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Single-crystal transformation of sphalerite CdS to cubic phaseCuxS
Applied Surface Science48/49 (1991) 535-539 North-Holland
535
Single-crystal transformation of sphalerite CdS to cubic phase Cu~S C a r m i F e l d m a n a, G u y D e u t s c h e r
a
and Enrique Grianbaum h
'~School of Physics and Astronomy. Rat'mond and Beverly Sackler Facul(v of Exact Sciences. t, Department of Ph.vsteal Electronics, Faeul O" of Engineering. Tel-Aviv Universio ~, TeI-Aviv 69978. Israel
Received 13 August 199f~; accepted for publication 30 October 1990
Thin films, 800-2000 ,~ thick, of single-crystal(100) sphalerite CdS, which were grown epitaxially on single-crystal(100) NaCI substrates and possessed a density of 1012-13 defects/cm2, were converted to Cu.,S by dipping them in a hot CuCI solution. Transmission electron microscopy of the converted samples revealed a new Cu,S cubic phase of high quality (106-7 defects/cm2), while the films remained single crystal despite the transformation process. In-situ TEM studies of the CdS to CutS transformation were carried out by heating CdS films with CuCI layers on their surfaces. They revealed that the initiation of transformation centres is accompanied by a stress relaxation, in which defect-free CdS grains are created, while the final film quality is determined by the elimination of these grains in the transformation process. The new Cu ~S cubic phase was found to be stable up to 330 o C, even in co-existencewith CdS.
1. Introduction CdS is one of the I I - V I semiconductor comp o u n d s that occurs in two crystalline phases: the wurtzite (hcp), which is the c o m m o n phase, and the sphalerite (fcc). The CuxS system possesses many phases [1]. Their structures are based on the hexagonal or the fcc symmetry of the sulphide c o m p o n e n t arrangement, while the inner structure of the lattice (or superlattice) is determined by the copper component, in accordance with its concentration (1.75 < x < 2.00). One method to obtain Cu2S, used in the preparation of C d S / C u 2 S heterojunctions, is to replace Cd atoms by Cu atoms in a CdS sample, as expressed in the following chemical exchange reaction: CdS + 2CuCI ~ CuzS + CdCI 2 which can be performed in a hot CuCI solution [2] or as a solid-state reaction [3]. This approach, which has the advantage of controlling the orientation of the Cu2S layers by the initial structure of the CdS layers, has been used widely [4-6] to transform samples of wurtzite CdS (0001) into topotaxially °oriented chalcocite and djurleite phases of Cu,.S (where 1.95 < x < 2.00).
This method was also a d o p t e d in this work, using transmission electron microscopy (TEM) for o b s e r v a t i o n s and in-situ t r a n s f o r m a t i o n s of single-crystal (100) sphalerite CdS lilms into cubic p h a s e C u , S films. Therefore it enabled us to learn about the transformation process and the thermal stability of the cubic phase Cu.,.S, as well as about the thermal stability of the C d S / C u x S junction. This report presents a part o f our research, carried out using a new approach in order to overcome the problem of the CuxS stability in co-existence with CdS and hence the C d S / C u . , S j u n c t i o n stability: the attainment of single-crystal C d S / C u x S junctions based on cubic phases of both materials CdS and CuxS.
2. Experimental methods The samples, single-crystal (100) sphalerite CdS films, were prepared [7,8] in high vacuum by epitaxial growth on single-crystal (100) NaCi substrates, floated off the substrates in water and collected on T E M grids. The double Au grids which held the self-supported samples, served for
'0169-4332/91/503.50 (,) 1991 - ElsevierScience Publishers B.V. (North-Holland)
536
(_: Feldman et al. / Single-co'stal transformation of sphalerite CdS to cubic phase Cu~S
T E M examination as well as for dipping the CdS samples in the hot CuCI solution in order to perform the transformation to Cu,.S. The samples, 800-2000 ,~, thick, possessed a density of 1012-13 defects (dislocations and stacking faults) per cm 2 and were found previously [9] to be thermally stable up to 550 o C. A saturated CuCI solution (80-98 ° C) was prepared by adding CuCI powder (99.999%) to purified water. NaC1 (99.999%) was added (NaCI: CuCI weight ratio 1 : 12-1 : 3) and dissolved in the solution in order to improve the extraction of the Cd atoms from the CdS samples during the reaction process. Three types of samples were prepared for TEM studies: (1) Fully or partly transformed CdS samples, obtained by dipping them in hot CuCI solution (for 1 s - 1 min). The CdCI 2 and remains from the solution were washed off by dipping the samples in alcohol. (2) Same samples as type 1, but unwashed. Therefore some non-reacted CuCI was also left on the surfaces of these samples. (3) Untransformed CdS samples with a small drop from the CuCt solution (without NaCI, at room temperature) on one of their surfaces. Samples of type 1 served for crystal and micro-structure analysis. Samples of type 2 and
type 3 were used for in-situ studies of the transformation process and the thermal stability, respectively. These studies were carried out by heating the samples indirectly, using a holder with a heating stage and facilities for temperature control, or directly by concentrating the microscope (Philips EM-300) electron beam (100 keV).
