Monolithic Integration of Diluted-Nitride III–VN Compounds on Silicon Substrates

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Monolithic Integration of Diluted-Nitride III–V-N
Compounds on Silicon Substrates: Toward the III–V/Si
Concentrated Photovoltaics
Olivier Durand, Samy Almosni, Yanping Wang, C. Cornet, A. L´etoublon,
C´edric Robert, Christophe Levallois, L. Pedesseau, Alain Rolland, J. Even, et
al.
To cite this version:
Olivier Durand, Samy Almosni, Yanping Wang, C. Cornet, A. L´etoublon, et al.. Monolithic
Integration of Diluted-Nitride III–V-N Compounds on Silicon Substrates: Toward the III–V/Si
Concentrated Photovoltaics. Energy Harvesting and Systems, DE GRUYTER, 2014, 1 (3-4),
pp.147-156. <10.1515/ehs-2014-0008>. <hal-01166477>
HAL Id: hal-01166477
https://hal.archives-ouvertes.fr/hal-01166477
Submitted on 22 Jun 2015
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EnergyHarvestingandSystems2014;aop
Special Issue Article
O. Durand*, S. Almosni, Y. Ping Wang, C. Cornet, A. Létoublon, C. Robert, C. Levallois,
L. Pedesseau, A. Rolland, J. Even, J.M. Jancu, N. Bertru, A. Le Corre, F. Mandorlo, M. Lemiti,
P. Rale, L. Lombez, J.-F. Guillemoles, S. Laribi, A. Ponchet and J. Stodolna
–
Monolithic Integration of Diluted-Nitride III V-N
Compounds on Silicon Substrates: Toward the
–
III V/Si Concentrated Photovoltaics
Abstract: GaAsPN semiconductors are promising material most of the microtwins and a bi-stepped Si buffer can be
for the development of high-efficiency tandem solar cells grown,suitabletoreducetheanti-phasedomainsdensity.
on silicon substrates. GaAsPN diluted-nitride alloy is stu- We also review our recent progress in materials develop-
died as the top-junction material due to its perfect lattice mentoftheGaAsPNalloyandourrecentstudiesofallthe
matching with the Si substrate and its ideal bandgap differentbuildingblockstowardthedevelopmentofaPIN
energy allowing a perfect current matching with the Si solar cell. GaAsPN alloy with energy bandgap around
bottom cell. The GaP/Si interface is also studied in order 1.8 eV, lattice matched with the Si substrate, has been
toobtaindefect-freeGaP/Sipseudo-substratessuitablefor achieved. This alloy displays efficient photoluminescence
the subsequent GaAsPN top junctions growth. Result atroom temperature and goodlight absorption.An early-
shows that a double-step growth procedure suppresses stageGaAsPNPINsolarcellprototypehasbeengrownon
aGaP(001)substrate.Theexternalquantumefficiencyand
*Correspondingauthor:O.Durand,UMRFOTON,CNRS,INSAde theI–Vcurveshowthatcarriershavebeenextractedfrom
Rennes,F-35708Rennes,France,E-mail:
the GaAsPN alloy absorber, with an open-circuit voltage
[email protected]
above 1 eV, however a low short-circuit current density
S.Almosni:E-mail:[email protected],Y.PingWang:
E-mail:[email protected],C.Cornet:E-mail: obtained suggests that GaAsPN structural properties need
[email protected],A.Létoublon:E-mail: further optimization. Considering all the pathways for
[email protected],UMRFOTON,CNRS,INSAde improvement, the 2.25% efficiency and IQE around 35%
Rennes,F-35708Rennes,France
obtained under AM1.5G is however promising, therefore
C.Robert,TyndallNationalInstitute,Cork,Ireland,E-mail:
validating our approach for obtaining a lattice-matched
[email protected]
dual-junction solar cellonsilicon substrate.
