Chemical vapor transport of CuInS2: Correlation of growth induced defect structure and photoactivity

Solar Energy Materials 13 (1986) 221-232 North-Holland, Amsterdam

221

CHEMICAL VAPOR TRANSPORT OF CulnS2: CORRELATION OF G R O W T H INDUCED DEFECT S T R U C T U R E AND P H O T O A C T I V I T Y H. GOSLOWSKY, S. FIECHTER, R. KONENKAMP and H.J. LEWERENZ Hahn- Meitner-lnstitut fiir Kernforschung Berlin, Bereich Strahlenchemie, D-IO00 Berlin 39, Germany Received 30 September 1985 Chemical vapor transport growth of CulnS 2 is described for a variety of transport agents. Best crystallinity and stoichiometry is obtained for crystals grown on the feed phase. Crystals grown with 12 show the highest photoactivity. Photocurrent spectroscopy allows the assignment of subbandgap features to In and S induced defects. Differences in photocurrent inversion potential upon illumination with light below and above the band gap energy are attributed to near surface changes in charge equilibria.

I. Introduction The recent attention directed at ternary group I - I I I chalcopyrites results from their promising optoelectronical properties. In particular the C u - I n compounds with direct energy gaps at 1.5 and 1 eV for the S and Se analogue, respectively, have been considered for application in solar cells [1-5]. Although efficient photovoltaic [1,2] and photoelectrochemical [3] solar cells have been developed with CuInSe2, a comparable success has not yet been achieved with the sulfide despite various attempts [4-6]. The origin of the difference in performance has not yet been clarified. In a separate investigation on the effect of surface modification on photoresponse of CuInS 2 [7], a strong influence of chemical treatment was found. In selected cases, it was possible to drastically improve the photoactivity. Although the relative improvements were pronounced, it was not possible to increase the materials performance to the stage considered necessary for efficient solar energy conversion. Obviously optimization was limited by bulk transport and recombination losses. Consequently, the present work focusses on the influence of bulk properties on the overall optoelectronical behavior of CuInS 2. The influential parameters are selected to be different semiconductor growth conditions. Changes were made with respect to the employed technique (chemical vapor transport or growth from the melt) and by variation of transport agents as well as pressure, temperature and postand pretreatments. At the present stage, the identification of traps a n d / o r recombination sites in the bulk appears necessary for improvement of the material. Spectral analysis, current voltage characteristics and time resolved photocurrent experiments [8,9] have been employed for that purpose. 0165-1633/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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H. Goslowsky et al. / Chemical vapor transport of CulnS 2

2. Experimental 2.1. Synthesis and crystal growth

CuInS 2 has been synthesized from stoichiometric amounts of the elements Cu (6N/Ventron), In (6N/Ventron) and S ( 6 N / V e n t r o n ) in evacuated (10 -3 Torr) and sealed quartz ampoules. Since the employed metals exhibited a substantial oxygen content, they were pretreated in Ar-flux: the molten Cu and In were reduced in 5 : 1 A r / H 2 gas stream at l l 0 0 ° C and 800°C, respectively. After the oxide layers had been removed, the metallic melts were cooled under rinsing with Ar. The sulphur was outgassed in high vacuum at 120°C. Since the reaction of S with the melt is known to be exothermic, the substances were placed in opposite sites of the reaction tube according to a geometry described by Bachmann [10]. Upon increasing the sulfur temperature to 450°C, the formation of metal sulfide films which especially prevent the evaporation of In, was observed during heating of the metals. The C u - I n phase diagram shows the C u - I n alloy of ratio 1 : 1 to melt at 680°C [11]. Therefore, the PBN-boat containing both metals was heated to this temperature. In analogy to the liquidus line of the pseudo-binary phase diagram C u I n - C u I n S e 2 [12] it might be assumed that - 80% of the formed CuInS 2 is dissolved in the CuIn melt at that temperature. After a reaction time of 3 h the temperature of the melt was raised to 1170°C (mpcolnS 2 < 1150°C [13]) and held for 3 h for completion of the reaction which yielded a metallic black ingot after cooling (cooling rate 150°/h). The described method has the advantage of avoiding high sulfur vapor pressures and ensures a successive reaction to CuInS 2. This prepared material was used as nutrient for chemical vapor transport (CVT) experiments with IC13, Br 2, 12, CuC1, CuBr and CuI as transport agents (concentration 1 m g / c m 3). These growth experiments were performed in evacuated and sealed quartz ampoules (diameter 22 mm, length 100-150 mm) in two zone furnaces where transport took place in a temperature gradient from 850°C to 750°C. 2.2. Photoresponse m e a s u r e m e n t s

