Noritoshi Maeda

Recent progress and future prospects of AlGaN-based high-efficiency deep-ultraviolet light-emitting diodes

In this paper, recent advances in AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) are demonstrated. 220–350-nm-band DUV LEDs have been realized by developing crystal growth techniques for wide-bandgap AlN and AlGaN semiconductors. Significant increases in internal quantum efficiency (IQE) have been achieved for AlGaN DUV emissions by developing low-threading-dislocation-density (TDD) AlN buffer layers grown on sapphire substrates. The electron injection efficiency (EIE) of the LEDs was also significantly increased by introducing a multiquantum barrier (MQB). We also discuss light extraction efficiency (LEE), which is the most important parameter for achieving high-efficiency DUV LEDs. We succeeded in improving LEE by developing a transparent p-AlGaN contact layer. The maximum external quantum efficiency (EQE) obtained was 7% for a 279 nm DUV LED. EQE could be increased by up to several tens of percent through the improvement of LEE by utilizing transparent contact layers and photonic nanostructures in the near future.

Enhanced light extraction in 260 nm light-emitting diode with a highly transparent p-AlGaN layer

AlGaN LEDs have been studied as efficient light sources in DUV. One of the central issues in DUV LEDs is their low light extraction efficiency owing to the absorption in a p-contacting layer and a metal electrode. We report the fabrication of a 260 nm LED containing a transparent p-AlGaN layer. The LED showed a relatively good current injection with an increase in forward voltage compared with a p-GaN LED. Its efficiency reached 2%, almost equivalent to that of the p-GaN LED. In addition, a nearly zero absorption in the p-contacting layer increased the light extraction efficiency by using a reflective metal electrode.

Direct Growth and Controlled Coalescence of Thick AlN Template on Micro-circle Patterned Si Substrate

High-density micro-circle patterned Si substrates were successfully fabricated for the direct overgrowth of thick AlN templates by using NH3 pulsed-flow multilayer AlN growth and epitaxial lateral overgrowth techniques. The experimental results show that an 8-μm-thick AlN template was grown at a very high growth rate on the substrates. The AlN template had full widths at half maximum of 0.23° and 0.37° for the (002) and (102) reflection planes in X-ray diffraction rocking curves. Atomic force microscopy and transmission electron microscopy confirmed that the roughness of the surface was low (3.5 nm) and the dislocation density was very low (1.5 × 108 cm−2 (screw), 3.7 × 108 (edge) cm−2).

Performance Improvement of AlN Crystal Quality Grown on Patterned Si(111) Substrate for Deep UV-LED Applications

An AlN template layer is required for growth of AlGaN-based deep ultraviolet light-emitting diodes (UV-LEDs). However, the crystal quality of AlN templates grown on both flat and patterned Si substrates has so far been insufficient for replacing templates grown on sapphire substrates. In this work, we grew a high-quality AlN template on 2 in. micro-circle-patterned Si substrate (mPSiS) with two different sizes and shapes through controlling the bias power of inductively coupled plasma (ICP) etching. The experimental results showed that the best AlN template was obtained on a large pattern size with a bow-angle shape and the template had X-ray rocking curves with full widths at half-maximum of 620 and 1141 arcsec for the (002) and (102) reflection planes. The threading dislocation density near surface of AlN template through transmission electron microscopy (TEM) estimation was in the order of 107 cm−2, which is the lowest dislocation density reported for a Si substrate to our knowledge. A strong single electroluminescence (EL) peak was also obtained for an AlGaN-based deep UV-LED grown on this template, means that it can be used for further developing high-efficiency deep UV-LEDs.

High external quantum efficiency (10%) AlGaN-based deep-ultraviolet light-emitting diodes achieved by using highly reflective photonic crystal on p-AlGaN contact layer

We increased the light-extraction efficiency (LEE) of AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) by introducing a highly reflective photonic crystal (HR-PhC) into the surface of the p-AlGaN contact layer, thereby achieving a high external quantum efficiency (EQE). A low-damage HR-PhC with a lattice period of approximately 250 nm was fabricated using nanoimprinting and dry etching. A reflective Ni/Mg p-type electrode was deposited on the HR-PhC layer using a tilted-evaporation method. The EQE of a conventional DUV LED with emission around 283 nm was increased from 4.8 to 10% by introducing the HR-PhC and the reflective Ni/Mg electrode. A simple estimation of the effective reflectance of the HR-PhC p-AlGaN contact layer with the Ni/Mg electrode indicated a value exceeding 90%.

