Bing-Cheng Shieh

Enhanced Light Output Power in InGaN Light-Emitting Diodes by Fabricating Inclined Undercut Structure

The InGaN-based light-emitting diode (LED) with an inclined undercut structure is fabricated through the photoelectrochemical two-step process to increase light extraction efficiency. In the first step the sidewall-undercut structure at the p-type and n-type GaN interface is created by selective wet oxidation on an n-type GaN surface in pure H 2 O solution. In the second step an inclined undercut structure through a crystallographic wet-etching process is formed by immersion in hot KOH solution. This crystallographic wet-etching process can remove the Ga 2 O 3 layer and form a {1011} p-type GaN stable plane, {1010} n-type GaN stable plane on the mesa sidewall. This inclined p-type GaN plane of LED structure can provide the higher overlap of incident light beam core and extraction core overlap on the mesa sidewall, and the total light output power of the treated LED is 2.10 times higher than the standard LED. Consequently, this inclined undercut LED structure is suitable for high-efficiency nitride-based LED application.

Chemical–Mechanical Lift-Off Process for InGaN Epitaxial Layers

An InGaN-based light-emitting diode (LED) structure was separated from a GaN/sapphire structure by inserting sacrificial Si-doped InGaN/GaN superlattice layers through a chemical–mechanical lift-off (CMLO) process. The CMLO process consisted of a band-gap-selective photoelectrochemical lateral wet etching process and a mechanical lift-off process. A lower elastic modulus and hardness of the lateral-etched LED structure were measured compared with the conventional LED structure, which indicated a weak mechanical property of the treated LED structure. The photoluminescence blue-shift phenomenon and the Raman redshift phenomenon indicated that the compressive strain from the bottom GaN/sapphire structure was released through the CMLO process.

InGaN light-emitting diodes with embedded nanoporous GaN distributed Bragg reflectors

InGaN-based light-emitting diodes (LEDs) with embedded conductive nanoporous GaN/undoped GaN (NP-GaN/u-GaN) distributed Bragg reflectors (DBRs) were demonstrated. Nanoporous GaN DBR structures were fabricated by pulsed 355 nm laser scribing and electrochemical etching processes. Heavily Si-doped n-type GaN:Si layers (n+-GaN) in an eight-period n+-GaN/u-GaN stack structure were transformed into a low-refractive-index, conductive nanoporous GaN structure. The measured center wavelength, peak reflectivity, and bandwidth of the nanoporous GaN DBR structure were 417 nm, 96.7%, and 34 nm, respectively. Resonance cavity modes of the photoluminescence spectra were observed in the treated LED structure with the nanoporous DBR structure.

InGaN-Based Light-Emitting Diodes With a Sawtooth-Shaped Sidewall on Sapphire Substrate

An InGaN light-emitting diode (LED) with a cone-shaped GaN structure and a sawtooth-shaped sapphire sidewall structure was fabricated through a laser-drilling process. The fabricated procedures consisted of a laser scribing/drilling process, a wet etching process, and a chip cleaving process. In the treated LED structure with the laser-drilling sawtooth-shaped sidewall, the light output power had a 16% enhancement compared to a conventional LED structure with a laser-scribing sidewall. A periodic high light emission intensity, with a 2.6- μm width spaced at regular intervals of 3.8 μm, was observed on the treated LED sidewall structure corresponding to the laser-drilling patterns. The LED structure consists of a laser-drilling sidewall and a cone-shaped GaN structure that increases the light extraction efficiency for high efficiency InGaN LED 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.

Fabrication of current confinement aperture structure by transforming a conductive GaN:Si epitaxial layer into an insulating GaOx layer.

We report here a simple and robust process to convert embedded conductive GaN epilayers into insulating GaOx and demonstrate its efficacy in vertical current blocking and lateral current steering in a working LED device. The fabrication processes consist of laser scribing, electrochemical (EC) wet-etching, photoelectrochemical (PEC) oxidation, and thermal oxidization of a sacrificial n(+)-GaN:Si layer. The conversion of GaN is made possible through an intermediate stage of porosification where the standard n-type GaN epilayers can be laterally and selectively anodized into a nanoporous (NP) texture while keeping the rest of the layers intact. The fibrous texture of NP GaN with an average wall thickness of less than 100 nm dramatically increases the surface-to-volume ratio and facilitates a rapid oxidation process of GaN into GaOX. The GaOX aperture was formed on the n-side of the LED between the active region and the n-type GaN layer. The wavelength blueshift phenomena of electroluminescence spectra is observed in the treated aperture-emission LED structure (441.5 nm) when compared to nontreated LED structure (443.7 nm) at 0.1 mA. The observation of aperture-confined electroluminescence from an InGaN LED structure suggests that the NP GaN based oxidation will play an enabling role in the design and fabrication of III-nitride photonic devices.

Reducing a Piezoelectric Field in InGaN Active Layers by Varying Pattern Sapphire Substrates

Internal quantum efficiency (IQE) and piezoelectric field (PZ) in InGaN light-emitting diodes (LEDs) were analyzed that were grown on a truncated pyramid (TP)- and a pyramid (P)-shaped pattern sapphire substrates. Lower flat-band voltage was measured at -8 V in the P-LED compared with the TP-LED (-12 V) that showed a low PZ was observed in the P-LED structure. The IQE value of the P-LED was measured as 86%, which is higher than that of the TP-LED (76%). High IQE, low PZ, and high light extraction efficiency were observed in the InGaN LED structure grown on the P-shaped pattern sapphire substrate.

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.

Bendable InGaN Light-Emitting Nanomembranes with Tunable Emission Wavelength

The integration of light-emitting diodes (LEDs) into the flexible devices has exhibited a great potential in the next-generation consumer electronics. In this study, we have demonstrated an exfoliated InGaN nanomembrane LED (NM-LED) separated from a GaN/sapphire substrate through an electrochemically wet etching process. The peak wavelengths blue-shifted phenomenon of the photoluminescence (PL) and the electroluminescence spectra were observed on the free-standing NM-LED compared to the nontreated LED with the same structure, which can be ascribed to the partial strain relaxation of the LED structure confirmed by the Raman spectra and the X-ray diffraction curves. A small divergent angle of the PL emission light has also been observed on the NM-LED. Moreover, the peak emission wavelength of this NM-LED can be even modulated from a red shift (521.7 nm) to a blue shift (500.4 nm) compared with that of the flat state (509.4 nm) while being curved convexly from top p-GaN:Mg side to bottom n-GaN:Si side. Our study provides an elegant way to develop a bendable light source with variable emission wavelengths through the mechanical deformation method.

InGaN light-emitting diodes with band-pass-filter-like GaN?:?Si nanoporous structures

InGaN-based light-emitting diodes (LEDs) with both GaN?:?Si nanoporous and air-gap structures were fabricated through a wet lateral etching (LE) process. Light output power of the LE-LED structure was enhanced by 58% compared with a non-treated LED structure, due to the increased light extraction from the GaN?:?Si nanoporous and air-gap structure. Optical transmittance of the structure was analysed using photoluminescence from the LED epitaxial layers. The transmittance of the LE-LED was measured to be 2.56 times for the blue emission and 0.43 times for the yellow emission, compared with the non-treated LED structure at a detection angle of 35? from the lateral direction. The optical properties of the GaN?:?Si nanoporous structure were similar to a band-pass filter with a 460?nm centre wavelength and a 70?nm bandwidth, which effectively enhanced the light-extraction efficiency in InGaN LEDs.