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Growth mechanism of 2H–WS2 thin films: a similar process to graphitization
Thin Solid Films 338 (1999) 75±80
Growth mechanism of 2H±WS2 thin ®lms: a similar process to graphitization O. Lignier, G. Couturier*, J. Salardenne CPMOH, CNRS UMR No. 5798, Universite de Bordeaux I, 351 Cours de la LibeÂration, 33405 Talence Cedex, France Received 29 January 1998; revised 19 June 1998; accepted 19 June 1998
Abstract The growth mechanism of 2H±WS2 thin ®lms for solar applications is investigated by X-ray diffraction and electron microscopy. It is shown that a Ni buffer layer increases the crystalline texture of the ®lms; a brief comparison is made between ®lms grown with and without a Ni buffer layer. A special attention is paid to the growth mechanism of 2H±WS2 ®lms on Ni-coated substrates where two different steps corresponding to the release of defects are clearly observed. The growth mechanism is then compared to the graphitization process of pyrocarbons. Also studied are the conductivity and the photoconductivity of the ®lms in relation to their crystallinity. It is shown that the photoconductivity is an increasing function of the crystallinity whereas the resistivity exhibits a maximum. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Growth; Graphitization; Thin ®lms; Solar applications; Electron microscopy; X-ray diffraction
1. Introduction Tungsten disul®de (WS2) belongs to the transition-metal dichalcogenide (TMDC) compounds. These compounds are known to crystallize in a layer type structure in which strongly bonded two-dimensional SZWZS sandwiches are weakly bonded to each other by van der Waals type forces. The applications of these materials are important: hydrodesulfurization catalysts, extractive metallurgy, solid lubricants, rechargeable battery [1±3]. The TMDC and WS2 are also very attractive semiconducting materials for photovoltaic applications [4]: ² WS2 is a semiconductor with an indirect band gap at < 1.3 eV and a direct one at < 1.78 eV that well match the solar spectrum. ² WS2 has a high optical absorption coef®cient in the visible range, that makes possible the use of relatively inexpensive thin ®lm technologies [5]. For an application of layered compounds in solar energy converting devices, ®lms with type II orientation, i.e. with basal (002) planes parallel to the substrate (~c '), have to prepared. Indeed, edge-oriented planes (~c==) exhibit recombination centres associated with dangling bonds of the tran* Corresponding author. Fax: +33 56846246; e-mail: [email protected].
sition metal and the chalcogenide atoms that dramatically damage the conversion ef®ciency. Different methods of preparation have been used to elaborate WS2 thin ®lms for photovoltaic applications: sulfurization of W or WO3 thin ®lms deposited by sputtering and evaporation techniques, chemical vapour deposition, reactive radio frequency (r.f.) sputtering, electrochemical deposition and solid-state reactions between sequentially deposited elements [6±12]. Recently, it has been shown that a large crystallite size of WSe2 (a TMDC material close to WS2) and a preferential orientation may be obtained by using nickel, chrome or nickel±chrome coated substrates. A model of van der Waals rheotaxy is proposed to explain the effect, based on the formation of an ultra thin metal±chalcogenide inter layer eutectics [13]. A similar effect of nickel on the crystallization of WS2 have also been observed [11,14±16]. In this paper, we mainly discuss the growth mechanism of WS2 deposited on silica or nickel-coated silica substrates; a comparison is made with the graphitization process of pyrocarbons. The resistivity and the photoconductivity of the ®lms in relation to their crystallinity are also discussed. 2. Experimental procedure Thin ®lms of tungsten sul®de (WSx) are deposited at room temperature by reactive r.f. magnetron sputtering.
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(98)00997-3
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diameter is made of a sintered WS2 powder (CERAC, 99.9% pure). The fused silica substrates are cleaned in successive baths of acetone and ethanol. Part of the substrates is nickel-coated, the nickel being deposited by r.f. magnetron sputtering. After deposition, the tungsten sul®de is heat treated in an open furnace under a dynamic argon atmosphere for 2 h. The S=W ratio of the as-deposited ®lms ranges from 6.5 to 4.5 for a r.f. power that ranges from 20 to 90 W. The as-deposited ®lms look like amorphous, except at high power where very small diffraction peaks are observed. The effect of several deposition and annealing parameters have been investigated within the ranges listed below: 1. deposition parameters of the tungsten sul®de WSx ² r.f. power: 20±90 W ² thickness of the as-deposited ®lm: 50±350 nm ² thickness of the nickel ®lm: 0±13 nm 2. annealing parameters ² annealing temperature: 873±1173 K ² pressure: 3±1000 Pa ² cooling rate
Fig. 1. Typical X-ray diffraction patterns for two different ®lms of thickness < 200 nm; (a) a ®lm grown on a silica substrate and (b) a ®lm grown on a Ni (5 nm)-coated substrate. For both ®lms the r.f. power is 60 W, the argon pressure during the annealing is 100 Pa and the cooling is free, note that a logarithmic scale is used vertically.
