Excitation dependence of sub-bandgap photoluminescence
from InGaAs/InAsP heterostructures*
Colleen Gillespie and Tim Gfroerer, Davidson College, Davidson, NC
Mark Wanlass, National Renewable Energy Laboratory, Golden, CO
Abstract
.
Heat
Blackbody Radiation
Experimental Setup
Valence Band
Semiconductor TPV
Converter Cells
Nd/Yag Laser
(1064 nm)
TPV Cells are designed to convert infrared blackbody radiation into
electricity. TPV conversion is ideal for combination heat and electricity
installations and for extremely reliable remote power applications.
Nominally Lattice-Matched Structure
Cryostat
0
10
FTIR
spectrometer
Sample
ND Filters
How TPV Cells Generate Electricity
Conduction Band
-
: Laser Light
E-Field
-
ELECTRON
The FTIR (Fourier Transform InfraRed) spectrometer is based on a
Michelson Interferometer, which splits the light, directs it along
differing paths, and then recombines it to produce an interferogram.
The Fourier transform of the interference pattern yields the intensity
of IR light as a function of energy.
Bandgap
PHOTON
(LIGHT)
Valence Band
HOLE
Avg band-to-band
slope = 1.01 +/- 0.01
-1
10
Eg = 0.80 eV at 77K
-2
10
10
Observed Behavior?
Low excitation: lowest
energy bonding configuration
78 K
90 K
100 K
110 K
120 K
-3
At high excitation, defect levels should be filled, saturating defectrelated recombination. Meanwhile, band-to-band recombination
should continue to grow (due to the relatively high density of
available band states).
-4
10
High excitation: Metastable
photo-induced configuration?
Avg. defect-related
slope = 1.32 +/- 0.03
-5
10
10
100
1000
Excitation Density (W/cm2)
E-Field
When a blackbody photon (with sufficient energy) is absorbed, an electron
is excited to the conduction band, leaving a hole in the valence band.
Caveat: if electrons recombine with holes before they are swept away by
the intrinsic electric field, TPV efficiency decreases.
Conduction Band
InGaAs
Valence Band
Defect levels
Typical Spectra
Normalized PL Intensity (a.u.)
Introduction: Defect Levels
Substrate
(InP)
: Luminescence
Integrated PL Intensity
Blackbody Radiator
Conduction
ConductionBand
Band
Results
10
0
10
-1
10
-2
10
-3
10
-4
10
-5
10
-6
0.4
Band-toband
Slightly Lattice-Mismatched Structure
577 W/cm
2
110 W/cm
2
52 W/cm
Defect-related
Note: all samples
are undoped
0.5
0.6
0.7
Energy (eV)
0.8
0.9
10
Note the super-linear enhancement in the defect-related emission with
increasing excitation (for comparison, the band-to-band increase is
approximately linear). This phenomenon is characterized by computing
integrated intensity and plotting against laser excitation.
One possible explanation for this unusual result is the photoinduced formation of a metastable defect configuration that could
have more defect states (as with the well-known DX Center in GaAs).
Avg. band-to-band
slope = 1.02 +/- 0.01
-1
10
Eg = 0.74 eV at 77K
Conclusions
-2
10
-3
10
78 K
90 K
100 K
110 K
120 K
-4
Avg. defect-related
slope = 1.35 +/- 0.01
10
• SBG:BB PL increases with excitation power in both latticematched and slightly lattice-mismatched structures.
• This unexpected behavior is currently attributed to the formation
of metastable photo-induced defect states.
• Next: look for evidence of photo-induced changes in the timedependence of defect-related recombination.
-5
10
10
100
1000
Excitation Density (W/cm2)
Interface Defect
Lattice defects produce new electronic states in the bandgap. Defect states
usually facilitate nonradiative recombination (which decreases TPV efficiency).
We are studying the weak IR light emitted by radiative defect states.
0
2
Integrated PL Intensity
Heat Source
Expected Behavior
ENERGY
Motivation:
Thermophotovoltaic (TPV) Power
Semiconductor-based thermophotovoltaic cells, which convert thermal radiation into electricity, show potential for an efficient and clean
source of energy. Indium-rich (x > 0.53) InxGa1-xAs alloys have small bandgaps, which are ideal for this process, but heterostructures
incorporating these alloys cannot be lattice-matched to InP substrates. The resulting defects 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 suggests that defect-related recombination in these structures saturates under relatively low excitation (I < 1 W/cm2) so
we would expect the sub-bandgap photoluminescence to saturate in this regime as well. In contrast, we find that this emission grows
with excitation power up to and exceeding 1000 W/cm2. This counterintuitive result suggests that the defect-related radiative
recombination process is more complex than simple recombination statistics would suggest. We are investigating this phenomenon
further to identify the mechanism for radiative recombination from these levels.
In two structures and at five different temperatures, the
defect-related recombination increases faster than the bandto-band recombination with increasing excitation.
* Project supported by the American Chemical
Society – Petroleum Research Fund