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The early stages of growth of crystalline, diamond-like films on Si(100) by pulsed laser evaporation of graphite
960
Surface
The early stages of growth of crystalline, by pulsed laser evaporation of graphite S. Ferrer,
F. Comin,
J.A. Martin,
L. Vazquez
diamond-like
Science 251/252
(1991) 960-964 North-Holland
films on Si( 100)
’ and P. Bernard
European Svnchrotron Radiation Facility. B. P. 1720, 38043 Grenoble Cedes. Frunce Received
1 October
1990; accepted
for publication
15 January
1991
Pulsed laser evaporation of graphite targets in an ultra high vacuum environment has been utilized to deposit ultra thin carbon films on Si(100) substrates. Analysis of the fine structure of the carbon Auger line reveals significant sp3 bonding. STM images show that the films are crystalline and that the surface lattice is hexagonal.
1. Intrusion Diamond is an extraordinary material since it has several extreme properties: it is the hardest material known, it has the highest thermal conductivity at room temperature, it is an excellent insulator and it is chemically very inert. Due to that, deposition of diamond films on various materials has been a challenging field of research that has accelerated noticeably in the last few years. Attempts to produce diamond films resulted in many cases in carbon films with intermediate properties between diamond and graphite. These are termed in the literature as diamond-like films. Cubic diamond has four sp3 bonds in a tetrahedral structure, Carbon atoms in graphite are bonded to three neighbours via sp’ bonds and the remaining p orbital forms a 71 electron band. Amorphous carbon contains a mixture of sp3, sp’ and sp’ bonds with no long range order. The most common methods to fabricate diamond crystallites (of the order of 1 ym in size) or diamond-like films on a variety of substrates are: (i) chemical vapor deposition techniques usually
’ Permanent address: Instituto de Ciencia de Materiales. CXII. Universidad Autonoma de Madrid. Cantoblanco. 28049 Madrid. Spain. 0039~6028/91/$03.50
$2 1991 - Elsevier Science
Publishers
plasma or thermally assisted and (ii) ion beam deposition techniques ]1,2]. Very often these techniques result in heavily hydrogenated carbon layers that are amorphous. Due to the inexistence of long range crystalline order, structural knowledge of those films is very scarce. In this paper we present results obtained with a relatively new method to produce carbon films [3]. A high power pulsed laser was used to ablate a graphite target under UHV. The evaporated carbon species were then let to deposit on a well characterized Si surface. It was found that the films exhibited an electronic structure with significant sp’ bonding. Also, STM data show that the films exhibit two-dimensional long range order and that the surface lattice is hexagonal.
2. Experimental
details
The experiments were carried out in an UHV chamber containing an hemispherical particle an electron gun for electron specanalyzer, troscopy and a small spot ion gun for ion scattering experiments and sputtering. The graphite targets were mounted in a precision manipulator facing the Si surface that was in the focus of the analyzer. In this way spectra could be recorded during growth. A retractable LEED optics and a
B.V. (North-Holland)
S. Ferrer et al. / Deposition of ultra thin carbon films on Si(lO0)
quartz balance were also installed. The Si(100) surfaces were cut from wafers with 9” of miscut and were prepared by sputtering and annealing cycles until a sharp single domain 1 x 2 pattern was obtained and the ratio of the C Auger line to the Si 91 eV line was less than 1 X 10T3. A Nd : YAG pulsed laser, frequency doubled, producing pulses of 10 ns and 200 mJ with a wavelength of 532 nm was focused onto the targets to achieve a power density of 10” W/cm2. The evaporated species consisted predominantly of carbon dimers as indicated by the mass analyzer and they had kinetic energies of a few eV [3]. STM images were taken ex-situ at atmospheric pressure with a commercial instrument based on a tube scanner. After imaging, the samples were reinserted in the UHV system to check the surface composition with AES.
