Square lattice nano-hole arrays with diameters and periodicities of 200 and 500 nm, respectively, are fabricated on InGaN/GaN blue light emitting diodes (LEDs) using electron-beam lithography and inductively coupled plasma reactive ion etching processes. Cathodoluminescence (CL) investigations show that light emission intensity from the LEDs with the nano-hole arrays is enhanced compared to that from the planar sample. The CL intensity enhancement factor decreases when the nano-holes penetrate into the multiple quantum wells (MQWs) due to the plasma-induced damage and the residues. Wet chemical treatment using KOH solution is found to be an effective method for light extraction from the nano-patterned LEDs, especially, when the nano-holes penetrate into the MQWs. About 4-fold CL intensity enhancement factor is achieved by the KOH treatments after the dry etching for the sample with a 250-nm deep nano-hole array.
Research and development of GaN-based light emitting diodes (LEDs) have grown rapidly in the last decades due to their promising applications such as back light units for flat panel display, signage, and general lighting [1]. Commercial blue and green LEDs employing InGaN/GaN multiple quantum wells (MQWs) are now available . However, there still are a lot of problems that need to be solved in order to achieve the high-efficiency and highbrightness LEDs that are expected to be widely used for general lighting technology instead of conventional light sources such as incandescent and fluorescent bulbs. For example, because of the total internal reflection, only a small fraction of the light generated in the active region of the LED can escape to the surrounding air, resulting in a low external quantum efficiency even if the internal quantum efficiency of the InGaN/GaN LED has been improved substantially by the advanced epitaxial growth techniques . This problem can also be resolved by introducing several techniques such as chip sidewall, surface roughening, patterned substrate, graded-refractive-index coating, and photonic crystal technologies. Photonic crystal LED is considered to be a promising approach because it is believed to improve the light extraction efficiency of the LED in two ways. One is the use of the photonic band gaps made by photonic crystal to prevent the guided modes emissions; only light generated in the band gap region can radiate outward since lateral propagation of the guided modes are prohibited in the band gap, thus improving extraction efficiency . The other consists in using photonic crystal to couple the guided modes to radiative modes and diffract those to the air .
Because of the chemical stability of the GaN material, a dry etching technique such as inductively coupled plasma reactive ion etching (ICP-RIE) is usually employed to fabricate the photonic crystal structures on the InGaN/GaN LEDs. Theoretically, the higher light extraction efficiency of the LED is expected from the deeper photonic crystal pattern regardless of the bandgap or the non-bandgap structures [8]. However, the experimental results reveal that light emission intensity is usually reduced when the etchedpatterns penetrate into the MQW active layer due to the plasma-induced damage . Therefore, most of the studies using photonic crystal for the InGaN/GaN LEDs focus on the patterning just on their top p-GaN layer. In this paper, we report on the improvement of cathodoluminescence (CL) of the InGaN/GaN blue LEDs with deep nano-hole arrays, which are penetrated into the MQW regions, fabricated using a combination of the ICP-RIE and wet chemical etching. The dry etching process allows the fabrication of the deep nanopatterns, while the wet chemical etching process is supposed to recover the plasma-induced damage and increase the sidewall areas of the nano-holes, which significantly contribute to the enhancement of the light extraction efficiency of the LED. In addition, we employ a CL mapping technique to investigate the light emission profile from the nano-patterned LEDs. Compared to other optical characterizations such as photoluminescence or electroluminescence, the CL mapping technique has advantages due to the fine probing capability of the electron beam.
Fig1
In summary, square lattice nano-hole arrays are fabricated on the InGaN/GaN blue LEDs by the e-beam lithography and ICP-RIE processes. CL investigations show that the light emission efficiency from the LED is enhanced by using the nano-hole patterns. However, the enhancement of CL intensity is decreased when the etched nano-patterns penetrate into the MQW layer due to the plasma-induced damages and the residues. It is found that wet chemical treatment with a KOH solution followed by the ICP etching significantly improve the CL intensity of the InGaN/GaN LEDs with the nano-hole arrays, especially with the deep nano-hole arrays which deeply penetrate into the InGaN/ GaN MQWs.
We first study the CL of the patterned LEDs after the ICP etching process with different etching depths. It reveals that all of the patterned LEDs showed a higher CL intensity compared to that of the as-grown LED. The enhancement of the CL intensity is increased with increasing etching depth, achieved a maximum 2-fold higher value at a depth around 150 nm and then decreased when the etching depth is deeper than this value. This CL enhancement trend of the patterned LED is reasonable because the thickness of the p-GaN layer in our InGaN/GaN LED is about 200 nm. Higher light extraction efficiency is expected at the deeper etching depth because more guided-modes in the LED structure are expected to have the opportunity to be diffracted to the air [8]. However, when the hole patterns come too near to or penetrate into the MQW layers, the high-energy ICP-RIE process can induce plasma damage effects that can degrade the emission property of the MQW active layer; this may be attributed to the observed CL intensity reduction from the deep-patterned LEDs.
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