The 2014 Nobel Prize in Physics

So, the next question, is why are these LEDs blue? And what if we want a different colour?

In the crystal, we have two types of particles flowing around. Electrons and holes. Holes aren't 'real' particles, but they still exist: much like bubbles in a bottle of water. It's an awkward analogy, but we can imagine two bottles of water: one that's completely full except for a few bubbles, and another that is higher in energy that's almost empty apart from a few drops of water. These drops of water sloshing around are the electrons moving through the crystal. What happens when an electron (water droplet) and hole (bubble) meet? They annihilate each other, and in the process give off a little spark of energy in the form of a piece of light. For gallium nitride, the light's blue because of the difference in energy between the electrons and the holes.

To get higher energy light (more blue->purple->ultra violet) we can replace some of the gallium with aluminium. To get lower energy light (green) we replace some of the gallium with indium. So far, so good.

One of many outstanding problems though is that LEDs have much poorer efficiencies when we want to emit green light. Lots of different combinations and permutations have been tried, but none are great, and we're still looking for a better solution–this might be in the form of finding a brand new material, or of growing nitride-based crystals in more unusual forms, such as nanowires (imagine a forest of crystals standing on a sapphire 'floor') or quantum dots (tiny little pyramids). This can help because electrons act in very different ways when they're confined in certain dimensions, e.g. if they can only move in along a straight line, and this can be exploited to make better devices. So lots of people are trying this, not just for green LEDs, but for all manner of different devices.

So this is what we do when we want to intentionally change the colour that's emitted. But there's lots that happens to unintentionally change the colour too, which is a problem if you're trying to make thousands of light bulbs that should all look the same! This is the result of much more subtle problems that exist on the very smallest atomic length scales...

Thanks for your comment, it really helps piece it together. I'm familiar with semiconductor/transistor theory but the article was light on details. Also, I was mostly lost reading Wikipedia. What I've found: there's quite a long list of band-gap semiconductors [1], and the blues fit in chronologically by coming after the reds/greens (Gallium-Arsenide GaAs stuff). The blues center around Gallium-Nitride (GaN) [2] semiconductors.

> They found out you could grow GaN by flowing hot gasses containing Ga and N on top of an artificial sapphire film, which would act as a template for the crystal to grow.

This must be what [3] refers to. Mix molten gallium with nitrogen at 100 atm, 1000 ˚C. Alternatively, mix gallium with ammonia. Get a powder of GaN, then vapor deposit it into layers.

> The problem is that GaN crystals and sapphire crystals are slightly different sizes (the gaps between their constituent atoms is different) so they don't match up exactly

Right, several articles mention matching lattice constants. Seems to be a big problem. In fact, [2] mentions that the first substrates used for growing GaN were sapphire, zinc oxide, and silicon carbide. A chart [4] shows lattice constants, which I don't fully understand, but GaN's 3.186 Å is pretty close to SiC's 3.086 Å. So this seems to make sense.

How do you compare a single lattice constant like ZnO: 4.580 Å with a pair like GaN's 3.186 Å, 5.186 Å?

[1] https://en.wikipedia.org/wiki/Light-emitting_diode#Ultraviol...

[2] https://en.wikipedia.org/wiki/Gallium_nitride

[3] https://en.wikipedia.org/wiki/Gallium_nitride#Bulk_substrate...

[4] http://sector7.xray.aps.anl.gov/calculators/crystal_lattice_...

> This must be what [3] refers to. Mix molten gallium with nitrogen at 100 atm, 1000 ˚C. Alternatively, mix gallium with ammonia. Get a powder of GaN, then vapor deposit it into layers.

Not quite. If you want something to search for, search for "metalorganic vapour phase epitaxy" (MOVPE) or "metalorganic vapour deposition" (MOCVD).

> How do you compare a single lattice constant like ZnO: 4.580 Å with a pair like GaN's 3.186 Å, 5.186 Å?

This is a harder question than it might seem!

You can easily calculate a lattice misfit as a percentage if the crystals are the same shape: (a_substrate - a_film)/a_film. If it's low, the films will be strained, if it's higher then the films will have to relax through some deformation process resulting in disruption and defects at the interface. It's a complex process, and there's no easy rule for what will happen (keyword to search for is "Matthews Blakeslee" who came up with a model to predict how thick a film could be for a given lattice misfit before you get these defects, but in practice it's quite limited).

Care must be taken to directly compare lattice parameters though. To pick a simple example, imagine you have one crystal with a lattice parameter exactly twice that of another. On paper, that'd be a lot of misfit, but because they tile perfectly in practice it might work really well. Likewise, you can imagine lining up two square crystals, you could imagine being able to line up the diagonal of one crystal with the sides of the other crystal if one lattice parameter if the ratio of their lattice parameters is 1:sqrt(2). So it's not as simple as just looking to see how similar two numbers are, you have to consider the geometry of the crystals too.

This is where it gets a little complicated. For your specific example of ZnO and GaN, the ZnO value you have is for cubic ZnO so its three lattice parameters are the same (a=b=c like the sides of a cube) which is why only one is quoted (a = 4.580 Å) whereas GaN is hexagonal (a=b!=c) which is why two are quoted (a = 3.186 Å, c = 5.186 Å).

[Aside: GaN is often grown on its c-plane, in which case we can neglect the c parameter for working out the lattice misfit. This is something that's difficult for me to explain in words, but if you're interested in understanding it a bit better, search for "Bravais lattices" so you more easily visualise what these lattice parameters refer to. This means we only need to consider the a values when working out the misfit.]

So you'd want to compare the 4.580 Å value to the 3.186 Å value and ignore the 5.18 6Å value. But because the GaN crystal is not just a different size but also different shape to the ZnO crystal (hexagonal vs. cubic), it's actually more complicated. However, luckily for you, ZnO also exists in a hexagonal form just like GaN and in that case has lattice parameters a ~= 3.25 Å and c ~= 5.21 Å, so the misfit between ZnO and GaN in this case would be about 2%?

If you're curious, it seems like people do grow ZnO on GaN and vice versa, so you picked a good example to ask about :)

[Edited a few times for clarity.]

I've been cursing Shuji Nakamura since about 2005. Suddenly every gizmo had a blue LED brighter than the sun. Took me months to realize that all that blue light was wreaking havoc on my sleep cycle.

> Blue ones are used, at least around my area, to decorate things.

Don't let the "blue LED" headline fool you. This discovery did not just enable blue LEDs but a whole range of nitride-based devices. This includes, but is not limited to:

* LEDs for a wide range of colours, including white LEDs for general lighting

* blue (and other colour) laser diodes (e.g. Blu-ray)

* solar cells (nitrides show good radiation resistance, making them of specific interest for space applications)

* high electron mobility transistors (power converters are a big application, but these are useful for a huge range of other applications too e.g. radar)

* potential for biosensors (it's non-toxic/biocompatible and can be functionalised)

* better UV emitters (and all that entails, e.g. water purification, or potentially lithography)

and more besides. It really is an enabling technology, though obviously there are alternatives for a lot of these applications too. Blue LEDs just started this all off.