Current Status of the Thermo-catalytic (HW) CVD of Thin Si Films for PV Applications

7/28/2019 Current Status of the Thermo-catalytic (HW) CVD of Thin Si Films for PV Applications

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Thin Solid Films 395 (2001) 298304

0040-6090/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved.PII: S 0 040- 6090 0 1 .0 1 2 7 7 - 9

Current status of the thermo-catalytic (hot-wire) CVD of thin siliconfilms for photovoltaic applications

Bernd Schroeder*, Urban Weber, Holger Seitz, Andrea Ledermann, Chandrachur Mukherjee

Department of Physics and Center of Materials Science, University of Kaiserslautern, P.O. Box 3049, D 67653 Kaiserslautern, Germany

Abstract

Thermo-catalytic (TC) or hot-wire (HW) chemical vapor deposition (CVD) is a promising technique for growing amorphousand microcrystalline silicon films with improved stability and high rates. In this paper we report on the photovoltaic (PV)applications of thin silicon films deposited by this method. After a short review of the history of PV applications of TCCVD,from the beginning in 1993, the main part of the paper deals with our research and development of films and interfaces neededfor the fabrication of different solar cell structures entirely by TCCVD. So far, our highest conversion efficiency is hs10.2% fora pin structure, containing only an intrinsic a-Si:H film deposited by TCCVD. Depositing the whole pin structure entirely byTCCVD, we have obtained hs8.8% until now. After development of a tunnel junction, the first tandem solar cell device, astacked pinpin structure has been recently produced showing hs7%. First attempts have been made for large-area deposition.In a 30=30 cm batch system, a-Si:H films with device grade photoelectronical properties and high thickness uniformity can be2

produced at a high rate of 6 Ays. When incorporating i-layers from this system into pin solar cells, a conversion efficiency of

hs(6.4"0.8)% was obtained on an area of 20=20 cm . Finally, we report on silicon wafer-based solar cells, where a-Si, mc-Si2

and epi-Si film emitters were deposited by TCCVD showing conversion efficiencies up to hs15.2%. 2001 Elsevier ScienceB.V. All rights reserved.

Keywords: Amorphous materials; Chemical vapor deposition (CVD); Deposition process; Heterostructures; Solar cells

1. Introduction

Hot-wire or thermo-catalytic chemical vapor deposi-tion of a-Si:H films was firstly reported by Wiesmannet al. w1x in 1979. The method is fairly simple inprinciple, and an inexpensive setup can be used for thedeposition of a-Si:H, mc-Si:H or a-SiX-alloy films.

TCCVD allows film growth from very low depositionrates, yielding smooth and compact films, up to veryhigh deposition rates, which can still be deposited withreasonable quality (no plasma instability or powderformation). This makes it easier to optimize sensitiveinterfaces in devices such as solar cells. However, thedeposition rate can be increased to 10 A ys or more in

less critical parts of the device. Although Matsumura

* Corresponding author. Tel.: q49-631-205-2377; fax: q49-631-205-3300.

E-mail address: [email protected] (B. Schroeder).

w2x and later Doyle et al. w3x, as well as Mahan et al.w4x, have shown that device quality material can bedeposited using this method, it took quite a long timebefore Papadopulos et al. w5x reported the first solar cellfabrication. He used a so-called superstrate (pin) struc-ture, where only the intrinsic layer was deposited byTCCVD. The device performance was hs4.3%. In the

following years, the conversion efficiency of pin (super-strate) as well as nip (substrate) solar cells with intrinsicTCCVD absorber films improved considerably. Theconversion efficiency was increased to hs6.8% in 1996w6,7x and hs10.2% w8x or hs9.8% w9x in 1998 for thepin or nip structure, respectively. In the nip cell yieldinghs9.8%, the i-layer was deposited with a very highrate of 16 Ays w9x. a-Si:H films deposited by TCCVD

at high substrate temperatures ()3008C) also showenhanced stability against light-induced degradationw8,10,11x. Therefore, the TCCVD method turns out tobe an interesting alternative to the plasma-enhanced


