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Illumination & Displays

Improving the high-current efficiency of LEDs

Understanding the origin of efficiency losses is key to developing the ultimate solid-state light source.
16 April 2009, SPIE Newsroom. DOI: 10.1117/2.1200904.1476

Indium gallium nitride (InGaN) has great potential in LED-based applications becasue of its ability to emit in the short-wavelength region, from near-UV to green. The emitting region is deposited as a quasi-2D quantum-well (QW) heterostructure composed of InGaN and gallium nitride. The past decade has seen tremendous progress in both epitaxial (or crystaline-layer) growth and InGaN LED design.

We recently demonstrated a wall-plug efficiency (i.e., the conversion of electrical to optical power as measured from the line source to the resulting emission) of about 60% for a blue ThinGaN® LED.1 The chip-level light-extraction efficiency of such devices has reached values in excess of 80%. However, the internal quantum efficiency for light generation by electron-hole carrier recombination remains lower. Even worse, the internal efficiency usually peaks around current densities considerably below the operating current and decreases monotonously towards higher currents, a phenomenon frequently called 'droop.' Understanding and reducing droop is crucial to reach the ultimate efficiency of InGaN-based LEDs. Various mechanisms that may cause this effect have been suggested, including carrier escape,2 losses due to dislocations,3 and the Auger effect.4,5

In our work, we validate that the efficiency drop at high current is QW internal. We compare temperature- and excitation-power-density-dependent resonant photoluminescence (PL) to electroluminescence (EL) using a green-emitting InGaN single-QW (SQW) LED structure. We measure PL and EL on the same ThinGaN chip. For PL excitation, a 405nm laser optically excites electron-hole pairs well below the band-gap energy of the GaN barrier.6 This enables us to reliably isolate QW-internal losses from losses due to parasitic carrier recombination outside the QW. Our experimental data strongly supports QW-internal loss.

All significant trends of the EL efficiency are followed perfectly by the resonant-PL efficiency (see Figure 1).

Figure 1. Internal efficiency of a green-emitting single-quantum-well (SQW) LED measured by electroluminescence (EL, empty boxes) compared to that measured in a resonant-photoluminescence (PL) experiment (solid boxes). Both experiments were performed at 300 (black) and 4K (red). Carrier generation and recombination in EL and PL are shown schematically in the conduction- and valence-band (CB/VB) diagrams in the insets. a.u.: Arbitrary units.

We can reproduce the dependence of the internal efficiency on current density, J, using a simple rate-equation model of the form J ~ A × n + B × n2 + C × n3, where n is the QW carrier density and A, B, and C are coefficients for nonradiative, radiative, and (nonradiative) Auger-like recombination, respectively. This model can describe the emission characteristics of green-emitting InGaN-based LED over a wide current-density range (see Figure 2). We thus identify a high-density QW-internal Auger-like process as the culprit for the high-current losses observed,7 with C=3.5×10−31cm6/s.

Figure 2. Emission power versus current density for a green-emitting multi-QW (MQW) LED. Different slopes can be attributed to different terms dominating the recombination-rate equation (e.g., solid blue, green, and orange lines). The corresponding recombination processes are shown in the band-diagram inset using the same colors.

Consequently, a decrease in InGaN active-layer carrier density is central to improving the high-current efficiency of InGaN-based light emitters. One established solution is the use of thick InGaN QWs.8–10 However, because of rapidly decreasing material quality of thick indium-rich InGaN films, no improvement over state-of-the-art multi-QW (MQW) LEDs could be shown. Alternatively, the active-layer carrier density can be reduced by enabling MQW operation. The applicability of this concept has been shown for devices over the full spectral range.11 We examined emission power versus current density for LEDs with different active-layer designs (see Figure 3). Consistent with simulations,6, 7 both thick QW and MQW exhibit reduced high-current saturation of emission power.

Figure 3. Comparison of simulated (dashed lines) to measured (solid lines) high-current emission-power saturation behavior of InGaN LED structures with different designs of the active layer: 2nm InGaN SQW (black), 10nm InGaN SQW (red), and 8-fold InGaN/GaN MQW (green). Both simulated and measured structures emit around 400nm. Conduction- and valence-band edges of the different active-layer designs are illustrated schematically in the insets.

We thus conclude that regardless of the concept employed, a decrease in active-layer carrier density is central to improving the high-current efficiency of InGaN-based light emitters. Band-to-band Auger recombination as the cause of these high-current losses is unlikely, as detailed many-particle calculations suggest.5 Phonon- and defect-assisted Auger recombination are possible. Direct experimental evidence, as well as further theoretical calculations, will need to clarify this issue. We are now focusing our efforts on optimizing MQW operation in nitride LEDs. This is very promising since it reduces the high-current saturation of the efficiency for all wavelengths from UV to green. It will bring us even closer to producing the ultimate solid-state light source.

Ansgar Laubsch, Matthias Sabathil, Martin Strassburg, Werner Bergbauer, Matthias Peter, Hans Lugauer, Norbert Linder, Berthold Hahn, Klaus Streubel
OSRAM Opto-Semiconductors GmbH
Regensburg, Germany