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Graphene Growth and Device Integration INVITED PAPER Graphene Growth and Device Integration This paper describes one of the emerging methods for growing grapheneVthe chemical vapor deposition methodVwhich is based on a catalytic reaction between a carbon precursor and a metal substrate such as Ni, Cu, and Ru, to name a few. By Luigi Colombo, Fellow IEEE, Robert M. Wallace, Fellow IEEE,andRodneyS.Ruoff ABSTRACT | Graphene has been introduced to the electronics devices in order to exceed the performance characteristics community as a potentially useful material for scaling elec- of current device applications as well as developing new tronic devices to meet low-power and high-performance devices, especially for flexible electronics, transparent targets set by the semiconductor industry international road- electrodes for displays and touch screens, photonic map, radio-frequency (RF) devices, and many more applica- applications, energy generation, and batteries [3], [12], tions. Growth and integration of graphene for any device is [13]. The first graphene films were only a few tens of challenging and will require significant effort and innovation to micrometers on the side and so the principal first issue in address the many issues associated with integrating the making graphene devices a reality is the development of a monolayer, chemically inert surface with metals and dielec- graphene of sufficient quality and size to meet the basic trics. In this paper, we review the growth and integration of physical and electronic properties. Since the isolation of graphene for simple field-effect transistors and present graphene from natural graphite, a number of techniques physical and electrical data on the integrated graphene with and processes have been studied and are in development to metals and dielectrics. form graphene materials for a variety of applications; an extensive and complete review of these processes has KEYWORDS | Chemical vapor deposition (CVD); electrochemical recently been summarized by Bonaccorso et al. [22] and a transfer; graphene; mobility; Raman spectroscopy; X-ray concise history of graphene is presented by Dreyer et al. photoelectron spectroscopy [23]. It is still too early to select the final graphene growth process for high-performance electronic devices since none of the processes meets all of the basic requirements. I. INTRODUCTION Today, there are a few sourcesofhigh-qualitygraphene Over the past seven years or so there has been a lot of films: 1) exfoliated graphene from natural graphite or excitement in using graphene for many electronic various other sources of graphite [24]; 2) graphene films applications among others [3]–[6]. Researchers have on SiC single crystals formed by the evaporation of silicon looked to graphene as a material with which to fabricate from either the carbon- or silicon-rich surfaces of SiC [25]; and 3) graphene grown on metal substrates by chemical vapor deposition (CVD) [11], [26]–[29]. The latter two graphene growth techniques have emerged as having the Manuscript received June 4, 2012; revised April 1, 2013; accepted April 17, 2013. Date highest potential for high-performance electronic applica- of publication May 24, 2013; date of current version June 14, 2013. This work was supported in part by the Nanoelectronic Research Initiative and the Southwest tions. There are other sources of ‘‘graphene,’’ for example, Academy of Nanoelectronics (SWAN–NRI) program. The work of R. M. Wallace was reduced graphene oxide or growth on various other metal also supported by an IBM Faculty award. The work of R. S. Ruoff was also supported by the W. M. Keck Foundation, the National Science Foundation, surfaces. However, these are either discontinuous, and the U.S. Office of Naval Research. ‘‘doped’’ with oxygen, or multilayered, as in the case of L. Colombo is with Texas Instruments Incorporated, Dallas, TX 75243 USA (e-mail: [email protected]). growth on metals with high carbon solubility, but these R. M. Wallace is with the University of Texas at Dallas, Richardson, TX 75080 USA may be suited for applications having less stringent (e-mail: [email protected]). R. S. Ruoff is with the University of Texas at Austin, Austin, TX 78712-0292 USA requirements [23], [30], [31]. (e-mail: [email protected]). Each of the graphene sources has its advantages and Digital Object Identifier: 10.1109/JPROC.2013.2260114 disadvantages: Graphene exfoliated from natural graphite 1536 Proceedings of the IEEE | Vol. 101, No. 7, July 2013 0018-9219/$31.00 Ó2013 IEEE Colombo et al.:GrapheneGrowthandDeviceIntegration is of very high quality, as established by carrier transport After the introduction of CVD graphene on Cu there properties, but the area of these films (flakes) is limited to has been a renewed focus in