Heat Treatment of Graphite Defects Produced by Irradiation with Ar Cluster Ions∗

We investigated defects produced on a graphite surface by bombardment with gas cluster ions consisting of hundreds or thousands of argon atoms, and examined the structural changes of these defects by heat treatment. Two types of defects, craters and dots, were observed in STM images of the ion-bombarded surfaces. The production of these defects depends on the average kinetic energy per constituent atom of the cluster ion, and the threshold energy for the production of the crater and dot are 5 eV and 1.5 eV, respectively. After heating an ion-bombarded sample to 560◦C in air, the craters changed to hexagonal pits and most of the dots disappeared, although a few (∼10%) dots changed to irregularly shaped pits. We describe the formation mechanisms of the two pit types on the basis of the oxidative etching of ion-induced defects. [DOI: 10.1380/ejssnt.2012.88]


I. INTRODUCTION
Ion-beam technologies are widely used in various fields such as semiconductor processing, micromachining, and surface analysis.The rapid development of electronic and optical devices requires an accuracy of within tenths of nanometers in etching depth.In secondary ion mass spectrometry (SIMS), depth profiling of chemical species with sub-nanometer resolution is now required.In order to fill these needs, the ion beam energy must be significantly reduced while maintaining the ion current, which is difficult to accomplish.
Gas cluster ions consist of gaseous atoms such as argon (Ar) aggregated by Van der Waals forces, and these clusters typically contain hundreds to thousands of atoms or molecules [1][2][3].The average kinetic energy per atom in a cluster ion is equal to the energy used to accelerate the cluster ion divided by the number of atoms in the cluster ion (called the cluster size).For example, when a cluster of 1000 atoms is accelerated to 10 keV, the average kinetic energy per atom of the cluster ion (E a ) is only 10 eV [4].Furthermore, when a cluster ion collides with a surface, chemical reactions such as sputtering or etching can be concentrated within a very thin layer by multiple collisions among the numerous atoms constituting the cluster ion.
We previously analyzed crater-like traces on graphite caused by Ar cluster ion bombardment, and obtained the following result: the kinetic energy per atom of the cluster ion used for bombardment almost completely determines the rate of trace production.A certain level of kinetic energy per atom or greater is required to produce the traces, and this threshold energy is determined by the cohesive energy of the carbon atoms that constitute the graphite.
The effects of ion bombardment on graphite have been reported by other groups [5][6][7][8].Most of these studies, however, employed monoatomic ions such as Ar + .
Bräucle et al. analyzed defects on graphite induced by the bombardment of carbon cluster ions (dominantly C + 60 ) [9,10].However, in that case, the supply of carbon atoms from the cluster itself to the defect sites may have affected the defect structure.With Ar cluster bombardment, on the other hand, the possibility of contamination is extremely low.In this study, we investigated the structural changes of traces, including craters produced on graphite, by heating in air.Furthermore, we discuss the mechanism of structural change on the basis of the oxidative etching of ion-induced traces.

II. EXPERIMENTAL
The system used for the generation and bombardment of Ar cluster ions is shown in Fig. 1.This system was composed of four chambers: the cluster generation chamber (source chamber), the ionization chamber, the sizeselection chamber, and the irradiating chamber (sample chamber).Ar gases introduced into the source chamber were aggregated into neutral clusters by Van der Waals' force after adiabatic expansion at the nozzle.Next, the neutral clusters were positively ionized by electron impact using a filament in the ionization chamber.These cluster ions were accelerated by an electrode installed in a subsequent stage.The cluster size was selected by a time-offlight (TOF) technique using a first and second deflector in the cluster size-selection chamber before bombarding the sample.Ar cluster ions with cluster sizes of 500 ∼ 2500 atoms/cluster were bombarded at accelerations of 3 keV or 7.5 keV and a dosage of 5×10 10 ions/cm 2 .The ion bombardment was carried out at room temperature, and the sample chamber vacuum was 5×10 −6 Pa.The sample was highly oriented pyrolytic graphite (HOPG; ZYH grade, NT-MDT Ltd.) cleaved in air.After the ion bombardment, the sample was exposed to the atmosphere, and heated at 560 • C for 20 min in air using an electric furnace (ANF-12, Asahi-rika, Ltd.).The heating temperature selection was based on a report by Chang et al. that the oxidative etching of HOPG without ion bombardment can occur at temperatures above 700 • C [11].Sample surfaces were observed before or after heat treatment using a scanning tunneling microscope (remodeled USM-1100, UNISOKU Ltd.) in constant-current mode.Mechanically    polished Pt-Ir tips were used.The typical tunneling current was 0.9-1.2nA and the bias voltage at the sample was 0.3-1.0V. Tunneling current-bias voltage characteristics (I-V curves) were measured over both ion-bombarded and un-bombarded areas.

