Beck, N.

Fejfar, A.

Fric, J.

Kocka, J.

Poruba, A.

Remes, Z.

Shah, A.

Stuchlikova, H. A.

Torres, P.

Vanecek, M.

Wyrsch, N.

Infoscience - École polytechnique fédérale de Lausanne

OPTICAL AND ELECTRICAL PROPERTIES OF UNDOPEDMICROCRYSTALLINE SILICON DEPOSITED BY THE VHF-GD WITHDIFFERENT DILUTIONS OF SILANE IN HYDROGENN. BECK*, P. TORRES*, J. FRIC**, Z. REMEˇS**, A. PORUBA**, HASTUCHL´IKOV ´A**, A. FEJFAR**, N. WYRSCH*, M. VANˇE ˇCEK**, J. KO ˇCKA**,A. SHAH** Institut de Microtechnique, Université de Neuchâtel, Rue A. L. Breguet 2, 2000Neuchâtel, Switzerland** Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10,162 00 Praha 6, Czech RepublicABSTRACTWe show that the optical and electrical properties of microcrystalline silicon (µc-Si:H)deposited by the VHF-GD technique at 110 MHz can considerably be tuned by changing thedilution ratio of silane to hydrogen.With increasing silane dilution we observe enhanced optical absorption for energiesbelow 2 eV due to the transition of the material from amorphous / microcrystalline mixtureto a pure microcrystalline phase. Simultaneously, the light scattering and the defectabsorption increases. Strong dilution also promotes the incorporation of impurities into thematerial, leading to a pronounced extrinsic behaviour as seen from the decrease of theactiviation energy of the electrical conductivity.The electrical properties were investigated in the dark by the Time of Flight technique.We measured drift mobilities at room temperature which slightly increase with dilution,reaching values of 3 cm2/Vs for electrons and 1.2 cm2/Vs for holes. The ratio betweenelectron and hole drift mobilities is found to be around 2 for all samples studied, similar tothat of crystalline silicon.Furthermore, post-transient Time of Flight measurements revealed detrimental electrondeep traps in low dilution material.INTRODUCTIONWith an enhanced optical absorption in the infrared region together with a lowsub-bandgap absorption, the microcrystalline silicon produced by the VHF-GD depositiontechnique is a promising material for solar cell applications [1]. The absorption properties ofsuch material is almost "crystalline-like" [2], but can also be tuned by changing thedeposition parameters [3].On the other hand, only very little is known, so far, about electrical transport in µc-Si:H;the collection mechanism for carriers in corresponding p-i-n cells is therefore still unclear.Entirely microcrystalline p-i-n solar cells with thicknesses of up to 3.6 µm [4] have beenshown to collect almost the totality of photo-generated carriers even though coplanartransport (photoconductivity and ambipolar diffusion length) is only slightly higher in suchµc-Si:H layers compared to amorphous silicon a-Si:H [5].With the aim to gain more insight into absorption and transport mechanisms ofmicrocrystalline silicon, we produced a series of µc-Si:H layers using different dilutionratios of silane in hydrogen for deposition. This enabled us to create materials with verydifferent qualities, ranging from purely amorphous to highly microcrystalline material. Theoptical absorption measurements as well as the transport properties parallel to the growthdirection reveal important changes within this series, mainly with respect to the density andactivity of deep defects, which will be presented and discussed in the present paper.0.0010.010.111 010010001 041 050.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4alphaalphaalphaalphaalpha(corr)alpha(cor)alpha(cor)alpha(cor)alpha(var)alpha(var)alpha(var)alpha(cor)alphaα [cm-1 ]Energy [eV]1.25%2.5%5%7.5%CPMPDST / Rc - S iPDS detectionlimit (samplethickness ≈2µm)Fig. 1 Optical absorption coefficient for the 110 MHzdilution series determined by PDS, CPM andTransmission / Reflection measurements: the transitionfrom amorphous to microcrystalline silicon takes place ataround 7.5% dilution. Crystalline silicon data [8] is alsoplotted for comparison. Note the difference between theCPM and PDS spectra for the 5% sample, discussed in thetext.EXPERIMENTALThe samples were deposited by the VHF-GD deposition technique at a plasma excitationfrequency of 110 MHz using a gas purifier [6]. Thereby the ratio of silane flow over totalgas flow [SiH4]/[SiH4 + H2] was varied between 1.25% and 7.5%. The sample deposited at7.5% dilution is amorphous, all the others are microcrystalline. With deposition ratesranging from less than 0.7 Å/s (1.25% dilution) to 3.5 Å/s (7.5% dilution) layer thicknessesbetween 1.6 and 1.9 µm were obtained. All films were deposited either on uncoated glass(Schott AF 45) or on chromium-coated glass (for Time of Flight). Semitransparent, smallsize (~2 mm2 area) chromium top contacts were deposited for TOF samples, coplanaraluminium contacts