C.Levallois:E-mail:[email protected],
L.Pedesseau:E-mail:[email protected], Keywords: III–V semiconductors, dilute nitride, molecu-
A.Rolland:E-mail:[email protected],J.Even:
lar beam epitaxy, multijunction solar cells, silicon
E-mail:[email protected],J.M.Jancu:
E-mail:[email protected],N.Bertru:
E-mail:[email protected],A.LeCorre:
DOI10.1515/ehs-2014-0008
E-mail:[email protected],UMRFOTON,CNRS,INSAde
Rennes,F-35708Rennes,France
F.Mandorlo:E-mail:[email protected],M.Lemiti:
E-mail:[email protected],INL,INSAdeLyon,Bâtiment
BlaisePascal,7AvenueJeanCapelle,69621VilleurbanneCedex, Introduction
France
P.Rale:E-mail:[email protected],L.Lombez:
To date, the highest efficiency conversions have been
E-mail:[email protected],J.-F.Guillemoles:
E-mail:[email protected],S.Laribi:E-mail: reached by using III–V monocrystalline multijunction
[email protected],InstitutdeRechercheetDéveloppementsur solar cell (MJSC) under concentrated sunlight. Spire
l’EnergiePhotovoltaïque(IRDEP),UMR7174-CNRS-EDF-ENSCP,EDF Semiconductors have pushed solar cell record to 42.3%,
R&D,6quaiWatier,78401ChatouCedex,France
in AM1.5D conditions, for terrestrial applications, with a
A.Ponchet:E-mail:[email protected],J.Stodolna:
metamorphic (lattice-mismatched) GaInP/GaAs/GaInAs
E-mail:[email protected],CEMES-CNRS,Universitéde
triple junction under solar concentration of 406 suns,
Toulouse,29rueJ.MarvigBP94347,31055ToulouseCedex4,
France and using a lightly doped n-type GaAs wafer. Moreover,
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2 O.Durandetal.:Diluted-NitrideIII–V-NCompoundsonSiliconSubstrates
aIII–Vtriplejunctioncoherentlygrown(latticematched) Monolithic growth of diluted-nitride
onto GaAs substrate has been performed by Solar III–V-N compounds on silicon
Junction. This solar cell has shown a 44% efficiency
substrates
under 947 suns (AM1.5D spectra) (Derkacs et al. 2012)
and contains a highly rewarded 1eV GaInAsNSb
diluted-nitride junction. However, maintaining the The aim of our work is to provide low-cost and high-
GaAs, or Ge, substrates to build these high-efficiency efficiency tandem cells grown on c-Si substrates, with
III–V solar cells, undoubtedly incurs a substantial cost merging both the monocrystalline Si approach and the
associated with such substrates. To realize the strategic high-efficiency monocrystalline multijunction approach
challengeofcostof0.25–0.5Euro/Wp,wehavechosento based on III–V materials. We propose to demonstrate the
usetheabundantlyavailableonearth,andthereforelow- proof-of-concept for a monolithic integration of high-effi-
cost,siliconmaterialasasubstrate.Indeed,atruemono- ciency multijunction CPV device on a low-cost monocrys-
lithic integration of the III–V compound semiconductor tallinesiliconsubstrateuponwhichaIII–Vlattice-matched
heterostructures with silicon is receiving great interest material is grown using molecular beam epitaxy (MBE).
since it will enable simultaneous both high-efficiency Among binary III–V materials, GaP presents the closest
and low-cost production. Moreover, this approach bene- lattice constant to Si (0.37% at 300 K). However, GaP is
fits from the high market throughput capabilities of the not useful due to its large (2.2eV) and indirect bandgap
silicon industry that is superior to any other material. whileadirectbandgapIII–Vcompoundishighlyrequired
Atandemsolarcell,madeofa1.7 eVIII–Vtopanda to obtain a thin-film absorber. Yet, nitrogen incorporation
1.1eVcrystallinesilicon(c-Si)bottomcell,would theore- in GaP not only enables for a perfect lattice matching on
tically reach an efficiency of 37%, under an AM 1.5G silicon but, with an appropriate adjustment of the N con-
(Kurtz, Faine, and Olson 1990). However, efficiency of tent(afewpercent),alsoenablesGaPNtodevelopadirect
MJSC is very sensitive to the structural defects such as bandgap character due to a strong reduction of the band-
misfit dislocations, appearing during metamorphic gapenergy,versustheNcontent,whichtherefore,canact
growth, since they dramatically reduce the carrier life- as efficient light absorber (Shan et al. 2000; Xin et al.
time, and thus the current extraction, and therefore 2000). Finally, our approach consists of the growth of
reduction of the solar cell performance. Therefore, com- GaAsPN quaternary compounds, lattice matched on Si,
bination of both the III–V and Si technologies through a whichbenefitsfromAsincorporationtoenhancethedirect
perfect lattice-matched epitaxial PV structure on silicon bandgap character (Chamings et al. 2008; Robert et al.
substrate would allow increasing significantly the effi- 2011), while loweringthe bandgap down to1.7eV.
