The Solid-State Lighting Science (SSLS) Energy Frontier Research Center (EFRC) works to advance the scientific foundation that underlies current and potential-future SSL technology, and to ultimately enable significant advances in the efficiency with which SSL is produced and used. We do this through the seven scientific research challenges:
Our second two scientific research challenges focus on better understanding emerging light-emitting materials architectures.
- InGaN Nanowires: The first light-emitting materials architecture is InGaN-based materials structured into nanowires, with a long-term goal of creating electroluminescent structures. Because of their narrow diameters, nanowires have the potential to be fabricated without line defects and can accommodate more strain and therefore a wider range of compositions, including those that might enable green, yellow, and perhaps even red light emission. In fact, as George Wang, or principal investigator for this research challenge, will tell you more about, we have recently begun working with a new so-called top-down fabrication method that promises much higher material quality than previously possible, and we have recently observed yellow-red electroluminescence from such materials.
- Quantum Dots and Phosphors: The second light-emitting materials architecture is quantum dots and phosphors for wavelength down conversion, particularly for narrow-linewidth red emision in the 610-615 nm wavelength range. That is the perfect SSL red. In the long run, however, we believe quantum dots have greater potential for high performance along with composition and wavelength tunability, and so they will be the focus of our attention going forward. In particular, we are focusing on the principal challenge associated with quantum dots for SSL: that their quantum yields decrease, sometimes permanently, when exposed to high temperatures and photon fluxes. We believe these instabilities are associated with strain and strain-induced defects. Going forward, we hope to unravel the mechanisms that underlie this yield-decrease phenomena.
Our last four scientific research challenges are focused on developing a foundational understanding or and exploring light-emission phenomena. To put these four in context, in the graphic above we present the equation that parses out light-emitting device’s power-conversion efficiency.
- A Joule efficiency is associated with resistive losses as carriers are transported from electrical contacts to the device’s active region
- An injection efficiency is associated with whether, once the carriers get to the active region, they (i) overshoot it, (ii) thermalize in it, or (iii) thermalize in it and then for whatever reason escape.
- An eternal quantum efficiency is associated with the fraction of carriers, N, that recombine radiatively rather than nonradiatively.
- The classical radiative process is just spontaneous emission (the BN2 term), as an electron recombines with a hole.
- The classical nonradiative processes are defect-mediated recombination (the AN term), in which electrons and holes are captured by deep levels, and Auger recombination (the CN3 term), in which the electron-hole recombination energy is taken up by another electron or hole rather than a photon.
d. Finally, there is extraction efficiency- the fraction of photons that are created in the device that escape.
All of these component efficiencies are important. However, terrific technological progress has been made recently with extraction efficiency, so this isn’t as big an issue anymore. And injection currents aren’t high enough yet for Joule efficiency to make it to our radar screen. So, thus far, we have chosen to focus on: injection efficiency and internal quantum efficiency.
3. Competing radiative and nonradiative processes: Mary Crawfordhas focused our first light-emission-related scientific research challenge: the competing radiative and nonradiative processes that determine injection efficiency and internal quantum efficiency. We’ve paid particular attention thus far to carrier overshoot and escape, and to spontaneous emission. In fact, one of the things we’ve demonstrated is that this spontaneous-emission B coefficient is quite complex- it is not, as is normally assumed, carrier-density-independent, but instead decreases with carrier density, and therefore could play a role in efficiency droop.
4. Defect-carrier interactions: Among nonradiative recombination processes, defect-mediated recombination, and the physics of the point defects that cause it, is one that we think is particularly important, because it is present at all InGaN compositions and, hence, is a key issue for the green-yellow gap. So, Andy Armstrong has taken up the second light-emission-phenomenon-related scientific research challenge, which is focused on defect-carrier interactions. One of the principles we recently demonstrated is the ability to perform depth-profiling of point defect densities and properties in real InGaN devices, not just in special-purpose heterostructures. This opens up exciting new opportunities for correlating device performance with underlying defect properties.
5. Enhanced spontaneous emission: It is one thing to design devices to accommodate the spontaneous emission rates that are determined by the materials themselves in traditional 2D planar quantum-well architectures. It is another thing to try to enhance these spontaneous emission rates by modifying the environment around the light-emitting material. One way to achieve this enhancement might be through surface plasmonics, where electron-hole-pair excitations couple to surface plasmons, which then couple to free-space photons. Another way might through photonic crystals. So, Igal Crener is working the third light-emission-phenomenon-related scientific research challenge, which is focused on novel ways to enhance spontaneous emission. In fact, we have recently demonstrated a tour-de-force fully 3D photonic crystal fabricated in GaN, an important step towards achieving such enhancements.
6. Stimulated emission: Finally, it is also one thing to live with spontaneous emission at all. Why not go beyond spontaneous emission, through the addition of cavities and coherent processes, such as lasing, to the mix? So, Art Fischer working to surmount the fourth light-emission phenomenon-related scientific research challenge, which is focused on going beyond spontaneous emission. In particular, we are looking at lasers with the possibility of the ultra-low-thresholds that manufacturers like for ultra-high-efficiency, such as nanowire or polarition lasers.