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Domain microscopy in chalcogenide alloy glass thin films
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Solid State Communications, Vol. 38, pp. 657-661. Pergamon Press Ltd. 1981. Printed in Great Britain.
DOMAIN MICROSCOPY IN CHALCOGENIDE ALLOY GLASS THIN FILMS C~-I. Chen, J.C. Phillips, K.L. Tai Bell Laboratories, Murray Hill, NJ 07974, U.S.A. and P2d. Bridenbaugh Bell Laboratories, Holmdel, NJ 07733, U.S.A.
(Received 29 October 1980 by A.G. Chynoweth ) We report the observation of vow well resolved polygonal domain (not island) trough networks in thin glass films. The linear troughs are observed by electron microscopy to be as narrow as 30 A and the hexagonal domain diameters are as large as 1000 (+ 1130)A. The composition dependence of the average diameters of the polygonal structures correlates well with the glass-forming tendency and agrees very well with the predictions of a recent topological model. ONE OF THE BASIC problems in condensed matter physics is the structural character of inorganic noncrystalline solids. Vast experimental efforts have been expended on this problem since Zachariasen began his classic paper [I] ong-SiO2 with the canny disclaimer "It must be frankly admitted that we know practically nothing about the atomic arrangement in glasses". By vapor deposition on very cold substrates or by splat quenching of alloys near a eutectic composition many materials can be prepared in non-crystalline form, but only a few of these can be heated to a temperature T = Tg ~ 2T,,/3 where they undergo a glass transition without prior recrystallization. The same exceptional materials can usually be supercooled and f'mally quenched into a non-crystalline state from the melt: these are "true glasses", as distinguished from merely amorphous solids. The topological principles underlying the glassforming tendency in covalent materials have recently been analyzed by Phillips [2]. He has shown that for certain especially favorable materials (the chalcogerdde alloys formed by combinating A = Ge or As with B = S or Se [and, to a lesser degree, Te]) it is possible to describe the factors that maximize the glass-forming tendency using concepts derived from classical (Lagrangian) mechanics. These concepts include mechanical constraints and degrees of freedom. They predict the most favorable compositions x = 0.40 and y = 0.17 for the glass-forming tendency in As~(S, Se)l_ x and G%(S, Se),_y alloys in good agreement with experiment [3]. An immediate consequence of the topological variational analysis is that near the optimal composition
the glass-forming alloys can form almost strain-free noncrystalline covalent networks if only short-range valence force-field interactions are considered. The accumulation of strain energies from residual long-range (e.g. van der Waals) interactions between distant atoms will eventually, however, determine the dimensions of the connected network and in true glasses can produce domains [4]. In this Letter we report the direct observation by electron microscopy of networks of these self-limited domains in thin trims. Our studies show that domain formation induced by strain energy accumulation m a~( be the fundamental barrier to recrystallization in bulk samples in the absence of eutectic (immiscibility) effects [5]. When both effects are present the topological one may still dominate in the chalcogenide alloys [2], which is why we have chosen to study these materials. Films of As2SemTea_ m (m = 3, 2, 15, 1, 0) glasses 600 A thick were prepared in commercial thermal evaporators at room temperature with a slow deposition rate of 3 5 - 6 0 Amin -1 on Si wafer substrates coated with a thin layer of polymer. The f'grns were peeled off intact from the substrate by dissolving the polymer in acetone and the free-standing f'rims remained planar and did not curl up. They were then examined in a JEM 200 B electron microscope under very low beam conditions (-< 10 -a Acm-2). (This condition was achieved by defocussing the second condenser lens. ) The fatiguing [6] or heating effects with exposures of 30 see where shown to be negligible compared to exposure to 0.05 W era-2 of ultraviolet light for 20 sec by comparison with Ag photodoping kinetics in GeySel_ ~ films which will be discussed elsewhere. Thus 657
658
DOMAIN MICROSCOPY IN CHALCOGENIDE THIN FILMS
Vol. 38, No. 7
Fig. I. The domain wall network of a 600 A f-tim ofg-As2Se a. The bright lines are deep troughs which are nearly normal to the plane of the Film. the domain structures which were observed are believed to be intrinsic to the films and are not radiation-induced artifacts. In the micrographs ofg-AszSe 3 films shown in Fig. 1 with thickness 600 A the domain network patterns are very well resolved and contain unique features never before reported for non-crystalline thin films. The patterns are dominated by polygons with hexagonal diameters of 1000 +- 1O0 A (r.m.s.). Many of the polygonal edges appear to be deep troughs that are normal to the Film surface and that may penetrate the fdm almost completely. The remaining edges are probably less deep but are still almost linear. The linearity and width ( ~ 30--40 A) of these troughs provide direct microscopic confirmation of the proposed mechanism of strain self-limited domain growth [5]. In the cases [7] of As2Se2Te and As2Te3 the deep troughs were replaced by a less deep and broader network, as shown in Fig. 2. As the Te concentration
increases still further, tile trough network between polygons becomes shallower and the features appearing in the micrographs become more complex. In the electron micrograph of AsaTe3, we first noticed the formation of many large ( " 2000 A) island hillocks. (These protrusions were evidenced by stereo microscopy and the existence of Fresnel fringes near the edges of the islands.) Very deep troughs (which may even be voids) are often found as cracks along parts of the island boundaries. Within the islands, isolated microvoids can be seen. The general features of As2Te 3 consist uf domains ( ~ 100 A in size) (size rather uniform) separated by a trough network 100 A in width. Since the troughs are so wide, the polygonal features of domains in As2Te3 are much less distinct titan in As2Se3. The deterioration of the trough network with the addition of Te reflects the breakdown of bonding constraints for this element compared to Se, and is correlated with a reduction in the #ass-forming tendency [2].
