Low Resistive and Low Absorptive Nitride-Based Tunnel junctions
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Low Resistive and Low Absorptive Nitride-Based Tunnel junctions Daichi Minamikawa1, Daiki Takasuka1, Masataka Ino1, Motoaki Iwaya1, Tetsuya Takeuchi1, Satoshi Kamiyama1 and Isamu Akasaki1.2 1 Faculty of Science and Technology, Meijo University, Nagoya 468-8502, Japan. 2 Graduate School of Engineering, Akasaki Research Center, Nagoya University, Nagoya 4648601, Japan ABSTRACT We have investigated two approaches for an alternative hole injection with a tunnel junction targeting deep UV-LEDs. One was an AlGaN-based tunnel junction. We fabricated the AlGaN-based tunnel junctions with various AlN mole fractions (0～0.2) grown on conventional blue-LEDs by MOVPE. A 7.5 nm heavily Mg-doped GaN/15 nm heavily Si-doped Al0.2Ga0.8N tunnel junction showed a large voltage drop, 5.31 V at 20 mA, under reverse bias. The other was a GaInN-based tunnel junction. We prepared Ga0.6In0.4N tunnel junctions with various thicknesses and Si doping levels grown on the blue LEDs by MOVPE. A 2 nm heavily Mgdoped Ga0.6In0.4N/3 nm heavily Si-doped GaN tunnel junction showed only 0.12 V drop at 20mA under reverse bias. Since an absorption of the thin GaInN tunnel junction was estimated to be less than 10 %, such a tunnel junction with small bandgap and thin layer thickness is a practical approach to obtain a low resistive and low absorptive hole injection in the deep UV-LEDs. INTRODUCTION The tunnel diode was invented by Esaki in 19581). While the diode shows not only a negative resistance under forward bias but also an ohmic characteristic under reverse bias. Therefore, the tunnel junction enables to inject holes from an n-layer to a p-layer and also minimize thicknesses of resistive p-layers, resulting in greater flexibilities for current injection. There are some devices utilizing the reverse-biased tunnel junctions. For example, multi-junction solar cells,2,3) and vertical cavity surface-emitting lasers4–7) with current confinement are realized. In addition, nitride-based tunnel junctions have been investigated and showed very low resistivity using GaInN-based materials by molecular beam epitaxy and metal organic vapor phase epitaxy (MOVPE).8,9) In addition, nitride-based multi-junction solar cells10), micro lightemitting diode (LED) arrays11), and LEDs with small current aperture6) have been realized so far. In the meantime, ultraviolet-LEDs, especially with deep ultraviolet (DUV) emissions have suffered from extremely high resistances of p-AlGaN layers and large absorptions of p-GaN contact layers. Thus an implementation of the nitride-based tunnel junction into the DUV-LED could be a solution for such issues. In principle, the tunnel junction can replace most part of the highly resistive p-Al0.6Ga0.4N layer with a reasonably conductive n-Al0.6Ga0.4N layer. In addition, the highly absorptive p-GaN contact layer can be eliminated and replaced with non-absorptive nAl0.6Ga0.4N contact layer for the hole injection. It is then really necessary to obtain low resistive and low absorptive nitride-based tunnel junctions targeting the DUV-LEDs, and tw
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