Bär, M.

Fischer, Ch. H.

Fuertes Marron, D.

Grimm, A.

Jung, Ch.

Kötschau, I.

Lauermann, I.

Lehmann, S.

Lux Steiner, M.Ch.

Pistor, P.

Rusu, M.

Schedel Niedrig, Th.

Sokoll, S.

Wiesner, S.

HZB Repository

MANUSCRIPT COVER PAGE FORME-MRS Symposium : Thin Film and Nano-Structured Materials for PhotovoltaicsPaper Number : 2042Title of Paper : CuGaSe2 – CuGa3Se5 phase transition in CCSVT-grown thinfilmsCorresponding Author: Sebastian LehmannFull Mailing Adress : Hahn-Meitner-Institut (HMI)Department Of Solar Energy Research (SE2)Glienicker Straße 10014109 BerlinGermanyTelephone : +49-(30)-8062 3243Fax : +49-(30)-8062 3199E-mail : sebastian.lehmann@hmi.deCuGaSe2 - CuGa3Se5 phase transition in CCSVT-grown thin filmsS. Lehmann1), M. Bär, D. Fuertes Marrón, S. Wiesner, M. Rusu, I. Kötschau, I. Lauermann,A. Grimm, S. Sokoll, Ch.-H. Fischer2), Th. Schedel-Niedrig, M.Ch. Lux-Steiner2)Department SE, Hahn-Meitner-Institut Berlin, Glienicker Straße 100, 14109 Berlin, GermanyCh. JungBessy GmbH, Albert Einstein  Strasse 15, 12489 Berlin, Germany1) Corresponding author: E-mail: sebastian.lehmann@hmi.de2) Additional affiliation: Freie Universität Berlin, 14195 Berlin, GermanyAbstractThe system CuGaSe2 - CuGa3Se5 in thin films has been investigated. Layer synthesis wascarried out by chemical close-spaced vapour transport (CCSVT) using Cu precursors onMo/soda-lime glass substrates. The extension of deposition times in a two-step process led tofinal film compositions with [Ga]/[Cu] ratios ranging from 1 to 3, allowing the study of thephase transition mentioned above. Films showing chalcopyrite (1:1:2), OVC (1:3:5) and two-phase were grown. X-ray emission spectroscopy and x-ray diffraction (XRD) techniques havebeen combined for a compositional and structural study of this material system probing bothbulk and near surface properties of the films. This analysis was also extended to the rear-surface investigation of selected two-phase thin films and complemented with surfacesensitive photoelectron spectroscopy (PES). From these results a growth model is presentedfor CuGa3Se5 formation in gallium-rich, CCSVT-grown CuGaxSey-films.IntroductionCopper-containing chalcopyrites are interesting materials for photovoltaic applications. Theycan be roughly clasified in low- (1.0 eV – 1.3 eV; CuInSe2, Cu(In,Ga)Se2 with low galliumcontent) and high-gap (1.3 eV – 2.4 eV; CuGaSe2, CuInS2, CuGaS2) materials. The lattergroup offers promising candidates for the realization of high-voltage solar cell devices and oftop cells in tandem configuration. Whereas the interface between low-gap absorbers andbuffer layers does not seem to be a critical issue for device performance, those based on wide-gap absorbers, suffer from electronic losses related to interfacial issues. It is generallyaccepted that the presence of secondary phases in the absorber surface stemming from copperdepletion (Ordered Vacancy Compound (OVC) defect chalcopyrite) play a major role in theinterface electronics with a positive impact in low-gap-based devices originating from band-gap widening and type inversion in the near surface region of the absorber [1]. For theparticular case of CuGaSe2, evidence of band gap widening has been reported [2] togetherwith deviation from stoichiometry pointing to formation of OVC-related phases. However, thepositive impact of OVC surface phases seems to be minor in wide-gap absorbers and there isadditionally a general lack of knowledge of fundamental properties regarding growth,structural and electronic properties at the interface of