Journal of Crystal Growth 89 (1988) 160—164 North-Holland, Amsterdam
THE PHASE RELATIONS IN THE SYSTEM Cu,In,Se Klaus J. BACHMANN Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695, USA
Hans GOSLOWSKY Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, USA
and Sebastian FIECHTER Hahn -Meitner-Institut Berlin, D-I000 Berlin 39, Germany
Received 15 April 1987~manuscript received in final form 21 January 1988
The phase relations in the system Cu,In,Se are evaluated by DTA and direct observations of the solid—liquid equilibria in the temperature range 800 T 1300 K. Five distinct miscibility gap regions where two liquids coexist in equilibrium with a solid phase are observed. Four of these regions extend from monotectics on the Cu—Se and In—Se binaries into the ternary field and tie to the homogeneity ranges about Cu 2Se, 1n25e3 and In4Se3, respectively. The fifth miscibility gap regions ties to the homogeneity range about CuInSe2 at — 943 K. Further clarification concerning the phase relations on the Cu2Se—In2Se3 pseudobinary is presented for the Cu2Se-rich section where four temperature invariant 3-phase lines are observed. In addition to the y—~transition a second solid state transformation b—s~exists for stoichiometric CuInSe2 in the disordered high temperature phase. A correction of the previously published phase relations at Xi~2~ < 0.2 is needed because of reactive interactions of the melt with unprotected fused silica enclosures used in the earlier work. The position of the second congruently melting ternary compound C on the Cu2Se—CuInSe, cut is slightly shifted from the previously reported composition Cu5lnSe4 to Cu151n4Se15, and the existence of an additional compound P is suggested that we tentatively link to a peritectic reaction at 1258 K between liquid at x = 0.105 and saturated solid solution of ln,Se1 in Cu,Se referred to as n-phase. A eutectic exists between P and C at 1208 K.
1. Infroduction The ternary compound semiconductor CuInSe2 is presently the subject of intense investigations in the context of thin film solar cells since solar power conversion efficiencies in excess of 10% of AM1 have been obtained routinely for Zn~Cd1 S/ CuInSe2 polycrystalline thin film cells that are stable with regard to degradation . Also, because of its appropriate energy gap of 1.05 eV [21, CuInSe2 holds promise as the low gap component in two-junction devices, e.g. in conjunction with amorphous Si1 ~ : H . A thorough understanding of the phase relations in the system Cu,In,Se is important as a guide for research on -
improved deposition techniques of CuInSe2 thin films. Also, it is essential for the evaluation of the relation between the optoelectronic properties of CuInSe2 and its native defect chemistry based on studies of bulk single crystals of well defined composition within the homogeneity range about this compound. Partial studies of the phase relations of the Cu2Se—In2Se3 pseudobinary and in the Se-rich corner of the ternary composition field were reported in refs. ,  and , respectively, to which preliminary results on the liquidus surface and the lines in the entire ternary field were added recently [7—9].In this paper we report a more detailed study that explains some of the incon-
0022-0248/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
K.J. Bachmann et aL
/ Phase relations in system
sistencies in the previously published data and that allow a more accurate description of the miscibility gaps and tie lines pertaining to the understanding and control of compound synthesis and crystal growth processes in the Cu,In,Se systern. Also, we include into this report DTA measurements in the In—Se binary system where phase diagrams conflicting in essential features were reported in the literature [10,11]. 2. Experimental The primary method of evaluation was differential thermal analysis using a measurement system designed and built in our laboratory. A schematic cross section of this apparatus is shown in fig. 1. A special self-built furnace incorporating a K filled heat tube of 600 mm length and 30 mm inner diameter was used with a spacing of 8 mm between the inner wall of the furnace core F and the outer wall of the isothermal furnace liner L. The heat tube was grounded and formed part of the shielding of the cavity in which the samples
