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Effect of Sb on the Properties 

of GaInP Top Cells 

Preprint 

J.M. Olson, W.E. McMahon, and S. Kurtz 

Presented at the 2006 IEEE 4th World Conference on 

Photovoltaic Energy Conversion (WCPEC-4) 

Waikoloa, Hawaii 

May 7–12, 2006 

Conference Paper 

NREL/CP-520-39903 

May 2006 
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EFFECT OF Sb ON THE PROPERTIES OF GaInP TOP CELLS* 

J.M. Olson, W.E. McMahon and Sarah Kurtz 

National Renewable Energy Laboratory, Golden, CO 80401 

ABSTRACT 

It is well known that the efficiency of GaInP/GaAs tandem solar cells is limited by the band gap of the GaInP 

top cell, which, in turn, is determined by the degree of 

compositional ordering in GaInP base layer. Attempts to 

raise the band gap by the addition of Al to the top cell 

have met with limited success due to the strong affinity 

between Al and oxygen. Here we investigate a different 

approach. It has been shown that the presence of antimony on the surface of GaInP during its growth suppresses the ordering process and increases the band 

gap. In this paper, we study the effects of Sb on the 

properties of GaInP top cells. We show that, in addition to 

raising the band gap of GaInP, it also increases the incorporation of Zn and changes the relative incorporation of 

Ga and In. These effects depend strongly on the substrate orientation, growth temperature and rate, and the 

Sb/P ratio in the gas phase. We show that the band gap 

of the GaInP top cell (and the Voc) can be increased 

without reducing the minority carrier collection efficiency. 

The implications of these results are presented and discussed. 

*This work has been authored by an employee or employees of the Midwest Research Institute under Contract No. DE-AC36-99GO10337 with the U.S. 

Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States 

Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, 

for United States Government purposes. 

INTRODUCTION 

The efficiency of a GaInP/GaAs tandem solar cell depends on the thickness and band gap of the GaInP top 

cell.[1, 2] For the growth conditions typically used to produce high quality electronic properties, the band gap of 

GaInP at a composition lattice-matched to GaAs is in the 

range of 1.8 eV. This band gap corresponds to partial 

ordering of Ga and In on the group III lattice sites. To 

achieve current matching with a 1.8eV band gap requires 

that the GaInP be thinned to some finite thickness on the 

order of 1μm.[1-4] However, for optimal efficiency both the 

thickness and band gap of the top cell should be increased. The highest efficiency occurs for a top-cell band 

gap of 1.9 eV, which, for GaInP, occurs when the Ga and 

In atoms are randomly located on the group III sublattice. 

Efforts to grow "disordered" GaInP with a 1.9 eV band gap 

have met with mixed success. One very effective way to 

decrease the ordering is to grow on GaAs(001) substrates 

with a X ̊ miscut towards [110] (abbreviated X ̊A) where X 

is in the range of 2 ̊ to 15 ̊.[5] Takamoto and coworkers 

used this fact to achieve an efficiency of 30.3%.[6] It has 

also been shown that the addition of various dopants can 

hinder the ordering process and increase Eg. For shallow 

dopants, the band gap increases only for dopant levels 

that adversely affect the minority carrier transport 

properties.[7] 

Antimony, on the other hand, can be used to suppress 

the ordering process[8, 9] A plot of this effect is show in 

Fig. 1. And since Sb is isoelectronic with P in GaInP, its 

effect on the minority carrier transport properties may be 

minimal. Hence it is the intent of this paper to examine the 

effect of Sb on the properties of GaInP top cells. 

EXPERIMENTAL DETAILS 

1.88

1.86

1.84

1.82

1.80

Band gap energy [ev]

0 10 20 30 40 50

Sb/P [ppm]

Figure 1 Band gap of GaInP as a function of Sb/P gas 

phase mole fraction. Substrate was GaAs(001) 6 ̊B. 

The growth temperature, rate and V/III ratio were 620 ̊C, 

5 μm/hr, and 300, respectively. 

All epilayer structures and devices were grown by lowpressure metal-organic chemical vapor deposition 

(MOCVD) using as sources triethylantimony (TESb), 

triethylgallium (TEG), trimethylindium (TMI), trimethylaluminum (TMA), phosphine (PH3), and arsine (AsH3). 

The cells were a simple n-on-p structure with a Zndoped AlGaInP back surface field (BSF) and a Se-doped 

AlInP window layer and GaAs contacting layer. They 

were grown on Zn-doped GaAs substrates. The substrate 

was GaAs(001) misoriented 6 ̊ towards (111)B (6 ̊B). 

Unannealed electroplated gold was used for the front and 

rear contacts. Unless otherwise stated, the cells were 

grown at 620 ̊C, at a rate of 5 μm/hr with a V/III ratio of 

300. 

1
The composition of ternary layers was determined 

from high-resolution x-ray diffraction measurements and 

fitting the results using a dynamical x-ray diffraction model 

from Bede Scientific, Inc. 

7

8

9

10 17

2

Hole concentration [cm -3 ]

0 10 20 30 40 50

Sb/P [ppm]

Fig. 3 Effect of Sb on the acceptor concentration in the 

Zn-doped base of an n/p GaInP top cell. 

