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Definitions

• the invention relates to an improved method for preparation of high-quality graphene on the surface (0001) of silicon carbide by superficial graphitisation of the compound in a stream of silicon atoms from an external sublimation source.
• Patent No. US 9150417 B2 discloses an invention relating to an improvement of the quality of graphene obtained by superficial graphitisation of silicon carbide. It consists in carrying out the graphitisation in a buffer gas atmosphere (e.g. argon), under a pressure of approx. 600 mbar _1 bar. Under these conditions, the sublimation rate of silicon atoms is significantly reduced, because after desorption, they have a finite probability of return onto the surface in the result of collisions with argon atoms [11]; alternatively, it may be understood in thermodynamic terms as a slowdown of the evaporation process after increasing enthalpy of this transition by a pressure-volume factor.
• a buffer gas atmosphere e.g. argon
• the graphitisation process occurs at significantly higher temperatures, of the order of 1450-1500 Q C.
• carbon atoms have higher mobility and form graphene layers characterized by higher quality and a relatively uniform thickness of approx. 1-2 monolayers [10].
• the graphene quality strongly depends on the quality of the substrate's surface.
• the buffer gases used in the process being described may be characterised by a purity of at most 6N (10 ⁇ 6 impurities). Under a gas pressure in the reaction chamber of approx. 1 bar, the partial pressure of the dopants amounts to approx. 10 "3 mbar, corresponding to an exposure of the surface to a stream of unknown particles of impurities having a gigantic volume of the order of 1000 L/s (1000 layers of impurities per second).
• Patent Application No. US 20110223094 Al describes an invention consisting in graphene synthesis on the surface of silicon carbide by placing two crystals: silicon and silicon carbide, in parallel, in some distance, in a vacuum chamber, then pumping off the air from the vacuum chamber to a pressure of the order of lxl0 "6 mbar, annealing the silicon crystal to a temperature of approx. 1200 Q C with a simultaneous annealing of the silicon carbide crystal to temperatures of 1500 Q C, 1600 Q C and 1700 Q C.
• the goal of the invention is to provide high-quality graphene with a low level of impurities and a basically defectless structure of the graphene crystalline lattice, i.e. a honeycomb structure, as well as a method for obtaining such high-quality graphene. It was surprisingly found that the increase in silicon partial pressure in the direct vicinity of the surface of silicon carbide subjected to graphitisation, achieved in accordance with the invention, advantageously slows down the process of sublimation of silicon atoms, leading to synthesis of a better ordered higher-quality graphene.
• the invention relates to a method for preparation of graphene on the surface of silicon carbide, characterised in that an SiC crystal with a crystallographic orientation of the surface (0001), is subjected to, consecutively: a) a pressure below lxlO "9 mbar; b) optionally annealing at a temperature from 300 Q C to 900 Q C under a pressure not higher than 1x10 " 8 mbar; c) optionally annealing at a temperature from 900 Q C to 1050 Q C in a stream of silicon atoms from an external sublimation source providing a nominal silicon growth rate from 0.5 A/min to 2.5 A/min; d) annealing at a temperature from 1300 Q C do 1800 Q C, under a pressure not higher than 5xl0 "7 mbar, in a stream of silicon atoms from an external sublimation source providing a nominal silicon growth rate from 0.5 A/min to 10 A/min.
• the invention also relates to a layer of graphene basically devoid of crystal defects, particularly on the surface of the SiC crystal, characterised in that it comprises from one to four, particularly from one to two, atomic layers forming a crystal lattice with a honeycomb structure, its diffraction spectrum obtained by low-energy electron diffraction having a diffraction pattern typical for the graphene on the SiC surface (0001), and the ratio of the maximum signal intensity to the minimum signal intensity (SNR), measured at room temperature, in the section between the two consecutive diffraction maxima connected with graphene is higher than 9.
• SNR minimum signal intensity
• the graphene according to the invention is obtained by the above-defined method according to the invention.
• graphene according to the invention is characterised in that its SNR value is higher than 9.8 for the annealing temperature higher than 1501 Q C in step d).
• the SNR value is higher than 11 for the annealing temperature lower than 1501 Q C in step d).
• the SNR value is higher than 13 for the annealing temperature lower than 1501 Q C in step d), the preparation method including step b) of the method according to the invention defined above.
• the SNR value is higher than 17 for the annealing temperature lower than 1501 Q C in step d), the preparation method including steps b) and c) of the method according to the invention defined above.
