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Growth of C60 thin films on mica by hot wall epitaxy
,. . . . . . . .
ELSEVIER
CRYSTAL GROWTH
Journal of Crystal Growth 156 (1995) 79-85
Growth of
C60 thin films on mica by hot wall
epitaxy
D. Stifter *, H. Sitter Institut f~r Experimentalphysik, Abteihng Festk&perphysik, Johannes Kepler Universitiit Linz, A-4040 Linz, Austria
Received 8 March 1995; manuscript received in final form 12 May 1995
Abstract
Thin C60 films were grown on mica by hot wall epitaxy. The influence of the individual growth parameters, like substrate, wall, and source temperature, on the surface morphology and crystalline quality of the C60 films was investigated using atomic force microscopy and high resolution X-ray diffraction. Under ideal growth conditions the full width at half maximum (fwhm) of the Ca)-(lll) X-ray rocking curve is about 170 arcsec indicating a more or less perfect monocrystalline growth of the C60 layer.
1. I n t r o d u c t i o n
The growth of thin C60 films on various substrate materials is object of several studies investigating the fundamental processes, as well as determining and optimizing the growth parameters. The existence of aligned {111} C60 planes parallel to the substrate surface could be proven especially for layered substrate materials, like mica or MoS z [1-8], Particularly mica is a promising material for the epitaxial growth of C60 with a lattice mismatch of 3.4% [1,2]. High resolution transmission electron microscopy studies [1,2] showed that a substrate temperature of 100°C is the onset of well orientated crystallographic growth of C60. The early stages of growth were investigated by atomic force microscopy studies [4,5] pointing out that growth starts with islands with the bare mica substrate in between. If the growth is continued
* Corresponding author.
new islands are formed until the surface is totally covered by C6o building extended flat terraces for the further growth [5]. In addition to atomic force and transmission electron microscopy only few studies were made using X-ray diffraction for the characterization of thin C60 films [6,7,9,14]. The full width at half maximum (fwhm) Aco of the (111)-C60 reflex in to direction (i.e. co-scan or rocking curve) serves as a criterion for the crystalline quality of the C6o epitaxial layer. So far values of fwhm have been reported varying from A~o = 2300 arcsec for 250 nm thick films grown on antimony [9] to Aco = 1100 and 720 arcsec for layers with a thickness of 200 and 250 nm grown on mica [6,7]. An 8 ~ m C60 film on mica showed a rocking curve fwhm of 3000 arcsec, which was grown in a system with an effusion cell producing a stable C6o flux of up to 2 , ~ / s using nitrogen as a transport gas from the source to the substrate [11]. The influence of the substrate temperature and the growth rate on twin formation and epitaxial orientation of grains in C6o films was studied measuring X-ray pole figures [12].
0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI I)022-0248(95)00250-2
80
D. Stifter, H. Sitter~Journal of Crystal Growth 156 (1995) 79-85
The applied growth methods varied from vapor deposition in high vacuum [2,7] to molecular beam epitaxy (MBE) and organic MBE [8-10]. In order to enhance the epitaxial ordering of the C6o layer a hot wall deposition was proposed [6]. We have grown C60 epilayers by an authentic hot wall epitaxy (HWE) setup, as defined by Otero [13]. The C6o films were characterized using high resolution X-ray diffraction, pointing out that the growth of 120 nm thick films with a fwhm Aoj of 210 arcsec is possible [14]. For thicker films the rocking curve of the C60-(111) reflex exhibited a more complex shape. This behavior can be explained by assuming a critical thickness for the C60 films. For films exceeding this thickness the growth gets worse with a mosaic spread of 0.9 °. It can be shown that the critical thickness depends on the substrate temperature and decreases with increasing growth temperature. In addition we present a set of experiments of C60 films grown on mica with the H W E system mentioned above, varying also the wall and source temperature.
MICA- SUBS'IRATE
TEMPERATURE PROFILE
'1 1 rll
0 --
/ QUARTZ TUBE
Tst~r.
Tsot~-BTwAtL
TEMPERATURE
2. Experimental procedure and setup
Fig. 1. Schematic sketch of our H W E system and the temperature profile caused by the three separate heaters. The C60 source material is evaporated at the bottom of the quartz tube and condenses on the mica substrate placed in the upper region of the tube.
The H W E system we used is schematically depicted in Fig. 1. It contains in contrast to other high vacuum growth systems, like MBE, an almost closed reactor. So it can be operated close to thermodynamic equilibrium, which is most important for van der Waals epitaxy. A quartz tube, with the source material at the bottom and the substrate on the top closing it tight with respect to the mean free path of the evaporated source molecules, is placed with three separated heaters into a high vacuum chamber. The region between source and substrate, called hot wall, guarantees a nearly uniform and isotropic flux of the molecules onto the substrate surface [15]. The advantage of such a system is the minimization of the loss of source material, what allows to obtain high growth rates at comparably high substrate temperatures. A separate oven for preheating of the substrates is also mounted in the vacuum chamber.
