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A new electrochemical technique for deposition of chevrel-phase compound MyMo6X8 thin films
Thin Solid Films, 206 (1991) 156- 160
156
A new electrochemical technique for deposition of chevrel-phase compound M,Mo,X, thin films Archana
Garg,
Physics Department,
Pankaj University
Garg and J. C. Garg
of Rajasthan,
Jaipur-302004
(India)
Abstract Chevrel-phase Cu,Mo,X, thin films have been deposited cathodically under galvanostatic conditions from an aqueous electrolyte bath (pH > 9) containing solutions of CuCI, MoCI,, Na,S,O,, Na,SeSO, and NH,OH. The films have been grown under different deposition conditions by varying (i) the deposition current density and (ii) the electrolyte bath composition. Energy-dispersive analysis of X-rays and X-ray diffraction studies showed that the as-deposited films were non-stoichiometric with rhombohedral structure and contained an additional phase of Cu,_ .Se along with Chevrel phases. The binary phase, however, disappeared on annealing at 300 “C for 5 min. The optical absorption studies showed that the bandgap of annealed films varies between 0.8 and 1.I eV and depends on the S/Se atomic ratio. The resistivity of the films varies from 0.2 to 0.3 R cm.
1. Introduction The class of ternary molybdenum chalcogenides M,Mo,X, where M is a metal, X a chalcogen and 0 < y < 4 (Chevrel phases) are of interest as most of them are metallic and a large number are superconductors with high T, and Hc2, and a wide variety of substitution for M and X is possible [l]. The structure of these compounds can be visualized as a slightly rhombohedrally distorted simple cubic lattice of Mo,X, units which themselves are distorted cubes, with the X atoms situated at the corners and the MO atoms placed approximately at the centre of the faces. The metal atom, e.g. lead or copper, is located in the channels running along the rhombohedral axes. When M is a large cation it is surrounded by eight Mo6X, units and the structure is a slightly distorted CsCl structure with M replacing caesium and an Mo,X, unit replacing chlorine so that the formula is M, Mo6X, (M = Pb). When M is a small cation there are two crystallographically different sites which are delocalized, each with six equivalent sites around the ternary (111) axis. Thus it is possible to introduce up to four M atoms so that the formula is Cu, Mo,X, with y = l-4 [2,3]. These compounds tend to undergo a structural transition to a triclinic structure at low temperatures (about 10 K) where M cations order. There have been a number of studies on the growth and characterization of these compounds in the form of single crystals, polycrystalline sintered lumps and wires [4, 51. However, systematic studies on samples in thin film form are rare. Electrodeposition is a very low cost process and is well suited for the deposition of Chevrelphase compound thin films onto a variety of substrates
0040-6090/91/$3.50
in the form of foils, wires and solenoids. Recently we reported this technique for depositing stoichiometric CuInSe, chalcopyrite thin films using a high pH aqueous alkaline bath and a single-parameter control of deposition current density, and have studied the structural and optical properties of the deposited films [6,7]. The object of the present study is to extend this technique to the growth of Chevrel-phase compound thin films containing copper as cation. In the present work Cu, Mo,( S, Se, _ ,Y)B thin films were electrodeposited for the first time and the effect of deposition parameters on structural and optical properties of these films are investigated.