The deposition is made in a mixture of 95% of argon and 5% of H2S at a pressure of 4 Pa. The target of 100 mm in
The ®lms are characterized by X-ray diffraction (XRD) in the u±2u geometry and also by transmission electron microscopy (TEM). Chemical composition is deduced from Rutherford backscattering spectroscopy (RBS). The thickness of the ®lms is given by a quartz balance and it is checked by RBS. The in-plane resistance is measured in a van der Pauw geometry [17], ohmic contacts are made of silver paste. Photoconductivity is measured using a lock-in ampli®er and a transversal arrangement.
3. Results 3.1. Growth mechanism Firstly, we present some results which emphasize the role of the Ni layer. In a second part, we discuss the growth mechanism of WS2 ®lms and its relation to the graphitization process. 3.1.1. In¯uence of the nickel layer on the crystallinity of the ®lms
Fig. 2. Detail of the (008) diffraction peak for a ®lm grown on a silica substrate.
In Fig. 1 are plotted typical XRD patterns for two ®lms; one is deposited on a silica substrate (a), the other on a Nicoated substrate (b). The conditions of preparation are rigorously the same for both ®lms. Only the diffraction peaks corresponding to the basal 002` planes, with ` 1; 2; ¼, of the 2H±WS2 phase are observed; thus a type-II orientation is obtained. In Fig. 1b, besides the WS2 phase, there are traces of the Ni±W alloy; the nickel-rich fcc
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increase of the c parameter in Fig. 3b at high annealing temperature are related to the decomposition of the WS2 that leads to the formation of a tungsten cubic phase (JCPDS: 4-0806). The growth mechanism of WS2 seems in fact very similar to a graphitization process; graphite and WS2 are both lamellar compounds. This point is now discussed. 3.1.2. WS2 growth and graphitization The graphitization process is characterized by a gradual shift of the layer spacing towards the ideal one of the graphite. The graphitic phase is usually preceded by a turbostratic phase which corresponds to a stacking disorder (small translations parallel to the plane or rotations about the ~c axis, normal to the plane) that increases the mean inter layer spacing. In some cases, the graphitization may be slightly different, that is the case for pyrocarbon in presence of titane where two phases are simultaneously observed; one turbostratic and one graphitic [19]. The growth of WS2 on silica substrates can be compared to this graphitization process; crystallites A and B mentioned above correspond to turbostratic and graphitic phases, respectively. The growth mechanism of WS2 ®lms on Ni-coated substrates seems very similar to the classical graphitization process of pyrocarbons. This hypothesis is corroborated by the following remarks. In Fig. 4a and Fig. 4b, we have plotted the intensity and the FWHM of the (008) diffraction peak vs. the c parameter for all the ®lms deposited on Ni-coated substrates. The ®lms correspond to different deposition and annealing parameters as mentioned above. Two steps are clearly observed: Fig. 3. Typical variations of the c parameter vs. the annealing temperature for (a) a ®lm grown on silica substrate, (b) a ®lm grown on Ni-coated substrate.
