ISSN 10637826, Semiconductors, 2011, Vol. 45, No. 5, pp. 623–627. © Pleiades Publishing, Ltd., 2011. Original Russian Text © A.A. Lebedev, N.V. Agrinskaya, S.P. Lebedev, M.G. Mynbaeva, V.N. Petrov, A.N. Smirnov, A.M. Strel’chuk, A.N. Titkov, D.V. Shamshur, 2011, published in Fizika i Tekhnika Poluprovodnikov, 2011, Vol. 45, No. 5, pp. 634–638.
CARBON SYSTEMS
LowTemperature Transport Properties of Multigraphene Films Grown on the SiC Surface by Sublimation A. A. Lebedev^, N. V. Agrinskaya, S. P. Lebedev, M. G. Mynbaeva, V. N. Petrov, A. N. Smirnov, A. M. Strel’chuk, A. N. Titkov, and D. V. Shamshur Ioffe Physical Technical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia ^email:
[email protected] Submitted November 2, 2010; accepted for publication November 8, 2010
Abstract—Multigraphene films grown by sublimation on the surface of a semiinsulating 6HSiC substrate have been studied. It is shown that pregrowth annealing of the substrate in a quasiclosed growth cell improves the structural quality of a multigraphene film. Ohmic contacts to the film have been fabricated, and the Hall effect has been studied at low temperatures. It is found that a 2D electron gas exists in the films. It is con cluded that the conductivity of the film is determined by defects existing within the graphene layer or at the interface between the graphene film and a SiC substrate. DOI: 10.1134/S1063782611050186
Graphene (GR) is presently considered as a real candidate for the microelectronics of the future [1]. Also of interest in this regard are films composed of several graphene layers (multigraphene) or ultrathin graphite films on various substrates, which may be more stable and easily obtainable than graphene itself. One of the ways to form GR is by thermal decom position of the surface (thermal destruction) of single crystal silicon carbide SiC in a vacuum or in argon atmosphere. The method consists in nonstoichiomet ric evaporation of silicon from the surface of single crystal SiC under hightemperature heating and for mation of a hexagonal lattice from residual carbon atoms on its surface [2]. In our previous report, we showed that multigraphene films can be grown on the SiC surface by using techniques and equipment for sublimation epitaxy [3]. The main problem of the thermal destruction method is the quality of the sili con carbide wafers it uses, because all surface defects and irregularities of a SiC crystal are inherited by the graphene film. It is known that, upon mechanical treatment of a crystal, nanoscratches with depths of 5–15 nm produced by abrasive grains during polishing are observed on its surface (Fig. 1a). There exist several methods for treatment of SiC wavers to remove the distorted surface layer. In the most widely used technique, silicon carbide wafers are etched in atmosphere of hydrogen at a temperature of 1400–1500°C [4]. However, this technique is inappli cable in a vacuum technology, because permanent use of gases in the vacuum chamber adversely affects the evacuation rate and residual pressure. As an alternative to the above method, one can use annealing of SiC substrates in a vacuum in a quasiclosed growth cell,
which can also remove the distorted surface layer with out using any gases. In this study, we examined the effect of highvac uum annealing in pregrowth processing on the quality and transport properties of multigraphene layers. We used semiinsulating 6HSiC (0001 ) wafers. To remove the distorted layer, the substrates were annealed in a quasiclosed growth cell at a temperature of 1300–1400°C and a residual pressure of ~10–6 Torr [5]. The annealing modified the surface of a starting substrate: as a result of annealing, defects induced by mechanical polishing were eliminated and atomically smooth steps with a height of a unit cell of the 6H poly type were formed (Fig. 1). Multigraphene was synthesized both on mechani cally polished substrates and on those subjected to a pregrowth thermal treatment. To obtain multi graphene layers, samples were again annealed, this time in an open growth cell in a highvacuum chamber at temperatures of 1400–1500°C. The presence of a multigraphene film upon high vacuum annealing in an open growth cell was con firmed by Auger electron spectroscopy and Raman spectroscopy. In both cases (multigraphene film on the surface after a mechanical polishing or pretreatment), only the peak of carbon was present in the Auger spectra. However, Raman spectra exhibited certain distinc tions. MicroRaman measurements were made in the y(xx)y backscattering configuration on a JobinYvon T64000 spectrometric installation with a confocal microscope at room temperature. The spectra were excited with an Ar+ laser (λ = 514.5 nm). To exclude any influence of local heating, which may shift phonon lines, the laser emission power incident onto a
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LEBEDEV et al. Intensity, arb. units λ = 514 nm T = 300 K
D
G 2D
~ ~
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Fig. 2. Raman spectra of multigraphene films: (a) synthe sized on a substrate subjected to mechanical polishing and (b) synthesized on a substrate annealed in a quasiclosed growth cell.
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Fig. 1. AFM topograph of the SiC surface: (a) after its mechanical treatment and (b) after its annealing in a quasiclosed growth cell.
sample was
