Physical and electrical properties of graphene grown under different hydrogen flow in low pressure chemical vapor deposition
© Hussain et al.; licensee Springer. 2014
Received: 2 May 2014
Accepted: 5 September 2014
Published: 2 October 2014
Hydrogen flow during low pressure chemical vapor deposition had significant effect not only on the physical properties but also on the electrical properties of graphene. Nucleation and grain growth of graphene increased at higher hydrogen flows. And, more oxygen-related functional groups like amorphous and oxidized carbon that probably contributed to defects or contamination of graphene remained on the graphene surface at low H2 flow conditions. It is believed that at low hydrogen flow, those remained oxygen or other oxidizing impurities make the graphene films p-doped and result in decreasing the carrier mobility.
Since the first report of the special characteristics of graphene, a planar sheet of sp2 hybridized carbon atoms arranged in a honeycomb lattice with hexagonal rings, the research areas of graphene have evolved tremendously from the field of fundamental physics [1, 2] to the application field in low-cost flexible transparent electronics , photovoltaics , or microelectronics devices [5–9]. However, to obtain large-area and high-crystalline graphene films, low-cost elaboration method is still a significant challenge. Micromechanical exfoliation of highly ordered pyrolytic graphite (HOPG) [10, 11], although yielding a good quality graphene, is not amenable to large scale production. For the nanotechnology and microelectronics industry, high quality films with low defect density, large-scale area, and high uniformity are required. The presence of grain boundaries, disorder, point defects, wrinkles, folds, tears and cracks, and so forth can scatter the charge carriers and have detrimental effects on the electronic, thermal, and mechanical properties of grapheme [12–14]. One of the most practical methods to produce graphene is chemical vapor deposition (CVD) [15–18]. Especially, the most uniform single layer graphene can be produced via CVD over a copper substrate . To obtain uniform graphene film using CVD, several factors influence its growth such as solubility of carbon in the metal-substrate, crystal structure of the metal, metals lattice plane, and other thermodynamic parameters like temperature, cooling rate, pressure of the system, type of catalyst, and the amount of flow of gases in the system. Hydrogen (H2) flow rate is another important parameter in CVD kinetics, and it can contribute to the improvement of graphene layer uniformity even on polycrystalline substrates. Hydrogen plays a dual role during growth on copper (Cu) substrate . Without the presence of H2, methane (CH4) used as a carbon precursor should chemisorb on the Cu surface to form active carbon species, that is, (CHx)s (x = 0 ~ 3), which subsequently react to form graphene. However, such dehydrogenation reactions are not thermodynamically favorable even on Cu surface . Molecular H2 more readily dissociates on Cu and produces active H atoms . These H atoms can promote activation of physisorbed CH4, and lead to production of surface bound (CH3)s, (CH2)s, or (CH)s radicals. It also controls the grains' shape and dimension by etching away the weak carbon-carbon bonds . So, changing the flow of H2 gas during growth steps affects the morphology of graphene film such as the sizes, shapes, and dimensions of graphene grain boundaries (e.g., edge, pits, zigzag) [19, 22].
In this article, we observed that the H2 flow during the low pressure CVD also had significant effect on the electrical properties of graphene. We found that in our process conditions, H2 seemed to act more like as a nucleation/growth promoter and a defect suppressor (oxygen-related functional groups) of CVD-grown graphene film in our process conditions.
The pressure of chamber during the growth
H2flow rate (sccm)
Total pressure/H2partial pressure (mTorr)
Results and discussion
As shown in Figure 1a, at low H2 flow rate (10 sccm), no nucleation occurs for smaller growth durations, e.g., t < 5 min. The spontaneous nucleation and subsequent growth of graphene occurs only when a critical level of supersaturation is reached. At lower H2 flow rates and hence at low partial pressure, this critical level of supersaturation is achieved only at longer growth durations. As the H2 flow rate increases, this threshold of supersaturation is achieved at relatively shorter growth durations. Further, our results also suggest that as the H2 flow increases the density of nuclei increases (Figure 1b,f). At the growth and segregation step (Figure 1c,g), the graphene growth rate is visibly influenced by the H2 flow rate, which is clearly visible in the SEM pictures (Figure 1b,f). Whereas when the H2 flow rate is very low (10 sccm), the nucleation and growth are reduced significantly, as compared in Figure 1b,c,f,g. The grown graphene films consist of irregular-shaped grains of different sizes at our CVD conditions. It has been reported that at low H2 partial pressure, the graphene domain feature irregular shapes . The domain and dislocation formation during graphene growth influences the electrical properties.
C1s XPS spectra display an asymmetric shape typical of graphitic sp2 carbon with binding energy of 284.1 to 284.6 eV. The remaining spectra can be adequately fitted by two different Gaussian contributions: C-C sp3 at hydroxyl C-OH (285.9 to 286.2 eV) and carbonyl C = O and carboxylic COOH (288.3 to 288.7 eV) groups [29, 30]. The sp2 hybridization is attributed to the carbon lattice, while sp3 is a result of oxygen-related functional groups like amorphous and oxidized carbon that probably contributed to defects or contamination . The sample grown at high H2 flow (500 sccm) shows higher sp2/sp3 peak ratio than that grown at 10 sccm in Figure 5 (b). A sharp graphitic peak sp2 is observed at 284.6 eV with FWHM of 0.69 eV, while sp3 peaks at hydroxyl C-OH and carboxylic COOH are at 285.3 and 288.1 eV, respectively. As for the sample grown at low H2 flow (10 sccm), the sp2/sp3 ratio decreases significantly, and sp2 peak is broadened, which reveals that graphene is oxidized and amorphized at the low H2 atmosphere. It is also thought that residual oxygen or oxidizing impurities , which are originally remained in the CVD chamber and gas feedstock, make the graphene films p-doped at low H2 atmosphere.
In the experiments, more than six devices were made. There is a trend in which the mobility of the graphene increases and the CNP shifts toward zero with increasing H2 flow rate. For the sample grown under 10 sccm of H2 flow, the CNP was at +31 V, but for the sample under 500 sccm H2 flow, the CNP was shifted to +10 V. Our results show that at low H2 flow all the film exhibited p-typed behavior . With the increasing the H2 flow, the synthesized graphene becomes closer to a pristine graphene, and its mobility increases. Figure 7c shows the charge carrier densities significantly change with different H2 flow. The changes in charge carrier density of graphene layers are related with changes in Fermi level of graphene layers. Thus, different H2 flow significantly modulates the Fermi level of graphene layers. The defects of graphene, which can be originated from the inevitable occurrence of residual oxidizing impurities in the chamber's atmosphere and gas feedstock, are suppressed under high H2 flow rate.
In conclusion, we observed that hydrogen flow during the low pressure chemical vapor deposition had significant effect not only on the physical properties but also on the electrical properties of graphene. Nucleation and grain growth of graphene increased at higher hydrogen flows. And, more oxygen-related functional groups like amorphous and oxidized carbon that probably contributed to defects or contamination of graphene remained on the graphene surface at low H2 flow conditions. It is believed that at low hydrogen flow, those remained oxygen or other oxidizing impurities make the graphene films p-doped and result in decreasing the carrier mobility. In our process conditions, H2 seemed to act more as a nucleation/growth promoter and a defect suppressor (oxygen-related functional groups) of CVD-grown graphene film.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (2010-0020207, 2012R1A1A2007211).
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