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

K.M. Jones, R.C. Reedy, and M.J. Romero 

Prepared for the 31st IEEE Photovoltaics Specialists 

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February 2005 • NREL/CP-520-37349 

Lattice-Matched GaNPAs-onSilicon Tandem Solar Cells 
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LATTICE-MATCHED GaNPAs-ON-SILICON TANDEM SOLAR CELLS 

J.F. Geisz, J.M. Olson, D.J. Friedman, K.M. Jones, R.C. Reedy, M.J. Romero 

National Renewable Energy Laboratory, Golden, CO 80401 USA 

ABSTRACTS 

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 twojunction tandem cell. We have demonstrated a monolithic

III-V-on-silicon tandem solar cell in which most of the III-V

layers are nearly lattice-matched to the silicon substrate.

The cell includes a 1.8 eV GaNPAs top cell, a GaP-based

tunnel junction (TJ), and a diffused silicon junction formed 

during the epitaxial growth of GaNP on the silicon

substrate. This tandem on silicon has a Voc of 1.53 V and 

an AM1.5G efficiency of 5.2% without any antireflection

coating. Low currents in the top cell are the primary 

limitation to higher efficiency at this point. 

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 twojunction cell [1]. Most of the previous work on such a

structure focused on III-V top cells that were latticemismatched 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]. This 

dislocation density decreases the electronic quality of the

top cell [3] 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 [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 highefficiency multijunction solar cells [7]. Such a cell would

have application in space power systems, as well as

terrestrial concentrators. In this paper, we present results

on the first lattice-matched GaNPAs-on-silicon tandem 

solar cells. 

TANDEM SOLAR CELL DESIGN 

We have fabricated the monolithic III-V-on-silicon 

tandem solar cell shown schematically in Fig. 1. The cell is 

functionally comprised of an n-on-p silicon bottom 

homojunction, a III-V tunnel junction, and a 1.8 eV n-i-p 

GaNPAs top junction. The epitaxial portions of the 

structure are composed mainly of GaNPAs and GaNP

layers that are lattice-matched to the single-crystal silicon 

substrate. A few thin layers were composed of latticemismatched GaP or GaAs for reasons to be discussed

below. 

Epitaxial III-V 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, and

dimethylhydrazine (DMH) sources. Prior to growth, the

silicon substrates were cleaned in an ammonia / hydrogen

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. 

By varying the lattice-matched composition of

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

eV can be achieved [7]. Nearly lattice-matched 

GaN0.04P0.86As0.1 layers with a bandgap of about 1.80 eV 

were used as the absorber layer in the top junction here. 

The GaP nucleation layer was grown without nitrogen 

because the DMH appears to react with the silicon surface

Fig. 1: Schematic of GaNPAs-on-silicon tandem solar cell 

1
Fig. 2: X-ray diffraction of GaNxP1-x grown on silicon taken in (004) reflection with corresponding TEM images. 

creating undesired silicon nitride regions which disrupt the

epitaxial nature of the growth. A thickness of 40 nm of

GaP on silicon is below the critical thickness. 

Using this nucleation scheme [8], high crystalline

quality was achieved only when the thicker GaNP layer

was closely lattice-matched to the silicon. Fig. 2 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. 

The electrical quality of GaNP(As) is highly dependent

on growth conditions. In particular, growth conditions that

minimize unintentional carbon and hydrogen 

contamination provide the highest carrier lifetimes [9]. 

Thus, the GaNPAs top junction was grown at 700°C and a

growth rate of 1 μm/h. Similar to the case of most 

GaInNAs material [10], GaNPAs grown to-date appears to 

have very short diffusion lengths, even when special care

is taken to minimize H and C contamination. Fig. 3 shows

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

increases dramatically with the depletion width. Thus, an 

n-i-p device employing field-aided collection was used to

maximize QE. Unlike MOCVD-grown GaInNAs material, 

intrinsic GaNPAs has relatively low carrier concentrations 

so that high QEs can be achieved in an n-i-p structure. 

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. The diffused junction 

formed in the p-type silicon during growth of the GaNP

layer was analyzed after selectively etching the III-V layers 

from the silicon using a 3 HCl : 1 HNO3 solution. Fig. 4 

shows the P concentration profile as measured by

Fig. 3: Spectral response of GaN0.02P0.98 grown on silicon

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

diffusion lengths. 

2
secondary ion-mass spectroscopy (SIMS), as well as the

net donor concentration profile as measured by

electrochemical capacitance-voltage (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. An annealed aluminum back contact formed a

back-surface field (BSF) in the bottom silicon junction. The 

aluminum was evaporated on the back after III-V growth,

then annealed 5 minutes at 700°C under H2. 

A good tunnel junction on silicon requires 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 [11]. P-type doping using Zn or C [9] in

GaNP is not well understood and appears problematic. Zn, 

though, is a common dopant in GaP, and requires low

temperatures for high incorporation, so a growth

temperature of 600°C was used for the p+

 side of the TJ. 

The slightly lattice-mismatched GaP layer in this tunnel

junction is likely to reduce the crystal perfection of the

whole structure if it is too thick, but a lattice-matched layer

of sufficiently high p-type doping for a TJ has not yet been

achieved. 

Fig. 4: Profile of donors and phosphorous atoms diffused

into silicon substrate during GaNP growth. 

A 70-nm-thick Se-doped GaAs contact layer was

used to make ohmic contact to electroplated gold at the

front. This GaAs layer was selectively etched between 

grids using an NH3OH : H2O2 : H2O solution. While we

have also achieved ohmic contacts to Se-doped GaP and

GaNP using annealed Pd/Ge/Pd, the required annealing 

seems to severely degrade the top junction. The high

lattice-mismatch between GaAs and silicon surely results

in significant defects in the GaAs layer, but sufficient

conduction through the layer is maintained. The extent of

these defects that propagate down into the GaNP(As) 

junction has not yet been characterized. 