The carrier concentration in the GaInP base and similar layers was obtained from capacitance-voltage (CV) 

measurements. 

Behavior of Sb-doped GaInP 

Besides the band gap there are a number of other aspects of GaInP that are affected by the presence of Sb. In 

Fig. 2 we show the effect of Sb on the composition x of 

GaxIn1-xP. For the B-miscut substrates the alloy becomes 

Ga-rich with Sb. For singular (001) surfaces there is virtually no effect and for substrate misoriented toward 

(111)A, the alloy becomes In-rich with Sb. These results 

have implications concerning the ordering mechanism as 

discussed below. 

0.53

0.52

0.51

0.50

Ga concentration x in Ga xIn1-x

P

0 5 10 15 20

Sb/P [ppm]

15ÞB

6ÞA

15ÞA

6ÞB

The Zn incorporation also increases with Sb concentration, as shown in Fig. 3. As the Sb/V ratio is increased 

from 0 to 30 ppm, it increases by a factor of ~2, and for 25 

ppm < Sb/P < 60 ppm, the Zn incorporation is approximately constant. 

The Se incorporation decreases with Sb concentration 

as shown in Fig. 4. Here we plot the sheet resistance of a 

Se-doped emitter of an n/p GaInP top cell. (Direct measurement of the electron concentration confirms this result.) Antimony and Se compete with P for group V sites 

on the surface during growth. Increasing the TESb or PH3

partial pressure will lower the Se incorporation. Hence it 

may be desirable to lower the TESb flux during the growth 

of the emitter. 

Antimony has a large effect on the morphology of the 

growing surface. This is particularly evident for layers 

grown on singular (001) surfaces. Without Sb, the (001) 

surface is composed of asymmetric hillocks, bounded by 

low angle (4 ̊) B-type facets, similar to that observed by 

Friedman et al. for ordered GaInP.[10] At high Sb concentrations the surface becomes very flat with wide (001) 

terraces separated by monolayer high steps (0.28 nm), as 

shown in Fig. 5. 

 This result strongly suggests that the main effect of 

Sb is to modify the step flow properties of the surface. 

However, the amount of Sb needed has been a matter of 

debate. The current literature suggests that there is closer 

to a full monolayer of Sb on the surface.. It should not 

take much Sb on the surface to modify the step flow if it 

were mainly concentrated on the steps. With this in mind 

we used the model of Muraki et al.[11] and SIMS to 

9ÞB

1000

900

800

700

600

Sheet resistance [ohms/

500

]

0 10 20 30 40 50

Sb/P [ppm]

Fig. 4 Sheet resistance of a Se-doped GaInP emitter as a 

function of antimony to phosphorus ratio. The emitter 

thickness is ~100 nm. The growth rate, temperature and 

V/III ratio are 5μm/hr, 620 ̊C and 600. 

0Þ

Fig. 2 Composition of GaxIn1-xP vs. Sb/P in the gas phase 

for various substrate orientations. The growth temperature is fixed at 620 ̊C, the V/III at 500 and the growth rate 

at 4.8 μm/hr. 

2
measure the Sb surface concentration under typical topcell growth conditions. The result is that the Sb surface 

concentration ΘSb in the range of 0.003 to 0.006, much 

less than a monolayer (see Fig. 6). 

Sb-doped GaInP Solar Cells 

Fig. 5 Atomic force microscopy image of singular 

GaInP(001) growth surface. The image size is 2 μm x 2 

μm. The height difference between distinct shades of 

grey is 0.28 nm, or half the lattice constant of GaInP. The 

growth temperature, rate and V/III were 620 ̊C, 2 μm/hr 

and 450.

A series of GaInP cells were grown with a variable 

amount of Sb present in the vapor phase during growth. 

All the cells had a nominal base thickness of 2μm and 

were grown at ~620 ̊C with a V/III of 300 and with a 

growth rate of 5 μm/hr. Both Eg and Voc increase with 

Sb/V, as shown in Fig. 7. The Voc, however, increases at 

a slower rate relative to Eg. This may indicate that the 

minority lifetime in the cell is decreasing with Sb, as seen 

by Fetzer et al.[9] The Jsc decreases with Sb/P (and Eg). 

However, the rate is slower than that expected theoretically, as shown in Fig. 8. 

1.88

1.86

1.84

1.82

1.80

Band gap energy [ev]

0 10 20 30 40 50

Sb/P [ppm]

Sb-doped GaInP/GaAs TANDEM CELLS 

An Sb-doped GaInP/GaAs tandem cell was fabricated 

with the top cell doped with Sb/P = 58 ppm,giving a band 

gap of 1.89 eV. It was lattice matched to underlying GaAs 

1015

1016

1017

1018

Concentration [cm

-3

]

2.5 3.0 3.5 4.0

Depth [μm]

30

0.1 0.1 00 0 0

1.43

1.41

1.39

1.37

1.35

Voc [V]

Fig. 7 Band gap energy and Voc of GaInP solar cells 

(with no ARC) versus the Sb/P in the gas phase during 

growth of the base. 

