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Surface microstructure and growth morphology of vacuum deposited a-As2S3 thin films
JOURNA
Journal of Non-Crystalline Solids 139 (1992) 222-230 North-Holland
L OF
NON-CR~LINE SOLIDS
Surface microstructure and growth morphology of vacuum deposited a-As2S 3 thin films N. Starbov, K. S t a r b o v a a n d J. D i k o v a Central Laboratory of Photographic Processes, Bulgarian Academy of Sciences, 1040 Sofia, Bulgaria Received 6 May 1991 Revised manuscript received 26 September 1991
Vacuum deposited a-As2S 3 thin films prepared under various experimental conditions are studied by conventional transmission and scanning electron microscopy. It is shown that general grain growth phenomena occur on the film free surface during the deposition process. The influence of the deposition rate, the substrate to evaporation source arrangement and the type of the substrate on the surface morphology is demonstrated. The formation of column-like internal film structure is shown by observations of the sample growth profiles. Thus, first evidence for the presence of microstructural inhomogeneity in vapor-deposited a-AszS3 thin films is obtained.
1. Introduction The physical and chemical properties of most vacuum deposited thin films differ from those of the corresponding bulk materials [1,2]. One of the main reasons for this difference is the specific thin film nucleation and growth phenomena which leads to microstructural aggregation of the vapor species condensed on the substrate in the form of densely arranged grains or columns. It seems that this process follows some universal trends since the granular or columnar structure is typical for crystalline and for a large number of amorphous thin films [3,4]. During the past decade, electron microscope investigations have shown that thin films of various chalcogenide glasses exhibit a well pronounced columnar structure [5-7]. It has been demonstrated further that these morphological peculiarities are responsible for some specific properties of the thin films of GexSel_ x and GexSl_x, e.g., photocontraction and anisotropic dry and wet chemical etching [6,8,9]. Simultaneously, it is strange and difficult to explain why, as an exception to the entire family of As and Ge
glasses, the vacuum deposited a-As2S 3 thin films grow in quite a different manner. It is a general assumption that the As2S 3 films are continuous and structureless, without granularity [10]. This assumption is tacitly accepted by many authors, although detailed growth morphology studies of vapor deposited As2S 3 have not been performed until now. However, structural investigations of the a-AszS 3 thin films are necessary since many of their basic properties do not differ substantially from those of the other vacuum deposited As or Ge glasses. It should be noted that literature reveals attempts to discuss the reasons for the different properties of thin films and bulk As2S 3 [11-14]. Any granularity effects in the films are excluded since the local molecular structure is accepted as a key for explanation of the differences between both types of samples. In this respect special attention is paid to the concentration of wrong and dangling bonds, bond lengths and bond angles. In fact the influence of these parameters on the behavior of the amorphous materials i s ' u n doubtedly very important. In our opinion, however, the specific properties of vacuum deposited
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
N. Starbov et al. / Vacuum deposited a-As2S 3 thin films
a-As2S 3 thin films could hardly be explained only from this point of view. Therefore, it is the aim of the present work to reveal the surface microstructure and growth morphology of vacuum deposited a-AszS 3 thin films.
2. Experimental procedure The experiments were performed with as-deposited As2S 3 films, 5-1000 nm thick, obtained
in a standard vacuum unit with conventional rotary and oil diffusion pump maintaining residual pressure in the order of 2 to 4 × 10 -4 Pa. High purity As2S 3 was used for the evaporation. The substrates were cleaned glass plates: virgin [15], coated with a thin polymer film [16], and covered by a thin film of vacuum deposited chromium [17]. In some cases, the condensation of As2S 3 was performed on a freshly cleaved mica surface. The deposition process was carried out on stationary as well as on planetary rotating substrates. For condensation on stationary substrates, a specially designed evaporation chamber allowed the preparation of films with different thicknesses at a constant deposition rate in one vacuum cycle. In this case a Knudsen vessel, indirectly heated to 170-190°C, was used as an evaporation source, allowing deposition rates up to 1 n m / s . The vapor incidence angle, measured with respect to the substrate normal, was 20 °. The evaporation of As2S 3 on planetary rotating substrates was performed by means of a directly heated source [18] in commercially available vacuum unit type UVN-2M-2 (USSR). During the film growth, the vapor incidence angle was continuously and periodically changed from 0 ° to 80 °. In this case deposition rates from 0.1 to 10 n m / s were attained by heating the evaporation source in the temperature range between 170 and 250°C. The structure of the As2S 3 films was regularly tested with X-ray and electron diffraction methods. It was found that the film structures were always amorphous despite changes in the evaporation and condensation conditions. The surface structure and growth morphology of the vacuum deposited samples were investi-
223
gated by conventional electron microscopic methods. The films for direct observation in the transmission electron microscope were obtained by condensation on pre-cleaned virgin glass plates and were removed from the substrates by means of distilled water. However, due to electron absorption and charging effects, direct imaging under the electron microscope can be applied for very thin specimens only [19]. To overcome this restriction, conventional replicas [20] were prepared from the film surface by consecutive vacuum deposition of Pt and C. It should be noted that, during the evaporation of carbon, photoand thermally initiated phenomena could occur in the AszS 3 [21]. For this reason comparative Formvar replicas were also prepared from a 0.5% CHC13 solution under safe light conditions. The metal-carbon and Formvar replicas were removed by means of 20 g/1 aqueous solution of N a O H at room temperature and then washed and fixed on electron microscope grids. In some experiments replicas of films partially dissolved in 0.1 g/1 N a O H at 20°C were also inspected under the electron microscope. The imaging of As2S 3 films and their replicas was performed under a transmission electron microscope JEOL, JEM 100 B at accelerating voltages 100 and 40 kV, respectively. Microfractographic technique [22,23] was applied for studying the growth morphology of aAszS 3 films. This technique requires cutting notches in the back side of the glass substrate before evaporation. Precautions were taken to avoid disturbance of the profiles during the fracture procedure. Further, 10 nm thick films of carbon and gold were consecutively sputtered on the studied surfaces. A scanning electron microscope JEOL, JSM T200 was used for inspection of the growth profiles.