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299B. Schroeder et al. / Thin Solid Films 395 (2001) 298304

chemical vapor deposition (PECVD) method. Recently,absorber layers with smaller band gaps, deposited byTCCVD, were also incorporated in solar cell structures.Lill et al. w12x reported on a pin structure, utilizing anintrinsic a-SiGe:H layer (Es1.5 eV) deposited bygTCCVD, which yielded a conversion efficiency ofhs6.4%. Solar cells have also been produced contain-ing mc-Si:H absorber layers produced by TCCVD show-ing hs4.4% w13x and very recently 4.6% w14x. Wanget al. w15x reported first on a nip mc-Si:H solar cellentirely fabricated by TCCVD, yielding hs1.7%, and,very recently, on nip a-Si:H solar cells entirely fabricatedby TCCVD, with initial efficiencies up to 8.7% w16x. Inthis paper we report on research and experimental detailsto realize a-Si:H pin solar cells, as well as pinpintandem structures, entirely deposited by TCCVD. First,results will be shown on the development of large-areadeposition for solar cell applications, as well as on thepreparation of silicon wafer-based hetero- and homo-

junction solar cell devices employing TCCVD.

2. Experimental details

The TCCVD method requires only a simple experi-mental setup, in which a well-defined flux of processgases is supplied by a shower-like gas inlet system anddirected to a hot-wire catalyzer system. W-shaped orparallel, grid-like w17x, tantalum or tungsten wires ofapproximately 0.5 mm diameter are commonly used ascatalyzers. Of course the lifetime of the catalyzer fila-ments is limited. To avoid silizidation, high temperaturecleaning procedures have to be applied, enabling adeposition of 2050 mm. Morrison et al. w18x haverecently reported some promising results on a new typeof filament, which can work for at least 200 mm of filmdeposition. For our experiments the silicon films forsolar cells as well as for materials studies were depositedin a three-chamber system with an additional load lockand transfer chamber, which allows to deposit whole pin structures without cross-contamination and withoutbreaking vacuum. For deposition details and for someadvantages of the use of tantalum over tungsten, seeWeber et al. w19x. The rear contacts of the cells were

fabricated at low rate (f10 Ays) by thermal evapora-tion of silver. To enhance the back reflection for somecells, an ITOyAg back reflector was also realized. IVcell characteristics were obtained at 258C under a lightsource giving 100 mWycm intensity and having an2

AM 1.5-like spectrum. Some cells have been cross-checked under an AM 1.5 solar simulator at the IPVJuelich, where quantum efficiency measurements havealso been carried out. Light soaking was performed bycontinuous illumination using fluorescent lamps (100mWycm ), keeping the cell temperature at approximate-2

ly 258C.

3. Results and discussion

3.1. Development of doped layers required for solar cell

application of TCCVD

3.1.1. n-type doping

Recently, Brogueira et al. w20x have extensively inves-tigated n- and p-type doping in TCCVD. However, fora-Si:H films they have used very high filament temper-atures, T , and moreover they used tungsten filaments.filWe investigated n-type doping, by using phosphine as adopant gas, at conditions with which we obtain devicequality intrinsic films, i.e. 15508C-T -16508C, ps1filPa, F(SiH )s15 sccm, Ts2008C. T was kept at this4 s squite low value since the use of TCO-coated glasssubstrates (Asahi U) in the pin (superstrate) solar cellstructure limits it to this temperature. As published indetail by our group w19x, n-type substitutional doping ina-Si:H by TCCVD is similar to the well-established

results obtained by PECVD w21x. Using prefabricatedTCOypy i substrates, where PECVD p- and i-layers hadbeen deposited on Asahi U substrates at the Institutefor Photovoltaics (IPV), FZ Juelich, we tested theperformance of TCCVD-deposited n-layers directly inthe solar cell device. By a thickness variation of the n-layer we reproduced the well-known result that, forthicknesses lower than 20 nm, the V and fill factorocdecrease due to a reduction in electric field within thecell, whereas for higher thicknesses, increasing up to 60nm, V remains constant, and the fill factor decreasesocslightly. As a result, an optimum n-layer thickness of

