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Observations on crystalline transformation in amorphous Ge thin films
Thin Solid Filtrt, . Elsevier Sequoia S.A., Lausanne - Printed in Switzerland
415
OBSERVATIONS ON C R Y S T A L L I N E T R A N S F O R M A T I O N IN A M O R P H O U S Ge T H I N FILMS E. !. ALESSANDRINI. R. J. GAMBINO AND J. D. KUPTSIS
I B M Thomas J. Watson Research Centc,. P.O. Box 218. Yorktown Heights, N . Y . 10598 ~ U.S.,4. )
(Received March 20. 1972: in revised form April I 1. 1972)
SUMMARY
The crystalline transformation of amorphous Ge films as a function of film thickness and environment has been studied using electron diffraction, electron microscope and electron microprobe techniques. Crystallization heat treatments were carried out in an electron diffraction unit on two series of films, one examined immediately following deposition and the other after being exposed to air at room temperature for several weeks. It was found in both series that crystallization is independent of film thickness. The transformation occurred at 375 °__+10 °C over a broad range o f thicknesses from 40 A to 260 A. On heating the films to 700 ~C, however, a difference in structure between the two film series was observed. Films exposed to air showed the formation of GeO2 in addition to Gc. This difference, analyzed by transmission electron microscopy and the clectron microprobe, could be attributed to a surface oxide and suggests that surface contamination does occur on aging at room temperature but does not alter the crystalline transformation of the amorphous material.
INTRODUCTION
Considerable interest has been shown in the amorphous to crystalline phase transformation of evaporated thin films not only from the standpoint of studying structural problems in semiconducting materials but also because these films exhibit device oriented properties 1-6. This investigation describes the structural aspects of the phase transformation in Ge films as a function of thickness and environment. Films were obtained over a range of thicknesses from 40 A to 260 A at about 40 h intervals. Since in very thin films surface contamination effects can dominate their properties, we also studied the effects of aging in an air ambient on the Thin Solid Films, 11 (1972) 415--422
4]6
I!. I. AI.ESSANI)RINI. R. J. (.,AMBIN(). J. IJ. KL I'lNIS
crystallization behaviour of the amorphous Ge films. "lw,o series of studies were carried out: (1) Films examined immediately following dcposition and (2) films exposed to air at room temperature for several weeks. This paper reports on evidence that the phase transformation is independent of both thickness and environment, but annealing at highcr temperatures results in a structure differcncc between the two series of films. EXPERIMENTAL P R O C E I ) t ; R E
The Gc lilms were prepared by vapor deposition from a tung>,tcn source. and five I00/~ carhorl subs/rates ~,upported on 200 mc~,h ( u electron microscope grids were coated during a >,ira,It deposition. S t l b ' , l l ' , : . t t c ' , ~.~,cl't.' fiscal hCk'~ltl',C I]]111> as thin as 40 /~ are not easily supported, and carbon was chosen since it would not react with Ge. A pivoting shutter made it possible to expose each grid to the vapor stream for a different period of time in order to obtain a different thickness on each substrate. The rate was monitored contintlously during deposition by means of a quartz crystal monitor. The thickness of the very thin tilms could not be accurately measured using the Tolansky method, therefore, it was used only to determine the thickness of the thickest films. This result served to chcck the calibration of the crystal monitor and thicknessc~ ranging from 40 A t~, 2~~0 A in increments of about 40 A were determined as a function of rate and time ofex posure to the vapor stream. The vacuum system had a base pressure of 10 " lorr which increased to about 5 × 10 ~' during evaporation. The deposition rate was of the order of 10 A."sec. Cwstallization heat treatments were carried out in the electron diffraction instrument on both series of films: those immediately after preparation and those that were aged in air at room temperature for several weeks. A heating cycle of I C:min was used, and the same rate was employed for all films. Transmission electron diffraction patterns were obtained of the fihns before, during and after heating up to temperatures as high as 700 (" in a ~acut, m of 7 × 10 - torr. On cooling to room temperature, films of both series that had been annealed at 700'(" were removed fiom the electron diffraction t, nil and examined in a Phillips 200 Electron Microscope. Bright anti dark field electron micrographs were taken. In addition, a comparison was also made between the annealed as deposited and the annealed aged films using electron microprobe techniques. An ARL. EMX-SM electron beam microprobe was used. EXPERIMENTAL RESULTS AND I)IS(TU,~ION
An electron diffraction paucrn taken of a Gc film unmediately after deposition, and a pattern obtained fi'om a film after aging can be seen in Fig. I. Fhe tilms were 260 /~ in thickness. Identical broad halo type diffraction patterns lhin Solid I')'lrn,~, 11 (I 972) 415 -422
CRYSTALLINE TRANSFORMATION IN AMORPHOUS Ge FILMS
417
(a)
[b) Fig. 1. Electron diffraction transmission pattern from an as deposited Ge film. The incident beam is nearly perpendicular to the film plane. (a) As deposited 260 A film; (b) aged 260 A film.