3. Results
3.1. Choice of electron microscope preparation technique Figure 1 shows transmission electron micrographs of a direct image (a), P t / C (b) and Form-
224
N. Starbov et al. / Vacuum deposited a-As2S 3 thin films
var (c) replicas of 10 nm thick A s 2 S 3 film deposited on a stationary virgin glass substrate. As seen in fig. l(a), the As2S 3 film is built up of individual grains separated by well defined intergrain boundaries. A similar image contrast has been also observed in other thin films of amorphous elemental and compound semiconductors including a-As2Se 3 [23], belonging with As2S3, to the same family of glasses. Simultaneously, the halo rings of the electron diffraction pattern in fig. l(a) is evidence for the amorphous structure of the films. Besides, a considerable loss of image sharpness was observed for film thicknesses above 30 nm due to the small depth of focus of the electron optical system as well as the enhanced electron absorption and scattering abilities of thicker samples [19]. In our opinion, the same factors are responsible for the lack of image
contrast of 50 nm thick samples in earlier experiments [24]. Figure l(d) shows a transmission electron micrograph of a 50 nm thick a-As2S 3 film obtained in the present work. The non-homogeneous film structure is clearly seen regardless of the low image contrast. O n the other hand, the similarity between the electron micrographs of the P t / C replica (fig. l(b)) and the direct image of the film (fig. l(a)) is obvious. Simultaneously, on the micrograph of the Formvar replica presented in fig. l(c) one can also resolve, although with low contrast, the grain-like morphology shown in figs. l(a) and (b). On this basis, it could be concluded that the photo- and thermoinduced phenomena occurring during the preparation of P t / C replicas do not influence substantially the surface structure of the samples studied. Therefore, the results ob-
Fig. 1. Transmission electron micrographs of 10 nm (a)-(c) and 50 nm (d) thick As2S 3 films obtained on stationary virgin precleaned glass substrates at deposition rate 0.1 nm/s: direct TEM images (a) and (d); Pt/C (b) and Formvar (c) replicas.
N. Starbov et al. / Vacuum deposited a-As2S3 thin films tained by comparing various sample p r e p a r a t i o n techniques justify the use of the conventional P t / C replicas [20] for studying the surface morphology of v a c u u m deposited a-As2S 3 thin films with differing thicknesses.
225
3.2. Influence o f the a-AseS 3 film thickness on the surface microstructure Figure 2 shows a series of electron micrographs of the P t / C replicas of a-As2S 3 films with
Fig. 2. Transmission electron micrographs of Pt/C replicas of a-As2S3 films obtained on stationary, coated with polymer glass substrates at deposition rate 0.1 nm/s. Film thickness: 5 (a), 10 (b), 25 (c), 50 (d), 100 (e) and 1000 nm (f).
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N. Starbov et al. / Vacuum deposited a-As2S3 thin films
thicknesses ranging from 5 to 1000 nm. The films were deposited on stationary glass substrates with an adhesive polymer sublayer at a rate of 0.1 n m / s . It is clearly seen that the film surface is characterized by a granular structure which strongly depends on the quantity of deposited As2S 3. Obviously the greater the film thickness, the larger the m e a n grain size. Probably a coalescence of neighboring grains proceeds during the film growth. As a result a transformation of the intergrain boundaries from high (figs. 2(a)-(c)) to low (figs. 2(d)-(f)) contrast occurs with increasing the film thickness. In addition, the micrographs (figs. 2(d)-(f)) show that substructural units are resolved in the individual grains. In the thinnest films, these subgrains are observed at higher magnifications a s seen in figs. l(a) and (b) for a 10 nm thick As2S 3 p r e p a r e d on a virgin glass substrate. It was established that a-As2S 3 films deposited on planetary rotating substrates also have a distinct grain surface morphology. In this case, similar to the condensation of a-As2S 3 on stationary substrates, the m e a n grain size also shows a tendency to increase with increasing film thickness. Further, it was found that the films obtained on planetary rotating substrates were slightly more fine grained, compared with those p r e p a r e d on stationary substrates. Analogous relationship have been established for thermally evaporated thin crystalline films [25]. The insufficient reproducibility of subgrain imaging in a-As2S 3 films
p r e p a r e d on rotating substrates should be noted. As seen on the micrograph in fig. 3(a), however, the subunits in the separate grains can be decorated by a short-time treatment in a strongly diluted solution of N a O H . The difference between the surface morphology of 100 nm thick as-deposited, control (fig. 3(b)) and etched (fig. 3(a)) samples is evident. Analogous morphological investigations were performed on thin films obtained by condensation of As2S 3 on glass substrates, either virgin or coated with a thin layer of vacuum deposited chromium, as well as on freshly cleaved mica. In these cases the formation of grain-like surface microstructure and its evolution with increasing film thickness was also found. It should be noted that similar structural peculiarities are typical for thin films of other amorphous semiconductor compounds. The results obtained [4] have shown that the m e a n grain size is sensitive not only to the film thickness but also to the deposition rate and the type of the substrate.