solid solution [Ni] and the NiW phases are well identi®ed [18]. The comparison of patterns (a) and (b) in Fig. 1 shows that ®lms deposited on Ni-coated substrates have more intense diffraction peaks and smaller full widths at half medium (FWHM). Consequently, these ®lms have a better preferential orientation with a larger coherence length Lc along the ~c axis (proportional to the inverse of FWHM). On the other hand, a detailed analysis of XRD reveals two types of crystallites, labelled A and B in Fig. 2, for ®lms grown on silica substrates whereas only one type is revealed in the case of Ni-coated substrates. The c lattice parameter of A crystallites depends on the annealing temperature, it decreases as the temperature increases, as shown in Fig. 3a. On the other hand, B crystallites have a c lattice parameter close to the single crystal one and it does not depend on the temperature. The c lattice parameter of ®lms grown on Nicoated substrates decreases as the annealing temperature increases and it tends to the crystal one, as shown in Fig. 3b. Both the vanishing of B crystallites in Fig. 3a and the
3.1.2.1. Step 1 It is essentially characterized by a decrease in the c parameter. It varies from < 1.26 to < 1.237 nm. The decrease of the FWHM points out a small increase of the coherence length Lc along the ~c axis. During this step, we observe a small increase of the intensity of the (008) diffraction peak which is a good indication for the preferential orientation of the layers. 3.1.2.2. Step 2 During this stage, the change of the c parameter is very small and it tends towards the ideal one of single crystals. From the literature, the c parameter of single crystals ranges from 1.2362 (JCPDS: 8-237) to 1.2323 nm [20]. This step is principally characterized by a large increase of the coherence length Lc along the ~c axis and of the preferential orientation as well. These results can be compared to those of the graphitization which has been extensively studied. The more recent results concerning this process are mainly due to Oberlin [21,22]. During the graphitization, the distortions in the sheets of layers are eliminated by step, each of them corre-
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Fig. 4. Variations of the intensity (a) and the full width at half maximum (b) of the (008) diffraction peak vs. the c parameter for ®lms grown on Nicoated substrates.
Fig. 5. A (002) lattice fringe micrograph obtained by TEM.
Fig. 6. Typical variations of the photoconductivity (a) and the resistivity (b) vs. the intensity of the (008) diffraction peak for a ®lm grown on Ni-coated substrate.
sponding to the release of a given type of defect. Using TEM, Oberlin has counted four different steps for the thermal graphitization of the anthracene-based carbons (AC). The ®rst stage (or the initial stage) corresponds to the formation of ¯at basal structural unity (BSU), the second one to the formation of distorted columns, the third one to the formation of distorted layers and the last one to the formation of ¯at layers. The ®rst two stages are characterized by a decrease of the inter layer spacing and a very slow increase of the coherence length Lc along the ~c axis. By XRD analysis, it is not possible to distinguish between these two stages, thus, we assume that the step 1, which is observed for WS2, is related to these two stages. The third stage of the graphitization process corresponds to an association edge-to-edge of the columns, i.e. to the release of in-plane defects. This stage corresponds to a very strong change of slope of Lc accompanied by a moderate increase of La which is the coherence length perpendicular to the ~c axis. By XRD analysis, only the diffraction peaks corresponding to the basal 002` planes of 2H±WS2 are
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FWHM are unchanged. This behaviour is related to the decomposition of WS2 which leads to the formation of a tungsten cubic phase observed by XRD. This phase has for consequence to give a disorientation of the crystallites. Below the decomposition temperature of WS2, the photoconductivity increases with the crystallinity whereas the resistivity goes through a maximum. A similar behaviour is obtained for all the ®lms as shown in Fig. 7a and Fig. 7b, where the photoconductivity and the resistivity versus the intensity of the (008) diffraction peak are plotted. In weakly doped polycrystalline WS2 ®lms, transport properties are usually well described by grain boundary models. According to the classical model of Seto [24], the photocurrent Iph may be written as Iph / Dpe2qVB =kT eqVa =kT 2 1 where Dp is the incremental carrier density created by the illumination, VB and Va are the barrier height and the applied voltage between two grains respectively. Assuming Va Vappl =Ng p kT=q where Vappl and Ng are the applied voltage across the ®lm and the number of grains respectively, the photocurrent Iph varies as / Dpe2qVB =kT Vappl =Ng :
Fig. 7. Variations of the photoconductivity (a) and the resistivity (b) vs. the intensity of the (008) diffraction peak for ®lms grown on Ni-coated substrates.
