Ren-Hao Jiang

Blue light-emitting diodes with a roughened backside fabricated by wet etching

The InGaN-based light-emitting diodes (LEDs) with a roughened patterned backside on the N-face GaN surface were fabricated through a crystallographic etching process to increase light-extraction efficiency. After laser decomposition, laser scribing, and a lateral crystallographic wet etching process at the GaN/Al2O3 interface, stable crystallographic etching planes were formed as the GaN {1011¯} planes that included an angle with the top GaN (0001) plane measured at 58°. The GaN buffer layer acted as the sacrificial layer for the laser decomposition process and the lateral wet etching process with a 26 μm/min etching rate. The LED with the inverted pyramidal N-face GaN surface close to the GaN/Al2O3 interface has a larger light-scattering process than the conventional LED. The light-output power of the LED with the backside roughened surface had a 47% enhancement when measured in LED chip form.

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.

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.

Fabricated InGaN Membranes through a Wet Lateral Etching Process

Epitaxial layers of InGaN light-emitting diodes (LED) were separated from undoped GaN/sapphire structures through a wet lift-off process. A 0.1-µm-thick Si-heavy-doped GaN:Si (n+-GaN) layer was inserted in the InGaN LED structure that acted as a sacrificial layer for a lateral wet etching process. The lateral etching rate of the n+-GaN sacrificial layer was 315 µm/h. The Fabry–Perot interferences of the lift-off InGaN LED membranes were observed in the angle-resolved photoluminescence spectra that indicated that the lift-off InGaN membranes had a flat etched surface. High light extraction efficiency, narrow divergent angle, and flat wet-etched GaN surface were observed on the lift-off InGaN membrane.

Micro-Square-Array InGaN-Based Light-Emitting Diode with an Insulated Ga2O3 Layer through a Photoelectrochemical Process

InGaN-based micro-square-array light emitting diode (MSA-LED) was fabricated by filling with an insulated Ga2O3 layer around the individual micro-square patterns for a metal interconnected process. The Ga2O3 layer formed at the mesa sidewall and the bottom etched surface of the n-type GaN layer in the LED structure through a selective photoelectrochemical (PEC) wet oxidation process in H2O solution. The 25- and 15-µm-square mesa patterns of the MSA-LED structures were defined by the plasma dry and the PEC wet etching processes that a conventional broad-area LED (BA-LED) was closed to the MSA-LED for comparison. The peak wavelength blueshift of the electroluminescence spectra and the enhancement of the light output power were measured at 1.0 nm/41% and 2.5 nm/22% for the 25- and 15-µm-MSA-LED, respectively, compared with the BA-LED. The reverse leakage current of both MSA-LED structures was about 2.5×10-11 A that was lower than the BA-LED (8.3×10-9 A) at -5 V reverse bias. The PEC Ga2O3 layer acted a passivation layer to prevent the leakage current from the mesa sidewall surface and an interconnect process in the MSA-LED structures.