Photoelectrochemical experiments were performed in the standard 3-electrode potentiostatic arrangement with a Pt counterelectrode and a saturated calomel reference electrode (SCE) connected to a Heka potentiostat G050-15. Photocurrents were measured in supporting electrolyte using lock-in technique (Brookdeal 9505). Illumination was provided by a W - I lamp (Oriel Corp.) and spectra were recorded using a Kratos 252 monochromator with lattices blazed at 500 nm (visible) and 1000 nm (IR) attached to a HP 85 desk computer. Time resolved photocurrent measurements were performed using a Lambda Physics pulsed excimer laser (wavelength 308 nm, pulse width 12 ns), a Tektronix D 7912 transient recorder, and a Hewlett Packard 214 voltage pulse generator. For these experiments the samples were sandwiched between an ohmic I n - G a / C u back contact and a semitransparent vacuum evaporated Au top contact. The voltage

H. Goslowsky et al. / Chemical vapor transport of CulnS 2

223

pulses applied to these electrodes had a typical duration of 100/~s and amplitudes, Va, in the range between 5 and 100 V. The laser pulses had a - 50/~s delay to the onset of Va and the resulting photocurrents were recorded across a 50 I2 load resistor. In the material investigated dielectric relaxation occurs on a much longer time scale than the experiment. Hence the applied voltage can be assumed to drop uniformly across the semiconductor and to add linearly to internal potentials. At the used laser wavelength, absorption is restricted to the surface-near region; therefore, this type of experiment is particularly useful for the determination of surface potentials [8,9]. Solutions were prepared from analytical grade chemicals and distilled water and potentials are refered to the saturated calomel electrode (SCE).

3. Results 3.1. Crystal growth

After solidification of the CuInS 2 melts polycrystalline ingots with crystallites of typical size between a few and several hundred micrometers were obtained. With CVT, a coalescence of charge was observed leading to the formation of platelets of up to 10 m m edge length and 1 m m thickness (fig. 1). The orientation of the shown crystals was [112]. Powder diffraction patterns of the crystals found on different locations in the ampoule after a typical CVT experiment are shown in fig. 2. Whereas the crystals

Fig. 1. CuInS2 single crystals obtained from a CVT experiment using InCl 3 as transport agent; scale division: 1 mm.

H. Goslowsky et al. / Chemical vapor transport of CulnS2

224

Fig. 2. Guinier analysis of crystals obtained after a typical CVT experiment; top: growth on top of the nutrient (CuInS 2 pattern); middle: growth near the middle of the ampoule (InES 3 (dominating) and CulnS 2 pattern); bottom: material grown opposite from the feed phase in the ampoule (CuInS 2 (dominating) and lnES 3 pattern).

grown on the nutrient show the pattern of CulnS 2, the actually transported materials have intergrown phases of In2S 3. The furthest transported crystals show a reduction in In 2S3 content. Because of different vapor pressures, the supersaturation of In and Cu occurs at different distances from the nutrient. In rich compounds are found closer to the feed material whereas Cu rich crystals appear at the end of the ampoule. Growth on the feed material is also observed. Crystals grown by C12 transport have intergrown a second phase determined as a Cu2S containing In2S 3 of composition Cuxlns_xSt2 ~ (0 ~ x ~< 1). Thermodynamical calculations of the equilibrium vapor phase according to the method of Nol~ng and Richardson [15] indicate significant differences in vapor phase composition, depending on the employed halogen (table 1). Based on these calculations, we propose the following reaction mechanism for the transport processes: CulnS2 (s) + 2 Hal2(g ) H InHal3(g ) + ½Cu~Hal3(g ) + S2(g )

(1)

(Hal = C1, Br, I). The equilibrium constants, Kp, calculated using the partial pressures given in table 1 D1/3 PlnHal3 " Cu3Hal 3 PS2

K0=

e2

(3)

Hal 2

differ considerably for the employed transport agents. For ideal transport, K p should be approximately 1. This condition is best fulfilled for transport with I2 (fig. 3), in accordance with successful CVT of CulnS 2 also described by other authors [16,17].