observed, thus no information about the La evolution can be obtained. Nevertheless, the step 2 in Fig. 4a and Fig. 4b are related to the third stage of graphitization. In the case of graphitization, the fourth stage is related to a very strong change in slope for La. In our case, such a change can not be observed. A (002) lattice fringe micrograph obtained by TEM analysis is given in Fig. 5. It corresponds to one of the most well crystallized ®lm (full circle in Fig. 4). We observe the presence of well parallel layers without stack disorder. Small period of distortions are observed, this is the signature of the third stage in a graphitization process. These distortions remain until the end of this stage [22]. 3.2. Electronic properties related to crystallinity Typical variations of the photoconductivity and of the resistivity vs. the intensity of the (008) diffraction peak are plotted in Fig. 6a and Fig. 6b, for a ®lm deposited on a Ni-coated substrate. The photoconductivity is measured at 1.96 eV (He±Ne laser); i.e. close to the ®rst excitonic peak [23]. For a high annealing temperature (1173 K), the intensities of the 2H±WS2 diffraction peaks decrease, whereas the
The number of grains is given by: Ng `=L where ` is the distance between the electrodes and L the grain size which varies as La: the coherence length perpendicular to the ~c axis. As a consequence, the photoconductivity increases as the crystallinity increases because photocarriers are created in WS2 grains only. The nickel plays a signi®cant role for the resistivity, principally for weakly crystallized ®lms (part [a ] of the curve in Fig. 7b). As-deposited ®lms have a metallic behaviour which is due to the nickel ®lm. During the annealing process, the nickel diffuses through the ®lm and at the end of the process, most of the nickel is present in small crystallites of the Ni±W alloy which have a metallic behaviour. The semiconductor-like behaviour of the ®lms after annealing shows that the metallic islands do not percolate. However, these islands can connect some WS2 crystallites to give an heterogeneous conductivity. A consequence of that is the large 1/f excess noise which is observed for all ®lms grown on Ni-coated substrates [25,26]. For well crystallized ®lms (part [b ] of the curve in Fig. 7b), the resistivity decreases as the crystallinity increases because L increases. 4. Conclusion Thin ®lms of WS2 grown on Ni-coated substrates exhibit only one type of turbostratic crystallites whereas without nickel two types of crystallites are observed. The growth mechanism of 2H±WS2 in presence of nickel clearly exhibits two steps and it is very similar to the graphitization
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of pyrocarbons. The graphitization is characterized by a gradual shift of the layer spacing towards the ideal one. Traces of the Ni±W alloy are observed in very small quantities. They contribute to the conductivity and their in¯uence is signi®cant for weakly crystallized ®lms. Well crystallized ®lms behave as homogeneous polycrystalline materials where the conductivity is grain boundary limited. The photoconductivity increases as the crystallinity increases because it is only related to the WS2 phase. References [1] O. Weisser, S. Landa, Sulphide catalyst, their properties and applications, Pergamon Press, Oxford, 1973. [2] I. Mejenes, J. Electrochem. Soc. 127 (1980) 1751. [3] M.B. Dines, Mat. Res. Bull. 10 (1975) 287. [4] H. Tributsh, J. Electrochem. Soc. 125 (1978) 1086. [5] A. JaÈger-Waldau, M.Ch. Lux-Steiner, E. Bucher, Solid State Phenomena 37±38 (1994) 479. [6] R. Bichsel, F. LeÂvy, Thin Solid Films 116 (1984) 367. [7] G. Chatzitheodorou, S. Fiechter, M. Kunst, J. Luck, W. Jaergermann, H. Tributsch, Mater. Res. Bull. 23 (1988) 1261. [8] P.A. Bertrand, J. Mater. Res. 4 (1989) 180. [9] A. Mallouky, J.C. BerneÁde, Thin Solid Films 154 (1987) 309. [10] S. Chandra, S.N. Sahu, Phys. Status Solidi A 89 (1985) 321.