RESULTS 

The current-voltage (IV) and quantum efficiency (QE)

performance of the GaNPAs-on-silicon tandem solar cell is

shown in Fig. 5. The QEs of the top and bottom junctions

were measured separately by light biasing. The IV current 

was calibrated using the integrated QE of the currentlimiting junction (top cell). The IV curves of the top and

bottom junctions are also shown separately in Fig. 5. 

These were measured in nearly identical devices except 

with inactive junctions. The bottom silicon junction was

simulated by doping all the epitaxial layers n-type. The top

junction and tunnel junction were simulated together by

growing the structure on an n-type silicon substrate. The 

QE of the single junctions agreed well with the light-bias 

measurements of the tandem cell. 

The tandem solar cell had a Voc of 1.53 V, fill factor of

54%, and Jsc of 6.3 mA/cm2

, resulting in an AM1.5G

efficiency of 5.2% without any antireflective coatings. The

filtered bottom junction had an internal QE of 90% below 

the band edge of the top cell, a fill factor of 67%, and Voc

of 536 mV. The AM1.5G Jsc of the bottom silicon junction

Fig. 5: (a) Light and dark (dashed) IV curves of tandem and single-junction cells. (b) Internal QE of tandem and singlejunction cells. Tandem results are light-biased to show both junctions.

3
CONCLUSION 

Fig. 6: CL image of top junction of tandem cell grown on

silicon. Some of the TDs are indicated by arrows. 

We have demonstrated the first lattice-matched III-V 

on silicon tandem solar cell. For such a tandem solar cell

to achieve efficiencies greater than current state-of-the-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 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. 

was 14.5 mA/cm2. The top junction had an internal QE of 

about 40%, a fill factor of 48% and a Voc of 1.09 V. The 

AM1.5G Jsc of the top GaNPAs junction was 5.7 mA/cm2

. 

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”, 

J. Appl. Phys. 68, 1990, pp. 1890. 

In a separate device, the GaP:Zn / GaNP:Se tunnel

junction used here was found to have a specific resistivity 

of about 1 Ω cm2

. This resistance may be acceptable for

one-sun operation, but is inadequate for concentrator

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

“Very Low Dislocation Density GaAs on Si Using 

Superlattices Grown by MOCVD”, J. Cryst. Growth 

107, 1991, pp. 479-482. 

Defects in the top cell grown on silicon were observed

using electron-beam-induced current (EBIC) and

cathodoluminescence (CL). The CL image shown in Fig. 6 

shows threading dislocations (TDs) at a density above

5x106

 cm-2 and misfit dislocations greater than 107 cm-2. In 

contrast, GaNPAs cells grown on GaP contained less than

105

 cm-2 dislocations. 

[3] C.L. Andre et al., “Impact of Threading Dislocations 

on both n/p and p/n Single Junction GaAs Cells 

Grown on Ge/SiGe/Si Substrates”, 29th IEEE PVSC, 

2002, pp. 1043-1046. 

[4] K.A. Bertness et al., “29.5%-Efficient GalnP/GaAs 

Tandem Solar Cells”, Appl. Phys. Lett. 65, 1994, pp. 

989-991. 

DISCUSSION 

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

“Effects of nitrogen on the band structure of GaNxP1-x

alloys”, Appl. Phys. Lett. 76, 2000, pp. 1267-1269. 

The top GaNPAs junction severely limits the current

of the tandem device at this point. The bottom silicon

junction, on the other hand, is quite respectable for its 

purpose here. To current match this top cell to the bottom 

Si cell, we need to increase the QE and/or reduce the

band gap of the top cell. 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. 

[6] H. Yonezu, “Control of structural defects in group 

III-V-N alloys grown on Si”, Semicond Sci Technol 17, 

2002, pp. 762-768. 

[7] J. Geisz, D.J. Friedman, and S. Kurtz, “GaNPAs Solar 

Cells Lattice-Matched to GaP”, 29th IEEE PVSC, 

2002, pp. 864-867. 

[8] J.F. Geisz et al., “Growth and Characterization of 

GaPNAs on Si”, Material Research Society 799, 

2003, pp. 27-31. 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. 

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

Metzger, “Unintentional carbon and hydrogen

incorporation in GaNP grown by MOCVD”, J. Cryst. 

Growth 259, 2003, pp. 223-231. 

[10] J.F. Geisz et al., “Photocurrent of 1 eV GaInNAs 

lattice-matched to GaAs”, J. Cryst. Growth 195, 1998, 

pp. 401-408. 

The tunnel junction remains a significant challenge. 

Both Se and Zn dopants tend to diffuse quite readily in

most III-V semiconductors. Further study of this tunnel 

junction is required to reliably fabricate GaNPAs-on-silicon

tandem cells. 

[11] K.M. Yu et al., “Nitrogen-induced increase of the 

maximum electron concentration in group III-N-V 

alloys”, Phys. Rev. B 61, 2000, pp. R13337-R13340. 

4
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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 a monolithic III-Von-silicon tandem solar cell in which most of the III-V layers are nearly lattice-matched to the silicon substrate. The 

cell includes a 1.8 eV GaNPAs top cell, a GaP-based tunnel junction (TJ), and a diffused silicon junction formed 

during the epitaxial growth of GaNP on the silicon substrate. This tandem on silicon has a Voc of 1.53 V and an 

AM1.5G efficiency of 5.2% without any antireflection coating. Low currents in the top cell are the primary limitation to 

higher efficiency at this point. 

15. SUBJECT TERMS

PV; latticed-matched; solar cells; silicon; tandem; two-junction device; III-V; tunnel junction (TJ); antireflection 

coating; 

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