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Electron microscopic observations of thermally induced transoformations in amorphous chalcogenide thin films
JOURNAL OF NON-CRYSTALLINESOLIDS2 (1970) 161-169 © North-Holland Publishing Co., Amsterdam
E L E C T R O N M I C R O S C O P I C OBSERVATIONS OF T H E R M A L L Y INDUCED T R A N S F O R M A T I O N S IN A M O R P H O U S C H A L C O G E N I D E T H I N FILMS B. G. BAGLEY and W. R. NORTHOVER Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey 07974, U.S.A.
Beam heating in the electron microscope was used to observe directly the thermally induced transformations in amorphous thin films of As2Se3-3As2Te3, As2Se3.2As~Te3, 2 As2Se3.As2Te3,and Asa0Te4sSilzGel0.A solid-fluid transition (softening) was observed in all four compositions. In one composition, 2 AszSea.As2Te3,neither phase separation nor crystallization was observed. Crystallization was observed in the other three compositions. Phase separation preceding this crystallization was not detected. I. Introduction Transformation studies have been made on bulk chalcogenide glasses of the following compositions: 2AszSea.AszTe3, As2Se3.2As2Te3, As2Se3" 3 As2Te3, and As3oTegsSi12Geao (ref. 1). These glassy semiconductors are of interest because of their unusual electrical properties (switching) x-3). We have concurrently investigated, using electron microscopy, the thermally induced transformations in vapor deposited thin films prepared from the same glasses. The object of this study was to determine the structure of the as-deposited thin films, and to observe directly the transformations which could be thermally induced by electron beam heating of the sample. The advantages of electron beam heating are that: (1) the transformation can be observed directly in the microscope as it proceeds (and interrupted at will) and (2) the sample, because it is only locally heated, maintains its shape and does not alloy with grids or substrates. The disadvantage is that sample temperatures are not known. The calorimetric studies 1), however, accurately determine transformation temperatures. The three thermally induced transformations of interest are: (1) the solidfluid transition, (2) crystallization, and (3) phase separation (amorphous phase immiscibility). The first two of these are easily detected in the electron microscope. The solid-fluid transition is identified by the observation of flow in the sample, and crystallization is revealed by the observation of crystalline reflections in the diffraction pattern and crystals in the micro161
162
B . G . BAGLEY A N D W . R . N O R T H O V E R
structure. The detection of phase separation can be more difficult, however. Phase separation may produce only subtle changes in the difffraction pattern, and changes in the microstructure will only be evident if the separating phases differ in electron density. The compositions studied here contain both heavy and light elements and it is assumed that phase separation, if it occurs, will lead to differences in electron density. Also, it is difficult to detect phase separation if the thickness of the film is more than a few times the diameter of the separated phase particle. Thus, heat treatments which cause a coarsening of the second phase aid in its identification. Calorimetric studies, such as those done on the bulk glasses1), are complementary to electron microscopy for the detection of phase separation; for phase separation results in energy as well as structural changes.
2. Experimental Thin films were formed by the flash evaporation of 5 mg of glass (appropriate composition) from a refractory metal heater in a vacuum of 10 -5 Torr. The glasses used were the same as those used in the concurrent investigations1), and their preparation is described there. The vapor was deposited on a fused silica substrate at room temperature. The resulting 200-400 A thick films were stripped from the substrate in water and examined in a Siemens electron microscope operating at 100 kV.
3. Results and discussion All four compositions as-deposited were: (1) amorphous, (2) showed no large chemical or structural inhomogeneities in the microstructure, and (3) deposited with a globular morphology on a scale of about 100 A. This globular structure masks any inhomogeneity on a scale finer than 100 A. Figs. 1 and 2 are typical of the microstructures and diffraction patterns obtained for the as-deposited thin films. When beam heated, all four compositions exhibited a solid-fluid transition; that is, the material was observed to soften and flow and the as-deposited globular morphology disappeared. Upon increasing the film temperature the compositions As 2Se 3 •2 As2We3, As2Se 3 • 3 As2Te 3, and As3oTe4sSil 2Gel o were observed to crystallize. A phase separation preceding or concurrent with the crystallization was not detected. Figs. 3, 4, 5 and 6 are the electron micrographs and diffraction patterns for partially crystallized samples of AsaoTegaSi 12Gelo and As2Se 3 • 3 As2Te 3. The micrograph and diffraction pattern for the composition As2Se 3 •2 As2Te 3 were similar to figs. 5 and 6 and are not shown. In the As-Se-Te system, the
ELECTRON MICROSCOPIC OBSERVATIONS
Fig. 1. Transmission electron micrograph of vapor deposited AsaoTeasSilzGel0.