characteristic of non-crystalline material 7 are observed. The diffraction patterns of these films upon heating to 375 °C (Fig. 2) show the onset of the crystalline transformation. This is seen more clearly in the recorded densitometer traces (Fig. 3) made from the patterns of the before and after heat treated films of Figs. l(a) and 2(a). The traces show a marked sharpening of the three halo peak widths Thin Solid Films, 1I (1972) 415-422
418
I!. I. AI.I.ISSAN1)RINI. R. J. GAMBINO. J. 1). KUPTSIS
Ca)
Fig. 2. Patterns fi'om the as grown G c films after heating to 3"75 ¢," for 10 rain. (a) Film treated i m m e d i a t e l y after d e p o s i t i o n : Ib) film aged in air for ,,cveral v, eck,.
7bin .S+olid Films. I1 ([972) 415 -422
CRYSTALLINE TRANSFORMATION IN AMORPHOUS
Ge
419
FILMS
v
0
5
tO FILM SCAN (ram)
15
20
(a) 0
"
h
i
5
I0 FILM SCAN (mm]
15
J(b) 20
Fig. 3. Densitometer traces from an as deposited 260 ,~, Ge film diffraction pattern. (a) Immediately after deposition (Fig. l(a)): (b) after heat treating to 375 "C for 10 min (Fig. 2(a)).
and resolution of the second one into the second (220) and third (311) reflections of the Ge diamond lattice. Crystallization is well defined and the same in both cases. Similar experiments were carried out as a function of specimen thickness at intervals of about 40/It from 260/It to 40/tt for both as deposited and aged films. Transformation as a function of decreasing thickness gave the same results as observed in Figs. 1 through 3. That is, crystallization from the amorphous phase was constant and well defined within experimental error and found to occur at a temperature o f 375 + 10 :C for all varying parameters of thickness and environment. It was when films from series one and two were annealed at 700 °C that a difference in film structure was observed. The films were annealed for 30 min, and diffraction patterns were recorded both at temperature and after vacuum cooling to room temperature. In films examined immediately after preparation, annealing resulted in grain growth and twinning. The electron diffraction patterns showed " f o r b i d d e n " reflections which could be accounted for by twinning and double diffraction of the Ge diamond lattice s (Fig. 4(a)). These twins were clearly visible in electron micrographs (Fig. 4(b)). Annealing of films from series two resulted in diffraction patterns which showed not only grain growth and twinning of the Ge, but also the presence of the tetragonal oxide GeO2 (Fig. 4(c)). This effect was examined in the electron microscope using both bright and dark field techniques. When a weak oxide reflection (arrow in Fig. 4(c)) in the diffraction pattern was used for image formation the dark lield micrograph showed that the oxide was not at grain or twin boundaries but was rather in the form of a skin over the Ge grains (Figs. 4(d) and 4(e)). Using the electron microprobe technique, it was found that the oxygen present on the surface of the films after room temperature exposure was bonded Thin Solid Films, 11 (1972) 415-422
420
I~. 1. AI.I'SSANI)RINh R. J. GAMBIN(). J. I). K[?P'FSIS
,,I)
(hi
Ic)
Fig. 4. Transmission electron diffraclion and electron rnicrograph of (;e films annealed at 700 ( al)er transformation. (a) Diffraction pattern, series one film: Ib) bright field micrograph: (el diffraction pattern series two tilm -arrow indicates (.;eO2 reflection: (d) bright tiekt micrograph: ( e ) t h e dark tield showing brightness over Ge ~rain when a weak (;cO 2 reflection i~ used lk)r image formation.