were placed. Three Pt/Pt—10%Rh reference grade thermocouples TC1 to TC3 of 0.25 mm diameter were used for measuring the cavity temperature T and the temperature difference ~ T between the sample under test S and a Ga reference R. An additional Pt/Pt—10%Rh thermocouple (TC4) provides the temperature control input signal to a Leeds & Northrup Electromax V controller interfaced with a L&N microprocessor controlled programmer. The DTA signals preamplified by 100 X were recorded with a resolution of 0.2°C/cm. Known transition temperature standards  established a calibration curve that was linear over the 500 T 11000 C range covered in this study. All temperature measurements were done versus the ice point and all transition temperatures refer to heating curves that were run at a maximum rate of 4°C/mm. Cooling curves were also recorded. Since substantial supercooling was observed in most cases, these curves were used only as a qualitative check of the transitions observed upon heating. The DTA data were supplemented by liquidus measurements based on direct observations of the temperature at which solid phase ceases the coexist with liquid phase upon heating. The samples used in this study were prepared from Cu2Se, CuInSe2 and In2Se3 synthesized from
the 6 N pure elements with additions of the excess elements for compositions off the Cu2Se—In2Se3 pseudobinary. Powdered mixtures of 100 p~m grain size were sealed in evacuated fused silica capsules with minimum residual empty volume and were air quenched to room temperature after homogenization at 20°C above the liquidus. For Cu-rich compositions close to the Cu—Se binary the inner walls of the capsules were coated with pyrolytic carbon to prevent reactive interaction and binding of the ternary mixture to the walls leading to cracking upon quenching. All —
~T- T - t RECORDER
data ±2°C reported in several in this individual study were runs.reproduced within 3. Results and discussion
Fig. 1. Schematic representation of the DTA furnace and the measurement and control system.
Fig. 2 shows the liquidus surface of the Cu,In,Se system as a projection of liquidus isotherms in the
K.J. Bachmann et al.
Phase relations in system C~u.In, Se
In—Se binary. The immiscible liquids forming the boundary of the 670°C plateau region tie a seetion of the homogeneity range about CuInSe2. A
thermal arrest at 670°C for a sample of nominal composition CuInSe2 was reported in ref. . The difficulties in reproducing this thermal arrest with
other samples of CuInSe2 by other authors [5,9] could be explained by a slightly off-stoichiometric composition of the sample used in ref. . Note that the miscibility gap associated with the 6700 C
in synthesizing homogeneous crystals of CuInSe2 80
plateau from Cu~In05 causes the alloy previously and Se [14,81. reported difficulties Fig. 3 shows DTA signals obtained at various compositions ondetailed the Cu7Se—In2Se1 pseudobinary where~ a more study of the phase
Cu 80 60 40 20 Se Fig. 2. Projections of the liquidus isotherms in the temperature range 500 T 1100°C onto the ternary composition field of the Cu.ln,Se system.
temperature range between 500 and 1100°C. Five distinct miscibility gaps where two liquids coexist with a solid phase exist. They correspond to plateaus in the liquidus surface. Four of these plateaus at 1105, 623, 720 and 520°C. respectively, extend from miscibility gaps on the Cu1~Se5and In1~Se~binaries into the ternary field. The plateaus at 1105 and 623°C emerge from monotectics of x 0.05 and x 0.31. and at x 0.525 and x 0.96, respectively on the =
relations on the Cu2Se rich section is needed to clarify on the previously published phase diagrams [4,7—9].For stoichiometric CuInSe2, three transformations are observed, i.e. a large peak at 998°C. a sharp peak at 814°C, and a weak peak at 850°C that at the scale of fig. 3 is barely visible in the heating curve, but is clearly resolved as a shoulder in the cooling curve. The melting transi-
Cu1 ~ binary  and are tied to the boundary of the homogeneity range about the Cu2Se solid. The plateaus at 720 and 520°C emerge from monotectics at y 0.98 and y 0.65, and at v 0.3 and v 0.04, respectively, On the In1 ~ binary [10,11] and tie to the boundaries of the homogeneity ranges about the compounds In2Se~ and In4Se3, respectively. Note that our DTA data support the correction of the In—Se phase diagram of ref.  presented in ref.  to the extent that the two liquids at monotectic temperature 520°C are not in equilibrium with In2Se, but tie to a more Se containing compound identified in ref.  as In4Se3. The fifth plateau in the liquidus surface at 670°C covers a large portion of the region labeled L1 + L4 in ref. , but is clearly disjoint from the plateau at 520°C tied to the
12 700 900
TEMPERATURE/C Fig. 3. DTA traces for 12 compositions selected on the Cu~Se—ln2Se~ pseudobinary. Values of ~ (I) 002. (2) 0.0~,(3) 0.086. (4) 0.09. (5) 0.155. (6) 0.164, (7) 0.173. (8) 0.21. (9) 0.42, (10) 0.46, (11)0.49, (12)0.5.