3.3. Influence o f the deposition rate and the substrate type on the surface morphology o f aA s 2 S 3 films
It was found that the surface morphology of a-As2S 3 films p r e p a r e d on stationary substrates was slightly influenced by deposition rates in the interval of 0.1-1 n m / s .
Fig. 3. Transmission electron micrographs of Pt/C replicas of 100 nm thick a-As2S 3 films obtained on planetary rotating, coated with polymer glass substrates at deposition rate 0.1 nm/s: treated 5 min in 0.1 g/l aqueous solution of NaOH at 20°C (a); as-deposited (b).
N. Starbov et al. / Vacuum deposited a-As2S 3 thin films
227
Fig. 4. Transmission electron micrographs of Pt/C replicas of 100 nm thick a-As2S 3 films obtained on planetary rotating, coated with polymer glass substrates at deposition rates of 0.1 (a), 1 (b) and 10 nm/s (c). T h e results o b t a i n e d for p l a n e t a r y r o t a t i n g s u b s t r a t e s a r e p r e s e n t e d in fig. 4 for glass p l a t e s c o a t e d with a p o l y m e r film. T h e figure shows e l e c t r o n m i c r o g r a p h s of P t / C r e p l i c a s o f 100 n m thick films o f a - A s 2 S 3 o b t a i n e d at d e p o s i t i o n
r a t e s of 0.1 (a), 1 (b) a n d 10 n m / s (c). A d e c r e a s e o f t h e m e a n g r a i n size with t h e i n c r e a s e o f the k i n e t i c e n e r g y o f the v a p o r species is clearly seen. This t e n d e n c y is o b s e r v e d for all types of subs t r a t e s a n d film t h i c k n e s s e s studied. S i m u l t a -
Fig. 5. Transmission electron micrographs of Pt/C replicas of 10 nm thick a-As2S3 films obtained at deposition rate 0.1 nm/s on stationary glass substrates: virgin (a), coated with polymer (b), covered with thin Cr film (c); on freshly cleaved mica (d).
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N. Starbov et al. / Vacuum deposited a-As2S 3 thin films
neously, the observed electron images reveal some differences in the surface morphology of the films depending on the substrate used. Due to the higher grain boundary contrast, the influence of the type of substrate on the surface microstructure is more pronounced in the thinner films, both for stationary and planetary rotating substrates. This difference is clearly seen in fig. 5, comparing the replicas from the surface of 10 nm thick As2S 3 samples p r e p a r e d at a deposition rate of 0.1 n m / s on stationary glass substrates, virgin (a), coated with polymer (b) or thin Cr film (c) as well as on freshly cleaved mica surface (d). Obviously, either the grain shape or the m e a n grain size are strongly influenced by the surface used for the a-As2S 3 condensation. The morphological peculiarities of a-As2S 3 thin films manifested above are inherent to their free surface. However, the processes occurring on this surface are known to be closely related to the internal microstructure and therefore to the growth morphology of the films [27]. 3.4. Growth morphology o f the a-As2S 3 f i l m s
On the scanning electron micrograph of a fractured surface shown in fig. 6(a) one can easily distinguish the glass substrate (III), the polymer film (II) and 1000 nm thick vapor deposited As2S 3 film (I) obtained on a stationary substrate at a deposition rate of 0.1 n m / s and vapor incidence
angle 20 °. It is seen that the As2S 3 is built up of individual columns running through the entire film thickness. As could be expected, the free surface of the film is granular. The mean grain diameter as observed in scanning (fig. 6(a)) and transmission (fig. 2(e)) electron mode is practically the same. Analogously, the As2S 3 films obtained on planetary rotating substrates are also characterized by columnar microstructure. In both cases randomly distributed voids exist in the films and these voids are evidence for the presence of free volume. Besides, it was established that on pre-cleaned glass substrates as well as on Cr sublayer, As2S 3 grows also in a columnar manner. In addition, the growth profiles of a-As2S 3 films prepared within the interval of deposition rates between 0.1 and 1 n m / s show practically the same morphological features. In the present study, no observations of the internal structure of films p r e p a r e d on rotating substrates at deposition rates higher than 1 n m / s were performed. However, having in view that the deposition rate strongly influences the film surface morphology, as demonstrated in fig. 4, a decrease of the m e a n column diameter at higher deposition rates could be expected. It might be noted, however, that until now the experimentally observed column inclination toward the substrate at high vapor incidence angles is regarded as the most reliable evidence for the columnar growth of thin films of chalcogenide
Fig. 6. Scanning electron micrographs of growth profiles and top surface of a-As2S3 films (I) deposited on coated with polymer (II) stationary glass substrates (III) at vapor incidence angle 20° (a) and 80° (b). Tilt angle 60° (a) and 45° (b).
N. Starbov et aL / Vacuum deposited a-As2S s thin films
glasses [5-7]. Figure 6(b) presents a scanning electron micrograph of a growth profile and the top surface of 1000 nm thick As2S 3 sample prepared at a deposition rate of 0.1 n m / s and vapor incidence angle 80 °. Evidently inclined columns are formed as a result of the oblique deposition. Simultaneously, a column elongation coinciding with the direction of the vapor beam is clearly seen. The difference in the growth morphology of As2S 3 samples obtained at vapor incidence angle 20 ° (fig. 6(a)) and 80 ° (fig. 6(b)) is unambiguous.