17.5

17.0

16.5

16.0

15.5

15.0

14.5

 Jsc [mAcm

-2

]

1.80 1.82 1.84 1.86 1.88

Band gap energy [eV]

 Theoretical Jsc

 Internal Jsc

Fig. 8 Theoretical and measured internal Jsc vs. band gap 

energy for the series of GaInP presented in Fig. 5.

30 0 0 0 0

 Sb

 Se

Fig. 6 SIMS profile of a GaInP layer grown with various 

combinations of Se and Sb. The black vertical lines are 

boundaries between various layers. The black(red) numbers refer to the input Sb(Se)/P ratio. Note that the Se 

incorporation decreases by a factor of ~2 in the presence 

of Sb. Analysis of the Sb rise(fall) profiles yields an Sb 

surface concentration ΘSb = 0.003(0.006). 

3
to within 0.01%. The thickness of the top cell base and 

emitter were 2 μm and 0.1 μm, respectively. The base of 

the GaAs bottom cell was 2.5-μm-thick and doped with Zn 

at a level of ~2e17 cm-3. A Zn-doped Al0.25Ga0.25In0.5P 

layer was used as the BSF for both subcells. The tunnel 

junction was a Se-doped GaInP/C-doped AlGaAs heterojunction similar to that proposed by Bedair et al.[12] The 

cell was patterned with a concentrator grid. The total and 

active areas of the device were 0.116 cm2

 and 0.103 cm2, 

respectively. A summary of the 1-sun AM1.5G results is 

shown in Table 1. 

The Voc of this device is equal to that achieved by Takamoto et al.[6] However, because of the large busbar 

obscuration, 1-sun Jsc for this cell is correspondingly low. 

Conversely, the active area Jsc for the top and bottom 

cells indicates that with appropriate grid design, the efficiency of this cell should exceed 31%. 

Table 1 Device parameters for an Sb-doped GaInP/GaAs 

tandem cell grown on GaAs. 

Voc 

[V] 

FF Jsc, 

Total 

area 

[mAcm-2] 

Jtop, 

active 

area 

[mAcm-2] 

Jbottom, 

active 

area 

[mAcm-2] 

2.48 0.8

7 

12.4 14.8 14.9 

CONCLUSION 

The properties of Sb-doped GaInP are very intriquing 

and yield clues as the fundamental mechanism of ordering in GaInP. Antimony is also an effective way to increase the band gap and Voc of a GaInP solar cell. This 

has important implications for very high efficiency twoand three-junction solar cells for concentrator applications. 

ACKNOWLEDGEMENTS 

We thank M. Young, A. Duda, and A. Kibbler for assistance in the preparation and measurement of these devices, C.S. Jiang for the AFM work, and R. Reedy for the 

SIMS studies. This work was funded by the U.S. Department of Energy, under Contract No. DE-AC36-

99GO10337. 

REFERENCES 

[1] S. R. Kurtz, P. Faine, and J. M. Olson, "Modeling 

of two-junction, series-connected tandem solar 

cells using top-cell thickness as an adjustable 

parameter," Journal of Applied Physics, vol. 68, 

pp. 1890, 1990. 

[2] J. M. Olson, S. R. Kurtz, A. E. Kibbler, and P. 

Faine, "A 27.3% efficient Ga0.5In0.5P/GaAs tandem solar cell," Appl Phys Lett, vol. 56, pp. 623-

625, 1990. 

[3] J. M. Olson, S. R. Kurtz, and A. E. Kibbler, "A 

21.8% GaInP2/GaAs tandem solar cell," presented at 18th IEEE Photovoltaic Specialists 

Conference, 1988. 

[4] K. A. Bertness, S. R. Kurtz, D. J. Friedman, A. E. 

Kibbler, C. Kramer, and J. M. Olson, "29.5%-

Efficient GalnP/GaAs Tandem Solar Cells," Appl 

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[5] S. R. Kurtz, J. M. Olson, D. J. Arent, M. H. Bode, 

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on (511)B GaAs substrates," Appl. Phys. Lett., 

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1997. 

[7] S. R. Kurtz, J. M. Olson, D. J. Friedman, A. E. 

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[8] J. K. Shurtleff, R. T. Lee, C. M. Fetzer, and G. B. 

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[9] C. M. Fetzer, R. T. Lee, G. B. Stringfellow, X. Q. 

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[10] D. J. Friedman, J. G. Zhu, A. E. Kibbler, J. M. 

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Appl Phys Lett, vol. 63, pp. 1774-1776, 1993. 

[11] K. Muraki, S. Fukatsu, Y. Shiraki, and R. Ito, 

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Levels in InGaAs/GaAs Quantum Wells," Appl 

Phys Lett, vol. 61, pp. 557-559, 1992. 

[12] S. M. Bedair, J. C. Roberts, D. Jung, B. F. 

Moody, N. A. El-Masry, and T. Katsuyama, 

"Analysis of p+-AlGaAs/n+-InGaP Tunnel Junction for High Solar Concentration Cascade Solar 

Cells," presented at 28th IEEE Photovoltaic Specialists Conference, Anchorage, Alaska, 2000.

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