high lifetime on n-type silicon wafers obtained - ISC Konstanz

HIGH LIFETIME ON N-TYPE SILICON WAFERS OBTAINED AFTER BORON DIFFUSIONAlexander Edler 1 , Johann Jourdan 1 , Valentin D. Mihailetchi 1 , Radovan Kopecek 1 , Rudolf Harney 1 , DanielStichtenoth 2 , Tilo Aichele 2 , Anett Grochocki 2 , Jan Lossen 21 International Solar Energy Research Center (ISC), Rudolf-Diesel-Str. 15, D-78467 Konstanz, Germany,alexander.edler@isc-konstanz.de; 2 Bosch Solar Energy AG, Wilhelm-Wolff-Str. 23, D-99099 Erfurt, Germany.ABSTRACT: For the formation of emitters or back surface fields by means of diffusion the influence of a <strong>high</strong>temperature processes on the bulk <strong>lifetime</strong> of Cz-Si wafers has to be understood. Here we present experiments ofsuch heat treatments over a temperature range from 850°C to 960°C employing <strong>lifetime</strong> samples of neighboringn-type and p-type Cz wafers. In a second experiment we investigated the influence of the wafer origin within theingot and compared 2 Cz-ingots from different feedstock material side by side. The results were measured usingthe QSSPC technique and µW-PCD <strong>lifetime</strong> mapping.1 INTRODUCTIONThe interest in n-type base material is ever increasingsince a few pioneering works [1,2,3] have pointed out itsadvantages for solar cell production. Slowly the scope ofsome solar cell manufacturers is turning towards n-typematerial and already today the three most advancedsilicon based cell concepts use such material. The wellknown tolerance over common metal impurities and lackof <strong>lifetime</strong> degrading boron-oxygen complexes make itwell suited for an industrial application [4]. An importantstep towards boron emitter formation, as well as theboron back surface field (BSF) formation in a standard p-type process is the understanding of boron diffusion andits effects on the <strong>lifetime</strong>. Contrary to the commonbelieve to degrade carrier <strong>lifetime</strong>s [5], in this paper wedemonstrate some examples of <strong>high</strong> temperatureprocesses, such as boron diffusion, that actually improvethe bulk properties. Alongside with boron diffusion wecompare the effects of pure nitrogen annealing andphosphorous diffusion in the relevant temperature regime(850°C-960°C) using n-type and p-type base materialfrom ingots grown by Bosch Solar Energy AG. In asecond step we chose solely n-type wafers from ingotsgrown from different quality feedstocks, and comparedtheir performance after exposure to <strong>high</strong> temperaturesusing the same process conditions as before. Themethods that are used can all be found in an industrialenvironment today, thus our results are of great relevancenot only for the general understanding but also for ourindustry partners.2 EXPERIMENTIn a first experiment neighboring, large area waferswere selected from an n-type ingot and from a p-typeingot grown under industrial conditions at Bosch SolarEnergy AG. The as-cut base resistivity was determined tobe in the range of around 2.5 Ωcm for n-type wafers and8.5 Ωcm for wafers of the p-type ingot. Values are takenbefore any processing steps so include the influence ofthermal donors. All wafers were chemically etched toremove the saw damage and subsequently divided inthree groups.The first one was the reference group and did not getany <strong>high</strong> temperature treatment. The second and thirdgroup were submitted to either boron diffusion orannealing under nitrogen atmosphere respectively.Process temperatures in the industrial type quartz tubefurnace were set to 850°C, 900°C, 930°C and 960°C. Forthe boron diffusion a BBr 3 source was used.Afterwards the wafers have undergone anotherchemical etching step in order to remove both the oxideand diffused layers. Hydrogenated silicon nitride(SiN x :H) layers were deposited by Plasma EnhancedChemical Vapour Deposition (PECVD) on both sides ofthe wafers as a passivation layer and then they weresubmitted to a firing step. Figure 1 shows the processsequences for both groups respectively.Test groupFigure 1Saw damage etchChemical cleaningHigh temperaturestep: Tempering,Boron diffusionRemoval ofdiffusion layersPassivation: frontand rearFigure 1 Process scheme for the preparation of<strong>lifetime</strong> samples from as cut wafers.3 RESULTS AND DISCUSSIONReference groupSaw damage