Vol. 38, No. 7
DOMAIN MICROSCOPY IN CHALCOGENIDE THIN FILMS
659
Fig. 2. The trough network o f 600 A f'flms o f ( a ) As~Se2Te and (b) As2Te3. Note the hillock at upper left in (b) which X-ray fluorescence suggests consists of amorphous As. In Fig. 3 we show the domain structure of As2Se3 films which are 300 A thick. At this stage the domain growth is incomplete and the troughs are irregular, With widths varying from distances which are too small to resolve to ~ 100 A. The polygonal character of the void network is much reduced at this stage and the boundary tension is poorly defined. In 1200 A films, on the other hand, the troughs shown in Fig. 1 begin to fade, and the contrast is much reduced, although many comer voids remain very deep. The microscopic interpretation of our observations is that the thin films are continuous (as they must be in order to be removed intact from the substrate) but that the domains are separated by very thin segments of material under stress. The segments are thin both vertically and laterally because coalescence of the islands is energetically unfavorable. The nearly vertical sides of the polygonal edges or troughs are fully covalently reconstructed and these edges (or domain surfaces) are exceptionally stable, as evidenced by their vertically and laterally linear structure. We have, of course, examined the diffraction patterns of'our films and have found no evidence of microcrystallinity; our diffraction results confirm those obtained previously [7]. Structure very similar to that presented here for g-As,Sea films is observed on similarly prepared
g-As2Sa films. When the films are deposited on transparent carbon films (which have some curvature because they are stretched across a grid of fine Mo wires) domain structure with the same polygonal diameters and trough dimensions is observed on some parts of the film but is absent in others. We attribute the failure of previous workers [8] to observe microstructure in 500 A films of g-As2Sa on transparent carbon to the presence of one or more factors (especially differences in substrate flatness, deposition rate or possibly film thickness) which we cannot determine from the published data [8]. The GeySet_y glass films do not exhibit the polygonal structure found for g-As2(S or Se), films, although the micrographs do indicate columnar growth which explains the giant photocontraction effect observed [6] n e a r y = 0.25. In the AsxSel_ x system maximum network stability in terms of chemical bond energies occurs at x = 0.4 [congruent melting point in the liquidus Tz(x), and a similar maximum in the glass transition temperature Tg(x)i, and this composition also topologically optimizes [2] the network strain energy. For Gey Se 1_5, chemical stability is maximized [4] at y = 1/3 while the strain energy is minimized [2] at y = 1/6. Competition between chemical and topological factors in Gey Se l_y alloys can lead to partial phase separation and substantial Se enrichment of broad interracial
660
DOMAIN MICROSCOPY IN CHALCOGENIDE THIN FILMS
Vol. 38, No. 7
Fig. 3. The early stage of domain growth in a 300 A film ofg-As2Se 3. In this case the troughs are wtder and much more irregular and include anomalies such as dogbones (small islands in troughs). regions for y < 1/3. The constraints associated with a large excess of two-fold coordinated Se atoms are insufficient to def'me polygonal domain walls because the linear disproportionation tension is small compared to the interfaciat strain energy. In this Letter we have reported the first observations of linear troughs which are very narrow compared to the domain size and film thickness (30 A compared to 1000 A). This very small ratio differentiates the void networks discussed here from those observed in a-Si, for example [9, 10]. The existence of glassy domain networks has far-reaching implications for the nature of the glass transition, which we will discuss elsewhere. Preliminary studies of Aso.~eSeo.6-~with let ~ 0.01 have shown that the width and linearity of the troughs degrade rapidly and depend on the sign of e only weakly even in this very small interval. This dependence on composition confirms the topological calculation [2] of the most favorable composition x = 0.4 and shows that vapor-phase molecules (such as
As4Se4, x = 0.5) cannot play a significant role in forming the polygonal trough pattern. Acknowledgement - We are grateful to Dale Jacobson for independently calibrating our mechanical measurements of film thickness by ion backscattering.