with a gap of 0.5 mm were used for dark conductivity and CPMmeasurements.The optical absorption of the materials was determined by Transmission/Reflectionspectroscopy, by Constant Photocurrent Method (CPM) and by Photothermal DeflectionSpectroscopy (PDS). The transport properties were studied by dark conductivitymeasurements at temperatures ranging from 25°C to 180°C. Time of Flight (TOF)measurements in both short-time and post transient modewere performed respecting thegu ide l ines  fo r  TOFmeasurements on µc-Si:Hpresented in [7].X - r a y  d i f f r a c t i o nmeasurements  on a l lmicrocrystalline samplespresented here show apreferential growth in the<220> direction. A mediumgrain size of around 200 Åand columnar shape of thegrains has already been foundin the past for other samplesproduced using similardeposition conditions [1].RESULTSOptical propertiesIt has already been shown[3] that the transition fromm i c r o c r y s t a l l i n e  t oamorphous material is abruptand typically takes placearound 7.5% dilution for thedeposition conditions usedhere. The optical absorptionspectra measured on thepresent series (represented infigure 1) confirms the drasticchange of optical absorptionwith a variation of the dilutionratio. Thereby, three distincteffects can be observed: the1 0-111 0-91 0-71 0-51 0-31.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4dark conductivity [1/Ωcm]1000/T  [K-1]5 %1.25%2.5%0.22 eV0.40 eV0.49 eVEact7.5%0.81 eVFig. 2 Measured dark conductivities as afunction of temperature for different dilutions.The sample with 7.5% dilution is amorphous, allthe others are microcrystalline. The activationenergies were determined at room temperature.transition from a-Si:H to µc-Si:H,scattering and the effect ofmixed-phase material.First, the main phenomenonobserved is of course the transitionfrom non-direct absorption inamorphous material with a Tauc gaparound 1.7 eV to the optical absorptionin the crystalline material with anindirect gap of about 1.1 eV. However,there are also significant changes in theabsorption curve within themicrocrystalline range of depositionconditions. As a second effect, for thesample with the highest dilution,further enhancement of opticalabsorption by scattering is apparent.We measured the scattered part of thetotal reflected light (and thus also ofthe penetrating light) and observed anincrease from about 8% at 650 nm (5%dilution sample) to more than 50% forthe highly diluted sample (1.25%dilution). As the scattering eventsresult in a longer effective optical pathof the photons, an enhanced apparent absorption is observed [9].Third, the difference between CPM and PDS deduced optical absorption (as clearly seenin the sample with 5% dilution) is due to the mixture of amorphous and crystalline phaseprevalent in this particular case. PDS sees the true optical absoprtion, whereas CPM curvecan be deconvoluted into two contributions: now, above approx. 1.7 eV the transport inamorphous tissue with higher photosensitivity gives an additional contribution to the"apparent optical absorption coefficient". Because of the calibration procedure used thisresults in an artificial shift of "α" to lower values for photon energies below 1.7 eV.When looking at the subbandgap optical absorption (below 1 eV), PDS cannot see thedifferences between the samples (due to its "detection limit"). On the other hand, CPMabsorption, which is not masked by the substrate, interface and surface absorption, candetect the changes in the defect-connected absorption: the subbandgap absorption connectedto defects is found to increase with increasing dilution ratios. This can indicate an increasednumber of dangling bonds, unsaturated by hydrogen, at the grain boundaries [3]. Thepossible role of oxygen impurities on the optical spectrum has not yet been studied,experiments are under preparation. However, post-transient TOF measurements performedon the same samples show a different trend. This apparent inconsistency will be addressedin more detail in the following section.Electrical propertiesEven though all layers were produced using a gas purifier in order to prevent theincorporation of impurities [6], the microcrystalline materials deposited with high dilutionsshow a pronounced extrinsic behaviour (Fig. 2) as is usually obtained in the presence of [O]contaminants. The measured dark conductivities vary from 1 x 10-7 1/Ωcm (typical for"truly" intrinsic µc-Si:H) to 1.3 x 10-5 1/Ωcm with activation energies ranging from 0.49 eVto 0.22 eV. In fact, the low deposition rates of the highly diluted µc-Si:H (0.67 Å/s for1.25% compared to 2.2 Å/s for 5% dilution) can possibly promote the incorporation ofoxygen as a donor impurity in the material and cause the observed shift of the Fermi