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Crystal structure of evaporated yttrium thin films
Journal of Crystal Growth 113 (1991) 181-185 North-Holland
181
Crystal structure of evaporated yttrium thin films C. Ferrater, A. Lousa Departament de Fisica Aplicada I Electrôn,ca, Universitat de Barcelona, Avda. Diagonal 647, E-08028 Barcelona, Spain
F. Badia Departament de FIsica Fonamental, Universitat de Barcelona, Avda. Diagonal 647, E-08028 Barcelona, Spain
X. Alcobé Serveis Cientlfico-Tecnics, Universitat de Barcelona, Avda. Mar11 i Franquès s/n, E-08028 Barcelona. Spain
and J.L. Morenza Departarnent de Fisica Aplicada i Electrônica Universitat de Barcelona, Avda. Diagonal 647, E-08028 Barcelona, Spain
Received 1 December 1990
The crystal structure of evaporated yttrium thin films has been studied using X-ray diffractometry. The structure of the films is strongly dependent on substrate temperature and thickness. Films are in most cases polymorphic, and other structures different from the yttrium bulk hcp have been observed. fcc crystallites appear only at high growth temperatures. A crystallographic ordering different from the simple fcc and hcp structures is dominant at low growth temperatures and even for very thin films grown at high temperatures.
1. Introduction Thin films may be obtained in crystalline forms other than those occurring in bulk. Y [1] and other rare earth metals like Tm, Tb, Dy, Ho, Er [2] and Gd [3], which exhibit hcp structure in the bulk form, may show fcc structure in thin film form. Both hcp and fcc structures are built from the successive stacking of close-packed atomic planes, but with different pile sequences. The pile sequence is ABABAB. in the hcp, and ABCABC. in the fcc structure [41.If the bonding energy only depends on the nearest neighbors, no significative energetic difference between both structures is expected. In recent years, yttrium and other rare earth . .
. .
0022-0248/91/503.50 © 1991
—
Elsevier Science ~blishers
metals have been frequently used in compositionally modulated thin films [5—8].If superlattice effects are not present, the X-ray diffractometer scans reveal the crystal structure of each separate material. A suitable knowledge of these structures is needed to evaluate the possible multilayer effects on the structure. However, the crystal structure of evaporated yttrium thin films is not well known. It has been reported that yttrium films 60 nm thick show hcp structure, while films 30 nm thick show both hcp and fcc structure [1]. A transformation from the hcp to the fcc structure with decreasing the composition modulation wavelength has been reported [6] for Cu—Y multilayers. However, a more systematic work is needed to establish how the experimental conditions de-
RV. (North-Holland)
I (2
( ~‘rritter
ci
of,
(‘TV oaf strucTure of ei’a~iorciteclvitriwfrl
liii,, filou
lermine the crystal structure of thin and very thin yttrium films. This knowledge would be useful to understand the observed yttrium peaks in multilayered structures, Recently, we have reported a preferred orientalion in the hcp (002) direction for yttrium in Fe— Y multilayers [8]. In the present work, we present the results of our study on the influence of thickness and growth temperature on the crystal structure of yttrium thin films. Deposition conditions that favor the formation of the hcp and fcc structures are established, and a staking sequence different from the typical hcp and fcc structures is deduced for very thin films and films grown at low temperature.
All of the peaks observed in the X-ray diffraction patterns are placed between 27° and 32°. and only second order reflections appear at higher angles. X-ray results of the saniples grown at 300°(’ with different thickness are shown in fig. 1. A different crystalline configuration is observed depending on I’ilm thickness. Films thicker than 60 nrn (curves A-- D) show four peaks. Three of them correspond to 100, 002 and 101 reflections of the well-known hcp yttrium hulk structure. ‘T’he relative intensities of these three peaks differ from those tabulated for powder yttrium, in which 101 is the most intense peak. The 002 hcp is the dominant peak in these samples. and the relative intensities of the other hexagonal peaks increase with increasing film thickness. The interplanar
2. Experimental
spacing corresponding to the peak between 100 and 002 hcp reflections is 0.305 nrn. and may he attributed to Ill fcc yttrium reflection [61. The relative intensity of this peak decreases with increasing film thickness. Full widths at half maximum (FWHM) of the rocking curves of these four peaks are between 4.9° and 9.2°, indicating that fcc crystallites are distributed around an unique
The films have been prepared by electron beam evaporation in a high vacuum. The evaporation rate and the thickness were controlled in situ by a cooled quartz crystal. Borosilicate glass substrates were placed 20 cm above the evaporation source, and heated by tungsten lamps. A slide shutter allows one to obtain, in the same evaporation. samples with different thickness. The deposition conditions were: pressure during evaporation 3 >< 10 mbar. substrate temperatures between 140 and 300°C. and deposition rate 0.2 nm/s. Films between 30 and 400 nni thick were deposited and covered in situ at room temperature with a thin
powder X-ray diffractometer, using Cu K~radiation, by means of 0—26 and rocking curve scans.