Table 1 Partial pressures of CVT relevant gas species at 825°C. Condensed phases: CulnS2(s), InHal3(l), In 2S3(s), Cu 2S(s). Transporting agent

PHah (atm)

PlnHal3 (atm)

PCu3Hal3 (atm)

PS2 (atm)

Equilibrium constant Kp

CI Br 1

7 . 0 x 10 - s 2 . 0 x 10 -4 3.0x10 -I

4.2×101 1.3 x 101 1.0xl0 ~

4 . 1 × 1 0 -6 4.3 x 10 -3 7 . 5 x 1 0 -4

3.4X 10 1 3.4X 10 1 2.8X10 1

3x1013 2x107 3X10 2

H. Goslowsky et aL / Chemical vapor transport of CulnS:

225

101

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~10 -z i/1 I,.l.I rt

.,~10-3 I--

.,,=E 13.

10-4

10-s 10-6

800

900 1000 1100 1200 TEHPERATURE [ K I

1300

Fig. 3. Temperature dependence of the partial pressures of gaseous species over CulnS 2 in the presence of iodine (3 mg/ml) calculated according to the method of Noli~ng and Richardson [15]. i

i

In2S3

"- "

!

I

~

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-~

I

500

xl000

"-,,..

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i

" 1

",,

"-..

t,00

i

I

~J I

'-

" I

600 700 800 WAVELENGTH / n m

I

9OO

1000

Fig. 4. Photocurrent spectra of CulnS 2 crystals grown in different locations within the ampoule compared to In2S3; (a) melt feed phase with In excess; (b) CuC1 transported material on top of the nutrient; (c) CuCI transported material deposited in the middle of the ampoule; electrolyte 1M KCL, electrode potential 0.4 V (SCE).

226

H. Goslowsky et al. / Chemical vapor transport of CulnS,

Because of the increasing concentration difference between the gas species InHal 3 and Cu3Hal 3 in going from 12 to C12, a shift in composition of the feed material and the transported crystals is expected. With C12- or Br2-transport, the nutrient becomes In deficient while the transported material will be enriched with In. The powder diffraction patterns of C12- or Br2-transported crystals do indeed show the existence of a second phase of In2S 3 structure (fig. 2). The highest degree of crystallinity and stoichiometry was observed for the CVT grown samples on top of the nutrient. Therefore, the properties of this material have been investigated predominantly.

3.2. Photoresponse The photocurrent spectra of crystals found in different places in the ampoule after a typical transport experiment are displayed in fig. 4 and compared to the melt grown feed material. The material grown on top of the nutrient exhibits a substantial subbandgap photoresponse and small photocurrents at energies hu > Eg, as is typical for photoconductors. The actually transported crystals (curve b) contain i

,

i

i

,

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,

i

i

CulnS2

g

u.i

tcontact c~ 3: Q..

-

0.8

'

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15

'

112 '

t4

1;6

ENER6Y/eV Fig. 5. Photocurrent spectra of CulnS 2 grown with different transport agents; electrolyte 1M KCL, electrode potential 0.5 V (SCE); for Bra-transport the photoresponse of electrolyte and metal (Au) front contact is compared; thickness of Au layer 100 A.

H. Goslowsky et at,. / Chemical vapor transport of CuInS 2

227

In2S 3 occlusions (see fig. 2). Therefore, the photocurrent of In2S 3 synthesized from the melt is also shown in fig. 4. CVT grown InES 3 shows a similar spectral dependence but the photocurrent is reduced by a factor of - 100 and subbandgap response occurs. The melt has usually a somewhat more sluggish spectral response. The material shown here has been found to exhibit an improved photoresponse when In is enriched. This spectral response (curve (a)) has been obtained after optimization of the growth procedure [18] which was made possible by correlating the subbandgap photoresponse of CVT crystals to particular lattice defects as will be shown below. The quantum efficiency measured with a H e - N e laser (X = 633 nm) approaches 80% for the best samples. The CVT crystals grown on the nutrient for different transport agents show significant spectral differences (fig. 5). Whereas the C12 and Br 2 transported material exhibits a substantial photoresponse for hp < Eg, 12 transport results in considerably improved spectral features. Negligible subbandgap absorption and a steep increase of the photocurrent at the energy gap are observed in this case, typical for direct transitions. -

