A preparation approach of exploring cluster ion implantation: from ultra-thin carbon film to graphene
© Wang et al.; licensee Springer. 2014
Received: 30 November 2013
Accepted: 5 April 2014
Published: 2 May 2014
Based on the extensive application of 2 × 1.7MV Tandetron accelerator, a low-energy cluster chamber has been built to explore for synthesizing graphene. Raman spectrum and atomic force microscopy (AFM) show that an amorphous carbon film in nanometer was deposited on the silicon by C4 cluster implantation. And we replaced the substrate with Ni/SiO2/Si and measured the thickness of Ni film by Rutherford backscattering spectrometry (RBS). Combined with suitable anneal conditions, these samples implanted by various small carbon clusters were made to grow graphene. Results from Raman spectrum reveal that few-layer graphene were obtained and discuss whether IG/I2D can contribute to explain the relationship between the number of graphene layers and cluster implantation dosage.
PACS: 29.20.-c; 29.25.Ni; 81.05.-t
KeywordsCarbon cluster Low-energy implantation Graphene Raman spectra
In the past of several decades, ion beam analysis (IBA) based on low-energy accelerator has developed to be a comprehensive particle analytical discipline system [1–4]. A further exploitation of what can be paid more attention has springed up on the functional materials , in situ observation for defects on semiconductor industry and the simulation of multi-ion irradiation environment. For instance, the energetic ion-solid interaction was taken as a classic model to characterize some structure information of superconductor at room temperature or high K by projecting MeV ions to impact on superconductive targets . In order to understand the influence induced by implanting multi-energy ions to the substrate, in particular several defects that lead to some phase transitions in matter, in situ characterization of these transients which can exhibit a clear physical image on changeable process of the structure was performed by the accelerator-transmission electron microscopy (TEM) interface system [7, 8]. For practical application of multi-particle irradiation, the purpose of fabricating the multi-ion irradiation stage associated with simulation of the realistic environment where some special materials or functional devices are used is scientific and effective [9, 10]. In a way, not only can ion beam analysis take full advantage of probing the stoichiometry but can also trace reasonable explanation on structure details of the matter .
In Wuhan University, the double 1.7 MV Tandetron accelerator was inherited from Physical Institution of Chinese Academy of Sciences in 2004. After several important maintenances and upgrades of facility, some primary ion beam analysis with terminal voltage at 1.2 MV can be performed in a good state, such as Rutherford backscattering spectrometry (RBS), elastic recoil detection analysis (ERDA), and nuclear reaction analysis (NRA). Besides, we have developed some extensive applications, including accelerator-TEM interface system  and double-ion beam radiation chamber and another new design of low-energy cluster chamber for ion implantation.
As another kind of ultra-thin carbon film, graphene is a promising material which is probable to replace silicon integration technique due to its advanced and novel physical properties [12, 13]. But a key issue for preparing larger scale and continuous thin film is always not comprehensively figured out. Garaj et al. and Baraton et al. have reported graphene synthesis by ion implantation at 30 keV  and 80 keV , respectively. But cluster ions have not been involved, especially in the case of lower energy implantation. Therefore, it is a reasonable attempt that can be attributed to much shallower penetration depth from low-energy cluster ions to dedicate to carbon atoms precipitation form the transition metal under subsequent thermal treatments. In this work, above low-energy cluster chamber is addressed to synthesis nanostructure carbon materials including ultra-thin film and graphene, expanding fundamental ion beam applications in this machine.
Low-energy cluster chamber
A source of negative ion by cesium sputtering (SNICS) can produce various negative ions from solid targets, such as B−, C−, Si−, P−, Fe−, Cu−, and Au−[16, 17], which can be implanted into the substrates after being accelerated up to the maximum 30 keV depending on the accelerator field. Selecting cluster ions with small size as projectiles to perform the process of low-energy ion implantation can form shallow layer architectures in the matrix, which is beneficial to fabricate ultra-shallow junction devices.
Results and discussion
Ultra-thin carbon film deposition
Few-layer graphene synthesis
It is an essential purpose that we designed this low-energy cluster chamber for graphene preparation. In the process of exploring some effective methods, after depositing carbon films with the scale of several nanometers on the silicon, we selected suitable substrates to succeed in achieving few-layer graphene. Uninstalling the decelerated field, we selected small carbon cluster ions to inject to the substrate below 30 keV. The substrate Ni/SiO2/Si with about 300 nm Ni film deposited Ni atoms onto silica by e-beam evaporating.