3. Results The films observed with the scanning electron microscope showed a smooth surface. The sample
tilting (~70 ) revealed the granular morphology of the films, with grain sizes between 70 and flO nrn. which increase with increasing film thickness.
10
-
(oo~)
10
10
~
TA l~1
~J\~J
~
~
10 10
20
25 .
(deg)
Fig. 1. X-ray diffractometer 9 ---29 scans corresponding to films . of differeni thickness grown at the same substrate icmperature (30000.). (A) 400 nm: (B) 240 on,: ((~)120 nm; (D( (1) nm: (F) 30 no’.
C. Ferrater et al.
/ Crystal structure
crystallographic direction (single textured), and
that hcp crystallites are distributed neither randomly nor with a single texture, but close to one of the three crystallographic directions with the lowest Miller indices. The film 30 nm thick grown at 300°C shows a qualitatively different 0—26 scan (curve E). The 100 and the 101 hcp reflections are not observed,
the similar 002 hcp and and the a new 111 reflection fcc reflections 0.300 have nmrecrystallization obtained athat rises theintensities structure between some weeks them. of process this later Moreover, film (fig. had was 2, taken curve metastable theatplace: 0—26 B)d =showed scan and the
intensity of the peak at d= 0.300 nm had increased and the 002 hcp reflection had disappeared, in such a way that the integrated intensity had been kept constant. Besides the thickness effect, there is also an important effect of growth temperature on films structure. Fig. 3 shows the 6—20 scans corresponding to samples 400 nm thick grown at different temperatures. It is observed that the 111 fcc reflection appears only at the two higher growth temperatures (curves A and B). At the lower growth temperatures, the reflection at d 0.300 =
183
ofevaporated yttrium thin films
10’
-___________________________________
(~)
10
-
-
>..~J
.4—’ —
10 2~‘ -10 102 102,
25
A (~I)(~)
A B C D
30
29 (deg)
35
Fig. 3. X-ray diffractometer 0—20 scans corresponding to films of the same thickness (400 nm) grown at different substrate temperatures: (A) 300°C:(B) 280°C:(C) 160°C:(D) 140°C.
nm emerges (curves C and D) and becomes dominant at sufficient low temperature (~140°C). With increasing growth temperature and film thickness, a shift of the hcp Y 002 peak towards higher scattering angles is observed (figs. I and 3), reducing the c parameter towards the bulk value.
On the contrary, any significative thickness dependence of (111) fcc interplanar spacing is observed.
4. Discussion 0.
X-ray diffraction patterns show that yttrium films are polycrystalline. Reflections at d 0.305 nm and d 0.300 nm, observed in some films, indicate the presence of other structures different from the hcp bulk yttrium. First, we will discuss the origin of these two reflections. From the analysis of the curves in fig. 3, it can be deduced that peaks at d 0.305 nm and d =
=
>5
ii)
Q) C 102.
=
=
0.300 nm have not originated by possible Y—Al 10
compounds at the interface. All these patterns
2
25
30
35
29 (deg) Fig. 2. X-ray diffractometer 0—20 scans corresponding to the same sample, 30 nm thick, grown at 300 ° C substrate temperature. obtained (A) immediately after deposition and (B) three weeks later,
correspond to samples that were covered with the same aluminum thickness at room temperature, so whatever interdiffusion effect would be the same for all of them. For this reason, any of the observed differences in the diffraction patterns can-
TI
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ii
a a
I (4
(
l”irratt’r ef a!.