• the method for preparation of graphene according to the invention is based on a replacement of buffer gases during graphitisation with a stream of silicon atoms originating from an external sublimation source of a high purity, to slow down the superficial sublimation of silicon atoms from the surface of silicon carbide.
• the exposure of the surface to impurities was reduced significantly.
• BEP beam equivalent pressure
• the BEP value for the impurities will amount to approx. 10 "7 mbar at worst, i.e. even 4 orders of magnitude less than in case of application of buffer gases.
• the density of the silicon atoms stream may be set basically to any value and thus the graphitisation temperature may be increased significantly.
• the graphene formed on the surface of silicon carbide In order for the graphene formed on the surface of silicon carbide to be characterised by desired properties, its perfect crystallographic ordering is necessary (the atoms must form a lattice with a honeycomb structure with a low concentration of defects, such as vacancies, dislocations or intergranular boundaries), high purity (a low concentration of impurities), and in consequence - an electron structure corresponding to theoretical predictions (a linear relation of electron dispersion near the K point of the reciprocal lattice), connected with a lack of incoherent dispersions.
• the density of the silicon atoms stream was defined so as to achieve almost equilibrium conditions of the process. Under such conditions, loss of silicon from the surface occurs very slow. Due to this fact, while using sufficiently high temperatures, the superficial carbon atoms have a sufficient thermal energy and time for the superficial system to be reorganised to an almost model graphene/SiC state.
• the method according to the invention leads to obtaining a very high- quality graphene devoid of crystalline defects directly on the insulating substrate of silicon carbide.
• An additional adjustments of the annealing time value and the stream of silicon atoms from an external sublimation source allow for obtaining synthesis of a single layer up to four atomic layers of graphene.
• Low energy electron diffraction was used for evaluation of quality of the prepared graphene.
• a suitable diffraction pattern shown in Fig. 1 (an image for electron energy of 156 eV), constitutes a parameter indicating the presence on the surface of graphene. In accordance with the invention, it is a diffraction pattern typical for the graphene on the SiC surface (0001).
• the gray vector is a vector connected with the substrate's surface (silicon carbide), while the white vector is a vector connected with the graphene being synthesised.
• Diffraction maxima connected with silicon carbide are located in the corners of the gray hexagon, while the maxima connected with graphene are located in the corners of the white hexagon.
• the most intense diffraction maxima originating from silicon carbide are surrounded by six maxima of a lower intensity (the hexagon around the most bright peaks), while the most intense diffraction maxima originating from graphene are surrounded by two strongest maxima located towards the centre of the diffraction image (the maximum connected with graphene and two secondary maxima form a triangle). Additional maxima, also forming a hexagon but not always visible, may be located around the maximum connected with graphene.
• a parameter which allows for evaluating the quality and the crystallographic order is constituted by a ratio of intensities of the diffraction maxima to the background level.
• a signal analysis should be carried out between the two consecutive diffraction maxima connected with graphene, as shown for example in Fig. 2.
• the starting surface of silicon carbide (which is subjected to the graphitisation process thereafter) by annealing under ultra-high vacuum at temperatures from 300 Q C to 900 Q C, and annealing under a vacuum higher then 5xl0 "7 mbar at a temperature from 900 Q C to 1050 Q C, combined with directing a stream of silicon atoms onto the sample, resulting in a nominal silicon growth rate of 0.5-2.5 A/min.
• the starting surface should be cooled to room temperature and its quality should be checked by diffraction methods (LEED). After the preparation, the surface is characterised by a (3x3) surface reconstruction, which is shown in the examples (A) below.
• the starting surface prepared by annealing in a stream of silicon atoms is characterised by a surface reconstruction of (3x3) type. Also, this surface is characterised by an almost perfect crystallographic ordering, which is proved by a very high number of observable diffraction maxima, their small transverse size, their high brightness and a low brightness of the background (a high ratio of the signal intensity to the background intensity, which proves a low number of crystallographic defects and amorphous areas of the surface).
• the surface prepared under vacuum is characterised by a reconstruction of (lxl) type and a low degree of crystallographic ordering, which is proved by a very high intensity of the background, a relatively low intensity of the diffraction maxima and a diffused transverse shape of the maxima.
• the starting surface prepared in a way described above was subjected to the graphitisation process at temperatures from 1300 to 1800 Q C in a stream of silicon atoms from an external sublimation source, corresponding to a nominal growth rate of silicon layers of 0.5-10 A/min. under a pressure in the vacuum chamber not exceeding 5xl0 "7 mbar.
