EFFICIENCY DROOP MODELINGTo effectively observe the efficiency droop, an Integrating Sphere was used to detect the output power of a blue GaN LED. The LED was inserted into the sample aperture and current was swept from 0.1mA to 111mA. The light emitted from the blue LED was directed through an optical fiber to a spectrometer and the optical output power was measured and collected for each value of applied current [Fig. 1].

The optical output power efficiency plotted in Figure 2 is consistent with previous findings regarding efficiency droop in InGaN/GaN blue LEDs. With a maximum efficiency of about 8.6%, there is a clear power loss in converting electrical power into optical power at as low as 10mW of input power. Furthermore, to achieve higher optical power outputs using GaN-based LEDs, the amount of input power needed is even less efficient. As a result, it is currently more efficient to use multiple low-powered LEDs than to use a single high-powered LED. However, the maximum efficiency of InGaN/GaN LEDs is still quite low, so using multiple low-power LEDs is not an effective solution to the issue. Therefore, to increase the efficiency of LED-based optics, the best approach is either to increase the optical intensity of low-power LEDs* or to improve the high-power efficiency droop issue**.

*See “Mg-Doped GaN” section in attached paperà improving p-doped GaN surface uniformity to increase optical power output.

**See “Hexagonal-to-Cubic Phase Transition of GaN” section in attached paperàNew method for effectively growing c-GaN in order to eliminate spontaneous polarization and increase photon emission efficiency for high-powered LEDs.

Light Emitting DiodesImproving Efficiency of Gallium-Nitride LEDs

Callan McCormickPhysics Department,

Colorado College, CO, USAEmail: c_mccormick@coloradocollege.edu

INTRODUCTIONLight emitting diodes are forward biased p-n junction devices that illuminate as a result of electron-hole recombination within semiconducting materials. Electrons from the n-doped semi-conducting material will drop down from the conduction band and recombine with holes from the valence band of the p-doped semi-conducting material in the active region of the diode. In direct bandgap semi-conductors, the electrons to drop down from the conduction band by releasing energy in the form of a photon (see figure below). The energy of the emitted photon is proportional to the bandgap energy of the semi-conducting material used in this p-n junction device by:

!"#$ ≈ !$&'(') =ℎ,-

where !"#$ is the bandgap energy; !$&'(') is the energy of the emitted photon; ℎ is Planck’s constant; , is the speed of light; and - is the wavelength of the emitted photon.

In 2014, the Physics Nobel Prize was awarded to Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura “for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources" [i]. This new form of LED takes advantage of the large energy band gap in Gallium Nitride (3.4 eV at room temp), which allows photons to be emitted at wavelengths in the UV spectrum range. To achieve the lower frequency blue light emission, the active region between the p-doped GaN and the n-doped GaN is grown with different concentrations of Indium-Gallium Nitride (InGaN) to create quantum wells (QWs) with smaller band gap energies. By varying the concentration of QWs within the active region of these GaN-based LEDs, the wavelength of the emitted photons can be tuned to lower frequencies. Unfortunately, the efficiency of these quantum wells begins to decrease significantly when higher input power is applied. Although GaN-based LEDs are widely used in numerous every-day applications such as TV screens, car headlights, indoor lighting, and outdoor lighting, the physical mechanism behind this efficiency droop in high powered LEDs is not fully understood.

[i]Class for Physics of the Royal Swedish Academy of Sciences, Sci. Backgr. Nobel Prize Phys. 2014 50005, 1 (2014).

ACKNOWLEDGMENTSThis material is based upon work supported by the National Science Foundation Faculty Early Career

Development (CAREER) Program under award number NSF ECCS 16-52871 CAR REU.This work was carried out in the Micro and Nanotechnology Laboratory and Frederick Seitz Materials Research Laboratory Central Research Facilities, University of Illinois at Urbana-Champaign, IL, USA. The author acknowledges support from Dr. Can Bayram (P.I.), Richard Liu, Hsuan-Ping Lee, and Lavendra Mandyam (Research Engineer).