3. Results aud Discussion 3.1. Electronic structure The nature of the C-C bonds in the deposited carbon films was investigated by analyzing the lineshape of the characteristic carbon Auger transition. It is well established since many years ago, that in some cases such as free electron metals or solids without localized electron bands, the lineshape of a KW Auger transition (V: valence band, K: core level) is similar to the self-convolution of the density of occupied states within the band [4]. Graphite is a typical example [5]. The calculated densities of states of graphite and diamond are in fact rather similar [6]. Referring the energies to the upper edge of the valence band, graphite shows a 7~ peak, derived from sp2 bonds at 1.5 eV, a major peak at 5.1 eV and two less intense peaks at 11 and 18 eV. In diamond the rr peak is absent, the major peak is at 4.5 eV and it has also two less intense peaks at - 11 and - 18 eV. Fig. 1 shows reference spectra of graphite cleaved in air before insertion into the system, of graphite after moderate Ar sputtering, and of synthetic single crystalline (111) diamond. The spectra were taken under identical conditions. The
220
Fig. 1. KW
240
260
961
eV
280
300
320
Auger spectra of graphite samples and of a diamond crystal.
modulation voltage was 2 VP,. At 4 eV at the right-hand side of the main minimum, a shoulder, indicated by a vertical dashed line, is clearly noticeable for the sputtered graphite, less resolved for the cleaved sample and it is absent in diamond. It corresponds to the self-convolution of the 7r band and it is a characteristic fingerprint of sp2 bonding. In less resolved spectra, the shoulder appears as a broadening of the high energy side of the spectrum compared with that of diamond. Typical values for the half width are 5.5 eV in diamond, 7-7.5 eV in graphite and 3.5 eV for a - 100 A thick Sic film on Si [7]. Electron irradiation causes the diamond surface to graph&e and the consequent appearance of the 7~ derived feature. At 8.5 eV of the main minimum, in the low energy side, the cleaved sample exhibits a maximum. At the sputtered sample there is a plateau. In the diamond sample the peak at 13.5 eV from the minimum, indicated by a dashed line has been traditionally taken as the most characteristic feature of diamond Auger emission [8]. In well prepared diamond samples, the intensity of this peak is somewhat higher that in our spectrum. The details in the Auger lineshape at around 10 eV from the main minimum are due to the folding of the major peak at around 5 eV in the density of states with that at around 10 eV. The differences between the diamond and graphite Auger spectra in this energy region originate from small dif-
S. Ferrer et al. / Deposition
962
8 layers of C
220
Fig. 2. Carbon
Fig. 3. Constant
240
260
260
300
eV Auger spectrum for a 12 A thick film grown a Si(100) substrate.
height
320
on
of ultrathin curbon jilms on Si( IO01 ferences in width and peak positions of the corresponding peaks of the density of states. In a previous work [7] it 0was found that for very thin carbon films (2-5 A) the Auger spectra of carbon was the characteristic one of silicon carbide (i.e. rather symmetrical lineshape and relatively small half width). Upon increasing thickness. the half width evolves from 3.5 to around 5.5 eV and the lineshape becomes less symmetrical. Fig. 2 shows a spectrum for a 0 carbon deposit with a thickness of around 12 A. It exhibits a weak maximum at 13.5 eV from the main minimum characteristic of diamond. and it has a half width on the high energy side of the minimum of 5.2 eV indicating a relatively small 7~contribution. Similar spectra have been obtained previously by other authors by using ion implantation tech-
STM image of an area of 67 X 67 A’ of a - 12 A thick film of carbon deposited on a Si substrate. parameters: sample bias 330 mV. current 1.4 nA. The surface crystallinity is apparent.
Imaging
S. Ferrer et al. / Depositionof ultra thin carbonfilm on Si(lo0)
niques to deposit carbon films [9]. Due to the similarity with the diamond spectrum, these films have been designated as corresponding to diamond-like films. 3.2. STM results STM images were taken in both topographic and constant height modes. The first operating mode was used to search for atomically flat patches of the surface and the constant height made was used for atomic resolution imaging. It was found that the preparation of the Si surfaces prior to carbon deposition (several hours annealing at 1200 K at 10-i’ Torr) was essential to obtain atomically flat areas on the carbon films.
