Toward a Monolithic 

Lattice-Matched III-V on 

Silicon Tandem Solar Cell 

September 2004 • NREL/CP-520-35323 

J.F. Geisz, J.M. Olson, and D.J. Friedman 

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TOWARD A MONOLITHIC LATTICE-MATCHED III-V ON SILICON TANDEM SOLAR CELL 

John F. Geisz, Jerry M. Olson, and Daniel J. Friedman 

National Renewable Energy Laboratory 

1617 Cole Boulevard, Golden, CO 80401 USA 

ABSTRACT: A two-junction device consisting of a 1.7–eV GaNPAs junction on a 1.1–eV silicon junction has the 

theoretical potential to achieve nearly optimal efficiency for a two-junction tandem cell. We have demonstrated some 

of the key components toward realizing such a cell, including GaNPAs top cells grown on silicon substrates, GaPbased tunnel junctions grown on silicon substrates, and diffused silicon junctions formed during the epitaxial growth 

of GaNP on silicon. These components have required the development of techniques for the growth of high 

crystalline quality GaNPAs on silicon by metal-organic vapor-phase epitaxy. 

Keywords: Multijunction Solar Cell, III-V Semiconductors, c-Si 

1 INTRODUCTION 

III-V semiconductors grown on silicon substrates 

are very attractive for lower-cost, high-efficiency 

multijunction solar cells. A two-junction device 

consisting of a 1.7–eV junction on a 1.1–eV silicon 

junction has the theoretical potential to achieve nearly 

optimal efficiency for a two-junction cell [1]. Prior to this 

work, virtually all of the work on such a structure focused 

on III-V top cells that were lattice-mismatched to the 

silicon bottom cell. Under the best conditions, this leads 

to a threading dislocation density on the order of 106

 cm-2

in the III-V top cell [2]. Under most conditions, this 

dislocation density decreases the electronic quality of the 

top cell to the point that a lattice-matched tandem solar 

cell with a less optimal band-gap combination (e.g. 

GaInP/GaAs) is much more efficient [3,4]. 

GaNxP1-x-yAsy, hereafter GaNPAs, is a direct-gap [5] 

III-V alloy that can be grown lattice-matched to Si with 

very low structural defect densities [6]. We have 

proposed the use of lattice-matched GaNPAs on silicon 

for high-efficiency multijunction solar cells [7]. Such a 

cell would have application in space power systems, as 

well as terrestrial concentrators. In this paper, we present 

the progress of our efforts toward realizing such a cell. 

2 GROWTH OF GaNP(As) ON SILICON 

Epitaxial GaNxP1-x-yAsy and GaNxP1-x layers were 

grown on single-crystal Czochralski silicon substrates by 

atmospheric-pressure metal-organic chemical vapor 

phase epitaxy (MOVPE) using triethylgallium, phosphine 

(PH3), t-butylarsine (TBA), and dimethylhydrazine 

(DMH) sources. Prior to growth, the silicon substrates 

were cleaned in an ammonia/peroxide solution, rinsed in 

deionized water, and spun dry. The native silicon oxide 

was removed in situ by annealing at 1000°C under H2. A 

thin (<40 nm) GaP nucleation layer was grown under a 

high partial pressure of PH3 (~10 torr) while reducing the 

temperature to 800°C at 100°C/min. A 0.5 μm thick 

GaN0.02P0.98 buffer layer was then grown at 700°C at 1 

μm/h. 

Figure 1: X-ray diffraction of GaNxP1-x grown on silicon taken in (004) reflection with corresponding TEM images. 
Using this nucleation scheme [8], high crystalline 

quality was achieved only when the thicker GaNP layer 

was closely lattice-matched to the silicon. Figure 1 shows 

transmission electron microscope (TEM) images and Xray diffraction results of various compositions of GaNP 

layers. The lattice-matched GaN0.02P0.98 layer shows low 

defect density away from the interface and X-ray peak 

widths as low as 24 arcsec. In spite of an imperfect 

nucleation scheme, the defects apparent at the GaP/Si 

interface appear not to propagate into the epilayer during 

the growth of the lattice-matched composition. 

