Journal of Non-Crystalline Solids 356 (2010) 299–303

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Journal of Non-Crystalline Solids

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

Low temperature deposition of boron-doped microcrystalline Si:H thin filmand its application in silicon based thin film solar cells

Ke Tao a,*, Dexian Zhang b, Jingfang Zhao b, Linshen Wang b, Hongkun Cai b, Yun Sun a

a Institute of Photo-electronics Thin Film Devices and Technique, Nankai University, PR Chinab Department of Electronic Science and Technology, Nankai University, PR China

a r t i c l e i n f o

Article history:Received 21 August 2009Received in revised form 27 November 2009Available online 5 January 2010

Keywords:Amorphous semiconductorsSiliconSolar cellsRaman scatteringX-ray diffractionChemical vapor depositionMicrocrystallinity

0022-3093/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.jnoncrysol.2009.12.001

* Corresponding author.E-mail address: patricktao2001@sina.com (K. Tao)

a b s t r a c t

Boron-doped hydrogenated microcrystalline silicon thin films (p-lc-Si:H) have been deposited by RF-PECVD method at different temperature, and the temperature dependence of growth kinetics and opto-electronic properties of p-lc-Si:H thin films have been studied. Both the deposition rate and the dark-conductivity of the p-lc-Si:H thin films drop down when the substrate temperature decreases. XRDand Raman measurements are used to characterize the micro-structure of p-lc-Si:H thin films preparedat different substrate temperature. Grain size of p-lc-Si:H thin films with different thickness as a func-tion of substrate temperature has been investigated. Amorphous silicon thin film solar cells with p–i–nstructures were fabricated on deposited boron doped lc-Si:H layers. The best cells performance isobtained for p-layers processed at 90 �C.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Nowadays, the sunlight enters into the amorphous silicon solarcells through p-layer for both n–i–p structure and p–i–n structure.Due to the reason that the mobility of photo-generate holes issmaller than that of electrons, it makes for the collection of holeswhen sunlight incident from p-layer. As the window layer, p-typematerials need to have larger optical band gap, lower light-absorp-tion coefficient and thickness. On the one hand, more sunlight canpass through the p-layer and enter into the intrinsic layer to gen-erate more charge carriers. On the other hand, the recombinationbetween the minority carrier of p-layer and holes collected fromintrinsic layer can be reduced. Thus, the output current of solarcells can be improved. In addition, the built-in electric filed be-tween p-layer and n-layer can be enhanced and the transportand collection of photo-carriers therefore the stability of solar cellscan be improved efficiently. Moreover, the p-type materials needto have larger conductivity so that the series resistance can be re-duced and the ohm contact can be formed between p-layer and theTCO. Several kinds of p-type materials have been developed includ-ing p-type a-Si:H, p-type a-SiC:H, p-type lc-Si:H, and so on.

As the window layer of solar cells, p-type a-SiC:H had once en-hanced the open circuit voltage (Voc) and efficiency (Eff) of solarcells. When carbon atoms are doped into the amorphous silicon,

ll rights reserved.

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the optical band gap of p-type materials can be increased [1]. In1980, the p-a-SiC:H was incorporated into a-Si:H solar cells byTawada, and the Voc was increased from 0.8 to 0.9 V [2]. However,p-a-SiC:H had itself disadvantage: the dark-conductivity and theoptical band gap were restricted reciprocally, due to the incorpo-rate of carbon atoms, furthermore, the defect state density of thematerial was large [3].

The electrical property of p-lc-Si:H had been studied exten-sively [4,5]. Compared with p-a-Si:H, they have greater doping effi-ciency, higher electrical conductivity, higher mobility, lower light-absorption coefficient, and they have received more and moreattention for the photovoltaic applications. Low activation energycan contribute to the enhancement of built-in potential while highconductivity can reduce series resistance.

Several deposition techniques have been established to preparep-lc-Si:H thin films, including ion beam mixing [6], plasma-en-hanced chemical vapor deposition (PECVD), and hot-wire chemicalvapor deposition (HWCVD) [7–9], among which PECVD appears tobe a promising deposition method for large-area thin film technol-ogy and has been employed for industrial applications, whileHWCVD has itself advantage: enhancing the deposition rate largelyand keeping the quality of the thin films.