163
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Fig. 2.
B . G . BAGLEY AND W . R. NORTFIOVER
Electron diffraction pattern o f the same area of As3oTe48Si12Gelo thin film as shown in fig. 1.
ELECTRON MICROSCOPICOBSERVATIONS
Fig. 3.
165
Transmission electron micrograph of vapor deposited Asa0Te4sSilzGelo beam heated in the electron microscope.
166
Fig. 4.
B.G.BAGLEY AND W.R.NORTHOVER
Electron diffraction pattern of the same area of As3oTe4sSil~Gelo thin film as shown in fig. 3.
ELECTRON MICROSCOPICOBSERVATIONS
Fig. 5.
167
Transmission electron micrograph of vapor deposited As~Se3.3 As~Te3 beam heated in the electron microscope.
168
Fig. 6.
B . G . BAGLEY AND W. R. NORTHOVER
Electron diffraction pattern o f the same area o f AszSez.3 AszTez thin film as shown in fig. 5.
ELECTRON MICROSCOPIC OBSERVATIONS
169
crystalline phase is a single phase solid solution which differs only slightly in density from the amorphous phase4). One composition, 2AszSe3-AszTe3, could be heated either slowly ( ~ 15 °C/min) or quickly (,,~ 100°C/rain), from room temperature up to a temperature at which the sample vaporized, without evidence of crystallization or phase separation. The inclusions observed in the as-prepared bulk glass 1) were not found in the vapor deposited thin films.
Acknowledgments We are indebted to A. D. Pearson for extensive discussions and Miss S. E. Koonce for electron microscopy.
References 1) B. G. Bagley and H. E. Bair, J. Non-Crystalline Solids 2 (1970) 155. 2) J. F. Dewald, A. D. Pearson, W. R. Northover and W. F. Peck, Jr., Electrochem. Soc. Meeting, Los Angeles, May 1962. 3) S. R. Ovshinsky, Phys. Rev. Letters 21 (1968) 1450. 4) T. N. Vengel and B. T. Kolomiets, Soviet Phys.-Tech. Phys. 2 (1957) 2314.
E L E C T R O N M I C R O S C O P I C OBSERVATIONS OF T H E R M A L L Y INDUCED T R A N S F O R M A T I O N S IN A M O R P H O U S C H A L C O G E N I D E T H I N FILMS B. G. BAGLEY and W. R. NORTHOVER Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey 07974, U.S.A.
Beam heating in the electron microscope was used to observe directly the thermally induced transformations in amorphous thin films of As2Se3-3As2Te3, As2Se3.2As~Te3, 2 As2Se3.As2Te3,and Asa0Te4sSilzGel0.A solid-fluid transition (softening) was observed in all four compositions. In one composition, 2 AszSea.As2Te3,neither phase separation nor crystallization was observed. Crystallization was observed in the other three compositions. Phase separation preceding this crystallization was not detected. I. Introduction Transformation studies have been made on bulk chalcogenide glasses of the following compositions: 2AszSea.AszTe3, As2Se3.2As2Te3, As2Se3" 3 As2Te3, and As3oTegsSi12Geao (ref. 1). These glassy semiconductors are of interest because of their unusual electrical properties (switching) x-3). We have concurrently investigated, using electron microscopy, the thermally induced transformations in vapor deposited thin films prepared from the same glasses. The object of this study was to determine the structure of the as-deposited thin films, and to observe directly the transformations which could be thermally induced by electron beam heating of the sample. The advantages of electron beam heating are that: (1) the transformation can be observed directly in the microscope as it proceeds (and interrupted at will) and (2) the sample, because it is only locally heated, maintains its shape and does not alloy with grids or substrates. The disadvantage is that sample temperatures are not known. The calorimetric studies 1), however, accurately determine transformation temperatures. The three thermally induced transformations of interest are: (1) the solidfluid transition, (2) crystallization, and (3) phase separation (amorphous phase immiscibility). The first two of these are easily detected in the electron microscope. The solid-fluid transition is identified by the observation of flow in the sample, and crystallization is revealed by the observation of crystalline reflections in the diffraction pattern and crystals in the micro161
162
B . G . BAGLEY A N D W . R . N O R T H O V E R
structure. The detection of phase separation can be more difficult, however. Phase separation may produce only subtle changes in the difffraction pattern, and changes in the microstructure will only be evident if the separating phases differ in electron density. The compositions studied here contain both heavy and light elements and it is assumed that phase separation, if it occurs, will lead to differences in electron density. Also, it is difficult to detect phase separation if the thickness of the film is more than a few times the diameter of the separated phase particle. Thus, heat treatments which cause a coarsening of the second phase aid in its identification. Calorimetric studies, such as those done on the bulk glasses1), are complementary to electron microscopy for the detection of phase separation; for phase separation results in energy as well as structural changes.
