In light of today's announcement of the 2014 Nobel laureates, and because of a discussion among colleagues about the physical significance of these devices, let me ask:

What is the physical significance of blue LEDs, which challenges had to be overcome to create them? Why are materials with the band gap necessary for blue light apparently so rare/difficult to manufacture?

I know it took decades to create blue LED after Holonyak discovered the first red ones, so there must have been some obstacles, which were maybe also important for other areas of research - otherwise I wouldn't understand why the inventors of the blue LED got a prize that the inventor of the first LED didn't. 

Wikipedia has something to say on the topic:


  Its development built on critical developments in GaN nucleation on sapphire substrates and the demonstration of p-type doping of GaN.


However, I'm asking myself why this is "critical" and why this was difficult.

The Nobel website scientific background is good. Basically, when you try to make gallium nitride, you usually end up with a material that is (1) chock-full of defects, and (2) n-doped (even when you were trying to p-dope it).
So blue LEDs required

The invention of MOCVD technology for growing crystals (early 1970s);
Finding the right recipe to grow good GaN by MOCVD (i.e., use a sapphire substrate, start with a low temperature step then switch to high temperature, etc.) (mid-1980s);
Finding the right recipe to grow p-type GaN (what dopant to use (Mg), in what concentration, and what annealing / treating recipe to use to make the Mg dopants actually work and reduce the number of unintended n-type dopants that were canceling it out) (early 1990s);
Once all that was in place, find good structures to make LEDs (e.g. if you can also grow InGaN then you can make quantum wells) (early-to-mid 1990s).

All these steps required not only painstaking trial-and-error but also lots of insightful analysis and careful measurements to diagnose the problems and discover how to fix them. :-D
Sidenote: I think it's really cool and exciting that this line of materials-science research is not finished yet. As you alloy more and more indium into indium-gallium-nitride, the defects get even worse and p-doping becomes even harder. There are now lots of people working on overcoming these problems. Each year it seems that someone comes up with a materials-processing breakthrough that allows them to use a few more percentage points of indium.
With enough indium, the bandgap would shift from blue to green (with MUCH more indium, it shifts all the way to infrared). So this research could potentially lead to a much more efficient green LED, and even better, the long-awaited green diode laser, which would have myriad applications e.g. in display technology. (You've seen green laser pointers, but these are complicated devices that use infrared lasers and nonlinear optics. A green diode laser, if it existed, would be cheaper, more rugged, smaller, and way more energy efficient.) Also, if you could use more indium, InGaN-GaN becomes a promising candidate material system for tandem solar cells.
