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Structural studies of amorphous semiconductors
JOURNALOF NON-CRYSTALLINESOLIDS 2 (1970) 347--357 © North-Holland Publishing Co., Amsterdam
STRUCTURAL
STUDIES OF AMORPHOUS
SEMICONDUCTORS*
A. BIENENSTOCK and F. BETTS Department of Materials Science, Stanford University, Stanford, California 94305, U.S.A. and S. R. OVSHINSKY Energy Conversion Devices, Inc., Troy, Michigan 48084, U.S.A.
X-ray diffraction radial distribution studies of amorphous samples of Ge~Tel x, with x = 0.11, 0.66 and 0.72, have been performed. These are characterized by first neighbor peaks at 2.7/~ and second neighbor peaks at 4.2 A.. The absence of a peak at the crystalline GeTe first neighbor position of 3 A is shown to imply that the local coordinations in the amorphous materials are different from that in the crystalline. In addition, thermally and flash lamp transformed Ovonic memory materials have been shown to contain crystalline Te in their conducting states, while the high resistance, as-prepared, states are amorphous. In some cases, crystalline GeTe is also present in the conducting state. The crystalline Te created by thermal transformation of the material can be revitrified through application of short duration, sharp cutoff light pulses. The flash lamp transformed material appears to contain larger, relatively well oriented crystallites which were not transformed by the light pulses.
1. Introduction This p a p e r describes structural studies o f the reversible m e m o r y type electrical t r a n s i t i o n 1) in a m o r p h o u s films o f G e - T e 2) based alloys. The films, as e v a p o r a t e d , are high resistance intrinsic-like semiconductors. A f t e r subj e c t i o n to a sufficiently high electrical field to b r i n g a b o u t a t r a n s i t i o n to a low resistance state, a n d a f u r t h e r delay d u r i n g which there is c u r r e n t flow, a p o r t i o n o f the m a t e r i a l is t r a n s f o r m e d to a s e m i p e r m a n e n t low resistance state. T h a t is, the m a t e r i a l r e m a i n s in a low resistance state w i t h o u t further a p p l i c a t i o n o f an electric field. T h e m a t e r i a l m a y be switched b a c k to a high resistance state t h r o u g h a p p l i c a t i o n o f a c u r r e n t pulse with a r a p i d t u r n off. This p a p e r r e p o r t s initial results o f studies designed to elucidate structural aspects o f the p h a s e t r a n s i t i o n in these materials.
* Work supported in part by the Advanced Research Projects Agency through the Center for Materials Research, Stanford University, by the Department of Materials Science, Stanford University, and by Energy Conversion Devices, Inc. 347
348
A. BIENENSTOCK, F. BETTS AND S. R. OVSH1NSKY
Initial efforts were based on the hypothesis that the memory effect involved the vitrification and devitrification of GeTe. A possible mechanism for the transformation was suggested by the structural model proposed by Hilton et al.3). On the basis of infrared and radial distribution studies, they deduced a chain structure in which coordination is two-fold, the interatomic distance is 2.57 A and the bond angle is approximately 110°. This should be contrasted with the only slightly distorted rocksalt structure of crystalline GeTe, where the coordination is six-fold, the interatomic separation is approximately 3 A and the bond angle is about 90° . This difference in local coordinations suggests that an appreciable portion of the difference in conductivities is due to a change in bonding with a corresponding change in the effective band gap. Unfortunately, the chain model of Hilton et al. a) can only be viewed as a conjecture. The infrared studies, in themselves, do not yield an unambiguous structure. The supporting radial distribution work was performed on a sample of the composition GeIsAS45Te4o. It shows a nearest neighbor peak at 2.50 A and a second neighbor peak at 4.02 A. The peak at 2.50 A would be appropriate for both As-Te and Ge-Te nearest neighbor pairs. Since As and Ge have approximately the same atomic scattering factors, it would be expected that the As-Te peaks would dominate the radial distribution at 2.50 A. On the other hand, the radial distribution shows a minimum at 3.1 A, indicating that there is very little bonding of the type associated with crystalline GeTe. This might, however, be accounted for by the small quantity of Ge contained in the sample. Doubt about a significant difference in the coordinations of amorphous and crystalline GeTe was also raised by the work of Chopra and Bahl4). They state: "The optical absorption edge is the same for both structures, suggesting the validity of the same energy band diagram for the 2 cases." It should be noted, however, that Tsu et al)) suggest that the band gap in crystalline GeTe is approximately 0.1-0.3 eV and that "... the observed optical absorption edges for our films at 0.7-1.0 eV are strongly Burstein-shifted". It is a crystalline absorption edge at approximately 0.8 eV to which Chopra and Bahl have compared their amorphous absorption edge measurements. The possibility of a significant Burstein shift in the crystalline absorption edge led us to believe that Chopra and Bahl's measurements do not rule out structural differences. The radial distribution work described in this paper is an attempt at clarifying this point. While the radial distribution work was underway, techniques for the reversible thermal transformation of appreciable volumes of samples were developed at Energy Conversion Devices, Inc. The second portion of this paper deals with X-ray diffraction identifications of the heat treatment products.
