ARTICLE IN PRESS

Physica B 405 (2010) 61–64

Contents lists available at ScienceDirect

Physica B

0921-45

doi:10.1

� Corr

E-m

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

Comparative study of the surface passivation on crystalline siliconby silicon thin films with different structures

Lei Zhao a,�, Hongwei Diao a, Xiangbo Zeng b, Chunlan Zhou a, Hailing Li a, Wenjing Wang a

a Solar Cell Technology Group, Key Laboratory of Solar Thermal Energy and Photovoltaic System of Chinese Academy of Sciences, Institute of Electrical Engineering,

The Chinese Academy of Sciences, Beijing 100190, Chinab Institute of Semiconductors, The Chinese Academy of Sciences, Beijing 100083, China

a r t i c l e i n f o

Article history:

Received 11 February 2009

Received in revised form

31 July 2009

Accepted 4 August 2009

PACS:

84.60.Jt

81.65.Rv

Keywords:

Silicon thin film

HIT solar cell

Surface passivation

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

016/j.physb.2009.08.024

esponding author.

ail address: zhaolei@mail.iee.ac.cn (L. Zhao).

a b s t r a c t

Si thin films with different structures were deposited by plasma enhanced chemical vapor deposition

(PECVD), and characterized via Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy.

The passivation effect of such different Si thin films on crystalline Si surface was investigated by

minority carrier lifetime measurement via a method, called microwave photoconductive decay (mPCD),

for the application in HIT (heterojunction with intrinsic thin-layer) solar cells. The results show that

amorphous silicon (a-Si:H) has a better passivation effect due to its relative higher H content, compared

with microcrystalline (mc-Si) silicon and nanocrystalline silicon (nc-Si). Further, it was found that

H atoms in the form of Si–H bonds are more preferred than those in the form of Si–H2 bonds to passivate

the crystalline Si surface.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

HIT (Heterojunction with intrinsic thin-layer) solar cell is verypromising due to that it combines the good properties ofcrystalline or multicrystalline silicon with the advantagesof thin-film silicon technology. By this technology, Sanyo hasobtained a record efficiency up to 23% [1]. HIT processes arecarried out under low temperatures (o200 1C), which can preventthe degradation of bulk quality that possibly happens in high-temperature cycling processes. And compared with conventionaldiffused cells, a much better temperature coefficient canbe obtained with a higher open-circuit voltage (VOC) [2,3]. Hence,HIT solar cells have attracted more and more interests in theworld [4,5].

In the HIT structure, junctions are formed not by doping partof a solid, but by growing thin films on surfaces. This makes thatHIT solar cells are basically surface-(or interface-) dominatedand the surface properties are absolutely critical for the deviceperformance. If the density of interface states is too high, VOC ofHIT solar cell will decrease greatly. In order to obtain goodinterface quality of the heterojunction, the intrinsic silicon thinfilm layer is inserted between the emitter and the base to

ll rights reserved.

passivate the heterojunction interface. In Sanyo’s research,intrinsic amorphous Si (a-Si:H) layer was adopted [1]. As weknow, silicon thin film can have different structures, such asa-Si:H, nanocrystalline silicon (nc-Si) and microcrystallineSi (mc-Si), corresponding to the different deposition conditionsby plasma enhanced chemical vapor deposition (PECVD), hotwire chemical vapor depositon (HWCVD), and so on. The latestresults show nc-Si and mc-Si were more stable than a-Si:H [6,7].So ones have introduced intrinsic silicon layers with otherstructures into HIT structures [8,9]. Kim et al. [10] investigatedthe effect of hydrogen dilution of the intrinsic layer on theperformance of the heterojunction solar cell, but the selectedhydrogen dilution range was narrow and the inner structuralchange of the silicon thin film with different hydrogen dilutionswas either not given. There is still a shortage of comparativestudy to show that which kind of silicon thin film is the bestchoice to passivate crystalline Si (c-Si) surface. The role ofthe structure and the H atoms distribution in Si thin films onthe passivation effect should also need more consideration.

In this study, Si thin films with different structures weredeposited on c-Si surface by PECVD. The obtained passivationeffect was investigated by minority carrier lifetime measurementvia the method of microwave photoconductive decay (mPCD). Bythe combined analysis with Raman and Fourier transform infrared(FTIR) spectroscopies, the relationship between the structure ofsilicon thin film and the passivation effect was given out.

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L. Zhao et al. / Physica B 405 (2010) 61–6462

2. Experimental

All the silicon thin films were deposited by a PECVD systemwith the plasma excitation frequency of 13.56 MHz via changingthe deposition conditions such as substrate temperature, gas flux,plasma excitation power, and deposition time. All the adoptedconditions were given in Table 1. Under each condition, twosamples were prepared at the same time on c-Si and quartzsubstrates, respectively. The one deposited on c-Si substrate wasused for FTIR and minority carrier lifetime measurements. Andanother one deposited on quartz substrate was prepared forRaman measurement. The utilized c-Si substrates were double-side polished Cz Si wafers with the resistivity of about 10O cmand the thickness of 600mm. Before the deposition, the Sisubstrates were cleaned via the standard RCA process, followedby 1% dilute HF solution dipping for 1 min to remove the nativeoxide layer. The quartz substrates were degreased in turn byacetone, alcohol, and DIE water cleaning, and then, were alsodipped by the dilute HF solution.