Fig. 4. Constant
height STM image of an area of 33 Imaging
X
Fig. 3 shows a current image of an area of 67 X 67 A* displaying rows of ordered atoms. The acquisition time was 30 s. The image is for a 12 A thick carbon film on a Si(100) substrate. As is clearly noticeable the film is crystalline. Images displaying ordering in patches as large as 140 x 140 A2 have been found. Fig. 4 shows a well ordered region of 33 x 33 A2. The acquisition time was 6 s. The data have been slightly Fourier filtered to enhance contrast. As may be seen the surface lattice displays hexagonal symmetry. It was found that the microscope could not be operated since it became unstable when the sample bias was lower than - 50 mV positive or negative. In some regions this threshold value could be as high as 700 mV. This is in contrast
33 AZ. The image shows an hexagonal
parameters:
963
sample bias - 550 mV, current
surface
5 nA.
lattice of the diamond-like
film.
964
S. Ferrer et al. / Deposition of ultra thin curhon films on Si(lWj
with graphite imaging were atomic resolution images were routinely taken at voltages between 5 and 10 mV. This finding was interpreted as being due to a lack of electrical conductivity of the diamond-like films caused by the existence of an electronic band gap. At present however a precise value of the band gap cannot be reported. A previously published work on STM imaging of carbon films prepared by magnetron sputtering [lo], revealed that the films were essentially amorphous since order was only observed in isolated patches of linear dimensions of 15 A at the most. It is worth emphasizing at this point, that amorphous films are the general rule for diamond-like deposits prepared by the usual techniques. The non-obvious result of obtaining ordering on a room temperature substrate by using the laser evaporation technique might be related to two facts: (i) the UHV environment ensures a very small hydrogen conta~nation on the films compared to standard non-UHV techniques and (ii) the kinetic energy of the C species at the vapor phase corresponds to an equivalent temperature of several ten thousands of degrees. This high translational energy compared with thermal evaporation energies may result in long diffusion paths on the surface leading to atomic ordering. It appears then, that laser evaporation is a promising technique for obtaining epitaxial and crystalline films.
4. Summary Carbon films laser evaporation
on Si(100) prepared by pulsed of graphite under UHV show
significant sp’ eshape of the crystalline with cated by STM
bonding as evidenced by the lincarbon Auger line and they are a hexagonal surface lattice as indidata.
This work has been partially supported by the European Community under the project ALADIN. L. Vazquez acknowledges a grant from the Spanish Department of Research.
References Yarbrough and R. Mrssrer. Science 247 (IWO) 68X. and S. Kasi, Science 239 ( 1988) 673. [3] P.K. Schenk, D.W. Bonnell and J.W. Hastie. J. Vat. Set. Technol. A 7 (1989) 1745. in: Low Energy Electrons and [41G. Ertl and J. Ktippers, Surface Chemistry, Monographs in Modern Chemistry. Vol. 4 (Verlag Chemie, Weinheim. 1974). M.B. Guseva, V.G. Bahaev and 0.y. PI V.V. Khostov, Rylova, Surf. Sci. 169 (1986) L253. [61 A. Zunger and R. Enlman. Phys. Rev. B 17 (197X) 647: G.S. Painter, D. Ellis and A.R. Luhinsky, Phys. Rev. B 4 (1971) 3610. F. C’omtn and S. I71J.A. Martin. L. Vazquez, P. Bernard, Ferrer. Appl. Phys. Lett. 57 (1990) 1742. ix1 P.G. Lurie and J.M. Wilson. Surf. Sci. 65 (1977) 476: B.B. Pate. Surf. Sci. 165 (1986) X3. 191 S. Kasi, H. Kang and J.W. Rabalais. Phya. Rev. Lert. 5Y (1987) 75. UOI B. Marchon, M. Salmeron and W. Siekhauc. Phbr. Kc\. B 39 (7989) 12907. [I] WA.