ciency, as well as reducing the overall cost of the PV Lattice-matched layers and slightly tilted substrates
multijunction cell. areusedtoovercometwoofthemaindifficultiesfacedby
Considering a perfect lattice matching, the first chal- thegrowthofIII–Vmaterialsonsiliconsubstrates:misfit
lenge for the III–V/Si solar cells is the control and sup- dislocations and anti-phase lattice defects, in order to
pressionofthecrystallinedefectsthatarisemainlyatthe obtain defect-free III–V materialsand to get large minor-
III–V/Si interface due to mismatch of local chemistry. ity carrier diffusion lengths for the PV applications
This leads to anti-phase domain (APD) characteristic of (Furukawa et al. 2002; Momose et al. 2001; Volz et al.
the growth of a polar material on a non-polar one, and 2011; Létoublon et al. 2011; Grassman et al. 2009). The
the microtwins (MT). Indeed, a very high level of struc- tandemGaAsPN/Sidouble-junctionsolarcellwillbeelec-
turalqualityisrequiredsothattheefficiencycouldreach trically connected with a tunnel junction (TJ), either Si-
at least 80% of the theoretical value. based or III–V based or hybrid (III–V/Si), depending on
Thispaperdescribes ourapproach inordertoobtain boththemodelingresultsandthestructuralpropertiesof
alattice-matchedtandemcellcomposedofa1.7 eVIII–V the III–V compound. Therefore, the final structure will
top and a 1.1eV c-Si bottom cell and the key material include a Si pn junction (1.1eV gap) grown on Si sub-
issues that until recently have prevented progress in this strate, followed by the TJ and the growth of top GaAsPN
field. First promising results are reported on the strategy p-i-nsubcell(1.7–1.8eV).Moreover,priortoanyGaAsPN
to analyze and control the structural defects for III–V growthontothesilicon,aGaPthinlayerhastobegrown
junction grown on GaP substrate, displaying a lattice by MBE to prevent the formation of structural defects.
constant close to the silicon. Finally, first very prelimin- Thestate-of-the-artofthisapproachisepitomizedby
ary results obtained on a p-i-n structure grown onto a theNationalRenewableEnergyLaboratory(NREL)which
GaP(001) substrate are reported. hasdemonstratedaMOCVD-growntandemsolarcellona
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O.Durandetal.:Diluted-NitrideIII–V-NCompoundsonSiliconSubstrates 3
Sisubstrate.Thelattice-matchedn-i-psolarcellconsisted to operate infar-from-equilibrium thermodynamic condi-
of a GaP N :Se/GaAsPN: 1.8eV/GaP N :Zn top tions. Finally, residual doping in MBE is very low which
0.98 0.02 0.98 0.02
cellanda1.1 eVdiffusedSibottomcell,withaGaP-based is mandatory for the development of diluted-nitride-
TJ (Geisz et al. 2005). The growth was done on Si sub- based high-efficiency solar cells. Both growth chambers
strates utilizing a miscut of 2° from the (001) orientated are linked by a UHV tunnel so that no surface contam-
toward (111). Though the dilute III–V nitride addresses ination can occur between both Si and III–V growth
the lattice mismatch, the efficiency achieved was about steps.
5.2%underAM1.5G(Geiszetal.2005)(withoutanyanti- ModelingisperformedusingtheATLASdevicesimu-
reflection coating), far from the theoretical maximum lator from Silvaco, including the TJ interconnection
(Kurtz et al. 1990). This low efficiency was likely due to between the top and the bottom subcells.
intrinsicdefectsintheGaAsPNorintheGaP/Siinterface,
which limit the minority carrier diffusion length in the
absorber inducing a low quantum efficiency in the Simulations and cell design
top cell.
Optimal composition of the GaAsPN
Experimental compound
Synchrotron X-ray diffraction has been performed on the The optimal composition of the GaAsPN has been thor-
D2AM (BM2) CRG beam line at European Synchrotron oughlydeterminedsinceitisnecessarytomatchthe1.7eV
Radiation Facility (ESRF). A large hybrid pixels detector bandgap requirement (or close to) while maintaining a
[14]hasbeenusedtoallowamuchfasterdatacollection perfect lattice matching between the III–V compound
than with traditional point detector. This is a crucial and the Si. To this end, the bandgaps of GaAsPN have
point for the acquisition of diffuse signal from crystal been calculated using sp3s*s tight-binding calculations
N
defects. The energy has been set at 16 keV in order to adapted for the description of nitrogen incorporation in a
limittheair-scatteringcontribution.Thebeamsizeatthe diluted regime (Chamings et al. 2008; Robert et al. 2011),
sample was about 0.2(cid:1)0.3 mm2 in vertical and horizon- asshowninFigure1wherethesolidblacklinerepresents
tal directions. the nitrogen composition, as a function of As content,
TEMhas beenperformedinplan-viewandcross-sec- which fulfills the requirement of GaAsPN lattice matched
tiongeometries([1–10]zoneaxis).Thinplateshavebeen tothesiliconsubstrate.Inthesamegraph,reddottedline
prepared for these two samples by mechanical polishing gives the calculated bandgap of the corresponding alloy.
andionthinning.Theexperimenthasbeenperformedon Forinstance,theGaAsPNalloywithanAscontentequalto
a Philips CM30 in high-resolution and conventional
modes, with an acceleration voltage of 300 kV.
