Hybrid optoelectronic devices based on blends of hard and soft semiconductors can combine the properties of the two material types, opening the possibility for devices with novel functionality and properties, such as cheap and scalable solution-based processing methods. However, the efficiency of such devices is limited by the relatively slow electronic communication between the material components that relies on charge transfer, which is susceptible to losses occurring at the hybrid interface.

A phenomenon called Förster resonant energy transfer (FRET) was recently theoretically predicted and experimentally observed in hybrid structures combining an inorganic quantum well with a soft semiconductor film. Förster resonant energy transfer is a radiationless transmission of energy that occurs on the nanometer scale from a donor molecule to an acceptor molecule. The process promotes energy rather than charge transfer, providing an alternative contactless pathway that avoids some of the losses caused by charge recombination at the interface.

Now researchers from the University of Cyprus and Cyprus University of Technology, along with colleagues from the University of Crete, Greece have conducted a comprehensive investigation on how various structural and electronic parameters affect FRET in structures of nitride quantum wells with light-emitting polymers. Based on their studies, the researchers discuss the process to optimize the energy transfer process and identify the limitations and implications of the Förster mechanism in practical devices. The work demonstrates the importance of understanding FRET in hybrid structures that could pave the way for developing novel devices such as high-efficiency LEDs and solar cells. The researchers present their work in a paper published this week in the Journal of Chemical Physics, from AIP Publishing.

In the top left is a schematic of Förster Resonant Energy Transfer from a near-surface nitride quantum well to a polymer overlayer. In the top right is fluorescence from solutions containing light emitting polymer materials. In the bottom left high resolution transmission electron microscope image from an InGaN/GaN quantum well. In the bottom right absorption and fluorescence spectra from various polymers used in our study. Credit: Grigorios Itskos/University of Cyprus, Cyprus

“Pioneering theoretical and experimental work has demonstrated that energy can be efficiently transferred across hybrid semiconductors via the Förster mechanism. However, our understanding is not complete and many material and structural parameters affecting FRET in such hybrids remain unexplored. Our work employs for a first time a comprehensive approach that combines fabrication, theoretical modeling and optical spectroscopy to fully understand FRET in a nitride quantum well-polymer hybrid structure,” said Grigorios Itskos, the primary researcher and an assistant professor from the Department of Physics at the University of Cyprus.

“We used a systematic approach to optimize the FRET efficiency by tuning various parameters of the nitride quantum well component. The process allowed us to study unexplored aspects of the mechanism and identify competing mechanisms that limit the energy transfer efficiency in hybrid planar structures. The outcome of our investigation can guide future efforts towards a rational design of hybrid geometries that can optimize FRET and limit competing losses to render FRET-based devices feasible,” he said.

Itskos noted that the researchers chose to study structures based on nitrides because the material is well-researched and is used in niche applications such as blue light emitting LEDs. “However, the functionality [of nitride structures] can be further increased by combining them with other soft semiconductors such as light-emitting polymers. The spectral tunability and high light-absorption and emitting efficiency of the polymers can be exploited to demonstrate efficient down-conversion of the blue nitride emission, providing a scheme for efficient hybrid LEDs,” Itskos said.

In the study, the researchers initially sought to produce and study near-surface nitride quantum wells to allow a close proximity with the light-emitting polymer deposited on their top surface.

“The nanoscale proximity promotes efficient interactions between the excitations of the two materials, leading to fast Förster transfer that can compete with the intrinsic recombination of the excitations,” Itskos explained. Förster resonant energy transfer is a strongly distance-dependent process which occurs over a scale of typically 1 to 10 nanometers. The contactless pathway of energy transmission could avoid energy losses associated with charge recombination and transport in hybrid structures.

Using a sequence of growth runs, theoretical modeling and luminescence spectroscopy (a spectrally-resolved technique measuring the light emission of an object), the researchers identified the way to optimize the surface quantum well emission.

“We studied the influence of parameters such as growth temperature, material composition, and thickness of the quantum well and barrier on the optoelectronic properties of the nitride structures. Increase of the quantum confinement by reducing the width or increasing the barrier of the quantum well increases the well emission. However, for high quantum well confinement, excitations leak to the structure surface, quenching the luminescence. So there is an optimum set of quantum well parameters that produce emissive structures,” Itskos said. He also pointed out that the studies indicate a strong link between the luminescence efficiency of the nitride quantum well with the FRET efficiency of the hybrid structure, as predicted by the basic theory of Förster. The correlation could potentially provide an initial and simple FRET optimization method by optimizing the luminescent efficiency of the energy donor in the absence of the energy acceptor material.