In this paper, fabrication of p-lc-Si:H thin films using RF-PECVDat different substrate temperature is described and the influence ofsubstrate temperature on the micro-structure and optoelectronicproperties of p-lc-Si:H thin films is investigated. Also, the photo-voltaic characteristic of p–i–n structure solar cells incorporated

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0.1deposition rate

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m)

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tenm

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Deposition temperature oC

dark-conductivity photo-conductivity

Fig. 1. The deposition rate, dark-conductivity and photo-conductivity of p-lc-Si:Hthin films (about 30–40 nm thickness) deposited on glass as a function of thedeposition temperature. The lines are drawn to guide the eye.

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300 K. Tao et al. / Journal of Non-Crystalline Solids 356 (2010) 299–303

with p-lc-Si:H thin film that deposited at low temperature arestudied.

2. Experimental

A serial of p-lc-Si:H thin films are deposited on 7059 glass sub-strate by RF (13.56 MHz) plasma-enhanced chemical vapor deposi-tion at different substrate temperature. Silane (SiH4), hydrogen(H2) and 0.5% diborane (B2H6) are used as the source gases. Thehydrogen dilution rate (R = SiH4/(SiH4 + H2)) is 0.5%, and the B2H6

concentration in silane is 0.5% in gas phase. The gas pressure andpower density are kept at 180 Pa and 350 mW/cm2. The substratetemperature is varied from 60 to 200 �C, and p-lc-Si:H films withdifferent thickness are prepared for different measurement byadjusting the deposition time. The p–i–n structures solar cells con-sisted of specular glass/SnO2/p-lc-Si:H/i-a-Si:H/n-lc-Si:H/Al arefabricated in a multi-chamber PECVD system. The deposition con-ditions of each layer are summarized in Table 1.

The thickness of films was measured using a surface profiler(AMBIOS XP-2), thereafter the deposition rate can be obtained.The micro-structure of the p-lc-Si:H films was characterized byX-ray diffractometer (XRD, Philips PANalytical X’Pert, CuKa), thenthe average crystallite size D of lc-Si:H can be estimated accordingto Debye–Scherrer formula [10]:

D ¼ Kk=b cos h

where K (�1) is a constant, k (=0.154187 nm) is the wavelengthof X-ray, b is the full width of half maximum (FWHM) of the dif-fraction peak, and the h is the diffraction angle. Raman scatteringis performed to characterize the structure of p-lc-Si:H films. The514 nm laser line is used for the excitation. AFM measurement isalso used to characterize the surface morphology of p-lc-Si:Hfilms. The electrical conductivity of the films is measured usingan Al coplanar contact electrode by Keithley 6430 Source/measureunit at room temperature. The light J–V characteristics of solar cellsare measured under AM1.5 solar simulator at 25 �C.

3. Results

As shown in Fig. 1, the deposition rate of p-lc-Si:H thin filmsdecreases monotonously from 2.1 to 1.2 nm/min when the sub-strate temperature decreases from 200 to 60 �C. At the same time,both the dark-conductivity and photo-conductivity of those filmsminish from the magnitude of 10�1 S/cm at 200 �C to 10�3 S/cmat 60 �C. In addition, it can be seen from Fig. 1 that the ratio ofphoto-conductivity to dark-conductivity becomes large as the sub-strate temperature decreases. This indicates that the doping effi-ciency of boron in the silicon matrix decreases.

The evolution of layer crystallinity of p-lc-Si:H thin filmsdeposited under different substrate temperature can be demon-strated with XRD spectrum. From the result of XRD given inFig. 2, it can be seen that the X-ray diffraction signal turns weakas the substrate temperature decreases. Furthermore, the diffrac-tion peak at (3 1 1) plane disappears when the temperature de-

Table 1The deposition conditions of each layer in the fabricated a-Si:H solar cells.

Parameters p-layer i-layer n-layerlc-Si:H a-Si:H lc-Si:H

Substrate temperature (�C) 60–200 270 270Flow rate (sccm) SiH4:B2H6:H2 =

6.4:0.64:121.6SiH4 = 40 SiH4:PH3:H2 =

3.3:3.3:76.8Gas pressure (Pa) 180 50 200Power density (mW/cm2) 350 70 140Thickness (nm) 20 400 35

creases to 150 �C. Indeed, the decrease of the diffraction signal atlow temperature must be related to the decrease of the crystallitesize. As shown in Fig. 2, the crystallite size calculated from (1 1 1)peak becomes smaller when temperature decreases.