349
STRUCTURAL STUDIES
2. Radial distribution studies of amorphous Ge~Tel _x alloys
2.1. EXPERIMENTALPROCEDURES AND DATA ANALYSIS Three films of amorphous GexTe,_x alloys were prepared at Energy Conversion Devices, Inc. by R. Nowicki. Sample 1 was in the form of a bulk powder, while samples 2 and 3 were thin films, of thicknesses about 34 #m and 50 #m, respectively, obtained by vapor deposition. Approximate values of x were obtained for each sample through X-ray emission measurements, based on comparisons with bulk samples. The values were 0.11, 0.66 and 0.72 respectively for samples 1, 2 and 3. The possible error in these numbers due to the finite thicknesses of samples 2 and 3 has not been determined. This error is not expected to be large, however, because both films are sufficiently thick to cause a factor of 10 attenuation of the appropriate wave length X-rays. X-ray diffraction patterns from these samples were obtained with molybdenum K~ radiation on a Picker diffractometer using a LiF diffracted beam m o n o c h r o m a t o r and a pulse-height analysis window sufficiently narrow to eliminate 12 components of the X-ray beam. The intensity data were obtained using the step scan technique with a step size of 0.267 ° ( 2 - 0 ) and 4000 counts per step. Measurements were made over the angular range 10 ° to 100 ° ( 2 - 0 ) , corresponding to a range in s (47r sin 0/2) of 1.54 to 13.68. Measurements of the passband of the monochromator indicated that, to a
1600 A
GE.rz TE2e
~',
1400
1200~ ~J
iooo
-
'.xx
-
800 -600 --
400 -200 -0
F I
I 2
I
3
I
4
I 5
I 6 4"fl"
I 7
I 8
I 9
t I0
I
II
I
12
SIN e
Fig. 1. Scaled,polarization corrected diffracted intensity as a function of s (= 4 n sin0/2). The dashed curve represents the scattering which is independent of atomic configurations.
350
A. BIENENSTOCK, F. BETTS AND $. R. OVSHINSKY
good approximation, the Compton scattering could be neglected in the data analysis. Fig. l shows the diffraction pattern from sample 3, after scaling and polarization corrections. Atomic radial distributions were obtained from the observed intensity data, I(s), by Fourier analysis using the relation 6) Smax
4~rEp (r) = 47rr2po +
s i(s) exp ( - as 2) sin (rs) ds.
(1)
Stain
Here, p (r) is the atomic radial distribution function in units of electrons ~) per N3. Po, its average value, is normally obtained from density measurements, i(s) is given by the equation
i(,) = r,oo,,-
x,s/(,)l
x;/. {,)j,
(=)
where x I is the mole fraction of component j, whilefj is its atomic or ionic scattering factor and Icorris the scaled, polarization and background corrected intensity. The exp ( - a s 2) in eq. (1) is an arbitrary temperature factor inserted to aid the convergence of the incomplete Fourier transform of eq. (1). a has 2 = 1.85. In evaluating eq. (2), the atomic been chosen in this work so that aSmax scattering factors of Cromer and WaberT) were used for Ge and Te. Dispersion corrections listed in the International Tables for X-ray Crystallography 8) were also included in the calculation. Sample densities necessary for the determination of Po were not determined experimentally. Instead, a density of 5.6 for sample 1 and 5.1 for samples 2 and 3 were used as a result of some considerations which will not be presented here. The effect of the approximate nature of the densities will be discussed below. Eq. (l) was evaluated numerically using the IBM 360 computer of the Stanford Computation Center. 2.2.
RADIAL DISTRIBUTION CURVES AND THEIR ANALYSIS
The resulting radial distribution curves for samples l, 2 and 3 are shown as figs. 2, 3 and 4 respectively. Before commenting on their analysis, it is worthwhile to indicate their shortcomings. All three curves show structure at r values less than 2 A. In principle, p(r) should be zero in this region. This structure indicates that there are errors in the data or their analysis. These errors are likely to influence the area and shapes of the peaks in the distribution, but should not have a major effect on the peak positions. The dotted line shows the contribution of the estimated Po term. It should be
STRUCTURALSTUDIES
351
50--
GEII TE.89 40
~" 2 0 -
,/
CI~
/
1' ' 1
Fig. 2.
I
2
3 RAOIUS
/
4
5
6
7
8
(Angstroms)
Calculated radial distribution for sample l, of composition OenTe89. The dashed
curve represents the contribution of the average electron density.
noted that the fluctuations about Po in the small r region are extremely small, indicating that it is the computed density which is in error. We have found, however, that densities approximately one-half those used would be needed to bring p(r) to more reasonable values. It is highly unlikely that the densities of the films are that small. Thus, we assume that there are other errors involved. Systematic attempts to determine the nature and the means of elimination of these errors are underway. It should also be noted that the effect of the temperature factor is to introduce a slight broadening of the radial distribution peaks. That is, we expect that the peaks are slightly broader than they would have been had we been able to extend the upper limit of the integral closer to infinity. In spite of the distributions' limitations, some of their features are well worth noting. Over the broad composition range studied, the first and second neighbor peaks always appear at 2.7 A and 4.2 A, respectively. These peak positions are not expected to be appreciably affected by the errors. This indicates that the bulk of the nearest neighbor and next nearest neighbor
352
A. B I E N E N S T O C K , F. B E T T S A N D S. R . O V S H I N S K Y
II/"
40 -
z/S!
OE6sTE34 Eo
3o
////
2O
RADIUS (Angstroms)
Fig. 3.
