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Solar Energy Materials & Solar Cells 93 (2009) 1524–1530

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/solmat

Phosphorus-doped silicon quantum dots for all-silicon quantum dot tandemsolar cells

X.J. Hao a,�, E.-C. Cho a, G. Scardera a, Y.S. Shen b, E. Bellet-Amalric c, D. Bellet d, G. Conibeer a, M.A. Green a

a ARC Photovoltaics Centre of Excellence, University of New South Wales, Sydney 2052, Australiab School of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australiac CEA-Grenoble, 17, rue des Martyrs 38054 Grenoble Cedex 9, Franced LMGP, INPG, BP 257, 38016 Grenoble Cedex 1, France

a r t i c l e i n f o

Article history:

Received 8 October 2008

Received in revised form

2 April 2009

Accepted 2 April 2009Available online 13 May 2009

Keywords:

Si quantum dot

Phosphorus

Doping

Dark resistivity

Solar cell

48/$ - see front matter & 2009 Elsevier B.V. A

016/j.solmat.2009.04.002

esponding author. Tel.: +6102 9385 6057; fax

ail address: xiaojing.hao@gmail.com (X.J. Hao

a b s t r a c t

Doping of Si quantum dots is important in the field of Si quantum dots-based solar cells. Structural,

optical and electrical properties of Si QDs formed as multilayers in a SiO2 matrix with various

phosphorus (P) concentrations introduced during the sputtering process were investigated for its

potential application in all-silicon quantum dot tandem solar cells. The formation of Si quantum dots

was confirmed by transmission electron microscopy. The addition of phosphorus was observed to

modify Si crystallization, though the phosphorus concentration was found to have little effect on

quantum dot size. Secondary ion mass spectroscopy results indicate minimal phosphorus diffusion from

Si QDs layers to adjacent SiO2 layers during high-temperature annealing. Resistivity is significantly

decreased by phosphorus doping. Resistivity of slightly phosphorus-doped (0.1 at% P) films is seven

orders of magnitude lower than that of intrinsic films. Dark resistivity and activation energy

measurements indicate the existence of an optimal phosphorus concentration. The photoluminescence

intensity increases with the phosphorus concentration, indicating a tendency towards radiative

recombination in the doped films. These results can provide optimal condition for future Si quantum

dots-based solar cells.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

Doping of Si quantum dots (QDs) has attracted much attentionrecently, in relation to solar cells [1,2], electroluminescent devices[3] and opto-electronic devices [4,5]. ‘‘All-Si’’ tandem solar cellsbased on Si QDs is one potential application of doped Si QDs [1,6].The concept of the ‘‘all-Si’’ tandem solar cells based on Si QDs isthe use of several solar cells of different bandgaps stacked on topof each other, with the highest bandgap cell uppermost andlowest on the bottom. The incident light is automatically filteredas it passes through the stack. Each cell absorbs the light that itcan most efficiently convert, with the rest passing through tounderlying lower bandgap cells [7].

Solar cells of different bandgaps can be achieved according tothe bandgap widening from quantum confinement effects. Si QDsembedded in a matrix of silicon dioxide are considered aspromising candidates for all-Si tandem solar cells since theyindicate a more reliable and tunable bandgap [6,8,9]. There areseveral methods for preparing Si QDs, such as ion implantation[10,11], PECVD [12,13] or magnetron sputtering [9,14]. For its

ll rights reserved.

: +6102 9385 5104.

).

application in solar cells, accurate control of size and density ofthe Si QDs is mandatory. In this work, the superlattice methodreported by Zacharias et al. [15] was adopted for the preparationof Si QDs. The Si QDs size is controlled independently by thesilicon-rich oxide (SRO) layer thickness equal to the desiredcrystal size, whereas the density of the Si dots can be varied by thecomposition of the SRO layers. This allows us to realize narrowsize-distributed and high-density Si QDs films.