Fig. 3 displays the Raman spectra of samples of Fig. 1. There is ashoulder peak near 513 cm�1 in the transverse optical (TO) modefor all the samples. However, when the temperature decreases, thisshoulder peak turns weak and almost disappears when thetemperature is 60 �C. This indicates that the crystalline fractionoriginated from microcrystals decreases gradually, and themicro-structure of the thin films translates from mixed-phase(microcrystalline + amorphous) to amorphous phase. Using Gauss-ian 3-peaks fit, the crystalline volume fraction Xc can be estimatedfrom the Raman spectra. In fact, Xc decreases gradually from 9.1 to5.5% when temperature changes from 200 to 60 �C.

In Fig. 4, the surface topology of 3 � 3 lm2 scan provided by anAFM in noncontact mode is shown for p-lc-Si:H thin films depos-ited on glass. The thicknesses of these films are 90, 107, 120, and130 nm respectively, corresponding to the substrate temperaturefrom 200 to 60 �C. It can be seen that when temperature decreasesfrom, the density of grain in the surface drops down. Fig. 5 hasshown the grain size calculated from AFM and XRD respectively,and RMS roughness of samples in Fig. 4 as a function of substrate

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Fig. 2. The XRD measurement of p-lc-Si:H thin films (about 30–40 nm thickness)deposited on glass under different substrate temperature. The inserted graphdescribes the change of the average crystallite size deduced using Debye–Scherrerformula from XRD spectrum as a function of substrate temperature. The dashed lineis drawn to guide the eye.

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Fig. 3. The Raman scattering spectra of the p-lc-Si:H thin films (about 30–40 nmthickness) deposited on glass under different substrate temperature.

K. Tao et al. / Journal of Non-Crystalline Solids 356 (2010) 299–303 301

temperature. The grain size recalculated from AFM measurementincreases from 27 nm at 200 �C to 39 nm at 90 �C, however, thegrain size decreases to 36 nm when the temperature decreases to60 �C. While the grain size recalculated from AFM measurementdecreases monotonously when the temperature decreases from200 to 60 �C, furthermore, the surface RMS roughness increasesmonotonously from 3.35 nm at 200 �C to 4.62 nm at 60 �C.

p–i–n structure a-Si:H solar cells are fabricated with the non-textured electrodes, and 20 nm thickness p-lc-Si:H thin films

Fig. 4. 3 � 3 lm2 AFM images of p-lc-Si:H thin films (the thicknesses of those films arsubstrate temperature from 60 �C to 200 �C.

deposited at 60, 90, 150 and 200 �C respectively are used as thewindow layer. The initial J–V curves of those solar cells are shownin Fig. 6. It can be seen that the open circuit voltage (Voc) of solarcells increase from 609 to 827 mV, as the temperature decreasefrom 200 to 90 �C. However, when the temperature decreases to60 �C respectively, the Voc decreases to 770 mV. The change trendof the short circuit current density (Jsc) of the solar cells is similarto that of Voc, and the highest efficiency (7.324%) is obtained whenthe p-lc-Si:H thin films is prepared at 90 �C.

4. Discussion

As we know that the growth of the microcrystalline silicon thinfilms begin firstly with an incubation layer that is amorphousphase. Then crystalline nuclei form in the incubation layer andgradually grow into small grains embedded in the amorphousphase, which is mixed-phase. Afterwards, the grains grow largerand larger until they come into contact, and, at last, the very largeconglomerates of microcrystalline silicon sometime labeled as‘‘columns” form, which is single microcrystalline phase [11,12].In this process, the surface RMS roughness measured by AFM in-creases firstly due to the presence of isolated large grains and getsto the maximum value, later, it decreases and trends to a saturatedvalue [13,14]. For p-lc-Si:H thin films, the growth mechanism issimilar to that of intrinsic microcrystalline silicon thin films. Atlower substrate temperature, the atomic hydrogen density is lowerand the surface diffusion probability of gas precursors is smaller

e 130 nm, 120 nm, 107 nm and 90 nm, respectively) deposited on glass under the

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Fig. 5. The grain size and RMS roughness of samples in Fig. 4 as a function ofsubstrate temperature from 200 �C to 60 �C. The square (j) and the pentacle (H)represent the grain size calculated from AFM and XRD respectively, and the triangle(N) represents the RMS roughness. The lines are drawn to guide the eye.