2. Experimental
details
The electrolyte bath mixture consisted of 10 ml CuCl (0.09 mM), 10 ml MoCl, (0.015 mM), 10 ml citric acid (2.5 mM). 10 ml Na,SeSO, (0.06 mM), 10 ml Na,S,O, (0.0, 3.5, 7.0 and 10.5 uM) and 10ml NH40H in distilled water. Partially unstable Na,SeSO, solution (0.06 mM) was prepared by dissolving elemental selenium in an aqueous solution of NazSO, (pH > 9) at 90 “C. The chemicals used were of AR grade. The electrolysis cell consisted of a Corning glass slide coated with indium tin oxide (ITO) (50 R cm) or titanium foils (99.99% pure, 0.1 mm thick, Aldrich make) as cathode and platinum metal foil as an anode mounted at a fixed distance apart. The pH of the bath was more than 9. The depositions were carried out for about 15 min in a galvanostatic mode in unstirred solution at room temperature by varying (i) the deposition current density between 0.6 and 1.2 mA cm-*, (ii) the quantity of CuCl
Lausanne
A. Gnrg et al. / Elecrrochemid
deposition of M,.Mo,
f’?OlJ
(ZZlJ
IOZZ
J
N
m
(SllNfl’EltrV)
AlISN31NI
X,
157
158
U 9
00
m
:
w
u
0 ln
U ln
fELLI
ag ln
N
W
W W
CL a
0 CI
( S1lNr-l
‘GkiV 1 AlISN31NI
A. Garg et al. / Electrochemical
(0.02-0.1 mM), (iii) the quantity of MoCl, (O.OlO.lOmM) and (iv) the quantity of Na,S,O, (3.510.5 PM). The deposition process involves the reaction of Cu+ and Mo5+ with Se*- and S*- ions in aqueous solution. The complexes of copper and molybdenum with NH,OH dissociate to give a controlled number of copper and molybdenum ions at the cathode which combine with the chalcogen ions (X2-) to form Cu, Mo,X, (X E Se or S). The Se*- and S2- ions are obtained from the Na,SeSO, and Na,S,O, solutions respectively according to the following reduction reactions [8] which are convective diffusion controlled: Na,SeSO, Na,S,O,
+ 2e --f NazSO, + 2e-+NazS0,
+ Se2+ S*-
It has been noted that films do not adhere well with the substrates when deposited for (i) current densities less than 0.6 mA cm-* and greater than 1.2 mA crne2, (ii) quantities of CuCl less than 0.09 mM and of MoCl, greater than 0.015 mM and (iii) a quantity of Na2S20, greater than 11 PM. The films were analysed by X-ray diffraction (XRD) using Fe Ka radiation lines using a Philips model PW 1840 diffractometer. Compositional studies were carried out by energy-dispersive X-ray analysis (EDAX) probe attached to a scanning electron microscope (Philips 505). The optical data were obtained in the spectral range 0.2-2.6 uM on a Hitachi 330 UV-visible-nearIR spectrophotometer. The electrical resistivity of the Cu-MO-S-Se films was measured using the van der Pauw method. Evaporated indium was used as an ohmic contact.
3. Results and discussion The Cu-MO-S-Se and Cu-MO-Se films grown by this technique adhere well with ITO-coated glass and titanium substrates for the optimized deposition parameters described above and are non-stoichiometric. XRD studies were carried out for as-deposited films and those annealed at 300 “C for 5 min. Figure 1 shows TABLE
1. Compositional
analysis
of the as-deposited
Cu-MO-S-Se
Sample
S/Se in electrolyte
J (mA cm-‘)
SO S,
0.000 0.058 0.116 0.232 0.000 0.058
0.8
S2
S, S, S,
deposition
XRD curves for as-deposited and annealed Cu-MoS-Se films at current density J = 0.8 mA cm-* with varying Na,S,O, concentration in the electrolyte (sample S, = 3.5 uM, sample S, = 7.0 uM and sample S, = 10.5 PM). It can be seen that as-deposited films contain a ( 111) reflection line corresponding to a binary phase of CuZ _-x Se which disappears on annealing (annealed sample S,). It is also evident from these curves that, as the S/Se atomic ratio in the electrolyte bath increases, the (202) reflection line corresponding to Cu-MO-Se becomes more prominent and its intensity is further enhanced on annealing. It suggests that addition of sulphur to the electrolyte bath increases the deposition of molybdenum ions at the cathode and is supported by the increase in the intensity of the (223) and (214) reflection lines with increase in S/Se ratio, belonging to Chevrel phases. Further, it has been observed that the (214) reflection line belonging to