An ultra-high vacuum (UHV) CVD technique has
beenchosen,fortheSigrowth.Boththinandthicklayers
with well-controlled doping levels can be realized with
this growth chamber (Quinci et al. 2013). Moreover, due
to the use of hydrogenated molecules(SiH , GeH …),the
4 4
presence of hydrogen-terminated surfaces increases
themobilityofspeciesatthegrowthfront,whichpermits
low-temperature operation (500–850°C, i.e. compatible
with GaP epilayers for Si/III–V overgrowth). Lastly,
the organization of atomic steps is easier to control as
compared to the H -free MBE growth. Monodomain
2
(bi-stepped) silicon surface is crucial for subsequent
GaP overgrowth avoiding the formation of APDs.
Concerning the group III–V compounds, we use a
Figure1 EvolutionofthenitrogencontentintheGaAsPNalloysas
MBE chamber which allows to grow samples at low tem-
afunctionofAsforanexactlatticematching(blackline).Bandgap
perature suitable for the diluted-nitrides growth. MBE energy for the corresponding GaAsPN alloys, determined with
providesaprecisecontroloftheinterface,thepossibility sp3d5s*s tight-bindingcalculations(redline)
N
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4 O.Durandetal.:Diluted-NitrideIII–V-NCompoundsonSiliconSubstrates
16% would need 5.5% of N to be lattice matched with Si, structural quality, and the dopant diffusion properties
giving an alloy with an energy bandgap of 1.63eV. between both III–V and Si materials.
According to this graph, and following the black dotted
line,therequirementfora1.7eVbandgapisfulfilledwith
a composition of lattice-matched GaAsxPyN(1–x–y) equal to Overall tandem cell structure
GaAs P N .
0.122 0.83 0.048
However, this lattice-matching condition can be The minority carrier diffusion length in diluted-nitride
slightly overcome without any plastic relaxation of the materials is known to be low, likely due to point defects
strain provided that the compound thickness is below a (Jussilaetal.2014;Dagnelundetal.2008),spatiallocaliza-
critical thickness. Following the Matthews and Blakeslee tion of carriers due to nitrogen fluctuating composition
formalism (1974) we have determined the critical thick- (Jandieri et al. 2012) or unintentional incorporation of
ness for the GaAsPN alloy grown onto Si substrate, impurities, requiring the use of the collection of field-
assuming a 1-µm threshold on critical thickness, and a aided carriers. Therefore, a thick intrinsic layer is thus
maximum of 4% nitrogen (allowing to keep reasonable used in the current top junction, leading to a PIN top
crystalline quality), the As content is equal to 9% subcell junction. The design of the top GaP/GaAsPN/GaN
(Almosni et al. 2013). According to tight-binding calcula- p-i-n junction (Rolland et al. 2014) has been theoretically
tions, this would give a 1.81 eV bandgap for the studied using Silvaco-ATLAS with carrier mobilities and
GaAs P N alloy, which is close to the expected lifetime values extracted from the literature (Geisz and
0.09 0.87 0.04
1.7 eV energy bandgap. Therefore, a targeted 1-μm-thick Friedman 2002; Fahy and O’Reilly 2003), as a function of
absorber 1.7–1.8eV bandgap GaAsPN layer can be easily theGaAsPNabsorberthickness.Theabsorptioncoefficient
grown on silicon substrate, in a pseudomorphic way, value used in the simulation has been extracted from
according to the critical thickness calculation. experiments, as shown in the following. As shown in
Rolland et al. (2014), an optimal top cell efficiency has
been found equal to 9.42% for a thickness about 1µm.