differently from that lormed after the high temperature h~;at trcatment. The thickest annealed and tmannealed films ( ~ 260 A) were evaluated for the presence o f oxygen. Oxygen K.~ radiation was detected only in the annealed film. Considering the magnitude o f the oxygen K7 cot, nts (after background correctionS. Thin 5,dtd kTlm.s. I ] I 1972) 4 [ 5 4_.'~"
CRYSTALLINE TRANSFORMATION IN AMORPHOUS
Ge
FILMS
421
oxygen was found to be a major constituent of the film. Visible cathodoluminescence was also observed from the annealed film and not in the case of the unannealed specimen. Compounds in particular oxides and sulfides are known to exhibit cathodoluminescence 9. Annealing the aged films apparently resulted in a change in the valence condition of the Ge to one which can be associated with compound formation. The chemical state of the Ge can be investigated by techniques such as ESCA (Electron Spectroscopy for Chemical Analysis), and it is the authors' intention to report on such a study in the near future. The sequence from an amorphous to a crystalline structure observed in both types of films has indicated that there is no justification for assuming a thickness dependence, in the range investigated here, as a determining factor for transformation. Work by other investigators 1° has shown that contamination as well as varying the vacuum pressure for film preparation can result in a change. In the work presented here, it should be noted that all films in a varying thickness series were made at the same time. In this way, the impurity content is not a factor for consideration. Surface contamination did result when films were aged in air, but this aging process did not affect the crystalline transformation temperature of any of the amorphous films. It was only when films were heated above the Ge crystallization temperature and where reaction of surface oxygen It could occur that its effect became evident. Then tetragonal GeO 2 as well as Ge appeared in the diffraction patterns. The intensity of the GeO2 reflections was found to vary with film thicknesses. As the Ge became thinner the oxide was present in a greater ratio as evidenced by an increase in the intensity of GeO 2 reflections and a decrease in those of Ge. There is evidence in the literaturC 2 that voids exist in most vapor deposited Ge films. We did not investigate void structure as the emphasis was on an electron diffraction study of crystallization. Diffraction patterns from the as deposited films did not resolve voids that might be present because of the inelastic scattering of electrons. Our results, however, did show that the oxide forms on the surface of the aged films. This suggests that any void structure, which may have been present, was closed to oxygen permeation. In summary, our work shows that thickness does not change the amorphous to crystalline transformation in Ge films provided that the films are prepared during the same evaporation. Surface contamination results when films are aged in an air ambient but does not alter the crystalline transformation temperature within experimental accuracy. Aging does significantly influence the film structure at higher temperatures. It is then that GeO2 appears as a second phase on the crystalline Ge film surface.
Thin Solid Films. 11 (1972) 415-422
422
E. 1. A I , E S S A N D R I N I , R. J. GAMBINO. J. D. KUPTSIS
ACKNOWLEDGEMENT
The authors are indebted to Mrs. S. Herd for the electron microscopy in this study. REFERENCES
I S . R . OVSlIINSKY. Ph.vs. Rer. Letters, 21 (1968) 1450. Z H. FRITZSCH• A.XI:.S. R. OVSH~NSKY.J. Non-('r.vst. Solids. 2 (197(I) 148. 3 T. TAKAMOm, R. ROY AYD G. J, McCARTllY. Mater. Res. Bull., 5 {1970) 529. 4 K . L . CHOPRA AND S. K. BAHI., J. Appl. Phys.. 40 (1969} 4174. 5 Y.P. GUPTA. J. Non-('ryst. Solids, 3 (197(I) 148. 6 N. V. GtTDKOVA. T. A. ZEVt~K~,. 1". N. MI~t)V~:IZ.EVA, R. A. Rtq3~,'SOtA AY,,I) T. N. S~RIZHI:','A. Soviet Phys. Crystal.. 15 (19711 747. 7 R. RoY. J. Non-Crvst. Solid~. 3 (1970) 33. 8 S. MkBER, Multiple Twinning and Pentagonal Structures in Germanium, IBM Res. Rept. )989. August 5. 1970. 9 J . D . BROWy. Fifty Years of Progress in Metallographic Techniques. AS'IM, .S'pec. l'ech. Pub/. No. 430 (1968) 374. 10 S.R. HERD. P. CHAt;DHAm AYD M. H. BRODSK¥. Metal Contact Induced Crystallization in Films of Amorphous Silicon and Germanium. I B M Res. Rept. 3666, December 28, 1~-~71,to be published in J. Non-Crvst. Solids. I 1 W. ROMANOWSKIAND D. Po'roczNA-PI-rRL'. Th#1 Solid Films. 8 ( 1971 ) 35. 12 T . M . DONOVANAND K. HEINEMANN. Phys. Rev. Letters. 27i19711 1794.