4. Discussion
As seen on the micrographs in figs. 1-5, the presence of a grain-like surface microstructure of a-As2S 3 thin films is revealed under transmission electron microscope. It is established that the granularity is inherent to every stage of film formation in a wide range of condensation conditions. The series of electron micrographs presented in fig. 2 shows a thickness driven grain growth which could be reasonably explained accepting a limited mobility of the condensed vapor species over the already deposited As2S 3 [1]. Simultaneously, the subgrains displayed (figs. 2(d)-(f)) show a relation to the film thickness similarly to the large grains. It must be noted that a similar hierarchy has been observed in the growth morphology of other optical thin films [2]. On the other hand, the decoration of subgrains in the films deposited on planetary rotating substrates by a treatment in strongly diluted alkaline solutions (fig. 3(a)) indicates that certain mass redistribution occurs on the film surface after the evaporation process has been completed. Beneath the surface, however, the structure is 'frozen' and was revealed by the etching procedure. This suggestion was additionaly supported by the observed contrast enhancement of the subgrains in specimens prepared on stationary substrates after etching in dilute alkaline solutions. Besides, the mean grain size is strongly dependent on the deposition rate (fig. 5) indicating that the nucleation stage play an essential role for the thin film formation. The higher deposition rate,
229
proportional to the nucleation rate [26], results in a smaller mean grain size. Therefore, similarly to other vapor-deposited amorphous compounds [4], the a-As2S 3 films studied follow some general trends of formation of the crystalline thin films [26]. As shown in fig. 5 the surface morphology of a-As2S 3 thin films is also determined by the substrate used for condensation. Obviously, all substrate surfaces studied are characterized by a specific distribution of active sites for nucleation and growth of As2S 3 from the vapor phase. Besides, the surface mobility of the condensed vapor spieces on the various substrates should also differ. The surface granularity observed determines an internal inhomogeneity of a-AszS 3 thin films in the form of column-like structure (fig. 6). Thus, similarly to the vacuum deposited amorphous As2Se 3 [5], GeSe 3 [6] and GeS 2 [7], the a-As2S 3 thin films also grow in a columnar manner at least under these experimental conditions. This type of growth means that, by contrast with the bulk samples of the same compound, these films are structurally inhomogeneous. Moreover, the condensation of a-As2S 3 at high vapor incidence angles is accompanied by a distinctly different thin film growth morphology characterized by columns inclined toward the subsrate. On this basis it is logical to suppose the existence of structure related properties of thin a-As2S 3 films prepared at various angles between the Substrate and the direction of the vapor beam. The investigations on this problem are in progress.
5. Conclusion
The surface inhomogeneity of vapor-deposited a-As2S 3 thin films is observed as grain-like mi-
crostructure following some general trends of thin film formation. The processes occurring on the film surface result in a columnar growth morphology. Both the surface and internal film structure are influenced by the conditions of evaporation and condensation. On this basis it might be concluded that the granularity and the columnar structure are inherent properties of a-As2S 3 thin
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A( Starbov et al. / Vacuum deposited a-As2S 3 thin films
films. Therefore, from a morphological point of view, vacuum-deposited a-As2S 3 is not an exception of the thin films of amorphous arsenic and germanium chalcogenide glasses as presently accepted. Moreover, the established morphological pecularities of the a-AsaS 3 films could be regarded as complementing and not contradicting the numerous data about their local structure.
References [1] A.G. Dirks and H.J. Leamy, Thin Solid Films 47 (1977) 219. [2] H.A. Macleod, J. Vac. Sci. Technol. A4 (1986) 418. [3] I. Esquivias, J. Sanz-Maudes, J. Sangrador and T. Rodriguez, J. Vac. Sci. Technol. A3 (1985) 1791. [4] J. Jurusik and L. Zdanowicz, Thin Solid Films 144 (1986) 241. [5] F. Jansen, Thin Solid Films 78 (1981) 15. [6] K.L. Chopra, L.K. Malhotra, K. Solomon Harshavardhan and Bhanwar Singh, in: Proc. Symp. on Inorganic Resist Systems, eds. D.A. Doane and H. Heller, Proceedings Volume 82-9, (The Electrochemical Society, Pennington, NJ, 1982)p. 129. [7] C.A. Spence and S.R. Elliot, J. Non-Cryst. Solids 97&98 (1987) 1215. [8] Bhanwar Singh, S. Rajagopalan, P.K. Bhat, D.K. Pandya and K.L. Chopra, Solid State Commun. 29 (1979) 167. [9] Bhanwar Singh, S. Rajagopalan, P.K. Bhat, D.K. Pandya and K.L. Chopra, J. Non-Cryst. Solids 35&36 (1980) 1053. [10] M. Janai, S. Oscar, S.G. Lipson and P.S. Rudman, Opt. Lett. 2 (1978) 51.