In order to realize the solar cell device, an active layer forcarrier separation is required, which may be fulfilled by impurity-doping of Si QD films. This can be either a grown or a diffused p–njunction or a p–i–n junction with the Si QD multilayers as the i-region. The latter requires careful control of the work functions(and therefore doping) of the p- and n-region but also means thatit is not essential for the Si QD multilayers itself to be doped. Theconcept of a Si QD junction is similar to that of a bulk Si p–njunction. This nano-scaled p–n junction can be created by dopingSi QDs. However, many regularities and definitions accepted forbulk semiconductors may not be simply transferred to the low-dimensional case. Nanocrystals or QDs often have zero impurityand defect concentrations. For a doped bulk semiconductor, adoping density of 1018 cm�3 is usually considered high. A commonsize in the application of all-Si quantum dot tandem solar cells is5 nm in diameter and the number of atom of this dot is in the

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range of 500–1000. Translating this doping concentration into aspherical nanocrystal of 5 nm in diameter, it can be calculated thatone nanocrystal with a doping density of 1018 cm�3 will have lessthan one dopant atom on average. Moreover, there are manydifficulties in doping QDs from various theoretical aspects[16–18]. For instance, if a very high concentration of dopant isincorporated into a QD, the formation energy of the dopant ishigher than in a larger chunk of the semiconductor [19]. It may bethermodynamically unfavorable for the dopant to remain as adopant in the nanocrystals. It may also be expelled from thesemiconductor ‘bulk’ onto the particle surface [17]. Indeed workfunction control by addition of dopants to Si QDs materials isunclear. Further investigation is required to break through thetechnology bottleneck of QDs application in all-Si quantum dottandem solar cells. Boron and phosphorus (P) are the mostcommonly used impurities for p-type and n-type Si, respectively.Studies on boron-doping Si QDs have been investigated in ourprevious experiments [20,21].

Previous attempts to identify P-doping into Si nanocrystalsmainly focused on Si nanocrystal monolayer films and theirphotoluminescence (PL) properties [22–24]. In our previousstudies, we have demonstrated that a sputtering method issuitable for an in-situ impurity-doping of SRO/SiO2 multilayerduring the deposition process. The effect of P-doping on theproperties of Si QDs in annealed SRO monolayer has beeninvestigated [25]. Phosphorus was found to improve Si crystal-lization of the annealed SRO monolayers by increasing the Sinanocrystal size, which is consistent with the reports of others[25–27]. This would influence the bandgap according to quantumsize effects. Few studies have dealt with the electrical propertiesof impurity-doped Si nanocrystals [28]. The investigation ofP-doped Si QD/SiO2 multilayer films is a necessary and importantstep towards realization of a nano-scaled p–n junction.

In this paper, P-doped Si QDs formed as multilayers in a SiO2

matrix have been fabricated. P-doped Si QDs were formed by co-sputtering of Si, SiO2 (fused quartz) and phosphorus pentoxide(P2O5) targets with a post-deposition anneal. Subsequently theeffect of phosphorus on the structural (dot size and crystallinity)and electrical properties and optical emission of Si QDs formed asmultilayers in a SiO2 matrix was investigated.

Table 1Chemical composition of SRO films with various P concentrations.

Sample ID Si (at%) O (at%) P (at%) C (at%) O/Si ratio

Intrinsic SRO 57.923 40.177 0 2.0 0.694

Lightly P-doped SRO 56.055 38.678 0.101 5.165 0.70

Heavily P-doped SRO 55.694 38.976 0.35 4.98 0.699

2. Experimental details

Si QD layers with various P concentrations were deposited byalternate deposition of P-doped silicon-rich oxide (SRO) and SiO2

layers by a co-sputtering technique. P-doped SRO was realized byco-sputtering of Si, SiO2 and P2O5 targets at room temperature.The O/Si ratio and P concentration in the SRO layers werecontrolled by adjusting the deposition rates of species from thesethree targets. Fifteen SRO/SiO2 bi-layers were deposited on Siwafer and quartz slide substrates for various characterizationpurposes. Individual SRO layers were around 5 nm in thickness,and were separated by SiO2 layers of about 6 nm in thickness. Allsamples featuring multiple SRO layers with various P concentra-tions were subjected to annealing at a temperature of 1100 1C for1 h. The annealed samples were then investigated in terms ofstructural, optical and electrical properties. For compositionanalysis, a single SRO layer of 120 nm thickness was preparedunder the same experimental conditions as the SRO layers in the15 SRO/SiO2 bi-layers. The chemical composition of this thickmonolayer was analyzed by X-ray photoelectron spectroscopy(XPS, Fisons ESCALAB 220i-XL) both on the surface and after 30 sof Ar+ ion sputter etching, with a monochromatic Al Ka