Thin Solid Films, 11 (1972) 415 422
415
OBSERVATIONS ON C R Y S T A L L I N E T R A N S F O R M A T I O N IN A M O R P H O U S Ge T H I N FILMS E. !. ALESSANDRINI. R. J. GAMBINO AND J. D. KUPTSIS
I B M Thomas J. Watson Research Centc,. P.O. Box 218. Yorktown Heights, N . Y . 10598 ~ U.S.,4. )
(Received March 20. 1972: in revised form April I 1. 1972)
SUMMARY
The crystalline transformation of amorphous Ge films as a function of film thickness and environment has been studied using electron diffraction, electron microscope and electron microprobe techniques. Crystallization heat treatments were carried out in an electron diffraction unit on two series of films, one examined immediately following deposition and the other after being exposed to air at room temperature for several weeks. It was found in both series that crystallization is independent of film thickness. The transformation occurred at 375 °__+10 °C over a broad range o f thicknesses from 40 A to 260 A. On heating the films to 700 ~C, however, a difference in structure between the two film series was observed. Films exposed to air showed the formation of GeO2 in addition to Gc. This difference, analyzed by transmission electron microscopy and the clectron microprobe, could be attributed to a surface oxide and suggests that surface contamination does occur on aging at room temperature but does not alter the crystalline transformation of the amorphous material.
INTRODUCTION
Considerable interest has been shown in the amorphous to crystalline phase transformation of evaporated thin films not only from the standpoint of studying structural problems in semiconducting materials but also because these films exhibit device oriented properties 1-6. This investigation describes the structural aspects of the phase transformation in Ge films as a function of thickness and environment. Films were obtained over a range of thicknesses from 40 A to 260 A at about 40 h intervals. Since in very thin films surface contamination effects can dominate their properties, we also studied the effects of aging in an air ambient on the Thin Solid Films, 11 (1972) 415--422
4]6
I!. I. AI.ESSANI)RINI. R. J. (.,AMBIN(). J. IJ. KL I'lNIS
crystallization behaviour of the amorphous Ge films. "lw,o series of studies were carried out: (1) Films examined immediately following dcposition and (2) films exposed to air at room temperature for several weeks. This paper reports on evidence that the phase transformation is independent of both thickness and environment, but annealing at highcr temperatures results in a structure differcncc between the two series of films. EXPERIMENTAL P R O C E I ) t ; R E
The Gc lilms were prepared by vapor deposition from a tung>,tcn source. and five I00/~ carhorl subs/rates ~,upported on 200 mc~,h ( u electron microscope grids were coated during a >,ira,It deposition. S t l b ' , l l ' , : . t t c ' , ~.~,cl't.' fiscal hCk'~ltl',C I]]111> as thin as 40 /~ are not easily supported, and carbon was chosen since it would not react with Ge. A pivoting shutter made it possible to expose each grid to the vapor stream for a different period of time in order to obtain a different thickness on each substrate. The rate was monitored contintlously during deposition by means of a quartz crystal monitor. The thickness of the very thin tilms could not be accurately measured using the Tolansky method, therefore, it was used only to determine the thickness of the thickest films. This result served to chcck the calibration of the crystal monitor and thicknessc~ ranging from 40 A t~, 2~~0 A in increments of about 40 A were determined as a function of rate and time ofex posure to the vapor stream. The vacuum system had a base pressure of 10 " lorr which increased to about 5 × 10 ~' during evaporation. The deposition rate was of the order of 10 A."sec. Cwstallization heat treatments were carried out in the electron diffraction instrument on both series of films: those immediately after preparation and those that were aged in air at room temperature for several weeks. A heating cycle of I C:min was used, and the same rate was employed for all films. Transmission electron diffraction patterns were obtained of the fihns before, during and after heating up to temperatures as high as 700 (" in a ~acut, m of 7 × 10 - torr. On cooling to room temperature, films of both series that had been annealed at 700'(" were removed fiom the electron diffraction t, nil and examined in a Phillips 200 Electron Microscope. Bright anti dark field electron micrographs were taken. In addition, a comparison was also made between the annealed as deposited and the annealed aged films using electron microprobe techniques. An ARL. EMX-SM electron beam microprobe was used. EXPERIMENTAL RESULTS AND I)IS(TU,~ION
An electron diffraction paucrn taken of a Gc film unmediately after deposition, and a pattern obtained fi'om a film after aging can be seen in Fig. I. Fhe tilms were 260 /~ in thickness. Identical broad halo type diffraction patterns lhin Solid I')'lrn,~, 11 (I 972) 415 -422
CRYSTALLINE TRANSFORMATION IN AMORPHOUS Ge FILMS
417
(a)
[b) Fig. 1. Electron diffraction transmission pattern from an as deposited Ge film. The incident beam is nearly perpendicular to the film plane. (a) As deposited 260 A film; (b) aged 260 A film.