/ Phase relations in system
K.J. Bachmann et al. I
peak at 924°C appears, in addition to the peaks at 782 and 905°C and the liquidus defining a 3-phase line P, C, The single 3-phase ltnes a, eutectic, 6 shown in ref.  at 905°C and
Cu5InSe4, eutectic, 6 shown in ref.  at 895°C are thus replaced by a more complex feature in fig. 4. Also, the congruent melting point reported in ref.  at composition x=0.165 is shifted to x 0.186, i.e. compound C is closer to the corn=
Cu2Se Xin2se3 CuInSe2 Fig. 4. Phase relations at the pseudobinary Cu2Se—In2Se3 for Xi~2s~ 0.5.
tion at 998°C agrees well with the previously reported value of refs. [7—9] and the peak at 814°C corresponds to the ordering of the cation sublattice from disordered high temperature 6 phase into the low temperature chalcopyrite structure y phase that was first reported in ref. . The weak feature at 850°C is interpreted by us as a second high temperature phase transformation 6/h’ in analogy to the case of CuInS2 where a zincblende-to-wurtzite structure 6/~ transition has been suggested in ref. . For a more detailed study of the phase relations in the ternary system Cu,In,S we refer to a forthcoming publication . At 0.22 Xjn2se2 0.5 (traces 9—12 in fig. 3), the liquidus exhibits a monotonic decrease in temperature. However, several additional solid state transformations appear at smaller x. At x 0.49 (trace 11), a sharp peak at 782°C is shown that is present in all DTA traces for x 0.09 and defines a 3-phase line a,6,y as shown in fig. 4 and previously identified in ef. . A second peak defining the lower boundary of the 6 phase region is present, in traces 11 and 12, but disappears at x 0.46 (trace 10). At x 0.42 (trace 9) a peak at 905°C appears in addition to the phase transformation at 782°C and the liquidus. The peak at 905°C occurs in the DTA traces 6 to 9 for cornpositions 0.16 x 0.42 and defines a second 3phase line a, P, 6. In the range 0.16 x 0.40, a
position Cu181n4Se15 than Cu5InSe4. The eutectic on the left side of C towards Cu2Se at 935°C is substantially above the eutectic at 780°C reported in ref. . Since the fused silica capsules used in the study of ref.  were not coated by carbon, the data at x <0.2 shown there are in error and are superseded by the phase diagram presented in fig. 4. In view of the difficulties in obtaining high resolution DTA traces at compositions close to Cu 2Se, the extent of the a-phase region of solid solutions of In 2Se3 in Cu 2Se and the precise nature and composition of compound P are presently not known, i.e. the dashed lines in fig. 4 are not determined experimentally, but represent merely an interpretation that is consistent with the presently known set of data. Also, the phase relations in the flat region of the liquidus at 0.20 x 0.23 are presently not understood and require further examination.