[11] O. Lignier, G. Couturier, J. Tedd, D. Gonbeau, J. Salardenne, Thin Solid Films 299 (1997) 45. [12] M. Regula, C. Ballif, J.H. Moser, F. LeÂvy, Thin Solid Films 280 (1996) 67. [13] G. Salitra, G. Hodes, E. Klein, R. Tenne, Thin Solid Films 245 (1994) 180. [14] C. Ballif, M. Regula, P.E. Schmid, M. Remskar, R. SanjineÈs, F. LeÂvy, Appl. Phys. A62 (1996) 543. [15] E. Gourmelon, O. Lignier, H. Hadouda, G. Couturier, J.C. BerneÁde, J. Tedd, J. Pouzet, J. Salardenne, Sol. Ener. Mater. Sol. Cel. 46 (1997) 115. [16] A. Ennaoui, S. Fiechter, K. Ellmer, R. Scheer, K. Diesner, Thin Solid Films 261 (1995) 124. [17] L.J. van der Pauw, Phillips Res. Reports 13 (1958) 1. [18] S.V. Nagender Naidu, A.M. Sriramamurthy, P. Rama Rao, J. Alloy Phase Diagrams 2(1) (1986). [19] A.S. Schwartz, J.C. Bokros, Carbon 5 (1967) 325. [20] W.J. Schutte, J.L. de Boer, F. Jellinek, J. Solid State Chem. 70 (1987) 207. [21] A. Oberlin, G. Terriere, J. Microsc. 18 (1973) 247. [22] A. Oberlin, Chem. Phys. Carb. 22 (1989) 1. [23] B.L. Evans, in P.A. Lee (Ed.), Physics and Chemistry of Materials with Layered Structures, Vol. 4, Optical and Electrical Properties, Reidel, Dordrecht, 1976. [24] J.Y.W. Seto, J. Appl. Phys. 46 (1975) 5247. [25] O. Lignier, G. Couturier, J. Salardenne, J. Appl. Phys. 82 (1997) 6110. [26] O. Lignier, Thesis, University of Bordeaux I, 1997.
Growth mechanism of 2H±WS2 thin ®lms: a similar process to graphitization O. Lignier, G. Couturier*, J. Salardenne CPMOH, CNRS UMR No. 5798, Universite de Bordeaux I, 351 Cours de la LibeÂration, 33405 Talence Cedex, France Received 29 January 1998; revised 19 June 1998; accepted 19 June 1998
Abstract The growth mechanism of 2H±WS2 thin ®lms for solar applications is investigated by X-ray diffraction and electron microscopy. It is shown that a Ni buffer layer increases the crystalline texture of the ®lms; a brief comparison is made between ®lms grown with and without a Ni buffer layer. A special attention is paid to the growth mechanism of 2H±WS2 ®lms on Ni-coated substrates where two different steps corresponding to the release of defects are clearly observed. The growth mechanism is then compared to the graphitization process of pyrocarbons. Also studied are the conductivity and the photoconductivity of the ®lms in relation to their crystallinity. It is shown that the photoconductivity is an increasing function of the crystallinity whereas the resistivity exhibits a maximum. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Growth; Graphitization; Thin ®lms; Solar applications; Electron microscopy; X-ray diffraction
1. Introduction Tungsten disul®de (WS2) belongs to the transition-metal dichalcogenide (TMDC) compounds. These compounds are known to crystallize in a layer type structure in which strongly bonded two-dimensional SZWZS sandwiches are weakly bonded to each other by van der Waals type forces. The applications of these materials are important: hydrodesulfurization catalysts, extractive metallurgy, solid lubricants, rechargeable battery [1±3]. The TMDC and WS2 are also very attractive semiconducting materials for photovoltaic applications [4]: ² WS2 is a semiconductor with an indirect band gap at < 1.3 eV and a direct one at < 1.78 eV that well match the solar spectrum. ² WS2 has a high optical absorption coef®cient in the visible range, that makes possible the use of relatively inexpensive thin ®lm technologies [5]. For an application of layered compounds in solar energy converting devices, ®lms with type II orientation, i.e. with basal (002) planes parallel to the substrate (~c '), have to prepared. Indeed, edge-oriented planes (~c==) exhibit recombination centres associated with dangling bonds of the tran* Corresponding author. Fax: +33 56846246; e-mail: [email protected].
sition metal and the chalcogenide atoms that dramatically damage the conversion ef®ciency. Different methods of preparation have been used to elaborate WS2 thin ®lms for photovoltaic applications: sulfurization of W or WO3 thin ®lms deposited by sputtering and evaporation techniques, chemical vapour deposition, reactive radio frequency (r.f.) sputtering, electrochemical deposition and solid-state reactions between sequentially deposited elements [6±12]. Recently, it has been shown that a large crystallite size of WSe2 (a TMDC material close to WS2) and a preferential orientation may be obtained by using nickel, chrome or nickel±chrome coated substrates. A model of van der Waals rheotaxy is proposed to explain the effect, based on the formation of an ultra thin metal±chalcogenide inter layer eutectics [13]. A similar effect of nickel on the crystallization of WS2 have also been observed [11,14±16]. In this paper, we mainly discuss the growth mechanism of WS2 deposited on silica or nickel-coated silica substrates; a comparison is made with the graphitization process of pyrocarbons. The resistivity and the photoconductivity of the ®lms in relation to their crystallinity are also discussed. 2. Experimental procedure Thin ®lms of tungsten sul®de (WSx) are deposited at room temperature by reactive r.f. magnetron sputtering.