Calculated radial distribution for sample 2, of composition Ge66Tea4. The dashed curve represents the contribution of the average electron density.
40-GE.72TEz8
/~
-
~ 3o-
t
o
,i/
/
20--
/
_
i..j -
o o.
/
/
~,
; ~ ,
8
RADIUS (Angstroms)
Fig. 4.
Calculated radial distribution for sample 3, of composition Ge72Te2s. The dashed curve represents the contribution of the average electron density.
S T R U C T U R A L STUDIES
353
distances are at approximately 2.6 and 4.1 A respectively. Thus, in no case do we see any indication of the bonding in crystalline GeTe, with nearest neighbor and next nearest neighbor separations of 3.0 and 4.2 to 4.3 A respectively. On the other hand, there is a noticeable broadening of the first neighbor peak in going from x=0.11 to x=0.66. This may be due to contributions of Ge-Te separations which are appreciably larger than 2.6 A. Alternatively, the broadening may be due to the above mentioned errors in the data or their analysis. In all three distributions, the area under the first peak suggests that the coordination is two or three fold, implying chain-like structures. As discussed above, however, these areas are uncertain due to the errors. In addition, there is an ambiguity in their analysis which is impossible to remove with X-ray measurements alone. That is, the nearest neighbor peak could be the superposition of Te-Te, G e - G e and Ge-Te nearest neighbor pairs. The contribution to the area from each pair is proportional to the product of the number of electrons in each atom. Without a knowledge of the number of each type of pair, the coordination cannot be determined uniquely. When it is possible that a single peak in the distribution has contributions from different types of pairs, it is impossible to determine the coordination uniquely without a knowledge of the relative number of each type of pair. This, of course, is the information to be determined. In summary, then, the most important result of this study is the conclusion that there is not an appreciable number of Ge-Te pairs in the amorphous materials at the interatomic separation associated with crystalline GeTe. This result is inconsistent with Chopra and Bahl's conclusion that the same energy band diagram is appropriate for the crystalline and amorphous materials. Our efforts are now aimed in two directions. First, we are concentrating on improving the quality of the radial distributions. Attempts will be made to measure the densities of the films. In addition, possible sources of systematic errors in the measured diffracted intensities are being studied. In addition, however, we are searching out more specific tools for the analysis of coordination changes. While quadrupole broadening of nuclear magnetic resonance lines would appear to be ideal, it is not well suited for Ge and Te. Hence, we are turning to an investigation of X-ray absorption spectroscopy techniques. With this tool, it may be possible to study the changes in coordination of Ge and Te separately when the samples are transformed from the amorphous to crystalline state.
3. X-ray diffraction identification of bulk crystallization products Fritzsche and Ovshinsky2) in their DTA studies of Ovonic memory ma-
354
A. BIENENSTOCK, F. BETTS AND S. R. OVSHINSKY
terials, have noted two types of behaviors in finely powdered bulk materials. In one series of materials, two exothermic peaks are evident in the first heating cycle. The low temperature peak is observed between 200 and 250°C, while a higher temperature peak is found between 250 and 300°C. In other materials, only one peak between 200 and 250°C is observed. We have studied a number of these materials, based on the Ge-Te system, by X-ray diffractometry. In all cases, the materials were, for the most part, amorphous prior to heating. Occasionally, extremely weak crystalline peaks were observed, indicating that a small number of crystallites were formed as the melt cooled. The heating products were determined in the following manner. Fritzsche supplied three types of samples: l) single peak samples in which DTA heating cycle was terminated immediately after the peaks, 2) double peak samples in which the cycle was terminated immediately after the first peak, 3) double peak samples in which the cycle was terminated after the second peak. Samples of types (l) and (2) always showed sharp Bragg peaks which were readily identifiable as crystalline Te, as well as an amorphous background. Samples of type (3) showed sharp Bragg peaks from crystalline Te and GeTe, as well as an amorphous background. Thus, the DTA peaks between 200 and 250°C can be associated with the crystallization of Te, while the higher temperature peaks, when they appear, are due to the crystallization of GeTe. 4. X-ray diffraction studies of thin films While studies of bulk samples indicated that the switching process is associated with crystallization, it is not justifiable to extrapolate bulk sample properties to explain thin film devices. Two objections to such an extrapolation are: 1) The recrystallization process requires separation of the Te and GeTe phases. Such phase separation might occur as the bulk melt cooled, or in the relatively slow heating associated with the DTA process. The films, as deposited, however, are most probably homogeneous. It is not at all apparent that phase separation can take place in the short switching times of the devices. 2) Given that crystallization and the corresponding phase separation can take place in the conversion of high resistivity to low resistivity material, it is not obvious that it would be possible to obtain the reverse transformation to the high resistance, amorphous state without a remixing of the material. In order to obtain more information about these questions, a partially completed program of measurements built around thin film samples was undertaken. In this section, the completed experiments will be described and experiments to be performed will be discussed.