2530 nm is found with respect to overall cell perform-ance. Using this optimum thickness, we obtained maxi-mum performance of cell structures using a doping levelof 1% PH in SiH .3 4

3.1.2. p-type window layers

For the use as p-type window layers in superstrate(pin) solar cells, two different materials have beeninvestigated, boron-doped amorphous siliconcarbon (p-a-SiC:H) and microcrystalline silicon (p-mc-Si:H). Forthe amorphous siliconcarbon alloy, two basic require-ments have to be met to apply these as window layersin pin solar cells: low absorption, i.e. high band gap

(E )1.95 eV) and high dark conductivity w)10y6Tauc(Vcm) x. We used methane and ethane as carbony1

precursor gases, and trimethylboron (TMB) as a dopantgas. Investigating the influence of various depositionparameters (for details see Koob et al. w22x), it turnsout that the absolute silane flow is the most crucialparameter for carbon incorporation which determinesthe band gap. In Fig. 1, the band gap of all undoped ornegligibly doped a-SiC:H films deposited in the courseof our investigations on the basis of methane, is shownas a function of silane flow. Deposition parameters arementioned in the figure itself. The band gap increases


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300 B. Schroeder et al. / Thin Solid Films 395 (2001) 298304

Fig. 1. Band gap of amorphous siliconcarbon alloy films preparedby TCCVD as a function of absolute silane flow.

Fig. 2. Dark conductivity vs. band gap for amorphous siliconcarbonalloy films prepared by TCCVD varying the doping gas concentrationc (TMB) and using two different silane flows. Other deposition par-gameters were F(CH )s50 sccm and p s25 Pa.4 g

significantly only at lower silane flows, and only then,

the carbon content in the films increases, as confirmedby Fourier transformed infrared (FTIR) measurementsand secondary ion mass spectrometry (SIMS). At thesame time, however, the deposition rate drops steadilywith decreasing silane flow, from approximately 1.5 Ays at F(SiH )s2 sccm to no deposition at all for zero4silane flow. These findings can only be understood ifwe assume that methane is not directly dissociated atthe hot filament, but by secondary gas phase reactions(collisions) with silicon-containing radicals or atomichydrogen. Therefore, it appears that silane-deficientconditions have to be employed for carbon incorporationin the film w22x. Additionally, a high filament tempera-ture (19008C) and high pressure (25 Pa) have to bechosen. Since band gap and dark conductivity need tobe optimized simultaneously for p-type a-SiC:H, a usefulvisualization is to display one against the other for acertain variation of deposition conditions. In Fig. 2,such a graph is shown for two different but low(starving) silane flows. The concentration of the dop-ant gas relative to silane c (TMB)sF(TMB)yF(SiH )g 4was varied. It should be noted that the basic require-ments on the band gap and dark conductivity of anamorphous siliconcarbon window layer can be met byusing TCCVD of silane, methane and TMB: E s1.98Tauc

eV; s s710 (Vcm) ; and E s300 meV havey6 y1D Abeen obtained. However, since the deposition parametersof the p-a-SiC:H layer are far away from device qualitya-Si:H deposition parameters, these films are void- anddefect-rich and are subject to a fast oxidation in air aswell as instabilities when employed in pin or pinpindevices.

Similar results were obtained using ethane instead ofmethane. However, carbon incorporation can be accom-plished more easily with ethane, such that lower filamenttemperatures (;17508C), lower pressures (;7 Pa) andhigher silane flows (;2 sccm) may be used. Still, these

parameters are far from the deposition conditions fordevice quality intrinsic material.

