Wan-Chun Huang

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.

InGaN light emitting solar cells with a roughened N-face GaN surface through a laser decomposition process

InGaN-based light-emitting solar cell (LESC) structure with an inverted pyramidal structure at GaN/sapphire interface was fabricated through a laser decomposition process and a wet crystallographic etching process. The highest light output power of the laser-treated LESC structure, with a 56% backside roughened-area ratio, had a 75% enhancement compared to the conventional device at a 20 mA operating current. By increasing the backside roughened area, the cutoff wavelength of the transmittance spectra and the wavelength of the peak photovoltaic efficiency had a redshift phenomenon that could be caused by increasing the light absorption at InGaN active layer.

InGaN light emitting diodes with a laser-treated tapered GaN structure.

InGaN light-emitting diode (LED) structures get an air-void structure and a tapered GaN structure at the GaN/sapphire interface through a laser decomposition process and a lateral wet etching process. The light output power of the treated LED structure had a 70% enhancement compared to a conventional LED structure at 20 mA. The intensities and peak wavelengths of the micro-photoluminescence spectra were varied periodically by aligning to the air-void (461.8nm) and the tapered GaN (459.5nm) structures. The slightly peak wavelength blueshift phenomenon of the EL and the PL spectra were caused by a partial compressed strain release at the GaN/sapphire interface when forming the tapered GaN structure. The relative internal quantum efficiency of the treated LED structure (70.3%) was slightly increased compared with a conventional LED (67.8%) caused by the reduction of the piezoelectric field in the InGaN active layer.

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 light emitting diodes with a nanopipe layer formed from the GaN epitaxial layer

A Si-heavy doped GaN:Si epitaxial layer is transformed into a directional nanopipe GaN layer through a laser-scribing process and a selectively electrochemical (EC) etching process. InGaN light-emitting diodes (LEDs) with an EC-treated nanopipe GaN layer have a high light extraction efficiency. The direction of the nanopipe structure was directed perpendicular to the laser scribing line and was guided by an external bias electric field. An InGaN LED structure with an embedded nanopipe GaN layer can enhance external quantum efficiency through a one-step epitaxial growth process and a selective EC etching process. A birefringence optical property and a low effective refractive index were observed in the directional-nanopipe GaN layer.

Step-Roughened N-face GaN Surface on InGaN Light-Emitting Diodes Using a Laser Decomposition Process

The step-inverted pyramidal structures were observed at the top and at the bottom of the truncated triangle-shaped patterned-sapphire substrate through a laser decomposition process and a wet etching process at the GaN/sapphire interface. A step-roughened N-face GaN surface was obtained for high light extraction purposes without using a conventional laser lift-off process. We cleaved a 2” LED wafer into two half-wafers to prepare for this experiment, with a mesa region of 540×210 µm 2 . The LED chips were treated by using a triple frequency ultraviolet Nd:YVO4 (355nm) laser for a front-side laser isolation process and a backside GaN decomposition process. The GaN buffer layer decomposed as Ga metal and N2 gas through the laser decomposition process. Half of the LED wafer was immersed in a hot KOH solution (KOH, 80oC) for a 15minute lateral crystallographic wet etching process that occurred at the laser treated stripe-line patterned regions. The RBPS-LED structure consisted of a truncated triangle-shaped patterned-sapphire substrate, a stepinverted pyramidal structure, and laser-scanning stripeline patterns to increase light extraction efficiency. A higher light scattering process occurred at the roughened N-face GaN surface and the GaN/air/patterned-sapphire interfaces. By increasing the pulse operating current up to 200mA (176 A/cm 2 ), the related efficiency droop and the peak wavelength blueshift phenomenon were almost the same in both LED structures. The LEDs with the stepinverted pyramidal structure and the GaN/airgap/patterned-sapphire interfaces had higher lightextraction efficiency for the nitride-based LEDs by leaving the sapphire substrates.