APS 2001 March Meeting Poster

1
Excitation-dependent transition between
defect-related and radiative recombination in
lattice-mismatched InGaAs/InAsP heterostructures
F.E. Weindruch and T.H. Gfroerer, Davidson
College
M.W. Wanlass, National Renewable
Energy Laboratory
Calibration to Obtain the Absolute Efficiency
Abstract
Derivatives of Best-Fit Curve
Lattice-mismatched (Indium-rich) InGaAs
heterostructures grown on InP substrates are
strong candidates for thermophotovoltaic cells,
devices that convert thermal radiation into
electricity.  We are studying a set of
incrementally lattice-mismatched InGaAs/InAsP
double heterostructures by measuring the
radiative efficiency as a function of excitation
power at 77K and comparing the rates of
defect-related (nonradiative) and radiative
recombination in these structures. We present
preliminary results on how the transition between
defect-dominated and radiative recombination
depends on lattice-mismatch.
Motivation Thermophotovoltaic (TPV) Power
Blackbody Radiation
Heat
The derivatives of the polynomial fit show where
the curvature of the relative efficiency
inflects. We scale the relative efficiency
curves to obtain 50 absolute efficiency at the
infection point.
Semiconductor TPV Converter Cells
Heat Source
Blackbody Radiator
TPV Cells are designed to convert infrared
blackbody radiation into electricity.
A Theoretical Model
Lattice-mismatched In-rich InGaAs on InP
Blackbody Radiation Absorbed
Bandgap vs. Alloy Composition
GaAs
Substrate
Severe Mismatch

Bandgap Energy (eV)
InAs
Atom Spacing (Angstroms)

Increasing the Indium concentration in the InGaAs
lowers the bandgap and increases the fraction of
blackbody radiation that is absorbed in the cell.
Comparing the Defect-Related and Radiative Rates
 
 
_at_ 50 Radiative Efficiency, n A/B Total Rate at
50 Efficiency An Bn2 2A2/B
Sample Structure
Threshold
Active Layer
Increasing Lattice Mismatch
Exceeding a threshold mismatch of 1 increases
the defect-related rate relative to the radiative
rate.
Conclusions

• Moderate mismatch does not affect the rate of
  defect-related recombination relative to the
  radiative rate.
• Large mismatch has an appreciable effect on this
  ratio.
• The threshold that distinguishes these two
  regimes is approximately 1 lattice mismatch.
• A change in the shape of the efficiency curve is
  observed for all mismatched samples relative to
  the lattice-matched case. The phenomenon is
  attributed to a change in the distribution of
  energy levels at defect sites. Further work is
  needed to test this hypothesis.

Acknowledgements and References
This project is supported by the Research
Corporation and the Petroleum Research Fund. T.
Saitoh, H. Iwadate and H. Hasegawa, Jpn. J. Appl.
Phys. 30, 3750 (1991).
Corresponding Author Tim Gfroerer Physics
Department, Davidson College Davidson, NC 28036
(tigfroerer_at_davidson.edu)