[2] W. Rabalais
Surface
The early stages of growth of crystalline, by pulsed laser evaporation of graphite S. Ferrer,
F. Comin,
J.A. Martin,
L. Vazquez
diamond-like
Science 251/252
(1991) 960-964 North-Holland
films on Si( 100)
’ and P. Bernard
European Svnchrotron Radiation Facility. B. P. 1720, 38043 Grenoble Cedes. Frunce Received
1 October
1990; accepted
for publication
15 January
1991
Pulsed laser evaporation of graphite targets in an ultra high vacuum environment has been utilized to deposit ultra thin carbon films on Si(100) substrates. Analysis of the fine structure of the carbon Auger line reveals significant sp3 bonding. STM images show that the films are crystalline and that the surface lattice is hexagonal.
1. Intrusion Diamond is an extraordinary material since it has several extreme properties: it is the hardest material known, it has the highest thermal conductivity at room temperature, it is an excellent insulator and it is chemically very inert. Due to that, deposition of diamond films on various materials has been a challenging field of research that has accelerated noticeably in the last few years. Attempts to produce diamond films resulted in many cases in carbon films with intermediate properties between diamond and graphite. These are termed in the literature as diamond-like films. Cubic diamond has four sp3 bonds in a tetrahedral structure, Carbon atoms in graphite are bonded to three neighbours via sp’ bonds and the remaining p orbital forms a 71 electron band. Amorphous carbon contains a mixture of sp3, sp’ and sp’ bonds with no long range order. The most common methods to fabricate diamond crystallites (of the order of 1 ym in size) or diamond-like films on a variety of substrates are: (i) chemical vapor deposition techniques usually
’ Permanent address: Instituto de Ciencia de Materiales. CXII. Universidad Autonoma de Madrid. Cantoblanco. 28049 Madrid. Spain. 0039~6028/91/$03.50
$2 1991 - Elsevier Science
Publishers
plasma or thermally assisted and (ii) ion beam deposition techniques ]1,2]. Very often these techniques result in heavily hydrogenated carbon layers that are amorphous. Due to the inexistence of long range crystalline order, structural knowledge of those films is very scarce. In this paper we present results obtained with a relatively new method to produce carbon films [3]. A high power pulsed laser was used to ablate a graphite target under UHV. The evaporated carbon species were then let to deposit on a well characterized Si surface. It was found that the films exhibited an electronic structure with significant sp’ bonding. Also, STM data show that the films exhibit two-dimensional long range order and that the surface lattice is hexagonal.
2. Experimental
details
The experiments were carried out in an UHV chamber containing an hemispherical particle an electron gun for electron specanalyzer, troscopy and a small spot ion gun for ion scattering experiments and sputtering. The graphite targets were mounted in a precision manipulator facing the Si surface that was in the focus of the analyzer. In this way spectra could be recorded during growth. A retractable LEED optics and a
B.V. (North-Holland)
S. Ferrer et al. / Deposition of ultra thin carbon films on Si(lO0)
quartz balance were also installed. The Si(100) surfaces were cut from wafers with 9” of miscut and were prepared by sputtering and annealing cycles until a sharp single domain 1 x 2 pattern was obtained and the ratio of the C Auger line to the Si 91 eV line was less than 1 X 10T3. A Nd : YAG pulsed laser, frequency doubled, producing pulses of 10 ns and 200 mJ with a wavelength of 532 nm was focused onto the targets to achieve a power density of 10” W/cm2. The evaporated species consisted predominantly of carbon dimers as indicated by the mass analyzer and they had kinetic energies of a few eV [3]. STM images were taken ex-situ at atmospheric pressure with a commercial instrument based on a tube scanner. After imaging, the samples were reinserted in the UHV system to check the surface composition with AES.