• high-quality graphene (described in detail above) forms on the surface, which is shown in the examples (B) below:
• Fig. 6 show the electron distribution in graphene in the vicinity of point K.
• the dispersion relation (dependence of energy on electron quasi-momentum, linearly proportional to the emission angle of the electron from the sample - vertical axis) should be linear around this point, which is evident for all samples, thus graphene has been formed on the surface of all samples.
• a parameter which allows for determining the impact of the surface ordering on the photoemission spectra is constituted by the background intensity.
• the lowest background intensity (formally the ratio of the signal intensity to the background intensity) was observed for sample B4 (the signal intensity between two branches of the linear dispersion relation is noteworthy).
• High-quality graphene according to the invention may be obtained by the method according to the invention for relatively broad range of values of the critical parameters, particularly the graphitisation temperature and the stream of silicon atoms. It was illustrated in the preferred embodiments described below, which should not be however identified with the full scope of the invention being claimed.
• high quality graphene was formed on the surface of the sample, which was confirmed by diffraction and spectroscopic tests.
• the exact structure of the surface varies, but all of them are characterised by a very high crystallographic ordering and electron structures of high quality.
• the sample 4 is characterised by a very high quality of the surface, which is evidenced by sharp bands in the dispersion relation and a very low background level. Additionally, two graphene layers occur on the sample, which is evident in the ARPES spectrum as a splitting of linear branches of the dispersion relation. This fact confirms that the method according to the invention allows for controlling the number of the graphene layers formed on the surface.
• the graphene sample obtained in Example 4 was analysed by scanning tunnelling microscopy at room temperature under vacuum (an Omicron RT-STM/AFM microscope, the microscope tip made of etched tungsten, polarisation voltage of 15 mV, tunnelling current of 100 pA).
• the results in the form of a microscopic image are shown in Fig. 10 A and B
• the perfect crystallographic structure of graphene is evident: the honeycomb structure.
• the unit cell of graphene is a single small hexagon; the change in the background intensity results from the effect of the subsurface layer (a buffer layer with a (6V3x6V3)R30 Q symmetry).
• the structure is characterised by a very low amount of any impurities or defects, confirming the high quality and crystallographic ordering on the surface of sample.
• the graphitisation process of the silicon carbide surface is carried out in a vacuum chamber ensuring a baseline pressure of the order of 1x10 " 6 Torr (see Fig. 1A in the cited document).
• the described method suffers from the same significant purity limitation as the method described in the Patent No. US 9150417 B2, in which the sublimation rate of silicon atoms is reduced by an application of buffer gases.
• the baseline pressure of the order of Ixl0 "5 -lxl0 -6 Torr corresponds to an exposure of the surface to unknown impurities of the order of l "10 L/s even before the beginning of annealing.
• the method according to the invention yields results different from the point of view of quality, thanks to application of a baseline pressure on the level of ultra-high vacuum, or approx. lxlO "10 mbar (an exposure of the order of 0.0001 L/s), as well as maintaining the pressure in the chamber during the annealing at a level better than 5xl0 "7 mbar (an exposure of the order of 0.5 L/s, or two orders of magnitude lower than in the method known from US 20110223094 Al).
• the method according to the invention allows for precise controlling of the value of the stream of silicon atoms and its change depending on the annealing temperature and time for the silicon carbide crystal. Thanks to precise control over the process parameters, the obtained results are significantly different than those obtained by the methods known earlier.
• the state of art does not provide the preliminary preparation of the silicon carbide surface.
• a preparation of the starting surface of silicon carbide subjected to graphitisation is carried out by annealing it at temperatures of 300 Q C-900 Q C under ultra-high vacuum, followed by annealing it at a temperature of 900-1100 Q C in a stream of silicon atoms, corresponding to a nominal rate of silicon growth of 0.5-2.5 A/min, under ultra-high vacuum, and then cooling the crystal to room temperature.
• the preliminary preparation results in removal of impurities from the surface of silicon carbide before commencing graphitisation, during which the impurities could diffuse to the interior of the crystal, as well as form structural defects in the graphene layer and also, it results in formation of a high crystallographic order ((3x3) reconstruction of the surface) and smoothing of step edges.
• the presented results confirm that the preliminary preparation of the surface affects the quality and the structure of the graphene obtained in the graphitisation process advantageously.
• the known method excludes a direct control over temperature of the silicon carbide surface.
• the method according to the invention executes such a control using an optical pyrometer directed onto the sample.
• a precise temperature control is very important for obtaining optimal and repeatable results.

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