A. Defects induced by Ar cluster ion bombardment
Figure 2(a) is an STM image of a graphite surface after bombardment with Ar cluster ions of 750 atoms/cluster at 7.5 keV (E a : 10 eV).Two types of defects were observed: crater-like defects about 10 nm in diameter (Fig. 2(b)) and dot-like defects about 2 nm in diameter (Fig. 2(c)).These dot-like defects have also been reported after bombardment with other ions such as Ar + and Ar 8+ [5,7].In contrast, the crater-like defect is uniquely produced by bombardment with gas cluster ions.The dependence of the rate of generation of these defects (the density of the defects observed divided by the ion dose) on the energy per atom of the cluster ion is shown in Fig. 3. Far fewer craters than dots were produced at the energies per atom examined here.We previously reported that the crater production increases to ∼80% when the energy per atom exceeds 40 eV/atom [12].The difference between the defect production rates can be explained as follows.When an Ar cluster ion hits the surface, the cluster is compressed at the surface and decomposed into many energetic Ar atoms.These Ar atoms can sputter away the carbon atoms parallel to the surface (lateral sputtering), forming the crater-like defects.On the other hand, most of the dot-like defects are formed by Ar atoms trapped between the basal planes of the graphite, since most of the dots disappeared by heating at low temperature such as 560 • C (reported later).Our results indicate that Ar atoms can penetrate into the basal planes at energies below 5 eV/atom.On the contrary, in the case of bombardment with a monoatomic ion such as Ar + , the threshold energy for an Ar atom to penetrate into the top layer of graphite was 42 eV, according to calculations using a classical trajectory method [13].Multiple collisions among the many Ar atoms of the cluster ion could significantly lower the energy per atom required for the penetration of the Ar atom [14].

B. Structural changes of the ion-induced defects by heating in air
The structure of the ion-induced defects was drastically changed by heating the sample at 560 • C for 20 min in air.The craters completely disappeared, and the number of dot-like defects was reduced by about 90%.Instead, hexagonally shaped pits with an average size of about 50 nm (Fig. 4   pits.About 10% of the dot-like defects became irregularly shaped pits, about 80% disappeared, and the rest (∼10%) remained unchanged.The depth of the hexagonal pits and the irregularly shaped pits were almost uniformly 1.0 nm and 0.3 nm, respectively, which correspond to thicknesses of 3 and 1 graphite layers, respectively.It was also confirmed that the graphite structure was fully maintained at the bottom of both pits (Figs.4(a) and 4(b), inset).Increasing the heating time to 200 min did not change the pit depth, but increased the pit area by approximately 50 times.
These two types of pits are basically created by oxidative etching of dot-like defects or craters.Carbon vacancies are intensively formed by displacement of surface carbon atoms at the craters.The etching mechanism of carbon vacancies has been explained [15] in terms of initial dissociative chemisorptions of O 2 at the dangling carbon bonds of a vacancy, followed by activated desorption of CO or CO 2 to remove the carbon atoms.This desorption creates new dangling bonds, and the etching continues in the same carbon plane by further O 2 reaction.The etching of the dot-like defects is initiated by oxidation of the graphite surface at a location that has been deformed by a trapped interstitial Ar atom below.The increase in reactivity at the deformed surface location results from a charge density increase in this region.The tunneling conductance near zero bias voltage at a small number of the dot-like defects was larger than in the flat regions, as shown in Fig. 5. Once an initial O 2 attack creates a vacancy on the basal plane, the rest of the defect can be etched in the same way as the craters.Judging from the magnified STM image in Fig. 2(c), the interstitial Ar atoms seem to be randomly distributed under the basal plane, which could result in the irregularly shaped pits.On the contrary, O 2 reactions initiate more symmetrically along the crater, and successive removal of carbon atoms can form hexagonally shaped pits that reflect the graphite structure.

IV. CONCLUSION
Crater and dot-like defects were formed on a graphite surface by bombardment with Ar cluster ions.The rate of production of these defects depends on the average kinetic energy per constituent atom of the cluster ion.When ion-induced defects were heated in air, the craters were etched to form hexagonally shaped pits.Most of the dots were formed by interstitial Ar atoms distributed underneath the basal plane, and disappeared during heating.A few of the dots were etched to form irregularly shaped pits.The depths of the hexagonal and irregular pits were approximately equal to the thicknesses of 3 and 1 graphite layers, respectively.Our results indicate that the etching depth can be controlled by adjusting the kinetic energy per atom of the bombarding cluster.Graphene, single layer graphite, has recently attracted much attention scientifically and technologically.The graphite etching behavior obtained here may be useful in fabricating graphene with uniformly controlled thickness.

Fig . 1 .
Fig .1.FIG.1: Schematic view of Ar cluster ion beam generation and the bombardment system.

FIG. 2 :
FIG. 2: STM image of a graphite surface after bombardment with Ar + 750 at 7.5 keV (a), a magnified STM image of a craterlike defect (b), and a dot-like defect (c).
(a)) and irregularly shaped pits with sizes of 5-10 nm (Fig. 4(b)) appeared.A statistical study of the number of defects and pits in the STM images clarified the structural changes of the defects during heating as follows: the crater-like defects almost all became hexagonal Volume 10 (2012) Tsukuda, et al.

Fig. 4 .
Fig.4.FIG.4: STM images of a graphite sample heated at 560 • C for 20 min after bombardment with (a) Ar + 750 at 7.5 keV and (b) Ar + 2500 at 7.5 keV, (c) depth profiles along a dashed line over a hexagonal pit and (d) an irregulaly shaped pit.

Fig. 5 .
Fig.5.FIG.5: The tunneling current-bias voltage characteristics measured over a dot-like defect and over a flat region in an STM image of a graphite surface bombarded with Ar + 2500 at 7.5 keV.