Light Emitting Diodes Improving Efficiency of Gallium-Nitride LEDs Callan McCormick Physics Department, Colorado College, CO, USA OLUTION EXAGONAL TO UBIC HASE Email: [email protected] S : H - -C P TRANSITION OF GAN Gallium Nitride is usually grown on sapphire (Al2O3) in its thermodynamically stable, INTRODUCTION GAN LEDS hexagonal (wurtzite) crystal phase with a bandgap energy of 3.4 eV at room Light emitting diodes are forward biased p-n junction devices Gallium Nitride is usually grown on sapphire (Al2O3) in its temperature. When hexagonal GaN (h-GaN) is deposited onto sapphire, the top plane that illuminate as a result of electron-hole recombination thermodynamically stable, hexagonal (wurtzite) crystal phase with tends to be highly polarized (along ‹0001› direction). For high power LEDs, polarization within semiconducting materials. Electrons from the n-doped a bandgap energy of 3.4 eV at room temperature. When hexagonal can increase the rate of Auger recombination, which is believed to contribute to the semi-conducting material will drop down from the GaN (h-GaN) is deposited onto sapphire, the top plane tends to be efficiency reduction in fully fabricated InGaN/GaN LED chips. conduction band and recombine with holes from the valence highly polarized (along ‹0001› direction). For high power LEDs, However, GaN can also be grown in its cubic phase (zincblende), which has no band of the p-doped semi-conducting material in the active polarization can increase the rate of Auger recombination, which is spontaneous polarization (i.e. lower Auger recombination rates within the QWs). Cubic region of the diode. In direct bandgap semi-conductors, the believed to contribute to the efficiency reduction in fully fabricated phase GaN (c-GaN) also has a smaller bandgap energy of 3.2 eV at room temperature. electrons to drop down from the conduction band by InGaN/GaN LED chips. Therefore, lower concentrations of Indium within the active region of the LED are releasing energy in the form of a photon (see figure below). However, GaN can also be grown in its cubic phase (zincblende), needed to effectively lower the frequency of emitted photons. Yet, c-GaN typically The energy of the emitted photon is proportional to the which has no spontaneous polarization (i.e. lower Auger requires low temperature deposition and is thermodynamically less stable than h-GaN bandgap energy of the semi-conducting material used in this recombination rates within the QWs). Cubic phase GaN (c-GaN) at room temperature. Thus, new deposition techniques are being explored for cubic p-n junction device by: also has a smaller bandgap energy of 3.2 eV at room temperature. GaN. ℎ, Therefore, lower concentrations of Indium within the active region ! ≈ ! = "#$ $&'(') - of the LED are needed to effectively lower the frequency of where !"#$ is the bandgap energy; !$&'(') is the energy of the emitted photons. Yet, c-GaN typically requires low temperature emitted photon; ℎ is Planck’s constant; , is the speed of light; and deposition and is thermodynamically less stable than h-GaN at - is the wavelength of the emitted photon. room temperature. Thus, new deposition techniques are being explored for cubic GaN. 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 EFFICIENCY DROOP MODELING gap in Gallium Nitride (3.4 eV at room temp), which allows To effectively observe the efficiency droop, an Integrating Sphere was photons to be emitted at wavelengths in the UV spectrum used to detect the output power of a blue GaN LED. The LED was range. To achieve the lower frequency blue light emission, inserted into the sample aperture and current was swept from 0.1mA the active region between the p-doped GaN and the n-doped to 111mA. The light emitted from the blue LED was directed through GaN is grown with different concentrations of Indium-Gallium an optical fiber to a spectrometer and the optical output power was Nitride (InGaN) to create quantum wells (QWs) with smaller measured and collected for each value of applied current [Fig. 1]. band gap energies. By varying the concentration of QWs within the active region of these GaN-based LEDs, the The optical output power efficiency plotted in Figure 2 is consistent wavelength of the emitted photons can be tuned to lower with previous findings regarding efficiency droop in InGaN/GaN blue Figure 1. LIV plot of Blue LED with applied frequencies. Unfortunately, the efficiency of these quantum LEDs. With a maximum efficiency of about 8.6%, there is a clear power current ranging from 0.1mA to 111mA. wells begins to decrease significantly when higher input loss in converting electrical power into optical power at as low as power is applied. Although GaN-based LEDs are widely used 10mW of input power. Furthermore, to achieve higher optical power ACKNOWLEDGMENTS in numerous every-day applications such as TV screens, car outputs using GaN-based LEDs, the amount of input power needed is This material is based upon work supported by the National Science Foundation Faculty Early Career headlights, indoor lighting, and outdoor lighting, the physical even less efficient. As a result, it is currently more efficient to use Development (CAREER) Program under award number NSF ECCS 16-52871 CAR REU. multiple low-powered LEDs than to use a single high-powered LED. This work was carried out in the Micro and Nanotechnology Laboratory and Frederick Seitz Materials mechanism behind this efficiency droop in high powered Research Laboratory Central Research Facilities, University of Illinois at Urbana-Champaign, IL, USA. The However, the maximum efficiency of InGaN/GaN LEDs is still quite low, LEDs is not fully understood. author acknowledges support from Dr. Can Bayram (P.I.), Richard Liu, Hsuan-Ping Lee, and Lavendra so using multiple low-power LEDs is not an effective solution to the Mandyam (Research Engineer). [i]Class for Physics of the Royal Swedish Academy of Sciences, issue. Therefore, to increase the efficiency of LED-based optics, the Sci. Backgr. Nobel Prize Phys. 2014 50005, 1 (2014). best approach is either to increase the optical intensity of low-power LEDs* or to improve the high-power efficiency droop issue**. REFERENCES *See “Mg-Doped GaN” section in attached paperà improving p-doped GaN Papers surface uniformity to increase optical power output. Class for Physics of the Royal Swedish Academy of Sciences, Sci. Backgr. Nobel Prize Phys. 2014 50005, 1 (2014). **See “Hexagonal-to-Cubic Phase Transition of GaN” section in attached paperà Figure 2. Efficiency plot of basic GaN-based R. 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