characteristic of non-crystalline material 7 are observed. The diffraction patterns of these films upon heating to 375 °C (Fig. 2) show the onset of the crystalline transformation. This is seen more clearly in the recorded densitometer traces (Fig. 3) made from the patterns of the before and after heat treated films of Figs. l(a) and 2(a). The traces show a marked sharpening of the three halo peak widths Thin Solid Films, 1I (1972) 415-422
418
I!. I. AI.I.ISSAN1)RINI. R. J. GAMBINO. J. 1). KUPTSIS
Ca)
Fig. 2. Patterns fi'om the as grown G c films after heating to 3"75 ¢," for 10 rain. (a) Film treated i m m e d i a t e l y after d e p o s i t i o n : Ib) film aged in air for ,,cveral v, eck,.
7bin .S+olid Films. I1 ([972) 415 -422
CRYSTALLINE TRANSFORMATION IN AMORPHOUS
Ge
419
FILMS
v
0
5
tO FILM SCAN (ram)
15
20
(a) 0
"
h
i
5
I0 FILM SCAN (mm]
15
J(b) 20
Fig. 3. Densitometer traces from an as deposited 260 ,~, Ge film diffraction pattern. (a) Immediately after deposition (Fig. l(a)): (b) after heat treating to 375 "C for 10 min (Fig. 2(a)).
and resolution of the second one into the second (220) and third (311) reflections of the Ge diamond lattice. Crystallization is well defined and the same in both cases. Similar experiments were carried out as a function of specimen thickness at intervals of about 40/It from 260/It to 40/tt for both as deposited and aged films. Transformation as a function of decreasing thickness gave the same results as observed in Figs. 1 through 3. That is, crystallization from the amorphous phase was constant and well defined within experimental error and found to occur at a temperature o f 375 + 10 :C for all varying parameters of thickness and environment. It was when films from series one and two were annealed at 700 °C that a difference in film structure was observed. The films were annealed for 30 min, and diffraction patterns were recorded both at temperature and after vacuum cooling to room temperature. In films examined immediately after preparation, annealing resulted in grain growth and twinning. The electron diffraction patterns showed " f o r b i d d e n " reflections which could be accounted for by twinning and double diffraction of the Ge diamond lattice s (Fig. 4(a)). These twins were clearly visible in electron micrographs (Fig. 4(b)). Annealing of films from series two resulted in diffraction patterns which showed not only grain growth and twinning of the Ge, but also the presence of the tetragonal oxide GeO2 (Fig. 4(c)). This effect was examined in the electron microscope using both bright and dark field techniques. When a weak oxide reflection (arrow in Fig. 4(c)) in the diffraction pattern was used for image formation the dark lield micrograph showed that the oxide was not at grain or twin boundaries but was rather in the form of a skin over the Ge grains (Figs. 4(d) and 4(e)). Using the electron microprobe technique, it was found that the oxygen present on the surface of the films after room temperature exposure was bonded Thin Solid Films, 11 (1972) 415-422
420
I~. 1. AI.I'SSANI)RINh R. J. GAMBIN(). J. I). K[?P'FSIS
,,I)
(hi
Ic)
Fig. 4. Transmission electron diffraclion and electron rnicrograph of (;e films annealed at 700 ( al)er transformation. (a) Diffraction pattern, series one film: Ib) bright field micrograph: (el diffraction pattern series two tilm -arrow indicates (.;eO2 reflection: (d) bright tiekt micrograph: ( e ) t h e dark tield showing brightness over Ge ~rain when a weak (;cO 2 reflection i~ used lk)r image formation.
