CuAl thin film metallic precursors were selenised using a tube furnace or a rapid thermal processor (RTP). A comparison is made between the two processes for slightly Cu rich films and best crystallographic and elemental properties are obtained for films selenised by RTP: it was found that ternary compound could only be formed using the RTP. In both cases a large amount of CuxSey grains are found to develop at the surface of the films. Only samples processed in the RTP showed cathodoluminscence excitation at 2.68 eV characteristics of the electronic bandgap. Al rich samples were used to study the effect of etching the CuxSey phases from the surface in order to reveal the underlying CuAlSe2 material

Aninat, Rémi

Edwards, Paul

Forbes, Ian

Luckert, Franziska

Martin, Robert

Miles, Robert

Zoppi, Guillaume

English

Northumbria Research Link

Rapid thermal processing of CuAlSe2G. Zoppi1 *, R. Aninat1, I. Forbes1, R. W. Miles1,F. Luckert2, P. Edwards2 and R. W. Martin21Northumbria Photovoltaics Applications Centre, Northumbria University, Ellison Building,Newcastle upon Tyne, NE1 8ST, UK2Department of Physics, SUPA, Strathclyde University, Glasgow, G4 0NG, UK* Corresponding Author: guillaume.zoppi@northumbria.ac.ukAbstractCuAl thin film metallic precursors wereselenised using a tube furnace or a rapidthermal processor (RTP). A comparison ismade between the two processes forslightly Cu rich films and bestcrystallographic and elemental propertiesare obtained for films selenised by RTP: itwas found that ternary compound couldonly be formed using the RTP. In bothcases a large amount of CuxSey grains arefound to develop at the surface of thefilms. Only samples processed in the RTPshowed cathodoluminscence excitation at2.68 eV characteristics of the electronicbandgap. Al rich samples were used tostudy the effect of etching the CuxSeyphases from the surface in order to revealthe underlying CuAlSe2 material.1. IntroductionThin film solar cells based on I-III-VI2semiconductors, such as Cu(In,Ga)(S,Se)2(CIGS) have reached record efficiency of20% [1]. The technology is currentlylimited by a p-type absorbing material witha relatively low bandgap (~1.15 eV) [2]compared to the theoretical optimum of1.5 eV [3] and by the use of a cadmiumbased n-type buffer layer [2]. Alternativebuffer layers can be successfully usedsuch as In2S3 [4], InxSey [5], ZnS [6], ZnSe[7], ZnO [8] and ZnMgO [9].CuAlSe2 (CAS) is another I-III-VI2semiconductor which has suitableproperties for use as a window layer.CuAlSe2 has an energy bandgap of 2.67eV [10], and crystallizes with thechalcopyrite structure making thesemiconductor a good lattice match for theunderlying CIGS absorber. Finally, CASfilm can be intrinsically doped either p or ntype by control of the Cu/Al ratio in the film[11, 12] yielding the possibility to createeither a wide bandgap absorber or windowlayer.However this material is difficult to growdue to the presence active Al. In this workwe investigate a route to produce CASfilms using a 2-stage process similar tothat use in the production of CIGS films[13] and we present initial results.2. ExperimentalCAS films were produced by a 2-stageprocess by means of selenisation ofmagnetron sputtered Cu-Al metallicprecursors. The Cu and Al layers weresequentially sputter deposited using highpurity (5N) targets onto unheated soda-lime glass or Mo-coated soda lime glasssubstrates, placed on a rotating substratetable. This was followed by selenisationeither in a tube furnace (slow process) orusing a rapid thermal processor (fastprocess). The slow process involvedselenisation in a mixed H2/N2 (10 mbar)and elemental selenium atmosphere attemperatures of up to 550°C (ramp 15°Cmin-1) for up to 120 min. The fast processinvolved heating the precursor up to 550°Cin 60-90 seconds and holding thistemperature for 1-5 min under high Arpressure (500-800 mbar) and elementalselenium. When necessary the films wereetched for 90 sec using a 10%concentrated KCN solution.The structural quality of the absorberlayers was examined using X-raydiffraction (XRD) carried out with aSiemens D-5000 diffractometer using aCuKradiation source (= 1.5406 Å). Inorder to obtain information