the Chevrel sulphide phase is merged into the (223) line corresponding to the Chevrel selenide phase. Table 1 depicts the compositional EDAX analysis of as-deposited films for different deposition current densities J and chalcogen ratios S/Se in the electrolyte. It can be seen that the deposited films are non-stoichiometric. Further, it should be noted that the MO atomic concentration in the film increases as the chalcogen ratio S/Se is enhanced and thus supports the XRD results cited above. Figure 2 shows typical XRD curves for as-deposited and annealed (300 “C for 5 min) film of Chevrel-phase Cu-MO-Se grown at J = 1.0 mA cme2. It can be seen that the (202) and (111) reflection lines belonging to Cu-MO-Se and Cu, _-xSe of equal intensity are present in both the curves. It suggests that the additional binary phase of Cu, _-xSe persists even after annealing of the films, a behaviour not observed in Cu-MO-S-Se annealed films. Since the molybdenum content in the Cu-MO-Se films as determined from EDAX analysis is low (Table 1) in comparison with that of the CuMO-S-Se films, we may conclude that the addition of sulphur plays an important role in enhancing the deposition of molybdenum ions. A similar behaviour has been noted during the deposition of CuInS, Se, _ x films using this technique [9]. films S/Se in film
Film composition
Cu,.,,Mo, Cu4.27M06.81
1.0
159
of M,Mo6 X8
,~%.a %
sZSe6
0.00 40
Cu,,, Mo7.,&Ge,0, cu wMo 7.s4S1 &e, 6s Ck,Mo, 41Ses 42 cu 3.89Mo 7.28S0.70Se6.13
0.08
0.14 0.18 0.00 0.1 I
A. Gwg
et (11./ Electrochemicrrl
J -0.8
of M,. Mo, X,
Se and Cu-MO-Se onto ITO-coated glass substrates and titanium foils. This technique is equally applicable to electrodepositing solid lubricant films of MoS, and CuMo,S, thin films onto a variety of substrates of different shapes. The studies on growth kinetics indicated that the deposition of molybdenum ions is sensitive to the chalcogen ratio S/Se in the electrolyte as well as to the deposition current density. More investigations are, however, required to prepare single-phase stoichiometric Cu-MO-S-Se and MoS, thin films.
s2 CuMoSSe
deposition
FILMS mAlcm2
I
Acknowledgments The authors would Scientific and Industrial cial support.
like to thank the Council of Research, New Delhi, for finan-
0
I
l”“OI
0.8
I
1.0 PHOTON
I
I
I.2 ENERGY
Fig. 3. (rhr)’ US. hrl plot for J = 0.8 mA cm-’ (sample Sz).
I 1.L
I
1 I.6
1
1 I.8
References
ha’ (eV)
Cu-MO-S-Se
film
deposited
at
t 0.
Fischer
and M. B. Maple (eds.). Superconductiviry in Temury Vols. I and II. in Topics in Currenf Physics, Vols. 32 and 34. Springer, Berlin, 1982. 2 R. Fliikiger, A. Junod, R. Baillif, P. Spitzli. A. Treyvaud, A. Paoli, H. Devantay and J. Muller, Solid Stute Commun., 23 ( 1977) 699. 3 R. Fliikiger and R. Baillif, in 0. Fischer and M. B. Maple (eds.), Superconductivity in Ternary Compounds, Vol. 1. Springer, Berlin, 1982. p. I 13. 4 W. Goldacker. S. Miraglia, Y. Hariharan, T. Wolf and R. Fliikiger, in A. F. Clark and R. P. Reed (eds.), Advunces in Cryogenic Engineering Materids, Vol. 34, Plenum, New York, NY, 1988, p. 655. 5 M. P. Janwadkar, V. S. Sastry, Y. Hariharan and T. S. Radhakrishnan. Jpn. J. Appl. Phys.. Suppl. 3, 26 ( 1987) 1287. 6 P. Garg, J. C. Garg and A. C. Rastogi, Proc. 2lst IEEE Photovoltuic Specidists Conf.. Floridu, 1990, IEEE, New York, 1990, p. 471. 7 P. Garg, J. C. Garg and A. C. Rastogi. Thin Solid Films. 192 ( 1990) Compounds,
Figure 3 shows a typical plot of the variation of ( ahv)2 with photon energy hv calculated from the optical absorption spectrum for the Cu-MO-S-Se (sample S,) film deposited at J = 0.8 mA cme2. The calculated value of the bandgap E, is 1.06 eV and varies between 0.8 and 1.1 eV as the deposition current density and S/Se ratio in the electrolyte are changed. The electrical conductivity measurements yielded a value of 0.2-0.3 fi cm for CuMO-S-Se thin films of different compositions.
4. Conclusions
L5.
8 M. S. Kazacos The present investigations trodeposit non-stoichiometric
showed the ability to electhin films of Cu-Mo-S-
and
B. Miller,
J. Elecrrochem.
Sot.,
127 (1980)
2378.
