TECHNOLOGY UV LEDs
which has a reflectivity of 92 percent at UVC wavelengths (280 nm to 100 nm). However, aluminium cannot form a p-type ohmic contact on p-type AlGaN, so we first insert a sub-nanometre- thick layer of nickel. Reflectivity with this combination is 64 percent, which is far superior to the conventional pairing of nickel and gold, which produces a reflectivity of just 30 percent. Note that another option for our highly reflective p-contact is a mesh-pattern electrode. If we decide to go down that road, we will need to insert a p-type, AlGaN hole-spreading layer beneath the electrode, which could be formed with a short- period superlattice. This structure could also help to combat an increase in forward voltage of about 5 V, which resulted from replacing p-GaN with p-AlGaN.
Our efforts at switching from p-GaN to p-AlGaN and adding a reflective contact were rewarded. With the new blocking layer in place, switching to the p-AlGaN contact layer increased EQE by about 50 percent, and a further efficiency gain of 70 percent resulted from the transparent p-AlGaN contact layer and highly reflective p-electrode (see Figure 7).
Figure 6. The aluminium composition in AlGaN can be reduced without a large impact on EQE. This indicates that an AlGaN layer, which is less absorbing than GaN, could lead to higher efficiencies in a modified device. The downside of p-AlGaN is a lower hole density, but this can be compensated with a better electron blocking layer. When researchers at RIKEN followed this path, their devices delivered far higher EQEs than before
Increasing light extraction
One route to making further improvements in LED light extraction is to introduce photonic nano-structures, such as two-dimensional photonic crystals or moth-eye patterns. We are investigating this possibility and introducing a connected- pillar AlN buffer beneath our devices. An array of AlN pillars should increase device efficiency by allowing light to propagate vertically along the array, and it should also enhance material quality, because the threading dislocation in the pillars should be quite low.
Our efforts in this direction begin by taking patterned sapphire substrates and growing connected, hexagonal-shaped AlN pillars on them by controlling the V/III ratio and growth temperature (see Figure 8). To reduce the threading dislocations in these structures, an ammonia pulse-flow method is employed in the initial stage of AlN pillar growth. Once the array is formed, we reduce the V/III ratio, so that the pillars merge to form a flat surface. The threading dislocation density in the pillars is low, according to cross-sectional images provided with a transmission electron microscope.
Figure 7. To take the EQE of its deep UV LEDs from about 3 percent to 5.5 percent, RIKEN’s researchers used p-AlGaN contact layer and switched from a Ni/Au contact to Ni/Al
These connected pillars have formed the foundation for 265 nm LEDs that deliver a continuous output of over 5 mW and have an EQE of a few percent. This work is still in its infancy, and we know that the external quantum efficiency of our devices will be far higher when we optimise the surface roughness of a connected pillar AlN buffer. We expect that by combining a transparent p-AlGaN contact layer with a connected-pillar AlN buffer, we will be able to increase the external quantum efficiency of our DUV LEDs to several tens of percent.
content AlGaN, multi-quantum barrier electron-blocking layer. The other issue that we faced – that the substrate reflects light back into the device – was addressed by introducing a new device architecture with a highly reflective p-electrode mirror. Why did we do this, rather than simply turning to a transparent electrode? Well, although that is a good approach for the blue LED, thanks to the availability of transparent ITO, no material can fulfil that role in the deep UV.
One candidate material for making the mirror is aluminium, 46
www.compoundsemiconductor.net October 2013
Figure 8. The array of hexagonal AlN pillars was formed by ammonia pulse-flow growth on patterned sapphire
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142