(1486.5 eV) X-ray source and a hemispherical energy analyzer.The X-ray source power is 10 kV�12 mA, and the analyzed area of

sample is �0.3 mm2. Similar chemical composition of SRO films(O/Si ratio) with various P concentrations was confirmed byFourier transform infrared spectroscopy (FTIR). The Si nanostruc-tural features were observed by transmission electron microscopy(TEM) using a Philips CM200 TEM with an accelerating voltage of200 kV. The TEM specimens were prepared by the small-anglecleavage technique [29]. The crystalline properties of the sampleswere also studied by glancing incidence X-ray diffraction (GIXRD)(Philips X’Pert Pro) using Cu Ka radiation (l ¼ 0.154 nm), operat-ing at a voltage of 45 kV and a current of 40 mA. The primaryoptics was defined using a 1/161 divergent slit in front of aparabolic mirror. The secondary optics consists of a parallel-platecollimator of 0.271 acceptance and a soller slit of 0.04 rad aperture.The glancing angle between the incident X-ray beam and thesample surface was set at 0.261 slightly above the critical angle. Inorder to investigate the location of the P atoms in Si QDs formedas multilayers in a SiO2 matrix, secondary ion mass spectroscopy(SIMS) analysis was performed using a 25 kV Bi+ TOF-SIMS IV(time-of-flight SIMS) system, ION-TOF with sputtering of 0.5 kVCs+ ions at a 451 incident angle. The beam currents were 1 pA and30 nA for the analysis gun and the sputtering gun, respectively.The photoluminescence of the Si QDs formed as multilayers in aSiO2 matrix was studied at room temperature using a 532 nmlaser as the excitation source.

3. Results and discussion

3.1. Structural properties

The composition of films, shown in Table 1, was estimated fromXPS measurement. The O/Si ratio of SRO layers in films withdifferent P concentrations is around 0.7 (atomic percent ratio). Pconcentrations range from 0 at% (atomic percent) to 0.35 at%.

The position of the principal absorbance, i.e., the asymmetricSi–O–Si stretching mode, is sensitive to the value of the O/Si ratio,and has been used to determine the composition of SiOX (0oXo2)[30]. Thus in this work FTIR was used to estimate the relative O/Siratio change of SRO films. FTIR spectra of SRO monolayers withvarious P concentrations (Fig. 1) show similar asymmetric O–Si–Ovibrational peak positions, which confirm the XPS results that thesamples with various P concentrations have the same O/Si ratio.However, considering the Si–O–Si asymmetric vibration peakposition is affected by its surrounding chemical environment, theas-deposited films of 120-nm-thick intrinsic SiO2 and 2 at%P-doped SiO2 were used to investigate the shift due to theinfluence of P in SiO2 on its structure. Fig. 2 shows that theasymmetric Si–O–Si vibration peaks of as-deposited SiO2 and SiO2

with 2 at% P are positioned at 1052 and 1051.2 cm�1, respectively.Since the resolution of the FTIR measurement is 1 cm�1, the peak-shift due to P-doping in SiO2 could be regarded as negligible.

The formation of Si QDs was observed by TEM. Fig. 3(a) showsa clear multilayer structure of the sample with 0.1 at% P afterannealing, where dark and light color layers correspond to Si QDand SiO2 layers, respectively. Fig. 3(b) shows a high magnificationimage, where lattice fringes embedded in the speckled patternindicate isolated Si QDs embedded in an amorphous SiO2. The

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1800-0.01

0.00

0.01

0.02

0.03

0.04

Abs

orba

nce

(a.u

.)

0 at.% P 0.1 at.% P 0.35 at.% P

Wavenumber (cm-1)1500 1200 900 600

Fig. 1. FTIR spectrum of as-deposited thick SRO layer with various P concentra-

tions.

1800

0.00

0.02

0.04

0.06

0.08

0.10

0.12

1051.21052

Abs

orba

nce

(a.u

.)

SiO2-asdep SiO2 (P_2at.%)-asdep

Wavenumber (cm-1)1500 1200 900 600

Fig. 2. FTIR spectrum of as-deposited thick layers of SiO2 and SiO2 with 2 at% P

addition at room temperature.

P-O5nm

20nm

Fig. 3. TEM of a Si QD film with 0.1 at% P (a) low magnification image showing Si

QDs/SiO2 multilayer structure and (b) high magnification image showing lattice

fringe.