REFERENCES 1. 2. 3. 4. 5. 6.
W.H. Zachariasen, J. Am. Chem. Soc. 54, 3841 (1932). J.C. Phillips, J. Non.Cryst. Solids 34, 153 (1979). M.B. Myers & E.J. Felty, Mat. Re~ Bull. 2, 535 (1967); R. Azoulay, H. Thibierge & A. Brenac, J. Non-Cryst. Solids 18, 33 (1975). J.C. Phillips, Phys. Rev. Lett. 42, 1151 (1979); Phy~ Rev. Lett. 45,905 (1980). D.R. Uhlmann, Treatise Solid State Chem. (Edited by N.B. Hannay), p. 293. Plenum, New York (1976). T.N. Clayton & R.J. Stadek, Phyg Rev. Lett. 42, 1482 (1979); B. Singh, S. Rajagopalar, P.K. Bhat,
Vol. 38, No. 7
7. 8. 9.
DOMAIN MICROSCOPY IN CHALCOGENIDE THIN FILMS
P.K. Raadya & K.L. Chapra, J. Non-Cryst. Solids 35-6, 1053 (1980). J. Chang & D.B. Dove, J. Non-Cryst. Solids 16, 72 (1974). J.P, De NeufviUe, S.C. Moss & S.R. Ovshinsky, or. Non-Cryst. Solids 13, 191 (1973). J.J. Hauser, Phys. Rev. 8B, 3817 (1973);J.C.
10.
661
Knights, G. Lucovsk-y & ILL Nemanich, J. Non-Cryst. Solids 32, 393 (1979). F.L. Galeener, Phys. Rev. Lett. 27,421 (1971); T.M. Donovan & K. Heinemann, Phyx Rev. Lett. 27, 1794 (1971); G.S. Carglll, Phyx Rev. Lett. 28, 1372 (1972).
Solid State Communications, Vol. 38, pp. 657-661. Pergamon Press Ltd. 1981. Printed in Great Britain.
DOMAIN MICROSCOPY IN CHALCOGENIDE ALLOY GLASS THIN FILMS C~-I. Chen, J.C. Phillips, K.L. Tai Bell Laboratories, Murray Hill, NJ 07974, U.S.A. and P2d. Bridenbaugh Bell Laboratories, Holmdel, NJ 07733, U.S.A.
(Received 29 October 1980 by A.G. Chynoweth ) We report the observation of vow well resolved polygonal domain (not island) trough networks in thin glass films. The linear troughs are observed by electron microscopy to be as narrow as 30 A and the hexagonal domain diameters are as large as 1000 (+ 1130)A. The composition dependence of the average diameters of the polygonal structures correlates well with the glass-forming tendency and agrees very well with the predictions of a recent topological model. ONE OF THE BASIC problems in condensed matter physics is the structural character of inorganic noncrystalline solids. Vast experimental efforts have been expended on this problem since Zachariasen began his classic paper [I] ong-SiO2 with the canny disclaimer "It must be frankly admitted that we know practically nothing about the atomic arrangement in glasses". By vapor deposition on very cold substrates or by splat quenching of alloys near a eutectic composition many materials can be prepared in non-crystalline form, but only a few of these can be heated to a temperature T = Tg ~ 2T,,/3 where they undergo a glass transition without prior recrystallization. The same exceptional materials can usually be supercooled and f'mally quenched into a non-crystalline state from the melt: these are "true glasses", as distinguished from merely amorphous solids. The topological principles underlying the glassforming tendency in covalent materials have recently been analyzed by Phillips [2]. He has shown that for certain especially favorable materials (the chalcogerdde alloys formed by combinating A = Ge or As with B = S or Se [and, to a lesser degree, Te]) it is possible to describe the factors that maximize the glass-forming tendency using concepts derived from classical (Lagrangian) mechanics. These concepts include mechanical constraints and degrees of freedom. They predict the most favorable compositions x = 0.40 and y = 0.17 for the glass-forming tendency in As~(S, Se)l_ x and G%(S, Se),_y alloys in good agreement with experiment [3]. An immediate consequence of the topological variational analysis is that near the optimal composition