The C60 source material was at first 99.4% pure. It had to be cleaned from solvents and impurities by subliming the material three times at 550°C under dynamical vacuum of 1 x 10 -6 mbar and by protecting it from visible light in order to prohibit photoinduced polymerization of the C60 molecules. About 200 mg of the cleaned material was loaded into the H W E system, which was enough to fabricate about 50 epilayers with an area of 1 cm 2 and an average thickness of 200 nm. As substrate material mica sheets were cut into pieces (15 x 15 mm2), cleaved in air with an adhesive tape and immediately transferred into the vacuum chamber of the H W E system. The substrates, before being loaded into the growth reactor, were preheated for one hour at 400°C in the separate oven to remove adhesives on the substrate surface. In our experiments the substrate temperature was varied from 100°C to
D. Stifter, H. Sitter/Journal of Crystal Growth 156 (1995) 79-85
200°C, the wall temperature from 340°C to 440°C and the source temperature from 360°C to 440°C. The residual gas pressure outside the growth reactor was 1 × 10 - 7 mbar when all ovens were heated. The growth time was varied from 1 to 5 h in order to obtain films of different thickness. The thickness of the films was measured afterwards with a mechanical surface profilometer by scanning the height of the step between the film and the region of the substrate which was covered by the substrate holder during growth. In order to measure the crystalline quality of the C60 films, high resolution X-ray diffraction (HRXD) was used taking the full width at half maximum (fwhm) of the rocking curve (to-scan) of the C60-(111) crystal reflex, i.e. measurements with the X-ray detector fixed at a Bragg angle 20 of a crystal reflex and varying the angle to, which is the angle between the incident beam and the sample surface. The fwhm of the rocking curve is sensitive to tilted grains and mosaic blocks, misfit dislocations and on lateral structures and surface corrugations [16]. The advantage of HRXD is, that it is a non-destructive method and takes a large region of the crystal (several mm 2) into account, compared to other methods like transmission electron microscopy. Our HRXD measurements were performed on a Philips high resolution diffractometer using Cu K a radiation filtered by a Ge four-crystal monochromator, and a 0.2 ° aperture slit mounted in front of the detector. The resolution in to direction (measuring at angles: to < 7° and 20 < 11°) was determined to be less then 20 arcsec. The scans were typically carried out with 11 arcsec stepwidth and an integration time up to one minute per step. Reciprocal space maps were measured by taking to-scans at different to/20 starting angles, scanning the reciprocal space in the region of the C60-(111) peak. The resolution in to/20 direction is more or less limited by the 0.2° aperture slit due to the orientation and size of our scanning probe in this region of the reciprocal space. Principles and details of reciprocal space mapping can be found elsewhere [17]. The surface morphology of the C60 films was studied using an atomic force microscope (Digital Instruments Nanoscope III) with a S i 3 N 4 needle.
81
The force exerted by the needle was about 100 nN.
3. Results and discussion
In a first set of experiments the source temperature was varied from 360°C to 440°C at a fixed substrate temperature of 140°C and a wall temperature of 400°C. The resulting growth rates are plotted in Fig. 2 on a logarithmic scale versus the inverse source temperature in a so-called Arrhenius plot. The results show, that the growth rate is proportional to the effusion rate, indicating that the growth mechanism is limited by the source. The enthalpy for the evaporation of C60 molecules A Hvap can be calculated from this data by evaluating the slope of the fitting curve. The value resulting from our experiments for the activation energy E a for evaporation of one C60 molecule is 1.17 + 0.03 eV. Identifying EaNa (Na = 6.022 × 1023 mo1-1) with the enthalpy AHv~p, then AHv~p= 113+3 kJ/mol in contrast to AH~ap = 159 kJ/mol published by Abrefah et al. [18]. This discrepancy cannot be explained by the uncertainity in our measurements of the absolute source temperature of about + 10°C, but has to be the result of a different mechanism, like a different impurity concentration in the source material.
440°C'N~ 4200C N ~ _,= _c
400°C 380°C
o
360°C
0.1 0.0013
o o;~14
o obls 1.rl'source[1/K]
o.ob16
0.0017
Fig. 2. Arrhenius plot of the growth rate versus the applied source temperature. The enthalpy AHw,, calculated from the slope is 113 kJ/mol.
D. Stifter, H. Sitter/Journal of Crystal Growth 156 (1995) 79-85
82
0.35' 0.30'
~ i~J C6o (111) rocking curve
0.25,
<8 o.2o: -r 0.15J
/
-
/ °'.s 4:0 4:s s:o s:s 6:0 6:s 7.0
/
o.lo:
o.o5" o.oo
16o
lho
16o
260
Temperature [°C] Fig. 3. fwhm of the C6o-(111) rocking curve versus the applied substrate temperature. The insert presents a typical rocking curve of a sample grown at a substrate temperature of 160°C.
As to the crystalline quality of C60 films, source temperatures between 380°C and 420°C resulting in growth rates between 0.2 and 0.5 , ~ / s led to samples with the best crystalline quality as described below. The influence of the substrate temperature on the growth and crystalline quality of C60 films grown in a hot wall system was studied by varying the substrate temperature in the range from 100°C
,--, 3O
~)
~ 2s >,
(a)
to 200°C at a fixed source and wall temperature of 400°C. The fwhm of the C60-(111) rocking curve of films with a thickness of about 120 nm is plotted in Fig. 3 versus the applied substrate temperature. The insert of Fig. 3 presents a typical rocking curve of a sample grown at a substrate temperature of 160°C. Varying the temperature from 100° to 180°C, it can easily be seen that there is a flat minimum at about 140°C. The point at 200°C is the result of a dramatic change of the line shape of the rocking curve. A broad peak of 0.9 ° width occurs, with a narrow one sitting on top. This superposition of two peaks with different width can also be observed for all the other substrate temperatures, but at a larger film thickness. The evolution of the line shape as a function of the film thickness is depicted in Fig. 4. It is demonstrated that the broad peak appears between 80 and 160 nm and becomes dominant for films with a larger thickness. Splitting the rocking curve into two superimposed peals and measuring the width of each peak separately, it can be seen from Fig. 5, where the widths of the peaks are plotted versus the film thickness, that the fwhm taken only from the broad peak (full circles) has an approximately constant value of
,-, 3O
80 nm
W
2s
(b)
160 nm
._~ 2o
20
~ 15 c ~
e.o lo
10
5
5 I 0
.
i
.
i
•
i
.
