Hot-Carrier Effects in Narrow Gap InAs &
GaSb Materials: Potential for Next Generation
Photovoltaics
V.R. Whiteside1, J. Tang1, S. Vijeyaragunathan1, H. Esmaielpour1, T.D. Mishima1,
M.B. Santos1, and I.R. Sellers1 1 University of Oklahoma
• InAs QW optical design
• Photoluminescence measurements: Temperature and Power
Dependence
• Hot Carrier Effects: Red and Blue laser excitation
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
1
InAs Based Optical Structures
InAs
AlAs.16Sb.84
InAs
AlAs.16Sb.84
InAs
50 nm Cap
InAs
2 nm
2.4 nm
2 nm
AlAs.16Sb.84
80x
2000 nm
GaAs Substrate
InAs
AlAs.16Sb.84
InAs
T664
50 nm Cap
10 nm
2.4 nm
10 nm
30x
2000 nm
GaAs Substrate
T673
• X and L valley energy separation:
• GaAs substrate as trial
InAs > GaAs
• AlAs.16Sb.84 latticed matched to InAs
Desirable to not pump higher energy
• InAs buffer layer 2 microns thick to
states
minimize strain
• Effective band gap tunable to 0.7 eV
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
Conduction & Valence Band Offsets
T673: 2.4 nm InAs QWs
10.0 nm AlAsSb Barrier
2.8
X =0.16
Ec
2.4
E3 QWs
1.770 eV
2.255 eV
E2 QWs
1.134 eV
http://hdl.handle.net/1802/12773, J.R. Pedrazzani
Thesis (Ph. D.)--University of Rochester. Institute of Optics, 2010
Energy (eV)
2.0
1.6
E1 QWs
0.454 eV
1.2
0.8
Ev
0.160 eV
0.4
0.0
x = 0.16
Indirect band gap ~ 1.7 eV
Direct band gap ~ 2.5 eV
Thin barriers expect direct band gap
absorption to dominate
0
150
300
450
600
750
Depth (Angstroms)
Multiple confined states:
Good for impact ionization
Complication for selective
extraction of hot carriers
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
900
InAs 2.4 nm Superlattice and MQW PL
8
T673 10 nm AlAsSb barrier
T664 2 nm AlAsSb barrier
Intensity (arb. u.)
7
2 nm Barrier
6
Max Temp 632 nm - 120 K
Max Temp 442 nm - 85 K
5
4
3
10 nm Barrier
PL Room Temp
50 meV increase in
Peak Energy
10 nm :: 2 nm
840 meV :: 790 meV
2
1
0
1380
1440
1500
1560
1620
1680
Wavelength (nm)
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
InAs MQW E vs T
Norm. Intensity (arb.u.)
Temp
Localization due to alloy fluctuations
typical of narrow well widths :
Temperature and Power Dependent
0.844
Peak Energy (eV)
0.840
1400
1450
1500
Wavelength (nm)
1550
T673 InAs QW 2.4 nm
AlAsSb barrier 10.0 nm
0.836
0.832
0.828
0.824
0.820
0.816
0
40
80
120
160
200
240
Temperature (K)
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
280
InAs MQW Temperature and Power
Dependence
λex = 633 nm
0.839
0.838
0.837
Minima in energy shifts observed
for lower power
localization effects observed
below 100 K
0.836
0.835
0.834
0.833
0.832
0.860
λex = 442 nm
0.831
0.830
0.829
90
120
150
180
210
240
Temperature (K)
λex = 633 nm
minima observed for all powers
Peak Energy (eV)
Energy (eV)
λex = 442 nm
P = 9.866 mW
P = 4.345 mW
P = 1.483 mW
0.855
41.28 mW
34 mW
21.15 mW
8.5 mW
0.850
0.845
0.840
0.835
0.830
Minima in energy shifts to lower
temperature as power decreases
0.825
0
50
100
150
200
250
Temperature (K)
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
300
InAs MQW Power Dependence
Peak Energy (eV)
0.855
Type II power dependence
based on triangular quantum well
ex = 442 nm
0.850
ε∝ 𝐼
0.845
Ee = const ε2/3 ≡ 𝑏𝐼1/3
0.840
0.835
77 K
90 K
150 K
225 K
295 K
0.830
0.825
Ledentsov et al., PRB Vol. 52, (19) 14058, C. Weisbuch, B. Vintner,
Quantum Semiconductor Structures, p. 20 (Academic, Boston, 1991)
0.840
0.815
0
10
20
30
40
Power (mW)
Low powers rapid change in peak energy
Higher powers leveling off of peak energy
77 K & 90 K power dependence
behaves more like type II
200 K and 295 K more like type I
Peak Energy (eV)
0.820
0.838
0.836
77 K
90 K
150 K
200 K
295 K
0.834
0.832
0.830
0.828
0.826
0.824
0.822
0.0
0.3
0.6
0.9
Power
1.2
1/3
1.5
1/3
(mW )
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
1.8
2.1
InAs QWs fitting methods
Ln(Intensity) (arb. u.)