STRUCTURAL STUDIES
355
A series of films of the one DTA peak type materials were vapor deposited onto glass microscope slides by J. Evans. Metallic electrodes were also deposited on the slide, underneath the film. The composition of these films has not been determined, but they can be characterized by their crystallization behavior. Film thicknesses were all of the order of 2 to 4 pro. It was verified that all films were amorphous as prepared. The initial experiments were performed on samples which were converted to the conducting state thermally. This was achieved by placing the slide on a hotplate, whose surface temperature was 400°C, for approximately 2 min in air. This process reduced the resistance of the film from l 0 7 ~ to 250 ~). The resulting X-ray diffraction pattern showed a number of intense sharp peaks, all of which could be associated with Te. In addition, extremely weak peaks associated with the electrodes were observed. The angular breadth of the diffraction peaks indicated an average crystallite size of at least 400 A perpendicular to the microscope slide. No determination of the crystallite size parallel to the slide could be made. A similarly treated sample, whose resistance was reduced from l07 fl to 500 ~ by heating was then reset to a final resistance of l06 ~) through subjection to one 500 ~sec light pulse by Evans, Helbers and Ovshinskyg). Their procedure is described in an accompanying paper. The resulting X-ray diffraction pattern showed a broad maximum typical of amorphous films. These results should be contrasted with those obtained from two samples transformed to the conducting state by J. Evans with the flash lamp technique. The first sample showed a film sheet resistance change from 10 7 ~ to 200 fl as a result of the flashing. The diffraction pattern was that of polycrystalline Te, but the ratios of intensities of different peaks were far from that expected for randomly oriented crystallites. What is more, the intensities of the peaks varied with rotation of the microscope slide about its normal. In one orientation of the slide, the (110) reflection is considerably stronger than the (101) reflection, although the latter peak is expected to be approximately three times as intense for a sample consisting of randomly oriented crystallites. As the sample was rotated through an angle of 90 ° about the normal, both peaks disappeared. We believe that these effects are due to needle crystallites oriented in the plane of the microscope slide. Reinvestigation showed that similar, but much less marked, effects can be observed in the thermally transformed samples. A second sample, which was first transformed from a film sheet resistance of 2 x l 0 6 ~ to 10 2 ~ by the flash lamp technique, was transformed back to a resistance of 9 x 105 f~ through a succession of short duration pulses. (See fig. 5 for a description of the change of resistance with pulsing.) The diffraction pattern from this sample shows an extremely weak (101) reflection
356
A. B I E N E N S T O C K ~ F. BETTS A N D S. R . O V S H I N S K Y
,°'I L~J (.~ Z
ACCUMULATED ENERGY " 4 - ' - -
I
I
I
i
i
IO s
txl 10 4 W I
10 3
10 z
I ACCUMULATED ENERGY--~ ]'~ I
I
I
I
I
1 _
Fig. 5. Record of measured sheet resistance of a thin film memory material as a function of flash lamp pulsing. The circles show the decrease of resistance through application of long duration pulses. The triangles show the increase of resistance through application of short duration, sharp cut-off pulses.
while the (110) peak is nearly as strong as it was for the conducting sample described in the preceding paragraph. This (110) peak could also be made to disappear by rotation of the sample. A model for these results would be one in which the smallest of the crystallites in the conducting samples are relatively randomly oriented, yielding the (101) reflection. These small crystallites are revitrified with the short duration pulses. Larger, relatively well oriented crystallites which account for the strong (110) reflection remain crystalline. These results indicate, first of all, that it is Te which crystallizes and vitrifies in the reversible memory transformation. They also show that it is possible to convert the system from one consisting of small crystallites of Te to an amorphous state in an extremely short time. This is shown quite clearly by the reconversion of the thermally treated samples to the amorphous state with the application of one short duration pulse. It should be noted that after the application of the first short duration pulse to the flash lamp converted
STRUCTURAL STUDIES
357
sample, there is no further appreciable increase o f resistance with pulsing. A model for this would be one in which the small crystallites are vitrified by the first short duration pulse, but the large crystallites are never revitrified. Finally, it should be noted that many more pulses are required to convert an evaporated high resistance film to the conducting state than are needed to return it to the a m o r p h o u s state. This effect m a y be explained by the necessity to phase separate the material in the process o f crystallization. One remaining p r o b l e m is to determine whether the material remains phase separated in the revitrification process. Attempts are n o w being made to determine this t h r o u g h transmission electron microscopy. We also hope to have, in the near future, a more complete picture o f crystallite orientation and sizes associated with these processes.
References 1) S. R. Ovshinsky, Control Engineering 11 (1964) 69; Talk at Intern. Colloq. on Amorphous and Liquid Semiconductors, Bucharest, 1967; U.S. Patent No. 3,271,591. 2) H. Fritzsche and S. R. Ovshinsky, J. Non-Crystalline Solids 2 (1970) 148, 393. 3) A. R. Hilton, C. E. Jones, R. D. Dobrott, H. M. Klein, A. M. Bryant and T. D. George, Phys. Chem. Glasses 4 (1966) 116. 4) K. L. Chopra and S. K. Bahl, Bull. Am. Phys. Soc. 14 (1968) 98. 5) R. Tsu, W. E. Howard and L. Esaki, Phys. Rev. 172 (1968) 779. 6) R. F. Kruh, in: Handbook of X-Rays, Ed. E. F. Kaelble (McGraw-Hill, New York, 1967). 7) D. T. Cromer and J. T. Waber, Acta Cryst. 18 (1965) 104. 8) International Tables for X-Ray Crystallography, Vol. 3 (Kynoch Press, Birmingham, 1962). 9) E. J. Evans, J. H. Helbers and S. R. Ovshinsky, J. Non-Crystalline Solids 2 (1970) 334.