.
.
.
.
.J .
•
3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 co [o]
3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
co [o1 ,~ 3O w
8" 25
,~ 3O
(c)
3 4 0 nm
W
2s
~ 2o
~ 2o
--
--
.o lo c
5
3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
[°]
5
0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 co [o]
Fig. 4. C6o-(11 l) rocking curves for layers grown at substrate temperatures around 160°C: The change of the shape with increasing film thickness is demonstrated.
D. Stifter, H. Sitter~Journal of Crystal Growth 156 (1995) 79-85 4000
Tsubstr.:
", 120°C
25
3500~' 3000-
-
83
~
\
104
I
8]
:, '1
=
\/
\
25002000- I 1500 •
-0---0---r"l--
1
broad peak narrow peak total peak
-
/ I
,~,,, 10. 140°C 00 ~ . xq~--
~
.0~
5-" 200°C O ~ • 0-
500•
2;0
a;o
•
4
5 -o-
:otJ
6
r"l ~
1000-
0 100
"-.•
,
4;o
i
s;o
600
0
~ " ~ w
0
o_J:'- ° 200
; 400
"------= 600
800
film thickness [nm]
film thickness [nm] Fig. 5. fwhm of the narrow peak (hollow circles), the broad peak (full circles) and of the total peak, which is the superposition of the two peaks (hollow squares).
i
i
' t = 240 nm
(a) 0.1 o
0.0 Ii
-0.1 I
'
I
-1.0
-1.5
-0.5
I
I
I
0.0
0.5
1.0
[o] '
(b)
1.5
Fig. 7. Ratio of the intensities 1c and ld, i.e. intensities of the narrow and broad peak respectively, as a function of the total film thickness.
about 3500 arcsec. This is also the case for the narrow peak with an fwhm of about 200 arcsec (hollow circles). The fwhm taken from the total peak, which is the superposition of the two peaks, exhibits a slope, indicating that the intensity of the broad peak increases with respect to the narrow one. In Figs. 6a and 6b reciprocal space maps measured on C60 films with a thickness of 240 and 1030 nm respectively are compared also demonstrating the evolution of the broad peak with regard to the narrow one sitting on top. In Fig. 6a, the narrow peak is more significant than the broad one. Its width is limited in to/20 direction by the width of the 0.2 ° receiving slit as already mentioned before. The width in to direc-
' t = 1630 nm
0.1
T
E 0.0
-0.1
-1.5
I
I
-1.0
-0.5
I
'
0.0
0.5
1.0
1.5
co [o] Fig. 6. Reciprocal space maps of the C6o-(111) reflex: (a) 240 nm thick sample, (b) 1030 nm thick sample. Separation between lines is 2 cps (a) and 3 cps (b) respectively. For these plots the maximum of the (111) reflex was taken as origin of the reciprocal space.
Fig. 8. Atomic force microscopy pictures: (a) surface of a 30 nm thick film below the critical thickness where the broad peak appears, exhibiting monomolecular terraces. (b) surface of a 500 nm thick film. Az values: (a) 15 nm, (b) 30 nm.
D. Stifter, H. Sitter/Journal of Crystal Growth 156 (1995) 79-85
84
tion is the width of the one-dimensional C60-(111) rocking curve. The slight rotation of the peak with respect to the axis is due to the orientation of the Ewaid sphere and scanning probe in this region of the reciprocal space. For the thicker C60 epilayer (Fig. 6b), the small peak is totally submerged by the broad one, which has a fwhm of about 1° in to direction. Splitting the total rocking curve into two superimposed peaks, as shown in the insert of Fig. 7, and calculating the intensity by taking the area under each separated peak, one can plot the ratio of these two intensities versus the film thickness for different substrate temperatures, as depicted in Fig. 7. I d is the intensity of the broad peak, whereas I~ is the intensity of the narrow one. It is demonstrated that for increasing substrate temperatures the ratio I c / I d is decreasing. For a fixed substrate temperature the ratio is also decreasing with increasing film thickness. This behavior could be explained and simulated by developing a simple phenomenological model [14]. According to this model, the film growth starts, after the initial island growth, in a more or less perfect mode. After exceeding a critical thickness t c, the growth mode changes and slightly tilted grains and mosaic blocks are formed during growth resulting in the enormous broadening of the rocking curve, whereas the perfect layer is almost undisturbed, originating the narrow peak superimposed on the broad one. The critical thickness t c depends on the applied substrate
temperature and decreases with increasing temperature. In Figs. 8a and 8b AFM pictures of the surfaces of a sample with a thickness below t c (Fig. 8a) and one of a film exceeding tc (Fig. 8b) are presented. The surface of the 30 nm thick film is covered by flat monomolecular terraces, whereas the surface of the second sample with a thickness of 500 nm exhibits a less ideal structure. A variation of the wall temperature from 340°C to 420°C at a fixed substrate and source temperature of 140°C and 400°C respectively, had no significant influence on the fwhm of the C6o films, but on the growtho rate. For 440°C the growth rate exceeds 0.8 A / s , which is due to a minimal condensation of C60 molecules on the wall. At such a high growth rate, the fwhm of the rocking curve is already influenced negatively, as it was observed for source temperatures over 420°C. In a last set of experiments the preheating time of the mica substrate was varied from 0 to 60 min, which led to no consequences regarding the crystalline quality of the samples. With the optimization of all the growth parameters described above a sample with the rocking curve depicted in Fig. 9 could be grown. The fwhm is about 170 arcsec, which is to our knowledge the smallest value reported so far in the literature for any C60 epilayer.