12
Maxwell-Boltzmann like distribution of carriers:
ex = 442 nm
10
T = 90 K
Ipl (hν) ∝ exp − hν 𝑘𝑏𝑇𝐻
8
6
Ipl - PL intensity
hν - photon energy
𝑇𝐻 - carrier temperature
4
2
0
0.75
0.80
0.85
0.90
0.95
1.00
1.05
A. Le Bris et al., Energy & Environmental Science DOI:10.1039/c2ee02843c
Energy (eV)
Increasing power → Increasing ΔT
HE slope of PL decreases
corresponds to increasing ΔT
Plaser = P1I1/2 + P2I + P3I3/2
Plaser - Laser power
I - Normalized Integrated intensity
P1, P2, P3 - fitting parameters
P1 - Shockley-Read-Hall recombination
P2 - Radiative recombination
P3 - Auger recombination
Hirst et al., IEEE J. of Photovoltaics, Vol. 4, No. 1, January 2013
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
InAs QWs Laser excitation comparison
24
442 nm above the barriers
4K
21
633 nm
AlAsSb
18
InAs
15
12
632.8 nm in the well
9
24
6
21
3
Power
0
0.80
0.85
0.90
0.95
1.00
Wavelength (nm)
High energy tail is more pronounced
when excited by higher energy
higher power blue laser.
Only a single QW is excited by blue laser
Typical change in slope of high energy
tail (broadening) as a function of power
ln(Intensity) (arb. u.)
ln(Intensity) (arb. u.)
442 nm
4K
18
15
12
9
6
3
0
0.80
0.85
Wavelength (nm)
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
0.90
InAs QWs Carrier Temperature
2
Equivalent Suns (1000 W/m )
200
180
160
40,000
60,000
80,000
0.860
4K
10K
30K
40K
60K
77K
90K
4K
140
120
100
90 K
80
Peak Energy (eV)
Temperature Difference (K)
20,000
41.28 mW
34 mW
21.15 mW
8.5 mW
0.855
0.850
0.845
0.840
0.835
0.830
60
0.825
2000
4000
6000
8000
2
Power (W/cm )
Low Temperature Regime:
Increasing ΔT wrt Power for all Temp
Temp increases slope of ΔT decreases
0
50
100
150
200
250
300
Temperature (K)
High Temperature Regime:
Holes delocalized
electron pileup
ΔT wrt Power nearly independent
Temp increases slope of ΔT becomes level
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
InAs QWs Carrier Temperature
2
6x10
6
5x10
6
4x10
6
3x10
6
2x10
6
1x10
6
20,000
Temperature Difference (K)
Itg. Intensity (arb. u.)
Equivalent Suns (1000 W/m )
41.28 mW
34 mW
21.15 mW
8.5 mW
0
0
50
100
150
200
250
300
Temperature (K)
40,000
60,000
80,000
250
200
150
295 K
100
90 K
80
90K
110K
130K
150K
225K
295K
60
2000
4000
6000
8000
2
Power (W/cm )
High Temperature Regime:
Low Temperature Regime:
Increasing ΔT wrt Power for all Temp
Temp increases slope of ΔT decreases
Holes delocalized
electron pileup
ΔT wrt Power nearly independent
Temp increases slope of ΔT becomes level
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
InAs QWs 633 nm Hot Carriers
100
24
41.28
mW
9.8 mW
34
8.5mW
mW
4.3 mW
21.15
mW
1.5 mW
mW
8.5
Low Energy Tail
ln(Intensity) (arb. u.)
50
0
Slope
4K
21
High Energy Tail
-50
18
15
12
9
6
3
-100
0
0.80
0.85
Wavelength (nm)
-150
0.844
0
50
100
150
200
250
300
Temperature (K)
Strong correlation Carrier Temperature/HE slope
and localization observed in PL peak energy
Peak Energy (eV)
0.840
-200
0.90
T673 InAs QW 2.4 nm
AlAsSb barrier 10.0 nm
0.836
0.832
0.828
0.824
0.820
0.816
0
Red laser pumps more QWs
weaker Hot Carrier Effects
40
80
120
160
200
Temperature (K)
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
240
280
Summary
Future measurements:
Repeat with 975 nm laser line to confirm trends that
have been observed with 442 and 633 nm lines
Magnetic field measurements to probe the nature
of localization confinement
Preliminary results for hot carrier optical studies:
InAs QW structure can be tuned to 0.7 eV band gap for hot carrier solar cells
Band offsets for AlAsSb/InAs superlattice structure are such that there are
energy states in QW suitable for Impact ionization
Blue laser shows more pronounced effects than red laser
Carrier temperature appears to be correlated to localization
Temperatures > 150 K leveling of carrier temperature wrt laser power
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
13