石墨烯的无序溅射方法

Deposition of various materials onto graphene without causing any disorder is highly  desirable for graphene applications. Especially, sputtering is a versatile technique to  deposit various metals and insulators for spintronics, and indium tin oxide to make  transparent devices. However, the sputtering process causes damage to graphene because  of high energy sputtered atoms. By flipping the substrate and using a high Ar pressure,  we demonstrate that the level of damage to graphene can be reduced or eliminated in dc,  rf, and reactive sputtering processes.

Graphene has attracted enormous scientific and engineering interest due to its  superior electronic, optical, thermal, mechanical properties1-9. The important properties of  graphene include a high intrinsic charge carrier mobility and a tunability of its properties  by applying a gate bias voltage. In order to make top-gate graphene field effect transistors  (G-FET), a high-k dielectric gate material needs to be deposited onto graphene without  damage, because the mobility of graphene is outstanding when graphene is pristine.10,11 The other interesting application is spintronics, in which an extremely long spin diffusion  length or spin lifetime is preferred possibly due to the weak spin-orbit coupling  strength.12,13 In order to achieve efficient spin injection, high quality ferromagnetic  contacts and tunnel barriers are essential without degrading the intrinsic properties of  graphene during the deposition. All of these emphasize the great importance of a nondestructive deposition technique in constructing graphene devices for both fundamental  studies and practical applications.

Although graphene is the strongest material ever known4,5, but since graphene is  only one monolayer of carbon atoms packed into a two-dimensional honeycomb lattice,  the C-C bonds are easy to be broken during the deposition. Therefore, only thermal  evaporation is known to be a non-destructive method of deposition onto graphene, which  greatly limits the choice of materials in graphene devices. Sputtering is a widely used  technique in industries for hard disk drives and for indium tin oxide coating as well as in  the spintronics community. However, high energy atom bombardment induces a large  amount of disorder into graphene, which greatly degrades the graphene’s property. There  have been a few studies based on sputtering onto graphene and all of them exhibited  significant disorder.

In this letter, we propose a defect-free sputter deposition technique onto graphene  by flipping the sample at high Ar pressure during the deposition in order to reduce the  energy of incident sputtering atoms. We investigate different sputtering techniques such  as dc, rf, and reactive sputtering, and study the level of damage of graphene by Raman  spectroscopy. By flipping the samples and sputtering at high Ar pressure (20 mTorr), we  successfully reduce the degree of disorder to a negligible level.

Fig1

In conclusion, we report a method for sputtering onto graphene without disorder  by flipping the sample holder in a high Ar pressure. Negligible disorder is observed from  the Raman spectra of graphene after the deposition of CoFe and Al by dc sputtering, and  the AFM images show a small roughness of the films on graphene. MgO deposited by rf  sputtering induces large disorder onto graphene due to its enhanced bombardment effect.  However, reactive sputtering of MgO onto graphene leads to a small disorder compared  to rf sputtering. The availability of sputtering for the deposition of various materials in the graphene system will pave a way of rigorous material engineering for various  graphene applications.

We check the uniformity of deposited materials onto graphene by the proposed  sputtering method. Figure 4 shows the atomic force microscopy (AFM) images of 4 nm  CoFe and 2 nm Al on graphene which were deposited by the flipping method with 20  mTorr Ar pressure. The mean roughness of CoFe on graphene is 0.432 nm, while that of  Al is 0.284 nm. The films show a good uniformity promising for practical applications.

Reactive sputtering is an alternative method to deposit high quality tunnel barriers  in a reactive gas mixture with Ar.22 We have deposited 1 nm MgO onto graphene by dc  reactive sputtering with the flipping method and the Raman spectra is shown in Fig. 3(d).  A small D peak is observed, which comes from the oxygen plasma due to the oxygen gas  mixture. By utilizing the proposed flipping method in high Ar pressure, a better quality of  graphene is obtained after the oxide deposition as compared to the previous reports14,16,23,  which will enable to use various oxide materials in graphene devices by sputtering.  Especially, highly spin filtering MgO tunnel barriers are of great importance for  spintronic applications. By replacing the O₂ gas with the N2 or other reactive gases,  various nitrides and other materials can be explored with graphene using reactive  sputtering.