4. Conclusions Ceo (111) ¢o- Scan FWHM: 170 arcsec
70 60
¢,/)
50
Tsubstr.: Twall :
1400C 380°C
Tsource: 4O0°C growth rate: 0.48A/s film thickness: 13Onto
30 C
--.~ 2o e'-
L__
lO
o i
4.75
I
I
I
5.00
5.25
5.50 =
i
I
I
5.75
6.00
6.25
[°1
Fig. 9. R o c k i n g curve of a s a m p l e grown u n d e r ideal conditions showing a rocking curve fwhm of 170 arcsec.
We have shown that hot wall epitaxy is an appropriate growth technique in order to obtain C60 epilayers with single crystal quality on mica. A detailed study pointed out that the film thickness, as well as the growth rate and substrate temperature have a strong influence on the crystalline quality of the samples. Having optimized the growth parameters of the system, epilayers with 170 arcsec fwhm of the C60-(111) rocking curve can be fabricated. In addition AFM pictures and reciprocal space maps were presented, also demonstrating the influence of the film thickness on the growth mode.
D. Stifter, H. Sitter~Journal of Crystal Growth 156 (1995) 79-85
Acknowledgements We thank H. Kuzmany and his group (University of Vienna) for introducing us into the exciting field of fullerenes and for providing us with C60 source material, O. Fuchs for the construction of the HWE setup, K. Schilcher (Institut t'fir Biophysik, University Linz) for the AFM measurements, A. Darhuber, E. Koppensteiner (Institut fiir Halbleiterphysik, University Linz), L. Palmetshofer, W. Plotz, E. Wirthl (Institut f'tir Festk/Srperphysik, University Linz), and P. Byszewski and his group (OBREP Warsaw) for technical assistance and helpful discussions. This work is partially supported by the Fond zur F6rderung der wissenschaftlichen Forschung in 0sterreich.
References [1] W. Krakow, N.M. Rivera, R.A. Roy, R.S. Ruoff and J.J. Cuomo, IBM Res. Rep. RC 17345 (#76621) 10/30/91 (1991). [2] W. Krakow, N.M. Rivera, R.A. Roy, R.S. Ruoff and J.J. Cuomo, Appi. Phys. A 56 (1993) 185.
85
[3] D. Schmicker, S. Schmidt, J.G. Skofronick, J.P. Toennies and R. Vollmer, Phys. Rev. B 44 (1991) 10995. [4] K. Tanigaki, S. Kuroshima, J. Fujita and T.W. Ebbesen, Appl. Phys. Lett. 63 (1993) 2351. [5] H.-G. Busmann, R. Hiss, H. Gaber and I.V. Hertel, Surf. Sci. 289 (1993) 381. [6] J.E. Fischer, E. Werwa and P.A. Heiney, Appi. Phys. A 56 (1993) 193. [7] S. Henke, K.H. Thiirer, J.K.N. Lindner, B. Rauschenbach and B. Stritzker, J. Appl. Phys. 76 (1994) 3337. [8] M. Sakurai, H. Tada, K. Saiki and A. Korea, Jpn. J. Appl. Phys. 30 (1991) L1892. [9] J.A. Dura, P.M. Pippenger, N.J. Halas, X.Z. Xiong, P.C. Chow and S.C. Moss, Appl. Phys. Lett. 63 (1993) 3443. [10] T. Ichihashi, K. Tanigaki, T.W. Ebbesen, S. Kuroshima and S. Iijima, Chem. Phys. Lett. 190 (1992) 179. [11] A. Fartash, Appl. Phys. Lett. 64 (1994) 1877. [12] S. Henke, K.H. Thiirer, S. Geier, B. Rausehenbach and B. Stritzker, Appl. Phys. A 60 (1995) 383. [13] A. Lopez-Otero, Thin Solid Films 49 (1978) 3. [14] D. Stifter and H. Sitter, Appl. Phys. Lett. 66 (1995) 679. [15] M.A. Herman and H. Sitter, Molecular Beam Epitaxy Fundamentals and Current Status, Ed. M.B. Panish, Springer Ser. Mater. Sci., Vol. 7 (Springer, Berlin, 1988) p. 44. [16] V. Hol~, J. Kubena, E. Abramof, K. Lischka, A. Pesek and E. Koppensteiner, J. Appl. Phys. 74 (1993) 1736. [17] E. Koppensteiner, G. Bauer, H. Kibbel and E. Kasper, J. Appl. Phys. 76 (1994) 3489. [18] J. Abrefah, D.R. Olander, M. Balooch and W.J. Siekhaus, Appl. Phys. Lett. 60 (1992) 1313.