differently from that lormed after the high temperature h~;at trcatment. The thickest annealed and tmannealed films ( ~ 260 A) were evaluated for the presence o f oxygen. Oxygen K.~ radiation was detected only in the annealed film. Considering the magnitude o f the oxygen K7 cot, nts (after background correctionS. Thin 5,dtd kTlm.s. I ] I 1972) 4 [ 5 4_.'~"
CRYSTALLINE TRANSFORMATION IN AMORPHOUS
Ge
FILMS
421
oxygen was found to be a major constituent of the film. Visible cathodoluminescence was also observed from the annealed film and not in the case of the unannealed specimen. Compounds in particular oxides and sulfides are known to exhibit cathodoluminescence 9. Annealing the aged films apparently resulted in a change in the valence condition of the Ge to one which can be associated with compound formation. The chemical state of the Ge can be investigated by techniques such as ESCA (Electron Spectroscopy for Chemical Analysis), and it is the authors' intention to report on such a study in the near future. The sequence from an amorphous to a crystalline structure observed in both types of films has indicated that there is no justification for assuming a thickness dependence, in the range investigated here, as a determining factor for transformation. Work by other investigators 1° has shown that contamination as well as varying the vacuum pressure for film preparation can result in a change. In the work presented here, it should be noted that all films in a varying thickness series were made at the same time. In this way, the impurity content is not a factor for consideration. Surface contamination did result when films were aged in air, but this aging process did not affect the crystalline transformation temperature of any of the amorphous films. It was only when films were heated above the Ge crystallization temperature and where reaction of surface oxygen It could occur that its effect became evident. Then tetragonal GeO 2 as well as Ge appeared in the diffraction patterns. The intensity of the GeO2 reflections was found to vary with film thicknesses. As the Ge became thinner the oxide was present in a greater ratio as evidenced by an increase in the intensity of GeO 2 reflections and a decrease in those of Ge. There is evidence in the literaturC 2 that voids exist in most vapor deposited Ge films. We did not investigate void structure as the emphasis was on an electron diffraction study of crystallization. Diffraction patterns from the as deposited films did not resolve voids that might be present because of the inelastic scattering of electrons. Our results, however, did show that the oxide forms on the surface of the aged films. This suggests that any void structure, which may have been present, was closed to oxygen permeation. In summary, our work shows that thickness does not change the amorphous to crystalline transformation in Ge films provided that the films are prepared during the same evaporation. Surface contamination results when films are aged in an air ambient but does not alter the crystalline transformation temperature within experimental accuracy. Aging does significantly influence the film structure at higher temperatures. It is then that GeO2 appears as a second phase on the crystalline Ge film surface.
Thin Solid Films. 11 (1972) 415-422
422
E. 1. A I , E S S A N D R I N I , R. J. GAMBINO. J. D. KUPTSIS
ACKNOWLEDGEMENT
The authors are indebted to Mrs. S. Herd for the electron microscopy in this study. REFERENCES
I S . R . OVSlIINSKY. Ph.vs. Rer. Letters, 21 (1968) 1450. Z H. FRITZSCH• A.XI:.S. R. OVSH~NSKY.J. Non-('r.vst. Solids. 2 (197(I) 148. 3 T. TAKAMOm, R. ROY AYD G. J, McCARTllY. Mater. Res. Bull., 5 {1970) 529. 4 K . L . CHOPRA AND S. K. BAHI., J. Appl. Phys.. 40 (1969} 4174. 5 Y.P. GUPTA. J. Non-('ryst. Solids, 3 (197(I) 148. 6 N. V. GtTDKOVA. T. A. ZEVt~K~,. 1". N. MI~t)V~:IZ.EVA, R. A. Rtq3~,'SOtA AY,,I) T. N. S~RIZHI:','A. Soviet Phys. Crystal.. 15 (19711 747. 7 R. RoY. J. Non-Crvst. Solid~. 3 (1970) 33. 8 S. MkBER, Multiple Twinning and Pentagonal Structures in Germanium, IBM Res. Rept. )989. August 5. 1970. 9 J . D . BROWy. Fifty Years of Progress in Metallographic Techniques. AS'IM, .S'pec. l'ech. Pub/. No. 430 (1968) 374. 10 S.R. HERD. P. CHAt;DHAm AYD M. H. BRODSK¥. Metal Contact Induced Crystallization in Films of Amorphous Silicon and Germanium. I B M Res. Rept. 3666, December 28, 1~-~71,to be published in J. Non-Crvst. Solids. I 1 W. ROMANOWSKIAND D. Po'roczNA-PI-rRL'. Th#1 Solid Films. 8 ( 1971 ) 35. 12 T . M . DONOVANAND K. HEINEMANN. Phys. Rev. Letters. 27i19711 1794.
Thin Solid Films, 11 (1972) 415 422