etchChemical cleaningPassivation: frontand rear3.1 n-type and p-type comparisonThe effective bulk <strong>lifetime</strong> was analyzed using µW-PCD mapping and QSSPC measurements at the center ofeach wafer. Figure 2 illustrates the <strong>lifetime</strong> results. Forsimplicity reasons only the <strong>high</strong>est and lowest processtemperatures are shown.The <strong>lifetime</strong> improvement over the whole areabecomes obvious for the n-type wafer when looking atthe <strong>lifetime</strong> maps. The p-type wafers on the other handtend to be negatively affected with increasing processtemperature. We see a correlation of the <strong>lifetime</strong> behaviorwith the change in the resistivity as it is also predicted bythe SRH theory for low injection levels.From the lateral patterns in figure 2 we deduct thateffects of the simple passivation method used here comeinto play and might limit the <strong>lifetime</strong>s.The <strong>lifetime</strong> of p-type minority charge carriers are

already initially lower, but especially on the processedReference Boron 850°C Boron 960°CN-typeP-type100µs 1000µsFigure 2: µW-PCD <strong>lifetime</strong> mapping of the n-type (left)and p-type (right) wafers that received boron diffusionwith the <strong>lifetime</strong> mapping of an initial wafer (withoutdiffusion).samples. However even if we compensate for thedifference in the diffusivity coefficient of the electronsand holes, significantly <strong>high</strong>er diffusion lengths ofminority charge carriers, as can be seen in figure 3, aremeasured on n-type wafers after the <strong>high</strong> temperatureprocess. These results indicate that boron diffusion canbe performed without degrading bulk <strong>lifetime</strong> in anindustrial solar cell process, and it demonstrates <strong>high</strong><strong>lifetime</strong> and <strong>high</strong> temperature stability of n-type wafers asneeded in a potential application. The solar cell resultsbased on these materials are presented at 2DO.2.2The substantial increase in <strong>lifetime</strong> on n-type substrates isobserved for both, samples with boron diffusion andFigure 3: Average values of effective <strong>lifetime</strong> of thepassivated wafer and corresponding diffusion length,deducted from µW-PCD mapping of the n-type(circles) and p-type (squares) wafers that received anannealing or either boron or POCl 3 diffusion stepand reference group.tempered samples and can therefore not be attributed toan impurity gettering effect of boron doped layers. Theresults for all temperatures are summarized in figure 3 inform of effective <strong>lifetime</strong> values and effective diffusionlengths.3.2 Comparison of ingot qualityHere we selected wafers from the top, middle andbottom region of two different n-doped Cz crystalsrespectively. Both crystals were intentionally doped togain a base resistivity of about 2.5 Ωcm. The differencebetween the crystals is the feedstock of different quality.One ingot was made from <strong>high</strong>er quality feedstock(hereafter A1) while the second one was made fromfeedstock of comparable quality as feedstock commonlyused in to dates industry processes (hereafter B1).We were interested in a side by side comparison of the<strong>high</strong> temperature effects on <strong>lifetime</strong> samples given thedifferent quality of the base material. The influence of theorigin of the wafer within the ingot was also underinvestigation. Groups of wafers from the differentpositions were processed under the same conditions. Infigure 4 we give a set of exemplary results from bothingots and all <strong>high</strong> temperature processes. Here we limitourselves to the the depiction of the middle region ofboth ingots but corresponding results have been found insamples from other parts of the respective ingot. Thesamples were prepared following the process sequenceoutlined in section 2.Reference P-Diffus. Tempering Boron 960°CB1 ingotA1 ingot150µ 700µ