leveltowards the conduction band edge. The layer deposited with 7.5% dilution shows values of     00.511.522.53280 300 320 340 360 380electronsdrift mobility [cm2 /Vs]T [K]1.25%2.5%5%7.5%     00.511.5280 300 320 340 360 380holesdrift mobility [cm2 /Vs]1.25%2.5%5%7.5%T [K]Fig. 3 Temperature dependency of electron and hole drift mobilities µDe and µDh asdetermined by Time of Flight.dark conductivity and activation energy that are typical for intrinsic amorphous material(Fermi level close to midgap).The characterisation of the carrier transport in the dark was made by the Time of Flighttechnique (TOF). Thereby, mainly two features designated this method as appropriate tostudy the carrier transport in our layers: first, the transport of electrons and holes can bemeasured independently and secondly the transport can be monitored in the growth directionof the layer, in a configuration close to that of a p-i-n cell. The results obtained for driftmobilities (µD) at room temperature are presented in figure 3. Even though the transportproperties vary less drastically than the optical properties, some of the observed features areremarkable. First of all, the drift mobilities clearly increase from 1 to 3 cm2/Vs (electrons)and from 0.4 to 1.2 cm2/Vs (holes) with increasing dilution at room temperature.Furthermore, in all measured µc-Si:H layers the drift mobilities for electrons are about twicethe hole drift mobilities; this ratio is in clear contrast with the much higher ratio observedfor a-Si:H (µDe/µDh ≈ 100). This observation strongly indicates that the drift mobility ofcarriers in µc-Si:H is not governed mainly by the amorphous tissue, as here band tailtrapping would give rise to the typical strong mobility asymmetry. Thus, other mechanismsrelated to grain boundaries and/or scattering must be taken into account.The measurements of drift mobilities of electrons and holes as a function of temperature(300 - 380 K) are plotted in figure 3. Whereas the drift mobility µD of electrons significantlydecreases with increasing temperature for all microcrystalline samples, the opposite isobserved for holes (except for the 1.25% dilution sample). The activation energies for holemobilities are low, 59 meV (5% dilution) and 21 meV (2.5% dilution), respectively.Thereby, numerous processes can influence the drift mobility with changing temperature:thermoionic emission of carriers over barriers [10], scattering effects (which can reduce themobility at high temperatures), but also deep trapping and tunneling may be involved. Thus,at this stage it is not possible to discriminate the dominant transport process in our µc-Si:Hmaterial and more extensive measurements will be necessary here.Whereas short-time TOF monitors the transit of the photogenerated carrier package throughthe layer, the extension of the TOF experiment to a much longer time scale (post-transientTOF [11], [12]) gives information about the density of states (DOS) in the gap. Thereby,electrons and holes which were deep-trapped during transit are thermally reemitted at longertimes and their collection becomes a direct fingerprint of the DOS. In our microcrystallinematerial the post transient currents from holes are featureless and no particular deep trapstates for holes can be observed. On the other side, for electron transients, very pronounceddetrapping at times around 10-3 - 10-4 s appears for low-dilution samples (see Fig. 4b and c),indicating thus the presence of efficient electron traps in the middle of the gap (about 0.5 eV10-1010-810-610-410-210-9 10-7 10-5 10-3 10-11.25 % dilutioncurrent [a.u.]time [s]a)SCLCelectronsholes 10-1010-810-610-410-210-9 10-7 10-5 10-3 10-12.5 % dilutioncurrent [a.u.]time [s]b)SCLCelectronsholes10-1010-810-610-410-210-9 10-7 10-5 10-3 10-15 % dilutioncurrent [a.u.]time [s]c)SCLCelectronsholes 10-1010-810-610-410-210-9 10-7 10-5 10-3 10-17.5 % dilution (amorphous)current [a.u.]time [s]d)SCLCelectronsholesFig. 4 Time of Flight post transient currents (in space charge limited mode SCLC) forelectrons and holes: the low dilution microcrystalline samples (fig.4c, but also b) show apronounced detrapping current for electrons, indicating the presence of a detrimental typeof electron deep trap in this material. Note that the characteristic time in the SCLC mode isthe extraction time which is different from the transit time used for drift mobilitydetermination.deep). As these traps are only active for electrons, we attribute them to positively chargeddefects at the grain boundaries. However, it is not clear at this stage, if they are due todangling bonds or to impurities (e.g. metal impurities or oxygen complexes). In addition, theobserved Fermi level shift (with increasing dilution) may also fill up defect states