* Corresponding author. Tel.: #39-081-772-3312; fax: #39-081-772-3344.E-mail address: tucci@portici.enea.it (M. Tucci).

Solar Energy Materials & Solar Cells 69 (2001) 175}185

CF�/O

�dry etching of textured crystalline silicon

surface in a-Si:H/c-Si heterojunctionfor photovoltaic applications

Mario Tucci*, Rosario De Rosa, Francesco RocaENEA - Research Center, Localita% Granatello-80055 Portici (Na), Italy

Received 24 January 2000; received in revised form 31 July 2000; accepted 5 November 2000

Abstract

We investigated a dry cleaning procedure of the crystalline substrate, both mono- and multicrystalline silicon, to leave an uncontaminated surface using an etching process involvingCF

�/O

�mixture. A detailed investigation was performed to "nd compatibility and optimisa-

tion of amorphous layer depositions both on #at and textured silicon by changing the plasmaprocess parameters. We found evidence that plasma etching acts by removing the native oxideand the damages of textured silicon and by leaving an active layer on silicon surface suitable forthe emitter deposition. SEM analysis con"rmed that it is possible to "nd plasma processconditions where no appreciable damages and change in surface morphology are induced. Byusing this process we achieved on amorphous crystalline heterostructure a photovoltaicconversion e$ciency of 13% on 51 cm� and 14.5% on 1.26 cm� active area. We also investigatedcompatibility of the process with industrial production of large area devices. � 2001 ElsevierScience B.V. All rights reserved.

Keywords: Dry etching; Heterojunction; Amorphous silicon

1. Introduction

The realisation of electronic devices based on c-Si heterojunctions using amorphoussilicon layers grown by plasma enhanced chemical vapour deposition (PECVD) seemsto be a very attractive technology due to its low cost, especially in case of large area

0927-0248/01/$ - see front matter � 2001 Elsevier Science B.V. All rights reserved.PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 4 0 9 - 8

applications such as solar cells [1,2] or thin "lm matrices [3]. One of the fundamentalcharacteristics is the low-temperature process needed during deposition of amorph-ous silicon. In fact, it can be e$ciently applied on any substrate that degrades at hightemperature, such as glass or a thin "lm [4]. Moreover, due to the very low thickness,the amorphous silicon layers involved in these heterostructures do not show anylight-induced degradation. Therefore, photodetector devices and solar cells obtainedwith this technology can achieve good and stable electrical properties.

Even though several research groups have investigated a-Si:H/c-Si heterostructurein order to produce low-cost and high-e$ciency solar cells and photodetectors in aneasy way, many problems still need solutions and improvements.

The surface of the substrate must be regular enough to allow conformal depositionof the thin a-Si layer in order to avoid shorts between the substrate and other layers.Any high-temperature step must be avoided after the deposition of amorphous siliconin order to preserve the junction integrity. One of the most critical steps in thefabrication of a-Si/c-Si heterostructure is the preparation of the crystalline siliconsurface before the amorphous silicon deposition. When the a-Si layer is deposited onc-Si, without any treatment before the deposition, the correct rectifying behaviour ofthe device is not well reproduced due to the presence of an insulating silicon oxidelayer on the surface of the wafers. A very e!ective method to reduce this SiO

�layer is

a bath in HF:DW (Deionised Water), (1}5):100, for few minutes [5]. Other wet etchingmethods are available, in particular, a very useful treatment for textured silicon isa bu!ered HF bath obtained with the following recipe: HF:HNO

�:DW at 1.5:1:30.

The wet treatment does not represent a good choice, especially in industrial produc-tion project. In fact, dry etching treatments simplify the cleaning procedure in solarcells production. Also, several dry chemical treatments were proposed in order topassivate the crystalline surface [5] before the amorphous layer deposition. But eachof them had the same goal: ideally terminating with a H atom of the dangling bonds ofthe atoms on silicon surface. This is not easy to obtain in the case of #at surfaces and itis very di$cult to achieve in the case of non regular surfaces such as textured silicon ormultisilicon. Then the surface treatments of the crystalline silicon before the amorph-ous layer deposition remain the key point to obtain a good junction with lessrecombination centres.