,
( rittal virutlurt’ of c’~aporaii’tI1110 oil 1/lit? ti/no
not he attributed to possible phenomena at the interface, The peak at ci = 0.305 nm may he attributed to Ill fcc yttrium, with a lattice parameter a = ().528 nm. This value is 2.5% higher than the theoretical one calculated on the basis of the hcp hulk struclure, assuming the same distance between the nearest neighbors in a close-packed plane. On the other hand, this value is lower than the one ohtamed in yttrium thin films deposited on formvard coated copper supporting grids [1] and slightl\ higher than the one obtained in Cu—Y multilayers [61. The peak at ci 0.300 nm observed in filnis grown at low temperature (fig. 3) and in the thinner film grown at high temperature (figs. I and 2). has also been observed in Fe—Y multilayered films [8], and was initially indexed as a 002 hcp reflection. From the present results it seems clear that this was not strictly correct. The position of the peak, between two near reflections caused by close-packed planes ((111) fcc and (002) hcp). suggests that it also corresponds to a closepacked planes stacking structure. Unfortunately, the absence of other reflections belonging to the same structure does not allow us to obtain more details about this structure. Nevertheless, our recent studies on powdered Fe—Y multilayers [9] indicate that this reflection may be considered as the Ill reflection of a set of fcc reflections, or as the 002 reflection of a set of hcp reflections, revealing the polytypic character of these stacking sequences. Froni the previous discussion, a structural evolution in the growing process can he deduced. In the early stages, yttrium grows by the stacking of close-packed planes (hcp, fcc and polytypic crystallites in 300°C films, and hcp and polytypic crystallites in 150°C films), in agreement with the general behavior of metals with hcp or fcc bulk structures when evaporated onto amorphous substrates [10]. But depending oii growth temperature. there is a different structure evolution in continuing the growing process. If the growth temperature is low enough (~150°C). 121 there are no significative structural changes. and the polytypic stacking remains dominant even for films 400 nm thick. If the growth temperature is
higher ( 300°(‘>. the hcp phase is unstable at the early stages. and if the growing process stops. hcp crystallites change to the polytypic phase. As film growth progresses, two remarkable phenomena (ICcur: the recrystallization of the initially grown polytype to a more ordered structure, and the growth of (111) textured fcc and (100) and (101) textured hcp crvstallites. that coexist with the doniinant (002) textured hcp structure.
5. Conclusion X-ray’ diffraction stLidies carried out on evaporated yttrium thin films deposited onto glass substrates showed tli~ the film structure depends on thickness and growth temperature. and may join three types of crystallites: hcp. fcc and those with a polytype structure, (002) textured hcp phase appeared in all of the films, and in most of them was the main one. (100) and (101) textured hcp. not observed in the thinner films (~30 nm), and (Ill) textured fcc crystallites appeared only’ in films grown at high enough temperature ( 300 cc). Polytypic crystallites with close-packed planes parallel to the substrate were characteristic of films with a low degree of crvstallinity, i.e.. very thin or low temperature ( 150°C’) grown films. Likewise, a spontaneous recrystallization at room temperature of hcp yttrium to polytypic yttrium was observed in the thinner film (30 nm grown at 300°C.
Acknowledgement The authors would like to thank Professor X. Solans of the Department of Crystallography for helpful discussions.
References III \~.K.Kaul and 0.15. Srivastava, 1. Less-( amman ~
137. -
Metals
-
B.M.S. Bist and 0.15~Snvastava, .1. l,ess—( ‘iinin,oi, Melals 33(1973)
99.
l~lA.F. (‘urron
and HG. (‘hleheck, J. Less’(’omnion Meials
27 (1972) 411.
C. Ferrater cial.
/
Crystal structure of evaporated yttrium thinfilms
141 RU. Evans. An Introduction to Crystal Chemistry (Cam’ bridge University Press. Cambridge, 1979) p. 80. [51 PP. Freitas, A.A. Gomes. MM. Goudinho and T.S. Plaskett, J. Magnetism Magnetic Mater. 83 (1990) 116. [61 R.W. Johnson. CC. Ahn and ER. Ratner. Phys. Rev. B40 (1989) 8139. [7] R.W. Johnson, CC. Ahn and ER. Ratner, AppI. Phys. Letters 54(1989) 795.
185
[8] F. Badia. C. Ferrater, A. Lousa. B. Martinez, A. Labarta and J. Tejada. J. AppI. Phys. 67 (1990) 5652. [9] C. Ferrater, A. Lousa. F. Badia. J. Tejada and iL. Morenza, to be published. 1101 L. Eckertová, Physics of Thin Films (Plenum, New York, 1986) p. 128.