As an alternative to the rather porous and unstable p-a-SiC:H, a p-type microcrystalline silicon material wasdeveloped by TCCVD. TMB was also used as a dopantgas. A hydrogen dilution of F(H ):F(SiH )s15:1 and2 4a low substrate temperature Tf1201808C were usedSfor growing these films. There are two problems withgrowing p-mc-Si:H films on textured TCO substrates:(i) the loss of transparency of the TCO layer during thegrowth of the p-mc-Si:H film, due to the high concen-tration of atomic hydrogen in such growth environments;and (ii) the attainment of the optimum crystallinefraction (2030%) to achieve fairly high s and trans-Dparency in thin p-mc-Si:H films (dF20 nm). In orderto solve the above problems, first a very thin p-a-Si:Hseed layer (df2 nm) was grown on TCO using a lowerT and a larger filament-to-substrate distance d result-fil filing in a moderate rate of rf12 A s . Then p-mc-y1d

Si:H layers were deposited at a higher T and lowerfilfilament-to-substrate distance. In 400-nm thick p-mc-Si:H films, a high conductivity (s f1 V cm ) withy1 y1Da low activation energy (E f62 meV) have beenAachieved. For thin films (df20 nm) suitable for solarcell applications, s f4.7=10 (Vcm) and E f80y2 y1D AmeV were obtained. More details about the developmentand properties of the p-mc-Si:H film are publishedelsewhere w23x.

3.2. Pin solar cells grown entirely by TCCVD

Taking into account the knowledge reported in Section3.1, superstrate (pin) solar cell structures have beenproduced entirely by TCCVD. p-a-SiC:H and p-mc-Si:Hfilms have been used alternatively for the window layer.Intrinsic layers were always deposited with amoderate hydrogen dilution of the process gas


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Fig. 3. AM 1.5 fill factor and blue and red fill factor (inset) of pinsolar cells entirely deposited by TCCVD as a function of the thicknessof the intrinsic layer.

Fig. 4. Schematic view of the stacked cell structure.

wF(H ):F(SiH )s1:3x, which shows optimum stability2 4

of a-Si:H films deposited at TF2008C by TCCVD (seesalso Bauer et al. w24x). Detailed investigations havebeen performed to find out the optimum depositiontemperature for the pin device, as the deposition of thep-window layer requires relatively low T , while the i-slayer quality improves with increasing T . Due to theslarge influence of the quality of the p-window layer oncell performance, the optimum T was found to be quiteslow, Tf1608C (for details see Weber et al. w19x). Thereswas no improvement in conversion efficiency when thei- and n-layer depositions were carried out at elevatedT compared to the p-layer deposition. As known fromsinvestigations of Bauer et al. w6,8x soft deposition of thei-layer near the py i-interface, the so-called p y i-interfaceengineering, has always improved the open circuit volt-age V and the fill factor FF of TCCVD solar cells. Asocshown in Fig. 3, the fill factor of the TCCVD pin solarcells hardly decreases up to i-layer thicknesses off600nm, indicating the high quality (large minority carrierdiffusion length) of our intrinsic layer material. For pinstructures containing the p-a-SiC:H window layer, themaximum initial conversion efficiency obtained untilnow is hs8.8% (J s15.6 mAycm , V s852 mV,2sc ocFFs66%) on an active area of 0.08 cm . However, we2

have also achieved, with similar conditions, 8.0% initial

efficiency on an area of 0.8 cm . For this cell we2

obtained an even higher fill factor of 69%; however,V and J were a bit smaller in this case. Additionally,oc sccell properties were found to be homogeneous on anarea of 5=5 cm . Only a few attempts have been2

undertaken to optimize the cells since, as described inSection 3.4, the cells containing the p-a-SiC:H windowlayer are quite unstable and show even irreversiblechanges. From our investigations of high performance(hf10%) pin solar cells containing only the i-layerdeposited by TCCVD, we know that the i-layer doesnot give any reason for this instability w8x.

In order to overcome the instability of our solar cells,mentioned above and reported in Section 3.4, micro-crystalline silicon was used as an alternative p-layermaterial (see also Mukherjee et al. w23x). The incorpo-ration of p-mc-Si:H layers in pin solar cells hasimproved both the open circuit voltage (to 870900mV) and the fill factor (up to 72%), compared with theuse of p-a-SiC:H layers. However, due to the incorpo-ration of large amounts of an amorphous phase into themc-material w23x, the transparency of the p-mc-Si:Hwindow layers is lower, which reduces J . At present,scthe parameters for the best pin solar cells with p-mc-Si:H are J s12.3 mAycm , V s873 mV, FFs72%,2sc ocresulting in an initial efficiency ofhs7.7%.

