Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session H5: Facing the Challenge of the LED Droop |
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Sponsoring Units: FIAP Chair: Jim Speck, University of California, Santa Barbara Room: Portland Ballroom 256 |
Tuesday, March 16, 2010 8:00AM - 8:36AM |
H5.00001: Auger Recombination in Indium Gallium Nitride: Experimental Evidence Invited Speaker: Progress in InGaN-based light-emitting diode (LED) technology has resulted in white-light emitters with efficiencies far exceeding those of conventional light sources such as tungsten-filament-based incandescence and mercury-vapor based fluorescence. Indeed, by now efficacies exceeding 150 lumens per Watt for InGaN-based phosphor-converted white LEDs are claimed, which represent a 90{\%} energy savings compared to the conventional incandescent (i.e., ``light bulb'') solution. However, these high performance levels are obtained under conditions of very low forward current-density for the InGaN LED and do not represent true operating conditions (nor cost-effective utilization) for the device. In order to reduce the cost (and thus increase market penetration of) solid-state lighting, more lumens per unit of semiconductor area are required which in practice necessitates higher drive current densities. Unfortunately, at these higher driver current densities, the internal quantum efficiency of InGaN-based LEDs is observed to decrease significantly. In the fall of 2007, researchers at the Advanced Laboratories of Philips Lumileds were the first to propose Auger recombination as the root-cause mechanism in InGaN which was behind this ``efficiency droop'' [1]. They further proposed to circumvent the problem by employing InGaN-based active region designs that maintain low carrier density, and demonstrated an LED device design that reaches a maximum quantum efficiency above 200 A/cm2, compared to $\sim $1-10 A/cm$^2$ for typical multiple-quantum-well heterostructures [2]. In this talk we will review the experimental evidence for Auger recombination in InGaN, beginning with the early work from 2007 and then considering additional work from more recent efforts to better understand the details behind this loss mechanism. \\[4pt] [1] Y. C. Shen, G. O. M\"{u}ller, S. Watanabe, N. F. Gardner, A. Munkholm, and M. R. Krames, ``Auger recombination in InGaN measured by photoluminescence'', Appl. Phys. Lett. 91, 141101 (2007). \\[0pt] [2] N. F. Gardner, G. O. M\"{u}ller, Y. C. Shen, G. Chen, S. Watanabe, W. G\"{o}tz, and M. R. Krames, ``Blue-emitting InGaN--GaN double-heterostructure light-emitting diodes reaching maximum quantum efficiency above 200 A/cm$^2$'', Appl. Phys. Lett. 91, 243506 (2007). [Preview Abstract] |
Tuesday, March 16, 2010 8:36AM - 9:12AM |
H5.00002: Auger recombination and free-carrier absorption in nitrides from first principles Invited Speaker: Solid-state optoelectronic devices in the blue/green part of the visible spectrum, based on group-III-nitride materials and their alloys, have a wide array of applications as well as the potential to replace incandescent and fluorescent light bulbs for general illumination. Progress in nitride light emitters research, however, is hampered by the efficiency droop effect, a severe drop in quantum efficiency at high drive currents that particularly affects devices emitting at longer wavelengths. The efficiency droop has been the subject of extensive research and several mechanisms have been proposed as its origin. One such mechanism is the Auger recombination process, a non-radiative recombination mechanism induced by free carrier scattering via the Coulomb interaction. An additional loss mechanism that affects laser devices in particular is the reabsorption of the generated light by free carriers in the device. We used first-principles calculations to study the direct as well as the indirect Auger recombination and free-carrier absorption processes, mediated by electron-phonon and alloy scattering, and identify their importance in nitride light emitters. Since the various loss processes are hard to decouple experimentally, first-principles calculations are an indispensable tool to investigate the various loss mechanisms in isolation and determine their significance. [Preview Abstract] |
Tuesday, March 16, 2010 9:12AM - 9:48AM |
H5.00003: Carrier transport, polarization matching, and efficiency droop in GaN-based visible LEDs Invited Speaker: Transport of electrons and holes in GaInN/GaN LEDs is known to be largely asymmetric, due to differences in carrier mobility and the ionization energy for donors and acceptors. Sheet charges at heterointerfaces of devices grown in the c-direction, which arise due to mismatch in spontaneous and piezoelectric polarization, further hinder transport by increasing barriers for carrier injection into quantum wells. The effect of these sheet charges upon capture of carriers by quantum wells is analyzed with a quantum-mechanical calculation of the dwell time of electrons and holes over quantum wells. Wavefunctions and dwell times are calculated using the k.p quantum transmitting boundary method with the wurtzite 8-band Hamiltonian with Burt-Foreman operator ordering. The effect of quantum well width upon dwell time is also considered. It is shown that both reduction of sheet charges by polarization matching and the increase of well width result in substantially longer dwell times, and therefore higher probability of capture. Experimental results of LEDs with polarization-matched active regions are presented. Such LEDs are shown to have improved efficiency throughout the entire range of forward currents. This indicates that a reduction in carrier density is not solely responsible for the improved efficiency, since reduced carrier density would also lead to a higher Shockley-Reed-Hall rate at low currents. The implications of this result are discussed in detail. Further, experimental results of LEDs with varying doping in the active region are presented. It is shown that a reduction in doping -- which has the effect of slowing electron transport -- results in an increase in efficiency at large forward currents. This result is explained in detail using modeling results. [Preview Abstract] |