Raman measurement was carried out at room temperature onthe Labram HR800 spectrometer, Jobin–Yvon corporation, France.The wavelength of the incident laser was 514 nm. An appropriatelow energy density was adopted to avoid crystallization of theamorphous phase.

FTIR measurement was executed in the vacuum at roomtemperature on the FTS-60V spectrometer, Bio-Rad corporation,USA, with a high intensity mercury lamp as the infrared lightsupply.

Semilab WT2000 tester, Hungary, was adopted to process theminority carrier lifetime measurement. The utilized wavelength ofthe incident light was 904 nm. During the measurement, the backsurfaces of the silicon substrates were passivated by 1% dilute HFacid solution. Such treatment combined with the large siliconsubstrate thickness of 600mm can minimize the back surfaceeffect.

Fig. 1. Raman spectra of the deposited silicon thin films.

3. Results and discussion

Fig. 1 gives out Raman spectra of the samples deposited onquartz substrates. In Raman spectrum of c-Si, Si–Si TO phononmode occurs at about 520 cm�1. For amorphous phase, due to itsdisorder structure, a dispersed package will appear in its Ramanspectrum, and the Si–Si TO phonon mode occurs at about480 cm�1. Such difference can be utilized to determine thestructure of the silicon thin film. Generally, Si thin film is amixture material of amorphous phase, crystalline phase, and

Table 1The deposition conditions for all the silicon thin films.

Temperature (1C) Pressure (Pa) Power (W)

Condition 1 200 120 150

Condition 2 150 150 150

Condition 3 100 150 50

Condition 4 100 200 100

Condition 5 200 150 100

Condition 6 150 200 50

Condition 7 150 120 100

Condition 8 150 170 5

Condition 9 50 120 5

Condition 10 50 150 50

Condition 11 100 170 150

Condition 12 50 170 100

Condition 13 200 200 5

Condition 14 50 200 150

Condition 15 200 170 50

interface phase. The corresponding Raman spectrum is thescattering sum of the above three phases and thus can beresolved into three Gauss function curves. The structure of siliconthin film can be deduced by the positions and the intensities ofthe three Gauss peaks [11].

Fig. 2 illustrates such deconvolution by Gauss function fitting.The right three peaks correspond to the Si–Si TO phonon

Flux of SiH4 (sccm) Flux of H2 (sccm) Deposition time (min)

4 135 20

7 45 20

7 180 30

4 45 20

1 180 60

1 135 40

9 90 20

4 180 20

1 45 60

4 90 45

1 90 60

7 135 30

7 90 30

9 180 20

9 45 20

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Fig. 2. Gauss function fitting of Raman spectrum of silicon thin film.Fig. 3. The passivation effect of silicon thin films with different structures on c-Si

surface, characterized by minority carrier lifetime, together with the crystalline

grain size in the silicon thin films, as a function of the corresponding Raman peak

shift of Si–Si TO phonon mode between the silicon thin film and c-Si.

Fig. 4. FTIR spectra of the silicon thin films deposited by Conditions 12, 14, and 15.

L. Zhao et al. / Physica B 405 (2010) 61–64 63

scattering of the amorphous phase, the interface phase, and thecrystalline phase from left to right, respectively. In general,the difference between the rightmost crystalline phase peak andthe position of Si–Si TO phonon mode in c-Si, 520 cm�1, called thepeak shift here, can be utilized to characterize the crystallinephase content qualitatively. The larger the peak shift is, the lessthe crystalline phase content is. That is, there will be moreamorphous phase in the thin film with larger peak shift.Furthermore, a correlation length model can be utilized tocalculate the crystalline grain size in the silicon thin film by thepeak shift according to the following formula [12]:

DoðDÞ ¼ Aa

D

� �gð1Þ

where, Do(D) is the Raman peak shift of Si–Si TO phonon modebetween silicon thin film and c-Si. D is the crystalline grain size,a ¼ 0.543 nm, is the lattice constant of c-Si. A ¼ 97.462 cm�1 andg ¼ 1.39, are both the fit parameters to describe the phononconfinement in crystalline grains. Thus, the crystalline grain size D

of all the samples can be obtained. The results were shown inFig. 3, where the deduced Raman peak shifts from Fig. 1were utilized as the horizontal ordinate, and the conditionnumbers were labeled close to the data points. It can be seenthat from Condition 1 to Condition 15, the crystalline grain size D

is gradually reduced from above 200 to only 1.5 nm. It meansthat silicon thin films with different structures, includingmc-Si (Conditions 1–5, D420 nm), nc-Si (Conditions 6–12,D: 5–10 nm) and a-Si:H (Conditions 13–15, D: 1–2 nm), havebeen prepared. One should note that the crystalline grain size D iscalculated as 1.03 nm according to the formula (1) forDo(D) ¼ 40 cm�1 in the perfect a-Si:H layer.

