Paper GR+AS+BI+PS+SS-ThA2
Plasma Enhanced ALD of Hafnium Oxide on Graphene Layer with Plasma Pretreatment

Thursday, October 31, 2013, 2:20 pm, Room 104 B

Graphene is the candidate of the future generation semiconductor material due to its high mobility of electrons and ultimately thin feature of the 2D structure. The use of graphene for CMOS technology to replace current silicon devices requires matching of each interface between the substrate, electrodes, channel, and dielectrics. Among these, growth of high dielectric constant film growth over graphene is not successful due to the chemically inert nature of the graphene surface. In order to have reactive chemical bond on graphene surface, there are trials of oxidation with ozone, etc.

Here, the effect of the oxygen plasma pretreatment on the graphene surface is examined in this study.

Graphene layer is prepared by peeling method from HOPG using adhesive tape. The domain size is 1 micron in width. (AFM topograph is shown in Fig.1) The pretreatment of the graphene surface is the exposure of O₂ ICP at 30 Pa for less than 1 min. This atomically modifies the topography ( fig . 2), and the chemistry of the surface (fig . 3). The XPS analysis indicates the many of the graphene 2D bonds are replaced by C-O or C-OH bond and the defects are increased.

The growth sequence of Hafnium oxide ALD consists of the exposure of metal precursor (Tetrakis Ethyl Methyl Amino Hafnium : TEMAH) with N₂ buffer flow, N₂ purge, and O₂ ICP at 30 Pa.

The chemical composition from XPS shows the film thickness is specifically controlled by ALD cycle number and it saturates at 4th cycle owing to the limited mean free path of photoelectrons .

The initial growth stage of the film with and without plasma pretreatment is compared for 2nd ALD cycle sample . (AFM in fig . 4 and 5). With pretreatment, the surface consists of 2-5 nm width dispersed nano -islands and 20 nm width HfO₂ mesa (film) of 1nm height. Mesas are separated by base graphene surface by 10 nm pitch. Without pretreatment, the surface is covered by closely packed 5-10nm width nano-islands, around 1nm of height . This comparison indicates the oxygen bonds introduced by O₂ plasma pretreatment contribute to the chemisorption of the precursor and successful 2D growth of HfO₂ in the initial stage. In contrast, inert graphene surface without pretreatment prohibits the interconnection of the physisorbed precursor with the surface and 3D island growth is preferred.

In summary, although the domain size of the HfO₂ is limited to around 20 nm, 2D mode growth is enabled by the introduction of O₂ plasma pretreatment . Further progress is necessary on the increase of the coverage of the film, minimizing the oxidation of the base graphene layer, and reducing nano-sized islands on the film.