A Comparison of Photoluminescence Imaging and
Confocal Photoluminescence Microscopy in the Study
of Diffusion near Isolated Extended Defects in GaAs
Tim Gfroerer
Davidson College, Davidson, NC
with Yong Zhang
University of NC @ Charlotte
and Mark Wanlass
National Renewable Energy Lab, Golden, CO
~ Supported by DARPA/MTO, the Charlotte Research Institute,
and the American Chemical Society – Petroleum Research Fund ~
Some of the experiments and analysis by . . .
Mac Read and Caroline Vaughan (’10)
Ryan Crum and Mark Crowley (’11)
Defect-Related Trapping and Recombination
Conduction Band
ENERGY
-
Defect Level
HEAT
HEAT
+
Valence Band
Electrons can recombine with holes by hopping through defect levels and releasing
heat. This loss mechanism reduces the efficiency of a solar cell.
Radiative Recombination and Efficiency
Conduction Band
-
light in
PHOTON
+
Valence Band
Radiative Rate ~ n x p
heat
light out
Radiative Efficiency = (light out) / (light in)
= (radiative rate) / (total recombination rate)
Photoluminescence Imaging
Experiment
Excitation-Dependent Images
2
2
Sample
(b) Iex ~ 1.2 W/cm
(a) Iex ~ 12 W/cm
Laser
Lowpass filter
100 m
1.00
0.90
0.92
0.83
0.83
0.75
0.75
0.68
100 m
0.60
0.66
2
Camera
2
(c) Iex ~ 0.12 W/cm
100 m
(d) Iex ~ 0.012 W/cm
0.75
0.30
0.69
0.28
0.63
0.25
0.56
0.23
0.50
100 m
0.20
Top View of Diffusion to a Dislocation
Simulated Images
Experiment
Simulation
1.00
0.90
0.92
0.83
0.83
0.75
0.75
0.68
100 m
2
100 m
1.00
0.90
0.92
0.83
0.83
0.75
0.75
0.68
0.66
0.60
0.60
0.66
2
2
(d) Iex ~ 0.012 W/cm
(c) Iex ~ 0.12 W/cm
2
(c) Iex ~ 0.12 W/cm
(b) Iex ~ 1.2 W/cm
(a) Iex ~ 12 W/cm
(b) Iex ~ 1.2 W/cm
100 m
2
2
2
2
(a) Iex ~ 12 W/cm
(d) Iex ~ 0.012 W/cm
0.75
0.30
0.69
0.28
0.75
0.30
0.63
0.25
0.69
0.28
0.23
0.63
0.25
0.56
0.56
0.23
0.50
0.20
0.50
0.20
100 m
Simulation Details
Generation, recombination, and diffusion
with augmented defect-related
recombination in dislocation pixel:


Defect
Radiative 


n(t ) Generation 


  recombination  recombination   Diffusion 
rate
t




rate
rate


d 2 (n)
LaplacianDiffusion  Dn 
dx 2
Theoretical Efficiency:
Efficiency 
Bn p
A (n  dp  p  dn)  B  n  p
J. Appl. Phys. 111, 093712 (2012).
Recombination Statistics:
1. Defect level distribution can be
tailored to achieve the best fit
2. Theory accounts for thermal
excitation out of traps
3. (# of e-s in conduction band) = n
can differ from
(# of holes in valence band) = p
4. (# of trapped e-s) = dn
can differ from
(# of trapped holes) = dp
Defect-Related Density of States
(a) Dislocation Pixel
(b) Bulk Pixels
8
2x10
6
6x10
-3
-1 -1
DOS /  (cm eV s )
8
2x10
6
4x10
8
1x10
6
2x10
7
5x10
0
0
Ec
Ev
-0.6
-0.3
0.0
0.3
0.6
Fractional Bandgap Energy
Ev
-0.6
Ec
-0.3
0.0
0.3
0.6
Fractional Bandgap Energy
Confocal Photoluminescence Microscopy
Experiment
Mirror
Lens
Spectrometer
Notch
Filter
Laser
Lens
Aperture
Efficiency Map
Lens
0.45
Sample
Translation
Stage
0.34
0.23
0.11
5 m
0
Excitation-dependent Confocal Maps
2
2
(b) Iex ~ 130 KW/cm
(a) Iex ~ 1100 KW/cm
0.050
0.25
0.038
0.19
0.025
0.13
0.012
5 m
0.06
5 m
0
0
2
2
(c) Iex ~ 11 KW/cm
(d) Iex ~ 1.5 KW/cm
0.45
0.008
0.34
0.006
0.23
0.004
0.11
5 m
0
5 m
0.002
0
Confocal Maps
Before
After Laser Modification
2
2
2
2
(b) Iex ~ 130 KW/cm
(a) Iex ~ 1100 KW/cm
(b) Iex ~ 130 KW/cm
(a) Iex ~ 1100 KW/cm
5 m
5 m
0.050
0.25
0.05
0.20
0.038
0.19
0.04
0.15
0.025
0.13
0.03
0.10
0.06
0.01
0.05
0
0
0
0.012
5 m
5 m
0
2
0.45
0.008
0.34
0.23
0
5 m
2
(c) Iex ~ 11 KW/cm
(d) Iex ~ 1.5 KW/cm
0.11
5 m
2
2
(c) Iex ~ 11 KW/cm
5 m
(d) Iex ~ 1.5 KW/cm
5 m
0.25
0.004
0.006
0.19
0.003
0.004
0.13
0.002
0.002
0.06
0.001
0
0
0
More Confocal Maps After Heating
High Magnification
2
2
(b) Iex ~ 11 KW/cm
(a) Iex ~ 42 KW/cm
20 m
5 m
5 m
2
2
(b) Iex ~ 130 KW/cm
(a) Iex ~ 1100 KW/cm
20 m
0.05
0.20
0.8
0.45
0.04
0.15
0.6
0.34
0.03
0.10
0.4
0.23
0.01
0.05
0.2
0.11
0
0
0
0
2
2
(c) Iex ~ 11 KW/cm
5 m
Low Magnification
2
(d) Iex ~ 1.5 KW/cm
5 m
2
(c) Iex ~ 3.4 KW/cm
(d) Iex ~ 1.5 KW/cm
20 m
20 m
0.25
0.004
0.19
0.003
0.13
0.100
0.008
0.075
0.006
0.002
0.050
0.004
0.06
0.001
0.025
0.002
0
0
0
0
Effective Diffusion Length (m)
Effective Diffusion Length
Before modification
After modification
10
1
10
3
10
4
10
5
10
2
Excitation (KW/cm )
6
Effective Diffusion Length (m)
Effective Diffusion Length
Before modification
After modification
Electrons?
10
Holes?
Point
Defects?
1
10
3
10
4
10
5
10
2
Excitation (KW/cm )
6
Top View of Confocal Measurement with
Diffusion to a Dislocation
Low-Excitation
+P
P
+P
-
L
P
+
P
P
D
P
Excitation &
Detection
-
P
+
P
P
P
P
Mid-Excitation
-
P
P
P
D Dislocation
+P
+P
+P
P
P
Pt. Defect
-
+P -
P
- +P
D -
L
-
+P
-
P
P
Electron
P
+
Hole
Top View of Confocal Measurement with
Diffusion to a Dislocation
High-Excitation
Mid-Excitation
+
P
+P
+P
+
P
+P
P
-
+P -
P
- +P
D -
L
-
+P
Excitation &
Detection
- + - ++ -+ P
+
- - + L - -+
+ - +
P
+ --+P -
P
P
D
P
P
P
P
P
P
D Dislocation
P
P
P
P
P
Pt. Defect
-
Electron
P
+
Hole
Conclusions
• Photoluminescence is a powerful tool for
examining the impact of defects
• Depletion of carriers near dislocations increases
with reduced illumination
• But bulk point defects inhibit diffusion at very
low excitation densities
• Understanding diffusion in the presence of
defects will facilitate better solar cell design