In literature, it is also reported that the use of special bu!er layers betweencrystalline silicon and the doped amorphous layer reduces recombination e!ects atinterface. Most of these bu!ers were obtained with di$cult processes involving criticalsteps such as deposition of a high-purity intrinsic layer [1].

In this paper we propose a di!erent point of view: try to obtain an active siliconsurface on which a doped amorphous silicon without any further bu!er layer can bedirectly deposited. With this aim, we developed a dry cleaning procedure based onradio frequency (RF) activated plasma of CF

�/O

�gas mixture. We studied this

plasma etching on several substrates, including textured multisilicon, in order toensure application to standard solar cell technology and industrial process. In par-ticular, we analysed the e!ect of the CF

�/O

�plasma on the silicon surface, as well as

the behaviour of the a-Si/c-Si heterojunction after di!erent plasma etching treatmentsand in particular we optimised etching time and temperature directly analysing the

176 M. Tucci et al. / Solar Energy Materials & Solar Cells 69 (2001) 175}185

photovoltaic parameters of the heterojunction obtained with this cleaning procedure.Good rectifying behaviour and encouraging conversion e$ciency were found for thesolar cell devices made on conditioned crystalline silicon surface.

2. Experiment

We investigated heterostructure solar cells based on a heterojunction formed bya p-type crystalline silicon wafer and an n-type amorphous silicon top layer. Wefocused our analysis on the dry cleaning procedure of the surface of the crystallinesilicon wafer. With this aim several substrates were experimented such as #at �1 0 0�oriented, p-type boron doped, 0.7}1.3 � cm resistivity, Czochralski silicon wafers,Bayer Cz silicon with the same orientation and doping, and "nally multi-crystallinegrown by Eurosolare S.p.A. The two last materials were textured with KOH etchingprocedure. The cleaning procedure was performed on crystalline wafer in two steps asfollows:

(1) 5 min in isoprophilic bath followed by nitrogen drying to remove dust from thesurface.

(2) dry etching in a commercial 13.56 MHz capacitive coupled glow discharge systemwith single UHV chamber using a RF activated CF

�/O

�plasma. The same

system was also used for PECVD of the amorphous silicon emitter layer of thesolar cell. The CF

�/O

�gas mixture had a 8% oxygen content, and the parameters

used during the etching process were the following: 200 mTorr working pressure,30 sccm gas #ow, 98 mW/cm� glow discharge power density, and temperatures inthe range 40}3203C.

The "rst step of the analysis was to measure the etching rate of CF�/O

�on #at

crystalline silicon. With this aim, a 500 nm thick aluminium layer was deposited bye-beam evaporation and patterned by a photolithographic step. Since aluminium doesnot react during the CF

�/O

�plasma exposure, a perfect step is obtained at the edge of

the pattern. Of course the depth of the step depends only on the plasma exposure time.At 2403C we found an etching rate of 275 nm/min. Di!erent etching times wereobtained at di!erent substrate temperatures. A linear trend of the etching rate versustemperature is obtained and presented in Fig. 1. In the following experiments, we "xedthe etching temperature at 2403C.

Re#ectance measurements using a Perkin-Elmer Lambda 9 spectrophotometer inthe range of 300}1100 nm and SEM analysis of the etched surface were done toinvestigate the optical and morphological properties of the wafer, in particular, in thecase of textured surface.

A 30 nm n� layer of amorphous silicon was deposited using a 200 mTorr workingpressure, 3203C substrate temperature, 20 sccm SiH

�and 10 sccm PH

�gas #ows, and

8.4 W/cm� power density. A grid shaped silver contact, deposited by e-gun evapor-ation, was used as the front electrode of the solar cell. No indium tin oxide (ITO)collecting layer was applied to avoid any possible contamination. The back contact

M. Tucci et al. / Solar Energy Materials & Solar Cells 69 (2001) 175}185 177

Fig. 1. Trend of the etching rate in CF�/O

�gas mixture as a function of the substrate temperature.