I

I

o.a !

i

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I

I

I

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CulnS 2

0.15 0

60 %

o

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0

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02

04

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ELECTRODE POTENTIAL(SCEJ/V Fig. 6. Potential dependence of the photocurrent of CulnS 2 grown with different transport agents; electrolyte 1M KCL.

228

H. Goslowsky et al.

/ Chemical vapor transport of CulnS e

Since these experiments were performed in supporting electrolyte in a standard electrochemical cell without redox species, the influence of a possible insoluble photocorrosion product on the spectral response was checked by comparing the electrolyte spectra with those obtained after evaporation of 100 ,~ Au onto the front of the crystals (dashed line in fig. 5). From the similarity of the overall features we conclude, that the monochromatic light intensity is small enough to allow quite accurate measurements without disturbance of the spectral features by a rapid growing sulphur or oxide layer [7]. The potential dependence of the photocurrent for illumination with white light is displayed in fig. 6 for the same set of samples as shown in fig. 5. For the C12 transported material zero dark current and almost linear photocurrent increase on both sides of the photocurrent zero is observed. The potential value for iph = 0 ( - 0 . 4 V SCE) corresponds well with the assumption of a mid gap position of the Fermi level deduced from flat band determination on p-CulnS 2 [5]. The photocurrents for Br 2 and 12 transport are approximately three orders of magnitude larger and show some rectifying behavior.

l

. . . . . . . . .

ch

F--

O F-0 "Tr~

• (18

• 19

•

•

,

1.2

•

i

t4

i 1.6

i

1.8

ENERGY/eV Fig. 7. Photocurrent spectra of IC13 transported crystals for different treatments; (a) annealing at 450°C in H2S/Ar atmosphere (1 : 10) for 3 h and subsequent annealing at 730°C for 2 h; (b) annealing at 450°C in H2S/Ar atmosphere (1 : 10) for 3 h; (c) untreated sample; (d) annealing in active vacuum at 650°C for 24 h; electrolyte 1M KCL, electrode potential 0.4 V (SCE).

229

H. Goslowsky et al. / Chemical vapor transport of CulnS 2

%

t/) I---

__i I-Z rw rw

C) "Cl)t2t.

Fig. 8. Potential dependence of photocurrent spectra for Br2-transported crystals with 100 ,~ Au front contact; the shaded rectangleindicates the photocurrent zero.

In order to get an estimate of the contribution of stoichiometry variations of the spectral features, IC13 transported material with only small subbandgap response and steep onset of interband transition was subjected to the following treatments: (i) The crystals were heated to 450°C in a H 2 S / A r 1 : 1 0 atmosphere for 3 h; (ii) additional heating for 2 h at 730°C; (iii) tempering for 24 h in active vacuum at 650°C. The resulting spectral features are displayed in fig. 7 and compared to the untreated samples (curve c). Upon annealing in H2S, the formerly small and broad subbandgap response develops into two well separated structures at E - 1.15 and -1.35 eV with a low energy shoulder on the energetically higher peak. Further annealing leads to a pronounced decrease of the low energy peak, the shoulder at - 1.3 eV vanishes resulting in a narrowing of the structure at 1.35 eV. The vacuum anneal produces a somewhat different pattern. Besides a change in photocurrent for h~, > Eg, the low energy peak of the double structure obtained after HES anneal is considerably broadened. The shoulder at - 1 . 3 eV remains almost unchanged (compare curves b and d). The position of the low energy shoulder is correlated with the low energy peaks in curves b and a. Besides the influence of the transport agent on the low energy photoresponse (fig. 5), a quite different behavior for illumination with photon energy above and below

230

H. Goslowsky et al. / Chemical vapor transport of CulnS 2 I

I

I

I

I

I

tA Z

_J

I

I

~

I---

k

I

100 ,~ Au

I-. . . . . . . . . . .