Figure 4: µW-PCD <strong>lifetime</strong> mapping of samples ofthe middle region of B1 ingot (left) and A1 ingot(right) wafers from the middle region that receivedboron diffusion with the <strong>lifetime</strong> mapping of aninitial wafer (without diffusion).As can be seen from figure 4 the ingots performcomparably well after these <strong>high</strong> temperature steps. Whatis remarkable is that even the production ingot B1yielded <strong>high</strong> <strong>lifetime</strong>s after phosphorous diffusion at850°C and also after boron diffusion at 960°C, which arecommon processing temperatures. The observations fromthe first part of the experiment are widely reproduced forthe case of n-type base material which emphasizes ourearlier findings. Again we suspect the reason for thesubstantial increase in bulk <strong>lifetime</strong> in the increase ofbase resistivity. The top part of an ingot usually containsa <strong>high</strong> amount of incorporated oxygen. The largeresistivity shift in the top part of the ingots A1 and B1can be explained by annealing of oxygen related defects.Figure 5: Shift of base resistivity for a <strong>lifetime</strong> samplefrom the test group in the process sequence asmeasured by QSSPC in the center of the wafers.3.3 Crystal defects triggering <strong>lifetime</strong> decayOn few samples originating from the very top of theA1 crystal we observed a pattern of concentric rings ofsignificantly reduced <strong>lifetime</strong> after processing. Agraphical representation of this effect is given in figure 6.Scalereduced 10x100µs 1000µsFigure 6: µW-PCD <strong>lifetime</strong> mapping of samples fromtop region of the A1 ingot after Boron diffusion at960°C (left) and nitrogen annealing at the sametemperature (right)We observed this phenomenon on similarly processedtop-region Cz-wafers from other manufacturers.Therefore we conclude its great relevance for thesuppliers of such material. We suspect a configuration ofgrown in intrinsic defects and defect clusters as at thecore of the problem [7,8,9]. It has been show in literaturethat a certain constellation of parameters can, duringcrystallization, cause similar characteristic defects. Thesedefects are known to act as a sink for impurity atoms andthus are believed to enhance formation of oxygencomplexes. We put this topic up for discussion and willcontinue research in this field.4 CONCLUSIONNo degradation of the bulk <strong>lifetime</strong> was observedafter boron diffusion or annealing in nitrogen on n-typeCz-Si wafers. Furthermore, an improvement of the bulk<strong>lifetime</strong> could be observed after <strong>high</strong> temperature step ascompared to the initial <strong>lifetime</strong>. This improvement is notattributed to a possible metal gettering effect of boronsince neighbouring wafers that received only thetempering step in nitrogen show the same behaviour.For p-type Cz-Si wafers, a different behaviour wasobserved: <strong>high</strong> temperature steps have a moderate impacton the bulk <strong>lifetime</strong> regardless of the process performed.Nevertheless, the <strong>lifetime</strong> values are significantly lowerthan those measured on n-type Cz-Si wafers resulting insignificantly <strong>high</strong>er diffusion lengths of minority chargecarriers on n-type wafers after the <strong>high</strong> temperatureprocess. Together with the results of the secondexperiment which widely reproduced the observationsmade for n-type wafers, the superiority of n-type Czsubstratesover p-type as base material for an applicationin a standard process with diffused boron emitter couldbe shown. By using large area (241cm²) wafers we coulddemonstrate that even a low cost feedstock material hasthe potential for a long-term stable <strong>high</strong> efficiencyn-type solar cell. The solar cell results based on thesematerials are presented at 2DO.2.2.ACKNOWLEDGEMENTSThis work is financially supported by Germangovernment (BMU) within EnSol project, contractnumber 0325120A.REFERENCES[1] J.E. Cotter et al., IEEE Transactions on Electron Devices,2006, Vol. 53, Issue 8, pp. 1893[2] J. Libal et al. Proceedings of the 20rd EPVSEC, Spain, 200,pp. 793[3] A. Cuevas et. al. Proc. 3rd WCPVEC, 2003, pp. 963[4] D. MacDonald et al., Appl. Phys. Lett., 2004, Vol. 85, no.18, pp. 4061[5] Michael Andreas Kessler et al. Semicond. Sci. Technol.2010, 25 055001[6] D. Macdonald et al.. Appl. Phys. Lett., 2006; 88, pp. 952[7] R. Falster and V. V. Voronkov. Materials Science andEngineering B, 73, Issues 1-3, 3 April 2000, pp. 87[8] H. Föll and B.O. Kolbesen, Jahrbuch der Akademie derWissenschaften in Göttingen (1976), pp. 27[9] Georg Müller et al., Crystal growth: from fundamentals totechnology Elsevier, 2004