X.J. Hao et al. / Solar Energy Materials & Solar Cells 93 (2009) 1524–15301526

estimated diameter of Si QDs is 4.670.5 nm. In order toinvestigate the effect of high-temperature annealing on thelocation of the P atoms in the Si QDs formed as multilayers in aSiO2 matrix, SIMS analysis was performed. In Fig. 4, SIMS profilesof Si and P show clear oscillation features in the annealed samplewith 0.1 at% P. Peaks and valleys of the Si profile correspond to SiQD layers and SiO2 layers, respectively. The P spectrum is in phasewith that of Si, which indicates that P is dominant in Si QD layersrather than in SiO2 layers. These oscillation features are inagreement with the deposition sequence discussed in Section 2.Therefore, it can be concluded that minimal P diffusion from SROlayers to adjacent SiO2 layers occurs during annealing.

More structural information on the effect of P addition on Sicrystallization (size and crystallinity) was obtained from GIXRD.Fig. 5 shows the X-ray diffractograms of SRO/SiO2 multilayer filmswith various P concentrations (0, 0.1 and 0.35 at%) annealed at1100 1C for 1 h. The contribution from the Si substrate has beensubtracted. Four clear peaks are evident at 21.01, 28.41, 47.41 and56.31. The first peak (2y ¼ 21.01) is very broad and is attributed toan amorphous SiO2 phase. The remaining peaks are very close to

the expected Bragg peaks of Si (111), (2 2 0) and (311). This resultconfirms that crystalline Si precipitates are formed within anamorphous SiO2 matrix during annealing. The volume averagesize of the Si QDs of the three samples, estimated from the peakwidths through the Scherrer formula [31,32] do not depend on theP dopant concentration and remain at around 4–5 nm, which is inreasonable agreement with the dot size observed from TEMimages (Fig. 3(b)). Comparing the diffraction patterns of SRO/SiO2

multilayer films with various P concentrations, the mostremarkable feature is that the integral GIXRD peak intensity ofSi (111), corresponding in first approximation to the crystallinevolume fraction, varies with the P concentration. It increases with

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0100

101

102

103

104

105

106

Inte

nsity

(cps

)

Sputtering time

P Si

500 1000 1500 2000

Fig. 4. SIMS profiles of Si QDs multilayer sample with 0.1 at% P (the sample was

annealed at 1100 1C for 1 h).

0

100

200

300

400

500

102 Theta (deg.)

Inte

nsity

(a.u

.)

0 at.% P

0.1 at.% P

0.35 at.% P

Si (111) a-SiO2

Si (220)

Si (113)

20 30 40 50 60 70

Fig. 5. GIXRD plots of Si QD multilayer samples with various P concentrations

annealed at 1100 1C for 1 h.

Al electrodeSpace l

filmd

Plain view Spiking upon sintering

w

Fig. 6. Schematic layout of the Al contacts on a film for dark resistivity

measurements.

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09

Res

istiv

ity (o

hm c

m)

X.J. Hao et al. / Solar Energy Materials & Solar Cells 93 (2009) 1524–1530 1527

slight P (0.1 at%) addition but decreases with heavy P (0.35 at%)addition. This may indicate that an optimal P concentration existsfor Si QD crystallization. This finding suggests a more complexbehavior to the accepted view that P encourages Si crystallization,which has previously been reported for the case of an annealedthick SRO monolayer [25–27]. With regard to the thick (�120 nm)SRO monolayer, impurity addition improves Si crystallization forseveral possible reasons: (1) precipitates formed by impurityatoms during annealing can serve as centers of heterogeneousnucleation of Si nanoinclusions and (2) diffusion coefficients maybe enhanced due to P addition. In the case of Si QDs formed asmultilayers in a SiO2 matrix, a possible reason for suppression inSi crystallization for higher P concentrations is P aggregation atthe SiQD/SiO2 matrix interface, which may slow down the Sidiffusion.

1.E+00

1.E+01

0P dopant level (at.%)

0.1 0.2 0.3 0.4

Fig. 7. Dark resistivity of Si QD films with various P concentrations.

