Chung-Chieh Yang

InGaN-Based Light-Emitting Diodes with a Cone-Shaped Sidewall Structure Fabricated Through a Crystallographic Wet Etching Process

The InGaN-based light-emitting diodes (LEDs) were fabricated through a crystallographic etching process to increase their light extraction efficiency. After the laser scribing and the selective lateral wet etching processes at the LED chip edge region, the stable crystallographic etching planes were formed as the GaN {1012} planes and had an including angle with the top GaN (0001) plane measured as 40.3°. The AlN buffer layer acted as the sacrificial layer for the lateral wet process with a 27.5 μm/h etching rate. The continuous cone-shaped sidewall (CSS) structure of the treated LED has a larger light-scattering area and higher light extraction cones around the LED chips. The LED with the CSS structure around the chip edge region has a higher light output power compared to a conventional LED when measured in LED chip form.

InGaN-Based Light-Emitting Diodes With Nanoporous Microhole Structures

InGaN-based light-emitting diodes (LEDs) with nanoporous microhole array (NMA) structures were fabricated through photoelectrochemical wet oxidation and oxide-removing processes. The average size of the nanoporous structure at the microhole regions was measured at 60-80 nm. Forward voltages were measured at 3.47 and 3.68 V for a standard LED (ST-LED) and an NMA-LED, respectively, the latter caused by the higher contact resistance at the nanoporous GaN:Mg surface. The light output power of the NMA-LED had a 40.5% enhancement compared with the ST-LED on nonencapsulated LEDs in chip form. The higher light scattering process occurred at the NMA structure on the GaN:Mg surface and at the ringlike patterns on the GaN:Si structure. The results were a higher light extraction efficiency and a larger divergent angle in the NMA-LED.

InGaN-Based Light-Emitting Diodes with a Multiple-Air-Gap Layer

InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) that have a multiple-air-gap (MAG) structure to increase light extraction efficiency and to reduce the piezoelectric field in an InGaN active layer are described. The LED structure was regrown on a GaN template layer that consists of the photoelectrochemical (PEC)-treated nanoporous GaN:Si layer with a bottom MAG structure and a top GaO x layer that acts as a self-assembled micromask for the epitaxial lateral overgrowth process. Electroluminescence (EL) spectra having a 5.0 nm blueshift phenomenon and a peak intensity of up to 52% enhancement in the MAG-LED compared to a standard light-emitting diode (ST-LED) at 20 mA operating current were obtained. The wavelength blueshift of the EL spectra, with an increasing injection current, in the MAG-LED (5.9 nm) was smaller than that in the ST-LED (8.1 nm). The wavelength blueshift of the micro-photoluminescence spectra, when varying the reverse-bias voltage from 0 to -13 V, was measured as 5.7 and 3.9 nm in the ST-LED and in the MAG-LED, respectively. A higher light extraction efficiency and a lower piezoelectric field in the InGaN wells were observed in the MAG-LED by inserting the PEC-treated MAG structure and a top GaO x layer.

Direct-Grown Air-Void Structure in the InGaN Light-Emitting Diodes

A high-efficiency InGaN light-emitting diode (LED) structure was grown on a silane (SiH4)-treated undoped-GaN layer with a thin in situ grown SiN∞ layer and a 3-D island structure. A lateral one-step epitaxial growth process was performed on the SiH4-treated GaN island structure to form a series-of-embedded-air-void (SEAV) structure. The SEAV structure prevented the dislocation from propagating to the top LED epitaxial layer that reduced the leakage current and increased the internal quantum efficiency of the treated InGaN LED. The light output power of the treated LED had a 68% enhancement compared with that of the standard LED at 20 mA. The high output power and the narrow divergent angle of the treated LED structure were caused by the high light scattering process on the SEAV structure.

Separating InGaN membranes from GaN/sapphire templates through a crystallographic-etch-limited process

InGaN membranes with light-emitting diode (LED) structures were separated from an undoped GaN nanorod structure on sapphire substrates through a chemical lift-off (CLO) process. The CLO processes consisted of a reducing diameter process on the GaN nanorods structure and a crystallographic wet-etching process on an N-face GaN surface. The N-face crystallographic-etching process was limited by the boundary of the GaN nanorods, where a InGaN active layer can prevent etching damage in a hot potassium hydroxide solution. The light output power of the lift-off LED membrane had a 2.28 times enhancement compared with a non-treated LED. A pyramidal-roughened structure was formed on the lift-off GaN surface to increase the light extraction efficiency. The free-standing InGaN LED membranes were realized through a crystallographic-etch-limited CLO process, which has the potential to replace the traditional laser lift-off process for vertical LEDs and be applied to flexible optoelectronic membrane applications.