3. Results The transformed C u , S films, of all three types, were found to consist of a single-crystal cubic phase Cu.~S with a fcc symmetry (fig. 1). The various transformation conditions affected only the quality (dislocation density) of the CuxS films and the CdS to Cu~S transformation rate. The conditions needed for obtaining high-quality (106- 7dislocations/cm 2) single-crystal cubic phase Cu~S (fig. 2a) were found to be as follows: solution temperature of 96°C, and dipping time of 25 s per 1000 A thick CdS for a full transformation of CdS in a CuCI solution with NaCI : CuCI weight ratio of 1 : 7. This ratio was determined as a compromise between full elimination of the Cd particles and a decrease in the transformation rate. Transformations carried out at lower temperatures of the CuCI solution led to samples of lower
800810
u
830840850
"= 8 7 0 8 8 0
700
740
780
5OO
540
580
400410
m 430440450
m 470480
300
340
380
i
!
II
100 bOO0010
140 m
030040050
180 m
070080
Fig. 1. TED and indexed patterns of the single-crystal cubic phase CuxS. The square spots in the indexed pattern correspond to double diffraction.
C. Feldman et aL / Single-crystal transformation of sphalerite CdS to cubic' phase Cu ~S
537
- The relative crystallographic orientations between the CdS and the C u , S are: (100) C d S [ I ( 1 0 0 ) C u , S
with
[001] CdS II [001] CuxS. - The structure of the C u , S phase is a cubic superlattice with a lattice p a r a m e t e r of a = 2 a o, where a 0 = 5.60 + 0.02 ,~. T E M micrographs of partly t r a n s f o r m e d samples (type 1 and type 2) showed a picture of u n t r a n s f o r m e d rectangular CdS grains in a CuxS matrix with their boundaries in the [001] a n d [010] directions (fig. 3). These CdS grains, which occupied most of the sample area at the early stage of the transformation, were almost without defects. Their identification in the T E M micrographs was confirmed by centred dark-field images, obtained by the selection of a characteristic CdS spot in the j o i n t diffraction pattern. In-situ heating of these samples (type 2) c o n t i n u e d the t r a n s f o r m a tion process by reducing the size of the CdS grains in the CuxS matrix until they vanished completely. In addition, small particles (few h u n dredths ,um in size) of Cd a p p e a r e d on the transformed surfaces (fig. 4), as for samples that h a d t r a n s f o r m e d in a CuCI solution without NaCI. All partly t r a n s f o r m e d samples gave rise to moir6 fringes at the CdS grain b o u n d a r i e s in the C u , S matrix. Fig. 2. (a) TEM micrograph of a single-crystal cubic phase CuxS film obtained at 96°C. The arrows are drawn along the path of a long dislocation. The sharp bent contours confirm the high crystalline perfection and symmetry. (b) TEM micrograph of Cu.~S film obtained at a low transformation temperature (85°C), showing a high density of defects (short dislocations).
quality: Cu.~S films prepared at 8 0 ° C possessed a defect concentration (short dislocations) of 109-1° per cm 2 (fig. 2b). A t temperatures higher than 9 6 ° C n o i m p r o v e m e n t was found, a n d actually the b u b b l i n g in the solution (before boiling) started to introduce some cracks into the thin CdS film. Diffraction p a t t e r n s from partly t r a n s f o r m e d CdS samples (type 1), which were composed of two patterns, one from the CdS a n d the other one from the CuxS, enabled us to c o m p a r e the two lattices and to conclude that:
l
.,~n,.. ~ m u,,,D~
Fig. 3. TEM micrograph showing rectangular defect-free CdS grains in a CupS matrix.