trying to grow large single hundreds of micrometers squared and is thus not scalable graphene layers on Pt, Ni, Co, and Ru among others [21], to meet high-volume manufacturing (HVM) requirements. [26], [29]. However, because of the higher C solubility in Multilayer graphene grown on SiC single crystals has been these metals and difficulty in etching for some of them, the shown to have excellent transport properties [32]–[34]. processes are not yet commonplace, even though there are Further, these films have the advantage of having definite thermal stability advantages in comparison to Cu. exceptional surface flatness, and it is for this reason that The quality of graphene grown by CVD, as determined by this material is preferred by many surface scientists to transport measurements, is now nearly equivalent to that study the basic physical properties of graphene. However, of graphene exfoliated from natural graphite on both SiO2 in order to satisfy the requirements of many electronic [44] substrates as well as on hexagonal boron nitride applications, from nanoelectronics to flexible electronics (hBN) [45]. The availability of large-area graphene grown and transparent conductive electrodes, it is necessary to on Cu affords the ability to form multilevel stacked 3-D form large individual single layers of graphene. At this devices that cannot be easily fabricated using traditional time, exfoliation of the full graphene films from SiC semiconductor materials with a low thermal budget. This substrates has not been achieved yet, and even if it could is all due to the ambipolar nature of graphene and the be achieved, as Hashimoto et al. suggest [35], it might be ability to grow and transfer to any substrate. impractical for many applications because of the SiC In order to take full advantage of graphene grown on substrate size limitations and, to a lesser extent, cost. The any electrically conductive substrate, it is critical to introduction of 450-mm silicon integrated circuit devel- developatransferprocessaswellasthesubsequent opment and production in the next decade or so will make cleaning and integration processes to ensure minimum the use of SiC as a source of graphene even more difficult graphene degradation so as to take advantage of the as- unless SiC graphene can be grown directly on these very grown material properties. Alternative growth methods on large Si wafers. nonconductive substrates are being explored [43], [46], The most challenging aspect of the use of graphene for [47] and may offer the prospect of high-quality material electronicdevicesistheabsenceofabandgap.Itiswell without the need for transfer methods. known that graphene develops a bandgap as its width Fabrication of electronic devices will require the narrows [36], and it would thus be desirable to be able to development of low defect density, highly ordered large- grow graphene nanoribbons (GNRs) in place. There are area graphene films, thin dielectric deposition processes several reports on the chemical synthesis of atomically that do not disturb the band structure of graphene, to precise GNRs [37]–[40], but there are no reports, to our select and develop contact materials and processes that knowledge, of GNR deterministicplacement.Thisisakey provide low contact resistance, and to develop associated research area that requires much more attention. integration processes such as etching and surface cleaning. Traditionally, processes such as CVD and atomic layer While thin dielectric deposition on Si is well developed deposition (ALD) have been a preferred method of film and understood, growth of scaled dielectrics on clean growth/deposition for many types of materials, and it graphene is facing some challenges. For example, because would be desirable if such growth techniques could be clean graphene is chemically inert, uniformly thin di- used for graphene. There have been several recent electrics, at the nanometer scale, have been difficult to attempts at the growth of graphene on Ni by CVD, but deposit. The chemical inertness of the graphene surface the films were mostly multilayers with only small patches requires some functionalizationVeither intentional or of monolayer graphite [41], [42]. Recently, bilayer unintentional (i.e., contamination). Growth of uniformly graphene synthesis at the underlying Ni/SiO2 interface deposited high-k dielectrics on hydrogen terminated Si has been reported with arbitrary carbon sources to the met a similar problem, and fortunately a thin oxide and/or exposed Ni surface [43]. an O–H terminated surface were found to fix the problem. Li et al. [11] on the other hand developed a CVD process Unfortunately graphene does not have a natural oxide and of graphene on Cu substrates where one can grow a so we have to look for different
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