9 P. Garg, A. Garg and J. C. Garg,
Thin Solid Films. 206
( 1991)
236.
156
A new electrochemical technique for deposition of chevrel-phase compound M,Mo,X, thin films Archana
Garg,
Physics Department,
Pankaj University
Garg and J. C. Garg
of Rajasthan,
Jaipur-302004
(India)
Abstract Chevrel-phase Cu,Mo,X, thin films have been deposited cathodically under galvanostatic conditions from an aqueous electrolyte bath (pH > 9) containing solutions of CuCI, MoCI,, Na,S,O,, Na,SeSO, and NH,OH. The films have been grown under different deposition conditions by varying (i) the deposition current density and (ii) the electrolyte bath composition. Energy-dispersive analysis of X-rays and X-ray diffraction studies showed that the as-deposited films were non-stoichiometric with rhombohedral structure and contained an additional phase of Cu,_ .Se along with Chevrel phases. The binary phase, however, disappeared on annealing at 300 “C for 5 min. The optical absorption studies showed that the bandgap of annealed films varies between 0.8 and 1.I eV and depends on the S/Se atomic ratio. The resistivity of the films varies from 0.2 to 0.3 R cm.
1. Introduction The class of ternary molybdenum chalcogenides M,Mo,X, where M is a metal, X a chalcogen and 0 < y < 4 (Chevrel phases) are of interest as most of them are metallic and a large number are superconductors with high T, and Hc2, and a wide variety of substitution for M and X is possible [l]. The structure of these compounds can be visualized as a slightly rhombohedrally distorted simple cubic lattice of Mo,X, units which themselves are distorted cubes, with the X atoms situated at the corners and the MO atoms placed approximately at the centre of the faces. The metal atom, e.g. lead or copper, is located in the channels running along the rhombohedral axes. When M is a large cation it is surrounded by eight Mo6X, units and the structure is a slightly distorted CsCl structure with M replacing caesium and an Mo,X, unit replacing chlorine so that the formula is M, Mo6X, (M = Pb). When M is a small cation there are two crystallographically different sites which are delocalized, each with six equivalent sites around the ternary (111) axis. Thus it is possible to introduce up to four M atoms so that the formula is Cu, Mo,X, with y = l-4 [2,3]. These compounds tend to undergo a structural transition to a triclinic structure at low temperatures (about 10 K) where M cations order. There have been a number of studies on the growth and characterization of these compounds in the form of single crystals, polycrystalline sintered lumps and wires [4, 51. However, systematic studies on samples in thin film form are rare. Electrodeposition is a very low cost process and is well suited for the deposition of Chevrelphase compound thin films onto a variety of substrates
0040-6090/91/$3.50
in the form of foils, wires and solenoids. Recently we reported this technique for depositing stoichiometric CuInSe, chalcopyrite thin films using a high pH aqueous alkaline bath and a single-parameter control of deposition current density, and have studied the structural and optical properties of the deposited films [6,7]. The object of the present study is to extend this technique to the growth of Chevrel-phase compound thin films containing copper as cation. In the present work Cu, Mo,( S, Se, _ ,Y)B thin films were electrodeposited for the first time and the effect of deposition parameters on structural and optical properties of these films are investigated.