538
C. Feldman et al. / Single-crystal transforn~ation of sphalerite CdS ~o cubic phase C u , S
4. Discussion
Fig. 4. TEM micrograph of an almost completely transformed film (type 2: obtained at 96 ° C and subsequently heated by the electron beam), showing one untransformed CdS grain (lower part of figure), one large and many small Cd particles (black spots) distributed on the film surface, and bent contours.
Samples of type 3 differed from samples of type 1 and type 2: The transformed CuxS regions showed a high density of small isolated dislocations ( ] 0 9-10 per cm2), while the CdS regions were composed of defect-free CdS grains, which were not restricted to the rectangular shape, and high defect areas, as in the original CdS films. Thermal stability examinations were carried out using samples of type 3. TEM micrographs and selected-area TED patterns of CuxS regions in the samples did not show any changes as the film temperature was increased up to 330 ° C. Furthermore, at all locations where the transformation had not been completed and could not be continued due to lack of CuCI, the CdS/CuxS boundaries also remained unchanged. At 330°C the diffraction spots became very weak (with a bright background due to increased inelastic scattering), and disappeared completely at 350 o C. TEM micrographs became also less clear at 330 o C as a semi-transparent liquid began to spread towards the untransformed CdS regions and started to perform a transformation. As the heating was ended, and the sample temperature was lowered below 330 ° C, the diffraction spots became clear again, but included additional spots, due to other CuxS phases.
The CuxS phase that was obtained in this work is different from any of the well known [10-13] structures of cubic phase CuxS. It corresponds to a new cubic phase of CuxS described only once by Kazinets et ai. [14]. In her work, this CuxS phase was grown epitaxially on a (100) single-crystal NaC! substrate from a synthesized compound of CuLgS composition. This confirms that our cubic CuxS diffraction pattern indeed corresponds to samples composed of Cu and S atoms, without any Cd in interstitial sites. The phenomenon of superlattice structure is well known [10] among the cubic phases of CuxS, as a = 5a 0 or a = 6a 0, where the values of a 0 change from one phase to another. The one that we have observed has a = 2a0; thus we can conclude that this cubic phase is composed of eight fcc cells (2 × 2 × 2) of sulphide from the original CdS, while the Cu atoms are distributed among them. The most interesting feature of the transformation process, which is observed in partly transformed samples, is the appearance of defect-free rectangular CdS grains in a Cu.~S matrix and not vice versa (i.e. CuxS grains in a CdS matrix). This points out that the transformation process is not carried out layer-by-layer (as expected) in the CuCI solution, but started at definite places, with a favoured transformation direction across the film thickness. It means that the transformation process is accompanied by another process which competes with the first one: Near each transformation centre in the CdS film a stress relaxation centre is formed, which exhibits itself as a defect-free CdS grain. This is caused by the transformation process, which shrinks (by recrystallisatien) the sulphide lattice as the Cd atoms are replaced by the Cu atoms, and gives the stressed CdS film (with its high defect concentration) the ability to be relaxed by moving all the defects away towards the CdS/CuxS boundaries. The stress relaxation is much faster than the transformation process in the temperature range of the solution. This can be deduced from partly transformed samples, where the CuxS matrix is
C. Feldman et al. / Single-crystal transformation of sphalerite CdS to cubic phase CuxS
very narrow between adjacent CdS grains, which occupy most of the sample area. These defect-free CdS grains play two important roles in the transformation process: (1) Th.qr boundaries within the CuxS matrix, where the transformation takes place, serve as channels for the out-diffusion of the Cd atoms. (2) Their presence prevents the creation of new transformation centres, which prefer the high defect CdS regions. This is confirmed by detection of moir6 fringes only at the CdS grain boundaries within the CuxS matrix, but not at their centres. The final step of the transformation process is the elimination of these grains by lateral growth of Cu.~S. This step is also the one which is responsible for the presence of dislocations in CuxS. Their length and density are determined by the size and density of the preceding defect-free CdS grains, formed near the transformation centres: In highquality CuxS films the low density of long dislocations is a consequence of large size CdS grains, while the high density of short dislocations in Cu.,.S, obtained at lower temperatures, is a direct consequence of a slower relaxation process which leads to small CdS grains and to a higher concentration of transformation centres.