ELSEVIER
CRYSTAL GROWTH
Journal of Crystal Growth 156 (1995) 79-85
Growth of
C60 thin films on mica by hot wall
epitaxy
D. Stifter *, H. Sitter Institut f~r Experimentalphysik, Abteihng Festk&perphysik, Johannes Kepler Universitiit Linz, A-4040 Linz, Austria
Received 8 March 1995; manuscript received in final form 12 May 1995
Abstract
Thin C60 films were grown on mica by hot wall epitaxy. The influence of the individual growth parameters, like substrate, wall, and source temperature, on the surface morphology and crystalline quality of the C60 films was investigated using atomic force microscopy and high resolution X-ray diffraction. Under ideal growth conditions the full width at half maximum (fwhm) of the Ca)-(lll) X-ray rocking curve is about 170 arcsec indicating a more or less perfect monocrystalline growth of the C60 layer.
1. I n t r o d u c t i o n
The growth of thin C60 films on various substrate materials is object of several studies investigating the fundamental processes, as well as determining and optimizing the growth parameters. The existence of aligned {111} C60 planes parallel to the substrate surface could be proven especially for layered substrate materials, like mica or MoS z [1-8], Particularly mica is a promising material for the epitaxial growth of C60 with a lattice mismatch of 3.4% [1,2]. High resolution transmission electron microscopy studies [1,2] showed that a substrate temperature of 100°C is the onset of well orientated crystallographic growth of C60. The early stages of growth were investigated by atomic force microscopy studies [4,5] pointing out that growth starts with islands with the bare mica substrate in between. If the growth is continued
* Corresponding author.
new islands are formed until the surface is totally covered by C6o building extended flat terraces for the further growth [5]. In addition to atomic force and transmission electron microscopy only few studies were made using X-ray diffraction for the characterization of thin C60 films [6,7,9,14]. The full width at half maximum (fwhm) Aco of the (111)-C60 reflex in to direction (i.e. co-scan or rocking curve) serves as a criterion for the crystalline quality of the C6o epitaxial layer. So far values of fwhm have been reported varying from A~o = 2300 arcsec for 250 nm thick films grown on antimony [9] to Aco = 1100 and 720 arcsec for layers with a thickness of 200 and 250 nm grown on mica [6,7]. An 8 ~ m C60 film on mica showed a rocking curve fwhm of 3000 arcsec, which was grown in a system with an effusion cell producing a stable C6o flux of up to 2 , ~ / s using nitrogen as a transport gas from the source to the substrate [11]. The influence of the substrate temperature and the growth rate on twin formation and epitaxial orientation of grains in C6o films was studied measuring X-ray pole figures [12].
0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI I)022-0248(95)00250-2
80
D. Stifter, H. Sitter~Journal of Crystal Growth 156 (1995) 79-85
The applied growth methods varied from vapor deposition in high vacuum [2,7] to molecular beam epitaxy (MBE) and organic MBE [8-10]. In order to enhance the epitaxial ordering of the C6o layer a hot wall deposition was proposed [6]. We have grown C60 epilayers by an authentic hot wall epitaxy (HWE) setup, as defined by Otero [13]. The C6o films were characterized using high resolution X-ray diffraction, pointing out that the growth of 120 nm thick films with a fwhm Aoj of 210 arcsec is possible [14]. For thicker films the rocking curve of the C60-(111) reflex exhibited a more complex shape. This behavior can be explained by assuming a critical thickness for the C60 films. For films exceeding this thickness the growth gets worse with a mosaic spread of 0.9 °. It can be shown that the critical thickness depends on the substrate temperature and decreases with increasing growth temperature. In addition we present a set of experiments of C60 films grown on mica with the H W E system mentioned above, varying also the wall and source temperature.
MICA- SUBS'IRATE
TEMPERATURE PROFILE
'1 1 rll
0 --
/ QUARTZ TUBE
Tst~r.
Tsot~-BTwAtL
TEMPERATURE
2. Experimental procedure and setup
Fig. 1. Schematic sketch of our H W E system and the temperature profile caused by the three separate heaters. The C60 source material is evaporated at the bottom of the quartz tube and condenses on the mica substrate placed in the upper region of the tube.
The H W E system we used is schematically depicted in Fig. 1. It contains in contrast to other high vacuum growth systems, like MBE, an almost closed reactor. So it can be operated close to thermodynamic equilibrium, which is most important for van der Waals epitaxy. A quartz tube, with the source material at the bottom and the substrate on the top closing it tight with respect to the mean free path of the evaporated source molecules, is placed with three separated heaters into a high vacuum chamber. The region between source and substrate, called hot wall, guarantees a nearly uniform and isotropic flux of the molecules onto the substrate surface [15]. The advantage of such a system is the minimization of the loss of source material, what allows to obtain high growth rates at comparably high substrate temperatures. A separate oven for preheating of the substrates is also mounted in the vacuum chamber.