STRUCTURAL
STUDIES OF AMORPHOUS
SEMICONDUCTORS*
A. BIENENSTOCK and F. BETTS Department of Materials Science, Stanford University, Stanford, California 94305, U.S.A. and S. R. OVSHINSKY Energy Conversion Devices, Inc., Troy, Michigan 48084, U.S.A.
X-ray diffraction radial distribution studies of amorphous samples of Ge~Tel x, with x = 0.11, 0.66 and 0.72, have been performed. These are characterized by first neighbor peaks at 2.7/~ and second neighbor peaks at 4.2 A.. The absence of a peak at the crystalline GeTe first neighbor position of 3 A is shown to imply that the local coordinations in the amorphous materials are different from that in the crystalline. In addition, thermally and flash lamp transformed Ovonic memory materials have been shown to contain crystalline Te in their conducting states, while the high resistance, as-prepared, states are amorphous. In some cases, crystalline GeTe is also present in the conducting state. The crystalline Te created by thermal transformation of the material can be revitrified through application of short duration, sharp cutoff light pulses. The flash lamp transformed material appears to contain larger, relatively well oriented crystallites which were not transformed by the light pulses.
1. Introduction This p a p e r describes structural studies o f the reversible m e m o r y type electrical t r a n s i t i o n 1) in a m o r p h o u s films o f G e - T e 2) based alloys. The films, as e v a p o r a t e d , are high resistance intrinsic-like semiconductors. A f t e r subj e c t i o n to a sufficiently high electrical field to b r i n g a b o u t a t r a n s i t i o n to a low resistance state, a n d a f u r t h e r delay d u r i n g which there is c u r r e n t flow, a p o r t i o n o f the m a t e r i a l is t r a n s f o r m e d to a s e m i p e r m a n e n t low resistance state. T h a t is, the m a t e r i a l r e m a i n s in a low resistance state w i t h o u t further a p p l i c a t i o n o f an electric field. T h e m a t e r i a l m a y be switched b a c k to a high resistance state t h r o u g h a p p l i c a t i o n o f a c u r r e n t pulse with a r a p i d t u r n off. This p a p e r r e p o r t s initial results o f studies designed to elucidate structural aspects o f the p h a s e t r a n s i t i o n in these materials.
* Work supported in part by the Advanced Research Projects Agency through the Center for Materials Research, Stanford University, by the Department of Materials Science, Stanford University, and by Energy Conversion Devices, Inc. 347
348
A. BIENENSTOCK, F. BETTS AND S. R. OVSH1NSKY
Initial efforts were based on the hypothesis that the memory effect involved the vitrification and devitrification of GeTe. A possible mechanism for the transformation was suggested by the structural model proposed by Hilton et al.3). On the basis of infrared and radial distribution studies, they deduced a chain structure in which coordination is two-fold, the interatomic distance is 2.57 A and the bond angle is approximately 110°. This should be contrasted with the only slightly distorted rocksalt structure of crystalline GeTe, where the coordination is six-fold, the interatomic separation is approximately 3 A and the bond angle is about 90° . This difference in local coordinations suggests that an appreciable portion of the difference in conductivities is due to a change in bonding with a corresponding change in the effective band gap. Unfortunately, the chain model of Hilton et al. a) can only be viewed as a conjecture. The infrared studies, in themselves, do not yield an unambiguous structure. The supporting radial distribution work was performed on a sample of the composition GeIsAS45Te4o. It shows a nearest neighbor peak at 2.50 A and a second neighbor peak at 4.02 A. The peak at 2.50 A would be appropriate for both As-Te and Ge-Te nearest neighbor pairs. Since As and Ge have approximately the same atomic scattering factors, it would be expected that the As-Te peaks would dominate the radial distribution at 2.50 A. On the other hand, the radial distribution shows a minimum at 3.1 A, indicating that there is very little bonding of the type associated with crystalline GeTe. This might, however, be accounted for by the small quantity of Ge contained in the sample. Doubt about a significant difference in the coordinations of amorphous and crystalline GeTe was also raised by the work of Chopra and Bahl4). They state: "The optical absorption edge is the same for both structures, suggesting the validity of the same energy band diagram for the 2 cases." It should be noted, however, that Tsu et al)) suggest that the band gap in crystalline GeTe is approximately 0.1-0.3 eV and that "... the observed optical absorption edges for our films at 0.7-1.0 eV are strongly Burstein-shifted". It is a crystalline absorption edge at approximately 0.8 eV to which Chopra and Bahl have compared their amorphous absorption edge measurements. The possibility of a significant Burstein shift in the crystalline absorption edge led us to believe that Chopra and Bahl's measurements do not rule out structural differences. The radial distribution work described in this paper is an attempt at clarifying this point. While the radial distribution work was underway, techniques for the reversible thermal transformation of appreciable volumes of samples were developed at Energy Conversion Devices, Inc. The second portion of this paper deals with X-ray diffraction identifications of the heat treatment products.
