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Threshold energy for the displacement of surface atoms in graphite
THRESHOLD ENERGY FOR THE DISPLACEMENT OF SURFACE ATOMS GRAPHITE* G. L. MONTET Argonne National Laboratory,
IN
and G. E. MYERS Argonne,
Illinois 60439, LJ.S.A.
(Received 20 July 1970)
Abstract-The energy required to displace a carbon atom from a site in the surface of a graphite crystal has been determined by the detection of surface vacancies resulting from the displacement process. Cleaved fragments, a few hundred Angstrom units thick from annealed natural crystals of graphite containing less than 10-l” vacancies per carbon atom were irradiated at room temperature with electrons of selected energies in the range from 100 to 200 keV. Preparation of these fragments by an etch-decoration technique and subsequent examination in an electron microscope indicated that irradiation by electrons possessing an energy of 150 keV was sufficient to produce vacancies in the surface of graphite; this corresponds to a threshold displacement energy of 3 I eV, which is the value found for production in the interior. It was found necessary to take extreme precautions to protect the surface from chemical attack.
1. INTRODUCTION
A fast particle traversing the lattice of a crystalline solid may lose energy by several processes, depending on the nature of the particle and its energy. In considering displacement of atoms from lattice sites, the most important means of energy loss is by elastic collisions with the atoms. If in an elastic collision the lattice atom receives an energy in excess of Ed, defined as the displacement energy, it is displaced from its normal lattice site. The displacement process, therefore, creates a lattice vacancy and an interstitial atom, since in most cases the recoiling atom comes to rest in a non-equilibrium site. The value of Ed is usually considerably larger than the energy required to form an interstitial-vacancy pair by a thermodynamically reversible process; this is because the recoil takes place before the neighboring atoms have an opportunity to relax. *Based on work performed under the auspices of the U.S. Atomic Energy Commission.
The process just described is called the primary process; the interaction between the incident particle and the lattice atom is the primary collision. In most cases of bombardment with heavy particles the primary recoil is also of importance. If the energy of the primary is considerably greater than E,!, the primary causes further atomic displacements of secondaries, and the secondaries, in turn, produce tertiaries, etc., until the energy of each particle has been degraded below Ed. Such displacement cascades account for most of the defects produced during irradiations with heavy particles. If or.e is interested in determining the threshold cascades energy Ed, these displacement introduce severe experimental difficulties; for this reason electrons of an energy too low to produce secondary displacements are usually chosen as the incident particles. Displacement energies are usually determined by the measurement of property typically of electrical resistance, changes, caused by the electron bombardment. Such
180
G. L. MONTET
measurements yield a value of the displacement energy dependent on the evaluation of various parameters occurring in the theoretical expression for the cross-section for atomic displacements. In the case of graphite technique the etch-decoration developed by Hennig [l] permits a direct observation of the vacancies resulting from the primary process, hence a direct determination of the threshold energy. The results obtained from experiments carried out with this technique have been reported [2]. In principle, the etch-decoration technique can be used to detect vacancies produced in the surface, as well as those produced in the interior, of the sample so that a determination can be made of the energy required to displace an atom in the surface layer. Conceivably, this energy may be different from the displacement energy within the crystal. This article reports on experiments undertaken to determine the threshold energy for the displacement of carbon atoms in the surface layers of graphite. 2. EXPERIMENTAL
An electron of energy sufficient to eject a carbon atom from its lattice position is in the relativistic range, and the equation governing the energy transfer from electron to target atom is Et
=
2 (E, + 2mc2)E, Me2
’
In this equation, E, and Et are tht energies of the electron and ejected atom, respectively, and m and M their respective rest masses; c is the velocity of light in vacuum. The displacement energy Ed is the minimum value of Et, which corresponds to a value of E, just sufficient to produce primary vacancies. The first series of experiments were carried out in the manner described earlier[2], with two important differences. Firstly, the electron irradiations were performed in a Cockcroft-Walton accelerator due to diffi-
and G. E. MYERS
culties in operating the van de Graaff accelerator at such low energies. The maximum energy attainable in the CockcroftWalton was 150 keV and irradiations were restricted to the range loo-150 keV. Secondly, in order to detect vacancies produced in the surface layer, it was necessary to electronbombard thin slices, a few hundred Angstrom units thick, cleaved from single crystals of natural graphite annealed at -2300°C to reduce the vacancy content to
1. Electron
micrograph
of etch pits produced
by chemical attack on surface &graphite
crystal.