5. Summary Single-crystal cubic phase CUxS films have been produced by dipping single-crystal sphalerite CdS films in a hot CuCI solution. The structure has been found to be related to the fcc symmetry with a superlattice parameter of 11.20 ,~,, while the exact chemical composition has yet to be determined. The transformation process has been found to be accompanied by a simultaneous stress relaxation process which creates defect-free CdS grains in a partly transformed CuxS sample. The transformation is completed by their elimination, while
539
their size and distribution, which are temperature-dependent, determine the defect concentration ir 'he transformed CuxS film. The main features of the cubic Cu~S phase are its thermal stability (up to 330 o C), as well as that of the C d S / C u ~S boundaries, and its high crystallographic quality (106-7 d i s l o c a t i o n s / c m z) when the transformation is carried out under the appropriate conditions. This result points out that our approach of a single-crystal cubic phase C d S / CuxS junction may solve the problem of junction stability in C d S / C u x S solar cells.
Acknowledgement This research was partially supported by Southern California Edison and The G o r d o n Centre for Energy Studies. Special thanks are due to Dr. J. Moyer for his continued support and interest in this work.
References [1] Landolt-B~rnstein, New Series, Volume 111/17/e-9.3.3. [21 F.A. Shirland, Adv. Et:ergy Convers. 6 (1966) 201. [3] T.S. te Velde, Energy Convers. 15 (1975) 111. [4] S. Oktik, G.J. Russell and J. Woods, Solar Cells 5 (1982) 231. [5] T.M. Razykov, V.I. Vialiy and M.A. Khodjaeva, Thin Solid Films 121 (1984) 1. [6] G. Gordillo, Solar Cells 14 (1985) 219. [7] D.B. Holt, Thin Solid Films 24 (1974) 1. [8] C. Feldman, MS Thesis, Tel-Aviv University (1984). [9] C. Feldman, G. Deutscher, Y. Lereah and E. Griinbaum, Inst. Phys. Conf. Ser. No. 76 (Institute of Physics, London-Bristol, 1985) section 6. [10] G. Donnay, J.D.H. Donnay and G. Kullerud, Am. Mineral. 43 (1958) 228. [11] S. Djurle, Acta Chem. Scand. 12 (1958) 1415. [12] L. Pierce and P.R. Buseck, Am. Mineral. 63 (1978) 1. [13] G.B. Gasymov, Yu.G. Asadov, G.G. Guseinov, M.A. Gezalov and N.V. Belov, Soy. Phys. Dokl. 23 (1978) 218. [14] M.M. Kazinets, I.V. Ivanova and R.B. Shafizade, Thin Solid Films 44 (1977) 331.
535
Single-crystal transformation of sphalerite CdS to cubic phase Cu~S C a r m i F e l d m a n a, G u y D e u t s c h e r
a
and Enrique Grianbaum h
'~School of Physics and Astronomy. Rat'mond and Beverly Sackler Facul(v of Exact Sciences. t, Department of Ph.vsteal Electronics, Faeul O" of Engineering. Tel-Aviv Universio ~, TeI-Aviv 69978. Israel
Received 13 August 199f~; accepted for publication 30 October 1990
Thin films, 800-2000 ,~ thick, of single-crystal(100) sphalerite CdS, which were grown epitaxially on single-crystal(100) NaCI substrates and possessed a density of 1012-13 defects/cm2, were converted to Cu.,S by dipping them in a hot CuCI solution. Transmission electron microscopy of the converted samples revealed a new Cu,S cubic phase of high quality (106-7 defects/cm2), while the films remained single crystal despite the transformation process. In-situ TEM studies of the CdS to CutS transformation were carried out by heating CdS films with CuCI layers on their surfaces. They revealed that the initiation of transformation centres is accompanied by a stress relaxation, in which defect-free CdS grains are created, while the final film quality is determined by the elimination of these grains in the transformation process. The new Cu ~S cubic phase was found to be stable up to 330 o C, even in co-existencewith CdS.
