Masafumi Jo

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%.

Growth of non-polar a-plane AlN on r-plane sapphire

Growth of non-polar AlN is crucial to the realization of polarization-free light-emitting diodes in deep UV range. The aim of this study was to investigate the growth condition for obtaining a flat a-plane AlN on r-plane sapphire. A thin AlN layer grown at lower temperature played an important role in protecting the sapphire surface. Both high temperature and low V/III ratio were necessary in terms of enhanced adatom diffusion, leading to the formation of a flat AlN buffer.

Recent progress of AlGaN-based deep-ultraviolet light-emitting diodes

The fundamental performance limits of coherent optical transmission systems can be observed by a simple optimization between the linear noise and the nonlinear noise generated within the system. Optical Phase Conjugation (OPC) is considered to be one of the promising techniques to compensate for optical fiber’s dispersion and nonlinearity that cause crosstalk between signals traveling through long-haul optical transmission systems, nonlinearity compensation can lead to significant information capacity and distance reach expansion of optical fiber transmission links. To get the full benefit from the deployment of OPC in optical transmission systems, a few considerations must be taken into account, such as: power profile symmetry, fiber’s dispersion slope and Polarization Mode Dispersion (PMD). In this contribution, we will present our simplified theoretical predictions of optical fiber transmission systems performance that deploy mid-link OPC and multiOPC and we will show that the introduction of multi-OPC in an optical transmission system will minimize the impact of uncompensated/nondeterministic signal-signal nonlinear interactions due to fiber’s PMD and signal-noise interactions. We will show wide range of simulation and experimental results that validate the theoretical predictions of system’s performance for various types of links: dispersion managed, dispersion unmanaged, discretely amplified systems and distributed Raman amplified systems. Also, we will present an extensive experimental study shows that the deployment of mid-link OPC can provide a significant reach improvement in asymmetric lumped optical fiber links when optimizing the span length.III-V semiconductors have a direct bandgap that can be tuned through alloy engineering and therefore appear as very interesting for solar-cells, solid-state lighting and high power applications. The performances of current devices may be increased through the use of nanostructures and nanowires which look promising for the integration of high efficiency devices. Nanowires exhibit great properties such as efficient strain relieving capability and large specific area. Growth on silicon substrates and core-shell structures can be considered as well. Still, the production of nanowire-based devices faces material challenges related to morphological, structural, optical and electrical properties which are very linked to the synthesis process. This presentation will focus on Hydride Vapor Phase Epitaxy, which is a growth process implemented in a hot wall reactor using chloride precursors, and showing unique features regarding the growth of III-V and III-Nitride nanowires. For example, self-catalyzed GaAs nanowires were grown on silicon at a fast growth rate (60 µm.h-1) exhibiting a constant zinc-blende crystalline phase, for the potential fabrication of GaAs-based photonic devices on Si. For III-Nitride materials, InGaN nanowires demonstrating the entire composition range were grown by using a method compatible with the standard GaCl-based GaN growth process. Photoluminescence coupled with transmission electron microscopy measurements showed that these nanowires could overcome the so-called green gap and stretch the limits of solar cells efficiency. By taking advantage of the large growth rates anisotropy resulting from the use of chloride precursors, we could freely tuned the shape of GaN wires on masked substrates with (sub)-micrometric apertures.W the popularization of data centre and other bandwidth hungry inter-connect applications, the desired capacity of short reach optical network has exponentially increased to 400 Gbit/s or even more. Recent standardization efforts for 400 G intradata center connections specify link lengths of up to 2 km. 8×56 Gb/s or 4x100 Gb/s could enable such 400 G networks. Relative to coherent detection. Intensity modulation/direct detection (IM/DD) is a good candidate in inter-connect due to its low cost. For 56 and up to 100 Gb/s signal generation, a few modulation formats or schemes, such as pulse-amplitude-modulation (PAM4), discrete multitone (DMT), duobinary and chirp-managed laser (CML) are proposed and experimentally demonstrated. However, considering cost, size and power comsuption, the modulation format should be optimized for different networks to meet different requirements. In this talk, we will discuss this issue how to optimize the modulation formats for different optical networks?

Impact of thermal treatment on the growth of semipolar AlN on m-plane sapphire

The interest in semipolar orientations has been increasing because the reduced piezoelectric field can improve the performance of nitride-based optoelectronic devices. However, the crystalline qual...

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.

Fabrication of an a-plane AlGaN quantum well on r-plane sapphire

Nonpolar AlGaN light-emitting diodes have been attracting much attention due to their potential for realizing optoelectronic devices with high efficiency and stability. However, few studies have been reported on the optical properties of nonpolar AlGaN quantum wells (QWs) on sapphire because of the difficulty in the growth. Here, we report the fabrication of an a-plane AlGaN QW on r-plane sapphire using high-temperature growth method. An Al(Ga)N layer with smooth morphology was obtained at high temperature and low V/III ratio compared to the conventional conditions. An NH3 flow rate played an important role in controlling the Al content of AlGaN at high temperature. An a-plane QW showed clear luminescence in the UVC range at room temperature.

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.