2. Electron
micrograph
of etch pit produced by electron irradiation crystal.
in interior of graphite
3. Electron
micrograph
of etch pits in surface of graphite electrons.
crystal after
irradiation
with 125 KeV
4. Electron
micrograph
of etch pits in surface of graphite _1--&--.--
crystal after
irradiation
with 150 KeV
DISPLACEMENT
OF SURFACE
fragments were covered by a copper grid or by a relatively thick layer of deposited gold while the sample was being bombarded by electrons. The grid was then removed mechanically-or the gold was removed by reaction with chlorine at temperatures high enough to volatilize gold chloride-and the etch-decoration procedure was carried out. Both the covered and uncovered areas of the specimen were then examined for etch pits in an attempt to detect any spurious etch pits which might be formed by chemical reaction with activated gases in the target region. ‘1’0 ascertain that electrons of a given energy might produce vacancies in the surface of graphite but not in the interior, irradiations of single crystals were carried out at various energies. The crystals were potted in silver and handled as described previously [2]. 3. RESULTS
AND DISCUSSION
Electron micrographs of two typical Cockcroft-Walton specimens are presented. Figure 1 shows a well-decorated area of a thin-flake specimen irradiated with 120 keV electrons (charge deposited -20 millicoulombs cm-“) in the accelerator; it is evident that a fairly high concentration of etch pits (it is believed these are due to chemical attack- see below) is revealed despite the presence of some debris on the surface. Figure 2 shows a well-decorated area of a thin slice cleaved from the interior of a single crystal irradiated with 150 keV electrons (charge deposited - 24 miliicoulombs cm-‘); it is evident that a low concentration of etch pits is revealed under the cleaner conditions of this experiment. The concentration, although low, is twenty times the background concentration obtained by etching and decorating an interior slice from an equivalent, unirradiated, single crystal; hence, we believe this low concentration of vacancies is formed near the threshold energy. Etch pits were detected in the surface of all
ATOMS
IN GRAPHITE
181
thin fragments irradiated, when satisfactory decoration could be obtained. The detection of pits in surfaces protected from the electron beam indicated that this result was anomalous and a program to study this phenomenon by irradiating specimens in the electron microscope was initiated. It was found that a high concentration of etch pits could be produced in the surface layer of unprotected fragments even at the lowest energy, 40 kV, obtainable in the microscope. On the assumption that these etch pits resulted from the action of ions formed in the residual gas in the microscope by activation by electrons, various methods of protecting the surface by films thin enough to allow electrons to penetrate but thick enough to stop ions were tried. All of these failed-even the deposition of a gold film about 2 km in thickness by means of vacuum evaporation at lOPi Torr failed to prevent the formation of a concentration of -lOei vacancies per carbon atom after one millicoulomb of charge was deposited. From these experiments it was concluded that the etch pits resulted from chemical attack of the graphite surface by ions formed in the adsorbed film on that surface. It may be calculated readily that an adsorbed film will be formed in about one second from a vacuum of about 10PiTorr, so that outgassing of the surface and subsequent protection of that surface must be carried out in an ultra-high vacuum system. By outgassing the graphite fragments at -1000°C to a pressure of - IO-” Torr, cooling at this pressure, and then depositing a relatively thick (-i pm to insure that a continuous film would he formed) layer of gold on the cleaved surface, it was possible to reduce the etch pit concentration concentration resulting from a 1 millicoulomb irradiation of 100 keV electrons to -lo-” per carbon atom. As a result of the above observations the tedious and rather involved procedure described below was adopted. The thin fragments, a few hundred Angstrom units thick, were prepared as described previously [2].