CuGaxSey device-quality material. Aslong as these OVC-like phases mediate the interface formation it is of highest importance tostudy this material system. In this investigation we focus our attention on the growth,compositional and structural properties of chemical close-spaced vapour transport (CCSVT)-grown CuGaxSey thin films as a function of gallium and selenium enrichment, as this isconsidered to be the key for optimizing the formation of the absorber/buffer interface. Bulk[x-ray fluorescence (XRF) and x-ray diffraction (XRD) in q-2q mode], near surface [soft x-ray emission spectroscopy (SXES) and grazing incident (GI)-XRD] and surface sensitive[photoelectron spectroscopy (PES)] methods allow the identification of compositionalvariations at these three different levels of information depth, which are correlated withstructural modifications.ExperimentalCuGaxSey thin films were grown by CCSVT [3] in a compositional range of 1 < x < 3. Thiswas performed by a variation of deposition times. For analysis of the rear-side a glasssubstrate was glued with silver-epoxy on the top side of the layer of selected structures(CuGaxSey/Mo/soda-lime glass). Afterwards the layer was lifted-off [4] under inert N2-atmosphere.X-ray fluorescence (XRF) for integral compositional studies was performed in a PhillipsMagiXPro PW2440 system using 18keV Rhodium K-radiation as excitation source and theSuperQ software for subsequent calculations. For this purpose Ga and Cu K-lines have beenused. Theoretical calculation of absolute attenuation lengths for these emission energies givesvalues of approx. 10mm [5]. Those are well above the overall layer thickness for the samples,typically 1-3mm, and hence provide integrated compositional information.Soft x-ray emission spectroscopy (SXES) analysis and photoelectron spectroscopy (PES)were carried out in the spectroscopic system “CISSY” (for a more detailed description see[6]) at the U41 PGM high-flux beam line at BESSY II. In this investigation SXES wasperformed with an excitation energy of Ephot.=1200eV for recording Cu and Ga L2 emissionlines using a SXES 300 Scienta Gammadata spectrometer. Referenced values for Ga L2 at1097.9eV and Cu L2 at 929.7eV [7] were used to set the energy scale. The attenuation lengthfor these emission energies is approx. 300nm [5]. This means that SXES provides moresurface related compositional information compared to XRF.A CLAM4 electron spectrometer (Thermo VG Scientific) was used for PES experiments. Thezero point of the energy scale for hn=174eV was set so that the Au 4f7/2 reference lineappeared at a binding energy of 84.00eV [8].Structural film characterization was performed by means of X-ray diffraction using a BrukerD8 Advance X-ray diffractometer with monochromatized Cu-Ka radiation including a Goebelmirror in the primary beam for parallel beam optics. The system was operated at 40kV and40mA in ? - 2? and Grazing Incident (GI) mode, the latter with an incident angle of 0.8°.ResultsThe films selected for SXES investigation show integral atomic [Ga]/[Cu] ratios as measuredby XRF of 1.0, 1.2, 1.7 and 2.6 respectively. Cu L2 and Ga L2 emission lines detected fromthe front surfaces of the layers are shown in Fig.1 where the Cu L2 peak was used tonormalise all spectra. The Ga L2 emission intensity increases with higher integral [Ga]/[Cu]ratio as expected.In order to compare vertical compositional gradients in the CuGaxSey (with x=1.7 and 2.6)samples, SXES spectra were additionally recorded from the rear surface after lift-off of theseCuGaxSey films, as shown