The passivation effect of such silicon thin films with differentstructures on c-Si surface was characterized by minority carrierlifetime measurement. Fujiwara et al. [13] observed that thepassivation effect was relative to the thickness of the silicon thinfilm. For a-Si:H, the passivation effect can be stabilized when itsthickness is larger than 4 nm. Here, all the silicon thin films weredeposited for an enough time (see Table 1) to make the thicknessmuch larger than 10 nm. So the stabilized passivation states werecharacterized. The results of the minority carrier lifetimemeasurement were also shown in Fig. 3. It can be clearly seenthat the minority carrier lifetime increases gradually from mc-Si,nc-Si to a-Si:H. The solid lines in Fig. 3 were used to guide eyes.The results show that more amorphous phase is helpful to obtain

the improved passivation effect, which can be attributed to thefact that there are more H atoms in a-Si:H than in mc-Si and nc-Si.The contribution of H atoms to the passivation effect has beenwell known [14,15]. Otherwise, there are still passivationdifferences among different a-Si:H thin films, which give rise tothe necessity to check the distribution of H atoms in the films.

Fig. 4 gives out the FTIR spectra of the samples obtained byConditions 12, 14, and 15 in the range of 1900–2250 cm�1. Thesilicon thin films have mainly two absorption modes in this range.One mode is the Si–H stretching mode occurring at 2000 and2100 cm�1. The other is the Si–H2 stretching mode occurring at2100 cm�1 [16]. Generally, a double-Gauss function fitting is usedto analyze such FTIR spectra. The intensity ratio of the Gauss peaknear 2000 cm�1 to the Gauss peak near 2100 cm�1 is utilized todisclose the chemical bond state of H atoms in the films.The larger the intensity ratio is, the more the Si–H bond contentis. The results of Fig. 4 show that there is a larger Si–H2 bondcontent in the samples obtained by Conditions 12 and 14 than in

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L. Zhao et al. / Physica B 405 (2010) 61–6464

the sample obtained by Condition 15. In fact, the aforementionedRaman results indicate that the sample obtained by Condition 12is nc-Si, and thus comprises a lot of interface phases therein. Si–H2

bonds distribute on the interfaces. For the sample obtained byCondition 14, during the deposition, a lot of yellow siliconpowders were observed. So its structure is loose and there are alot of voids to provide positions for Si–H2 bonds. Whereas, thesample obtained by Condition 15 is perfectly compact a-Si:Hmaterial with Si–H bonds mostly. The results in Fig. 3 show thatthe minority carrier lifetime obtained by Condition 15 is thehighest. So it can be concluded that Si–H bonds are morebeneficial than Si–H2 bonds for the surface passivation of c-Si.Some others’ studies confirmed the above results. Fujiwara et al.[13] determined the growth process of the a-Si:H layer on c-Sisubstrate based on real-time spectroscopic ellipsometry (SE) andinfrared attenuated total reflection (ATR) spectroscopy. The depthprofiles of Si-H2 and Si-H contents were achieved. A maximumSi-H2 content of 27 at% was seen at the a-Si:H/c-Si interface.The Si–H2 content decreased gradually as the a-Si:H layer grew.The Si–H content otherwise increased at the same time. As aresult, the a-Si:H/c-Si heterojunction solar cell performance waslow originally, and improved gradually with the increase of thea-Si:H thickness. When the thickness of the a-Si:H layer was up to4 nm, the Si–H2 and Si–H contents were stabilized, theperformance of the a-Si:H/c-Si heterojunction solar cell reachedthe best. The authors believed that large Si–H2 contentindicated poor network formation in the layer and resulted inlow solar cell performance. Gielis et al. [15] also obtained similarresults. Obviously, the original Si–H2 content depends on thestructure of the silicon thin film. Hence, in order to passivatethe heterojunction interface effectively in HIT solar cells, theinserted intrinsic layer must be chosen as the compact a-Si:Hlayer with low Si–H2 bonds and high Si–H bonds.

4. Conclusions

The passivation effect of Si thin films with different structureson crystalline Si surface was investigated by minority carrierlifetime measurement, combined with Raman spectroscopy, andFourier transform infrared (FTIR) spectroscopy. Compared with

nanocrystalline silicon (nc-Si) and microcrystalline (mc-Si),amorphous silicon (a-Si:H) thin film had a better passivationeffect due to its relative higher H content. Furthermore, it wasfound that H atoms in the form of Si–H bonds were preferred thanthose in the form of Si–H2 bonds, because Si–H2 bonds indicatedpoor network formation in the thin film. Thus, in order topassivate the heterojunction interface effectively in HIT solar cells,the inserted intrinsic layer must be chosen as the compact a-Si:Hlayer with low Si–H2 bonds and high Si–H bonds.

Acknowledgments

This work was supported by the 863 High Technology ResearchProgram of China (Grant no 2006AA05Z405) and the directorfoundation of IEE, CAS.

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