By varying the lattice-matched composition of 

GaNxP1-x-yAsy, direct bandgaps in the range of 1.5 to 2.0 

eV were achieved [7]. Figure 2 shows the optical 

absorption coefficent of a series of GaNPAs layers grown 

lattice-matched on GaP substrates. 

The electrical quality of GaNP(As) has been shown 

to be highly dependent on growth conditions. In 

particular, growth conditions that minimize unintentional 

carbon and hydrogen contamination provide the highest 

carrier lifetimes [9]. These conditions were used for 

optimal performance of the GaNPAs top cells. 

3 SILICON BOTTOM CELL 

During the epitaxial growth of GaNPAs on silicon at 

700°C, significant diffusion of the III-V atoms into the 

silicon occurs. When a 0.5-μm GaN0.02P0.98 layer is 

grown on a p-type, B-doped silicon (1-10 Ω-cm) 

substrate, an n-p homojunction is created in the silicon. 

But when the same layer is grown on an n-type, P-doped 

silicon (1-10 Ω-cm) substrate, no silicon homojunction is 

created. This is because a greater concentration of P than 

Ga diffuses into the silicon from the III-V epilayers. By 

selectively etching the III-V from the silicon using a 3 

HCl : 1 HNO3 solution, we were able to analyze the 

diffused junction formed in the p-type silicon during 

growth of the GaNP layer. Figure 3 shows the P 

concentration profile as measured by secondary ion-mass 

spectroscopy (SIMS), as well as the donor concentration 

profile as measured by electrochemical capacitancevoltage (CV) measurements. From these data, we deduce 

that the thickness of the diffused emitter in this cell is 

approximately 2000 Å. This thickness will vary with time 

and temperature of the III-V growth. 

We have fabricated a functioning silicon cell from 

this diffused silicon homojunction. The front side was 

contacted with a gold grid on a thin Se-doped GaAs 

contact layer grown on the 0.5-μm Se-doped GaN0.02P0.98

layer. An annealed aluminum back-contact formed a 

back-surface field. For some devices, the aluminum was 

evaporated on the back after III-V growth, then annealed 

5 minutes at 700°C under H2. For other devices, an 

Al/Mo bilayer was evaporated onto the back before III-V 

growth. The back-surface field was then formed during 

the high-temperature III-V growth. This technique 

eliminated the need for an extra annealing step and helps 

reduce post-growth diffusion in the structure. Figure 4 

shows the internal quantum efficiency (QE) and currentvoltage (IV) characteristics of this silicon cell. The cell 

containing only a 0.5-μm GaNP buffer has an internal 

quantum efficiency of 90% below the band edge of the 

GaNP, a fill factor of 71%, and Voc of 549 mV. The 

relatively thick emitter and defects at the III-V/Si 

interface (see Fig. 1) are likely to result in poor blue 

response of this junction; but this is not a major concern, 

because in the full tandem structure, the high energy light 

1015

1016

1017

1018

1019

1020

Concentration (cm-3)

0.00 0.05 0.10 0.15 0.20

Depth (μm)

 Donors by CV

 [P] by SIMS

 Acceptors in substrate

Figure 3: Profile of donors and phosphorous atoms diffused 

into silicon substrate during GaNP growth. 

-20

-10

0

10

20

Current (mA/cm2

)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Voltage (V)

1.0

0.8

0.6

0.4

0.2

0.0

Internal QE

1.0 1.5 2.0 2.5 3.0 3.5

Energy (eV)

Filter Al anneal 0.5 μm GaNP after growth

GaNPAs top cell during growth

Figure 4: Light and dark (dashed) IV curves of Si 

homojunction cells formed during III-V growth. The inset 

graph shows internal QE of the same cells. 