Tunnel junction This is encouraging since the low carrier mobility and
short carrier lifetime have been used in the model, while
The main approach to connect electrically two cells in a improvement of the structural quality of the GaP-based
MJSC is to use a two-terminal structure where a TJ, or diluted-nitride materials is an ongoing research topic to
Esakidiode(Esaki1958),isplacedbetweenthetwocells, optimizethematerialqualityandreducethedefectdensity.
offering a low resistance due to its high doping
((cid:3)1.1019 cm−3) and good optical transparency due to its
lowthickness((cid:3)50nm).Toobtainoptimalperformances,
Main experimental results and
leading to a good voltage addition from the stacked sub-
cells, abrupt dopants profile should be obtained, and discussion
thus diffusion of dopants in the TJ should be minimized,
andtheTJstructurehastobecompatiblewiththeoverall
Structural defect analyses of the GaP/Si
growthconditions.Inparticular,usingmodel,twodiffer-
entTJshavebeeninvestigatedandfoundsuitableforthe interface
1.7 eV/1.1eVtargetedtandemcellstructure:ahybridGaP
(nþ)/Si(pþ)TJandaSi(nþ)/Si(pþ)one,consideringa A diluted-nitride compound cannot be grown directly
n-type Si absorber in the bottom junction. Considering onto the Si substrate, and it is therefore necessary to
uniform doping of 5(cid:1)1019 cm−3 on both junction sides obtain GaP/Si defect-free pseudo-substrates, consisting
(reachable experimentally according to Hall measure- of a thin GaP layer pseudomorphically grown onto Si,
ments on doped GaP and Si layers), theoretical current suitable for the subsequent GaAsPN growth. This part
densities as large as 5.5(cid:1)102mA/cm2 in the GaP(nþ)/Si describes the key material issues which have prevented
(pþ)TJcaseand4.4(cid:1)105mA/cm2fortheSi(nþ)/Si(pþ) until recently the progress in the III–V/Si growth, some
one have been calculated, suitable for the tandem cell analytical development we have used to analyze the
application (Rolland et al. 2014). Obviously, the final structural defects. Finally, some possible routes are pro-
choice between both will be depending on the actual posed to get rid of the defects.
experimental structural properties attained for each TJ, Perfect coherent growth of thin GaP layers onto sili-
in particular the maximum doping level, the material consubstrateshasalreadybeenreported(NguyenThanh
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O.Durandetal.:Diluted-NitrideIII–V-NCompoundsonSiliconSubstrates 5
etal.2012),usingbothhigh-resolutiontransmissionelec- To obtain a bi-stepped Si surface, we have studied
tronmicroscopy(HRTEM) andXRDexperiments,without the Si homoepitaxy on substrate having different misor-
anydislocationmisfit.However,twoothertypesofstruc- ientationsalongthe[110]direction(0°,1°,2°,4°,and6°),
turaldefectsneedtobeeliminated,theMTsandtheanti- with the same growth conditions. The Si buffer layer
phase boundaries (APBs) separating APDs. surfacequalitywassteadilyevaluatedthroughtheatomic
XRDanalysesareperformedontheD2AMBM2beam- surface reconstruction using in-situ Reflection High-
line at the ESRF in Grenoble, France. Reciprocal space Energy Electron Diffraction (RHEED) system and post-
mappings (RSMs) around the GaP(002) Bragg reflection growth Atomic Force Microscopy (AFM) analysis. As a
have been recorded using a two-dimensional large area main result, the silicon growth on 6°-off misorientated
hybridpixeldetector(XPADthirdgeneration)(Beraretal. Si substrates is found to provide the smoother silicon
2009). The RSMs reveal streaks characteristic of the pre- surface, while displaying bi-steps suitable for avoiding
senceofMTs,whicharesmalldomainsofrotatedcrystal any APD formation at the early stages of the growth
inside the main GaP phase (Nguyen Thanh et al. 2013). (Quinci et al. 2013). This is a promising result for the
According to the XRD analyses, they are totally sup- suppression of APD at the GaP/Si interface.
pressed at high-temperature growth (around 580°C) as
showninFigure2.However,surfaceroughnessisdrama-
tically high at this growth temperature so that growth Ohmic contacts on GaP
improvement is necessary to obtain flat surface without
anyMTs.Tothisend,wearecurrentlyworkingondiffer- Since electrical carriers should be collected efficiently,
ent growth recipes, involving a two-step growth strategy ohmic contacts constitute an important part of the PV
(with different growth temperatures) with different device. Low-resistance ohmic contacts are necessary in
growth modes including migration-enhanced epitaxy ordertolowerasmuchaspossibleanyadditionaleffects
(MEE), which avoid a 3D GaP-nucleation growth, and on the device. Obtaining a good ohmic contact requires
MBE. Indeed, a 10-nm-thin GaP layer MEE made at low the use of an appropriate contact alloy which can be
temperature followed by a 35-nm-thick GaP layer MBE difficult for a p-type large bandgap material such as
grown at higher temperature already showed very pro- GaP, for instance. The main properties required for
mising results with no evidence of any MTs by XRD ohmic contacts are linear, symmetrical I–V characteristic
experiments and displayed a smaller surface roughness. and highly reliable and reproducible stable behavior,
Details of these results and the growth procedure will be which imposes an annealing technological procedure.