in Fig.2 normalised to Cu L2 intensities. For further comparisonsthe nearly stoichiometric sample with an integral x=1.0 served as a reference, assuming ahomogeneous distribution of gallium and copper, at least throughout the film thickness probedby SXES. The normalised Ga L2 reference spectra multiplied by the XRF-determined[Ga]/[Cu] ratios is shown in Fig.2 (squares) together with the detected spectra both from front(dotted) and rear surface (solid). It can be observed that the Ga L2 intensity measured fromfront surface is higher than that recorded for the rear surface. This fact indicates that the frontsurface region is gallium richer than the backside of the thin films. When comparingmeasured with calculated spectra for the x=2.6 film it turns out that the integral compositionlies at mid-values between the contributions determined from front and rear sides pointingeither to a double layered structure or a vertical gallium gradient through the layer. For thex=1.7 film the rear-side Ga L2 emission line shows nearly the same intensity as the calculatedcurve. The measured front side peak intensity clearly exceeds that from the calculation. Thisstrengthens the assumption of a bi-layer structure formation which is further supported byXRD measurements, as discussed in the following. Detailed calculations of absolute[Ga]/[Cu] ratios revealed from SXES spectra, which should include the effect of both surfaceroughness and compositional gradient related issues, are in progress.XRD measurements in ? - 2? and grazing incident (GI) mode were performed obtainingstructural information from the bulk and near surface regions. The GI angle was set to 0.8° inorder to achieve comparable information depths as those probed by SXES, as discussed above[5]. Only two phases, attributed to CuGaSe2 chalcopyrite [9] and CuGa3Se5 Ordered VacancyCompound (OVC) [10] have been observed in all ? - 2? measurements (see Fig.3).Evidence of additional phases like Cu2-xSe or Ga2Se3 has not been found in any of theinvestigated films. For CuGaxSey film compositions close to the 1:1:2 stoichiometry withx=1.0-1.2 the XRD spectra only show peaks which can be attributed to the 1:1:2 phase withchalcopyrite structure. For gallium-richer layers (x=1.7-2.6) additional diffraction peaksattributed to the 1:3:5 OVC phase also occur. Their contribution increases with higher overallgallium content. No 1:1:2 related peaks are detected for x=3 (see Fig.3). The shift ofidentically indexed reflexes to higher diffraction angles of 1:3:5 peaks than the corresponding1:1:2 peaks is associated with a reduction of the lattice parameters (CuGa3Se5: a0=0.550nmand c0=1.095nm [10] CuGaSe2: a0=0.561nm and c0=1.1032nm [9]).Fig.4 shows the highest intensity peak of both phases, the (112), comparing ? - 2? and GI-XRD measurements. No features from 1:3:5 phase have been observed for x=1.0-1.2 neitherin the bulk nor in the near surface region. In turn, for x=3.0 no 1:1:2 phase related peaks havebeen observed within the resolution of this method. The two films with x=1.7 and 2.6, wherethe 1:3:5 and 1:1:2 phases coexist, are of special interest for the establishment of the growthmodel. For this purpose  the (112) peaks of both phases were fitted (see Fig.4) with a PearsonVII function including Cu-Ka1 and -Ka2  contributions. The intensity ratios of (112)1:3:5/(112) 1:1:2 varies between ? - 2? and GI modes with a higher 1:3:5 contribution formore surface sensitive GI measurements.The CuGaxSey thin film with x=1.2 is moderately gallium rich, but even surface sensitive GI-XRD