technology LEDs
L
ED light bulbs have many great attributes. Their efficiency trounces that of the incumbent source, the incandescent bulb, and unlike their energy- efficient rival, the compact fluorescent, they are not ridden with mercury and reach full brightness in an instant. However, sales are poor, because retail prices are very high – on average an LED bulb sells for $28, according to the UK-based market analyst IMS Research.
To slash LED bulb prices, manufacturers must trim every major contribution to the overall cost. The packaged LED is the obvious place to start, since it accounts for almost 60 percent of the total bill of materials, according to a recent report from the US Department of Energy. If the efficiency of these chips can be increased, that will not only trim the price of the light bulb, thanks to a reduction in the number of LEDs needed to produce a given power; it will also reduce running costs, and in turn make the purchase more attractive.
White LED prices are very high, because the production process involves coating red, yellow or green phosphors on blue-emitting chips to produce white emission via colour mixing. This combination of chips and phosphors limits device efficiency, yield and reliability. But it is possible to tackle these issues by building novel, phosphor-free GaN-based dot-in-a-wire white LED. This technology promises to unlock the door to very-high-efficiency, more affordable light bulbs, according to our team at McGill University, Montreal, Canada.
Broadband emission
If solid-state lighting products are to be competitive, they must deliver high-performance in the blue, green and red spectral range. Today, blue LEDs built from GaN-based quantum wells are a relatively mature, high- performance technology, but their longer wavelength green, yellow and red cousins produce relatively low quantum efficiencies. What’s more, the device efficiency plummets at increasing current densities, a weakness that is commonly referred to as ‘efficiency droop’.
The low quantum efficiency of these green, yellow and red LEDs – and their severe efficiency droop – stems from material characteristics associated with III-nitride planar heterostructures, such as polarization fields and high densities of defects and dislocations. These traits
are behind the unique carrier dynamics found in conventional III-nitride quantum-well LEDs, and have been claimed to be the root cause for electron leakage or overflow, Auger recombination, and poor hole transport in this type of device.
In contrast, our one-dimensional nanowire heterostructure LEDs do not suffer from many of these issue, which plague their planar counterparts. They can be built with drastically reduced dislocations and polarization fields, and they can enhance light extraction efficiency, thanks to far larger surface-to-volume ratios.
Conventional wisdom indicates that the way to realise green and red emission with nitride devices is to embed the InGaN quantum wells or ternary wires in GaN nanowire structures. But this approach is flawed: Only a small proportion of injected carriers transfer to the lateral surfaces of the wire, due to relatively poor carrier confinement in the nanoscale heterostructures; and the non-radiative carrier recombination on the wire surfaces significantly degrades the quantum efficiency of the device.
Our unique InGaN/GaN dot-in-a-wire heterostructures address this critical issue. In our case, InGaN quantum dots are incorporated in defect-free GaN nanowires that provide three-dimensional carrier confinement, a pre- requisite for ultra-high-efficiency emission (see figure 1(a)). On top of this, our novel nanostructures offer unprecedented colour tunability. The size and the composition of the dots govern the emission wavelengths, and it is possible to create intrinsic white- light sources from single GaN nanowires by varying the structural properties of the dots during a single epitaxial growth process.
We have employed a scanning electron microscope to acquire images of our InGaN/GaN dot-in-a-wire arrays that are grown directly on silicon (111) substrates by radio-frequency plasma-assisted MBE (see Figure 1 b). Catalyst-free nanowires, which are vertically aligned to the substrate and exhibit excellent size uniformity, form spontaneously under nitrogen-rich conditions.
A scanning transmission electron microscope can uncover more detailed images of our devices (see Figure 1 c). This tool reveals multiple InGaN quantum dots near the wire centre, due to strain-induced self- organization. The composition of these dots can be
April / May 2012
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