1. Introduction CdS is one of the I I - V I semiconductor comp o u n d s that occurs in two crystalline phases: the wurtzite (hcp), which is the c o m m o n phase, and the sphalerite (fcc). The CuxS system possesses many phases [1]. Their structures are based on the hexagonal or the fcc symmetry of the sulphide c o m p o n e n t arrangement, while the inner structure of the lattice (or superlattice) is determined by the copper component, in accordance with its concentration (1.75 < x < 2.00). One method to obtain Cu2S, used in the preparation of C d S / C u 2 S heterojunctions, is to replace Cd atoms by Cu atoms in a CdS sample, as expressed in the following chemical exchange reaction: CdS + 2CuCI ~ CuzS + CdCI 2 which can be performed in a hot CuCI solution [2] or as a solid-state reaction [3]. This approach, which has the advantage of controlling the orientation of the Cu2S layers by the initial structure of the CdS layers, has been used widely [4-6] to transform samples of wurtzite CdS (0001) into topotaxially °oriented chalcocite and djurleite phases of Cu,.S (where 1.95 < x < 2.00).
This method was also a d o p t e d in this work, using transmission electron microscopy (TEM) for o b s e r v a t i o n s and in-situ t r a n s f o r m a t i o n s of single-crystal (100) sphalerite CdS lilms into cubic p h a s e C u , S films. Therefore it enabled us to learn about the transformation process and the thermal stability of the cubic phase Cu.,.S, as well as about the thermal stability of the C d S / C u x S junction. This report presents a part o f our research, carried out using a new approach in order to overcome the problem of the CuxS stability in co-existence with CdS and hence the C d S / C u . , S j u n c t i o n stability: the attainment of single-crystal C d S / C u x S junctions based on cubic phases of both materials CdS and CuxS.
2. Experimental methods The samples, single-crystal (100) sphalerite CdS films, were prepared [7,8] in high vacuum by epitaxial growth on single-crystal (100) NaCi substrates, floated off the substrates in water and collected on T E M grids. The double Au grids which held the self-supported samples, served for
'0169-4332/91/503.50 (,) 1991 - ElsevierScience Publishers B.V. (North-Holland)
536
(_: Feldman et al. / Single-co'stal transformation of sphalerite CdS to cubic phase Cu~S
T E M examination as well as for dipping the CdS samples in the hot CuCI solution in order to perform the transformation to Cu,.S. The samples, 800-2000 ,~, thick, possessed a density of 1012-13 defects (dislocations and stacking faults) per cm 2 and were found previously [9] to be thermally stable up to 550 o C. A saturated CuCI solution (80-98 ° C) was prepared by adding CuCI powder (99.999%) to purified water. NaC1 (99.999%) was added (NaCI: CuCI weight ratio 1 : 12-1 : 3) and dissolved in the solution in order to improve the extraction of the Cd atoms from the CdS samples during the reaction process. Three types of samples were prepared for TEM studies: (1) Fully or partly transformed CdS samples, obtained by dipping them in hot CuCI solution (for 1 s - 1 min). The CdCI 2 and remains from the solution were washed off by dipping the samples in alcohol. (2) Same samples as type 1, but unwashed. Therefore some non-reacted CuCI was also left on the surfaces of these samples. (3) Untransformed CdS samples with a small drop from the CuCt solution (without NaCI, at room temperature) on one of their surfaces. Samples of type 1 served for crystal and micro-structure analysis. Samples of type 2 and
type 3 were used for in-situ studies of the transformation process and the thermal stability, respectively. These studies were carried out by heating the samples indirectly, using a holder with a heating stage and facilities for temperature control, or directly by concentrating the microscope (Philips EM-300) electron beam (100 keV).
3. Results The transformed C u , S films, of all three types, were found to consist of a single-crystal cubic phase Cu.~S with a fcc symmetry (fig. 1). The various transformation conditions affected only the quality (dislocation density) of the CuxS films and the CdS to Cu~S transformation rate. The conditions needed for obtaining high-quality (106- 7dislocations/cm 2) single-crystal cubic phase Cu~S (fig. 2a) were found to be as follows: solution temperature of 96°C, and dipping time of 25 s per 1000 A thick CdS for a full transformation of CdS in a CuCI solution with NaCI : CuCI weight ratio of 1 : 7. This ratio was determined as a compromise between full elimination of the Cd particles and a decrease in the transformation rate. Transformations carried out at lower temperatures of the CuCI solution led to samples of lower
800810
u
830840850
"= 8 7 0 8 8 0
700
740
780
5OO
540
580
400410
m 430440450
m 470480
300
340
380
i
!