cannot prove the existence of the 1:3:5 structure in the near surface region of thesample. Additional PES measurements were performed, recording Cu 3p, Se 3d and Ga 3dlines (not shown). The surface composition was calculated from integrated line intensities(after fitting with a Voigt function) and corresponding cross sections [11]. From these resultsa surface composition of CuGa3.1Se5.6 at least for the first 2-3 monolayers surveyed by thistechnique [12] is determined. This indicates that a copper depleted surface region of the type1:3:5 is formed in moderately gallium rich CCSVT-grown samples, in accordance to previousresults [13].DiscussionA growth model of gallium rich phases in CCSVT-grown CuGaxSey thin films can beproposed based on these results. The use of bulk (XRF and XRD in q-2q mode), near surface(SXES and GI-XRD) and surface sensitive (PES) methods allows the identification ofcompositional variations at these three different levels, which are correlated with structuralmodifications. A few monolayer thick copper depleted surface region is formed on bulk singlephase 1:1:2 films. This surface layer promotes the formation of a 1:3:5 near-surface regionwith further gallium and selenium enrichment for x>1.7. This transition is directional from thefront to the bottom of the films. No evidence of 1:1:2 remnants was found for an integral bulkcomposition of x=3.0. Hints of surface copper depletion in 1:3:5 phase suggests this to be anintrinsic surface relaxation mechanism, which may in turn lead to a gallium- and selenium-richer OVC phase formation with further gallium and selenium enrichment, particularly 1:5:8,as has been already reported e.g. in [14] for PVD grown samples.ConclusionsThe transition between the 1:1:2 phase and the 1:3:5 phase in CCSVT-grown CuGaxSey thinfilms has been monitored. A bi-layer structure consisting of a near surface 1:3:5 phasecovering a 1:1:2 bulk phase has been found for 1.7<x<2.6. The transition region betweenthese two phases moves towards the rear side with increasing gallium content, until thecomplete layer shows OVC structure for x=3.0. Additionally, copper depletion has beenfound at the surface of the samples.AcknowledgementThe authors gratefully thank M. Kirsch for technical support.References[1]U. Rau, M. Turcu, Mat. Res. Soc. Symp. Proc. Vol. 763(2003)B8.8.1[2] A. Meeder, L. Weinhardt, R. Stresing, D. Fuertes Marrón, R. Würz, S.M. Babu, Th.Schedel-Niedrig, M.Ch. Lux-Steiner, C. Heske, E. Umbach, J. Phys. Chem. Solids64(2003)1553[3] M. Rusu, S. Wiesner, D. Fuertes Marrón, A. Meeder, S. Doka, W. Bohne, S. Lindner, Th.Schedel-Niedrig, CH. Giesen, M. Heuken, M.Ch. Lux-Steiner, Thin Solid Films 451-452(2004)556[4] D. Fuertes Marrón, A. Meeder, S. Sadewasser, R. Würz, C.A. Kaufmann, Th. Glatzel, Th.Schedel-Niedrig, M.Ch. Lux-Steiner, Journ. Appl. Physics 97(2005)094915-1[5] B.L. Henke, E.M. Gullikson, and J.C. Davis. X-ray interactions: photoabsorption,scattering, transmission, and reflection at E=50-30000 eV, Z=1-92; Atomic Data and NuclearData Tables Vol. 54 (No.2), 181-342 (July 1993), internet, http://www-cxro.lbl.gov/optical_constants/atten2.html[6] I. Lauermann, M. Bär, A. Ennaoui, U. Fiedeler, Ch-H. Fischer, A. Grimm, I. Kötschau,M.Ch. Lux-Steiner, J. Reichardt, B.R. Sankapal, S. Siebentritt, S. Sokoll, L. Weinhardt, O.Fuchs, C. Heske, C. Jung, W. Gudat, F. Karg, T.P. Niesen, Mat. Res. Soc. Symp. Proc. Vol.763, B4.5.1 (2003)[7] Berkeley Laboratories X-ray properties of elements reference data booklet; internethttp://xdb.lbl.gov[8] D. Briggs and M.P. Seah, Practical