# Presentation: 381520

## (Inorganic) Solid-State Lighting:The Promise and the Materials Challenges

- Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Security Administration under Contract DE-AC04-94AL85000.

**Joseph Wright, "An Experiment on a Bird in the Air Pump" (1768)**

**Jeff Y Tsao**
**Sandia National Laboratories**

**Notes:**
I’d like to talk to you today about Solid-State Lighting.  By Solid-State Lighting I mean the use of solid-state, semiconductor light-emitting diodes or lasers to produce white light for illumination.  Not to produce colored light for Signals or Displays, but to produce white light for Illumination.
There are two general classes of semiconductor materials that appear to be possible for Solid-State Lighting.
Organic materials are one possibility, and Milan Stolka will be giving an overview of organic solid-state lighting in the talk right after this one.  Anil Duggal also gave an overview of organic solid-state lighting yesterday afternoon.
This talk is an overview of the second possibility, inorganic solid-state lighting.  George Craford gave an overview of inorganic solid-state lighting this morning.  In fact, it was for laying the foundations for this technology that George, Russ Dupuis and Nick Holonyak were just awarded the National Medal of Technology.
Now, it’s anybody’s guess how the two materials possibilities will evolve.  Perhaps both will succeed, perhaps neither will succeed.  But, as George discussed this morning, inorganic solid-state lighting is generally considered to be more mature and its possibilities more definite.  However, to be safe, I would consider both possibilities to be rather immature.
So this talk is on inorganic solid-state lighting.  I’ve divided the talk into three parts.  First, I’ll say a few words about lighting history and how solid-state lighting might add a new chapter to that history.  Then I’ll say a few words about various approaches to solid-state lighting.  Finally, I’ll say a few words about some of the challenges that will be faced if inorganic solid-state lighting is to be successful, with a particular focus on materials challenges.

## History of Lighting Technology

**Notes:**
To put solid-state lighting in perspective, here I show a condensed history of modern lighting technologies, over the 200-yr time span from 1850, projected out to 2050.  The metric I’ve used is the one that’s normally used, luminous efficacy, in lumens per watt, which is basically the efficiency with which either chemical or electrical power is converted into light, convolved with the human eye’s sensitivity to that light.  For a four-color white light source composed of red, yellow, green and blue components, 100% power conversion efficiency would give approximately 380 lm/W, 50% power conversion efficiency would give about 190 lm/W, and so on.
One can see how terribly inefficient Fire is – even the most efficient gas lamps only achieve about 1 lm/W efficacy, or about 1⁄4 of a percent power conversion efficiency.
Incandescence was a huge improvement – but even it is limited to roughly 20 lm/W, or about 5% power conversion efficiency.
And, of course, Fluorescence and high-intensity discharges were yet another big improvement – they have the potential to approach 100 lm/W, or about 25% power conversion efficiency.
The latest technology, developing rapidly on this 200-yr time scale, is Solid-State Lighting.
Here, I show the precursors to that technology – the colored LEDs upon which white solid-state lighting is based.  One can see, beginning in the 1960’s or so, red LEDs gradually developing, with GaP, AlGaAs, and now AlInGaP, materials.  And, beginning in the 1990’s the much more rapid development of green and blue LEDs, initially with SiC and ZnSe materials, but now dominated by GaN-based materials.  Because these single-wavelength LEDs are so much more efficient than their filtered incandescence equivalents, we are now right in the middle of a massive replacement of all colored lighting applications with LEDs.
White light is a little trickier, and we are currently only at about 30lm/W or so.  However, what has gotten a lot of people excited about solid-state lighting is that there does not seem to be any fundamental reason why these white efficiencies couldn’t be as high as 50% or so.  In fact, the highest-efficiency optoelectronic devices now are high-power infrared semiconductor lasers, with power conversion efficiencies approaching 70%.  If they can do that in the infrared, then 50% doesn’t seem like such a stretch in the visible.  If solid-state lighting does make it to 50% efficiency, then it would be roughly two times more efficient than Fluorescence, and ten times more efficient than Incandescence.