The C60 source material was at first 99.4% pure. It had to be cleaned from solvents and impurities by subliming the material three times at 550°C under dynamical vacuum of 1 x 10 -6 mbar and by protecting it from visible light in order to prohibit photoinduced polymerization of the C60 molecules. About 200 mg of the cleaned material was loaded into the H W E system, which was enough to fabricate about 50 epilayers with an area of 1 cm 2 and an average thickness of 200 nm. As substrate material mica sheets were cut into pieces (15 x 15 mm2), cleaved in air with an adhesive tape and immediately transferred into the vacuum chamber of the H W E system. The substrates, before being loaded into the growth reactor, were preheated for one hour at 400°C in the separate oven to remove adhesives on the substrate surface. In our experiments the substrate temperature was varied from 100°C to
D. Stifter, H. Sitter/Journal of Crystal Growth 156 (1995) 79-85
200°C, the wall temperature from 340°C to 440°C and the source temperature from 360°C to 440°C. The residual gas pressure outside the growth reactor was 1 × 10 - 7 mbar when all ovens were heated. The growth time was varied from 1 to 5 h in order to obtain films of different thickness. The thickness of the films was measured afterwards with a mechanical surface profilometer by scanning the height of the step between the film and the region of the substrate which was covered by the substrate holder during growth. In order to measure the crystalline quality of the C60 films, high resolution X-ray diffraction (HRXD) was used taking the full width at half maximum (fwhm) of the rocking curve (to-scan) of the C60-(111) crystal reflex, i.e. measurements with the X-ray detector fixed at a Bragg angle 20 of a crystal reflex and varying the angle to, which is the angle between the incident beam and the sample surface. The fwhm of the rocking curve is sensitive to tilted grains and mosaic blocks, misfit dislocations and on lateral structures and surface corrugations [16]. The advantage of HRXD is, that it is a non-destructive method and takes a large region of the crystal (several mm 2) into account, compared to other methods like transmission electron microscopy. Our HRXD measurements were performed on a Philips high resolution diffractometer using Cu K a radiation filtered by a Ge four-crystal monochromator, and a 0.2 ° aperture slit mounted in front of the detector. The resolution in to direction (measuring at angles: to < 7° and 20 < 11°) was determined to be less then 20 arcsec. The scans were typically carried out with 11 arcsec stepwidth and an integration time up to one minute per step. Reciprocal space maps were measured by taking to-scans at different to/20 starting angles, scanning the reciprocal space in the region of the C60-(111) peak. The resolution in to/20 direction is more or less limited by the 0.2° aperture slit due to the orientation and size of our scanning probe in this region of the reciprocal space. Principles and details of reciprocal space mapping can be found elsewhere [17]. The surface morphology of the C60 films was studied using an atomic force microscope (Digital Instruments Nanoscope III) with a S i 3 N 4 needle.
81
The force exerted by the needle was about 100 nN.
3. Results and discussion
In a first set of experiments the source temperature was varied from 360°C to 440°C at a fixed substrate temperature of 140°C and a wall temperature of 400°C. The resulting growth rates are plotted in Fig. 2 on a logarithmic scale versus the inverse source temperature in a so-called Arrhenius plot. The results show, that the growth rate is proportional to the effusion rate, indicating that the growth mechanism is limited by the source. The enthalpy for the evaporation of C60 molecules A Hvap can be calculated from this data by evaluating the slope of the fitting curve. The value resulting from our experiments for the activation energy E a for evaporation of one C60 molecule is 1.17 + 0.03 eV. Identifying EaNa (Na = 6.022 × 1023 mo1-1) with the enthalpy AHv~p, then AHv~p= 113+3 kJ/mol in contrast to AH~ap = 159 kJ/mol published by Abrefah et al. [18]. This discrepancy cannot be explained by the uncertainity in our measurements of the absolute source temperature of about + 10°C, but has to be the result of a different mechanism, like a different impurity concentration in the source material.
440°C'N~ 4200C N ~ _,= _c
400°C 380°C
o
360°C
0.1 0.0013
o o;~14
o obls 1.rl'source[1/K]
o.ob16
0.0017
Fig. 2. Arrhenius plot of the growth rate versus the applied source temperature. The enthalpy AHw,, calculated from the slope is 113 kJ/mol.
D. Stifter, H. Sitter/Journal of Crystal Growth 156 (1995) 79-85
82
0.35' 0.30'
~ i~J C6o (111) rocking curve
0.25,
<8 o.2o: -r 0.15J
/
-
/ °'.s 4:0 4:s s:o s:s 6:0 6:s 7.0
/
o.lo:
o.o5" o.oo
16o
lho
16o
260
Temperature [°C] Fig. 3. fwhm of the C6o-(111) rocking curve versus the applied substrate temperature. The insert presents a typical rocking curve of a sample grown at a substrate temperature of 160°C.