Surface Analysis, Vol. 1, John Wiley & Sons, (1990)[9] Joint Commitee of Powder Diffraction Spectra 35-1100[10] G.Marin, S. Tauleigne, S.M. Wasim, R. Guevara, J.M. Delgado, C. Rincón, A.E. Mora,G. Sánchez Pérez. Mat. Res. Bull. 33(1998)1057[11] J.J. Yeh, I. Lindau, Atomic Data and Nuclear Data Tables, Vol. 32, No. 1, 1985[12] M.P. Seah, W.A. Dench, Surf. Interf. Anal. 1(1979)2[13] R. Würz, M. Rusu, Th. Schedel-Niedrig, M.Ch. Lux-Steiner, H. Blum, M. Hävecker, E.Kleimenov, A. Knop-Gericke, R. Schlögl, Surf. Science 580(2005)80[14] S. Nishiwaki, S. Siebentritt, M.Ch. Lux-Steiner, Mat. Res. Soc. Symp. Proc.Vol.763(2003)219figure captionsFig.1: Cu L2 (left) and Ga L2 emission lines (right) observed by SXES (hn=1200eV) fromfront-side measurements of CCSVT grown CuGaxSey thin films with different integralXRF-determined [Ga]/[Cu] ratios x. All spectra were normalised to the Cu L2intensity. An increase in the intensity of the Ga L2 emission is clearly correlated withthe integral gallium content of the layer.Fig.2: Comparison of SXES Cu L2 (left) and Ga L2 (right) emission lines for two gallium richCuGaxSey thin films with integral [Ga]/[Cu] ratios of x=1.7 (upper panels) and 2.6(lower panels) respectively, as observed from front (dotted) and lifted-off rear sides(solid). All spectra were normalised to the Cu L2 line intensity. Calculated Ga L2lines(squares) were obtained by multiplying normalised reference spectra (x=1.0) byXRF-determined [Ga]/[Cu] ratios.Fig.3: XRD ? -2 ? scans of CuGaxSey thin films with XRF-determined integral compositionsin the range of 1<x<3. Single phase 1:1:2 (lower panel) [8] (vertical lines) and singlephase 1:3:5 [9] (filled circles) (upper panel) for x=1.0 and x=3.0 respectively areobserved. Both phases are identified for intermediate compositions.Fig.4: Comparison of (112) reflexes of thin CuGaxSey films, in the integral XRF-determinedcompositional range of 1<x<3, measured in ? -2 ? - (lower panels) and GI-mode (0.8°)(upper panels). For bi-layer samples two (112) peaks can be resolved, the (112) of the1:3:5 phase at higher (2?~28.1°) and the (112) of the 1:1:2 phase at lower (2 ? ~27.7°)diffraction angles. These two films (x=1.7/2.6) show a higher contribution of the 1:3:5phase in surface sensitive mode (upper panels).Figure 1Figure 2925 930 9350123   1090 1095 1100 11050123Cu L2 emission line Ga L2 emission line XRF [Ga]/[Cu]~2.6 XRF [Ga]/[Cu]~1.7 XRF [Ga]/[Cu]~1.2 XRF [Ga]/[Cu]~1.0counts [arb.u.]  emission energy [eV]0123  rear-side front-sideXRF [Ga]/[Cu]~1.7     hn=1200eV  counts/sec [arb.u.]0123  rear-side  front-side Ga L2 calculated for [Ga]/[Cu]=1.7XRF [Ga]/[Cu]~1.7hn=1200eV   925 930 9350123 rear-side front-sideXRF [Ga]/[Cu]~2.6     hn=1200eVemission energy [eV]1090 1095 1100 11050123Ga L2 emission lineCu L2 emission line  rear-side  front-side Ga L2 calculated for [Ga]/[Cu]=2.6XRF [Ga]/[Cu]~2.6hn=1200eV  Figure 3Figure 4  q - 2 qq - 2 qq - 2 qq - 2 qGI 0.8°GI 0.8°GI 0.8°GI 0.8°q - 2 qGI 0.8°              27 28 29XRF [Ga]/[Cu]=2.6 XRF [Ga]/[Cu]=3.0XRF [Ga]/[Cu]=1.7XRF [Ga]/[Cu]=1.2XRF [Ga]/[Cu]=1.0  27 28 29 observed simulatd peak fittings 27 28 29 27 28 29 27 28 29counts [arb.u.]2 Theta [°]  20 25 30 45 50 55 2 Theta [°]counts [log / arb.u.]   CuGaSe2 [8]  CuGa3Se5 [9] XRF [Ga]/[Cu] = 2.6XRF [Ga]/[Cu] = 1.0XRF [Ga]/[Cu] = 1.7XRF [Ga]/[Cu] = 3.0   

CuGaSe2 CuGa3Se5 phase transition in CCSVT grown thin films

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CuGaSe2 CuGa3Se5 phase transition in CCSVT grown thin films

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