~ II II
a a a a
a a a
a a
181
Crystal structure of evaporated yttrium thin films C. Ferrater, A. Lousa Departament de Fisica Aplicada I Electrôn,ca, Universitat de Barcelona, Avda. Diagonal 647, E-08028 Barcelona, Spain
F. Badia Departament de FIsica Fonamental, Universitat de Barcelona, Avda. Diagonal 647, E-08028 Barcelona, Spain
X. Alcobé Serveis Cientlfico-Tecnics, Universitat de Barcelona, Avda. Mar11 i Franquès s/n, E-08028 Barcelona. Spain
and J.L. Morenza Departarnent de Fisica Aplicada i Electrônica Universitat de Barcelona, Avda. Diagonal 647, E-08028 Barcelona, Spain
Received 1 December 1990
The crystal structure of evaporated yttrium thin films has been studied using X-ray diffractometry. The structure of the films is strongly dependent on substrate temperature and thickness. Films are in most cases polymorphic, and other structures different from the yttrium bulk hcp have been observed. fcc crystallites appear only at high growth temperatures. A crystallographic ordering different from the simple fcc and hcp structures is dominant at low growth temperatures and even for very thin films grown at high temperatures.
1. Introduction Thin films may be obtained in crystalline forms other than those occurring in bulk. Y [1] and other rare earth metals like Tm, Tb, Dy, Ho, Er [2] and Gd [3], which exhibit hcp structure in the bulk form, may show fcc structure in thin film form. Both hcp and fcc structures are built from the successive stacking of close-packed atomic planes, but with different pile sequences. The pile sequence is ABABAB. in the hcp, and ABCABC. in the fcc structure [41.If the bonding energy only depends on the nearest neighbors, no significative energetic difference between both structures is expected. In recent years, yttrium and other rare earth . .
. .
0022-0248/91/503.50 © 1991
—
Elsevier Science ~blishers
metals have been frequently used in compositionally modulated thin films [5—8].If superlattice effects are not present, the X-ray diffractometer scans reveal the crystal structure of each separate material. A suitable knowledge of these structures is needed to evaluate the possible multilayer effects on the structure. However, the crystal structure of evaporated yttrium thin films is not well known. It has been reported that yttrium films 60 nm thick show hcp structure, while films 30 nm thick show both hcp and fcc structure [1]. A transformation from the hcp to the fcc structure with decreasing the composition modulation wavelength has been reported [6] for Cu—Y multilayers. However, a more systematic work is needed to establish how the experimental conditions de-
RV. (North-Holland)
I (2
( ~‘rritter
ci
of,
(‘TV oaf strucTure of ei’a~iorciteclvitriwfrl
liii,, filou
lermine the crystal structure of thin and very thin yttrium films. This knowledge would be useful to understand the observed yttrium peaks in multilayered structures, Recently, we have reported a preferred orientalion in the hcp (002) direction for yttrium in Fe— Y multilayers [8]. In the present work, we present the results of our study on the influence of thickness and growth temperature on the crystal structure of yttrium thin films. Deposition conditions that favor the formation of the hcp and fcc structures are established, and a staking sequence different from the typical hcp and fcc structures is deduced for very thin films and films grown at low temperature.
All of the peaks observed in the X-ray diffraction patterns are placed between 27° and 32°. and only second order reflections appear at higher angles. X-ray results of the saniples grown at 300°(’ with different thickness are shown in fig. 1. A different crystalline configuration is observed depending on I’ilm thickness. Films thicker than 60 nrn (curves A-- D) show four peaks. Three of them correspond to 100, 002 and 101 reflections of the well-known hcp yttrium hulk structure. ‘T’he relative intensities of these three peaks differ from those tabulated for powder yttrium, in which 101 is the most intense peak. The 002 hcp is the dominant peak in these samples. and the relative intensities of the other hexagonal peaks increase with increasing film thickness. The interplanar
2. Experimental
spacing corresponding to the peak between 100 and 002 hcp reflections is 0.305 nrn. and may he attributed to Ill fcc yttrium reflection [61. The relative intensity of this peak decreases with increasing film thickness. Full widths at half maximum (FWHM) of the rocking curves of these four peaks are between 4.9° and 9.2°, indicating that fcc crystallites are distributed around an unique
The films have been prepared by electron beam evaporation in a high vacuum. The evaporation rate and the thickness were controlled in situ by a cooled quartz crystal. Borosilicate glass substrates were placed 20 cm above the evaporation source, and heated by tungsten lamps. A slide shutter allows one to obtain, in the same evaporation. samples with different thickness. The deposition conditions were: pressure during evaporation 3 >< 10 mbar. substrate temperatures between 140 and 300°C. and deposition rate 0.2 nm/s. Films between 30 and 400 nni thick were deposited and covered in situ at room temperature with a thin
powder X-ray diffractometer, using Cu K~radiation, by means of 0—26 and rocking curve scans.