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(98)00997-3
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diameter is made of a sintered WS2 powder (CERAC, 99.9% pure). The fused silica substrates are cleaned in successive baths of acetone and ethanol. Part of the substrates is nickel-coated, the nickel being deposited by r.f. magnetron sputtering. After deposition, the tungsten sul®de is heat treated in an open furnace under a dynamic argon atmosphere for 2 h. The S=W ratio of the as-deposited ®lms ranges from 6.5 to 4.5 for a r.f. power that ranges from 20 to 90 W. The as-deposited ®lms look like amorphous, except at high power where very small diffraction peaks are observed. The effect of several deposition and annealing parameters have been investigated within the ranges listed below: 1. deposition parameters of the tungsten sul®de WSx ² r.f. power: 20±90 W ² thickness of the as-deposited ®lm: 50±350 nm ² thickness of the nickel ®lm: 0±13 nm 2. annealing parameters ² annealing temperature: 873±1173 K ² pressure: 3±1000 Pa ² cooling rate
Fig. 1. Typical X-ray diffraction patterns for two different ®lms of thickness < 200 nm; (a) a ®lm grown on a silica substrate and (b) a ®lm grown on a Ni (5 nm)-coated substrate. For both ®lms the r.f. power is 60 W, the argon pressure during the annealing is 100 Pa and the cooling is free, note that a logarithmic scale is used vertically.
The deposition is made in a mixture of 95% of argon and 5% of H2S at a pressure of 4 Pa. The target of 100 mm in
The ®lms are characterized by X-ray diffraction (XRD) in the u±2u geometry and also by transmission electron microscopy (TEM). Chemical composition is deduced from Rutherford backscattering spectroscopy (RBS). The thickness of the ®lms is given by a quartz balance and it is checked by RBS. The in-plane resistance is measured in a van der Pauw geometry [17], ohmic contacts are made of silver paste. Photoconductivity is measured using a lock-in ampli®er and a transversal arrangement.
3. Results 3.1. Growth mechanism Firstly, we present some results which emphasize the role of the Ni layer. In a second part, we discuss the growth mechanism of WS2 ®lms and its relation to the graphitization process. 3.1.1. In¯uence of the nickel layer on the crystallinity of the ®lms
Fig. 2. Detail of the (008) diffraction peak for a ®lm grown on a silica substrate.
In Fig. 1 are plotted typical XRD patterns for two ®lms; one is deposited on a silica substrate (a), the other on a Nicoated substrate (b). The conditions of preparation are rigorously the same for both ®lms. Only the diffraction peaks corresponding to the basal 002` planes, with ` 1; 2; ¼, of the 2H±WS2 phase are observed; thus a type-II orientation is obtained. In Fig. 1b, besides the WS2 phase, there are traces of the Ni±W alloy; the nickel-rich fcc
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increase of the c parameter in Fig. 3b at high annealing temperature are related to the decomposition of the WS2 that leads to the formation of a tungsten cubic phase (JCPDS: 4-0806). The growth mechanism of WS2 seems in fact very similar to a graphitization process; graphite and WS2 are both lamellar compounds. This point is now discussed. 3.1.2. WS2 growth and graphitization The graphitization process is characterized by a gradual shift of the layer spacing towards the ideal one of the graphite. The graphitic phase is usually preceded by a turbostratic phase which corresponds to a stacking disorder (small translations parallel to the plane or rotations about the ~c axis, normal to the plane) that increases the mean inter layer spacing. In some cases, the graphitization may be slightly different, that is the case for pyrocarbon in presence of titane where two phases are simultaneously observed; one turbostratic and one graphitic [19]. The growth of WS2 on silica substrates can be compared to this graphitization process; crystallites A and B mentioned above correspond to turbostratic and graphitic phases, respectively. The growth mechanism of WS2 ®lms on Ni-coated substrates seems very similar to the classical graphitization process of pyrocarbons. This hypothesis is corroborated by the following remarks. In Fig. 4a and Fig. 4b, we have plotted the intensity and the FWHM of the (008) diffraction peak vs. the c parameter for all the ®lms deposited on Ni-coated substrates. The ®lms correspond to different deposition and annealing parameters as mentioned above. Two steps are clearly observed: Fig. 3. Typical variations of the c parameter vs. the annealing temperature for (a) a ®lm grown on silica substrate, (b) a ®lm grown on Ni-coated substrate.