349
STRUCTURAL STUDIES
2. Radial distribution studies of amorphous Ge~Tel _x alloys
2.1. EXPERIMENTALPROCEDURES AND DATA ANALYSIS Three films of amorphous GexTe,_x alloys were prepared at Energy Conversion Devices, Inc. by R. Nowicki. Sample 1 was in the form of a bulk powder, while samples 2 and 3 were thin films, of thicknesses about 34 #m and 50 #m, respectively, obtained by vapor deposition. Approximate values of x were obtained for each sample through X-ray emission measurements, based on comparisons with bulk samples. The values were 0.11, 0.66 and 0.72 respectively for samples 1, 2 and 3. The possible error in these numbers due to the finite thicknesses of samples 2 and 3 has not been determined. This error is not expected to be large, however, because both films are sufficiently thick to cause a factor of 10 attenuation of the appropriate wave length X-rays. X-ray diffraction patterns from these samples were obtained with molybdenum K~ radiation on a Picker diffractometer using a LiF diffracted beam m o n o c h r o m a t o r and a pulse-height analysis window sufficiently narrow to eliminate 12 components of the X-ray beam. The intensity data were obtained using the step scan technique with a step size of 0.267 ° ( 2 - 0 ) and 4000 counts per step. Measurements were made over the angular range 10 ° to 100 ° ( 2 - 0 ) , corresponding to a range in s (47r sin 0/2) of 1.54 to 13.68. Measurements of the passband of the monochromator indicated that, to a
1600 A
GE.rz TE2e
~',
1400
1200~ ~J
iooo
-
'.xx
-
800 -600 --
400 -200 -0
F I
I 2
I
3
I
4
I 5
I 6 4"fl"
I 7
I 8
I 9
t I0
I
II
I
12
SIN e
Fig. 1. Scaled,polarization corrected diffracted intensity as a function of s (= 4 n sin0/2). The dashed curve represents the scattering which is independent of atomic configurations.
350
A. BIENENSTOCK, F. BETTS AND $. R. OVSHINSKY
good approximation, the Compton scattering could be neglected in the data analysis. Fig. l shows the diffraction pattern from sample 3, after scaling and polarization corrections. Atomic radial distributions were obtained from the observed intensity data, I(s), by Fourier analysis using the relation 6) Smax
4~rEp (r) = 47rr2po +
s i(s) exp ( - as 2) sin (rs) ds.
(1)
Stain
Here, p (r) is the atomic radial distribution function in units of electrons ~) per N3. Po, its average value, is normally obtained from density measurements, i(s) is given by the equation
i(,) = r,oo,,-
x,s/(,)l
x;/. {,)j,
(=)
where x I is the mole fraction of component j, whilefj is its atomic or ionic scattering factor and Icorris the scaled, polarization and background corrected intensity. The exp ( - a s 2) in eq. (1) is an arbitrary temperature factor inserted to aid the convergence of the incomplete Fourier transform of eq. (1). a has 2 = 1.85. In evaluating eq. (2), the atomic been chosen in this work so that aSmax scattering factors of Cromer and WaberT) were used for Ge and Te. Dispersion corrections listed in the International Tables for X-ray Crystallography 8) were also included in the calculation. Sample densities necessary for the determination of Po were not determined experimentally. Instead, a density of 5.6 for sample 1 and 5.1 for samples 2 and 3 were used as a result of some considerations which will not be presented here. The effect of the approximate nature of the densities will be discussed below. Eq. (l) was evaluated numerically using the IBM 360 computer of the Stanford Computation Center. 2.2.
RADIAL DISTRIBUTION CURVES AND THEIR ANALYSIS
The resulting radial distribution curves for samples l, 2 and 3 are shown as figs. 2, 3 and 4 respectively. Before commenting on their analysis, it is worthwhile to indicate their shortcomings. All three curves show structure at r values less than 2 A. In principle, p(r) should be zero in this region. This structure indicates that there are errors in the data or their analysis. These errors are likely to influence the area and shapes of the peaks in the distribution, but should not have a major effect on the peak positions. The dotted line shows the contribution of the estimated Po term. It should be
STRUCTURALSTUDIES
351
50--
GEII TE.89 40
~" 2 0 -
,/
CI~
/
1' ' 1
Fig. 2.
I
2
3 RAOIUS
/
4
5
6
7
8
(Angstroms)
Calculated radial distribution for sample l, of composition OenTe89. The dashed
curve represents the contribution of the average electron density.
noted that the fluctuations about Po in the small r region are extremely small, indicating that it is the computed density which is in error. We have found, however, that densities approximately one-half those used would be needed to bring p(r) to more reasonable values. It is highly unlikely that the densities of the films are that small. Thus, we assume that there are other errors involved. Systematic attempts to determine the nature and the means of elimination of these errors are underway. It should also be noted that the effect of the temperature factor is to introduce a slight broadening of the radial distribution peaks. That is, we expect that the peaks are slightly broader than they would have been had we been able to extend the upper limit of the integral closer to infinity. In spite of the distributions' limitations, some of their features are well worth noting. Over the broad composition range studied, the first and second neighbor peaks always appear at 2.7 A and 4.2 A, respectively. These peak positions are not expected to be appreciably affected by the errors. This indicates that the bulk of the nearest neighbor and next nearest neighbor
352
A. B I E N E N S T O C K , F. B E T T S A N D S. R . O V S H I N S K Y
II/"
40 -
z/S!
OE6sTE34 Eo
3o
////
2O
RADIUS (Angstroms)
Fig. 3.
Calculated radial distribution for sample 2, of composition Ge66Tea4. The dashed curve represents the contribution of the average electron density.