II
100 bOO0010
140 m
030040050
180 m
070080
Fig. 1. TED and indexed patterns of the single-crystal cubic phase CuxS. The square spots in the indexed pattern correspond to double diffraction.
C. Feldman et aL / Single-crystal transformation of sphalerite CdS to cubic' phase Cu ~S
537
- The relative crystallographic orientations between the CdS and the C u , S are: (100) C d S [ I ( 1 0 0 ) C u , S
with
[001] CdS II [001] CuxS. - The structure of the C u , S phase is a cubic superlattice with a lattice p a r a m e t e r of a = 2 a o, where a 0 = 5.60 + 0.02 ,~. T E M micrographs of partly t r a n s f o r m e d samples (type 1 and type 2) showed a picture of u n t r a n s f o r m e d rectangular CdS grains in a CuxS matrix with their boundaries in the [001] a n d [010] directions (fig. 3). These CdS grains, which occupied most of the sample area at the early stage of the transformation, were almost without defects. Their identification in the T E M micrographs was confirmed by centred dark-field images, obtained by the selection of a characteristic CdS spot in the j o i n t diffraction pattern. In-situ heating of these samples (type 2) c o n t i n u e d the t r a n s f o r m a tion process by reducing the size of the CdS grains in the CuxS matrix until they vanished completely. In addition, small particles (few h u n dredths ,um in size) of Cd a p p e a r e d on the transformed surfaces (fig. 4), as for samples that h a d t r a n s f o r m e d in a CuCI solution without NaCI. All partly t r a n s f o r m e d samples gave rise to moir6 fringes at the CdS grain b o u n d a r i e s in the C u , S matrix. Fig. 2. (a) TEM micrograph of a single-crystal cubic phase CuxS film obtained at 96°C. The arrows are drawn along the path of a long dislocation. The sharp bent contours confirm the high crystalline perfection and symmetry. (b) TEM micrograph of Cu.~S film obtained at a low transformation temperature (85°C), showing a high density of defects (short dislocations).
quality: Cu.~S films prepared at 8 0 ° C possessed a defect concentration (short dislocations) of 109-1° per cm 2 (fig. 2b). A t temperatures higher than 9 6 ° C n o i m p r o v e m e n t was found, a n d actually the b u b b l i n g in the solution (before boiling) started to introduce some cracks into the thin CdS film. Diffraction p a t t e r n s from partly t r a n s f o r m e d CdS samples (type 1), which were composed of two patterns, one from the CdS a n d the other one from the CuxS, enabled us to c o m p a r e the two lattices and to conclude that:
l
.,~n,.. ~ m u,,,D~
Fig. 3. TEM micrograph showing rectangular defect-free CdS grains in a CupS matrix.
538
C. Feldman et al. / Single-crystal transforn~ation of sphalerite CdS ~o cubic phase C u , S
4. Discussion
Fig. 4. TEM micrograph of an almost completely transformed film (type 2: obtained at 96 ° C and subsequently heated by the electron beam), showing one untransformed CdS grain (lower part of figure), one large and many small Cd particles (black spots) distributed on the film surface, and bent contours.
Samples of type 3 differed from samples of type 1 and type 2: The transformed CuxS regions showed a high density of small isolated dislocations ( ] 0 9-10 per cm2), while the CdS regions were composed of defect-free CdS grains, which were not restricted to the rectangular shape, and high defect areas, as in the original CdS films. Thermal stability examinations were carried out using samples of type 3. TEM micrographs and selected-area TED patterns of CuxS regions in the samples did not show any changes as the film temperature was increased up to 330 ° C. Furthermore, at all locations where the transformation had not been completed and could not be continued due to lack of CuCI, the CdS/CuxS boundaries also remained unchanged. At 330°C the diffraction spots became very weak (with a bright background due to increased inelastic scattering), and disappeared completely at 350 o C. TEM micrographs became also less clear at 330 o C as a semi-transparent liquid began to spread towards the untransformed CdS regions and started to perform a transformation. As the heating was ended, and the sample temperature was lowered below 330 ° C, the diffraction spots became clear again, but included additional spots, due to other CuxS phases.
