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)
Patten Priestley (Davidson ’03)
and Malu Fairley (Spelman ’03)
Pete Campbell
(Davidson ’03)
Motivation: TPV converters
1.6
GaAs
1.4
Normalized Intensity
InP
1.2
Bandgap (eV)
1.0
Substrate
1.0
Severe
Mismatch
0.8
0.6
T=1300 C
0.8
0.6
0.4
0.2
0.4
0.2
5.6
InAs
5.7
5.8
5.9
6.0
Lattice parameter (Angstroms)
6.1
0.0
0.0
0.5
1.
0
1.5
2.0
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.
2.5
Variable-Bandgap Lattice-Mismatched Stuctures
Undoped InAsyP1-y, 30 nm
Undoped InxGa1-xAs, 1.5 μm
Undoped InAsyP1-y buffer, 1 μm
Undoped InAsyP1-y step-grade region:
0.3 μm/step (~ -0.2% LMM/step), n steps
Undoped InP substrate
Efficiency Measurements
CW YAG laser
1 Watt @ 1064 nm
Photodiode
Variable Temp
Cryostat
Lowpass
Filter
: Laser Light
Sample
ND Filters
: Luminescence
light in = heat + light out
heat
light in
radiative efficiency = light out / light in
light out
10
16
10
12
-3
-1
Density of States (cm eV )
Defect-Related Density of States
10
8
10
4
10
0
0.0
0.1
Valence
Band
0.2
0.3
0.4
Energy (eV)
0.5
0.6
Conduction
Band
The distribution of defect levels within the bandgap can be represented by
a density of states (DOS) function as shown above.
Radiative Efficiency Measurements
light
80
60
40
20
0
heat
21
EV
Energy
EV Energy EC
60
Log(DOS)
Radiative Efficiency (%)
80
Log(DOS)
100
Eg = 0.80 eV
Log(DOS)
Radiative Efficiency (%)
100
40
EV Energy EC
20
Eg = 0.68 eV
EC
0
23
25
19
10
10
10
10
e-h Pair Gen. / Recombination (cm-3s-1)
18
20
22
24
10
10
10
10
-3 -1
e-h Pair Gen. / Recombination (cm s )
Radiative efficiency measurements at 77K. The theoretical fits correspond to
the defect-related DOS functions indicated in the inset graphs.
Defect-Related Transition
Probabilities
-
P ~ (0.5)10 ~ 10-3
P ~ 10-1
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.
Temperature Dependence
Radiative Efficiency
0.8
1.0
77K
131K
185K
239K
296K
0.6
0.4
0.2
0.0
77K
131K
185K
239K
296K
0.8
Radiative Efficiency
1.0
Eg ~ 0.77 eV
23
0.6
0.4
0.2
0.0
25
19
21
10
10
10
10
e-h Pair Gen. / Recombination (cm-3s-1)
10
Eg ~ 0.56 eV
18
10
20
22
10
10
24
10
26
-3 -1
e-h Pair Gen. / Recombination (cm s )
Temperature dependence of radiative efficiency vs. excitation, showing how
the SRH and Auger mechanisms depend on Indium concentration.
Auger Recombination
8
8
6
Eg ~ 0.71 eV
4
cm /s )
Eg ~ 0.56 eV
6
-28
2
2
C ( 10
C ( 10
-28
185 K
239 K
296 K
4
Eg ~ 0.65 eV
6
cm /s )
6
Eg ~ 0.77 eV
1
1
0.8
0.8
0.6
0.6
0.4
0.4
40
60
80
-1
1 / kT (eV )
100
0.5
0.6
0.7
0.8
Bandgap Energy (eV)
Temperature and bandgap dependence of the Auger coefficient C. The CHSH
(band-to-band) mechanism dominates Auger recombination in low-bandgap InGaAs.
Sub-Bandgap Photoluminescence
10
10
0
0
10
-2
10
-3
10
-4
x = 0.53
x = 0.60
0.4
0.5
0.6
Normalized PL Intensity
Normalized PL Intensity
10
-1
T = 77K
x = 0.72
x = 0.78
T = 77K
0.7
0.8
0.9
10
-1
10
-2
10
-3
10
-4
0.3
0.4
0.5
0.6
0.7
0.8
Energy (eV)
Energy (eV)
FTIR spectra showing a deep transition in the lattice-matched material that
abates and then disappears with increasing [In].
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).