40-GE.72TEz8
/~
-
~ 3o-
t
o
,i/
/
20--
/
_
i..j -
o o.
/
/
~,
; ~ ,
8
RADIUS (Angstroms)
Fig. 4.
Calculated radial distribution for sample 3, of composition Ge72Te2s. The dashed curve represents the contribution of the average electron density.
S T R U C T U R A L STUDIES
353
distances are at approximately 2.6 and 4.1 A respectively. Thus, in no case do we see any indication of the bonding in crystalline GeTe, with nearest neighbor and next nearest neighbor separations of 3.0 and 4.2 to 4.3 A respectively. On the other hand, there is a noticeable broadening of the first neighbor peak in going from x=0.11 to x=0.66. This may be due to contributions of Ge-Te separations which are appreciably larger than 2.6 A. Alternatively, the broadening may be due to the above mentioned errors in the data or their analysis. In all three distributions, the area under the first peak suggests that the coordination is two or three fold, implying chain-like structures. As discussed above, however, these areas are uncertain due to the errors. In addition, there is an ambiguity in their analysis which is impossible to remove with X-ray measurements alone. That is, the nearest neighbor peak could be the superposition of Te-Te, G e - G e and Ge-Te nearest neighbor pairs. The contribution to the area from each pair is proportional to the product of the number of electrons in each atom. Without a knowledge of the number of each type of pair, the coordination cannot be determined uniquely. When it is possible that a single peak in the distribution has contributions from different types of pairs, it is impossible to determine the coordination uniquely without a knowledge of the relative number of each type of pair. This, of course, is the information to be determined. In summary, then, the most important result of this study is the conclusion that there is not an appreciable number of Ge-Te pairs in the amorphous materials at the interatomic separation associated with crystalline GeTe. This result is inconsistent with Chopra and Bahl's conclusion that the same energy band diagram is appropriate for the crystalline and amorphous materials. Our efforts are now aimed in two directions. First, we are concentrating on improving the quality of the radial distributions. Attempts will be made to measure the densities of the films. In addition, possible sources of systematic errors in the measured diffracted intensities are being studied. In addition, however, we are searching out more specific tools for the analysis of coordination changes. While quadrupole broadening of nuclear magnetic resonance lines would appear to be ideal, it is not well suited for Ge and Te. Hence, we are turning to an investigation of X-ray absorption spectroscopy techniques. With this tool, it may be possible to study the changes in coordination of Ge and Te separately when the samples are transformed from the amorphous to crystalline state.
3. X-ray diffraction identification of bulk crystallization products Fritzsche and Ovshinsky2) in their DTA studies of Ovonic memory ma-
354
A. BIENENSTOCK, F. BETTS AND S. R. OVSHINSKY
terials, have noted two types of behaviors in finely powdered bulk materials. In one series of materials, two exothermic peaks are evident in the first heating cycle. The low temperature peak is observed between 200 and 250°C, while a higher temperature peak is found between 250 and 300°C. In other materials, only one peak between 200 and 250°C is observed. We have studied a number of these materials, based on the Ge-Te system, by X-ray diffractometry. In all cases, the materials were, for the most part, amorphous prior to heating. Occasionally, extremely weak crystalline peaks were observed, indicating that a small number of crystallites were formed as the melt cooled. The heating products were determined in the following manner. Fritzsche supplied three types of samples: l) single peak samples in which DTA heating cycle was terminated immediately after the peaks, 2) double peak samples in which the cycle was terminated immediately after the first peak, 3) double peak samples in which the cycle was terminated after the second peak. Samples of types (l) and (2) always showed sharp Bragg peaks which were readily identifiable as crystalline Te, as well as an amorphous background. Samples of type (3) showed sharp Bragg peaks from crystalline Te and GeTe, as well as an amorphous background. Thus, the DTA peaks between 200 and 250°C can be associated with the crystallization of Te, while the higher temperature peaks, when they appear, are due to the crystallization of GeTe. 4. X-ray diffraction studies of thin films While studies of bulk samples indicated that the switching process is associated with crystallization, it is not justifiable to extrapolate bulk sample properties to explain thin film devices. Two objections to such an extrapolation are: 1) The recrystallization process requires separation of the Te and GeTe phases. Such phase separation might occur as the bulk melt cooled, or in the relatively slow heating associated with the DTA process. The films, as deposited, however, are most probably homogeneous. It is not at all apparent that phase separation can take place in the short switching times of the devices. 2) Given that crystallization and the corresponding phase separation can take place in the conversion of high resistivity to low resistivity material, it is not obvious that it would be possible to obtain the reverse transformation to the high resistance, amorphous state without a remixing of the material. In order to obtain more information about these questions, a partially completed program of measurements built around thin film samples was undertaken. In this section, the completed experiments will be described and experiments to be performed will be discussed.