The CuxS phase that was obtained in this work is different from any of the well known [10-13] structures of cubic phase CuxS. It corresponds to a new cubic phase of CuxS described only once by Kazinets et ai. [14]. In her work, this CuxS phase was grown epitaxially on a (100) single-crystal NaC! substrate from a synthesized compound of CuLgS composition. This confirms that our cubic CuxS diffraction pattern indeed corresponds to samples composed of Cu and S atoms, without any Cd in interstitial sites. The phenomenon of superlattice structure is well known [10] among the cubic phases of CuxS, as a = 5a 0 or a = 6a 0, where the values of a 0 change from one phase to another. The one that we have observed has a = 2a0; thus we can conclude that this cubic phase is composed of eight fcc cells (2 × 2 × 2) of sulphide from the original CdS, while the Cu atoms are distributed among them. The most interesting feature of the transformation process, which is observed in partly transformed samples, is the appearance of defect-free rectangular CdS grains in a Cu.~S matrix and not vice versa (i.e. CuxS grains in a CdS matrix). This points out that the transformation process is not carried out layer-by-layer (as expected) in the CuCI solution, but started at definite places, with a favoured transformation direction across the film thickness. It means that the transformation process is accompanied by another process which competes with the first one: Near each transformation centre in the CdS film a stress relaxation centre is formed, which exhibits itself as a defect-free CdS grain. This is caused by the transformation process, which shrinks (by recrystallisatien) the sulphide lattice as the Cd atoms are replaced by the Cu atoms, and gives the stressed CdS film (with its high defect concentration) the ability to be relaxed by moving all the defects away towards the CdS/CuxS boundaries. The stress relaxation is much faster than the transformation process in the temperature range of the solution. This can be deduced from partly transformed samples, where the CuxS matrix is
C. Feldman et al. / Single-crystal transformation of sphalerite CdS to cubic phase CuxS
very narrow between adjacent CdS grains, which occupy most of the sample area. These defect-free CdS grains play two important roles in the transformation process: (1) Th.qr boundaries within the CuxS matrix, where the transformation takes place, serve as channels for the out-diffusion of the Cd atoms. (2) Their presence prevents the creation of new transformation centres, which prefer the high defect CdS regions. This is confirmed by detection of moir6 fringes only at the CdS grain boundaries within the CuxS matrix, but not at their centres. The final step of the transformation process is the elimination of these grains by lateral growth of Cu.~S. This step is also the one which is responsible for the presence of dislocations in CuxS. Their length and density are determined by the size and density of the preceding defect-free CdS grains, formed near the transformation centres: In highquality CuxS films the low density of long dislocations is a consequence of large size CdS grains, while the high density of short dislocations in Cu.,.S, obtained at lower temperatures, is a direct consequence of a slower relaxation process which leads to small CdS grains and to a higher concentration of transformation centres.
5. Summary Single-crystal cubic phase CUxS films have been produced by dipping single-crystal sphalerite CdS films in a hot CuCI solution. The structure has been found to be related to the fcc symmetry with a superlattice parameter of 11.20 ,~,, while the exact chemical composition has yet to be determined. The transformation process has been found to be accompanied by a simultaneous stress relaxation process which creates defect-free CdS grains in a partly transformed CuxS sample. The transformation is completed by their elimination, while
539
their size and distribution, which are temperature-dependent, determine the defect concentration ir 'he transformed CuxS film. The main features of the cubic Cu~S phase are its thermal stability (up to 330 o C), as well as that of the C d S / C u ~S boundaries, and its high crystallographic quality (106-7 d i s l o c a t i o n s / c m z) when the transformation is carried out under the appropriate conditions. This result points out that our approach of a single-crystal cubic phase C d S / CuxS junction may solve the problem of junction stability in C d S / C u x S solar cells.
Acknowledgement This research was partially supported by Southern California Edison and The G o r d o n Centre for Energy Studies. Special thanks are due to Dr. J. Moyer for his continued support and interest in this work.
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