about thesurface morphology and the compositionof the layers, secondary electron imageswere taken and wavelength dispersive X-ray (WDX) analysis was performed using aCameca SX100 electron microprobemodified for cathodoluminescence (CL).3. Results & DiscussionThe CuAl precursors were grown eitherslightly Cu-rich (Cu/Al = 1.04) on Mocoated glass as potential p-type absorber(type A) or Al rich (Cu/Al = 0.75) on glassas potential n-type window layers (type B).The layers were terminated with a 5 nmthick Cu layer to reduce the oxidation ofthe precursors. The thickness of theprecursors was around 600 nm. The XRDpattern of a type A film is shown in figure 1(a). The phases presents are Al, Cu andCuAl2, indicating partial mixing of the twoelements. No oxides were detected in thebulk of the film.Col 1 vs Col 4Col 1 vs Col 52 Theta (deg)30 35 40 45 50Intensity**AlCuCuAl 2CuAl 2(b)(a)Cu 9Al 4Cu 9Al4Figure 1. XRD patterns of type A (a) andtype B (b) CuAl precursor. Peaks marked(*) arise from the substrate.The XRD patterns of selenised type Afilms are shown in figure 2 for both theslow and fast process. Using the slowprocess it is possible to form some CASmaterial however the film is dominated byCuxSey phases and unreacted CuAl. Whenusing the faster heating process the CASphase dominates the film crystallographyand all the precursor is found to react withthe chalcogen. The CAS film is shown tocrystallize with the chalcopyrite structurewhich is indicated by the presence of thesmaller (103) reflection.2 Theta (deg)30 35 40 45 50 55NormalizedIntensity(b)(a)(112)(200)(004)**(220)(204)(312)(116)Cu3Se2CuSeCu2-xSeCu 9Al4 + Cu6Al4 (103)(211) , Figure 2. XRD patterns of a selenisedCuAl film (type A) using a slow process (b)and a fast process (b). Peaks marked (*)arise from the substrate.Figure 3 shows the surface morphology ofthe selenised type A films and WDX linescans (marked as a white line insecondary electron images). The flat andsmooth surface of the precursor film (notshown here) gives way to a film whichdisplays large numbers of surface featuresand not the granular structure usuallyfound in thin film solar cell absorbers.Figure 3. SEM micrographs (left), and WDX line scans (marked in the SEM images) ofsurface features (middle) and bulk film composition (right) of selenised CuAl films for a slowprocess (top) and fast process (bottom) The lines in the plots of the elemental compositiondisplay the average content of all points for each element (Cu: dashed, Al: dotted and Se:dashed-dotted) and act as a guide for the eyes.The middle column of figure 3 shows thecomposition of surface features while theright column gives the composition of thebulk of the film. For the slow process thesurface features are consistent with eitherCuSe or Cu3Se2 in accordance with theXRD measurements; while for the fastprocess varied composition is detected:some CuSe elements, some nearly pureSe grains and some mixed ternaryCuAlSe2 grains.The bulk composition of the films is shownin the right column of figure 3. The scanscorresponding to the slow process clearlyindicate that the Se incorporation is closeto zero with an average film composition ofCu0.41Al0.54Se0.05 (with an uncertainty of0.01). On the other hand the line scancorresponding to the CuAl film selenisedusing the fast process shows an averagecomposition of Cu0.25Al0.23Se0.52, indicatinga good homogeneity of the ternary CAScompound. However care should be takenbecause other line scans showed lessuniform compositions.All films showed a high level of oxygencontent and levels up to four times higherin the films selenised using the slowprocess. However no oxide phase eitherCu or Al based where detected on theXRD patterns.1.5 2.0 2.5 3.00100200300400500600CL @ 300K12kV, 20 nACLintensity[arb.u.]Energy [eV]800 700 600 500Wavelength [nm]Figure 4. Room temperature CL spectrumof type (A) CuAl precursor film selenisedusing the fast process.In addition to the compositional data, CLwas measured for the CAS layersproduced by the fast process. The roomtemperature spectrum is displayed infigure 4. A peak at 2.68 eV with a full widthat half maximum of 57 meV dominates thespectrum. This peak corresponds to bandedge luminescence of the CuAlSe2 layer.No CL spectrum could be obtained for thelayer formed in the slow process since itconsists mainly of copper and aluminium.2 Theta (deg)30 35 40 45 50 55NormalisedIntensity(b)(a)(112)(200)(004)(220)(204)(312)(116)(103)(211)+ CuSe2x CuAl+ ++XFigure 5. XRD patterns normalizedagainst the (112) reflection of CAS before(a) and after etching (b).