2. Experimental Thin films were formed by the flash evaporation of 5 mg of glass (appropriate composition) from a refractory metal heater in a vacuum of 10 -5 Torr. The glasses used were the same as those used in the concurrent investigations1), and their preparation is described there. The vapor was deposited on a fused silica substrate at room temperature. The resulting 200-400 A thick films were stripped from the substrate in water and examined in a Siemens electron microscope operating at 100 kV.
3. Results and discussion All four compositions as-deposited were: (1) amorphous, (2) showed no large chemical or structural inhomogeneities in the microstructure, and (3) deposited with a globular morphology on a scale of about 100 A. This globular structure masks any inhomogeneity on a scale finer than 100 A. Figs. 1 and 2 are typical of the microstructures and diffraction patterns obtained for the as-deposited thin films. When beam heated, all four compositions exhibited a solid-fluid transition; that is, the material was observed to soften and flow and the as-deposited globular morphology disappeared. Upon increasing the film temperature the compositions As 2Se 3 •2 As2We3, As2Se 3 • 3 As2Te 3, and As3oTe4sSil 2Gel o were observed to crystallize. A phase separation preceding or concurrent with the crystallization was not detected. Figs. 3, 4, 5 and 6 are the electron micrographs and diffraction patterns for partially crystallized samples of AsaoTegaSi 12Gelo and As2Se 3 • 3 As2Te 3. The micrograph and diffraction pattern for the composition As2Se 3 •2 As2Te 3 were similar to figs. 5 and 6 and are not shown. In the As-Se-Te system, the
ELECTRON MICROSCOPIC OBSERVATIONS
Fig. 1. Transmission electron micrograph of vapor deposited AsaoTeasSilzGel0.
163
164
Fig. 2.
B . G . BAGLEY AND W . R. NORTFIOVER
Electron diffraction pattern o f the same area of As3oTe48Si12Gelo thin film as shown in fig. 1.
ELECTRON MICROSCOPICOBSERVATIONS
Fig. 3.
165
Transmission electron micrograph of vapor deposited Asa0Te4sSilzGelo beam heated in the electron microscope.
166
Fig. 4.
B.G.BAGLEY AND W.R.NORTHOVER
Electron diffraction pattern of the same area of As3oTe4sSil~Gelo thin film as shown in fig. 3.
ELECTRON MICROSCOPICOBSERVATIONS
Fig. 5.
167
Transmission electron micrograph of vapor deposited As~Se3.3 As~Te3 beam heated in the electron microscope.
168
Fig. 6.
B . G . BAGLEY AND W. R. NORTHOVER
Electron diffraction pattern o f the same area o f AszSez.3 AszTez thin film as shown in fig. 5.
ELECTRON MICROSCOPIC OBSERVATIONS
169
crystalline phase is a single phase solid solution which differs only slightly in density from the amorphous phase4). One composition, 2AszSe3-AszTe3, could be heated either slowly ( ~ 15 °C/min) or quickly (,,~ 100°C/rain), from room temperature up to a temperature at which the sample vaporized, without evidence of crystallization or phase separation. The inclusions observed in the as-prepared bulk glass 1) were not found in the vapor deposited thin films.
Acknowledgments We are indebted to A. D. Pearson for extensive discussions and Miss S. E. Koonce for electron microscopy.
References 1) B. G. Bagley and H. E. Bair, J. Non-Crystalline Solids 2 (1970) 155. 2) J. F. Dewald, A. D. Pearson, W. R. Northover and W. F. Peck, Jr., Electrochem. Soc. Meeting, Los Angeles, May 1962. 3) S. R. Ovshinsky, Phys. Rev. Letters 21 (1968) 1450. 4) T. N. Vengel and B. T. Kolomiets, Soviet Phys.-Tech. Phys. 2 (1957) 2314.