2. Experimental
details
The electrolyte bath mixture consisted of 10 ml CuCl (0.09 mM), 10 ml MoCl, (0.015 mM), 10 ml citric acid (2.5 mM). 10 ml Na,SeSO, (0.06 mM), 10 ml Na,S,O, (0.0, 3.5, 7.0 and 10.5 uM) and 10ml NH40H in distilled water. Partially unstable Na,SeSO, solution (0.06 mM) was prepared by dissolving elemental selenium in an aqueous solution of NazSO, (pH > 9) at 90 “C. The chemicals used were of AR grade. The electrolysis cell consisted of a Corning glass slide coated with indium tin oxide (ITO) (50 R cm) or titanium foils (99.99% pure, 0.1 mm thick, Aldrich make) as cathode and platinum metal foil as an anode mounted at a fixed distance apart. The pH of the bath was more than 9. The depositions were carried out for about 15 min in a galvanostatic mode in unstirred solution at room temperature by varying (i) the deposition current density between 0.6 and 1.2 mA cm-*, (ii) the quantity of CuCl
Lausanne
A. Gnrg et al. / Elecrrochemid
deposition of M,.Mo,
f’?OlJ
(ZZlJ
IOZZ
J
N
m
(SllNfl’EltrV)
AlISN31NI
X,
157
158
U 9
00
m
:
w
u
0 ln
U ln
fELLI
ag ln
N
W
W W
CL a
0 CI
( S1lNr-l
‘GkiV 1 AlISN31NI
A. Garg et al. / Electrochemical
(0.02-0.1 mM), (iii) the quantity of MoCl, (O.OlO.lOmM) and (iv) the quantity of Na,S,O, (3.510.5 PM). The deposition process involves the reaction of Cu+ and Mo5+ with Se*- and S*- ions in aqueous solution. The complexes of copper and molybdenum with NH,OH dissociate to give a controlled number of copper and molybdenum ions at the cathode which combine with the chalcogen ions (X2-) to form Cu, Mo,X, (X E Se or S). The Se*- and S2- ions are obtained from the Na,SeSO, and Na,S,O, solutions respectively according to the following reduction reactions [8] which are convective diffusion controlled: Na,SeSO, Na,S,O,
+ 2e --f NazSO, + 2e-+NazS0,
+ Se2+ S*-
It has been noted that films do not adhere well with the substrates when deposited for (i) current densities less than 0.6 mA cm-* and greater than 1.2 mA crne2, (ii) quantities of CuCl less than 0.09 mM and of MoCl, greater than 0.015 mM and (iii) a quantity of Na2S20, greater than 11 PM. The films were analysed by X-ray diffraction (XRD) using Fe Ka radiation lines using a Philips model PW 1840 diffractometer. Compositional studies were carried out by energy-dispersive X-ray analysis (EDAX) probe attached to a scanning electron microscope (Philips 505). The optical data were obtained in the spectral range 0.2-2.6 uM on a Hitachi 330 UV-visible-nearIR spectrophotometer. The electrical resistivity of the Cu-MO-S-Se films was measured using the van der Pauw method. Evaporated indium was used as an ohmic contact.
3. Results and discussion The Cu-MO-S-Se and Cu-MO-Se films grown by this technique adhere well with ITO-coated glass and titanium substrates for the optimized deposition parameters described above and are non-stoichiometric. XRD studies were carried out for as-deposited films and those annealed at 300 “C for 5 min. Figure 1 shows TABLE
1. Compositional
analysis
of the as-deposited
Cu-MO-S-Se
Sample
S/Se in electrolyte
J (mA cm-‘)
SO S,
0.000 0.058 0.116 0.232 0.000 0.058
0.8
S2
S, S, S,
deposition
XRD curves for as-deposited and annealed Cu-MoS-Se films at current density J = 0.8 mA cm-* with varying Na,S,O, concentration in the electrolyte (sample S, = 3.5 uM, sample S, = 7.0 uM and sample S, = 10.5 PM). It can be seen that as-deposited films contain a ( 111) reflection line corresponding to a binary phase of CuZ _-x Se which disappears on annealing (annealed sample S,). It is also evident from these curves that, as the S/Se atomic ratio in the electrolyte bath increases, the (202) reflection line corresponding to Cu-MO-Se becomes more prominent and its intensity is further enhanced on annealing. It suggests that addition of sulphur to the electrolyte bath increases the deposition of molybdenum ions at the cathode and is supported by the increase in the intensity of the (223) and (214) reflection lines with increase in S/Se ratio, belonging to Chevrel phases. Further, it has been observed that the (214) reflection line belonging to the Chevrel sulphide phase is merged