0.01

0.1

1

10

100

α(μm-1)

1.5 2.0 2.5 3.0 3.5 4.0

Energy (eV)

N As

(%) (%)

5 30

4 24

3 17

2 7

0 0

increasing N

Figure 2: Absorption coefficient of GaP and GaNPAs layers 

grown on GaP substrates as determined from transmission 

and spectral ellipsometry data. 
will be collected in a GaNPAs top cell, rather than the 

silicon cell. Here, the high-energy light is just filtered by 

the GaNP layers above the junction. The other Si cell 

shown contains a filter simulating a 1.8-eV GaNPAs cell. 

This GaNPAs filter is identical to the cells discussed in 

section 5 (see Fig. 7), but is doped entirely n-type. This 

Si device has an AM1.5G Jsc of 15 mA/cm2

. 

The red responses of these silicon cells are quite 

satisfactory for our purposes at this point. This can be 

improved in the future, if necessary, by using higher 

quality float-zone silicon substrates or optimizing the 

back-surface field. 

4 TUNNEL JUNCTION 

If the n-on-p silicon homojunction described above 

is to be used in a tandem cell, a p+-on-n+ highly 

conductive tunnel junction must be developed. This will 

require extremely high doping levels in GaP or GaNP 

materials near a sharp interface. Se doping in GaNP is 

very efficient and high electron concentrations can be 

achieved, similar to Se doping of GaNAs [10]. P-type 

doping using Zn or C [9] in GaNP is not as well 

understood. Zn is a common dopant in GaP but requires 

low temperatures for high incorporation. Unfortunately, 

both Se and Zn dopants tend to diffuse quite readily. 

Using an n-type silicon substrate, we are able to 

study tunnel junctions grown on silicon without forming 

a silicon junction. A Zn-doped GaP on Se-doped GaNP 

structure resulted in a fairly low-resistance (0.14 Ω cm2

) 

ohmic tunnel junction, but in this case, an annealed 

Pd/Zn/Pd contact was deposited directly onto the 

GaP:Zn. The diffusion of Zn from the metal contact most 

likely improved the tunnel junction, because an identical 

tunnel junction using unannealed Au / GaAs:Zn contacts 

was non-ohmic. The same tunnel junction was used in 

cells described in the following sections, but they did not 

function properly. Clearly, increased p-type doping is 

needed to form a functioning tunnel junction in a tandem 

structure. 

5 GaNPAs TOP CELL 

GaNPAs top cells were grown on GaP [7] and 

silicon substrates as discussed earlier. Ti/Pd/Al/Pd/Au 

was used as a back-contact to the n-type silicon 

substrates. Annealed Pd/Zn/Pd was used to contact the ptype GaP substrates. Gold grids were deposited onto a 

thin (70 nm) highly doped GaAs contacting layer on the 

front of the cell. The GaAs contact layer was then 

selectively etched from between the grid fingers. 

Similar to the case of most GaInNAs material [11], 

GaNPAs grown to-date appears to have very short 

diffusion lengths, even when special care is taken to 

minimize H and C contamination. Figure 5 shows how 

the spectral response of GaN0.02P0.98 grown on silicon 

increases dramatically with the depletion width. Thus, pi-n or n-i-p devices employing field-aided collection are 

used to maximize QE. Unlike the GaInNAs material, the 

GaNPAs can be easily grown with very low carrier 

concentrations to achieve thick intrinsic layers in these 

devices, and thus, high QEs. 

Because GaNP growth on p-type silicon always 

seems to result in a diffused homojunction, we must 

include a tunnel junction to study a single-junction n-i-p 

GaNPAs top cell grown on silicon. This is the same 

structure as the tandem shown in Fig. 7 except the silicon 

is n-type. We have previously studied n-i-p GaNPAs cells 

grown on p-type GaP [7], but these are grown to a 

slightly different composition to lattice-match to GaP, 

rather than silicon. We have also fabricated p-i-n 

GaNPAs cells on n-type silicon. Characteristic results of 

these different types of GaNPAs are shown in Fig. 6. 

P-i-n GaN0.02P0.98 cells grown on silicon with a band 

gap of 1.98 eV have achieved internal quantum 

efficiencies of 60%, fill factors of 60%, and Voc of 1.2 V. 