4. Swnmary and conclusions
The phase relations in the system Cu,In,Se are evaluated by DTA and direct observations of the solid—liquid equilibria in the temperature range 800 T 1300 K. Five distinct miscibility gap regions where two liquids coexist in equilibrium with a solid phase are observed corresponding to plateaus at 1283, 993, 943, 886 and 793 K in the liquidus surface. Four of these regions extend from monotectics on the Cu—Se and In—Se binaries into the ternary field and tie to the homogeneity ranges about Cu2Se, In2Se3 and In4Se3, respectively. The fifth miscibility gap region ties to the homogeneity range about CuInSe2 at 943 K. Further clarification concerning the phase relalions on the Cu2Se—In2Se3 pseudobinary is
KJ. Bachmann et al.
Phase relations in system Cu, In, Se
presented for the Cu2Se-rich section where four temperature invariant 3-phase lines a—6—y at 1055 K, a—C—h’ at 1178 K, P—C—k at 1197 K and P—eutectic—C at 1223 K are observed. In addition to the y—6 transition causing a sharp DTA signal at 1087 K, a weaker signal at 1123 K is observed for stoichiornetric CuInSe2, which we interpret as a second solid state transformation 6—k in the disordered high temperature phase. A correction of the previously published phase relations at Xin2se~<0.2 is needed because of reactive interactions of the melt with unprotected fused silica enclosures used in the earlier work. The position of the second congruently melting ternary cornpound C on the Cu 2Se—CuInSe2 cut is slightly shifted from the previously reported composition Cu5InSe4 to Cu181n4Se15, and the existence of an additional compound P is suggested, which we tentatively link to a peritectic reaction at 1258 K between liquid at x 0.105 and saturated solid solution of In2Se3 in Cu2Se referred to as a-phase. A eutectic exists between P and C at 1208 K. =
Acknowledgement This work has been supported by SERI Contract XL4-04041-1.
References [11 R.A. Mickelsen and W.S. Chen. in: Proc. 16th IEEE Photovoltaic Specialists Conf., 1982, p. 781. [21 P. Lange, H. Neff, ML. Fearheiley and K.J. Bachmann, Phys. Rev. B31 (1985) 4074.  C.F. Gay, Proc. SPIE 543 (1985) 6. [41L.S. Palatnik and El. Rogacheva, Doklady 12 (1967) 503.  J.C.W. Folmer, J.A. Turner. R. Noufi and D. Cahen. J. Electrochem. Soc. 132 (1985) 1319. [61TI. Koneshova. A.A. Babitsyna and VT. Kalinnikov. Izv. Akad. Nauk SSSR, Neorg. Mater. 18 (1982) 1483.  M.L. Fearheiley and K.J. Baclsmann, in: Proc. Symp. on
      
Materials and New Processing Technologies for Photovoltaics, ECS PubI. 8311, Eds. V.J. Kapur and J. Amick (Electrochem. Soc., Princeton, NJ. 1983) p. 469. K.J. Bachmann, ML. Fearheiley. Y.H. Shing and N. Tran, AppI. Phys. Letters 44 (1984) 407. ML. Fearheiley, Solar Cells 16 (1986) 91. M. Hansen, Constitution of Binary Alloys, 2nd ed. (McGraw-Hill, New York, 1958) p. 450. A. Likforman and M. Guittard. Compt. Rend. (Paris) C279 (~974)33. Ml. Pope and M.D. Judd, Differential Thermal Analysis (Heyden, London, 1977) p 38. K.D. Becker and S. Wagner. Phys. Rev. B27 (1983) 5240. H. Haupt and K. Hess, in: Ternary Compounds 1977, Inst. Phys. Conf. Ser. 35, Ed. GD. Holah (Inst. Phys., London—Bristol, 1977) p. 5. D.J. Chakrabarti and D.E. Laughlin, Bull. Alloy Phase Diagrams 2 (1981) 305. H. Goslowsky and K.J. Bachmann. J. Electron. Mater., submitted.