Tuesday, March 16, 2010 9:48AM - 10:24AM |
H5.00004: The contribution of carrier localisation to efficiency droop in GaN LEDs Invited Speaker: One of the most significant problems preventing the widespread adoption of Solid State Lighting is the reduction in efficiency at high drive currents: so called ``efficiency droop''. A number of mechanisms have been proposed for explaining this phenomenon for example Auger recombination. However, the reason InGaN LEDs work, even though the dislocation density is high, is widely believed to due to carrier localisation. We propose that modification of carrier localisation may also control the droop. In this paper we discuss three localisation mechanisms which may be relevant to efficiency droop. In an InGaN/GaN QW the active region is strained and is also a random alloy. We have shown theoretically that random alloy fluctuations localise the holes on a 1-2 nm length scale (localisation mechanism 1). In addition, monolayer and bilayer steps on the upper InGaN/GaN QW interface localise the electrons on a 5-10 nm lateral length scale (mechanism 2). In addition, some InGaN QWs (depending on the growth conditions) exhibit a QW network structure with gross thickness fluctuations. These localise electrons and holes at room temperature on a typically 100 nm lateral length scale (mechanism 3). There are two related reasons carrier localisation may contribute to efficiency droop. First, localised carriers are in local potential minima. As the current density increases, carriers may fill these dot like regions and became delocalised, enabling them to diffuse to dislocations, reducing the light emission and resulting in efficiency droop. Second, in polar and semi-polar materials, as the current density increases, the electric field across the QW decreases, which reduces the size of the confining local potential wells, allowing the carriers to become delocalised. Experimentally we have found that the efficiency droop is significantly different in QWs with localisation mechanisms 1 plus 2 operating relative to those in which all three mechanisms operate. [Preview Abstract] |
Tuesday, March 16, 2010 10:24AM - 11:00AM |
H5.00005: On the importance of radiative and Auger losses in GaN-based quantum wells Invited Speaker: Non-radiative carrier losses due to Auger recombinations have been suggested as a possible reason for the efficiency droop in GaN-based laser diodes [1]. This hypothesis is based on the observation that measured efficiencies can be reproduced using the classical power law for the density dependence of the loss current, $J=AN+BN^2+CN^3$, with an Auger constant $C\approx 10^{-31}-10^{-30}\,cm^6/s$. Auger losses can only be deduced indirectly from the overall loss if all other loss processes are know. Thus, it is not clear whether they are indeed responsible for the droop or whether it is an alternative loss process with a similar density dependence, like, maybe, density activated defect recombination. To investigate this we use fully microscopic many-body models to calculate absorption/gain as well as carrier losses due to radiative and Auger recombinations. These models have been shown to give excellent quantitative agreement with the experiment for materials ranging from the mid-IR to less than one micron [2]. These models have shown that the classically assumed density and temperature dependencies for the loss processes are generally far from reality, especially at densities relevant for laser operation [2]. In particular, in this regime the density dependence of Auger losses usually becomes less than cubic. This makes the use of the simple power laws rather questionable. Using the microscopic analysis we find for a typical InGaN/GaN system that carrier losses are dominated by radiative recombinations for all relevant densities [3]. At densities at which the onset of droop has been observed the Auger losses contribute only about $0.1\%$. Fits to the microscopically calculated losses yield for radiative losses $B=3.5\times 10^{-12}cm^2/s$ in agreement with traditional estimates. However, for Auger losses one finds $C=3.5\times 10^{-34}cm^6/s$ which is far too small to reproduce the experiment. Thus, we do not think that the direct Auger processes investigated here are responsible for the droop. Preliminary investigations lead us to believe that in general also phonon-assisted Auger processes or Auger processes including higher bulk electron bands are not strong enough to explain the droop.\\[4pt] [1] Y.C. Shen, et al. Appl. Phys. Lett. {\bf 91}, 141101 (2007).\\[0pt] [2] J. Hader, et al., Appl. Phys. Lett. {\bf 94}, 061106 (2009). J.V. Moloney, et al., Laser \& Photon. Rev. {\bf 1}, 24 (2007). J. Hader, et al., IEEE J. Quantum Electron. {\bf 44}, 185 (2008).\\[0pt] [3] J. Hader, et al., Appl. Phys. Lett. {\bf 92}, 261103 (2008). [Preview Abstract] |
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