3.3. Development of a-Si:Hya-Si:H tandem solar cells

As known from the well-established PECVD method,a-Si:H-based solar cells require stacked structures toobtain high stabilized efficiencies. In Fig. 4, a schematicview of the stacked (tandem) cell structure is shown,which we realized for the first time employing entirelythe TCCVD method.

The development of a tunnel or recombination junc-tion between two pin or nip devices with at least onemicrocrystalline layer is the key to realize tandem solarcells with a low series resistance and a high FF. For thetandem cells described here, the n-layer of the top cell(n ) was always microcrystalline. We have developed1two types of tunnel junctions: n -mc-Si:Hyp -a-SiC:Hq q

and n -mc-Si:Hyp -mc-Si:H (for details see Weber etq q

al. w25x). In order to match the current of the top andbottom junctions, the top junction thickness was syste-matically varied. Using a highly dense top i-layer (i )1w25x, the thickness of this layer could be reduced to 43nm. Together with a bottom absorber layer thickness of450 nm, a well-balanced cell structure has been realizedw25x.

In Fig. 5 the IV characteristics of two tandemstructures are depicted. The maximum initial conversion


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Fig. 5. IV characteristics of two a-Si:Hya-Si:H tandem cells. Cellno. 2 was measured at IPV Julich, after some degradation.

Fig. 6. Spectral response measurement of an a-Si:H ya-Si:H TCCVDtandem structure (cell no. 1) with same absorber material and thick-nesses of 43 and 450 nm for the top and bottom absorber, respectively.

efficiency obtained so far for a TCCVD tandem structureis hs7.0% (J s7.6 mAycm , V s1.63 V,2sc oc

FFs57%). However, fill factors up to 71% have alreadybeen obtained. Cell no. 2 with hs7%, which has anITOyAg back contact, was measured at IPV Julich

when some irreversible degradation had already takenplace. Fig. 6 shows the spectral response of a TCCVDtandem cell which demonstrates that the currents of thetwo junctions are nearly balanced: J s7.35 mAycm ;top 2scJ s7.14 mAycm . Attempts have been made tobottom 2scimprove the current by using absorber layers of differentband gap in the top and bottom junctions, but until nowwe have not achieved a device improvement.

3.4. Degradation behavior of TCCVD solar cells

It has been shown by Bauer et al. w8x that pin solarcells, containing an i-layer deposited with moderatehydrogen dilution at low T , exhibit a similar degrada-Stion behavior as cells prepared by PECVD using highhydrogen dilution for i-layer deposition. However, thepin as well as the pinpin solar cells entirely fabricatedby TCCVD are still unstable. As reported in detailelsewhere w19,25x, considerable degradation takes place,not only under light-soaking conditions but also withoutillumination. Especially, devices containing p-a-SiC:H

window layers appear to be very unstable, and alsoshow an irreversible degradation, which we attribute tostructural instabilities and acceptor deactivation in thisporous and less dense alloy material. Replacing the p-a-SiC:H window layer by p-type mc-Si:H material insingle and tandem junctions improves the stability. Thedegradation of devices containing p-mc-Si:H layers aremostly reversible, since these cells can be annealed.Much more research has to be carried out to optimizethe TCCVD-based solar cells. Especially, the preparationof the substrate (nip) type structure, which allows theuse of high T , needs to be performed in order to makeS

use of the higher stability potential of the TCCVDmaterial.