The n-i-p GaNPAs cells grown on GaP with a band gap 

of 1.82 eV have also achieved internal QEs of 60% and 

fill factors of 60%, but lower Voc of 1.0 V due to the 

lower band gap. Both of these Vocs relative to their band 

gaps are lower than we would expect for high-quality 

solar cell materials. The n-i-p GaNPAs cells grown on 

silicon gave similar QE results, but an apparent bucking 

junction from an inadequate tunnel junction results in a 

lower fill factor and increased series resistance. The 

tunnel junction used here was the same as the successful 

tunnel junction discussed in section 4, but without the Zn 

diffused from the metal contact. 

The largest AM1.5G Jsc that we have achieved in a 

III-V top cell on Si is about 6.4 mA/cm2

 using 1.8-eV 

GaNPAs. To current match this top cell to the bottom Si 

0.01

0.1

1

Spectral Response @ 2.5 eV

0.01 0.1 1

Depletion Width (μm)

 undoped GaNP

 GaNP:Te

 GaNP:Se

SR = 1- e-2.1w

Figure 5: Spectral response of GaN0.02P0.98 grown on 

silicon for a variety of n-type doping levels. The fit implies 

α~2.1 μm-1 consistant with Fig. 2. 

-10

-5

0

5

10

Current (mA/cm2

)

0.0 0.5 1.0 1.5 2.0

Voltage (V)

1.0

0.8

0.6

0.4

0.2

0.0

Internal QE

1.5 2.0 2.5 3.0 3.5

Energy (eV)

 n-i-p GaNPAs cell on GaP

 p-i-n GaNP cell on Si

 n-i-p GaNPAs cell + TJ on Si

Figure 6: IV curves of single-junction GaNP(As) cells. 

The inset graph shows the internal QE of the same cells. 
cell, we need to increase the QE and/or reduce the band 

gap. Reducing the band gap by increasing nitrogen and 

arsenic concentration tends to result in lower-quality 

material and also reduces the Jsc of the bottom cell. If a 

field-aided device is used, increasing the thickness while 

reducing the carrier concentration in the intrinsic layer 

may help increase the QE. 

 Increasing the diffusion length in these materials is 

the preferable method of increasing the QE. Growth 

conditions may be further optimized to reduce 

unintentional carbon and/or hydrogen impurities. 

Improved nucleation of GaP on silicon may also help by 

reducing the defect densities that are currently on the 

order of 107

 cm-2, as observed by electron-beam-induced 

current (EBIC) images. 

Another problem with the current GaNPAs top cells 

is an extremely high emitter sheet resistance. The 

GaNP(As) emitter materials grown so far have very low 

mobility and must be highly doped to achieve a sheet 

resistance of even 10 kΩ/sq. This loss mechanism 

becomes even more pronouced under concentration. 

6 TANDEM 

We put these components together into the tandem 

device shown in Fig. 7. Figure 8 shows the performance 

of this tandem cell, along with the performance of singlejunction cells grown in nearly identical structures. This 

tandem cell is not well current-matched; the top GaNPAs 

junction limits the current. As discussed above, this topcell current must be increased. It is clear that the tandem 

cell has a voltage boost over the single-junction top cell. 

The poor tunnel junction affects the tandem cell in the 

same way that it affects the single-junction top cell. 

7 CONCLUSION 

We have demonstrated the first lattice-matched IIIV on silicon tandem solar cell. For such a tandem solar 

cell to achieve efficiencies greater than current state-ofthe-art silicon cells and compete with GaInP/GaAs/Ge 

solar cells, the performance of the top GaNPAs junction 

and tunnel junction must be further improved. These 

improvements will require better control over nucleation 

of III-V on silicon substates, doping of GaP and 

GaNP(As) materials, and material quality of GaNP(As). 