[11] K. Tanaka, in: Proc. Int. Conf. on Structure and Excitation of Amorphous Solids, eds. G. Lucovsky and F.L. Galeener, AIP Conf. Proc. Number 31 (AIP, New York, 1976) p. 148. [12] A.J. Apling, A.J. Leadbetter and A.C. Wright, J. NonCryst. Solids 23 (1977) 369. [13] R J . Nemanich, G.A.N. Connell, T.M. Hayes and R.A. Street, Phys. Rev. B18 (1978) 6900. [14] D.J. Treacy, U. Strom, P.B. Klein, P.C. Taylor and T.P. Martin, J. Non-Cryst. Solids 35&36 (1980) 1035. [15] A. Savov, B. Tomerova and V. 'Platikanova, Bulgarian Authorship Certificate Nol 84027 (1989). [161 S. Kitova, H. Alexandrova and J. Malinowski J. Image. Technol. 15 (1989) 126. [171 K. Petkov, Vacuum 34 (1984) 1065. [18] B. Mednikarov, J. Vac. Sci. Technol. B7 (1989) 561. [19] J. Heydenreich and W. Neumann, Elektronenmikroskopie in der Festkoerperphysik, (Electron Microscopy in Solid State Physics), eds. H. Bethge and J. Heydenreich, (VEB, Berlin, 1982) ch. 2, pp. 41-61. [20] D.E. Bradley, Br. J. Appl. Phys. 5 (1954) 96. [21] J.S. Berkes, S.W. Ing Jr. and W.J. Hillegas, J. Appl. Phys. 42 (1971) 4908. [22] H.J. Leanly and A.G. Dirks, J. Phys. D: Appl. Phys. 10 (1977) L95; J. Appl. Phys. 49 (1978) 3430. [23] J.C. Philips, Physics Today Feb. (1982) 27. [24] J.P. De Neufville, S.C. Moss and S.R. Ovshinsky, J. Non-Cryst. Solids 13 (1973/74) 191. [251 B. Mednikarov and J. Malinowski, Commun. Dept. Chem., Bulg. Acad. Sci. 10 (1977) 64. [26] J.P. Hirth and G.M: Pound, in: Condensation and Evaporation - Nucleation and Growth Kinetics (Progress in Materials Science, Vol. 11) ed. B. Chalmers (Pergamon, Oxford, 1963) ch. 3. [271 D.J. Srolovitz, J. Vac. Sci. Technol. A4 (1986) 2925.
Journal of Non-Crystalline Solids 139 (1992) 222-230 North-Holland
L OF
NON-CR~LINE SOLIDS
Surface microstructure and growth morphology of vacuum deposited a-As2S 3 thin films N. Starbov, K. S t a r b o v a a n d J. D i k o v a Central Laboratory of Photographic Processes, Bulgarian Academy of Sciences, 1040 Sofia, Bulgaria Received 6 May 1991 Revised manuscript received 26 September 1991
Vacuum deposited a-As2S 3 thin films prepared under various experimental conditions are studied by conventional transmission and scanning electron microscopy. It is shown that general grain growth phenomena occur on the film free surface during the deposition process. The influence of the deposition rate, the substrate to evaporation source arrangement and the type of the substrate on the surface morphology is demonstrated. The formation of column-like internal film structure is shown by observations of the sample growth profiles. Thus, first evidence for the presence of microstructural inhomogeneity in vapor-deposited a-AszS3 thin films is obtained.
1. Introduction The physical and chemical properties of most vacuum deposited thin films differ from those of the corresponding bulk materials [1,2]. One of the main reasons for this difference is the specific thin film nucleation and growth phenomena which leads to microstructural aggregation of the vapor species condensed on the substrate in the form of densely arranged grains or columns. It seems that this process follows some universal trends since the granular or columnar structure is typical for crystalline and for a large number of amorphous thin films [3,4]. During the past decade, electron microscope investigations have shown that thin films of various chalcogenide glasses exhibit a well pronounced columnar structure [5-7]. It has been demonstrated further that these morphological peculiarities are responsible for some specific properties of the thin films of GexSel_ x and GexSl_x, e.g., photocontraction and anisotropic dry and wet chemical etching [6,8,9]. Simultaneously, it is strange and difficult to explain why, as an exception to the entire family of As and Ge
glasses, the vacuum deposited a-As2S 3 thin films grow in quite a different manner. It is a general assumption that the As2S 3 films are continuous and structureless, without granularity [10]. This assumption is tacitly accepted by many authors, although detailed growth morphology studies of vapor deposited As2S 3 have not been performed until now. However, structural investigations of the a-AszS 3 thin films are necessary since many of their basic properties do not differ substantially from those of the other vacuum deposited As or Ge glasses. It should be noted that literature reveals attempts to discuss the reasons for the different properties of thin films and bulk As2S 3 [11-14]. Any granularity effects in the films are excluded since the local molecular structure is accepted as a key for explanation of the differences between both types of samples. In this respect special attention is paid to the concentration of wrong and dangling bonds, bond lengths and bond angles. In fact the influence of these parameters on the behavior of the amorphous materials i s ' u n doubtedly very important. In our opinion, however, the specific properties of vacuum deposited
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
N. Starbov et al. / Vacuum deposited a-As2S 3 thin films
a-As2S 3 thin films could hardly be explained only from this point of view. Therefore, it is the aim of the present work to reveal the surface microstructure and growth morphology of vacuum deposited a-AszS 3 thin films.