STRUCTURAL STUDIES
355
A series of films of the one DTA peak type materials were vapor deposited onto glass microscope slides by J. Evans. Metallic electrodes were also deposited on the slide, underneath the film. The composition of these films has not been determined, but they can be characterized by their crystallization behavior. Film thicknesses were all of the order of 2 to 4 pro. It was verified that all films were amorphous as prepared. The initial experiments were performed on samples which were converted to the conducting state thermally. This was achieved by placing the slide on a hotplate, whose surface temperature was 400°C, for approximately 2 min in air. This process reduced the resistance of the film from l 0 7 ~ to 250 ~). The resulting X-ray diffraction pattern showed a number of intense sharp peaks, all of which could be associated with Te. In addition, extremely weak peaks associated with the electrodes were observed. The angular breadth of the diffraction peaks indicated an average crystallite size of at least 400 A perpendicular to the microscope slide. No determination of the crystallite size parallel to the slide could be made. A similarly treated sample, whose resistance was reduced from l07 fl to 500 ~ by heating was then reset to a final resistance of l06 ~) through subjection to one 500 ~sec light pulse by Evans, Helbers and Ovshinskyg). Their procedure is described in an accompanying paper. The resulting X-ray diffraction pattern showed a broad maximum typical of amorphous films. These results should be contrasted with those obtained from two samples transformed to the conducting state by J. Evans with the flash lamp technique. The first sample showed a film sheet resistance change from 10 7 ~ to 200 fl as a result of the flashing. The diffraction pattern was that of polycrystalline Te, but the ratios of intensities of different peaks were far from that expected for randomly oriented crystallites. What is more, the intensities of the peaks varied with rotation of the microscope slide about its normal. In one orientation of the slide, the (110) reflection is considerably stronger than the (101) reflection, although the latter peak is expected to be approximately three times as intense for a sample consisting of randomly oriented crystallites. As the sample was rotated through an angle of 90 ° about the normal, both peaks disappeared. We believe that these effects are due to needle crystallites oriented in the plane of the microscope slide. Reinvestigation showed that similar, but much less marked, effects can be observed in the thermally transformed samples. A second sample, which was first transformed from a film sheet resistance of 2 x l 0 6 ~ to 10 2 ~ by the flash lamp technique, was transformed back to a resistance of 9 x 105 f~ through a succession of short duration pulses. (See fig. 5 for a description of the change of resistance with pulsing.) The diffraction pattern from this sample shows an extremely weak (101) reflection
356
A. B I E N E N S T O C K ~ F. BETTS A N D S. R . O V S H I N S K Y
,°'I L~J (.~ Z
ACCUMULATED ENERGY " 4 - ' - -
I
I
I
i
i
IO s
txl 10 4 W I
10 3
10 z
I ACCUMULATED ENERGY--~ ]'~ I
I
I
I
I
1 _
Fig. 5. Record of measured sheet resistance of a thin film memory material as a function of flash lamp pulsing. The circles show the decrease of resistance through application of long duration pulses. The triangles show the increase of resistance through application of short duration, sharp cut-off pulses.
while the (110) peak is nearly as strong as it was for the conducting sample described in the preceding paragraph. This (110) peak could also be made to disappear by rotation of the sample. A model for these results would be one in which the smallest of the crystallites in the conducting samples are relatively randomly oriented, yielding the (101) reflection. These small crystallites are revitrified with the short duration pulses. Larger, relatively well oriented crystallites which account for the strong (110) reflection remain crystalline. These results indicate, first of all, that it is Te which crystallizes and vitrifies in the reversible memory transformation. They also show that it is possible to convert the system from one consisting of small crystallites of Te to an amorphous state in an extremely short time. This is shown quite clearly by the reconversion of the thermally treated samples to the amorphous state with the application of one short duration pulse. It should be noted that after the application of the first short duration pulse to the flash lamp converted
STRUCTURAL STUDIES
357
sample, there is no further appreciable increase o f resistance with pulsing. A model for this would be one in which the small crystallites are vitrified by the first short duration pulse, but the large crystallites are never revitrified. Finally, it should be noted that many more pulses are required to convert an evaporated high resistance film to the conducting state than are needed to return it to the a m o r p h o u s state. This effect m a y be explained by the necessity to phase separate the material in the process o f crystallization. One remaining p r o b l e m is to determine whether the material remains phase separated in the revitrification process. Attempts are n o w being made to determine this t h r o u g h transmission electron microscopy. We also hope to have, in the near future, a more complete picture o f crystallite orientation and sizes associated with these processes.
References 1) S. R. Ovshinsky, Control Engineering 11 (1964) 69; Talk at Intern. Colloq. on Amorphous and Liquid Semiconductors, Bucharest, 1967; U.S. Patent No. 3,271,591. 2) H. Fritzsche and S. R. Ovshinsky, J. Non-Crystalline Solids 2 (1970) 148, 393. 3) A. R. Hilton, C. E. Jones, R. D. Dobrott, H. M. Klein, A. M. Bryant and T. D. George, Phys. Chem. Glasses 4 (1966) 116. 4) K. L. Chopra and S. K. Bahl, Bull. Am. Phys. Soc. 14 (1968) 98. 5) R. Tsu, W. E. Howard and L. Esaki, Phys. Rev. 172 (1968) 779. 6) R. F. Kruh, in: Handbook of X-Rays, Ed. E. F. Kaelble (McGraw-Hill, New York, 1967). 7) D. T. Cromer and J. T. Waber, Acta Cryst. 18 (1965) 104. 8) International Tables for X-Ray Crystallography, Vol. 3 (Kynoch Press, Birmingham, 1962). 9) E. J. Evans, J. H. Helbers and S. R. Ovshinsky, J. Non-Crystalline Solids 2 (1970) 334.