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Experiments by . . .

Recombination in low-bandgap InGaAs Tim Gfroerer Davidson College, Davidson, NC with Mark Wanlass National Renewable Energy Lab, CO ~ Supported by Bechtel Bettis, Inc. and the American Chemical Society – Petroleum Research Fund ~. Experiments by. Colleen Gillespie (Davidson ’06). Pete Campbell

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Experiments by . . .

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  1. Recombination in low-bandgap InGaAsTim GfroererDavidson College, Davidson, NCwith Mark WanlassNational Renewable Energy Lab, CO~ Supported by Bechtel Bettis, Inc. and the American Chemical Society – Petroleum Research Fund ~

  2. Experiments by . . . Colleen Gillespie (Davidson ’06) Pete Campbell (Davidson ’03) Patten Priestley (Davidson ’03) and Malu Fairley (Spelman ’03)

  3. 1.0 0.8 0.6 0.4 0.2 0.0 Motivation: TPV converters T=1300 C Normalized Intensity 0.0 0.5 1.0 1.5 2.0 2.5 Energy (eV) Increasing the Indium concentration in the InGaAs lowers the bandgap and increases the fraction of blackbody radiation that is absorbed in the cell.

  4. Variable-Bandgap Lattice-Mismatched Stuctures

  5. Efficiency Measurements CW YAG laser 1 Watt @ 1064 nm Photodiode Variable Temp Cryostat Lowpass Filter Sample ND Filters : Laser Light : Luminescence light in = heat + light out radiative efficiency = light out / light in heat light in light out

  6. ENERGY Defect-Related Density of States Valence Band Conduction Band The distribution of defect levels within the bandgap can be represented by a density of states (DOS) function as shown above.

  7. Radiative Efficiency Measurements light heat Radiative efficiency measurements at 77K. The theoretical fits correspond to the defect-related DOS functions indicated in the inset graphs.

  8. - - - + + + Defect-Related Transition Probabilities P ~ 10-1 P ~ (0.5)10 ~ 10-3 P ~ (0.5)16 ~ 10-5 P ~ 10-5 P ~ 10-3 P ~ (0.5)4 ~ 10-1 The probability P of transitions involving phonon emission depends on the number of phonons required, which is determined by the position of the defect level in the gap.

  9. Temperature Dependence Temperature dependence of radiative efficiency vs. excitation, showing how the SRH and Auger mechanisms depend on Indium concentration.

  10. Auger Recombination Temperature and bandgap dependence of the Auger coefficient C. The CHSH (band-to-band) mechanism dominates Auger recombination in low-bandgap InGaAs.

  11. Sub-Bandgap Photoluminescence FTIR spectra showing a deep transition in the lattice-matched material that abates and then disappears with increasing [In].

  12. Four Conclusions • Deep defect levels → shallow near-bandedge states with increasing [In]. • The CHSH Auger mechanism is dominant in this alloy. • Sub-gap PL from deep (Ea > 0.2 eV) levels ↓and then disappears with increasing [In]. • Structure-less sub-gap cathodoluminescence supports assignment of this band to point defects. Three References • T.H. Gfroerer, L.P. Priestley ('03), F.E. Weindruch ('01), and M.W. Wanlass, APL 80, 4570 (2002). • T.H. Gfroerer, L.P. Priestley ('03), Malu Fairley (‘03), and M.W. Wanlass, JAP 94, 1738 (2003). • T.H. Gfroerer, C.E. Gillespie (‘05), J.P. Campbell (‘03), and M.W. Wanlass, JAP 98, 093708 (2005).

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