## Lighting and Energy Consumption

**Lighting and Energy Consumption**

**Courtesy of NASA**

**Notes:**
And if they do make it there, to 50% efficiency, and if manufacturing costs scale as one might expect, then the grand dream of solid-state lighting would be as illustrated here.  To someday replace every lamp on Earth, all the way from the smallest indicator lamp, to your room and office lights, to the largest football-stadium-sized floodlight, with semiconductor lamps.
This would lead to significant reductions in energy consumption.
In the U.S., energy consumption, in electricity-equivalent units, is about 10,000 TWh/yr.  Of that, about 1/3 is consumed as electricity, and of that 1/5 is consumed by lighting.  So 1/15 of all energy consumed is due to lighting.
At a growth rate of about 1.5%/yr, by the year 2025, the U.S. will be consuming about 1,000 TWh/yr of electricity for lighting.  If half of that could be saved through the use of solid-state lighting, then that would be 500 TWh/yr.  At a market price of about 6 cents/kWh, that would be a savings of about $30B/yr.  Worldwide, all these numbers are larger by roughly a factor 3-4x, so the worldwide savings would be closer to $100B/yr.

## Lighting and the Human Visual Experience

**Joseph Wright, "An Experiment on a Bird in the Air Pump" (1768)**

**Notes:**
These are large numbers, but even they don’t convey the full impact of solid-state lighting.  In my mind, the most important long-term impact of solid-state lighting could very well be how it enhances the quality the human visual experience.
I like to show this painting by Joseph Wright, because it is such a nice illustration of how important light is to us.  Lighting illuminates, of course, as this scene is illuminated by the moonlight from the window at the upper right. But by selectively illuminating some parts of the scene rather than others, it focuses our attention and interest.  And by the warmness or, in this case, coolness, of the white light, it alters our emotional response to scenes, and affects whether we interpret what we see with comfort or, as in this case, with some apprehension.
With semiconductor-based solid-state lighting, it is possible to imagine lights that are independently and digitally controlled in space, time, intensity and color temperature to best enhance a particular visual experience -- something that is impossible to do with current lighting technologies.  In fact, one might anticipate that in the long term, solid-state lighting won’t save us energy at all.  As with other technologies that enhance our lives, we’ll use more, rather than less, of it.  But in using more of it, presumably our overall productivity and quality of life will also improve.