2. Experimental procedure The experiments were performed with as-deposited As2S 3 films, 5-1000 nm thick, obtained
in a standard vacuum unit with conventional rotary and oil diffusion pump maintaining residual pressure in the order of 2 to 4 × 10 -4 Pa. High purity As2S 3 was used for the evaporation. The substrates were cleaned glass plates: virgin [15], coated with a thin polymer film [16], and covered by a thin film of vacuum deposited chromium [17]. In some cases, the condensation of As2S 3 was performed on a freshly cleaved mica surface. The deposition process was carried out on stationary as well as on planetary rotating substrates. For condensation on stationary substrates, a specially designed evaporation chamber allowed the preparation of films with different thicknesses at a constant deposition rate in one vacuum cycle. In this case a Knudsen vessel, indirectly heated to 170-190°C, was used as an evaporation source, allowing deposition rates up to 1 n m / s . The vapor incidence angle, measured with respect to the substrate normal, was 20 °. The evaporation of As2S 3 on planetary rotating substrates was performed by means of a directly heated source [18] in commercially available vacuum unit type UVN-2M-2 (USSR). During the film growth, the vapor incidence angle was continuously and periodically changed from 0 ° to 80 °. In this case deposition rates from 0.1 to 10 n m / s were attained by heating the evaporation source in the temperature range between 170 and 250°C. The structure of the As2S 3 films was regularly tested with X-ray and electron diffraction methods. It was found that the film structures were always amorphous despite changes in the evaporation and condensation conditions. The surface structure and growth morphology of the vacuum deposited samples were investi-
223
gated by conventional electron microscopic methods. The films for direct observation in the transmission electron microscope were obtained by condensation on pre-cleaned virgin glass plates and were removed from the substrates by means of distilled water. However, due to electron absorption and charging effects, direct imaging under the electron microscope can be applied for very thin specimens only [19]. To overcome this restriction, conventional replicas [20] were prepared from the film surface by consecutive vacuum deposition of Pt and C. It should be noted that, during the evaporation of carbon, photoand thermally initiated phenomena could occur in the AszS 3 [21]. For this reason comparative Formvar replicas were also prepared from a 0.5% CHC13 solution under safe light conditions. The metal-carbon and Formvar replicas were removed by means of 20 g/1 aqueous solution of N a O H at room temperature and then washed and fixed on electron microscope grids. In some experiments replicas of films partially dissolved in 0.1 g/1 N a O H at 20°C were also inspected under the electron microscope. The imaging of As2S 3 films and their replicas was performed under a transmission electron microscope JEOL, JEM 100 B at accelerating voltages 100 and 40 kV, respectively. Microfractographic technique [22,23] was applied for studying the growth morphology of aAszS 3 films. This technique requires cutting notches in the back side of the glass substrate before evaporation. Precautions were taken to avoid disturbance of the profiles during the fracture procedure. Further, 10 nm thick films of carbon and gold were consecutively sputtered on the studied surfaces. A scanning electron microscope JEOL, JSM T200 was used for inspection of the growth profiles.
3. Results
3.1. Choice of electron microscope preparation technique Figure 1 shows transmission electron micrographs of a direct image (a), P t / C (b) and Form-
224
N. Starbov et al. / Vacuum deposited a-As2S 3 thin films
var (c) replicas of 10 nm thick A s 2 S 3 film deposited on a stationary virgin glass substrate. As seen in fig. l(a), the As2S 3 film is built up of individual grains separated by well defined intergrain boundaries. A similar image contrast has been also observed in other thin films of amorphous elemental and compound semiconductors including a-As2Se 3 [23], belonging with As2S3, to the same family of glasses. Simultaneously, the halo rings of the electron diffraction pattern in fig. l(a) is evidence for the amorphous structure of the films. Besides, a considerable loss of image sharpness was observed for film thicknesses above 30 nm due to the small depth of focus of the electron optical system as well as the enhanced electron absorption and scattering abilities of thicker samples [19]. In our opinion, the same factors are responsible for the lack of image
contrast of 50 nm thick samples in earlier experiments [24]. Figure l(d) shows a transmission electron micrograph of a 50 nm thick a-As2S 3 film obtained in the present work. The non-homogeneous film structure is clearly seen regardless of the low image contrast. O n the other hand, the similarity between the electron micrographs of the P t / C replica (fig. l(b)) and the direct image of the film (fig. l(a)) is obvious. Simultaneously, on the micrograph of the Formvar replica presented in fig. l(c) one can also resolve, although with low contrast, the grain-like morphology shown in figs. l(a) and (b). On this basis, it could be concluded that the photo- and thermoinduced phenomena occurring during the preparation of P t / C replicas do not influence substantially the surface structure of the samples studied. Therefore, the results ob-
Fig. 1. Transmission electron micrographs of 10 nm (a)-(c) and 50 nm (d) thick As2S 3 films obtained on stationary virgin precleaned glass substrates at deposition rate 0.1 nm/s: direct TEM images (a) and (d); Pt/C (b) and Formvar (c) replicas.
N. Starbov et al. / Vacuum deposited a-As2S3 thin films tained by comparing various sample p r e p a r a t i o n techniques justify the use of the conventional P t / C replicas [20] for studying the surface morphology of v a c u u m deposited a-As2S 3 thin films with differing thicknesses.
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3.2. Influence o f the a-AseS 3 film thickness on the surface microstructure Figure 2 shows a series of electron micrographs of the P t / C replicas of a-As2S 3 films with
Fig. 2. Transmission electron micrographs of Pt/C replicas of a-As2S3 films obtained on stationary, coated with polymer glass substrates at deposition rate 0.1 nm/s. Film thickness: 5 (a), 10 (b), 25 (c), 50 (d), 100 (e) and 1000 nm (f).