REFERENCESPapersClass for Physics of the Royal Swedish Academy of Sciences, Sci. Backgr. Nobel Prize Phys. 2014 50005, 1 (2014).R. Liu and C. Bayram, J. Appl. Phys. 120, 25106 (2016).C. Bayram, J.A. Ott, K.T. Shiu, C.W. Cheng, Y. Zhu, J. Kim, M. Razeghi, and D.K. Sadana, Adv. Funct. Mater. 24, 4492 (2014).A. Hospodkova, J. Oswald, M. Zikova, J. Pangrac, K. Kuldova, K. Blazek, G. Ledoux, C. Dujardin, and M. Nikl, J. Appl. Phys. 121, 214505 (2017).J. Iveland, L. Martinelli, J. Peretti, J.S. Speck, and C. Weisbuch, Phys. Rev. Lett. 110, 1 (2013).H. Pinto, R. Jones, J.P. Goss, and P.R. Briddon, J. Phys. Condens. Matter 21, 402001 (2009).J.-D. Hwang, Z.-Y. Lai, C.-Y. Wu, and S.-J. Chang, Jpn. J. Appl. Phys. 44, 1726 (2005).K. Sato, M. Shikida, Y. Matsushima, T. Yamashiro, K. Asaumi, Y. Iriye, and M. Yamamoto, Sensors Actuators A Phys. 64, 87 (1998).

BooksA. H. Edwards, The Physics and Chemistry of SiO2 and the SiO2 Interface, 2nd ed. (Plenum Press, New York, 1988).M.A. Omar, Elementary Solid State Physics: Principles and Applications (Addison-Wesley Publishing, 1975).

GAN LEDSGallium Nitride is usually grown on sapphire (Al2O3) in its thermodynamically stable, hexagonal (wurtzite) crystal phase with a bandgap energy of 3.4 eV at room temperature. When hexagonal GaN (h-GaN) is deposited onto sapphire, the top plane tends to be highly polarized (along ‹0001› direction). For high power LEDs, polarization can increase the rate of Auger recombination, which is believed to contribute to the efficiency reduction in fully fabricated InGaN/GaN LED chips.However, GaN can also be grown in its cubic phase (zincblende), which has no spontaneous polarization (i.e. lower Auger recombination rates within the QWs). Cubic phase GaN (c-GaN) also has a smaller bandgap energy of 3.2 eV at room temperature. Therefore, lower concentrations of Indium within the active region of the LED are needed to effectively lower the frequency of emitted photons. Yet, c-GaN typically requires low temperature deposition and is thermodynamically less stable than h-GaN at room temperature. Thus, new deposition techniques are being explored for cubic GaN.

Figure 1. LIV plot of Blue LED with applied current ranging from 0.1mA to 111mA.

Figure 2. Efficiency plot of basic GaN-based blue LED. The optical output efficiency reaches a maximum of 8.6% at about 10mW of electrical input power.

SOLUTION: HEXAGONAL-TO-CUBIC PHASETRANSITION OF GAN

Gallium Nitride is usually grown on sapphire (Al2O3) in its thermodynamically stable, hexagonal (wurtzite) crystal phase with a bandgap energy of 3.4 eV at room temperature. When hexagonal GaN (h-GaN) is deposited onto sapphire, the top plane tends to be highly polarized (along ‹0001› direction). For high power LEDs, polarization can increase the rate of Auger recombination, which is believed to contribute to the efficiency reduction in fully fabricated InGaN/GaN LED chips.However, GaN can also be grown in its cubic phase (zincblende), which has no spontaneous polarization (i.e. lower Auger recombination rates within the QWs). Cubic phase GaN (c-GaN) also has a smaller bandgap energy of 3.2 eV at room temperature. Therefore, lower concentrations of Indium within the active region of the LED are needed to effectively lower the frequency of emitted photons. Yet, c-GaN typically requires low temperature deposition and is thermodynamically less stable than h-GaN at room temperature. Thus, new deposition techniques are being explored for cubic GaN.