withelectrons, making them inactive for electron trapping (and invisible for TOF) even thoughthey remain present. Thus, we cannot exclude the presence of such states in highly dilutedmaterials on the basis of our TOF measurements. This also means, that our observation ofimportant trap state density in low-dilution material is not necessarily contradictory to theoptical measurements by CPM, which indicate a higher density of deep defect states forhigh-dilution material. CPM data reflects the optical transitions from gap states below theFermi level (occupied by electrons), hence, CPM sees different defect states thanpost-transient TOF.CONCLUSIONSWe have shown that the optical and electrical properties of µc-Si:H prepared by glowdischarge can strongly be influenced by changing the dilution ratio of silane to hydrogen.Thereby, not only structural changes are involved, but also an enhanced incorporation ofimpurities at low deposition rates (typical for high dilution materials), leading to depositionof material with an extrinsic character.Spectral dependence of the optical absorption coefficient reflects the material changesthat occur due to an increased dilution: it directly shows the transition from amorphous tomicrocrystalline material and also reflects residual amorphous fractions. In the subbandgapregion absorption measurements are sensitive to defects and impurities. To extract morequantitative data, more has to be known about the influence of oxygen and on the distortionof absorption spectra due to light scattering.Time of Flight measurements on the µc-Si:H samples show a strongly increased driftmobility of holes when compared to typical a-Si:H. The measured values increase withhigher dilution ratios from 1 to 3 cm2/Vs for electrons and from 0.4 to 1.2 cm2/Vs for holes.The ratio between electron and hole mobilities observed at room temperature is about 2 andis close to that of crystalline silicon; this clearly indicates a transport mechanism which isdifferent from the multiple trapping transport found in a-Si:H.By using the post-transient TOF technique we have also found in low-dilution materialan efficient type of deep electron trap in the middle of the gap whose origin is not yet clear.Further studies are necessary in order to clarify if these traps are due to dangling bonds or toimpurities in the material. Furthermore, the Fermi level shift which accompanies the dilutioneffect, can render "invisible" certain defect states for TOF by filling them up with electrons;thus a direct interpretation of the results becomes difficult.ACKNOWLEDGMENTSThis work was supported by the Swiss National Science Foundation under grantsFN-39377 and FN-45696.95 and by the Swiss Federal Office of Energy under Researchgrant EF-REN (93)032.REFERENCES[1] J. Meier, S. Dubail, R. Flückiger, D. Fischer, H. Keppner and A. Shah, Proc. of the 1stWCPVEC, Hawaii, 405 (1994).[2] N. Beck, J. Meier, J. Fric, Z. Remes, A. Poruba, R. Flückiger, J. Pohl, A. Shah and M.Vane˘c˘ek, J. Non-Cryst. Sol. 198-200, 903 (1996).[3] U. Kroll, J. Meier, A. Shah, S. Mikhailov and J. Weber, J. Appl. Phys. 80, 4971(1996).[4] J. Meier, P. Torres, R. Platz, S. Dubail, U. Kroll, J.A. Anna Selvan, N. PellatonVaucher, C. Hof, D. Fischer, H. Keppner, A. Shah, K.-D. Ufert, P. Giannoulès, J.Koehler, to be published in Mater. Res. Soc. Symp. Proc. (San Francicso, 1996).[5] M. Goerlitzer, N. Beck, P. Torres, J. Meier, N. Wyrsch and A. Shah, J. Appl. Phys. 80,5111 (1996).[6] P. Torres, J. Meier, R. Flückiger, U. Kroll, J.A. Anna Selvan, H. Keppner, A. Shah,S.D. Littelwood, I.E. Kelly and P. Giannoulès, Appl. Phys. Lett. 69, 1373 (1996).[7] N. Wyrsch, M. Goerlitzer, N. Beck, J. Meier and A. Shah, to be published in Mater.Res. Soc. Symp. Proc. (San Francicso, 1996).[8] M. A. Green, M. J. Keevers, Progress in Photovoltaics: Research and Applications 3,189 (1995).[9] M. Vane˘c˘ek, D. Cervinka, M. Favre, H. Curtins and A. Shah, Mater. Res. Soc. Symp.Proc. 192, 639 (1990).[10] J. Y. W. Seto, J. Appl. Phys. 46, 5247 (1975).[11] G. F. Seynhave, R.P. Barclay, G.J. Adriaenssens, J.M. Marshall, Phys. Rev. B39,10196 (1989).[12] J. Koc˘ka, M. Vane˘c˘ek, P. Machác˘ek, A. Fejfar, E. ˘Sípek, Ho-The-Ha, I. Pelant, J. Fric,J. Rosa, Z. Reme˘s, A. Poruba, M. Konagai and W. Kusian, Proc. of the 1st WCPVEC,Hawaii, 437 (1994).

Optical and Electrical Properties of Undoped Microcrystalline Silicon Deposited by the VHF-GD with Different Dilutions of Silane in Hydrogen

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Optical and Electrical Properties of Undoped Microcrystalline Silicon Deposited by the VHF-GD with Different Dilutions of Silane in Hydrogen

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Infoscience - École polytechnique fédérale de Lausanne