Fig. 2. Sketch of the heterostructure device used for dry etching optimisation.

was ensured by a screen printed deposition of an aluminium silver alloy typically usedin a solar cell process followed by an annealing step at 6003C. Of course this backcontact was realised before any plasma treatment and amorphous layer deposition,since a high temperature was used in the annealing procedure. Finally an annealingstep for 10 min at 2503C under nitrogen #ux was performed before electrical measure-ments to enhance the ohmic behaviour of the contact between amorphous n-typelayer and silver grid [6].

In Fig. 2, a cross-section of the device used in this investigation is shown. Photovol-taic characteristics, under standard AM1.5 conditions using a Spectrolab solar simu-lator as well as quantum yield (QY) pro"le were evaluated to extract informationabout the status of the heterojunction after a di!erent etching time.

By using the cleaning process described above we were able to fabricate large areaheterojunction solar cells at low temperature using steps compatible with industrialproduction techniques. To evaluate the maximum e$ciency of the solar cell involvingthis kind of cleaning procedure, we "nally realised several samples with a more

178 M. Tucci et al. / Solar Energy Materials & Solar Cells 69 (2001) 175}185

Fig. 3. Total and averaged di!used re#ectance in the range of 300}1100 nm as a function of the etching timein CF

�/O

�gas mixture.

e$cient collection window than that described above. In particular, we deposited bysputtering and/or e-beam, a 90 nm ITO layer on the n-type amorphous emitter. Thislayer behaved as antire#ecting coating and low resistivity layer. Finally, a screenprinted silver grid optimised for low-temperature conditioning was deposited. Opti-misation of drying procedure of the front grid silver paste was the last step of thepresent work.

Preliminary results and applications of this cleaning technique for heterojunctionsbetween amorphous silicon and multicrystalline textured silicon are reported at theend of the paper.

3. Results and discussion

3.1. Surface analysis

The main aim of this investigation was to "nd a cleaning procedure for texturedcrystalline silicon that can be used in industrialisation processes applying heterojunc-tions. Therefore, we focused our attention on the surface of this material after etchingtreatments. In the initial stages, the e!ect of etching on the textured crystalline siliconsurface was monitored by re#ectance measurements. In particular, we performed totaland di!use re#ectance measurements on samples treated from 1 min up to 20 min, andwe found that the di!use component represents the major contribution to the totalre#ectance because of a pyramidal shape on the textured surface. In Fig. 3, the linearbehaviour of both total and di!used re#ectance, averaged in the range of300}1100 nm, as a function of etching time are shown. The increasing values of there#ectance suggest an anisotropic e!ect of the etching process, since a change inmorphology of the surface occurs. To get a better understanding, SEM analyses wereperformed on c-Si textured wafers that con"rmed the re#ectance results. We can argue

M. Tucci et al. / Solar Energy Materials & Solar Cells 69 (2001) 175}185 179

Fig. 4. (a), (b), (c), (d) SEM pictures of the textured silicon surface, respectively, after 1�, 5�, 10�, 20� etchingtimes in CF

�/O

�gas mixture.

that the incoming #ux of CF�/O

�plasma species acts more on the lateral faces of the

pyramids, than on the peaks and the valleys of the surface. Then a change in the shapeof the textured pro"le became evident increasing with etching time. After 20 min,a planar area within the valleys and a sharper tip on the top of pyramids started toappear. In Fig. 4, the e!ects of the increasing etching time are reported. In particular,SEM pictures of the textured silicon surface after 1, 5, 10 and 20 min are shown. InFig. 5 SEM pictures of some areas such as tips and holes after 20 min of etching arepresented. From the pictures, in particular, Fig. 4(c), it is quite evident that an increasein hole formation occurs at the base of the pyramidal faces due to the presence of dustthat enhances the erosion process. Due to the isotropy of the dry etching process,e!ects arising form lattice defects, such as dislocations, vacancy and impurities haveno relevance in the surface morphology after the etching.

180 M. Tucci et al. / Solar Energy Materials & Solar Cells 69 (2001) 175}185

Fig. 5. (a), (b) SEM pictures of some particulars on the textured silicon surface after 20� etching time inCF

�/O

�gas mixture.