182
G. L. MONTET
These fragments were caught on copper electron microscope grids, placed on a tungsten ribbon, outgassed and protected in an ultra-high vacuum system, as described immediately above, and then irradiated with high energy electrons through the gold coating. (It can readily be calculated that this will not degrade the energy of the electrons significantly [3].) S ome of these irradiations were carried out on the Cockroft-Walton accelerator using electrons of energies from 130 to 150 keV, but more samples were irradiated in a high-voltage electron microscope using electrons of energies of 125, 150, 175, and 200 keV, since a beam of higher intensity was attainable and the conditions of irradiation were more easily controlled. The pertinent results of one of the better runs in which 3 millicoulombs of charge was deposited in each specimen (-300 millicoulombs per cm2 compared to -20 millicoulombs per cm2 in the Cockcroft-Walton) are displayed in Figs. 3 and 4. The low concentration, 3 X lo-” per carbon atom, of pits in the surface irradiated at 125 keV, Fig. 3, is probably the background concentration due to residual adatoms. This being the case, the concentration 1.3 X lo-’ pits per carbon atom shown in Fig. 4 represents an increase of lOen V/C caused by the 150 keV irradiation; hence, the threshold energy for displacement from the surface is s 31 eV, from equation (1). This conclusion is strengthened when it is observed that the thick monitor samples yielded an interior concentration of
and
G. E. MYERS
slow rate that only a very long irradiation would result in a detectable concentration. Due to operating difficulties with the machine it was deemed impracticable to attempt such long irradiations. These experiments thus indicate that the threshold energy for displacement of carbon atoms from the planar surfaces of graphite, and from the interior, is Ed=31
eV.
(2)
This value is obtained from the CockcroftWalton data as well as from the high-voltage electron microscope data. The threshold energy for atomic displacements in bulk graphite is in essential agreement with that given earlier[2] (33 eV), since the estimated precision of each measurement is +-1 eV. (It is also in fair agreement with the value reported by Iwata and Nihara[4]). It is felt, nevertheless, that the reason for the discrepancy most likely lies in the determination of the energy of the electrons. It is very difficult to calibrate the energy of the van de Graaff beam in this low range and it may well be that an energy of -160 keV for the van de Graaff is equivalent to -150 keV for the Cockcroft-Walton and the high energy electron microscope; all of the beam energies are known only to -+5 per cent. The essential equality of the threshold displacement for carbon atoms in the surface layer and in the interior of a graphite crystal is readily understood. In a more isotropicallybonded material it would be expected that the threshold energy for displacement of an atom from the surface should be lower than the energy of displacement from the interior because fewer bonds must be broken in the former case; however, the bonding in the layer-structured graphite is such that the bonding of an atom in a surface layer is, to a first approximation, essentially equivalent to the bonding of an atom in an interior layer -the only difference being a small van der Waals’ type contribution. On this basis it is
DISPLACEMENT
OF SCRFACE
to be expected that the surface displacement energy and the bulk displacement energy should be very nearly equal, differing at most by a few per cent; this is in accord with our observations. It would be interesting to apply this, or a similar, technique to the determination of threshold energies for displacement of atoms in the surface and in the interior of an isotropically bound material in an attempt to find the expected difference. The etchdecoration technique depends to a great extent on the anisotropic properties of layered substances and it is difficult to discern its extension to isotropic substances; in any case, our experience has demonstrated that it is necessary to exercise great care in
ATOMS
attempting
IX3
IN (iRAPHI7‘E
to detect vacancies in the surface.
Acknowledgments-
We are indebted to I‘. Klippert and A. Youngs for the irradiations carried out in accelerator and to I.. the Cockcroft-Walton Michels for those carried out in the high voltage electron mic-roscope. We thank K. Huebrner and K. Karnpwirth for the use of their ultra-hi@ V;ICUL~I~~ system. We are gralel’ul to K. Blankensh~p for preparation of some of the s;m~plr:s and for assistance in carrying out some of the runs at the (:ockcroft-WaltoIl accelerator
1. Hennig 2. Montet
REFERENCES G. K., Appl. Phy. f.ptt. 4. .X2 ( IM4). G. I,., Cnrbon 5, I9 (IUG). M. J. and Seltzer S. M., NASA SP3012
3. Berger (1964). 4. Iwata ‘1‘.and Nihar-a ‘I‘., Phy.
f.~t/. 23, 631
(1966).