3. Results aud Discussion 3.1. Electronic structure The nature of the C-C bonds in the deposited carbon films was investigated by analyzing the lineshape of the characteristic carbon Auger transition. It is well established since many years ago, that in some cases such as free electron metals or solids without localized electron bands, the lineshape of a KW Auger transition (V: valence band, K: core level) is similar to the self-convolution of the density of occupied states within the band [4]. Graphite is a typical example [5]. The calculated densities of states of graphite and diamond are in fact rather similar [6]. Referring the energies to the upper edge of the valence band, graphite shows a 7~ peak, derived from sp2 bonds at 1.5 eV, a major peak at 5.1 eV and two less intense peaks at 11 and 18 eV. In diamond the rr peak is absent, the major peak is at 4.5 eV and it has also two less intense peaks at - 11 and - 18 eV. Fig. 1 shows reference spectra of graphite cleaved in air before insertion into the system, of graphite after moderate Ar sputtering, and of synthetic single crystalline (111) diamond. The spectra were taken under identical conditions. The
220
Fig. 1. KW
240
260
961
eV
280
300
320
Auger spectra of graphite samples and of a diamond crystal.
modulation voltage was 2 VP,. At 4 eV at the right-hand side of the main minimum, a shoulder, indicated by a vertical dashed line, is clearly noticeable for the sputtered graphite, less resolved for the cleaved sample and it is absent in diamond. It corresponds to the self-convolution of the 7r band and it is a characteristic fingerprint of sp2 bonding. In less resolved spectra, the shoulder appears as a broadening of the high energy side of the spectrum compared with that of diamond. Typical values for the half width are 5.5 eV in diamond, 7-7.5 eV in graphite and 3.5 eV for a - 100 A thick Sic film on Si [7]. Electron irradiation causes the diamond surface to graph&e and the consequent appearance of the 7~ derived feature. At 8.5 eV of the main minimum, in the low energy side, the cleaved sample exhibits a maximum. At the sputtered sample there is a plateau. In the diamond sample the peak at 13.5 eV from the minimum, indicated by a dashed line has been traditionally taken as the most characteristic feature of diamond Auger emission [8]. In well prepared diamond samples, the intensity of this peak is somewhat higher that in our spectrum. The details in the Auger lineshape at around 10 eV from the main minimum are due to the folding of the major peak at around 5 eV in the density of states with that at around 10 eV. The differences between the diamond and graphite Auger spectra in this energy region originate from small dif-
S. Ferrer et al. / Deposition
962
8 layers of C
220
Fig. 2. Carbon
Fig. 3. Constant
240
260
260
300
eV Auger spectrum for a 12 A thick film grown a Si(100) substrate.
height
320
on
of ultrathin curbon jilms on Si( IO01 ferences in width and peak positions of the corresponding peaks of the density of states. In a previous work [7] it 0was found that for very thin carbon films (2-5 A) the Auger spectra of carbon was the characteristic one of silicon carbide (i.e. rather symmetrical lineshape and relatively small half width). Upon increasing thickness. the half width evolves from 3.5 to around 5.5 eV and the lineshape becomes less symmetrical. Fig. 2 shows a spectrum for a 0 carbon deposit with a thickness of around 12 A. It exhibits a weak maximum at 13.5 eV from the main minimum characteristic of diamond. and it has a half width on the high energy side of the minimum of 5.2 eV indicating a relatively small 7~contribution. Similar spectra have been obtained previously by other authors by using ion implantation tech-
STM image of an area of 67 X 67 A’ of a - 12 A thick film of carbon deposited on a Si substrate. parameters: sample bias 330 mV. current 1.4 nA. The surface crystallinity is apparent.
Imaging
S. Ferrer et al. / Depositionof ultra thin carbonfilm on Si(lo0)
niques to deposit carbon films [9]. Due to the similarity with the diamond spectrum, these films have been designated as corresponding to diamond-like films. 3.2. STM results STM images were taken in both topographic and constant height modes. The first operating mode was used to search for atomically flat patches of the surface and the constant height made was used for atomic resolution imaging. It was found that the preparation of the Si surfaces prior to carbon deposition (several hours annealing at 1200 K at 10-i’ Torr) was essential to obtain atomically flat areas on the carbon films.