A~

cu

i,i

In - G a

]

t'Y

I--

- - V a - +,50V

Z

i,i

- - V , =-S0V

CE u

0 0 "I13_

/

I

0.2

1

I

0.6

I

I

-

t

I

1 1.4 TIME / I~s

I

I

1.8

Fig. 9. Transient photoconductivity profiles of a A u / n - C u l n S 2 structure for two bias voltages; illumination: excimer laser (Lambda Physics); pulse duration 12 ns, X = 308 nm.

Eg is observed. The potential dependence of the photocurrent spectrum of Br2-transported CulnS 2 with Au front contact (see dashed line in fig. 5) is shown in fig. 8. The shaded rectangle represents the photocurrent zero and the data clearly show a change of sign of the photocurrent upon inverting the electrode potential. It is also obvious, however, that the subbandgap response changes sign at 0 V electrode potential whereas for hv > Eg fully anodic currents are observed up to - 1.2 V and complete inversion is observed at - 1.5 V where the photocurrent is purely cathodic. In fig. 9 we show the results of the time-resolved photocurrent measurements performed on Br2-transported materials whose spectra are displayed in fig. 8. The potentials shown in fig. 9 refer to the rear electrode (In-Ga). It is obvious from fig. 9 that the photocurrent transients behave differently with time depending on the sign of the bias voltage. For + 50 V, an exponential decay is observed whereas bias inversion produces a minimum at about 700 ns. Both signals approach zero current at - 1.6/ts but have different signs in the time regime t > 450 ns. 4. Discussion

The results displayed in figs. 2 and 3 indicate that crystals which are actually transported using Br 2 and CuC1 exhibit a heterogeneous composition with phases of In2S 3 and possibly Cu2_xS incorporated in the CulnS 2 host as the equilibrium constants in table 1 indicate strong variation in vapor phase composition for the gaseous phases on the right hand side of eq. (1). Associated with irregularities in stoichiometry, the crystallinity of these samples is relatively poor. The material grown on top of the nutrient exhibits considerably better stoichiometry (fig. 2) and crystallinity (fig. 1) and appears more suited for the analysis of defect induced photoeffects. The materials found on the nutrient by variation of the transport agent also show quite different photoresponse behavior. For CuC1 transport, the photocurrent spec-

H. Goslowsky et al. / Chemical vapor transport of CulnS e

231

trum in fig. 4 indicates that photoconductivity dominates. The subbandgap response is higher compared to excitation with hv > Eg, because of the deeper penetration of the light which results in a conductivity increase. The near surface absorption for h~, > Eg leads to a considerably lower photocurrent as the bulk resistivity remains high. A substantial decrease in resistivity and a corresponding photocurrent increase is found for Br 2 and 12 transport. Whereas with Br2, a still pronounced subbandgap response is observed (fig. 5), the corresponding signal is virtually absent for 12 transport. The differences in the photoactivity are similarly revealed in fig. 6 where the CuCl-transported samples exhibit zero dark current over a wide potential range. The Br 2- and I2-transported samples show some rectifying behavior and larger anodic than cathodic photocurrents. Mostly intrinsic conductivity, most pronounced for CuCl-transported crystals, is evidenced by the almost midgap position of the Fermi level (see above). The results shown in fig. 7 have to be viewed in the context of recent experiments by Wiedemeier [19]. From his mass spectroscopy experiments and own thermochemical calculations CulnS 2 most likely decomposes according to the equation CuInS 2 ~ Cu2S(S ) + InzS(g) + 1S2(g ). We therefore interprete the structure obtained upon annealing in vacuum (curve d) to defects associated with deficiencies of In and S in the lattice. The annealing procedures in H2S atmosphere can be assigned to restoration of some S-induced defects. The successive decrease of the low energy peak (curves b and a) indicates the healing of defects resulting from S deficiencies by extended exposure to H : S at elevated temperature. We therefore assign the maximum at 1.15 eV to a S-related defect as well as the shoulder at 1.3 eV. The maximum at 1.35 eV is believed to be associated with In-induced defects. The data in fig. 8, obtained with Br2 transport, show a variation in the photocurrent inflection potential depending on whether subbandgap or above bandgap illumination prevails. The photocurrent inversion for hu < Eg occurs at the expected potential of - - 0 . 4 V (SCE) indicating close to ideal behavior expected for this intrinsic semiconductor. For excitation with h p > Eg, the photocurrent inversion is shifted negatively by approximately 0.8 V and occurs around - 1 . 2 V (SCE). It appears that for illumination with light of energy hp > Eg, which is predominantly absorbed in the surface near region, an additional space charge is introduced [20]. This excess charge leads to an asymmetry of anodic and cathodic photocurrents thus shifting the photocurrent inflection potential. The sign of the introduced charge is positive. This view is further supported by our interpretation of the time resolved photocurrent measurements. For positive bias of the rear electrode, surface field and applied field have the same direction and are thus superposing to a unipolar field profile in the region probed by the light induced carriers. The sign reversal observed for the transient photocurrent using Va = - 5 0 V, can be explained by two field regions of opposite polarity. In the region closest to the surface the carriers drift in the strong surface field which is independent of the applied bias, while carriers