published in a forthcoming paper. For n-type GaP substrate, we have chosen to use (Ni/
TheappearanceofAPDsisalsoakeyconcernofany Au/Ge) contact structure, with an optimized annealing
growthofapolarmaterial,suchasGaP,ontoanon-polar temperature at 530°C, based on the previous results
material, such as silicon. APBs are interfaces between described by Peternai et al. (2003). Ohmic contact with
two domains corresponding to each other through the a symmetrical I–V behavior has been obtained with a
exchange of Ga and P atoms. In order to achieve an very good reproducibility of the mechanical stability.
APD-free GaP layer, the obtaining of a bi-stepped Si sur- Moreover, a low contact resistivity of 10−5Ωcm2 has
faceisrequired,insteadofsingle-stepped,orself-annihi- been obtained on n-GaP substrate (n¼1(cid:1)1018 cm−3),
lation of all APDs must be guaranteed (Volz et al. 2011). which validates the electrical contact. As far as the
APD characterization has been performed using X-ray p-type GaP is concerned, based on the work reported by
and TEM techniques (Nguyen Thanh et al. 2012). Baojun Enke, and Fujia (1997) and Zhang et al. (1997)
Figure2 XRDRCM(performedattheESRFsynchrotron)aroundthe(002)reflectionon20-nm-thickGaP/Sisamplesgrownat(left)350°C,
(middle)450°C,and(right)580°C.ThetwostreaksunderneaththeGaP(002)reflectionarecharacteristicsoftheMT.TheRSMsshowthe
reductionoftheMT–inducedstreakswiththegrowthtemperatureincrease
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6 O.Durandetal.:Diluted-NitrideIII–V-NCompoundsonSiliconSubstrates
another multimetal film made of Pd/Zn/Pd stacking has
105
been employed, with a higher annealing temperature at
550°C. As in the n-type doping case, suitable ohmic con-
tact characteristics have been obtained with, again, a nit) 104
u
very low-specific contact resistivity of 10−5 Ωcm2 on p- b.
GaP substrate (p¼1(cid:1)1018 cm−3). y (ar 103
sit
n
nte 102
Preliminary results on heterostructures D i
R
grown on GaP(001) X 101
In order to obtain a reproducible incorporation of nitro- 100 68.4 68.6 68.8 69.0 69.2 69.4
gen and to improve the optoelectronic properties of our 2θ (°)
material, a two-step process has been used. This
Figure3 XRDmeasurementofa100-nmGaAsPNlayeronGaP(001)
approach has been developed by Harris et al. (2007) on
with a 10-nm GaP cap layer. The XRD pattern has been recorded
the InGaAsN(Sb) material. It consists in growing the
aroundtheGaP(004)Braggreflection
GaAsPN diluted-nitride alloy at low growth temperature
(480°C), which minimizes the point defects generation
and any small phase separation (Harris et al. 2007).
Annealed
Then, an annealing step (800°C, 1min) suppresses 100 Not annealed
someoftheremainingpointdefectsusuallyencountered
intheGaPN-basedalloys(Jussilaetal.2014),leadingtoa nit)
dramatic improvement of the optical properties (Aho et b. u
al. 2013, 2014; Harris et al. 2007). Following this ar
y ( 10–1
approach, 100-nm-thick GaAsPN layers capped with GaP sit
n
(10 nm) have been grown by MBE on GaP(001) substrate e
nt
in order to evaluate the diluted-nitride material’s struc- L i
P
tural and optical qualities. The composition has been
10–2
estimated by cross-modeling results obtained by tight-
binding calculation of the GaAsPN energy bandgap as a
function of its composition with experimental XRD and 1.4 1.6 1.8 2.0 2.2
PLresultsthatgiveusaccesstothelatticeparameterand Energy (eV)
the emission energy, respectively. A GaAs P N
0.085 0.90 0.015 Figure 4 RT PL measurements of a 100-nm GaAsPN layer on GaP
compound lattice matched with the GaP substrate and (001) with a 10-nm GaP cap layer, before (blue) and after (red) an
displayinga1.78 eVenergyphotoluminescence(PL)peak annealingstep(800°C,1 min.)