Fig. 4. Constant
height STM image of an area of 33 Imaging
X
Fig. 3 shows a current image of an area of 67 X 67 A* displaying rows of ordered atoms. The acquisition time was 30 s. The image is for a 12 A thick carbon film on a Si(100) substrate. As is clearly noticeable the film is crystalline. Images displaying ordering in patches as large as 140 x 140 A2 have been found. Fig. 4 shows a well ordered region of 33 x 33 A2. The acquisition time was 6 s. The data have been slightly Fourier filtered to enhance contrast. As may be seen the surface lattice displays hexagonal symmetry. It was found that the microscope could not be operated since it became unstable when the sample bias was lower than - 50 mV positive or negative. In some regions this threshold value could be as high as 700 mV. This is in contrast
33 AZ. The image shows an hexagonal
parameters:
963
sample bias - 550 mV, current
surface
5 nA.
lattice of the diamond-like
film.
964
S. Ferrer et al. / Deposition of ultra thin curhon films on Si(lWj
with graphite imaging were atomic resolution images were routinely taken at voltages between 5 and 10 mV. This finding was interpreted as being due to a lack of electrical conductivity of the diamond-like films caused by the existence of an electronic band gap. At present however a precise value of the band gap cannot be reported. A previously published work on STM imaging of carbon films prepared by magnetron sputtering [lo], revealed that the films were essentially amorphous since order was only observed in isolated patches of linear dimensions of 15 A at the most. It is worth emphasizing at this point, that amorphous films are the general rule for diamond-like deposits prepared by the usual techniques. The non-obvious result of obtaining ordering on a room temperature substrate by using the laser evaporation technique might be related to two facts: (i) the UHV environment ensures a very small hydrogen conta~nation on the films compared to standard non-UHV techniques and (ii) the kinetic energy of the C species at the vapor phase corresponds to an equivalent temperature of several ten thousands of degrees. This high translational energy compared with thermal evaporation energies may result in long diffusion paths on the surface leading to atomic ordering. It appears then, that laser evaporation is a promising technique for obtaining epitaxial and crystalline films.
4. Summary Carbon films laser evaporation
on Si(100) prepared by pulsed of graphite under UHV show
significant sp’ eshape of the crystalline with cated by STM
bonding as evidenced by the lincarbon Auger line and they are a hexagonal surface lattice as indidata.
This work has been partially supported by the European Community under the project ALADIN. L. Vazquez acknowledges a grant from the Spanish Department of Research.
References Yarbrough and R. Mrssrer. Science 247 (IWO) 68X. and S. Kasi, Science 239 ( 1988) 673. [3] P.K. Schenk, D.W. Bonnell and J.W. Hastie. J. Vat. Set. Technol. A 7 (1989) 1745. in: Low Energy Electrons and [41G. Ertl and J. Ktippers, Surface Chemistry, Monographs in Modern Chemistry. Vol. 4 (Verlag Chemie, Weinheim. 1974). M.B. Guseva, V.G. Bahaev and 0.y. PI V.V. Khostov, Rylova, Surf. Sci. 169 (1986) L253. [61 A. Zunger and R. Enlman. Phys. Rev. B 17 (197X) 647: G.S. Painter, D. Ellis and A.R. Luhinsky, Phys. Rev. B 4 (1971) 3610. F. C’omtn and S. I71J.A. Martin. L. Vazquez, P. Bernard, Ferrer. Appl. Phys. Lett. 57 (1990) 1742. ix1 P.G. Lurie and J.M. Wilson. Surf. Sci. 65 (1977) 476: B.B. Pate. Surf. Sci. 165 (1986) X3. 191 S. Kasi, H. Kang and J.W. Rabalais. Phya. Rev. Lert. 5Y (1987) 75. UOI B. Marchon, M. Salmeron and W. Siekhauc. Phbr. Kc\. B 39 (7989) 12907. [I] WA.
[2] W. Rabalais