TECHNOLOGY LEDs
Blue and red LEDs excel in this regard, but the same cannot be said for their green-emitting cousins. At modest currents, such as 20 mA, their efficiency is significantly inferior. And when the current is cranked up, they are more prone to droop, so their efficiency plummets faster.
These weaknesses have several origins, all associated with flaws in material characteristics. To produce a green LED, engineers use a similar growth process to that employed for making a blue emitter, but increase the indium content in the InGaN quantum well. Unfortunately, this increases the crystalline strain in the well, leading to the generation of crystalline defects that peg back the efficiency of the green LED.
This device is fabricated from the hexagonal, or wurtzite, phase of the wide bandgap material, which features strong internal fields. These fields cause a shift in wavelength as the current is increased, due to a phenomenon known as the quantum confined Stark effect. This is particularly severe in green LEDs.
As well as the shift in emission wavelength, there is a pulling apart of electrons and holes, which pile up on opposite ends of the well. This impairs the radiative recombination efficiency. Turning to narrower wells can bring the charge carriers closer together, but may have very damaging side affects associated with the increase in carrier density. Auger recombination is known to increase with the cube of carrier density, and if, as many believe, this is the primary cause of droop, turning
Figure 3. LEDs formed in the stripes of a silicon wafer produce green emission
to thinner wells will fail to improve the performance of a green-emitting LED at meaningful current densities.
So, as you can see, the green gap is a very tricky problem. Researchers are trying to solve it, however, with some taking radical steps that will allow a lowering of the carrier concentrations in the quantum wells. Since it is difficult to see how one could do this with polar material, groups are trying different approaches, such as growing devices on non-polar faces of hexagonal GaN. And at the NSF-funded Smart Lighting Engineering Research Center at Rensselaer Polytechnic Institute we are taking a similar, but distinct tack, by turning to the cubic phase of GaN.
January / February 2014
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