IN
and G. E. MYERS Argonne,
Illinois 60439, LJ.S.A.
(Received 20 July 1970)
Abstract-The energy required to displace a carbon atom from a site in the surface of a graphite crystal has been determined by the detection of surface vacancies resulting from the displacement process. Cleaved fragments, a few hundred Angstrom units thick from annealed natural crystals of graphite containing less than 10-l” vacancies per carbon atom were irradiated at room temperature with electrons of selected energies in the range from 100 to 200 keV. Preparation of these fragments by an etch-decoration technique and subsequent examination in an electron microscope indicated that irradiation by electrons possessing an energy of 150 keV was sufficient to produce vacancies in the surface of graphite; this corresponds to a threshold displacement energy of 3 I eV, which is the value found for production in the interior. It was found necessary to take extreme precautions to protect the surface from chemical attack.
1. INTRODUCTION
A fast particle traversing the lattice of a crystalline solid may lose energy by several processes, depending on the nature of the particle and its energy. In considering displacement of atoms from lattice sites, the most important means of energy loss is by elastic collisions with the atoms. If in an elastic collision the lattice atom receives an energy in excess of Ed, defined as the displacement energy, it is displaced from its normal lattice site. The displacement process, therefore, creates a lattice vacancy and an interstitial atom, since in most cases the recoiling atom comes to rest in a non-equilibrium site. The value of Ed is usually considerably larger than the energy required to form an interstitial-vacancy pair by a thermodynamically reversible process; this is because the recoil takes place before the neighboring atoms have an opportunity to relax. *Based on work performed under the auspices of the U.S. Atomic Energy Commission.
The process just described is called the primary process; the interaction between the incident particle and the lattice atom is the primary collision. In most cases of bombardment with heavy particles the primary recoil is also of importance. If the energy of the primary is considerably greater than E,!, the primary causes further atomic displacements of secondaries, and the secondaries, in turn, produce tertiaries, etc., until the energy of each particle has been degraded below Ed. Such displacement cascades account for most of the defects produced during irradiations with heavy particles. If or.e is interested in determining the threshold cascades energy Ed, these displacement introduce severe experimental difficulties; for this reason electrons of an energy too low to produce secondary displacements are usually chosen as the incident particles. Displacement energies are usually determined by the measurement of property typically of electrical resistance, changes, caused by the electron bombardment. Such
180
G. L. MONTET
measurements yield a value of the displacement energy dependent on the evaluation of various parameters occurring in the theoretical expression for the cross-section for atomic displacements. In the case of graphite technique the etch-decoration developed by Hennig [l] permits a direct observation of the vacancies resulting from the primary process, hence a direct determination of the threshold energy. The results obtained from experiments carried out with this technique have been reported [2]. In principle, the etch-decoration technique can be used to detect vacancies produced in the surface, as well as those produced in the interior, of the sample so that a determination can be made of the energy required to displace an atom in the surface layer. Conceivably, this energy may be different from the displacement energy within the crystal. This article reports on experiments undertaken to determine the threshold energy for the displacement of carbon atoms in the surface layers of graphite. 2. EXPERIMENTAL
An electron of energy sufficient to eject a carbon atom from its lattice position is in the relativistic range, and the equation governing the energy transfer from electron to target atom is Et
=
2 (E, + 2mc2)E, Me2
’
In this equation, E, and Et are tht energies of the electron and ejected atom, respectively, and m and M their respective rest masses; c is the velocity of light in vacuum. The displacement energy Ed is the minimum value of Et, which corresponds to a value of E, just sufficient to produce primary vacancies. The first series of experiments were carried out in the manner described earlier[2], with two important differences. Firstly, the electron irradiations were performed in a Cockcroft-Walton accelerator due to diffi-
and G. E. MYERS
culties in operating the van de Graaff accelerator at such low energies. The maximum energy attainable in the CockcroftWalton was 150 keV and irradiations were restricted to the range loo-150 keV. Secondly, in order to detect vacancies produced in the surface layer, it was necessary to electronbombard thin slices, a few hundred Angstrom units thick, cleaved from single crystals of natural graphite annealed at -2300°C to reduce the vacancy content to
1. Electron
micrograph
of etch pits produced
by chemical attack on surface &graphite
crystal.