“Our studies also indicated that electronic doping of the interlayer between the nitride quantum well and the polymer film reduces the efficiency of FRET. This constitutes a potential limitation for the implementation of such hybrid structures in real-world electronic devices, as electronic doping is required to produce efficient practical devices. Further studies are needed to establish the exact influence of doping on FRET,” Itskos noted.

He said the team’s next step is to perform a systematic study of hybrid structures based on doped nitride quantum wells to investigate the mechanisms via which electronic doping affects the characteristics of the Förster resonant energy transfer.

The use of sapphire in the manufacturing of Light Emitting Diodes (LEDs) is covered in the second part of a two part series.

BY WINTHROP A. BAYLIES and CHRISTOPHER JL MOORE, BayTech-Resor LLC, Maynard, MA

In Part 1 of this article, we discussed the optical and mechanical properties of sapphire and its use in the mobile device industry. In part 2, we will discuss the use of sapphire in the LED process including some of the newer technologies that produce these devices.

Solid state lighting (or “LED bulbs” as they are commonly known) have become a mainstream product in our culture. Their longer life time and lower power usage (along with the banning of incandescent bulbs) have ensured that more and more consumers are moving to this type of lighting. Like a fluorescent light (where the white light is produced by a phosphor coating excited by the excited gas molecules) solid state lights use a phosphor excited by the short wavelength light emitted by an LED. What you may not know is that about 8 out of every 10 LED bulbs sold uses sapphire as the starting material for their manufacturing process.

As we summarized in part 1, sapphire has some good points: hard, strong, optically transparent and chemically inert (there is a reason high end watches use sapphire crystals) and some bad points: hard, strong, and chemically inert (which is why sapphire crystals are more expensive than glass). What we did not discuss is that single crystal sapphire has turned out to be an ideal material on which to grow the layers of material needed to make an LED.

As FIGURE 1 shows an LED is made by growing epitaxial layers of Gallium Nitride (GaN), AlGan or InGaN on a substrate. Ideally one would use GaN as the substrate material (similar to growing epitaxial Si on Si for integrated circuits) as this would result in the highest quality material and thus the most efficient LED’s. Unfortunately GaN substrates are very difficult to make in any reasonable size and the costs have ruled out using this approach except in certain niche markets. The three main substrate alternatives have been silicon (Si), silicon carbide (SiC) and sapphire.

As a substrate material Si would be expected to be the best choice due to its high quality, low cost and ready availability. To date, the quality of GaN type layers grown on Si has not been sufficient for large scale manufacturing processes. Work continues on improving this process and although it may one day dominate the process it currently remains a small part of the business.

SiC substrates are higher cost than Si but have been successfully used for LED manufacturing processes. Much of the LEDs produced by Cree (who also manufacture SiC substrates) use this type of substrate. However, the higher cost and limited availability of 6 inch SiC material means that the majority of LED producers use sapphire.

Thus sapphire substrates account for the majority of LED devices produced [1]. Although not as cheap as Si they are cheaper than SiC, available from a number of manufacturers and are able to survive the high temperature processes needed to produce a short wavelength LED. FIGURE 2 schematically shows the production process for a typical non-patterned sapphire wafer.

The sapphire production process starts when a seed crystal and a mixture of aluminum oxide and crackle (un-crystallized sapphire material) is heated in a crucible. Included in this mix is a cookie-sized seed crystal which forms the pattern to be replicated as the crystal grows. Each furnace manufacturer has its own special recipe which heats the material using a specific temperature/ time profile based on the size of melt and the type of crystal to be grown. Once the correct growth temperature is reached the melt is cooled (this process can take two weeks depending on the amount of sapphire being produced) using another set of carefully controlled time/temperature profiles. When done correctly, the cookie-sized seed grows and produces a single-crystal sapphire boule. (FIGURE 3). In reality, two weeks is a long time and any number of can go arise during this process including gas bubbles, mechanical faults such as cracks and contamination. Each of these problems affects the sapphire and its crystal properties. Each crystal fault can become a nucleation site for defects in the epitaxy grown on wafers produced from the boule. There is a clear correlation between the time taken to grow a boule and the potential quality of the boule produced. Many of the problems encountered in the upscaling of the sapphire production process have come from trying to grow large boules at high speeds.