the glass-forming alloys can form almost strain-free noncrystalline covalent networks if only short-range valence force-field interactions are considered. The accumulation of strain energies from residual long-range (e.g. van der Waals) interactions between distant atoms will eventually, however, determine the dimensions of the connected network and in true glasses can produce domains [4]. In this Letter we report the direct observation by electron microscopy of networks of these self-limited domains in thin trims. Our studies show that domain formation induced by strain energy accumulation m a~( be the fundamental barrier to recrystallization in bulk samples in the absence of eutectic (immiscibility) effects [5]. When both effects are present the topological one may still dominate in the chalcogenide alloys [2], which is why we have chosen to study these materials. Films of As2SemTea_ m (m = 3, 2, 15, 1, 0) glasses 600 A thick were prepared in commercial thermal evaporators at room temperature with a slow deposition rate of 3 5 - 6 0 Amin -1 on Si wafer substrates coated with a thin layer of polymer. The f'grns were peeled off intact from the substrate by dissolving the polymer in acetone and the free-standing f'rims remained planar and did not curl up. They were then examined in a JEM 200 B electron microscope under very low beam conditions (-< 10 -a Acm-2). (This condition was achieved by defocussing the second condenser lens. ) The fatiguing [6] or heating effects with exposures of 30 see where shown to be negligible compared to exposure to 0.05 W era-2 of ultraviolet light for 20 sec by comparison with Ag photodoping kinetics in GeySel_ ~ films which will be discussed elsewhere. Thus 657
658
DOMAIN MICROSCOPY IN CHALCOGENIDE THIN FILMS
Vol. 38, No. 7
Fig. I. The domain wall network of a 600 A f-tim ofg-As2Se a. The bright lines are deep troughs which are nearly normal to the plane of the Film. the domain structures which were observed are believed to be intrinsic to the films and are not radiation-induced artifacts. In the micrographs ofg-AszSe 3 films shown in Fig. 1 with thickness 600 A the domain network patterns are very well resolved and contain unique features never before reported for non-crystalline thin films. The patterns are dominated by polygons with hexagonal diameters of 1000 +- 1O0 A (r.m.s.). Many of the polygonal edges appear to be deep troughs that are normal to the Film surface and that may penetrate the fdm almost completely. The remaining edges are probably less deep but are still almost linear. The linearity and width ( ~ 30--40 A) of these troughs provide direct microscopic confirmation of the proposed mechanism of strain self-limited domain growth [5]. In the cases [7] of As2Se2Te and As2Te3 the deep troughs were replaced by a less deep and broader network, as shown in Fig. 2. As the Te concentration
increases still further, tile trough network between polygons becomes shallower and the features appearing in the micrographs become more complex. In the electron micrograph of AsaTe3, we first noticed the formation of many large ( " 2000 A) island hillocks. (These protrusions were evidenced by stereo microscopy and the existence of Fresnel fringes near the edges of the islands.) Very deep troughs (which may even be voids) are often found as cracks along parts of the island boundaries. Within the islands, isolated microvoids can be seen. The general features of As2Te 3 consist uf domains ( ~ 100 A in size) (size rather uniform) separated by a trough network 100 A in width. Since the troughs are so wide, the polygonal features of domains in As2Te3 are much less distinct titan in As2Se3. The deterioration of the trough network with the addition of Te reflects the breakdown of bonding constraints for this element compared to Se, and is correlated with a reduction in the #ass-forming tendency [2].
