Defect states with an occupation-dependent lattice configuration in zinc-doped Ga0.58In0.42P on GaAs
T. H. Gfroerer and D. G. Hampton
Davidson College, Davidson, NC 28035
M. W. Wanlass
National Renewable Energy Laboratory, Golden, CO 80401
While lattice-matched Ga0.51In0.49P on GaAs has the ideal bandgap for the top converter in triplejunction GaAs-based solar cells, more complex designs will benefit from flexibility in the
composition of this alloy. Changes in the gallium/indium ratio are accompanied by latticemismatch relative to GaAs substrates, so higher dislocation densities are expected in alternative
gallium/indium epitaxial alloys. We report deep level transient spectroscopy (DLTS) and
photocapacitance measurements on metamorphic, Zn-doped Ga0.58In0.42P. The experimental
evidence, including thermally-activated non-exponential capture and escape and an augmented
optical threshold energy, indicates that hole traps with an occupation-dependent lattice
configuration are present in this device. Similar behavior has been linked with Zn-doping in
GaAsP, raising the suspicion that zinc may also be involved in the AX-like complex reported here.
Motivation: Multi-junction solar cells
DLTS and Photocapacitance
Photocapacitance optical cross-section
Depletion Layer
GaAs bandgap
1.00
0.75
0.50
-
-
-
-
+-
P
+-
T = 77K
+
-11
10
+
+
Depletion with bias
Visible
0.25
0.00
400
-
+
800
1200
1600
2000
2400
Cross Section (cm )
-1
-2
1.25
+
+
+
+
+
2
1.50
Solar Spectral Irradiance (Wm nm )
- - - - - - - N+ - - - -
2.1eV GaInP
Deep level transient spectroscopy (DLTS) and photocapacitance
employ transient capacitance measurements on diodes during
and/or after the application of a bias pulse to monitor the
capture/emission of carriers into/out of defect-related traps.
-12
10
EThreshold = 0.70 eV
DLTS Capture and Escape
Wavelength (nm)
Higher energy photons are absorbed in higher bandgap alloys,
reducing the heat loss caused by excess photon energy
relative to the gap.
Conventional Exponential
Stretched Exponential
0.7
-2
0.05 mm n+ (S) GaAsP Contact
0.01 mm n+ (S) GaInP Emitter
GaInP Interrupt
2 mm p (Zn) GaInP Base
T = 143K
Capacitance Change (a.u.)
Device Structure
10
5 mm p+ (Zn) GaAsP Step Grade Layer and Contact
0.9
1.1
Energy (eV)
1.3
The optical escape threshold energy of 0.70 eV is significantly higher than
the thermal escape energy of 0.32 eV, supporting an occupation-dependent
lattice configuration model like the one shown below.
Changing lattice configuration model
-3
10
(Zn) GaAs Substrate
The Stretched Exponential:
C(t )  Ae
Schematic of the test structure (not to scale).
0.0
0.2
 kt d
0.4
0.6
0.8
1.0
Time (s)
Experimental Setup
Conventional and stretched exponential fits to the
capture transient measured at 143 K, demonstrating the
non-exponential response and the suitability of the
stretched exponential analysis.
(0.70 eV)
(0.32 eV)
(0.21 eV)
5
10
Capture
In the occupation-dependent model, the energy of the system changes as a
function of the local configuration of the lattice. This means that lattice
vibrations, or phonons, can push the defect level closer to the valence
band, but photons, which carry very little momentum, have no effect on the
defect level. As a result, the optical escape energy is much larger than the
thermal activation energies.
Ea = 0.21eV
4
10
3
Conclusion and Acknowledgement
-1
Rate (s )
10
2
10
Escape
Defect centers with an occupation-dependent lattice configuration (namely,
DX centers) are relatively common in n-type III-V alloys, particularly near
bandgap crossover. This report of analogous behavior in p-type material
suggests that acceptor complexes (i.e. AX centers) may also form in Zndoped high-bandgap GaInP and related alloys. If this interpretation is
correct, Zn-related AX centers may limit the performance of these alloys
in photovoltaic cells.
1
10
Ea = 0.32eV
0
10
The pulse generator applies reverse and pulse (toward zero)
biases to the sample while the capacitance meter reads the
resulting change in capacitance as a function of time. For DLTS,
we study how dark measurements change with temperature. For
photocapacitance, we study how cold measurements change with
the energy of illumination from the monochromator.
70
75
80
85
90
95
-1
1/kT (eV )
Arrhenius plot of the stretched exponential rates (k)
used to fit the capture and escape transients. We
obtain thermal activation energies of 0.21 eV for
capture and 0.32 eV for escape.
The authors would like to thank J. J. Carapella
for performing the MOVPE growth and processing
the devices. Acknowledgment is made to the
Davidson Research Initiative, funded by The
Duke Endowment, and the donors of the American
Chemical Society – Petroleum Research Fund for
support of this research.