As to the crystalline quality of C60 films, source temperatures between 380°C and 420°C resulting in growth rates between 0.2 and 0.5 , ~ / s led to samples with the best crystalline quality as described below. The influence of the substrate temperature on the growth and crystalline quality of C60 films grown in a hot wall system was studied by varying the substrate temperature in the range from 100°C
,--, 3O
~)
~ 2s >,
(a)
to 200°C at a fixed source and wall temperature of 400°C. The fwhm of the C60-(111) rocking curve of films with a thickness of about 120 nm is plotted in Fig. 3 versus the applied substrate temperature. The insert of Fig. 3 presents a typical rocking curve of a sample grown at a substrate temperature of 160°C. Varying the temperature from 100° to 180°C, it can easily be seen that there is a flat minimum at about 140°C. The point at 200°C is the result of a dramatic change of the line shape of the rocking curve. A broad peak of 0.9 ° width occurs, with a narrow one sitting on top. This superposition of two peaks with different width can also be observed for all the other substrate temperatures, but at a larger film thickness. The evolution of the line shape as a function of the film thickness is depicted in Fig. 4. It is demonstrated that the broad peak appears between 80 and 160 nm and becomes dominant for films with a larger thickness. Splitting the rocking curve into two superimposed peals and measuring the width of each peak separately, it can be seen from Fig. 5, where the widths of the peaks are plotted versus the film thickness, that the fwhm taken only from the broad peak (full circles) has an approximately constant value of
,-, 3O
80 nm
W
2s
(b)
160 nm
._~ 2o
20
~ 15 c ~
e.o lo
10
5
5 I 0
.
i
.
i
•
i
.
.
.
.
.
.J .
•
3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 co [o]
3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
co [o1 ,~ 3O w
8" 25
,~ 3O
(c)
3 4 0 nm
W
2s
~ 2o
~ 2o
--
--
.o lo c
5
3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
[°]
5
0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 co [o]
Fig. 4. C6o-(11 l) rocking curves for layers grown at substrate temperatures around 160°C: The change of the shape with increasing film thickness is demonstrated.
D. Stifter, H. Sitter~Journal of Crystal Growth 156 (1995) 79-85 4000
Tsubstr.:
", 120°C
25
3500~' 3000-
-
83
~
\
104
I
8]
:, '1
=
\/
\
25002000- I 1500 •
-0---0---r"l--
1
broad peak narrow peak total peak
-
/ I
,~,,, 10. 140°C 00 ~ . xq~--
~
.0~
5-" 200°C O ~ • 0-
500•
2;0
a;o
•
4
5 -o-
:otJ
6
r"l ~
1000-
0 100
"-.•
,
4;o
i
s;o
600
0
~ " ~ w
0
o_J:'- ° 200
; 400
"------= 600
800
film thickness [nm]
film thickness [nm] Fig. 5. fwhm of the narrow peak (hollow circles), the broad peak (full circles) and of the total peak, which is the superposition of the two peaks (hollow squares).
i
i
' t = 240 nm
(a) 0.1 o
0.0 Ii
-0.1 I
'
I
-1.0
-1.5
-0.5
I
I
I
0.0
0.5
1.0
[o] '
(b)
1.5
Fig. 7. Ratio of the intensities 1c and ld, i.e. intensities of the narrow and broad peak respectively, as a function of the total film thickness.
about 3500 arcsec. This is also the case for the narrow peak with an fwhm of about 200 arcsec (hollow circles). The fwhm taken from the total peak, which is the superposition of the two peaks, exhibits a slope, indicating that the intensity of the broad peak increases with respect to the narrow one. In Figs. 6a and 6b reciprocal space maps measured on C60 films with a thickness of 240 and 1030 nm respectively are compared also demonstrating the evolution of the broad peak with regard to the narrow one sitting on top. In Fig. 6a, the narrow peak is more significant than the broad one. Its width is limited in to/20 direction by the width of the 0.2 ° receiving slit as already mentioned before. The width in to direc-
' t = 1630 nm
0.1
T
E 0.0
-0.1
-1.5
I
I
-1.0
-0.5
I
'
0.0
0.5
1.0
1.5
co [o] Fig. 6. Reciprocal space maps of the C6o-(111) reflex: (a) 240 nm thick sample, (b) 1030 nm thick sample. Separation between lines is 2 cps (a) and 3 cps (b) respectively. For these plots the maximum of the (111) reflex was taken as origin of the reciprocal space.
Fig. 8. Atomic force microscopy pictures: (a) surface of a 30 nm thick film below the critical thickness where the broad peak appears, exhibiting monomolecular terraces. (b) surface of a 500 nm thick film. Az values: (a) 15 nm, (b) 30 nm.
D. Stifter, H. Sitter/Journal of Crystal Growth 156 (1995) 79-85
84
tion is the width of the one-dimensional C60-(111) rocking curve. The slight rotation of the peak with respect to the axis is due to the orientation of the Ewaid sphere and scanning probe in this region of the reciprocal space. For the thicker C60 epilayer (Fig. 6b), the small peak is totally submerged by the broad one, which has a fwhm of about 1° in to direction. Splitting the total rocking curve into two superimposed peaks, as shown in the insert of Fig. 7, and calculating the intensity by taking the area under each separated peak, one can plot the ratio of these two intensities versus the film thickness for different substrate temperatures, as depicted in Fig. 7. I d is the intensity of the broad peak, whereas I~ is the intensity of the narrow one. It is demonstrated that for increasing substrate temperatures the ratio I c / I d is decreasing. For a fixed substrate temperature the ratio is also decreasing with increasing film thickness. This behavior could be explained and simulated by developing a simple phenomenological model [14]. According to this model, the film growth starts, after the initial island growth, in a more or less perfect mode. After exceeding a critical thickness t c, the growth mode changes and slightly tilted grains and mosaic blocks are formed during growth resulting in the enormous broadening of the rocking curve, whereas the perfect layer is almost undisturbed, originating the narrow peak superimposed on the broad one. The critical thickness t c depends on the applied substrate
temperature and decreases with increasing temperature. In Figs. 8a and 8b AFM pictures of the surfaces of a sample with a thickness below t c (Fig. 8a) and one of a film exceeding tc (Fig. 8b) are presented. The surface of the 30 nm thick film is covered by flat monomolecular terraces, whereas the surface of the second sample with a thickness of 500 nm exhibits a less ideal structure. A variation of the wall temperature from 340°C to 420°C at a fixed substrate and source temperature of 140°C and 400°C respectively, had no significant influence on the fwhm of the C6o films, but on the growtho rate. For 440°C the growth rate exceeds 0.8 A / s , which is due to a minimal condensation of C60 molecules on the wall. At such a high growth rate, the fwhm of the rocking curve is already influenced negatively, as it was observed for source temperatures over 420°C. In a last set of experiments the preheating time of the mica substrate was varied from 0 to 60 min, which led to no consequences regarding the crystalline quality of the samples. With the optimization of all the growth parameters described above a sample with the rocking curve depicted in Fig. 9 could be grown. The fwhm is about 170 arcsec, which is to our knowledge the smallest value reported so far in the literature for any C60 epilayer.