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thicknesses ranging from 5 to 1000 nm. The films were deposited on stationary glass substrates with an adhesive polymer sublayer at a rate of 0.1 n m / s . It is clearly seen that the film surface is characterized by a granular structure which strongly depends on the quantity of deposited As2S 3. Obviously the greater the film thickness, the larger the m e a n grain size. Probably a coalescence of neighboring grains proceeds during the film growth. As a result a transformation of the intergrain boundaries from high (figs. 2(a)-(c)) to low (figs. 2(d)-(f)) contrast occurs with increasing the film thickness. In addition, the micrographs (figs. 2(d)-(f)) show that substructural units are resolved in the individual grains. In the thinnest films, these subgrains are observed at higher magnifications a s seen in figs. l(a) and (b) for a 10 nm thick As2S 3 p r e p a r e d on a virgin glass substrate. It was established that a-As2S 3 films deposited on planetary rotating substrates also have a distinct grain surface morphology. In this case, similar to the condensation of a-As2S 3 on stationary substrates, the m e a n grain size also shows a tendency to increase with increasing film thickness. Further, it was found that the films obtained on planetary rotating substrates were slightly more fine grained, compared with those p r e p a r e d on stationary substrates. Analogous relationship have been established for thermally evaporated thin crystalline films [25]. The insufficient reproducibility of subgrain imaging in a-As2S 3 films
p r e p a r e d on rotating substrates should be noted. As seen on the micrograph in fig. 3(a), however, the subunits in the separate grains can be decorated by a short-time treatment in a strongly diluted solution of N a O H . The difference between the surface morphology of 100 nm thick as-deposited, control (fig. 3(b)) and etched (fig. 3(a)) samples is evident. Analogous morphological investigations were performed on thin films obtained by condensation of As2S 3 on glass substrates, either virgin or coated with a thin layer of vacuum deposited chromium, as well as on freshly cleaved mica. In these cases the formation of grain-like surface microstructure and its evolution with increasing film thickness was also found. It should be noted that similar structural peculiarities are typical for thin films of other amorphous semiconductor compounds. The results obtained [4] have shown that the m e a n grain size is sensitive not only to the film thickness but also to the deposition rate and the type of the substrate.
3.3. Influence o f the deposition rate and the substrate type on the surface morphology o f aA s 2 S 3 films
It was found that the surface morphology of a-As2S 3 films p r e p a r e d on stationary substrates was slightly influenced by deposition rates in the interval of 0.1-1 n m / s .
Fig. 3. Transmission electron micrographs of Pt/C replicas of 100 nm thick a-As2S 3 films obtained on planetary rotating, coated with polymer glass substrates at deposition rate 0.1 nm/s: treated 5 min in 0.1 g/l aqueous solution of NaOH at 20°C (a); as-deposited (b).
N. Starbov et al. / Vacuum deposited a-As2S 3 thin films
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Fig. 4. Transmission electron micrographs of Pt/C replicas of 100 nm thick a-As2S 3 films obtained on planetary rotating, coated with polymer glass substrates at deposition rates of 0.1 (a), 1 (b) and 10 nm/s (c). T h e results o b t a i n e d for p l a n e t a r y r o t a t i n g s u b s t r a t e s a r e p r e s e n t e d in fig. 4 for glass p l a t e s c o a t e d with a p o l y m e r film. T h e figure shows e l e c t r o n m i c r o g r a p h s of P t / C r e p l i c a s o f 100 n m thick films o f a - A s 2 S 3 o b t a i n e d at d e p o s i t i o n
r a t e s of 0.1 (a), 1 (b) a n d 10 n m / s (c). A d e c r e a s e o f t h e m e a n g r a i n size with t h e i n c r e a s e o f the k i n e t i c e n e r g y o f the v a p o r species is clearly seen. This t e n d e n c y is o b s e r v e d for all types of subs t r a t e s a n d film t h i c k n e s s e s studied. S i m u l t a -
Fig. 5. Transmission electron micrographs of Pt/C replicas of 10 nm thick a-As2S3 films obtained at deposition rate 0.1 nm/s on stationary glass substrates: virgin (a), coated with polymer (b), covered with thin Cr film (c); on freshly cleaved mica (d).