With regard to depositing Ni film onto silica but not silicon substrate, it was reported that the silicon oxide at a thickness of 300 nm can enhance scattered signals of Raman resonance spectrum drastically because photon can evoke continuous interferences at the interface between Ni and silica . All the matrixes were implanted with the same dosage at 8 × 1015 cm−2 by ion implantation consisting of different cluster sizes at 20 keV. After implantation, these samples were annealed from room temperature to 900°C and dwell time was 60 min, then cooled down to room temperature naturally at 2.0 torr.
In previous studies, we have achieved some carbon cluster mass spectra at the different extractor voltages of sputtering ion source in this machine and paid much attention to investigate Raman characterizations of graphene by means of the carbon cluster ion implantation [18, 23]. Because the cluster ion current can be influenced from cluster size and extractor bias strongly, selecting small carbon cluster ions to carry out implantation is out of more time consumptions. However, more defects can be produced by cluster C1 implantation instead of saving time. For example, implantation time is about 8.5 h for cluster C8 at 20 keV in this work, but the IG/I2D ratio is the smallest which indicates that the graphene quality is better than that in the other smaller cluster sizes.
Simply, E0 is cluster energy, and every atom of Cn cluster can be allocated as homogeneous energy of E0/n. Therefore, in comparison with C1, C n (n > 1) has more sophisticated interactions with the substrate, involving in non-linear damage effect and atomic self-sputtering effect [24, 25]. During such low-energy shallow ion implantation, carbon atom contents in Ni film may reach up to saturation at certain implantation dosage, which is significant for cluster aggregation to interact with the substrate. Graphene nucleation on the transition metal has been investigated to a theoretical growth issue that strongly depends on segregation and precipitation on the grain boundaries of the substrate after thermal treatment , no matter how to prepare graphene, by chemical vapor deposition (CVD) or ion implantation [14, 15, 20, 21]. Baraton et al. have proposed that the anneal temperature from 900°C to 725°C, half of the carbon atoms were removed to grain boundaries of Ni surface to form graphene; that is to say, 4 × 1015 cm−2 and 8 × 1015 cm−2 of carbon concentration on the surface are in agreement with monolayer and bilayer graphene , respectively. However, it is not successful to control the number of graphene layers accurately by regulating the contents of implantation carbon atoms. We always seek to graphene synthesis with fewer defects by low-energy cluster ion technique; larger cluster size C n (n > 10) under suitable energy is more likely to develop this process. But we have to take the atomic self-sputtering effect and more sophisticated cluster-matter interaction into consideration. More investigations are probable to promote the nucleation mechanism of graphene including ion-matter interaction, crystal quality of the substrate, anneal temperature, and other details about growth conditions.
We have developed a low-energy cluster chamber on the base of extensive application for the double 1.7 MV Tandetron accelerator, which was used to explore for graphene synthesis. In our previous work, a kind of amorphous ultra-thin carbon film was fabricated by projecting C4 cluster ions to the silicon at 14 keV, and the RMS is about 5.10 nm. Another substrates Ni/SiO2/Si whose thickness was measured at 227.3 nm by RBS were implanted with the same carbon atoms at 8 × 1015 cm−2 by several kinds of small clusters C n (n = 1, 2, 4, 6, 8); after annealing, Raman spectra indicate that few-layer graphene was prepared successfully. And the ratio IG/I2D shows that the number of graphene layers cannot be controlled by implantation dosage purely but are associated with carbon atoms precipitation and segregation from inside to the surface grain boundaries of the substrate during thermal treatment. From ultra-thin carbon film to graphene by means of the similar cluster ion implantation technique, it is conductive for cluster implantation of light elements to develop low-energy shallow ion implantation in semiconductor industry.
atomic force microscopy
elastic recoil detection analysis
ion beam analysis
nuclear reaction analysis
Rutherford backscattering spectrometry
root mean square roughness
transmission electron microscopy.
This work was supported by the National Natural Science Foundation of China under grant 11350110206 and the Fundamental Research Funds for the Central Universities under the contract (No. 201120202020005). And we sincerely appreciated for help from Professor Liu (firstname.lastname@example.org) who proposed some constructive suggestions for experimental design.
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