1. Time-Resolved Photoluminescence Spectroscopy of InGaAs/InP Heterostructures* Colleen Gillespie and Tim Gfroerer, Davidson College, Davidson, NC Mark Wanlass, National Renewable Energy Laboratory, Golden, CO Abstract Semiconductor-based thermophotovoltaic cells, which convert thermal radiation into electricity, show potential for an efficient and clean source of energy. InGaAs alloys are ideal for this process because of their small bandgap energies. However, defects in the material can give rise to new electronic levels within the bandgap. These defect states usually provide non-radiative recombination paths (which decrease the conversion efficiency of photovoltaic cells), but we have found a deep level in the near-lattice matched samples that allows for radiative recombination. Previous work shows that sub-bandgap emission grows super-linearly with excitation power up to and exceeding 1000 W/cm 2. This unusual result suggests that the defect-related radiative process is complex. We hypothesized that the electrons and holes are physically separated at the defect sites, so that increasing excitation leads to an increase in electron-hole wavefunction overlap and an increased recombination rate. If the electrons and holes are spatially separated, the resulting transition times should generally be much longer than those of the band-to-band transitions. We have measured the sub-bandgap and band-to- band recombination rates and find that our experimental results are inconsistent with the spatial separation model. Conclusions Band-to-band (B-B) transient PL reveals radiative (  1 = 47ns) and defect-related (  2 = 920ns) mechanisms as expected. Sub-band gap (SBG) and B-B response times are similar (approximately 1 microsec). Results are inconsistent with spatial separation hypothesis. The similiarity in SBG and B-B response indicates that the mechanisms are more closely related than expected. * Project supported by the American Chemical Society – Petroleum Research Fund Hypothesis: Spatially-separated Recombination τ 1 = 47 ns τ 2 = 920 ns System response time = 18 ns Transient PL If the sub-bandgap recombination is spatially-separated, then the rate would be much slower than spatially-direct band-to-band recombination. We measured the band-to-band recombination rates shown above. However, the sub-bandgap signal is ~1000 times weaker and could not be detected with this system. : Laser Light : Luminescence Nd/Yag Laser (1064 nm) ND Filters AOM Cryostat @ 77 K Sample Lock-in Photodetector Band-pass filter Fast Pre-amp New Experimental Setup: Frequency Response So we used a fast lock-in amplifier to measure the frequency response of the band-to-band and sub-bandgap signals. The lock-in amplifier is more sensitive than the boxcar averager that we used to measure transient PL. Frequency Response Results Response time ~ 1/(1000 KHz) ~1 microsec While this is not a very precise test, it clearly shows the similarity in response times between the band-to-band and sub-bandgap signals, which is inconsistent with the spatial separation hypothesis. f modulation 8f modulation Time Lock-in signal Frequency-dependent Lock-in Signal When the modulation frequency of the laser is increased, a signal with a slower response time will not be able to keep up, resulting in a smaller measured amplitude. Motivation: TPV Cells and How they Generate Electricity When a blackbody photon (with sufficient energy) is absorbed, free charge carriers are created, which move to generate current. Caveat: if electrons recombine with holes before they are swept away by the intrinsic electric field, TPV efficiency decreases. Conduction Band Valence Band PHOTON (LIGHT) Bandgap PHOTON (LIGHT) Bandgap ELECTRON HOLE + + E-Field E- ELECTRON HOLE +++ ++ ++ +++ E-Field E- AbsorptionCurrent (good!) or Recombination (bad!) ENERGY - - - - - Defect-related emission is observed in the photoluminescence (PL) spectrum. But the super-linear growth with increasing excitation (for comparison, the band-to-band increase is approximately linear) is not consistent with the expected saturation noted above. Previous Work: Spectra and Results Sub- bandgap (SBG) Band-to- band (B-B) Introduction: Defect Levels and Expected Behavior InGaAs Interface Defect Substrate (InP) Lattice defects produce new electronic states in the bandgap. Defect states facilitate nonradiative recombination (which decreases TPV efficiency). But at high excitation, defect levels should be filled, saturating this loss mechanism. - Conduction Band Valence Band Conduction Band Energy -- ------ -- - -- ----------- ++++++++++++++++++++++++++++++++++++++++++++++++++++++ +++++++++ - +++++++++ Low Excitation: One possible explanation for this behavior is that sub-bandgap recombination is taking place between spatially separated electrons and holes. The electrons and hole wavefunctions are separated in physical space. Wavefunction position hole electron Indirect transition + + + ENERGY - High Excitation: At a high excitation, there is an increased density of electrons and holes, which leads to an increased electric field and an increased electron-hole wavefunction overlap. Recombination would occur at a faster rate, explaining the super-linearity of the sub-bandgap PL. Wavefunction position Increased E-field Overlap increases More direct transition ENERGY + + + + + + - -