Multidimensional characterization, Landau levels and Density of States in epitaxial graphene grown on SiC substrates
© Camara et al; licensee Springer. 2011
Received: 13 September 2010
Accepted: 14 February 2011
Published: 14 February 2011
Using high-temperature annealing conditions with a graphite cap covering the C-face of, both, on axis and 8° off-axis 4H-SiC samples, large and homogeneous single epitaxial graphene layers have been grown. Raman spectroscopy shows evidence of the almost free-standing character of these monolayer graphene sheets, which was confirmed by magneto-transport measurements. On the best samples, we find a moderate p-type doping, a high-carrier mobility and resolve the half-integer quantum Hall effect typical of high-quality graphene samples. A rough estimation of the density of states is given from temperature measurements.
It is now widely accepted that graphene-based devices are promising candidates to complement silicon in the future generations of high-frequency microelectronic devices. To this end, the most favourable technique to produce graphene for industrial scale applications seems to be epitaxial graphene (EG) growth. This can be done by chemical vapour deposition on a metal [1, 2] or by heating a SiC wafer up to the graphitisation temperature [3–6]. In the first case, the disadvantage is the need to transfer the graphene film on an insulating wafer. In the second case, the SiC wafer plays the role of the insulating substrate without any need for further manipulation. Of course, to be suitable for the microelectronics industry, these EG layers must be continuous and homogeneous at the full wafer scale or, at least, on surfaces large enough to process devices.
On the Si-face of 6H or 4H SiC substrates, graphitisation at high temperature in an Ar atmosphere close to atmospheric pressure shows promising results for on-axis substrates. In this way, single-layer epitaxial graphene (SLEG) has already been grown at the full wafer scale [7, 8] but an open issue remains the 6√3 SiC surface reconstruction which is a C-rich buffer monolayer on top of the SiC substrate. The first "real" graphene layer on top of this buffer layer is strained, not at all free-standing, strongly coupled to the C-rich buffer, heavily n-type doped, with a low-carrier mobility. On the contrary, on the C-face of the same SiC substrates, there is no need of a C-rich buffer layer at the interface before growing the first graphene layer [9–12]. In this way, the mobility could reach 30,000 cm2/V s in the work of Ref. .
For a long time, whatever the growth technique, the uniformity and quality of the EG was not good enough to find evidence of the so-called "half integer" quantum Hall effect (QHE). However, recently, large SLEG areas have been produced on the C-face of on-axis SiC substrates and, on such monolayer graphene, the carriers were holes with mobility close to the one found in mechanically exfoliated graphene films on SiO2/Si . Consequently, the QHE could be demonstrated . This shows clearly the advantage and quality of SLEG grown on the on-axis C-face of a SiC wafer over the on-axis Si-face. However, for further integration of graphene with current SiC technology, 8° off-axis substrates should be also considered since they constitute the standard in modern SiC industry .
In this work, we compare the results of graphene growth on semi-insulating, on axis and and 8° off-axis, 4H-SiC substrates. The quality, uniformity and size of the growth products will be compared using optical microscopy (OM), scanning electron microscopy (SEM), atomic force microscopy (AFM) and micro-Raman spectroscopy (μR). Then, Hall effect measurements will be done at different temperature in order to extract the density of states in the epitaxial monolayers.
Graphene growth, microscopy and Raman studies
Tens of similar monolayer islands grown on, both, on axis and off-axis substrates were probed by Raman spectroscopy. We used the 514 nm laser line of an Ar-ion laser for excitation and got very similar features. At the micrometer size, all spectra reveal that the islands are of the same nature and very homogeneous. First, the D-band, which usually indicates the presence of disorder or edges defects, is very weak and the Raman signature is extremely close to the one found for exfoliated graphene on SiO2/Si . Second, the 2D-band appears at low frequency (2685 cm-1) which is strong evidence that there is no strain at the layer to substrate interface (i.e. almost a free-standing SLEG layer). Third, this 2D-band can be fitted with a single Lorentzian shape with a FWHM of 30 cm-1. Fourth, the ratio I 2D/I G between the integrated intensities of the 2D-band and the G-band is high, which suggests weak residual doping in the order of 3 to 6 × 1012 cm-2. Altogether, these Raman and microscopy measurements tend to demonstrate the almost free-standing low-doped and continuous character of the grown layers [12, 19].
Electrical transport measurements
Then transport measurements were done at low temperature on the different samples, using a maximum magnetic field of 13.5 T. The contact geometry allowed simultaneous measurement of, both, the longitudinal and transverse voltages with the current flowing between two injection contacts at the flake extremities. In both series of samples, from the sign of the Hall voltage, we found that the carriers were holes (in agreement with other results published on the C-face [13, 14]). The holes concentration ranged from 1 × 1012 to 1 × 1013 cm-2 at low temperature, with a weak temperature dependence.
For the low doped layers, the transverse resistance exhibits now quantized Hall plateaus, clearly governed by the sequence R K /4(N + 1/2) in which R K = h/e 2 is the Von Klitzing constant  and N = 0, 1, 2... As already known, this peculiar sequence of resistance values is the well-known quantum transport signature of the monolayer graphene Landau levels . In Figure 4(b).we show the longitudinal and Hall resistance values for such a low-doped SLEG device with hole concentration n s = 1.2 × 1012 cm-2 and mobility μ ~5000 cm2/V s at T = 1.6 K. At B = 12 T, the longitudinal resistance cancels while the transverse resistance tends to 12.9 kΩ which is the expected value for the N = 0 plateau (R K /2).
The temperature dependence of ρ xx (B) is shown in Figure 5a, between 1.6 and 44 K. In this temperature range, an activated behaviour is found for the resistivity: ρ xx ~exp(-E a/k B T) of the N = 0 plateau. This activation energy E a is the energy separation between the Fermi energy E F and the delocalised states of the N = 1 Landau level. In Figure 5b we plot the resistivities values ρ xx taken at different magnetic fields in the vicinity of the R K /2 plateau. The activation energy E a varies from 0.7 to 3.3 meV between B = 10 and 13 T, which remains much smaller than the distance between the first and the second Landau level (~120 meV at B = 10 T). This indicates that the Fermi energy is firmly pinned by localised states. E a has been calculated by taking into account only temperatures above 6 K. At lower temperatures, there is an additional contribution to the conductivity, which is visible in Figure 5b as a change in the slope. We attribute this additional contribution to hopping.
In principle, from the activation energy, we can reconstruct the density of state ρ(E). The filling factor is calculated from B = 10 to 13 T, each filling factor change Δν at a given magnetic field corresponding to a density variation Δn s = n sΔν/ν. The Fermi energy shifts by ΔE a to compensate for the density variation and the mean value for the density of states at energy ~E a is given by ρ(E) = Δn s/ΔE a.
To summarize, we have shown the possibility to grow large islands of monolayer graphene on the C-face of on-axis and 8° off-axis commercial 4H-SiC wafers. The graphene layers are continuous, almost free-standing and show quantum transport properties comparable with high-quality, low-doped, exfoliated graphene. We show evidence of half-integer QHE specific of graphene monolayer and give a first estimate of the density of states in the magnetic field.
atomic force microscopy
half-width at half-maximum
quantum Hall effect
scanning electron microscopy
single-layer epitaxial graphene.
This work was supported by the French ANR ("GraphSiC" Project No. ANR-07-BLAN-0161). We acknowledge the EC for partial support through the RTN ManSiC Project, and the Spanish Government through a grant Juan de la Cierva.
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