Solution-processed Li-Al layered-double-hydroxide platelet structures for high efficiency InGaN light emitting diodes.

High-oriented Li-Al layered double hydroxide (LDH) films were grown on an InGaN light-emitting diode (LED) structures by immersing in an aqueous alkaline Al(3+)- and Li+-containing solution. The stand upward and adjacent Li-Al LDH platelet structure was formed on the LED structure as a textured film to increase the light extraction efficiency. The light output power of the LED structure with the Li-Al LDH platelet structure had a 31% enhancement compared with a conventional LED structure at 20 mA. The reverse leakage currents, at -5V, were measured at -2.3 × 10(-8) A and -1.0 × 10(-10)A for the LED structures without and with the LDH film that indicated the Li-Al LDH film had the insulated property acted a passivation layer that had potential to replace the conventional SiO2 and Si3N4 passivation layers. The Li-Al LDH layer had the textured platelet structure and the insulated property covering whole the LED surface that has potential for high efficiency InGaN LED applications.

Higher Light-Extraction Efficiency of Nitride-Based Light-Emitting Diodes with Hexagonal Inverted Pyramids Structures

Self-assembled hexagonal inverted pyramid (HIP) structures were formed at the mesa-edge region in the InGaN-based light emitting diodes (LEDs). The HIP structures consisted of the top p-type GaN:Mg layers and the bottom InGaN active layers, and they were fabricated through a bandgap-selective photoelectrochemical (PEC) wet-etching process in a 2.2 M potassium hydroxide solution. In the HIP-LED structures, the light output power was 1.6 times higher and the divergent angle was reduced to 146° compared to a standard LED without PEC treatment. The light emission extracted from the InGaN active layer was scattered in a normal direction through the hexagonal inverted pyramid structures located at the 10 μm wide mesa-edge regions without depositing any transparent metal contact layer.

Optical Properties of InGaN-Based Light Emitting Diodes Fabricated Through Dry and Wet Mesa Etching Process

InGaN-based light emitting diodes (LEDs) were fabricated through a photoelectrochemical (PEC) wet mesa etching process to replace the conventional dry mesa etching process. The undercut structures were formed from a bandgap-selective lateral wet etching process that occurred at the InGaN/GaN multiple-quantum-well layers. By measuring the selective-area microphotoluminescence spectra focused on the mesa edge region, the blueshift wavelength of the photoluminescence spectrum in the wet mesa etched light emitting diode (WME-LED) was 9.1 nm (55 meV) that was compared to the conventional dry etching LED. The relative internal quantum efficiencies of WME-LED were calculated as 13.7% (at the first region), 21.8% (at the second region), and 24.5% (at the third region) from the mesa center to the edge. The flatband voltage of the WME-LED was - 13 V to balance the piezoelectric field, calculated as -1.17 MV/cm, in the InGaN active layer. However, we did not observe any flatband voltage in the conventional LED up to -19 V (piezoelectric field larger than -1.9 MV/cm). By forming the bending undercut structure on p-type GaN:Mg layer, the lattice mismatch induces a compressed strain and a piezoelectric field in the InGaN active layer that can be partially released in the WME-LED by using a PEC wet mesa etching process.

Wet Mesa Etching Process in InGaN-based Light Emitting Diodes

A photoelectrochemical wet mesa etching (WME) process was used to fabricate InGaN-based light emitting diodes (LEDs) as a substitute for the conventional plasma mesa dry etching process. The p-type GaN:Mg layer, InGaN active layer, and n-type GaN:Si layer were etched through a sequential photoelectrochemical oxidation and oxide-removing process to define the mesa region. The higher lateral wet-etching rate (∼3.4 μm/h) of the InGaN active layer was observed to form a wider undercut structure which has 42.7% light output power enhancement compared to a conventional LED fabricated with the plasma dry etching process. The reverse current of a WME-LED was suppressed by avoiding plasma damage during the dry mesa etching process.

InGaN Light-Emitting Diode with a Nanoporous/Air-Channel Structure

High-efficiency InGaN light-emitting diode (LED) with an air-channel structure and a nanoporous structure was fabricated. The air-channel structure was formed through an epitaxial regrowth process on a dry-etched undoped GaN nanorod structure. The GaN:Si nanoporous structure embedded in treated LED structures was fabricated through a photoelectrochemical wet etching process in an oxalic acid solution. Light output powers were enhanced 1.48- and 1.75-fold for the LEDs with an air-channel structure and with a nanoporous/air-channel structure, respectively, in comparison with that of a conventional LED structure. The air-channel structure and the nanoporous GaN:Si structure in the treated LED structures provided high-light-extraction structures.