ACKNOWLEDGEMENTS 

The authors would like to thank R. Reedy, K. Jones, 

M. Romero, A. Duda, C. Kramer, and M. Young for their 

contributions to this work. This research was funded by 

the United States Department of Energy under Contract 

No. DE-AC36-99GO10337.

REFERENCES 

[1] S. R. Kurtz, P. Faine, J. M. Olson, J. Appl. Phys. 68 

(1990) 1890. 

[2] T. Soga, H. Nishikawa, T. Jimbo, M. Umeno, J. 

Cryst. Growth 107 (1991) 479. 

[3] J. M. Olson, T. Gessert,M. M. Al-Jassim, 

Proceedings 18th IEEE Photovoltaic Specialists 

Conference, (1985) 552. 

[4] J. M. Olson, S. R. Kurtz, A. E. Kibbler, P. Faine, 

Appl. Phys. Lett. 56 (1990) 623. 

[5] H. P. Xin, C. W. Tu, Y. Zhang, A. Mascarenhas, 

Appl. Phys. Lett. 76 (2000) 1267. 

[6] H. Yonezu, Semicond. Sci. Tech. 17 (2002) 762. 

[7] J. Geisz, D. J. Friedman, S. Kurtz, Proceedings 29th 

IEEE Photovoltaic Specialists Conference, New 

Orleans, (2002) 864. 

[8] J. F. Geisz, J. M. Olson, W. E. McMahon, T. 

Hannappel, et al., Proceedings Material Research 

Society, Boston, MA, Vol. 799 (2003) 27. 

[9] J. F. Geisz, R. C. Reedy, B. M. Keyes,W. K. 

Metzger, J. Cryst. Growth 259 (2003) 223. 

[10] K. M. Yu, W. Walukiewicz, W. Shan, J. W. Ager, J. 

Wu, E. E. Haller, J. F. Geisz, D. J. Friedman, J. M. 

Olson, Phys. Rev. B 61 (2000) R13337. 

[11] J. F. Geisz, D. J. Friedman, J. M. Olson, S. R. Kurtz, 

B. M. Keyes, J. Cryst. Growth 195 (1998) 401. 

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0

10

20

Current (mA/cm2

)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Voltage (V)

1.0

0.8

0.6

0.4

0.2

0.0

IQE

1.0 1.5 2.0 2.5 3.0 3.5

Energy (eV)

Si

GaNPAs

 GaNPAs tandem

 GaNPAs + TJ with inactive bottom cell

 Si cell with inactive top cell

Figure 8: Light and dark (dashed) IV curves of tandem and 

single-junction cells. The inset shows internal QE. 

Al metal contact

diffused Si:Al back-surface-field

p-type Si:B substrate

diffused n-type Si emitter

 40 nm GaP nucleation layer

0.5 μm GaN0.02P0.98:Se buffer layer

0.2 μm n+ GaN0.02P0.98:Se

0.1 μm p+ GaP:Zn

0.2 μm p-type GaN0.02P0.98:Zn

1.0 μm intrinsic undoped GaNPAs

0.1 μm n-type GaN0.02P0.98:Se

70 nm N+ GaAs:Se

Au grid

n-i-p

top cell

tunnel

junction

silicon

bottom

cell

Figure 7: Schematic of full tandem structure. 
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Toward a Monolithic Lattice-Matched III-V on Silicon Tandem Solar 

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14. ABSTRACT (Maximum 200 Words)

A two-junction device consisting of a 1.7-eV GaNPAs junction on a 1.1-eV silicon junction has the theoretical 

potential to achieve nearly optimal efficiency for a two-junction tandem cell. We have demonstrated some of the 

key components toward realizing such a cell, including GaNPAs top cells grown on silicon substrates, GaP-based 

tunnel junctions grown on silicon substrates, and diffused silicon junctions formed during the epitaxial growth of 

GaNP on silicon. These components have required the development of techniques for the growth of high 

crystalline quality GaNPAs on silicon by metal-organic vapor-phase epitaxy. 

15. SUBJECT TERMS

PV; multijunction solar cell, III-V semiconductors, c-Si. 

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