At this point in the process you have a boule which in fact has the wrong crystal orientation for growing GaN epitaxy. Unlike the Si crystal growth process where the cylindrical boules can be ground to size and then cut into wafers, sapphire boules are often cored at right angles to the boule axis. Some companies produce sapphire using a silicon like process [2] but the majority of sapphire produced has to be cored. Thus the next step in the process is to “core-drill” a boule to produce one or more smaller round cylinders (ingots) depending on the original boule size and the size of wafers to be produced.

The ability to grow large sized boules on a regular basis is not in question; most important is how much of that boule is bubble-, crack- and impurity-free. In some cases the boules are inspected with various metrology techniques to determine which sections of the boule can be used and which cannot. The section of the boules not used is recycled into the original growth process (unless contaminated). Obviously if one is producing 6 inch wafers larger volumes of the boule need to be defect free than if one is producing 2 inch or 4 inch. Currently most of the LEDs produced are produced on 4 inch wafers with a few newer 6 inch lines and a number of older 2 inch lines. 8 inch sapphire wafers do exists but are not in mass production at this time.

The process after this is very similar to that used in the silicon industry to produce the wafers which will be used as substrates. A diamond saw (remember, Sapphire is a very hard material) is used to cur the ingot into a number of thin disc shapes by cutting perpendicular to the ingot’s long sides. Each of these discs is then ground to its final size, surface-ground and mechanically and chemically polished to produce sapphire substrates. These substrates, after cleaning, can be used as starting material for the epitaxial process used to produce the LED structure. FIGURE 4 shows some pictures of typical 2, 4 and 5 inch sapphire substrates. As discussed earlier the more defect free the surface is the better the quality of epitaxial film that can be grown. The video listed in reference [3] produced by GTAT shows many of the steps discussed above.

Recently one further step has been taken to produce what are called patterned sapphire substrates (PSS). The multiple quantum well layer shown in Fig. 1 is the layer that generates light in an LED. As you can imagine this light is emitted in all directions. However, once packaged most LED’s emit light from only one surface of the device. In the case of Fig. 1, a typical package collects the light emitted from the top of the device. This of course means that all of the light emitted in any other direction is wasted. In particular, since sapphire is transparent, little of the light emitted toward the substrate can be used.

One obvious solution to this would be to coat the substrate with something that reflects the light (i.e. metal). Unfortunately this interferes with the epitaxial layer growth process, producing poor devices. One partial solution to the reflection problem is to pattern the sapphire surface such that it reflects light. This pattern can be a series of microscopic pyramidal structures or more rounded bump like structures on the surface. FIGURE 5 shows top and side view SEM pictures of some of the patterns produced by manufacturers. These patterns scatter the light and reflect some of it back towards the surface of the device increasing the light output from the LED. In addition to increasing the apparent light output a number of manufacturers have claimed that epitaxial layers grown on patterned substrates is of better quality than that grown on bare sapphire substrates.

Patterned substrates can be produced by the manufacturer of the sapphire substrates. However, factories now exist which begin with a non-patterned substrate and produce specific patterns (normally via chemical etch) for specific LED manufacturers.

Once valued only as a gemstone, sapphire is now an engineered material with a wide variety of industrial uses. These two article have concentrated on its use in mobile devices for everything from camera lens covers to touch sensors and touch screens to the starting material on which most of the solid state lights produced are made. Cost of the material continues to be a limiting factor in its widespread adoption for certain industries. However, as the technology for producing sapphire matures material costs are decreasing and in some ways sapphire substrates have become a commodity rather than a rarity.

Additional reading and viewing material

1. http://rubicontechnology.com/sites/default/files/Opportu- nities%20for%20Sapphire%20White%20Paper-Rubicon%20 Technology.pdf
2. http://www.arc-energy.com/products-services/CHES/Foundations/1
3. https://www.youtube.com/embed/mHrDXyQGSK0