TECHNOLOGY CONFERENCE REPORT
this device less susceptible to droop, the decline in the efficiency of a nitride LED at higher current densities. What’s more, he explained that the blue LED has a higher electrical efficiency than a green-emitting variant, despite its higher bandgap; and that the green emitting, optically pumped structure does not suffer from an electron-hole imbalance, which is detrimental to a conventional green LED.
Trials of this novel green emitter have involved the growth of epistructure featuring a 40 period multi-quantum well. This is estimated to deliver an external quantum efficiency of 50 percent. To boost green emission and prevent significant output in the blue, three of these multi-quantum well structures were stacked on top of one another.
From LEDs to lasers A radical alternative for solid-state lighting is to use lasers, rather than LEDs, as the primary light source. Jonathan Wierer from Sandia National Laboratories, Albuquerque, outlined this proposal, which could use a laser to pump a phosphor. He explained that he and his co-workers have tried this, and found that the colour quality is comparable to that produced by LED pumping: “Despite spiky spectra, you get good colour rendering.” Wierer also reminded the
performance of the company’s green and blue laser diodes developed in the lab. According to Avramescu, the company’s green lasers, which emit around 520 nm, now deliver a kink-free output up to 250 mW and have a wall-plug efficiency of 8.7 percent at 150 mW. Meanwhile, the company’s blue lasers produce an output of up to 4 W, and at 1.6 W they have a conversion efficiency of 30 percent.
Avramescu pointed out that the main applications that Osram intends to target with its green lasers are small projectors, speciality lighting, assistance with surveying and head-up displays. Small projectors, which could soon feature in mobile phones, combine red, green and
green-emitting variant, despite its higher bandgap; and that the
“
audience that this approach is already being pursued by the German automaker BMW, which is developing laser-based headlights.
The attraction of lighting with lasers, rather than LEDs, is that the former class of device is not plagued with droop. This opens the door to high efficiency at high current densities, so fewer chips are needed in a fixture, and those that are employed require less thermal management. Although Weirer calculates that LEDs will get more efficient, he believes that lasers will also improve, so laser-based lighting will continue to have the potential to offer the greatest efficacies in the future.
Delegates gained insight into the efficiency of some of the latest lasers in a talk given by Osram’s Adrian Avramescu, who revealed significant advances in the
green emitting, optically pumped structure does not suffer from an electron-hole imbalance, which is detrimental to a conventional green LED.
blue lasers with a two-axis mirror to form a technology known as flying-spot laser projection. “You build your image up pixel by pixel,” explained Avramescu.
Requirements for lasers used in small projectors include an output of about 80 mW, or 20 lumens. Blue lasers that the company launched in 2009 meet this, while the direct green variants that hit the market in 2012 fall short of the mark, producing 50 mW, equating to 12-13 lumens. In addition, the emission profile from these chips is not the ideal Guassian profile that is preferred for projection applications, due to interactions with the substrate.
All these weaknesses have now been remedied in the lab, thanks to various efforts at understanding the behaviour of the green laser diode. First-generation devices produced a sub-linear output
42
www.compoundsemiconductor.net October 2013 ”
above 50 mW, and investigating this decline in efficiency led the engineers at Osram to discover that the laser’s differential gain varies with temperature. Self-heating and a high threshold current are partly to blame, along with imperfection injection.
Material gains
Research efforts were also directed at material issues. Dark spots were found with dimensions of 1-20 µm, alloy fluctuations were uncovered at 50 nm to 500 nm length scales, and transmission electron microscopy uncovered defects in the quantum wells, such as dislocations.
The blue LED has a higher electrical efficiency than a
Improving the quality of the material and making proprietary modifications to laser design enabled the fabrication of higher output green lasers with improved beam quality. At 80 mW – the output power required for small projectors – efficiency is 7.5 percent at 25 °C and 7 percent at 60 °C.
Avramescu also explained that Osram’s more powerful blue laser is suitable for projectors in homes and offices, which require 2000 lumen sources. In 2012, the company launched a 1.4 W blue laser in a TO56 package, and now it has raised the bar to 2.5 W at 25 °C.
Packaging is very important at these power densities, and by optimising this it is even possible to crank the output up to 4 W. However, Avramescu explained that this involves driving the chip in an “over- stressed” regime.
It will be interesting to see how laser performance improves over the next year, and whether efforts to use this source for general lighting take off. Progress occurring over this timeframe will not be reported at ICNS, because this is a biannual meeting, but the international nitride community will get together next year at the International Workshop on Nitride Semiconductors. This will be held in Poland, in the last week of August, 2014.
© 2013 Angel Business Communications. Permission required.
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