3.5. Large-area deposition by TCCVD for solar cell

application

In order to use the thermo-catalytic CVD for industrial

production, it has to be shown that uniform and devicequality large-area deposition is feasible. Therefore, thescale-up of TCCVD of a-Si:H-based solar cells has beenstudied. Among other things, knowledge is required ofhow the gas supply and filament geometry affect thedeposition process. In a detailed investigation w17x theinfluence of gas flow, as well as the effect of multiplefilament geometry on the uniformity and the quality ofthe a-Si:H films, are measured and partly simulated bysimple uniformity calculations. In a 30=30 cm depo-2

sition system, device quality a-Si:H-material, depositedat rs6.0 Ays and with a thickness uniformity ofd

"2.5% on a nearly circular area with a diameter ofapproximately 20 cm, could be achieved using a specialgas shower head and a filament grid of six tantalumwires with a filament-to-filament distance of 4 cm(details see Ledermann et al. w17x). Fig. 6 shows acomparison of the measured and simulated thicknessdistribution. This setup was used to deposit the i-layersof a-Si:H pin solar cells. The p-layers were depositedby PECVD, and the n-layers were made by TCCVD indifferent deposition systems. Although there were twoairbreaks at the py i- and the iyn-interfaces, we couldfabricate solar cells with initial efficiencies ofhf(6.4"0.8)% on an area of 20=20 cm at this early2

stage of development.

3.6. Fabrication of silicon wafer-based solar cells with

emitters deposited by TCCVD

Hetero- and homojunction silicon solar cells havebeen produced by depositing n- and p-type amorphous,


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Fig. 7. Comparison of simulated and measured thickness variation forlarge area deposition.

Fig. 8. IV characteristics and solar cell parameters of a n-a-Si yp-c-Si heterojunction solar cell.

microcrystalline and epitaxially grown emitters on p-and n-type c-Si wafers, respectively, by TCCVD. Detailsof deposition and epitaxial growth are published else-where w26,27x. p-a-Si:Hyn-c-Si, p-mc-Si:Hyn-c-Si andn-a-Si:Hyp-c-Si heterojunction as well as epi-n-Si yp-c-Si homojunction solar cell devices have been manufac-tured. For the last three mentioned devices, conversionefficiencies of 1011% were achieved for quite simpledevice structures. Dark IyV-measurements show that theV and FF of solar cells on textured substrates areOCmostly limited by the emitterybasis-interface defectdensity (Fig. 7), which can be reduced by a HW

hydrogen treatment of the silicon wafer prior to emitterdeposition w28x. As shown in Fig. 8, a conversionefficiency of 15.2% (active area) has been obtained bydepositing an n-a-Si:H emitter on textured p-c-Si wafers.

4. Conclusions

First, a review is given on different solar cells, wherea-Si:H or mc-Si:H-films fabricated by TCCVD havebeen incorporated so far. In the main part of the paperit is reported how we obtained pin and pinpin solarcell structures entirely deposited by TCCVD. At thepresent state of development, pin superstrate solar cellswith hs8.8% (initial), and pinpin tandem structureswith hs7% (after some degradation), were achieved.Due to instabilities of the doped layers, especially thep-a-SiC:H window layer, and as a consequence of device interfaces, the solar cells still show a largedegradation, which is only partly reversible. Firstattempts have been undertaken to show that large-areadeposition is feasible. In a large-area deposition system,device quality a-Si:H material with a high thicknessuniformity has been deposited at a high rate, andhs(6.4"0.8)% (initial) has been obtained on an area

of 20=20 cm for devices where the i-layer was2deposited in this system. Finally, we report on hetero-and homo-junction devices where different emitters weredeposited by TCCVD on c-Si-wafers. A maximumconversion efficiency of hs15.2% (active area) for an-a-Si:Hyp-c-Si hetero junction solar cell structure wasachieved.

Acknowledgements

The authors would like to thank Dr R.O. Dusane(now IIT, Bombay) for his contributions to the doped

microcrystalline films, Dr A.R. Middya(

now Universityof Syracuse, USA) for contributions to the developmentof tandem structures, Dr H. Stiebig (Institute for Pho-tovoltaics (IPV), FZ Julich) for helpful discussions and

control I (V) and spectral response measurements, andDr S. Bauer (Schott Glas) for carrying out uniformitymeasurements with reflection spectrometry. We aregrateful to the German Federal Ministry of Economicsand Technology (BMWI, grant no. 0329 811 0) andAngewandte Solartechnik(ASE) GmbH, Product CentrePhototronics, Putzbrunn, Germany, for financial support.

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Current Status of the Thermo-catalytic (HW) CVD of Thin Si Films for PV Applications