Fig. 6. Short-circuit current and open-circuit voltage of c-Si/a-Si:H heterostructure solar cells as a functionof the etching time in CF

�/O

�gas mixture.

3.2. Photovoltaic properties

The previous analysis of the surface morphology explains the observed decrease ofphotovoltaic parameters in the case of high etching times as presented in Figs. 6 and 7.In fact, the #atness in the valley increases the re#ectance of the surface, and thesharpening of the peaks produces electrical shorts through the amorphous siliconlayers. Both will lead to a reduction in <

��and in J

��. Moreover for shorter etching

times, a decrease in "ll factor occurs due to the carrier recombination via defects at theinterface between the amorphous and crystalline materials as reported in Fig. 7. Thosedefects can be related to both a residual oxide layer at the interface and a damagedsilicon surface. The decrease in <

��values with increasing etching time is also related

M. Tucci et al. / Solar Energy Materials & Solar Cells 69 (2001) 175}185 181

Fig. 7. Conversion e$ciency and "ll factor of c-Si/a-Si:H heterostructure solar cells as a function of theetching time in CF

�/O

�gas mixture.

Fig. 8. Quantum Yield measurements of c-Si/a-Si:H heterostructure solar cells as a function of the etchingtime in CF

�/O

�gas mixture.

to an increase of defect density induced by the plasma on the surface of the crystallinesilicon wafer that shifts the Fermi level position toward mid-gap in the crystalline side.This is re#ected in the reduction of the built-in voltage of the heterojunction.

During the etching process, two e!ects must be taken into account. The "rst is thenative oxide removal and the second is the silicon oxidation and removal of thatoxide. The latter will leave an oxide layer thinner than the native one on the surface.For longer plasma duration, there is no oxide "lm growth on the silicon surface afteran initial reaction layer is formed [7].

We con"rm this assumption by QY measurements performed in short circuitcondition and the results are presented in Fig. 8. The reduction in the collection of thespectral component due to an increase of re#ectance is evident. In particular, the QY

182 M. Tucci et al. / Solar Energy Materials & Solar Cells 69 (2001) 175}185

performed at di!erent etching time does not change in the shape, but only inamplitude.

On the other hand, for the CF�/O

�plasma, the amount of carbon radicals is

negligible because carbon species are oxidised in the gas phase and at surface, in fact,the presence of carbon species dramatically increases the recombination e!ect atinterface [7,8].

A trade-o! is needed in the choice of etching time to obtain the best conditioning ofthe surface. In fact, we seek to reduce the oxide thickness and remove the damagedsurface layer present after the cutting and texturing steps, but we also need to preservethe low re#ectance of the surface. A reduction in defect formation can avoid a pinningof the Fermi level at the crystalline silicon surface in the midgap position correspond-ing to a maximum of the recombination rate of the photogenerated carriers. Thenincreasing depletion in the crystalline region after junction formation is expected [9].

By "xing the etching time to be 1� 30�� for this cleaning procedure, we obtain a goodcompromise between the short-circuit current and open-circuit voltage of the solarcells. Then from these results we can argue that the proposed cleaning procedureleaves the surface of crystalline silicon covered with a very short thick silicon oxidethat does not reduce the carrier transport across the heterojunction and behaves asa protective layer, reducing the possibility of hydrogen permeation in the crystallinesubstrate during the amorphous layer growth [10]. Previous experiments performedon heterojunction solar cells, in which we used hydrogen dry etching as cleaningprocedure of similar p-type crystalline silicon textured surface before amorphouslayers deposition, showed lower <

��values [9]. Hydrogen motion, in fact, is one of

the most damaging process is that occurs on crystalline silicon during exposure tohydrogen plasma, in particular, during a heterojunction formation with amorphouslayer. In fact, the formation of hole traps (charge states), as a consequence of hydrogenpermeation and acceptor deactivation in the crystalline p-type semiconductor, is a notparticularly stable phenomenon thermally [10]. On the crystalline surface, it producesa shift of the Fermi level toward mid-gap that in#uences the amorphous n-type layerproperties and this is re#ected in the higher defect density at interface betweenamorphous and crystalline silicon [9].