## Solid-State Lighting Approaches

**Sunlight**

**Incandescence**

**Computer Monitor**

_**Phosphor**_
_**Down-**_
_**Conversion**_

_**Color**_
_**Mixing**_

**Notes:**
Still, these are all dreams right now.  We are very far from a solid-state light that is 50% efficient, and whose light can be simultaneously controlled in space, time and color temperature.  There are a lot of challenges that must be overcome.
To see what some of these challenges are, let’s start by noting that semiconductors are very good at creating light at one wavelength, but not at creating the distribution of wavelengths that one needs for white light.  Fortunately, the human eye can be fooled, and it turns out it doesn’t take very many discrete wavelengths to create a reasonable quality white light.  In fact, the four wavelengths shown here as grey dots along the perimeter of this chromaticity diagram – 460, 530, 570 and 610nm -- are sufficient to create white light of high enough quality for almost all applications.  And, by mixing these four colors with various intensities, one can produce light of any color within these dashed boundaries, and white light of any color temperature along this Planckian black-body equivalent curve, all the way from the warm 2850K light associated with incandescence, to the 4800K light associated with sunlight, to the cool 6500K light associated with computer monitors.
Still, you do need at least four wavelengths, and in general there are two approaches to doing that.  The first approach is the phosphor down-conversion approach shown at the upper left.  You start with a purple or near-UV LED, then down-convert into a distribution of red, yellow, green and blue colors using phosphors.  The second approach is the color mixing approach shown at the lower left.  You start with red, yellow, green and blue LEDs, then optically mix them to create white.  There are also hybrid approaches, but for the purpose of keeping things simple, I’m only going to talk about these two extremes.
The phosphor down-conversion approach is the one that is currently most popular, because it is so simple, and because for its semiconductor LED it depends only on purple or near-UV wavelengths at which LEDs are already reasonably efficient.  However, because of the Stokes-shift loss from the wavelength down-conversion, the electricity-to-light conversion efficiency of this approach is likely to be limited to 25% or so, which will only be slightly better than that of fluorescence, and probably not enough to make a significant reduction in energy consumption.  Plus, in this approach, the color temperature of the white light is fixed at the factory, and cannot be controlled at point of use.
The color-mixing approach is the one that is currently least popular, because it is more complex, and because it depends on semiconductor LEDs at all four wavelengths, at some of which efficient LEDs don’t yet exist.  However, this is probably the only approach that might enable power conversion efficiencies approaching 50%.  And it is the only approach that would enable the color temperature of the white light to be controlled at point of use.
So, in the long-term, the color-mixing approach holds the greatest promise for a significant reduction in energy consumption, and for enhancing the human visual experience. So, in the last part of this talk, I’d like to discuss some of the challenges associated with this approach, with a special focus on the heart of that approach – the semiconductor material.

## SSL Materials Candidates

**Notes:**
Let’s start by asking what inorganic semiconductor materials are the possible candidates for solid-state lighting.
This graph maps two key physical properties, bandgap and hardness, of all the binary octet semiconductor materials – the IV-IVs down here, the III-Vs slightly higher, the II-VIs up here, and the I-VIIs way up here.  These are arguably the two most important physical properties that determine whether a material is a possible candidate for solid-state lighting.  The bandgap of the material needs to be in the right range to emit red, yellow, green and blue light.  And the hardness of the material needs to be in the right range as well.  If it is too soft, then its degradation rate during operation is likely to be too fast; if it is too hard, then it is likely to be very difficult to process.  So the candidate materials are within this narrow band of color here, and between the two dashed white lines.
In a way, this map just quantifies what we already know – that there aren’t very many candidates.
The IV-IVs have bandgaps that are just too narrow, and, even worse, as indicated by the open circles, they are all indirect gap with very low luminescence efficiencies.
As the ionicity of bonding increases, bandgaps increase but also the materials get softer.  So the I-VIIs, which have very ionic bonds, have bandgaps that are too wide, and they are also all too soft.
The II-VIs have bonds that are not quite as ionic, so they have narrower bandgaps and are a little harder.  In fact, you’ll note that Zn-based materials, ZnO-ZnS-ZnSe-ZnTe, have bandgaps that are nearly perfectly matched to solid-state lighting.  However, they are all just a little too soft, and experience has shown that their degradation rates are just a little too high.  Still, they are close, and it isn’t inconceivable that there be some breakthrough in extending degradation rates in these materials.
The III-Vs have bonds that are even less ionic, so again they have even narrower bandgaps and are a little harder.  In fact, one might argue that the III-Vs are not quite ionic enough – if they were just a tad more ionic, their curve would be shifted upwards and to the left a little, and it would be “easy” materials like GaAs and InP that would be perfectly matched to solid-state lighting.  Still, there are three families of III-V materials with bandgaps that span the solid-state lighting range, and have reasonable hardnesses.
The first family is the GaN-GaP-GaAs family.  The disadvantages to this family are, first, that GaP is an indirect-gap semiconductor, as indicated by the open circle and, second, that GaP and GaAs are cubic, while GaN is hexagonal, so there is a crystallography mismatch that must be overcome.
The second family is the AlP-GaP-InP family.  The disadvantages to this family are, first, that GaP and AlP are indirect-gap semiconductors, as indicated by the open circles and, second, that they don’t quite span the solid-state lighting range of wavelengths.  As we know from experience, they are very good in the deep red, but aren’t very good if pushed into the light red, much less into the yellow, green and blue.
The third family is the AlN-GaN-InN family.  This family doesn’t have any obvious disadvantages, and, as we all know, is the family on which the most attention has been focused.
So what I’d like to do next is take a closer look at challenges associated with the GaN-InN family of materials.  That isn’t to say that the ZnO-ZnS-ZnSe-ZnTe family, or the AlP-GaP-InP family, or the GaN-GaP-GaAs family, of materials aren’t still possibilities.  Breakthroughs in these materials are possible.  But the GaN-InN family is the most likely possibility.

