Exploring defect level position and occupation
in GaAsP via recombination analysis
KyungTaek Lim and Tim Gfroerer, Davidson College, Davidson, NC
Mark Wanlass, National Renewable Energy Lab, Golden, CO
Abstract
When electron-hole pairs are created in a semiconductor, recombination can occur either radiatively or nonradiatively. We measure the
radiative efficiency (the fraction of recombination that is radiative) as a function of laser excitation intensity and temperature to
characterize recombination mechanisms and predict how defects will impact solar cell performance. In a 1.6eV-bandgap GaAsP
heterostructure, we observe complex changes in the photoluminescence spectrum and radiative efficiency with excitation and temperature.
The changes suggest that two defect bands may be contributing to recombination in this alloy. Further evidence for the presence of these
bands is obtained by modeling the defect level occupation and recombination statistics. The analysis suggests that thermal activation out of
shallow defect levels can improve the performance of solar cells incorporating GaAsP.
Luminescence Spectra at Low Excitation
E-Field
5
Intensity (Abs. Unit)
-
ELECTRON
ABSORPTION
HOLE
+
E-Field
+
-ENERGY
ENERGY
- -
-
Trapping
kT
1.65
1.70
1.75
1.80
Radiative recombination reveals a Defect-Related (D-R) transition approximately
40 meV below the Band-to-Band (B-B) emission. The D-R transition is primarily
radiative at 78K, but becomes primarily nonradiative at higher temperatures.
-
Escape
-
Band to Band
(Radiative )
Recombination
8
7
10
B-B
78K
120K
165K
10
Defect
Level
∆p∆n is the product of hole and
electron densities in the valence
and conduction bands, respectively.
Defect-related recombination is
assumed to be purely non-radiative
so we do not include the 78K
results in the fit.
Temperature-dependent defect
level occupation allows for thermal
activation out of shallow traps.
The least error between theory
and experiment is obtained with
the defect-related density of
states (DOS) shown in the inset,
which matches our interpretation
of the data.
23
10
120K
165K
207K
250K
297K
22
10
32
34
10
10
36
10
The Low-Density Defect Band
At 120K
At 77.7K
Conduction Band
- -
-
-
-
-
Trapping
D-R
B- B Radiative
Recombination
6
10
-
-
Photons
Heat
Defect Related
Recombination
5
10
PHOTON
+ + + + +
Valence Band
At low temperature, electrons can recombine with holes by hopping through
defect levels and releasing heat. This loss of energy as heat reduces the
efficiency of a solar cell. At high temperature, trapped electrons can
escape to conduction band and then be swept away to create electricity or
drop down to valence band by emitting photons.
Experimental Setup
4
10
1.60
1.65
1.70
1.80
The radiative efficiency increases dramatically with laser power at 165K, while
the D-R band is still prominent in the 78K spectrum. This behavior suggests that
another, lower-density defect level is contributing to the recombination under
low excitation at 165K.
Valence Band
Valence Band
The hypothetical low-density defect level which reduces the radiative efficiency at
low excitation and high temperature is not an effective recombination center at
78K. Perhaps there is an energetic barrier that surrounds these defects, inhibiting
occupation at 78K.
Efficiency Results:
Band Gap Energy = 1.75 eV
The Higher-Density Shallow Band
At 120K
At 77.7K
At 165K
Conduction Band
• Excitation-dependent radiative
efficiency measurements are consistent
with the luminescence spectra
1.0
Conduction Band
Conduction Band
•Radiative efficiency drops dramatically
with temperature between 78K and
120K via 2 mechanisms:
0.8
--- - - -
- - - - - -
•Second, transitions through another,
less-dense defect level begin to
contribute to nonradiative
recombination
In our experiment, laser light is incident upon the semiconductor sample,
producing luminescence. We collect this emitted light and focus it onto a
photodiode for efficiency measurements, or into the spectrometer for
spectral analysis.
1.75
Energy (eV)
•First, radiative recombination via the
shallow D-R band becomes nonradiative
•Radiative efficiency increases again
above 120K as carriers are thermally
activated out of the shallow D-R band.
0.6
Defect Related
Recombination
kT
Photons
-
-
-
kT
Defect Related
Recombination
kT
Heat
B- B Radiative
Recombination
0.4
T 78
T120
T165
T207
T250
T290
0.2
Valence Band
0.0
22
10
23
10
24
10
25
10
-3 -1
Recombination Rate (cm s )
Valence Band
Valence Band
The D-R recombination at 78K is radiative – the D-R luminescence is as strong as
the B-B emission and the radiative efficiency is relatively high. At 120K and above,
radiative D-R recombination quenches, producing heat. However, thermal activation
out of the shallow level enhances B-B recombination at 165K and above.
Conclusions, Significance, and Acknowledgement
•
•
•
•
•
EC
Energy
-6
B-B
D-R
EV
pn (cm )
Conduction Band
Laser Power:
2
74 W/cm
24
10
30
B-B
D-R
25
10
10
Luminescence Spectra at High Excitation
Intensity (Abs. Intensity)
Conduction Band
Valence Band
4.
Energy (eV)
Conduction Band
+ + + + +
3.
3
1.60
At High Temperature
HEAT
D-R
10
+
At Low Temperature
Defect-Related
(Non-Radiative)
Recombination
4
10
+
Defect-Related Trapping and Recombination
- -
2.
10
When a photon is absorbed, an electron is excited into the conduction band,
leaving a hole behind in the valence band. Then an internal electric field
sweeps the electrons and holes away, creating current through the solar cell
and providing electricity.
HEAT
Laser Power:
2
2.83 W/cm
D-R
2
Valence Band
-
1.
B-B
Radiative Efficiency
PHOTON
10
B-B
DOS
Conduction Band
-3 -1
78K
120K
165K
The Model:
B-B
D-R
Recombination Rate (cm s )
Motivation: Solar Cell Operation
--
Theoretical Fit
We observe complex changes in the radiative efficiency of GaAsP with excitation and temperature.
The changes suggest that 2 defect bands may be contributing to recombination.
The presence of 2 bands, with expected characteristics, is confirmed by modeling the defect level occupation and recombination statistics.
Thermal activation out of shallow defect levels can reduce the negative impact of these states on GaAsP-based solar cell performance.
We thank Jeff Carapella for growing the test structure and
we acknowledge the Donors of the American Chemical Society – Petroleum Research Fund for supporting this work.