Gallium nitride (GaN) is a wide band gap material (which means it is practically an insulator) with plentiful promise in high speed and high power electronics, and optical engineering applications. The problem however is that this material is very expensive. Sophisticated techniques currently used to make this material include methods such as metal organic chemical vapor deposition (MOCVD), and molecular beam epitaxy (MBE). These methods are not only energy intensive, but also expensive and result in slow growth of this material. Thankfully these techniques result in high quality gallium nitride (GaN), which is essential for engineering applications. The cost however is prohibitive, and the technical expertize and the carbon-footprint is certainly a non-neglible problem. The question is: would it be possible to come up with a technique using which several grams (~30 grams!) of high quality GaN can be made in a short time (say 1-4 hours).

A reliable way of making nitrides involves the use of ammonothermal methods, wherein ammonia is used as the nitrogen source. At high temperatures, ammonia cracks and becomes an excellent source of reactive nitrogenous species. These reactive species can be made to react with Gallium, to yield GaN. The principle seems straightforward, however in practice there is a substantial challenge here. A crust of GaN forms on the surface of Ga, when ammonothermal reaction is carried out using Ga and reactive nitrogenous species. This means that only Ga present on the surface is converted to GaN; and the rest of the Ga remains unreacted and unused. This leads to wastage of most of the gallium that is put within the reaction chamber. Considering that gallium is a precious metal, this is a problem that needs to be addressed. We averted this problem by resorting to some clever chemistry. Bismuth (Bi) came to our rescue here. A Ga-Bi mixture can be used as the starting material, instead of pure Ga. When this was done, a simple wetting-sinking mechanism kicked in during the GaN synthesis reaction, which ensured the consumption of all the Ga. Briefly, what Bi does is "coat" the new GaN particles which form on the melt-surface. The particles formed on the surface becomes heavier due to Bi-loading, and sink to the bottom; exposing fresh Ga, which then reacts. This continues till all of Ga is consumed. This also ensures that the reactions were rapid (<4 hours), and high yield (~100%)!! Hence this technology circumvented a central problem associated with GaN synthesis using ammonothermal technique. Now we are capable of making several grams (~30 grams) in just 4 hours, at a cost that is 10% the market cost!

Further more, we used a variant of this technology to make efficient light emitting materials, such as Europium doped GaN. Europium is a rare earth element, and sits cosily within the GaN matrix. The f electrons of Eu do not participate in chemical bonding with the lattice, which ensures that the f-f electron transitions are similar (in many ways) to the atomic spectra of Eu! The lattice/matrix within which Eu is sitting should be good at harnessing incident light, and transfering the energy absorbed to the rare earth ion, which is the light emitter. Not all lattices are capable of doing this. In fact coming up with a good optical lattice for rare earth dopants is non-trivial. For example, even in GaN, the slightest amount of oxygen could destroy its ability to be a good host. We got the conditions just right to obtain good red emission (wavelength ~ 620 nm) from Eu:GaN synthesized using ammonothermal techniques. This material can now be put to good use in devices such as light emitting diodes and lasers. You are welcome to talk to us in case you would like to make a device out of this material.

Ref: Journal of Crystal Growth, Vol. 316, 90-96 (2011).