into the (223) line corresponding to the Chevrel selenide phase. Table 1 depicts the compositional EDAX analysis of as-deposited films for different deposition current densities J and chalcogen ratios S/Se in the electrolyte. It can be seen that the deposited films are non-stoichiometric. Further, it should be noted that the MO atomic concentration in the film increases as the chalcogen ratio S/Se is enhanced and thus supports the XRD results cited above. Figure 2 shows typical XRD curves for as-deposited and annealed (300 “C for 5 min) film of Chevrel-phase Cu-MO-Se grown at J = 1.0 mA cme2. It can be seen that the (202) and (111) reflection lines belonging to Cu-MO-Se and Cu, _-xSe of equal intensity are present in both the curves. It suggests that the additional binary phase of Cu, _-xSe persists even after annealing of the films, a behaviour not observed in Cu-MO-S-Se annealed films. Since the molybdenum content in the Cu-MO-Se films as determined from EDAX analysis is low (Table 1) in comparison with that of the CuMO-S-Se films, we may conclude that the addition of sulphur plays an important role in enhancing the deposition of molybdenum ions. A similar behaviour has been noted during the deposition of CuInS, Se, _ x films using this technique [9]. films S/Se in film
Film composition
Cu,.,,Mo, Cu4.27M06.81
1.0
159
of M,Mo6 X8
,~%.a %
sZSe6
0.00 40
Cu,,, Mo7.,&Ge,0, cu wMo 7.s4S1 &e, 6s Ck,Mo, 41Ses 42 cu 3.89Mo 7.28S0.70Se6.13
0.08
0.14 0.18 0.00 0.1 I
A. Gwg
et (11./ Electrochemicrrl
J -0.8
of M,. Mo, X,
Se and Cu-MO-Se onto ITO-coated glass substrates and titanium foils. This technique is equally applicable to electrodepositing solid lubricant films of MoS, and CuMo,S, thin films onto a variety of substrates of different shapes. The studies on growth kinetics indicated that the deposition of molybdenum ions is sensitive to the chalcogen ratio S/Se in the electrolyte as well as to the deposition current density. More investigations are, however, required to prepare single-phase stoichiometric Cu-MO-S-Se and MoS, thin films.
s2 CuMoSSe
deposition
FILMS mAlcm2
I
Acknowledgments The authors would Scientific and Industrial cial support.
like to thank the Council of Research, New Delhi, for finan-
0
I
l”“OI
0.8
I
1.0 PHOTON
I
I
I.2 ENERGY
Fig. 3. (rhr)’ US. hrl plot for J = 0.8 mA cm-’ (sample Sz).
I 1.L
I
1 I.6
1
1 I.8
References
ha’ (eV)
Cu-MO-S-Se
film
deposited
at
t 0.
Fischer
and M. B. Maple (eds.). Superconductiviry in Temury Vols. I and II. in Topics in Currenf Physics, Vols. 32 and 34. Springer, Berlin, 1982. 2 R. Fliikiger, A. Junod, R. Baillif, P. Spitzli. A. Treyvaud, A. Paoli, H. Devantay and J. Muller, Solid Stute Commun., 23 ( 1977) 699. 3 R. Fliikiger and R. Baillif, in 0. Fischer and M. B. Maple (eds.), Superconductivity in Ternary Compounds, Vol. 1. Springer, Berlin, 1982. p. I 13. 4 W. Goldacker. S. Miraglia, Y. Hariharan, T. Wolf and R. Fliikiger, in A. F. Clark and R. P. Reed (eds.), Advunces in Cryogenic Engineering Materids, Vol. 34, Plenum, New York, NY, 1988, p. 655. 5 M. P. Janwadkar, V. S. Sastry, Y. Hariharan and T. S. Radhakrishnan. Jpn. J. Appl. Phys.. Suppl. 3, 26 ( 1987) 1287. 6 P. Garg, J. C. Garg and A. C. Rastogi, Proc. 2lst IEEE Photovoltuic Specidists Conf.. Floridu, 1990, IEEE, New York, 1990, p. 471. 7 P. Garg, J. C. Garg and A. C. Rastogi. Thin Solid Films. 192 ( 1990) Compounds,
Figure 3 shows a typical plot of the variation of ( ahv)2 with photon energy hv calculated from the optical absorption spectrum for the Cu-MO-S-Se (sample S,) film deposited at J = 0.8 mA cme2. The calculated value of the bandgap E, is 1.06 eV and varies between 0.8 and 1.1 eV as the deposition current density and S/Se ratio in the electrolyte are changed. The electrical conductivity measurements yielded a value of 0.2-0.3 fi cm for CuMO-S-Se thin films of different compositions.
4. Conclusions
L5.
8 M. S. Kazacos The present investigations trodeposit non-stoichiometric
showed the ability to electhin films of Cu-Mo-S-
and
B. Miller,
J. Elecrrochem.
Sot.,
127 (1980)
2378.
9 P. Garg, A. Garg and J. C. Garg,
Thin Solid Films. 206
( 1991)
236.