solid solution [Ni] and the NiW phases are well identi®ed [18]. The comparison of patterns (a) and (b) in Fig. 1 shows that ®lms deposited on Ni-coated substrates have more intense diffraction peaks and smaller full widths at half medium (FWHM). Consequently, these ®lms have a better preferential orientation with a larger coherence length Lc along the ~c axis (proportional to the inverse of FWHM). On the other hand, a detailed analysis of XRD reveals two types of crystallites, labelled A and B in Fig. 2, for ®lms grown on silica substrates whereas only one type is revealed in the case of Ni-coated substrates. The c lattice parameter of A crystallites depends on the annealing temperature, it decreases as the temperature increases, as shown in Fig. 3a. On the other hand, B crystallites have a c lattice parameter close to the single crystal one and it does not depend on the temperature. The c lattice parameter of ®lms grown on Nicoated substrates decreases as the annealing temperature increases and it tends to the crystal one, as shown in Fig. 3b. Both the vanishing of B crystallites in Fig. 3a and the
3.1.2.1. Step 1 It is essentially characterized by a decrease in the c parameter. It varies from < 1.26 to < 1.237 nm. The decrease of the FWHM points out a small increase of the coherence length Lc along the ~c axis. During this step, we observe a small increase of the intensity of the (008) diffraction peak which is a good indication for the preferential orientation of the layers. 3.1.2.2. Step 2 During this stage, the change of the c parameter is very small and it tends towards the ideal one of single crystals. From the literature, the c parameter of single crystals ranges from 1.2362 (JCPDS: 8-237) to 1.2323 nm [20]. This step is principally characterized by a large increase of the coherence length Lc along the ~c axis and of the preferential orientation as well. These results can be compared to those of the graphitization which has been extensively studied. The more recent results concerning this process are mainly due to Oberlin [21,22]. During the graphitization, the distortions in the sheets of layers are eliminated by step, each of them corre-
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Fig. 4. Variations of the intensity (a) and the full width at half maximum (b) of the (008) diffraction peak vs. the c parameter for ®lms grown on Nicoated substrates.
Fig. 5. A (002) lattice fringe micrograph obtained by TEM.
Fig. 6. Typical variations of the photoconductivity (a) and the resistivity (b) vs. the intensity of the (008) diffraction peak for a ®lm grown on Ni-coated substrate.
sponding to the release of a given type of defect. Using TEM, Oberlin has counted four different steps for the thermal graphitization of the anthracene-based carbons (AC). The ®rst stage (or the initial stage) corresponds to the formation of ¯at basal structural unity (BSU), the second one to the formation of distorted columns, the third one to the formation of distorted layers and the last one to the formation of ¯at layers. The ®rst two stages are characterized by a decrease of the inter layer spacing and a very slow increase of the coherence length Lc along the ~c axis. By XRD analysis, it is not possible to distinguish between these two stages, thus, we assume that the step 1, which is observed for WS2, is related to these two stages. The third stage of the graphitization process corresponds to an association edge-to-edge of the columns, i.e. to the release of in-plane defects. This stage corresponds to a very strong change of slope of Lc accompanied by a moderate increase of La which is the coherence length perpendicular to the ~c axis. By XRD analysis, only the diffraction peaks corresponding to the basal 002` planes of 2H±WS2 are
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FWHM are unchanged. This behaviour is related to the decomposition of WS2 which leads to the formation of a tungsten cubic phase observed by XRD. This phase has for consequence to give a disorientation of the crystallites. Below the decomposition temperature of WS2, the photoconductivity increases with the crystallinity whereas the resistivity goes through a maximum. A similar behaviour is obtained for all the ®lms as shown in Fig. 7a and Fig. 7b, where the photoconductivity and the resistivity versus the intensity of the (008) diffraction peak are plotted. In weakly doped polycrystalline WS2 ®lms, transport properties are usually well described by grain boundary models. According to the classical model of Seto [24], the photocurrent Iph may be written as Iph / Dpe2qVB =kT eqVa =kT 2 1 where Dp is the incremental carrier density created by the illumination, VB and Va are the barrier height and the applied voltage between two grains respectively. Assuming Va Vappl =Ng p kT=q where Vappl and Ng are the applied voltage across the ®lm and the number of grains respectively, the photocurrent Iph varies as / Dpe2qVB =kT Vappl =Ng :
Fig. 7. Variations of the photoconductivity (a) and the resistivity (b) vs. the intensity of the (008) diffraction peak for ®lms grown on Ni-coated substrates.
