Band-to-band and sub-bandgap
cathodoluminescence from GaAsP/GaInP
epistructures grown on GaAs substrates
Tim Gfroerer
Davidson College, USA
M.J. Romero, M.M. Al-Jassim, and M.W. Wanlass
National Renewable Energy Lab, USA
~ Supported by the Amer. Chem. Soc. – Petroleum
Research Fund and the U.S. Dept. of Energy ~
Motivation: Lattice-Mismatched
Multi-Junction Solar Cells
High bandgap
Medium bandgap
Low bandgap
Sample Structure (not to scale)
GaInP window
GaAsxP(1-x) active layer
Lattice-matched GaInP barrier
GaAsP buffer
GaAsP Lattice-mismatch step grade
GaAs Substrate
Spatially-Integrated CL Spectra
8
Cathodoluminescence (a.u.)
10
7
10
SBG
GaAsxP1-x
BB
T = 77K
I ~ 0.5nA
6
10
x = 0.844
5
10
Shifted
Vertically
For clarity
4
10
x = 0.927
BB
3
10
SBG
2
10
x = 1.00
1
10
1.2
1.4
1.6
Energy (eV)
1.8
2.0
Plan-View Band-to-Band CL at 300K
14 mm
[As] = 0.844
Eg = 1.76 eV
14 mm
35 mm
[As] = 1.000
Eg = 1.43 eV
[As] = 0.927
Eg = 1.62 eV
Two Color Plan-View CL at 300K
[As] = 0.927
Eg = 1.62 eV
Red = SBG
Blue = BB
[As] = 0.844
Eg = 1.76 eV
Two Color Plan-View CL at 77K
[As] = 0.927
Eg = 1.62 eV
Red = SBG
Blue = BB
[As] = 0.844
Eg = 1.76 eV
Beam-Energy Dependence of CL Spectrum
CL (a.u)
-1
10
[Ga] = 1.00
T = 77K
0
BB
15 KeV
20 KeV
30 KeV
10
SBG
CL (a.u.)
0
10
-1
10
BB
Substrate
SBG
-2
-2
10
-3
10
10
-3
10
1.2
[Ga] = 0.844
T = 300K
15 KeV
20 KeV
30 KeV
1.3
1.4
1.5
Energy (eV)
1.6
1.4
1.5
1.6
1.7
Energy (eV)
1.8
1.9
2.0
Cross-Sectional CL at 77K
-0.5
0.0
CL Intensity
30.00
30.00
Position (microns)
Position (microns)
0.0
0.5
SBG
1.0
3000
BB
1.5
CL Intensity
700
SBG
BB
1.0
3000
1.5
[As] = 0.844
T = 77K
Substrate
[As] = 0.927
T = 77K
600
0.5
2.0
800
Wavelength (nm)
900
Substrate
600
700
800
Wavelength (nm)
900
Beam-Current Dependence of CL spectrum
5
10
BB
4
CL (a.u.)
10
3
Beam Current
50 pA
200 pA
400 pA
1000 pA
10
Barrier
2
10
1
10
SBG slope = 1.31
6
10
Gaussian amplitude (a.u.)
SBG
[As] = 0.844
T = 77K
5
10
BB slope = 0.95
4
10
1.4
1.6
1.8
Energy (eV)
2.0
2.2
2
3
10
10
Excitation (pA)
Conclusions
• A broad SBG peak is observed ~0.15eV below
the B-B emission in all structures
• Spatial correlation of B-B and SBG plan-view CL
depends strongly on temperature
• The SBG emission is associated with the lower
interface of the heterostructure
• Excitation current dependence points to a
spatially indirect recombination mechanism
Future Work
• Detailed photoluminescence study: excitation
intensity, temperature, and time dependence