4. Conclusions Ceo (111) ¢o- Scan FWHM: 170 arcsec
70 60
¢,/)
50
Tsubstr.: Twall :
1400C 380°C
Tsource: 4O0°C growth rate: 0.48A/s film thickness: 13Onto
30 C
--.~ 2o e'-
L__
lO
o i
4.75
I
I
I
5.00
5.25
5.50 =
i
I
I
5.75
6.00
6.25
[°1
Fig. 9. R o c k i n g curve of a s a m p l e grown u n d e r ideal conditions showing a rocking curve fwhm of 170 arcsec.
We have shown that hot wall epitaxy is an appropriate growth technique in order to obtain C60 epilayers with single crystal quality on mica. A detailed study pointed out that the film thickness, as well as the growth rate and substrate temperature have a strong influence on the crystalline quality of the samples. Having optimized the growth parameters of the system, epilayers with 170 arcsec fwhm of the C60-(111) rocking curve can be fabricated. In addition AFM pictures and reciprocal space maps were presented, also demonstrating the influence of the film thickness on the growth mode.
D. Stifter, H. Sitter~Journal of Crystal Growth 156 (1995) 79-85
Acknowledgements We thank H. Kuzmany and his group (University of Vienna) for introducing us into the exciting field of fullerenes and for providing us with C60 source material, O. Fuchs for the construction of the HWE setup, K. Schilcher (Institut t'fir Biophysik, University Linz) for the AFM measurements, A. Darhuber, E. Koppensteiner (Institut fiir Halbleiterphysik, University Linz), L. Palmetshofer, W. Plotz, E. Wirthl (Institut f'tir Festk/Srperphysik, University Linz), and P. Byszewski and his group (OBREP Warsaw) for technical assistance and helpful discussions. This work is partially supported by the Fond zur F6rderung der wissenschaftlichen Forschung in 0sterreich.
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85
[3] D. Schmicker, S. Schmidt, J.G. Skofronick, J.P. Toennies and R. Vollmer, Phys. Rev. B 44 (1991) 10995. [4] K. Tanigaki, S. Kuroshima, J. Fujita and T.W. Ebbesen, Appl. Phys. Lett. 63 (1993) 2351. [5] H.-G. Busmann, R. Hiss, H. Gaber and I.V. Hertel, Surf. Sci. 289 (1993) 381. [6] J.E. Fischer, E. Werwa and P.A. Heiney, Appi. Phys. A 56 (1993) 193. [7] S. Henke, K.H. Thiirer, J.K.N. Lindner, B. Rauschenbach and B. Stritzker, J. Appl. Phys. 76 (1994) 3337. [8] M. Sakurai, H. Tada, K. Saiki and A. Korea, Jpn. J. Appl. Phys. 30 (1991) L1892. [9] J.A. Dura, P.M. Pippenger, N.J. Halas, X.Z. Xiong, P.C. Chow and S.C. Moss, Appl. Phys. Lett. 63 (1993) 3443. [10] T. Ichihashi, K. Tanigaki, T.W. Ebbesen, S. Kuroshima and S. Iijima, Chem. Phys. Lett. 190 (1992) 179. [11] A. Fartash, Appl. Phys. Lett. 64 (1994) 1877. [12] S. Henke, K.H. Thiirer, S. Geier, B. Rausehenbach and B. Stritzker, Appl. Phys. A 60 (1995) 383. [13] A. Lopez-Otero, Thin Solid Films 49 (1978) 3. [14] D. Stifter and H. Sitter, Appl. Phys. Lett. 66 (1995) 679. [15] M.A. Herman and H. Sitter, Molecular Beam Epitaxy Fundamentals and Current Status, Ed. M.B. Panish, Springer Ser. Mater. Sci., Vol. 7 (Springer, Berlin, 1988) p. 44. [16] V. Hol~, J. Kubena, E. Abramof, K. Lischka, A. Pesek and E. Koppensteiner, J. Appl. Phys. 74 (1993) 1736. [17] E. Koppensteiner, G. Bauer, H. Kibbel and E. Kasper, J. Appl. Phys. 76 (1994) 3489. [18] J. Abrefah, D.R. Olander, M. Balooch and W.J. Siekhaus, Appl. Phys. Lett. 60 (1992) 1313.