observed, thus no information about the La evolution can be obtained. Nevertheless, the step 2 in Fig. 4a and Fig. 4b are related to the third stage of graphitization. In the case of graphitization, the fourth stage is related to a very strong change in slope for La. In our case, such a change can not be observed. A (002) lattice fringe micrograph obtained by TEM analysis is given in Fig. 5. It corresponds to one of the most well crystallized ®lm (full circle in Fig. 4). We observe the presence of well parallel layers without stack disorder. Small period of distortions are observed, this is the signature of the third stage in a graphitization process. These distortions remain until the end of this stage [22]. 3.2. Electronic properties related to crystallinity Typical variations of the photoconductivity and of the resistivity vs. the intensity of the (008) diffraction peak are plotted in Fig. 6a and Fig. 6b, for a ®lm deposited on a Ni-coated substrate. The photoconductivity is measured at 1.96 eV (He±Ne laser); i.e. close to the ®rst excitonic peak [23]. For a high annealing temperature (1173 K), the intensities of the 2H±WS2 diffraction peaks decrease, whereas the
The number of grains is given by: Ng `=L where ` is the distance between the electrodes and L the grain size which varies as La: the coherence length perpendicular to the ~c axis. As a consequence, the photoconductivity increases as the crystallinity increases because photocarriers are created in WS2 grains only. The nickel plays a signi®cant role for the resistivity, principally for weakly crystallized ®lms (part [a ] of the curve in Fig. 7b). As-deposited ®lms have a metallic behaviour which is due to the nickel ®lm. During the annealing process, the nickel diffuses through the ®lm and at the end of the process, most of the nickel is present in small crystallites of the Ni±W alloy which have a metallic behaviour. The semiconductor-like behaviour of the ®lms after annealing shows that the metallic islands do not percolate. However, these islands can connect some WS2 crystallites to give an heterogeneous conductivity. A consequence of that is the large 1/f excess noise which is observed for all ®lms grown on Ni-coated substrates [25,26]. For well crystallized ®lms (part [b ] of the curve in Fig. 7b), the resistivity decreases as the crystallinity increases because L increases. 4. Conclusion Thin ®lms of WS2 grown on Ni-coated substrates exhibit only one type of turbostratic crystallites whereas without nickel two types of crystallites are observed. The growth mechanism of 2H±WS2 in presence of nickel clearly exhibits two steps and it is very similar to the graphitization
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of pyrocarbons. The graphitization is characterized by a gradual shift of the layer spacing towards the ideal one. Traces of the Ni±W alloy are observed in very small quantities. They contribute to the conductivity and their in¯uence is signi®cant for weakly crystallized ®lms. Well crystallized ®lms behave as homogeneous polycrystalline materials where the conductivity is grain boundary limited. The photoconductivity increases as the crystallinity increases because it is only related to the WS2 phase. References [1] O. Weisser, S. Landa, Sulphide catalyst, their properties and applications, Pergamon Press, Oxford, 1973. [2] I. Mejenes, J. Electrochem. Soc. 127 (1980) 1751. [3] M.B. Dines, Mat. Res. Bull. 10 (1975) 287. [4] H. Tributsh, J. Electrochem. Soc. 125 (1978) 1086. [5] A. JaÈger-Waldau, M.Ch. Lux-Steiner, E. Bucher, Solid State Phenomena 37±38 (1994) 479. [6] R. Bichsel, F. LeÂvy, Thin Solid Films 116 (1984) 367. [7] G. Chatzitheodorou, S. Fiechter, M. Kunst, J. Luck, W. Jaergermann, H. Tributsch, Mater. Res. Bull. 23 (1988) 1261. [8] P.A. Bertrand, J. Mater. Res. 4 (1989) 180. [9] A. Mallouky, J.C. BerneÁde, Thin Solid Films 154 (1987) 309. [10] S. Chandra, S.N. Sahu, Phys. Status Solidi A 89 (1985) 321.
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