2. Electron
micrograph
of etch pit produced by electron irradiation crystal.
in interior of graphite
3. Electron
micrograph
of etch pits in surface of graphite electrons.
crystal after
irradiation
with 125 KeV
4. Electron
micrograph
of etch pits in surface of graphite _1--&--.--
crystal after
irradiation
with 150 KeV
DISPLACEMENT
OF SURFACE
fragments were covered by a copper grid or by a relatively thick layer of deposited gold while the sample was being bombarded by electrons. The grid was then removed mechanically-or the gold was removed by reaction with chlorine at temperatures high enough to volatilize gold chloride-and the etch-decoration procedure was carried out. Both the covered and uncovered areas of the specimen were then examined for etch pits in an attempt to detect any spurious etch pits which might be formed by chemical reaction with activated gases in the target region. ‘1’0 ascertain that electrons of a given energy might produce vacancies in the surface of graphite but not in the interior, irradiations of single crystals were carried out at various energies. The crystals were potted in silver and handled as described previously [2]. 3. RESULTS
AND DISCUSSION
Electron micrographs of two typical Cockcroft-Walton specimens are presented. Figure 1 shows a well-decorated area of a thin-flake specimen irradiated with 120 keV electrons (charge deposited -20 millicoulombs cm-“) in the accelerator; it is evident that a fairly high concentration of etch pits (it is believed these are due to chemical attack- see below) is revealed despite the presence of some debris on the surface. Figure 2 shows a well-decorated area of a thin slice cleaved from the interior of a single crystal irradiated with 150 keV electrons (charge deposited - 24 miliicoulombs cm-‘); it is evident that a low concentration of etch pits is revealed under the cleaner conditions of this experiment. The concentration, although low, is twenty times the background concentration obtained by etching and decorating an interior slice from an equivalent, unirradiated, single crystal; hence, we believe this low concentration of vacancies is formed near the threshold energy. Etch pits were detected in the surface of all
ATOMS
IN GRAPHITE
181
thin fragments irradiated, when satisfactory decoration could be obtained. The detection of pits in surfaces protected from the electron beam indicated that this result was anomalous and a program to study this phenomenon by irradiating specimens in the electron microscope was initiated. It was found that a high concentration of etch pits could be produced in the surface layer of unprotected fragments even at the lowest energy, 40 kV, obtainable in the microscope. On the assumption that these etch pits resulted from the action of ions formed in the residual gas in the microscope by activation by electrons, various methods of protecting the surface by films thin enough to allow electrons to penetrate but thick enough to stop ions were tried. All of these failed-even the deposition of a gold film about 2 km in thickness by means of vacuum evaporation at lOPi Torr failed to prevent the formation of a concentration of -lOei vacancies per carbon atom after one millicoulomb of charge was deposited. From these experiments it was concluded that the etch pits resulted from chemical attack of the graphite surface by ions formed in the adsorbed film on that surface. It may be calculated readily that an adsorbed film will be formed in about one second from a vacuum of about 10PiTorr, so that outgassing of the surface and subsequent protection of that surface must be carried out in an ultra-high vacuum system. By outgassing the graphite fragments at -1000°C to a pressure of - IO-” Torr, cooling at this pressure, and then depositing a relatively thick (-i pm to insure that a continuous film would he formed) layer of gold on the cleaved surface, it was possible to reduce the etch pit concentration concentration resulting from a 1 millicoulomb irradiation of 100 keV electrons to -lo-” per carbon atom. As a result of the above observations the tedious and rather involved procedure described below was adopted. The thin fragments, a few hundred Angstrom units thick, were prepared as described previously [2].
