Bridging The Green Gap With A New Foundation
Relaxed InGaN substrates open the door to high quality lighting and full colour displays based on III-nitride LEDs
BY AMÉLIE DUSSAIGNE FROM CEA/LETI AND DAVID SOTTA FROM SOITEC
The majority of commercial white LEDs are based on a blue-emitting chip that pumps a phosphor. This combination has generated tremendous sales, but it does have two major weaknesses: an energy loss of typically 25 percent, associated with the phosphor down conversion; and a colour rendering index that is not that high.
Both these drawbacks can be addressed by direct colour mixing, using LEDs with a variety of emission wavelengths. The ideal way to accomplish this is to use one material system that spans the blue to red, because this simplifies colour control. Do this, and the technology could also find deployment in high-brightness displays.
Fulfilling this dream is not easy, however. The main issue is the material. Although the InGaN alloy can theoretically cover the whole visible range, its quantum efficiency plummets above 500 nm as emission shifts from green to red. Switching to the AlGaInP material family allows the production of high-performance amber and red LEDs, but in this case efficiency takes a nose dive as emission stretches to wavelengths shorter than 570 nm.
The 500 nm to 570 nm spectral range, where it is difficult to realise efficient emission, has its own name: the green gap. For GaN-based LEDs, lack of success stems from a reduction in material quality with increasing indium content in the InGaN alloy. This issue can be traced back to low miscibility of indium in GaN, and the high lattice mismatch between the GaN buffer layer and the InGaN quantum wells.
Our team at CEA/Leti and Soitec has tackled this issue head on with a novel substrate technology that allows InGaN quantum wells to combine a high indium content with great material quality. This is accomplished with substrates that are better suited to the growth of InGaN than the most common one used for nitride LEDs, sapphire.
Growth of GaN-based LEDs on sapphire often begins with the deposition of a GaN buffer layer. That’s not a great start, as the buffer layer leads to strong compressive strain. Making matters worse, when InGaN quantum wells are added, the strain gets even higher. Compounding these issues, internal electric fields in the InGaN/GaN heterostructures hamper LED efficiency when wells are widened, or indium content increased.
The electric field can be eliminated by switching to non-polar GaN planes. But it’s not a great solution, as more indium is needed to realise the same emission wavelength.
If concerns over electric fields are put to one side, and the focus placed on limiting the strain, one way forward is to insert AlGaN layers in an InGaN buffer or an InGaN active zone. This has been tested, but no reports of highly efficient LEDs have followed.
Our idea is to grow the LED on a relaxed InGaN substrate, as this reduces the strain between well and buffer. Additional merits of this platform are an increase in the indium incorporation rate and a reduction in the internal electric field for the same emission wavelength.
This particular substrate is not available - but we have something similar, an InGaN pseudo-substrate called InGaNOS, based on Soitec’s Smart Cut technology. Development of this foundation, formed by transferring a thin layer of InGaN to a sapphire substrate, has been motivated by efforts to reduce the efficiency droop in high-brightness blue LEDs. However, through our work, led by CEA-LETI, we are showcasing the capability of this platform in another arena – its potential to form the base for structures emitting at long wavelengths. Emission can span blue to amber when InGaN LEDs are grown on an InGaNOS substrate, thanks to the increase in the indium incorporation rate.
Forming the foundation
Our starting material for making InGaN pseudo-substrates is an InGaN donor template. This template is formed by growing a 3 mm-thick GaN buffer layer on a 100 mm sapphire substrate, followed by an InxGa1-xN layer that is typically 200 nm-thick and has an indium content, x, of between 1.5 percent and 8 percent. This InGaN donor template, which is strained due to lattice mismatch between GaN-on-sapphire and the InxGa1-xN layer, is hydrogen implanted. In parallel, a compliant layer is deposited onto a sapphire substrate, before molecular bonding unites the InGaN donor template and sapphire handle substrate.
The next step is to transfer the donor strained InxGa1-xN layer to the compliant layer of the sapphire handle substrate, using Smart Cut technology. Photo-lithography and dry etching follow to create strained patterns, which are then relaxed, thanks to the compliant layer. This relaxation process proceeds via successive thermal annealings (see Figure 1 for an overview of the entire process).