## GaN-InN Materials Challenges

**GaN-InN Materials Challenges**

**Notes:**
So let’s take a closer look at the GaN-InN family of materials.  Here I show, in the bottom figure, a plot of internal radiative quantum efficiencies of LEDs where the active light-emitting layers are fabricated from the GaN-InN family of materials. Pure GaN emits at roughly 370nm.  Adding 40% InN gets you to the red, and in between you get blue, green and yellow.
The data points are from Nichia and LumiLeds, and are not the most up-to-date numbers, particularly in the purple and UV.  But they serve to illustrate two key issues.
The first issue is that, at all wavelengths, the internal radiative efficiencies are a lot lower than unity.  If we’re targeting a 50% electricity-to-usable-light power conversion efficiency, then these internal radiative efficiencies must be very close to unity.  The reason is that there will be lots of other losses that will be almost impossible to reduce, especially related to light extraction from the chip, but also including power conditioning even before the electricity hits the chip.
The second issue is that, although the internal radiative efficiency does increase from the UV to the purple and to the blue, it decreases in the green, and is nearly zero in the yellow and red.  This is of course why the dominant current approach to white light creation is phosphor down-conversion of light from blue, purple or near-UV LEDs.
To see at one more level of detail what might be going on, just above I show a plot of radiative and non-radiative electron-hole recombination rates.  The numbers were estimated using data from Kyoto University, Nichia, Linkoping University, and UCSB.  The data in the two graphs aren’t from the same devices or structures, so they can’t be compared.  However, they do give a good idea of the trends.
Now, the internal radiative efficiency is the result of a dynamic competition between radiative and non-radiative recombination, so if radiative recombination were much faster than non-radiative recombination, internal radiative efficiency would be near unity.  Instead, radiative recombination is slower, rather than faster, than non-radiative recombination, throughout the wavelength spectrum.  So the key challenge is to reverse this ordering.  Now, you can either do that by increasing the radiative recombination rate, or by decreasing the non-radiative recombination rate.
In fact, radiative recombination in pure GaN, here at roughly 365nm, is relatively fast.  Radiative recombination coefficients generally increase with bandgap, so GaN, with the widest bandgap, has the highest radiative recombination coefficient of all the III-V compound semiconductor.  So, aside from going to stimulated emission devices, it’s not likely that this rate is going to be increased by much.  The problem is that non-radiative  recombination is even faster, probably because of dislocations and other defects associated with growth of GaN on the commonly-used but lattice-mismatched sapphire substrates.  So, a major materials challenge is to develop lattice-matched substrates so that low defect-density InGaN can be grown.  And, even better, to develop such substrates at the four different lattice constants needed for four different InGaN compositions – one for blue, one for green, one for yellow, and possibly one for orange!
Interestingly, you’ll notice that, even without lattice-matched substrates, non-radiative recombination is already decreasing as one adds In.  That’s commonly thought to be due to the chemical immiscibility of GaN and InN, and to the tendency for InGaN alloys to phase separate into InN-rich and GaN-rich regions.  The problem is that as this happens, local strain develops, because GaN and InN have such different lattice constants.  The strain causes large piezoelectric fields, which spatially separate the electrons and holes within the InN-rich regions, slowing radiative recombination.  Eventually, even though non-radiative recombination is also getting slower, radiative recombination is getting slower faster, and the internal radiative efficiency goes to zero.
So it’s not yet clear whether in the long run, phase separation will be viewed as good or bad.  If you’re stuck with lattice-mismatched substrates and a high density of dislocations and other defects, then you need phase separation to slow non-radiative recombination, and the challenge will be to somehow reduce the piezoelectric fields to speed up radiative recombination.  But if lattice-matched substrates can be developed, then you probably don’t want phase separation, and the challenge will be to somehow, through non-equilibrium growth techniques, overcome the natural tendency for such phase separation.

