High-quality nanodisk of InGaN/GaN MQWs fabricated by neutral-beam-etching and GaN regrowth: towards directional micro-LED in top-down structure

Abstract

A nanodisk array of blue InGaN/GaN multiple quantum wells was made using neutral beam etching (NBE) followed by GaN regrowth. The NBE-fabricated nanodisk presented a vertical and highly smooth sidewall surface where the InGaN well layers were easily distinguished even with a scanning electron microscope. A high interface quality without any voids or obvious defects was obtained between the nanodisk and the regrown-GaN layer. The nanodisk after regrowth presented a smaller blueshift of photoluminescence emission energy (12 meV) and a substantially higher and almost constant internal quantum efficiency of ~50% over three orders of magnitude of excitation laser power when compared to the nanodisk before regrowth. This study shows that the process of NBE nanodisk etching followed by GaN regrowth represents a promising step forward in the development of truncated cone-shaped directional micro-LEDs with a buried active region in a top-down structure.

Hydrogen iodide (HI) neutral beam etching characteristics of InGaN and GaN for micro-LED fabrication

“…On the other hand, R-value of NBE was significantly less than that of Cl 2 plasma etching (210%) as compared with the previous report [37]. Previous studies on using Cl 2 NBE for fabricating μLED reported that the surfaces recovered after GaN regrowth [15] and the EQE was stable at low current density (1 A cm −2 ) [14]. Thus, our results prospected that the etching interface of HI NBE can recover as well or better than that of Cl 2 NBE because it does not have any etching residue.…”

“…Etching 15%-indium-content InGaN/GaN MQW blueemitting (≈440 nm) μLEDs with chlorine gas has a low etching rate due to the InGaN layer (4.5 nm min −1 for GaN and 2 nm min −1 for In 0.15 GaN) [14] and forms indium residue [15]. This etching residue comes from low-volatility indium chloride remaining on the surface, and it causes recombination centers and disturbs the regrowth process [15]. Moreover, if red-light-emitting μLEDs are to be etched, the indium content ratio increases from 15% to 35%.…”

Preferential crystal orientation etching of GaN nanopillars in Cl2 plasma

“…The advantage of the top-down approach is that it offers more flexibility in creating arrays of nanowires with different shapes, dimensions, and densities on a wafer scale. However, plasma patterning is known to cause damage and rough surfaces, which prevent high-quality surface regrowth [21,22] while the regrowth of the quantum wells on the nanowire sidewalls for core/shell emitting structures is favored if m-oriented facets are present. One of the greatest challenges in patterning high aspect ratio GaN nanowires is achieving high GaN-tomask etch selectivity while maintaining high-quality etched surfaces.…”

“…Most studies addressing this etch selectivity issue use Ni-based hard masks obtained by lift-off lithography, since they offer high selectivity for deep GaN etching (>15) [16][17][18]23]. However, for certain applications, the use of metal masks may not be desired due to potential contamination and hard mask roughness issues [22]. For industrial applications, the use of dielectric masks is preferred.…”

Microdisk-Type Multicolor Semipolar Nitride-Based Light-Emitting Diodes

“…Moreover, because GaN has a refractive index of 2.35, which is higher than that of air, the escape cone angle of GaN-based LEDs is approximately 25.18°, as determined using Snell’s law. , GaN-based LEDs are expected to show a light extraction efficiency (LEE) of up to a few percent. Thus, the light escape cone angle of the disk-type LEDs prepared using the dry-etching process can be increased by using a cylindrical shape that can emit light at all angles because of the light emission direction perpendicular to the surface. − However, during the fabrication of a disk-type LED structure, the dry-etching process can produce major crystal defects, which act as nonradiative recombination centers and significantly decrease the internal quantum efficiency of the LED structure despite the increase in its LEE. , In particular, the emission efficiency of an LED decreases significantly with a decrease in its size because of the presence of dry-etched sidewall defects in it. , However, because semipolar GaN films can be etched by chemical etchants, semipolar GaN-based microdisk (MD)-type LEDs fabricated via wet etching are expected to exhibit improved LEE without substantial dry-etching damage for applications in micro-LED display light sources. This is because, in the wet-etching process, the semipolar GaN film is etched by a chemical etchant to avoid dry-etching damage .…”