Mg doping on structural defects in AlGaN layers grown by metalorganic chemical vapor deposition

Influence of Mg doping on structural defects in Al0.13Ga0.87N layers grown on sapphire substrates by metalorganic chemical vapor deposition were studied using transmission electron microscopy. By increasing the Mg source flow rate, the reduction of dislocation density occurred up to the Mg source flow rate of 0.103 μmol/min. While the vertical type inversion domain boundaries (IDBs) were observed in the Al0.13Ga0.87N layers grown with the low Mg source flow rate, the IDBs in the Al0.13Ga0.87N layers grown with the high Mg source flow rate have horizontally multifaceted shapes. The change of polarity by the IDBs of horizontal type also resulted in the 180° rotation of pyramidal defects within the same AlGaN layer.