Gallium Nitride Blue LEDs
First, I would like to congratulate those who have been working on blue GaN LEDs for so long, in some cases nearly 50 years, for their progress so far. These days, there is virtually no reason at all to buy a tungsten-filament incandescent bulb (at least of the common globular shape), and even compact-fluorescents (CFL's) are hard to justify.
Ever since 2008, I have been considering isotopes, and how to use them to improve processes and devices. It's been a lonely task, because virtually every chemist or physicist views "isotopes" as merely atoms with a different number of neutrons, and thus a different atomic weight. Yes, they are indeed that, but they are so much more.
I scan the literature, recently phys.org, to figure out how to modify the makeup of isotopes in ordinary-isotope materials to enhance characteristics and behaviors. Until a few weeks ago, I simply didn't realize how much trouble that the GaN blue LED industry was having doping crystalline GaN with magnesium to provide 'holes'.
I've long been aware that elemental (stable) magnesium isn't merely "magnesium". Magnesium in nature consists of 78.99% Mg-24 isotope, 11.01% Mg-26, and 10.00% Mg-25.
Moreover, I was well aware that it was only the Mg-25 isotope whose nucleus posses 'nuclear spin': Mg-24 and Mg-26 have both an even number of protons, and an even number of neutrons. But Mg-25 is different: its nucleus contains an odd (not even) number of neutrons, and so it has a slight 'wobble'. The unpaired neutron can be thought of as orbiting around the positively-charged rest of the nucleus, so that rest of the nucleus behaves like a positive electric charge, itself spinning around the center-of-mass of the whole structure. And as every physicist should know (my degree is in Chemistry, from MIT), a charge traveling in a circle causes a magnetic dipole to exist.
Zinc, similarly, is made up of isotopes. Only 4.1% of natural, stable zinc is Zn-67 and it has a nuclear 'spin'. The rest is Zn-64, Zn-66, Zn-68, and Zn-70, and none of them have nuclear 'spin'. And I notice that some early work on GaN LEDs used Zinc as a p+ dopant. It worked, I suppose, but somehow it was abandoned early on, since magnesium worked better. Why? Could that be because 10% is greater than 4.1% ? Well, THAT can be fixed!
Doing some more research, I also notice that the radius of gallium atoms is 130 picometers. The radius of zinc atoms is 135 picometers. And the radius of magnesium atoms is 150 picometers. So I can certainly understand the difficulty they had packing a 150 picometer-radius magnesium atom into a position suitable for a 130 picometer gallium atom. They must have used a shoe-horn to pack the magnesium into the spot! Zinc's 135 picometers looks far more easily matched!
How do you get isotopes? Check out Isoflex.com They have Mg-25 and Zn-67.
Merry Christmas. And you're welcome!
Jim Bell "I invent with isotopes"
My understanding is that it is very hard to pack more Mg into the lattice. So, the solubility of Mg is less.
Notice that the X-axis for the graph says: "Ge, Zn, Mg concentration in the melt". And the Y-axis is labeled, "Hole concentration cm-3"
The Mg points on the graph cut off at a concentration of "0.2 atomic %" in GaN : Mg has reached its solubility limit!. More zinc can be put into a given GaN crystal, but at the same concentration seems to be about 2.5x less effective at providing p-carriers. (holes)
Zn has not: It continues to 3 atomic %.
Now, notice how the Mg numbers "meet" the Zn numbers, at about 0.2 atomic %.
It looks to me like the leftmost data point for Zn is a bit lower than the equivalent rightmost data point for Mg. How much lower? I haven't looked precisely, but I would imagine the vertical difference reflects the fact that the spin isotope Mg-25 is 10% atom/atom, compared with only 4.1% atom/atom for Zn-67 in natural-isotope Zn. So Zn seems to be about 2.5x less effective at doping than Mg, and that may be due to the difference in percentage of 'spin'-containing isotopes. This might be quite simple.
So, at least preliminarily, the figure-of-merit is "how many spin-containing dopant atoms can be put into the crystal structure".
Therefore, as far as I can see, if you put 100% Zn-67 into the GaN lattice, you will get 24x positive carriers (holes) as the CURRENT number of ordinary-isotope Zn dopant atoms. And that's 10x the number of positive carriers as natural-isotope Mg. So even limiting the Zn to 0.2 atomic %, you'd get an increase to 5 x (10*19) holes, 10x greater than can currently be achieved with ordinary-isotope-mix magnesium.