## Solid-State Lighting “Sub-Technologies”

**Solid-State Lighting “Sub-Technologies”**

- U.S. Patent Data compiled by Perspectives
- “Seeded” lexical VxInsight map by K Boyack and JY Tsao (Sandia)

**Notes:**
Finally, let me back away from materials just a bit, and take an expanded view of solid-state lighting.
Here I show a map of a reasonably comprehensive database of about 1,000 U.S. patents in the area of solid-state lighting, categorized into the sub-technologies that were identified in last year’s U.S. solid-state lighting roadmap.  The map was created using an iterative process of manual categorization and automated placement through lexical similarity metrics.  That basically means that patents that are close together on the map share a lot of common words, and patents that are far apart on the map don’t share very many common words.
There are three overall sub-technology areas.
Area 1, which has to do with semiconductor substrates, buffers and epitaxy, are the white dots down in the lower left.  It’s the most compact area, indicating that there is a lot of overlap between the sub-areas within it.
Area 2, which has to do with semiconductor devices and processing, are the green dots down in the lower right.  Processing and LED devices are pretty closely placed, indicating that there is a lot of overlap between these two sub-areas.  However, surprisingly, Directional Emitters is way down here, closer to epi processes than to LEDs.  This sort of makes sense, since Directional Emitters and Lasers are fairly epi intensive.  The most basic building blocks for lasers -- separate electrical and optical confinement, current blocking, distributed Bragg reflectors -- all make use of novel epi engineering of some form or another.  One might imagine, though, as lasers become more mature, that their center of gravity might move upwards towards processing.
Area 3, which has to do with phosphors and encapsulants, lamps, luminaires, and systems, are the blue dots along the top.  Phosphors and Encapsulants, together with Systems, are the two mountains at the upper left.  Their closeness to each other makes some sense, since it is the choice of phosphors that determines color quality, which in turn determines the classes of system applications one might penetrate.  Lamps are, not surprisingly, very closely coupled to LEDs – the way one packages an LED into a lamp, face-up, face-down, etc., is intimately connected to LED design.  Luminaires are connected to both LEDs and Lamps, which makes sense, since how one shapes and delivers light, through light pipes, etc., depends on both the LEDs and Lamps.  Luminaires are pretty far from Directional Emitters, but again, this is probably because of the immaturity of Directional Emitters.
Now, all of these sub-technology areas have major challenges.
In Area 2, light extraction is a really big challenge.  Device design for high-temperature high-current operation is another one.
In Area 3, phosphors, especially a red/orange phosphor that emits near 610nm, but with a sharp cut-off at longer wavelengths where the human eye is less sensitive, is a major challenge.  Understanding the psychology and physiology of the interaction of the human visual system with lights that are simultaneously controlled in space, time, intensity and color temperature is another challenge.
But the biggest challenges are probably in Area 1 -- materials.  Again, it may or not be possible, but if solid-state lighting is to fulfill its promise -- to significantly reduce energy consumption while improving the quality of the human visual experience – then we need to be able to produce all four colors (orange, yellow, green and blue) directly from semiconductors.  In principle, we ought to be able to do that; in practice, we’re nowhere near being able to do that.