3.2. Electrical properties

The resistivity of the Si QD material is an important parameterfor photovoltaic application. In bulk Si, the concentration ofdopants has a significant effect on its resistivity. Accordingly, it is

necessary to investigate the effect of phosphorus concentration onresistivity of Si QDs formed as multilayers in a SiO2 matrix.Samples deposited on quartz substrates were used for lateralresistivity measurements. The SiO2 layer between adjacent Si QDlayers in all samples is around 6–7 nm, which should serve toconfine lateral carrier transport to Si QD layers. Ohmic contactswere obtained by depositing aluminium (Al) contacts by thermalevaporation, followed by sintering at a temperature lower thanthe Al–Si eutectic temperature (500–550 1C) to allow the Al tospike down into the film [33]. The layout of the Al contacts on afilm is shown schematically in Fig. 6. The dark current was thenmeasured at forward and backward voltages using a Keithley 617electrometer. The dark resistivity is defined as

rdark ¼Vdw

Il(1)

where d is the thickness of the Si QD active layers, w is the lengthof Al pad and l is the spacing of Al pads. As shown in Fig. 7, theintroduction of a slight amount of P (0.1 at%) leads to a significantvariation in the room-temperature dark resistivity of the filmsfrom 108O cm for the undoped sample to 101O cm for the samplewith 0.1 at% P. The resistivity of the sample with 0.1 at% P is sevenorders of magnitude lower than that of the undoped sample. Thisdramatic decrease in resistivity may indicate effective P-doping inSi QDs. However, the resistivity of the sample with a heavier Pconcentration (0.35 at%) does not decrease as much as that in thelight P concentration (0.1 at%) film. The resistivity of the samplewith P concentration of 0.35 at% is only five orders of magnitudelower compared to the undoped sample. The abrupt resistivityincrease upon an increase in the P concentration from 0.1 at% to0.35 at% may result from the saturation of P in the Si QDs and thus

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1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09

1.E+10

1.E+11

1.E+12

1.E+13

2.70

R (o

hm)

0 at.% P0.1 at.% P0.35 at.% P

Ea: 0.527 eV

Ea: 0.149 eV

Ea: 0.101 eV

2.90 3.10 3.301000/T (K-1)

Fig. 8. Temperature dependence of the resistance R of the Si QD films with various

P concentrations.

1.E+06

1.E+07

2.70

R (o

hm)

0.35 at.% P - spacing 1

0.35 at.% P - spacing 2

1000/T (K-1)

2.90 3.10 3.30

Fig. 9. Temperature dependence of the resistance R of the Si QD film with 0.35 at%

P using contacts with different spacing distances.

4500.0

0.5

1.0

1.5

2.0

2.5

3.0

0.1 at.% P

PL

inte

nsity

(a.u

.)

Wavel

0.45 at.% P

30 min 1 hour 3 hours

600 750 900 1050 450 600

Fig. 10. Photoluminescence of Si QD films with various P concen

X.J. Hao et al. / Solar Energy Materials & Solar Cells 93 (2009) 1524–15301528

P may aggregate around the Si QD/SiO2 matrix interfaces. Fromthis point of view, the dependence of resistivity on Pconcentrations appears linked to the structural findings fromGIXRD.

The temperature dependence of the measured resistance (R)was also investigated at a voltage Vp5 V in the temperature (T)range of 290pTp383 K. Fig. 8 presents plots of the measuredtemperature dependence of resistance (R) for various Pconcentrations. The plots of log R vs 1000/T for all three samplesare close to linear. The values of the activation energy Ea calculatedfrom the relation R � expðEa=kTÞ are indicated in Fig. 8. As thedoping concentration increases from 0 to 0.1 at%, the activationenergy decreases from 0.527 to 0.101 eV. This decrease in activationenergy indicates that the Fermi energy moves towards theconduction bands and the observed resistivity decrease is aconsequence of an increase in carrier concentration, whichsuggests effective n-type doping of the Si QDs. However, a furtherP concentration increase to 0.35 at% increases the activation energyto 0.149 eV. This slight increase of activation energy may implynegative effect on P-doping of Si QDs. The possible reasons for thischange are saturation of P in Si, and/or degradation of Sicrystallinity as discussed in relation to GIXRD measurements.

The contact resistances for the above measurements weredetermined using the method proposed by Reeves and Harrison(the transmission line model (TLM)) [34]. The TLM methodinvolves measurement of the resistance between several pairs ofcontacts that have identical areas, but are separated by differentspacings. Similar measurements were performed for all thesamples indicated in Fig. 7. It was found that the contactresistances do not exceed 5% of the total resistance of the film.For comparison, Fig. 9 presents plots of the temperaturedependence of R for films containing 0.35 at% P, conductedbetween pairs of contacts with different spacings. It can beobserved that the contact resistances neither contributesignificantly to the measured resistance nor influence thetemperature dependence of R.