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neously, the observed electron images reveal some differences in the surface morphology of the films depending on the substrate used. Due to the higher grain boundary contrast, the influence of the type of substrate on the surface microstructure is more pronounced in the thinner films, both for stationary and planetary rotating substrates. This difference is clearly seen in fig. 5, comparing the replicas from the surface of 10 nm thick As2S 3 samples p r e p a r e d at a deposition rate of 0.1 n m / s on stationary glass substrates, virgin (a), coated with polymer (b) or thin Cr film (c) as well as on freshly cleaved mica surface (d). Obviously, either the grain shape or the m e a n grain size are strongly influenced by the surface used for the a-As2S 3 condensation. The morphological peculiarities of a-As2S 3 thin films manifested above are inherent to their free surface. However, the processes occurring on this surface are known to be closely related to the internal microstructure and therefore to the growth morphology of the films [27]. 3.4. Growth morphology o f the a-As2S 3 f i l m s
On the scanning electron micrograph of a fractured surface shown in fig. 6(a) one can easily distinguish the glass substrate (III), the polymer film (II) and 1000 nm thick vapor deposited As2S 3 film (I) obtained on a stationary substrate at a deposition rate of 0.1 n m / s and vapor incidence
angle 20 °. It is seen that the As2S 3 is built up of individual columns running through the entire film thickness. As could be expected, the free surface of the film is granular. The mean grain diameter as observed in scanning (fig. 6(a)) and transmission (fig. 2(e)) electron mode is practically the same. Analogously, the As2S 3 films obtained on planetary rotating substrates are also characterized by columnar microstructure. In both cases randomly distributed voids exist in the films and these voids are evidence for the presence of free volume. Besides, it was established that on pre-cleaned glass substrates as well as on Cr sublayer, As2S 3 grows also in a columnar manner. In addition, the growth profiles of a-As2S 3 films prepared within the interval of deposition rates between 0.1 and 1 n m / s show practically the same morphological features. In the present study, no observations of the internal structure of films p r e p a r e d on rotating substrates at deposition rates higher than 1 n m / s were performed. However, having in view that the deposition rate strongly influences the film surface morphology, as demonstrated in fig. 4, a decrease of the m e a n column diameter at higher deposition rates could be expected. It might be noted, however, that until now the experimentally observed column inclination toward the substrate at high vapor incidence angles is regarded as the most reliable evidence for the columnar growth of thin films of chalcogenide
Fig. 6. Scanning electron micrographs of growth profiles and top surface of a-As2S3 films (I) deposited on coated with polymer (II) stationary glass substrates (III) at vapor incidence angle 20° (a) and 80° (b). Tilt angle 60° (a) and 45° (b).
N. Starbov et aL / Vacuum deposited a-As2S s thin films
glasses [5-7]. Figure 6(b) presents a scanning electron micrograph of a growth profile and the top surface of 1000 nm thick As2S 3 sample prepared at a deposition rate of 0.1 n m / s and vapor incidence angle 80 °. Evidently inclined columns are formed as a result of the oblique deposition. Simultaneously, a column elongation coinciding with the direction of the vapor beam is clearly seen. The difference in the growth morphology of As2S 3 samples obtained at vapor incidence angle 20 ° (fig. 6(a)) and 80 ° (fig. 6(b)) is unambiguous.
4. Discussion
As seen on the micrographs in figs. 1-5, the presence of a grain-like surface microstructure of a-As2S 3 thin films is revealed under transmission electron microscope. It is established that the granularity is inherent to every stage of film formation in a wide range of condensation conditions. The series of electron micrographs presented in fig. 2 shows a thickness driven grain growth which could be reasonably explained accepting a limited mobility of the condensed vapor species over the already deposited As2S 3 [1]. Simultaneously, the subgrains displayed (figs. 2(d)-(f)) show a relation to the film thickness similarly to the large grains. It must be noted that a similar hierarchy has been observed in the growth morphology of other optical thin films [2]. On the other hand, the decoration of subgrains in the films deposited on planetary rotating substrates by a treatment in strongly diluted alkaline solutions (fig. 3(a)) indicates that certain mass redistribution occurs on the film surface after the evaporation process has been completed. Beneath the surface, however, the structure is 'frozen' and was revealed by the etching procedure. This suggestion was additionaly supported by the observed contrast enhancement of the subgrains in specimens prepared on stationary substrates after etching in dilute alkaline solutions. Besides, the mean grain size is strongly dependent on the deposition rate (fig. 5) indicating that the nucleation stage play an essential role for the thin film formation. The higher deposition rate,
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proportional to the nucleation rate [26], results in a smaller mean grain size. Therefore, similarly to other vapor-deposited amorphous compounds [4], the a-As2S 3 films studied follow some general trends of formation of the crystalline thin films [26]. As shown in fig. 5 the surface morphology of a-As2S 3 thin films is also determined by the substrate used for condensation. Obviously, all substrate surfaces studied are characterized by a specific distribution of active sites for nucleation and growth of As2S 3 from the vapor phase. Besides, the surface mobility of the condensed vapor spieces on the various substrates should also differ. The surface granularity observed determines an internal inhomogeneity of a-AszS 3 thin films in the form of column-like structure (fig. 6). Thus, similarly to the vacuum deposited amorphous As2Se 3 [5], GeSe 3 [6] and GeS 2 [7], the a-As2S 3 thin films also grow in a columnar manner at least under these experimental conditions. This type of growth means that, by contrast with the bulk samples of the same compound, these films are structurally inhomogeneous. Moreover, the condensation of a-As2S 3 at high vapor incidence angles is accompanied by a distinctly different thin film growth morphology characterized by columns inclined toward the subsrate. On this basis it is logical to suppose the existence of structure related properties of thin a-As2S 3 films prepared at various angles between the Substrate and the direction of the vapor beam. The investigations on this problem are in progress.
5. Conclusion
The surface inhomogeneity of vapor-deposited a-As2S 3 thin films is observed as grain-like mi-
crostructure following some general trends of thin film formation. The processes occurring on the film surface result in a columnar growth morphology. Both the surface and internal film structure are influenced by the conditions of evaporation and condensation. On this basis it might be concluded that the granularity and the columnar structure are inherent properties of a-As2S 3 thin
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films. Therefore, from a morphological point of view, vacuum-deposited a-As2S 3 is not an exception of the thin films of amorphous arsenic and germanium chalcogenide glasses as presently accepted. Moreover, the established morphological pecularities of the a-AsaS 3 films could be regarded as complementing and not contradicting the numerous data about their local structure.
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