position has been obtained. The lattice matching was
verified through X-ray diffraction experiments, and PL
at room temperature close to the targeted 1.7 eV energy enhancement of the PL intensity with a reduction of the
has been obtained (Figure 3). The thickness fringes (also FWHM from 264 to 190 meV. The low energy tail is the
called Pendellösung fringes thickness) that can be seen characteristic ofthe presence of localdensity of states in
ontheXRDdiagrampresentedinFigure3indicateahigh the bandgap, typical of N-induced alloy fluctuation and
crystalline quality along the overall sample thickness. point defects (Zhang et al. 2000). Moreover, the GaAsPN
The annealing step has little effect on the XRD diagram measuredabsorptionspectrumisclosertoadirectband-
figures (not shown here), meaning that the lattice con- gapmaterialthananindirectbandgapone(Figure5).The
stant remains the same, but has a strong impact on the optical properties enhancement upon annealing is well
optical properties asshown in Figure 4,likely dueto the knownintheInGaAsN(Sb)systemgrownatlowtempera-
nitrogencontenthomogenizationandthehealthofsome ture which reinforces the validity of the two-stepprocess
point defects not reachable through XRD experiments approach for the GaAsPN growth. However, after these
Aho et al. 2013, 2014; Harris et al. 2007). Indeed, the preliminary, while promising, results, thorough growth
annealed sample presents a blue-shift of the PL peak and annealing studies are necessary in order to fully
energy from 1.78 to 1.9 eV, while displaying a 50 times optimizetheopticalpropertiesoftheGaAsPNcompound.
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O.Durandetal.:Diluted-NitrideIII–V-NCompoundsonSiliconSubstrates 7
The first structure consists of a 300-nm n-doped (Si
doped, 1(cid:1)1018 cm−3) GaP bottom layer, subsequently of
–1m) 100 a 1-µm-thick GaAsPN absorber followed by a 250-nm p-
µ
nt ( doped (Be doped, 1(cid:1)1018 cm−3) GaP window layer. The
e
ci ohmiccontactsonthefrontandbacksurfaceshavebeen
effi described in section “Ohmic contacts on GaP”. The front
o
c
n contact consists of a 5(cid:1)5 mm2 grid with 10-µm large
ptio 10–1 GaAsPN fingers each spaced by 300µm. The structure has been
r GaAs
o
bs GaP annealed at 800°C for 1min in order to improve the
A
Si optical properties of the diluted-nitride compounds.
Figure 6(b) shows the I–V characteristic of the diode
1.2 1.6 2.0 2.4 2.8 3.2
Energy (eV) under an AM1.5G illumination, without any concentra-
tion. The efficiency of this diode is 1.6%, which is an
Figure 5 Absorption spectra of the 1-µm-thick GaAsPN compound
encouragingresulttakingintoaccountthatthisstructure
as compared with GaAs direct bandgap, and GaP and Si indirect
(thickness, growth and composition of the absorbing
bandgapcompounds
materials, layer doping, and contacts) has not been
fully optimized, so far. Indeed this cell displays a 1.18V
Theseoptimizations,alongwiththeuseofanti-reflection
open-circuitvoltage(withtheoreticaloptimalvaluebeing
coating material, constitute a clear pathway for further
around 1.5 V) which consists of the state-of-the-art
improvement.
