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Study of Radiative and Heat-Generating Recombination in GaAs Ryan Crum and Tim Gfroerer, Davidson College, Davidson, NC Mark Wanlass, National Renewable Energy Lab, Golden, CO Abstract Light- and heat-generating recombination decrease the efficiency of GaAs-based solar cells. In this investigation, we couple results from recent radiative studies with new measurements of heat generation. This coupling of results is important because heat-generating recombination dominates at low excitation while radiative recombination (luminescence) dominates at higher laser intensities. In past studies of diffusion in semiconductors, images of luminescence were used to examine the diffusion of carriers away from the excitation position. But with complementary heating profiles, we are able to ascertain the underlying carrier density more accurately. Since heat-generating recombination scales linearly with the density, while radiative recombination is proportional to the density squared, we show that the heating profile is comparable to the square root of the luminescence profile. We have also studied the diffusion of heat (i.e. thermal diffusion) in GaAs, but we will need to account for heat loss in order to compare our results with theoretical predictions. Motivation Light- and heat-generating recombination decrease the efficiency of GaAs-based solar cells because the release of light/heat reduces the energy available for generating electricity. Heat-generating recombination tends to occur in the presence of defects (and their associated energy levels) while light-generating recombination is more likely when defect levels are not present. Diffusion of Heat in GaAs Experimental Setup Thermal Diffusivity of GaAs Conclusions We found that the diffusion of carriers is best represented by the temperature difference rather than the luminescence. When analyzing luminescence, the square root of the signal should be used to obtain an accurate measurement of the carrier density. Our measurement of the thermal diffusivity constant in GaAs differs from the accepted value by an order of magnitude. We believe that the error in our measurement may be due to local heat loss. * Salazar, Agustin, Arantza Mendioroz, Raquel Fuente, and Ricardo Celorrio, J. Appl. Phys. 107, 043508 (2010). Acknowledgements Conduction Band Valence Band ENERGY Defect Level HEAT + - Conduction Band Valence Band LIGHT + - Heat-Generating RecombinationLight-Generating Recombination 100 μm When we analyze a sequence of thermal images where the laser alternates between on and off in consecutive frames, we observe a relatively rapid diffusion of heat radially outward over time. Hence, we study the frame coincident with the first laser pulse to obtain the most accurate representation of the position-dependent heat generation. When we study the evolution of the temperature profile following a single pulse (rather than a sequence of pulses), local heat loss appears to dominate over thermal diffusion. Analyzing the Outward Diffusion of Carriers In past studies, images of luminescence (radiative recombination) have been used to monitor the diffusion of electrons and holes away from the excitation position. In our study, we measure the luminescence profile along with the temperature gradient due to heat generation. We obtain temperature profiles that are broader than the profiles of luminescence. This observation is consistent with how these mechanisms depend on carrier density: heat-generating recombination should be directly proportional to the density of carriers while radiative-recombination is proportional to the density of carriers squared. Since the carrier density decreases with distance from the excitation position, radiative recombination dominates at short distances while heat-generating recombination becomes more important with increasing distance. We test this hypothesis by plotting the square root of the luminescence, which matches the temperature profile surprisingly well. The steady-state thermal profile for a laser that is chopped at frequency f can be used to determine the thermal diffusivity of the material. The natural logarithm of the temperature difference plotted verses the radial distance should be linear with a slope equal to –(π*f/D) -1/2. * We obtain an experimental value of 0.038 cm 2 /sec for the thermal diffusivity (D), while the accepted value for D in GaAs is 0.3 cm 2 /sec. We suspect that the error may be due to heat loss (as shown below). The 18mm focal length aspheric lens focuses the 0.34W (only 0.17W is absorbed) Gaussian laser beam to a 4 m FWHM spot on the sample. In order to optimize the speed of the thermal imaging, a shutter was used to synchronize the laser excitation with individual frames from the FLIR T300 thermal camera (30 Hz). We thank Jeff Carapella for growing the test structures and we acknowledge the Donors of the American Chemical Society – Petroleum Research Fund for supporting this work. Single Pulse Evolution

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