3. Results The films observed with the scanning electron microscope showed a smooth surface. The sample
tilting (~70 ) revealed the granular morphology of the films, with grain sizes between 70 and flO nrn. which increase with increasing film thickness.
10
-
(oo~)
10
10
~
TA l~1
~J\~J
~
~
10 10
20
25 .
(deg)
Fig. 1. X-ray diffractometer 9 ---29 scans corresponding to films . of differeni thickness grown at the same substrate icmperature (30000.). (A) 400 nm: (B) 240 on,: ((~)120 nm; (D( (1) nm: (F) 30 no’.
C. Ferrater et al.
/ Crystal structure
crystallographic direction (single textured), and
that hcp crystallites are distributed neither randomly nor with a single texture, but close to one of the three crystallographic directions with the lowest Miller indices. The film 30 nm thick grown at 300°C shows a qualitatively different 0—26 scan (curve E). The 100 and the 101 hcp reflections are not observed,
the similar 002 hcp and and the a new 111 reflection fcc reflections 0.300 have nmrecrystallization obtained athat rises theintensities structure between some weeks them. of process this later Moreover, film (fig. had was 2, taken curve metastable theatplace: 0—26 B)d =showed scan and the
intensity of the peak at d= 0.300 nm had increased and the 002 hcp reflection had disappeared, in such a way that the integrated intensity had been kept constant. Besides the thickness effect, there is also an important effect of growth temperature on films structure. Fig. 3 shows the 6—20 scans corresponding to samples 400 nm thick grown at different temperatures. It is observed that the 111 fcc reflection appears only at the two higher growth temperatures (curves A and B). At the lower growth temperatures, the reflection at d 0.300 =
183
ofevaporated yttrium thin films
10’
-___________________________________
(~)
10
-
-
>..~J
.4—’ —
10 2~‘ -10 102 102,
25
A (~I)(~)
A B C D
30
29 (deg)
35
Fig. 3. X-ray diffractometer 0—20 scans corresponding to films of the same thickness (400 nm) grown at different substrate temperatures: (A) 300°C:(B) 280°C:(C) 160°C:(D) 140°C.
nm emerges (curves C and D) and becomes dominant at sufficient low temperature (~140°C). With increasing growth temperature and film thickness, a shift of the hcp Y 002 peak towards higher scattering angles is observed (figs. I and 3), reducing the c parameter towards the bulk value.
On the contrary, any significative thickness dependence of (111) fcc interplanar spacing is observed.
4. Discussion 0.
X-ray diffraction patterns show that yttrium films are polycrystalline. Reflections at d 0.305 nm and d 0.300 nm, observed in some films, indicate the presence of other structures different from the hcp bulk yttrium. First, we will discuss the origin of these two reflections. From the analysis of the curves in fig. 3, it can be deduced that peaks at d 0.305 nm and d =
=
>5
ii)
Q) C 102.
=
=
0.300 nm have not originated by possible Y—Al 10
compounds at the interface. All these patterns
2
25
30
35
29 (deg) Fig. 2. X-ray diffractometer 0—20 scans corresponding to the same sample, 30 nm thick, grown at 300 ° C substrate temperature. obtained (A) immediately after deposition and (B) three weeks later,
correspond to samples that were covered with the same aluminum thickness at room temperature, so whatever interdiffusion effect would be the same for all of them. For this reason, any of the observed differences in the diffraction patterns can-
TI
aa aa a a a a a a a a a a a a
ii
a a
I (4
(
l”irratt’r ef a!.