GaAsPN solar cells (see Table 1), with however a low
short-circuit current (2.9 mA/cm2). Figure 6(a) shows the
external quantum efficiency (EQE). The absorption edge
Early-stage results on PIN diodes grown on
of the materials corresponds well to the bandgap esti-
GaP(001) substrate
mated by PL around 1.9 eV showing that carriers have
been extracted from the GaAsPN absorber layer. The
In order to develop the top junction, GaAsPN/GaP PIN absorption edge above 2.2eV corresponds to the GaP
diodes have been grown on a n-type GaP(001) substrate. indirect bandgap. The overall low EQE might be due to
Table1 Deviceresultsreview
Reference Substrate PVstructure Absorberthickness(µm) J (mA/cm2) V (V) FF(%) η(%)
sc oc
Geisz,FriedmanandKurtz(2002) GaP SinglePIN 0.64 5.8 1.12 60 3.8
Geiszetal.(2005) Si Tandem 1 6.3 1.53 54 5.2
Thiswork GaP SinglePIN 1 2.9 1.18 51 1.6
Thiswork GaP SinglePIN 0.3 3.77 0.89 71 2.25
100
90 (a) 4 (b) JSC = 2.92 mA/cm2
80 2m) VOC = 1.18 V
70 A/c 2 FF = 50%
%)60 y (m η = 1.6%
E (50 nsit 0
IQ3400 ent de –2
r
20 ur
C
10 –4
0
1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 –1.0 –0.5 0.0 0.5 1.0
Energy (eV) Bias (V)
Figure6 (a)IQEand(b)J–VunderunconcentratedAM1.5GsolarspectrumofaGaP/GaAsPN1µm/GaPPINjunctiongrownonGaP(001)
substrate
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8 O.Durandetal.:Diluted-NitrideIII–V-NCompoundsonSiliconSubstrates
N localized states and non-radiative traps which reduce modificationofthespacechargeregion(SCR)positionin
the minority carrier diffusion length of the absorber as it the solar cells when the absorber is n doped. This result
has already been reported in diluted-nitride compounds seems to indicate that the absorber is intrinsically p
(Geisz, Friedman, and Kurtz 2002) and which could doped, with a doping density high enough to play a
explain the current density increase in the reverse bias role in the position of the SCR, meaning that the
regime of the I–V curve (Figure 6(b)). This low overall GaAsPN structural properties need an optimization to
EQE might also be due to the presence of ZnO complex reduce its intrinsic carrier density. This study will be
thatisalmostunavoidableinGaPsubstrate(Montgomery publishedinaforthcomingpaper.Wearecurrentlywork-
et al. 2011), therefore reducing the carrier diffusion ing on the absorber growth optimization, the annealing
length. Another issue might be due to the p-GaP layer step, the GaP passivation layers, and the anti-reflecting
surface recombination velocity, which could explain the coating.Moreover,usingfrontelectricalcontactsforboth
low EQE in the blue region (over 2.6 eV) (Lu et al. 2012). p and n sides would be necessary to estimate the actual
Therefore,inordertoimprovetheefficiencyofthose efficiency of this top-junction solar cell, without the GaP
solarcells,wehavestudiedasecondstructurewitha30- substrate influence.
nm-thin GaP window layer (as compared to the 250-nm-
thick previous one), a 0.3-µm-thick GaAsPN absorber (as
Conclusions
compared to the 1-µm-thick previous one) and a GaP
substrate which has been thinned down to 250 µm (as
compared to the 400-µm-thick previous one). With this Wehavereportedpromisingbuildingblocksforthedevel-
structure, an efficiency of 2.25%, a FF of 71% opment of GaAsPN/Si-based dual-junction solar cells.
(Figure 7(b)), which consists of the state-of-the-art, and Considering the GaP/Si(001) platform development, a
an IQE around 35% (Figure 7(a), under AM1.5G, have good control of the Si initial surface (bi-stepped, smooth,
been obtained (theoretical maximum of around 16%), and contaminant-free) for subsequent GaP overgrowth
confirming this way for efficiency improvement, even if through Si homoepitaxy on a 6°-off Si substrate obtained
a systematic study of each individual parameter effect inaUHVCVD-MBEgrowthclusterhasbeendemonstrated.
(GaP window layer, low diffusion length of the minority Such a bi-stepped surface is known to dramatically lower
carriers in the absorber, low diffusion length of the car- theAPDdensityinthesubsequent GaPthinlayergrowth.
rier in the substrate) still to be done in order to fully We have also demonstrated a MT-free growth of GaP thin
interpret the IQE. However, the general issue in this layer on Si at high growth temperature, however with a
structure is the still low short-circuit current (3.77 mA/ rough GaP surface. Moreover, a two-step growth strategy
cm2). Recent IQE measurements performed on solar cells showed very promising results with no evidence of any
with p- and n-doped absorber show an increase of the MTsbyXRDexperimentswhiledisplayingasmallsurface
photoncollectionwithenergyabove3eVforthen-doped roughness. One of the main issues for the dual-junction
sample while the p-doped sample behaves like the PIN solarcelldevelopmentisobtaininganefficient TJ, model-
structure. This collection increase can be explained by a ing of which has shown high theoretical current densities
100
90 (a) 4 (b) VOC = 0.88 V
80 m²) JSC = 3.77 mA/cm2
70 A/c 2 FF = 71.2%
m η = 2.25%
%) 60 y (
E ( 50 nsit 0
IQ 3400 ent de –2
r
r
20 u
C
10 –4
0
1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 –1.0 –0.5 0.0 0.5 1.0
Energy (eV) Bias (V)
Figure7 (a)IQEand(b)J–VunderunconcentratedAM1.5Gsolarspectrum,ofaGaP/GaAsPN300nm/GaPPINjunctiongrownonGaP(001)
substrate
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