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TECHNOLOGY UV LEDs


available in large sizes and it is easy for it to be separated from the LED epitaxial structure with wet chemical etching. That step is essential, because silicon absorbs the device’s emission, and once it has been removed, it is possible to fabricate vertical LEDs


“ Silicon foundations


Silicon is a very attractive substrate for UV LEDs. It is cheap, it is available in large sizes and it is easy for it to be separated from the LED epitaxial structure with wet chemical etching. That step is essential, because silicon absorbs the device’s emission, and once it has been removed, it is possible to fabricate vertical LEDs. Such structures can also be made with LEDs formed on sapphire, but the substrate has to be removed with laser lift-off, and this damages the material.


We have developed a fabrication process for making DUV LEDs on silicon (see Figure 9 for the device structure). It consists of AlGaN LED growth on silicon, wafer bonding to a heat sink, silicon wafer removal, dry etching of AlN and fabrication of a mesh electrode. With this approach, the biggest challenge is to grow crack-free AlN on silicon, due to the significant difference in the thermal expansion coefficient of these two materials. One way that we have addressed this is to fabricate an AlN buffer on silicon by ammonia pulse-flow growth. Within 1 µm of buffer growth, we obtained a crack-free film with a low threading dislocation density. This provided a platform for the growth of a range of DUV LEDs on silicon that featured an InAlGaN quantum well and produced emission from 284 nm to 300 nm.


We have also explored epitaxial lateral overgrowth for the deposition of crack-free AlN on silicon. Again, we began by depositing an AlN layer on silicon using an ammonia pulse-flow method, before fabricating a stripe pattern on this binary and then growing the ELO layer. With this approach, we realised a low threading dislocation density, 3 µm-thick AlN buffer layer on silicon and demonstrated a series of DUV LEDs featuring AlGaN quantum wells emitting between 256 nm and 278 nm. We are now planning to remove the substrate and develop vertical DUV LEDs. This should lead to devices with low


QCLs at RIKEN


In addition to RIKEN’s development of DUV LEDs spanning 220 nm to 350 nm, the leading research institute has a programme devoted to the development of terahertz quantum-cascade lasers.


One notable success is the record for the highest temperature, stable operation of lasers emitting in the 2-4 THz range. Thanks to the use of a novel quantum-cascade structure, the researchers fabricated a 1.9 THz device that can operate at 160K.


The team is also developing QCLs based on the pairing of GaN and AlGaN, which have the potential to cover the 5 Thz to 12 THz range and operate at relatively high temperatures. The GaAs-based material system is not ideal for operating at these frequencies, due to phonon interactions.


RIKEN’s researchers are the first group to report spontaneous emission by current injection from an AlGaN-based QCL structure. Their device has a 150 period active region based on GaN and Al0.2


Ga0.8 N and emits at 1.37 THz. October 2013 www.compoundsemiconductor.net 47 ”


Figure 9. Moving from a sapphire substrate to one made with silicon aids the development of vertical LEDs. A damaging laser lift-off process has to be used to separate the epitaxial stack from sapphire, but with silicon a wet chemical etching approach can be employed to yield a pristine chip


operating voltages and high light extraction that will help the UV LED industry to serve many applications that would benefit from an affordable solid-state, portable source of light.


© 2013 Angel Business Communications. Permission required.


Further reading Sachie Fujikawa et al. Appl. Phys. Express 4 061002 (2011) Takuya Mino et al. Appl. Phys. Express 4 09210 (2011)


Silicon is a very attractive substrate for UV LEDs. It is cheap, it is


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