How does diffusion affect radiative efficiency measurements?

1. Laser Excitation Area Luminescence Area Absorption Event Recombination Event Electrons Holes How does diffusion affect radiative efficiency measurements? Caroline Vaughan and Tim Gfroerer, Davidson College, Davidson, NC Mark Wanlass, National Renewable Energy Lab, Golden, CO Motivation Focusing Analysis Abstract 2m Defect-related Recombination Radiative Recombination Conduction Band Conduction Band Defect-related recombination can lower the efficiency of many semiconductor devices. We measure the radiative efficiency (the ratio of emitted to incident light) as a function of excitation laser power to investigate defect-related recombination in GaAs. Images of the emitted light reveal isolated dark regions where the radiative efficiency is reduced. When the laser is focused onto one of these defective regions, the dependence of the radiative efficiency on excitation power is dramatically different. Using these results, we model the distribution of defect-related energy levels near and far from the isolated defect. But our defect-related recombination model is incomplete because it does not account for diffusion. Our radiative efficiency measurements depend strongly on laser focusing, suggesting that diffusion is important. We seek to understand how diffusion modifies our experimental results. - - Defect Level ENERGY HEAT HEAT LIGHT + + Valence Band Valence Band Non-radiative recombination occurs when an electron or hole is trapped in a defect state and releases heat instead of light during recombination. Experimental Setup Defect Analysis Even though the radiative efficiency should only depend on laser power per area, we find a large shift in the efficiency curve when changing the spot size of the laser. We need to understand this effect in order to interpret our focused efficiency measurements. Diffusion Using the efficiency curves, we model the distribution of defect-related energy levels near and far from the isolated defect. Near the defect, we find a much larger density of states function over τ, where τ equals the time it takes for the energy level to trap an electron or hole. Our model yields the lowest error when the defects cause electron traps – electronic states near the conduction band edge Ec. Raditative Efficiency At high excitation, the diffusion length is small, so the luminescence area is comparable to the excitation area. At lower excitations the diffusion length is longer, so the effective recombination area is greater (see below). Conclusions High Excitation Low Excitation 100 μm • When the laser is focused on a defect, we observe a dramatic difference in the way that the radiative efficiency depends on laser power. • Analysis of the radiative efficiency suggests that the GaAs has many trapping energy states near the conduction band and the increased DOS/t (density of these states over the trapping time) near the defect allows for more non-radiative recombination here. • We observe a puzzling dependence of the radiative efficiency on laser spot size and hypothesize that it may be due to the diffusion of carriers. Diffusion could have a large effect for the small laser spot, but would have a negligible effect when the spot size is much bigger than the diffusion length. Efficiency curves are measurements of the radiative efficiency (emitted / absorbed light) as a function of laser power per area. Acknowledgments We thank Jeff Carapella for growing the test structures and the Donors of the American Chemical Society – Petroleum Research Fund for supporting this work.