,
( rittal virutlurt’ of c’~aporaii’tI1110 oil 1/lit? ti/no
not he attributed to possible phenomena at the interface, The peak at ci = 0.305 nm may he attributed to Ill fcc yttrium, with a lattice parameter a = ().528 nm. This value is 2.5% higher than the theoretical one calculated on the basis of the hcp hulk struclure, assuming the same distance between the nearest neighbors in a close-packed plane. On the other hand, this value is lower than the one ohtamed in yttrium thin films deposited on formvard coated copper supporting grids [1] and slightl\ higher than the one obtained in Cu—Y multilayers [61. The peak at ci 0.300 nm observed in filnis grown at low temperature (fig. 3) and in the thinner film grown at high temperature (figs. I and 2). has also been observed in Fe—Y multilayered films [8], and was initially indexed as a 002 hcp reflection. From the present results it seems clear that this was not strictly correct. The position of the peak, between two near reflections caused by close-packed planes ((111) fcc and (002) hcp). suggests that it also corresponds to a closepacked planes stacking structure. Unfortunately, the absence of other reflections belonging to the same structure does not allow us to obtain more details about this structure. Nevertheless, our recent studies on powdered Fe—Y multilayers [9] indicate that this reflection may be considered as the Ill reflection of a set of fcc reflections, or as the 002 reflection of a set of hcp reflections, revealing the polytypic character of these stacking sequences. Froni the previous discussion, a structural evolution in the growing process can he deduced. In the early stages, yttrium grows by the stacking of close-packed planes (hcp, fcc and polytypic crystallites in 300°C films, and hcp and polytypic crystallites in 150°C films), in agreement with the general behavior of metals with hcp or fcc bulk structures when evaporated onto amorphous substrates [10]. But depending oii growth temperature. there is a different structure evolution in continuing the growing process. If the growth temperature is low enough (~150°C). 121 there are no significative structural changes. and the polytypic stacking remains dominant even for films 400 nm thick. If the growth temperature is
higher ( 300°(‘>. the hcp phase is unstable at the early stages. and if the growing process stops. hcp crystallites change to the polytypic phase. As film growth progresses, two remarkable phenomena (ICcur: the recrystallization of the initially grown polytype to a more ordered structure, and the growth of (111) textured fcc and (100) and (101) textured hcp crvstallites. that coexist with the doniinant (002) textured hcp structure.
5. Conclusion X-ray’ diffraction stLidies carried out on evaporated yttrium thin films deposited onto glass substrates showed tli~ the film structure depends on thickness and growth temperature. and may join three types of crystallites: hcp. fcc and those with a polytype structure, (002) textured hcp phase appeared in all of the films, and in most of them was the main one. (100) and (101) textured hcp. not observed in the thinner films (~30 nm), and (Ill) textured fcc crystallites appeared only’ in films grown at high enough temperature ( 300 cc). Polytypic crystallites with close-packed planes parallel to the substrate were characteristic of films with a low degree of crvstallinity, i.e.. very thin or low temperature ( 150°C’) grown films. Likewise, a spontaneous recrystallization at room temperature of hcp yttrium to polytypic yttrium was observed in the thinner film (30 nm grown at 300°C.
Acknowledgement The authors would like to thank Professor X. Solans of the Department of Crystallography for helpful discussions.
References III \~.K.Kaul and 0.15. Srivastava, 1. Less-( amman ~
137. -
Metals
-
B.M.S. Bist and 0.15~Snvastava, .1. l,ess—( ‘iinin,oi, Melals 33(1973)
99.
l~lA.F. (‘urron
and HG. (‘hleheck, J. Less’(’omnion Meials
27 (1972) 411.
C. Ferrater cial.
/
Crystal structure of evaporated yttrium thinfilms
141 RU. Evans. An Introduction to Crystal Chemistry (Cam’ bridge University Press. Cambridge, 1979) p. 80. [51 PP. Freitas, A.A. Gomes. MM. Goudinho and T.S. Plaskett, J. Magnetism Magnetic Mater. 83 (1990) 116. [61 R.W. Johnson. CC. Ahn and ER. Ratner. Phys. Rev. B40 (1989) 8139. [7] R.W. Johnson, CC. Ahn and ER. Ratner, AppI. Phys. Letters 54(1989) 795.
185
[8] F. Badia. C. Ferrater, A. Lousa. B. Martinez, A. Labarta and J. Tejada. J. AppI. Phys. 67 (1990) 5652. [9] C. Ferrater, A. Lousa. F. Badia. J. Tejada and iL. Morenza, to be published. 1101 L. Eckertová, Physics of Thin Films (Plenum, New York, 1986) p. 128.
~ II II
a a a a
a a a
a a