0.00 0.15 0.30 0.45 0.60 0.75 0.900

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60oC, Eff=6.044% 90oC, Eff=7.324% 150oC, Eff=5.551% 200oC, Eff=4.390%

Fig. 6. The initial J-V characteristics for pin solar cells with p-lc-Si:H thin filmsdeposited at different substrate temperature.

302 K. Tao et al. / Journal of Non-Crystalline Solids 356 (2010) 299–303

[15,16]. So the gas precursors cannot diffuse for a long distance onthe surface to find energetically stable sites to form ordered struc-ture. Finally part of them are desorbed or etched away, and the restpart form less ordered structure which is amorphous. Thus, the gasprecursors which directly fall on the nuclei only contribute to thelc-Si growth. As a result, the deposition rate and crystalline vol-ume fraction of p-lc-Si:H thin films is smaller when substratetemperature is lower. In addition, the dark-conductivity is smallerfor the lack of conductive crystalline grains.

The dependence of grain size as a function of temperature aredifferent as shown in Fig. 5, however, they are not inconsistent.In fact, there are two kind of grain size: one is calculated fromAFM, which is used to describe surface topography; and the otheris computed from XRD measurement using Debye–Scherrer for-mula. When the thickness of film is small, the crystalline nucleican not contact with each other and the crystallite size is depen-dent on the thickness of film. So, given the fixed thickness, samplesdeposited under lower substrate temperature have smaller crystal-lite size and crystalline volume fraction. As the substrate tempera-ture increases, the crystalline size and crystalline volume fractionof thin films augment, which is similar to the intrinsic microcrys-

talline silicon [17]. When the thickness of films becomes large,the crystalline grains have contacted with each other, especiallyfor samples deposited under high temperature, as shown inFig. 4. According to the discussion above, the RMS roughness showsa degressive trend. After contacting with each other, the grains be-gin the emulative growth, and column structures are formed. Forsamples deposited at 150 and 200 �C, they are thought to be onthe growth of ‘‘column structure”. While, for samples depositedat 60 and 90 �C, the grains just partly aggregate, thus they havelarge surface roughness and grain size.

When the thin p-lc-Si:H layers deposited under different sub-strate temperature, were incorporated in a-Si:H solar cells, Theefficiency of solar cells changes as a function of substrate temper-ature. It can be explained that when the temperature is higher, thecrystalline volume fraction of the p-lc-Si:H thin films is larger, andthe mobility band gap is smaller. A larger mismatch of band gapbetween p-layer and i-layer induce to a lower built-in potential,as a result the Voc is smaller. The decrease of Jsc can be attributedto larger carrier recombination at the i/p interface [18]. When thetemperature decreases to 60 �C, the crystalline volume fraction ofthe p-lc-Si:H thin films is very small, and the conductivity islow, as described above. Thus, the activation energy increases,and the built-in potential also decreases, which induce to a lowerVoc and Jsc.

5. Conclusion

The temperature dependence of micro-structure and optoelec-tronic properties of p-lc-Si:H thin films has been studied. Thedeposition rate, crystalline fraction as well as conductivity of p-lc-Si:H thin films diminish as the substrate temperature decreasesfrom 200 to 60 �C. The grain size of films with different thickness asa function of substrate temperature has been investigated. Whenp-lc-Si:H thin films deposited at different temperature are incor-porated into p–i–n structure solar cells, the best performance of so-lar cells is obtained with p-layer prepared at 90 �C.

Acknowledgements

The authors are grateful to Changjian Li, Yanfeng Ma and Bo Liufor helpful discussions and AFM, Raman measurements. Also, theauthors highly appreciate China National high-tech research devel-opment plan (2006AA05Z422), China Tianjin applied basic researchprojects and cutting-edge technology (08JCYBJC13100) and ChinaTianjin Science and Technology research plan (06YFGPGX08000).

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