Over 10% EQE AlGaN deep-UV LED using transparent p-AlGaN contact layer (Conference Presentation)

AlGaN deep ultraviolet light-emitting diodes (DUV-LEDs) are attracting much attention for a wide variety of applications, however, the efficiency of DUV-LED is still low suppressed by low light-extraction efficiency (LEE). Transparent contact layer is considered to be necessary in order to obtain high LEE in AlGaN DUV LEDs. In this work, we demonstrate over 10% external quantum efficiency (EQE) in an AlGaN DUV-LED by using transparent p-AlGaN contact layer and highly reflective p-type electrode. We fabricated AlGaN quantum well (QW) DUV LEDs with transparent p-AlGaN contact layers on AlN/sapphire templates. EQEs were compared between LEDs with Ni/Al highly reflective electrode and with conventional Ni/Au electrode. The transparency of the p-AlGaN contact layer was confirmed to be more than 97 %. The maximum EQE for 261 nm LEDs with Ni/Al and Ni/Au electrodes were approximately 2 and 3.3%, respectively. We confirmed that the LEE was increased by about 1.7 times. We also fabricated flip-chip (FC) UVC LED module with transparent p-AlGaN contact layer and reflective electrode. The FC LED module was encapsulated to increase LEE. The emission wavelengths were 276 nm. The EQE value under the forward current of 120 mA was increased from 2.7 to 8.6% by increasing an LEE. The output power of approximately 60 mW was obtained under the forward current of 150 mA. The EQE value was maximally increased up to 10.8%. LEE was estimated to be increased from 8.6 % to 25.5 % by introducing LEE enhancement structure.

Recent progress of AlGaN Deep-UV LED by improving light-extraction efficiency

We demonstrated significant improvement of light-extraction efficiency (LEE) of AlGaN ultraviolet (UV) C light-emitting diodes (LEDs) by using transparent p-AlGaN contact layer and highly-reflective p-type electrode.

Direct growth of thick AlN template on micro-circle patterned-si substrate

A 8-μm-thick AlN template has been successfully directly grown on micro-circle patterned-Si substrate. Low surface roughness of 3.5 nm and both screw and edge dislocation densities are in the order of 10⁸/cm² have been obtained.

Improvement of light-extraction efficiency of deep-UV LEDs using transparent p-AlGaN contact layer

We demonstrated deep-ultraviolet light-emitting diodes (DUV-LEDs) with emission wavelengths at around 285 nm using transparent p-AlGaN contact layer and reflective p-type electrode. The reflectivity of p-type electrode was increased from 30% to approximately 70% by introducing Ni(1nm)/Al metal layers. The external quantum efficiency (EQE) of the 288 nm LED was increased from 1.9% to 2.5% by replacing conventional p-GaN contact layer by transparent p-AlGaN contact layer. The increase of light extraction efficiency (LEE) was estimated to be by 1.5 times. We finally achieved EQE of 5% in DUV-LED by improving both of LEE and electron injection efficiency (EIE). The EQE of DUV-LED would be much increased by optimizing the device structure and by combining with other approaches.

Marked efficiency enhancement of 250 nm-band AlGaN Deep-UV LEDs using multiquantum-barrier

Deep-ultraviolet (DUV) light-emitting diodes (LEDs) have a wide range of potential applications such as sterilization, water purification, medicine and biochemistry, and so on. We have recently developed 220–350 nm AlGaN or InAlGaN DUV-LEDs[1–5]. However, the efficiency of the AlGaN DUV-LED is not yet so high, due to inferior light extraction efficiencies (LEE), as well as, low electron injection efficiency (EIE) caused by quite low hole concentrations of p-type AlGaN. In this work, we demonstrated dramatic increase of efficiency of 250 nm-band AlGaN DUV-LEDs by introducing multiquantum-barrier (MQB) [6].