(b)(a)Figure 6. Secondary electron micrographsof CAS films before (a) and after etching(b).The XRD pattern for a type B precursor isshown in figure 1 (b). Although the filmswere grown Al rich (Cu/Al = 0.75) the onlydetected phase was Cu9Al4. Type Bprecursor films were exclusively selenisedusing the fast process and in order toremove the CuxSey phases, a cyanide etchwas performed. After this treatment thedark grey films evolved into yellow filmswhich are characteristic of a wide bandgapmaterial. Figure 5 shows the XRD patternsfor a CAS film on glass pre and post-etching. After etching most peakscorresponding to the CuxSey phasesdisappeared indicating the effectiveness ofthe cyanide etch.The effect of etching is also observed onthe surface morphology of the films. Figure6 displays the corresponding secondaryelectron micrographs for an unetched andetched layer. The non uniform surface withlarge agglomerate of CuxSey grains givesway to a smoother film after etching.Compact, micron sized grains arerevealed. However the films turned out tobe pinholed and extremely thin preventingeffective optical or electricalcharacterization.4. ConclusionCuAlSe2 films were produced by a 2-stageprocess using a rapid thermal annealing ofmagnetron sputtered CuAl metallicprecursor. Films crystallize in thechalcopyrite structure but surfacecoverage is dominated by CuxSey grains.As with CuIn(Se,S)2 those grains caneasily be removed when performing acyanide etch. Further experiments are inprogress in order to increase the surfacecoverage of the CAS layer by using thickerfilms and also by altering the etchingrecipe.AcknowledgmentsPart of this work was funded by theEPSRC SUPERGEN programme“Photovoltaics for the 21st Century”.References[1] M. A. Green, K. Emery, Y. Hishikawa,and W. Warta, Progress in Photovoltaics:Research and Applications 17 (2009) 85-94[2] I. Repins, M. A. Contreras, B. Egaas,C. DeHart, J. Scharf, C. L. Perkins, B. To,and R. Noufi, Progress in Photovoltaics:Research and Applications 16 (2008) 235-239[3] J. J. Loferski, Journal of AppliedPhysics 27 (1956) 777-784[4] N. Naghavi, S. Spiering, M. Powalla, B.Cavana, and D. Lincot, Progress inPhotovoltaics: Research and Applications11 (2003) 437-443[5] M. Konagai, Y. Ohtake, and T.Okamoto, Symposium on Thin Films forPhotovoltaic and Related DeviceApplications, at the 1996 MRS SpringMeeting, San Francisco, Ca (1996) 153-163[6] M. A. Contreras, T. Nakada, M. Hongo,A. O. Pudov, and J. R. Sites, Proceedingsof the 3rd World Conference onPhotovoltaic Energy Conversion, Osaka,Japan (2003) 2LN-C08[7] A. Ennaoui, S. Siebentritt, M. C. Lux-Steiner, W. Riedl, and F. Karg, SolarEnergy Materials and Solar Cells 67(2001) 31-40[8] M. Bar, C. H. Fischer, H. J. Muffler, B.Leupolt, T. P. Niesen, F. Kang, and M. C.Lux-Steiner, Proceedings of the 29th IEEEPhotovoltaic Specialists Conference, NewOrleans, USA (2002) 636-639[9] A. Hultqvist, C. Platzer-Bjorkman, T.Torndahl, M. Ruth, and M. Edoff,Proceedings of the 22nd EuropeanPhotovoltaic Solar Energy Conference,Milan, Italy (2007) 2381-2384[10] S. Marsillac, T. B. Wahiba, C. ElMoctar, J. C. Bernede, and A. Khelil, SolarEnergy Materials and Solar Cells 71(2002) 425-434[11] L. Barkat, M. Morsli, C. Amory, S.Marsillac, A. Khelil, J. C. Bernede, and C.El Moctar, Thin Solid Films 431-432(2003) 99-104[12] Y. B. K. Reddy and V. S. Raja,Materials Chemistry and Physics 100(2006) 152-157[13] P. Nasikkar, G. Zoppi, I. Forbes, R.W. Miles, P. J. Dale, L. M. Peter, J. D.Major, and K. Durose, Proceedings of the3rd Photovoltaic Science Application andTechnology (PVSAT) Conference andExhibition, Durham, UK (2007) 81-84

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Rapid thermal processing of CuAISe2

Rapid thermal processing of CuAISe2

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