Vol. 38, No. 7
DOMAIN MICROSCOPY IN CHALCOGENIDE THIN FILMS
659
Fig. 2. The trough network o f 600 A f'flms o f ( a ) As~Se2Te and (b) As2Te3. Note the hillock at upper left in (b) which X-ray fluorescence suggests consists of amorphous As. In Fig. 3 we show the domain structure of As2Se3 films which are 300 A thick. At this stage the domain growth is incomplete and the troughs are irregular, With widths varying from distances which are too small to resolve to ~ 100 A. The polygonal character of the void network is much reduced at this stage and the boundary tension is poorly defined. In 1200 A films, on the other hand, the troughs shown in Fig. 1 begin to fade, and the contrast is much reduced, although many comer voids remain very deep. The microscopic interpretation of our observations is that the thin films are continuous (as they must be in order to be removed intact from the substrate) but that the domains are separated by very thin segments of material under stress. The segments are thin both vertically and laterally because coalescence of the islands is energetically unfavorable. The nearly vertical sides of the polygonal edges or troughs are fully covalently reconstructed and these edges (or domain surfaces) are exceptionally stable, as evidenced by their vertically and laterally linear structure. We have, of course, examined the diffraction patterns of'our films and have found no evidence of microcrystallinity; our diffraction results confirm those obtained previously [7]. Structure very similar to that presented here for g-As,Sea films is observed on similarly prepared
g-As2Sa films. When the films are deposited on transparent carbon films (which have some curvature because they are stretched across a grid of fine Mo wires) domain structure with the same polygonal diameters and trough dimensions is observed on some parts of the film but is absent in others. We attribute the failure of previous workers [8] to observe microstructure in 500 A films of g-As2Sa on transparent carbon to the presence of one or more factors (especially differences in substrate flatness, deposition rate or possibly film thickness) which we cannot determine from the published data [8]. The GeySet_y glass films do not exhibit the polygonal structure found for g-As2(S or Se), films, although the micrographs do indicate columnar growth which explains the giant photocontraction effect observed [6] n e a r y = 0.25. In the AsxSel_ x system maximum network stability in terms of chemical bond energies occurs at x = 0.4 [congruent melting point in the liquidus Tz(x), and a similar maximum in the glass transition temperature Tg(x)i, and this composition also topologically optimizes [2] the network strain energy. For Gey Se 1_5, chemical stability is maximized [4] at y = 1/3 while the strain energy is minimized [2] at y = 1/6. Competition between chemical and topological factors in Gey Se l_y alloys can lead to partial phase separation and substantial Se enrichment of broad interracial
660
DOMAIN MICROSCOPY IN CHALCOGENIDE THIN FILMS
Vol. 38, No. 7
Fig. 3. The early stage of domain growth in a 300 A film ofg-As2Se 3. In this case the troughs are wtder and much more irregular and include anomalies such as dogbones (small islands in troughs). regions for y < 1/3. The constraints associated with a large excess of two-fold coordinated Se atoms are insufficient to def'me polygonal domain walls because the linear disproportionation tension is small compared to the interfaciat strain energy. In this Letter we have reported the first observations of linear troughs which are very narrow compared to the domain size and film thickness (30 A compared to 1000 A). This very small ratio differentiates the void networks discussed here from those observed in a-Si, for example [9, 10]. The existence of glassy domain networks has far-reaching implications for the nature of the glass transition, which we will discuss elsewhere. Preliminary studies of Aso.~eSeo.6-~with let ~ 0.01 have shown that the width and linearity of the troughs degrade rapidly and depend on the sign of e only weakly even in this very small interval. This dependence on composition confirms the topological calculation [2] of the most favorable composition x = 0.4 and shows that vapor-phase molecules (such as
As4Se4, x = 0.5) cannot play a significant role in forming the polygonal trough pattern. Acknowledgement - We are grateful to Dale Jacobson for independently calibrating our mechanical measurements of film thickness by ion backscattering.
REFERENCES 1. 2. 3. 4. 5. 6.
W.H. Zachariasen, J. Am. Chem. Soc. 54, 3841 (1932). J.C. Phillips, J. Non.Cryst. Solids 34, 153 (1979). M.B. Myers & E.J. Felty, Mat. Re~ Bull. 2, 535 (1967); R. Azoulay, H. Thibierge & A. Brenac, J. Non-Cryst. Solids 18, 33 (1975). J.C. Phillips, Phys. Rev. Lett. 42, 1151 (1979); Phy~ Rev. Lett. 45,905 (1980). D.R. Uhlmann, Treatise Solid State Chem. (Edited by N.B. Hannay), p. 293. Plenum, New York (1976). T.N. Clayton & R.J. Stadek, Phyg Rev. Lett. 42, 1482 (1979); B. Singh, S. Rajagopalar, P.K. Bhat,
Vol. 38, No. 7
7. 8. 9.
DOMAIN MICROSCOPY IN CHALCOGENIDE THIN FILMS
P.K. Raadya & K.L. Chapra, J. Non-Cryst. Solids 35-6, 1053 (1980). J. Chang & D.B. Dove, J. Non-Cryst. Solids 16, 72 (1974). J.P, De NeufviUe, S.C. Moss & S.R. Ovshinsky, or. Non-Cryst. Solids 13, 191 (1973). J.J. Hauser, Phys. Rev. 8B, 3817 (1973);J.C.
10.
661
Knights, G. Lucovsk-y & ILL Nemanich, J. Non-Cryst. Solids 32, 393 (1979). F.L. Galeener, Phys. Rev. Lett. 27,421 (1971); T.M. Donovan & K. Heinemann, Phyx Rev. Lett. 27, 1794 (1971); G.S. Carglll, Phyx Rev. Lett. 28, 1372 (1972).