3.3. Top contact optimisation

Better collection of the photocurrent of the solar cell can be obtained using a 90 nmITO layer and a silver grid-shaped top contact because the resistivity of ITO is notvery low: 2�10��� cm. Therefore in the following, the investigated heterostructurecell is Ag/ITO/a-Si:H/c-Si. The choice of silver is done on the basis of previous worksin which we demonstrated the endurance of the Ag/ITO contact during annealingprocess up to 3003C [6]. With this emitter contact, we obtained the followingphotovoltaic parameters for a small area device:

<��

"576 mV; J��

"35.32 mA/cm�; FF"72.45%;

EFF"14.74%; Active area"1.26 cm� ;

M. Tucci et al. / Solar Energy Materials & Solar Cells 69 (2001) 175}185 183

Fig. 9. Comparison of internal quantum e$ciency of c-Si/a-Si:H heterostructure solar cells grown ontextured mono- and multicrystalline silicon.

Table 1Silicon substrate, active area and photovoltaic parameters of the samples reported in Fig. 9

c-Si substrate Active area (cm�) <��

(mV) J��

(mA/cm�) FF (%) EFF (%)

Monocrystalline 51 591 34.8 62.6 13Multicrystalline 45 554 31.5 52.7 9.2

In the case of large area application, this metallic grid must be deposited by screenprinted process because thicker metallic layers are needed to carry high photocurrentfrom the solar cell. The commercial, screen printed pastes have the problem ofannealing temperature needed to dry the solvents present in the mixture. But amorph-ous/crystalline silicon heterojunction cannot be annealed at temperature higher than3003C for prolonged time, because of the evolution of hydrogen [11] and di!usion ofmetal from top grid into n-type amorphous layer. This second e!ect can be reducedwith the ITO layer [6], but the "rst one increases the recombination centres density.We found a good compromise in a special paste that can be dried at temperature lessthan 3003C, and showed a conductivity of 2.23�10��� cm.

The dry CF�/O

�plasma cleaning procedure was also successfully applied on cast

multicrystalline silicon and results obtained were encouraging. In Fig. 9, a comparisonbetween di!erent heterostructure solar cells fabricated using this cleaning procedure isreported. In particular, Fig. 9 shows the internal quantum e$ciency of two large areaheterojunctions. One is grown on a monocrystalline textured substrate, and the otheron textured multicrystalline silicon. In Table 1 the photovoltaic parameters, the activearea and the crystalline material used in the samples are summarised. Optimisation ofthe grid design is needed to increase the conversion e$ciency.

4. Conclusions

In this work we examined a dry cleaning treatment using a RF plasma in a CF�/O

�gas mixture. This process is proposed in order to reduce the e!ect of the native oxide

184 M. Tucci et al. / Solar Energy Materials & Solar Cells 69 (2001) 175}185

layer and damages from the silicon surface. With this cleaning procedure, it is possibleto achieve high-quality heterojunctions between amorphous and textured monocrys-talline silicon and textured multicrystalline silicon. We found that care is needed in theetching process in particular with regard to the choice of the etching time in order tooptimise the photovoltaic properties of the heterojunctions. SEM analysis and re#ec-tance measurements of the treated surface are reported for the evaluation of thebehaviour of the cleaning procedure. Optimum etching time is found by the evalu-ation of photovoltaic parameters measured on several samples treated with di!erentetching times. E$ciency of 13% on 51 cm� active area has been obtained, cleaning thewafer with plasma treatment in a CF

�/O

�gas mixture for 1� 30�� and with the aid of

a ITO antire#ection coating and special low-temperature dried silver printed grid.Then we can conclude remarking that the achieved results can be easily transferred

to industrial production of full low-temperature heterostructure solar cells.

Acknowledgements

The authors are grateful to Dr. M.L. Addonizio for SEM pictures, to Dr. L. Pirozzifor screen printed process and I. Nasti for etching rate measurements.

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M. Tucci et al. / Solar Energy Materials & Solar Cells 69 (2001) 175}185 185