232

H. Goslowsky et al. / Chemical vapor transport of CulnS:

generated deeper in the bulk follow the applied field. The temporal correlation is such that drift in the surface field dominates the beginning of the current transient, i.e. for t < t R. In agreement with the results shown in fig. 8 this suggests that the surface layer of the semiconductor is electron depleted. The origin of the illumination induced space charge is due to the near surface absorption of light with energy h u > Eg most likely associated with surface or interface states which are empty in the dark but can be filled under illumination leading to a change in the charge equilibria at the surface state/semiconductor boundary. The elucidation of the actual mechanism needs further investigation and it appears that the surface states depend critically on the crystal preparation procedure as different shifts of the photocurrent inversion potential have been observed for preparation with other transport agents.

Acknowledgement The authors thank Prof. H. Tributsch and H.-M. K~hne for stimulating discussions. The work has been supported in part by a BMFT grant No. 03E-8375-A.

References [1] [2] [3] [4] [5] [6] [7] [8] [91 [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

R.A. Michelsen and W.S. Chen, 15th IEEE Photovoltaic Spec. Conf. (1981) p. 800. J.L. Shay, S. Wagner and H.M. Kasper, Appl. Phys. Lett. 27 (1975) 89. S. Menezes, H.J. Lewerenz and K.J. Bachmann, Nature 305 (1983) 615. M. Robbins et al., J. Electrochem. Soc. 125 (1978) 831. H.J. "Lewerenz, H. Goslowsky and F.A. Thiel, Solar Energy Mater. 9 (1983) 159. D. Cahen, Y. Mirowsky and R. Tenne, Solid State Chem. 3 (1983) 173. H. Goslowsky, H.M. Kiahne, H. Neff, R. K/Stz and H.J. Lewerenz, Surf. Sci. 149 (1985) 191. R.A. Street, Phys. Rev. B 27 (1983) 4924. R. Krnenkamp and H.J. Lewerenz, J. Electrochem. Soc. 132 (1985) 2297. K.J. Bachmann, Proc. Mat. and New Proc. Techn. Photovolt., Vol. 11 (1983) p. 469. L. Gmelin, Handbuch der anorg. Chem., Vol. 63 (Verlag Chemie, 1968). K.J. Bachmann, Proc. Mat. and New Proc. Techn. Photovolt. 11 (1983) p. 469. F.A. Thiel, J. Electrochem. Soc. 129 (1982) 1570. B.I. Nolang and M.W. Richardson, J. Cryst. Growth 34 (1976) 198; J. Cryst. Growth 34 (1976) 205. Ch. Sun, H. Hwang, Ch. Len, L. Lin and B. Tseng, Jap. J. Appl. Phys. 19 (1980) Suppl. 19-3. C. Paorici, L. Zanotti and M. Curti, Crystal Res. and Techn. 17 (1982) 917. H. Goslowsky, S. Fiechter, K.-D. Husemann and H.J. Lewerenz, submitted. H. Wiedemeier and R. Santandrea, Z. Anorg. Allg. Chem. 497 (1983) 105. K. Hauffe and H.J. Engell, Z. Elektrochem. 56 (1952) 366.