3.3. Photoluminescence properties

The effect of P concentration on the optical emission propertyof Si QDs formed as multilayers in a SiO2 matrix was widelyinvestigated for thick SRO monolayer films [22–25]. Until now fewstudies have been done on the P-doped Si QD/SiO2 multilayerfilms. The PL of Si QDs formed as multilayers in a SiO2 matrix withvarious P concentrations (0, 0.1 and 0.45 at%) were studied in thiswork in terms of annealing time. The sample with 0.45 at% P has

x140

0 at.% P

x15

ength (nm)750 900 1050 450 600 750 900 1050

trations and anneal conditions (1100 1C from 30 min to 3 h).

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not been discussed until now. Its structural characteristic agreeswith the trend observed in Fig. 5. All three samples were annealedat 1100 1C with different annealing times (30 min, 1 h and 3 h). Asshown in Fig. 10, the PL peak position and intensity stronglydepend on the P concentration. With increases in annealing time,the PL peak of the undoped Si QD multilayer film is red shifted andthe intensity peak increases. The PL peak of the 0.1 at% P sample isslightly red shifted with increasing annealing time, while itsintensity remains almost constant. This may indicate that Pencourages Si crystallization as less time is needed to establish SiQDs during high-temperature annealing. However, compared withthe above 0 and 0.1 at% P-doped samples, the PL of the heavilyP-doped (0.45 at%) multilayer film has no obvious shift and itsintensity decreases with increase in annealing time. On the otherhand, the PL intensity increases with P concentration under thesame annealing conditions. Enhanced PL intensity with anincreasing P concentration has been previously reported [22,23].P may terminate dangling bonds at the Si QD/SiO2 matrixinterfaces. Moreover, the observed trend may be due to adecrease in the mechanical stress (arising from heat treatment)because of the close values of the thermal expansion coefficientsof the Si inclusions and the SiO2 matrix [24]. Another possiblemechanism is the considerable increase in the probability ofinterband radiative transitions and hence of the PL intensity [35].

4. Conclusions

P-doped Si QD multilayer films were fabricated by RFmagnetron sputtering of Si, SiO2 and P2O5 with a post-depositionanneal. The effect of P concentration on Si QDs formed asmultilayers in a SiO2 matrix was investigated in terms of structural,electrical and optical properties. It was found in SIMS spectra thatthere is minimal P diffusion from SRO layers to adjacent SiO2 layersduring high-temperature treatment, and most of the P remains inthe original deposited SRO layer. Structural investigations indicatethat an optimal P concentration exists for crystallization of Si QDs,whereas the P concentration has little effect on Si QDs size. There isan optimal P concentration for resistivity. With a slight increase inP concentration, a dramatic decrease in resistivity was obtained. Incontrast, too much P causes an increase in resistivity, which mayresult from saturation of P in Si and P segregation around theinterface of Si QDs. Moreover, there exists an optimal P concentra-tion that results in the lowest activation energy. The activationenergy, estimated from the temperature-dependent resistance,decreases with slight P-doping, which suggests that the decrease inresistivity is the consequence of an increase in carrier concentra-tion. It indicates effective n-type doping of the Si QDs. A furtherincrease in the P concentration was found to increase the activationenergy. This slight increase of activation energy, along with theincrease in resistivity, may indicate a negative doping effect.Possible explanations are the saturation of P in Si and degradationof Si crystallization or the formation of P metal precipitates. The PLintensity was observed to increase with an increase in Pconcentration, indicating a tendency towards radiative recombina-tion in the doped films. The trend of PL peak position under variousannealing conditions is strongly dependent on P concentration. Inconclusion, the properties of Si QD formed as multilayers in a SiO2

matrix strongly depend on P dopant concentration.

Acknowledgements

The authors acknowledge the Photovoltaics Centre of Excel-lence supported by the Australian Research Council as well as theStanford University Global Climate and Energy Project (GCEP).

Xiaojing Hao also gratefully acknowledges the Endeavour Inter-national Postgraduate Research Scholarship (EIPRS) for financialsupport of her Ph.D. study.

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