TECHNOLOGY LEDs
Going Green with cubic GaN
Conventional green LEDs are plagued by strong internal electric fields, which impair recombination and make it difficult to address droop. The solution is switch to growth on the cubic phase of GaN, which is free from internal fields and has a smaller bandgap, making it easier to reach longer wavelengths.
By MARK DURNIAK and CHRISTIAN WETZEL from RENSSELAER POLYTECHNIC INSTITUTE
THE SOLID-STATE LIGHTING REVOLUTION is now upon us. Businesses and homeowners are trimming their electricity bills, and by 2030 an ever-increasing uptake of LED bulbs could lead to savings as high as $30 billion, according to the US Department of Energy.
Most consumers that are currently investing in solid-state lighting are buying bulbs that employ a highly efficient blue LED to excite a phosphor, which then re-emits photons of lower energy, usually in the yellow spectral region. Colour mixing of blue and yellow creates a white-emitting luminaire.
Figure 2. Model of the cubic/hexagonal GaN interface
This approach has the merit of simplicity, but it is not ideal. One weakness is the inherent conversion efficiency ceiling, stemming from the Stokes loss in the down-conversion of high-energy blue photons to lower energy yellow photons. In addition, the colour temperature or ‘warmness’ of the white light
is fixed, governed by the mix of phosphors.
These limitations can be lifted with luminaires based on the colour mixing of several LEDs, such as those emitting in the red, green and blue. With this approach, which can involve three or more LEDs, it is possible to construct a high-quality, colour-tuneable light source. The Philips HUE bulb is an example of this, which allows users to pick the colour they want by running an app on their smart phone.
Figure 1. Growth of hexagonal GaN in ‘V-shaped’ trenches leads to the formation of cubic GaN
54
www.compoundsemiconductor.net January / February 2014
Another advantage of the colour mixing approach is that it can achieve higher theoretical efficiencies than phosphor conversion. The efficacy of this form of lighting is then governed by the efficiencies of the LEDs.
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 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169