182
G. L. MONTET
These fragments were caught on copper electron microscope grids, placed on a tungsten ribbon, outgassed and protected in an ultra-high vacuum system, as described immediately above, and then irradiated with high energy electrons through the gold coating. (It can readily be calculated that this will not degrade the energy of the electrons significantly [3].) S ome of these irradiations were carried out on the Cockroft-Walton accelerator using electrons of energies from 130 to 150 keV, but more samples were irradiated in a high-voltage electron microscope using electrons of energies of 125, 150, 175, and 200 keV, since a beam of higher intensity was attainable and the conditions of irradiation were more easily controlled. The pertinent results of one of the better runs in which 3 millicoulombs of charge was deposited in each specimen (-300 millicoulombs per cm2 compared to -20 millicoulombs per cm2 in the Cockcroft-Walton) are displayed in Figs. 3 and 4. The low concentration, 3 X lo-” per carbon atom, of pits in the surface irradiated at 125 keV, Fig. 3, is probably the background concentration due to residual adatoms. This being the case, the concentration 1.3 X lo-’ pits per carbon atom shown in Fig. 4 represents an increase of lOen V/C caused by the 150 keV irradiation; hence, the threshold energy for displacement from the surface is s 31 eV, from equation (1). This conclusion is strengthened when it is observed that the thick monitor samples yielded an interior concentration of
and
G. E. MYERS
slow rate that only a very long irradiation would result in a detectable concentration. Due to operating difficulties with the machine it was deemed impracticable to attempt such long irradiations. These experiments thus indicate that the threshold energy for displacement of carbon atoms from the planar surfaces of graphite, and from the interior, is Ed=31
eV.
(2)
This value is obtained from the CockcroftWalton data as well as from the high-voltage electron microscope data. The threshold energy for atomic displacements in bulk graphite is in essential agreement with that given earlier[2] (33 eV), since the estimated precision of each measurement is +-1 eV. (It is also in fair agreement with the value reported by Iwata and Nihara[4]). It is felt, nevertheless, that the reason for the discrepancy most likely lies in the determination of the energy of the electrons. It is very difficult to calibrate the energy of the van de Graaff beam in this low range and it may well be that an energy of -160 keV for the van de Graaff is equivalent to -150 keV for the Cockcroft-Walton and the high energy electron microscope; all of the beam energies are known only to -+5 per cent. The essential equality of the threshold displacement for carbon atoms in the surface layer and in the interior of a graphite crystal is readily understood. In a more isotropicallybonded material it would be expected that the threshold energy for displacement of an atom from the surface should be lower than the energy of displacement from the interior because fewer bonds must be broken in the former case; however, the bonding in the layer-structured graphite is such that the bonding of an atom in a surface layer is, to a first approximation, essentially equivalent to the bonding of an atom in an interior layer -the only difference being a small van der Waals’ type contribution. On this basis it is
DISPLACEMENT
OF SCRFACE
to be expected that the surface displacement energy and the bulk displacement energy should be very nearly equal, differing at most by a few per cent; this is in accord with our observations. It would be interesting to apply this, or a similar, technique to the determination of threshold energies for displacement of atoms in the surface and in the interior of an isotropically bound material in an attempt to find the expected difference. The etchdecoration technique depends to a great extent on the anisotropic properties of layered substances and it is difficult to discern its extension to isotropic substances; in any case, our experience has demonstrated that it is necessary to exercise great care in
ATOMS
attempting
IX3
IN (iRAPHI7‘E
to detect vacancies in the surface.
Acknowledgments-
We are indebted to I‘. Klippert and A. Youngs for the irradiations carried out in accelerator and to I.. the Cockcroft-Walton Michels for those carried out in the high voltage electron mic-roscope. We thank K. Huebrner and K. Karnpwirth for the use of their ultra-hi@ V;ICUL~I~~ system. We are gralel’ul to K. Blankensh~p for preparation of some of the s;m~plr:s and for assistance in carrying out some of the runs at the (:ockcroft-WaltoIl accelerator
1. Hennig 2. Montet
REFERENCES G. K., Appl. Phy. f.ptt. 4. .X2 ( IM4). G. I,., Cnrbon 5, I9 (IUG). M. J. and Seltzer S. M., NASA SP3012
3. Berger (1964). 4. Iwata ‘1‘.and Nihar-a ‘I‘., Phy.
f.~t/. 23, 631
(1966).