METHOD FOR PREPARATION OF HIGH-QUALITY GRAPHENE ON THE SURFACE OF SILICON CARBIDE

Abstract

A 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 is disclosed.

Claims

1. 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 110.sup.9 mbar; b) optionally annealing at a temperature from 300 C. to 900 C. under a pressure not higher than 110.sup.8 mbar; c) optionally annealing at a temperature from 900 C. to 1050 C. in a stream of silicon atoms from an external sublimation source providing a nominal silicon growth rate from 0.5 /min to 2.5 /min; d) annealing at a temperature from 1300 C. do 1800 C., under a pressure not higher than 510.sup.7 mbar, in a stream of silicon atoms from an external sublimation source providing a nominal silicon growth rate from 0.5 /min to 10 /min.

2. 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.

3. Graphene according to claim 2, characterised in that it is obtained by the method defined in claim 1.

4. Graphene according to claim 3, characterised in that the SNR value is higher than 9.8 for the annealing temperature higher than 1501 C. in step d).

5. Graphene according to claim 3, characterised in that the SNR value is higher than 11 for the annealing temperature lower than 1501 C. in step d).

6. Graphene according to claim 3, characterised in that the SNR value is higher than 13 for the annealing temperature lower than 1501 C. in step d), the preparation method including step b) defined in claim 1.

7. Graphene according to claim 3, characterised in that the SNR value is higher than 17 for the annealing temperature lower than 1501 C. in step d), the preparation method including steps b) and c) defined in claim 1.

Description

EXAMPLE A1

[0034] The method for preparation of sample 1: introduction to the vacuum chamber, pumping off the chamber to a pressure <110.sup.9 mbar, annealing of the sample at a temperature increased gradually from 100 to 850 C. (degassing), annealing of the sample at a temperature 950 C. under vacuum (without the stream of silicon atoms) for a time t=10 minutes.

EXAMPLE A2

[0035] The method for preparation of sample 2: introduction to the vacuum chamber, pumping off the chamber to a pressure <110.sup.9 mbar, annealing of the sample at a temperature increased gradually from 100 to 850 C. (degassing), annealing of the sample at a temperature 950 C. in a stream of silicon atoms resulting in a nominal silicon growth rate of 0.5-2.5 /min for a time t=10 min.

[0036] Then, tests of the obtained surface were carried out. The results are presented in FIG. 4.

[0037] The tests were carried out by low energy electron diffraction using two samples prepared according to examples A1 and A2. The results in the form of diffraction patterns for electron energy equal to 142 eV are shown in FIG. 4.

[0038] The starting surface prepared by annealing in a stream of silicon atoms is characterised by a surface reconstruction of (33) 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 (11) 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.

[0039] B. Graphitisation of Pre-Prepared SiC Surface.

[0040] The starting surface prepared in a way described above was subjected to the graphitisation process at temperatures from 1300 to 1800 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 /min. under a pressure in the vacuum chamber not exceeding 510.sup.7 mbar. After the preparation, high-quality graphene (described in detail above) forms on the surface, which is shown in the examples (B) below:

EXAMPLE B1

[0041] The method for preparation of sample 1: introduction to the vacuum chamber, pumping off the chamber to a pressure <110.sup.9 mbar, annealing of the sample at a temperature increased gradually from 100 to 850 C. (degassingno preparation of the starting surface (33)), annealing of the sample at a temperature 1450 C. (without the stream of silicon atoms) for a time t=10 minutes.

EXAMPLE B2

[0042] The method for preparation of sample 2: introduction to the vacuum chamber, pumping off the chamber to a pressure <110.sup.9 mbar, annealing of the sample at a temperature increased gradually from 100 to 850 C. (degassingno preparation of the starting surface (33)), annealing of the sample at a temperature 1450 C. in a stream of silicon atoms resulting in a nominal silicon growth rate of approx. 4 /min for a time t=10 minutes.

EXAMPLE B3

[0043] The method for preparation of sample 3: after preparation of the starting surface characterised by a (33) surface reconstruction, as described above: annealing of the sample at a temperature 1450 C. (without the stream of silicon atoms) for a time t=10 minutes.

EXAMPLE B4

[0044] The method for preparation of sample 4: after preparation of the starting surface characterised by a (33) surface reconstruction, as described above: annealing of the sample at a temperature 1450 C. in a stream of silicon atoms resulting in a nominal silicon growth rate of approx. 4 /min for a time t=10 minutes.

[0045] Diffraction Studies of the Obtained Samples

[0046] The tests were carried out by low energy electron diffraction at room temperature using two samples prepared according to schemes B1-B4. The results in the form of diffraction patterns for electron energy equal to 156 eV are shown in FIG. 5.

[0047] It was found that the surface prepared by preparation of the starting surface of (33) type and annealing in a stream of silicon atoms is characterised by a distinctly better crystallographic ordering. The diffraction maxima are well-formed, have perfect shapes and high intensities, while the background is practically invisible. This is an evidence of a high ordering degree and a minimum concentration of all defects in the graphene layer and amorphous areas on the surface of sample.

[0048] Spectroscopic Studies of the Obtained Samples

[0049] The tests were carried out by angle-resolved photoemission spectroscopy in the UV-radiation range (UV-ARPES) at room temperature using two samples prepared according to schemes B1, B3 and B4. The results in the form of spectra presenting the intensity distribution of photoelectrons sputtered by the UV radiation vs. the emission angle (vertical axis) and kinetic energy (horizontal axis), which allows for imaging of electronic structures of materials, are shown in FIG. 6.

[0050] The spectra presented in FIG. 6 show the electron distribution in graphene in the vicinity of point K. As predicted theoretically, the dispersion relation (dependence of energy on electron quasi-momentum, linearly proportional to the emission angle of the electron from the samplevertical axis) should be linear around this point, which is evident for all samples, thus graphene has been formed on the surface of all samples.

[0051] 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).

[0052] 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.

EXAMPLE 1

[0053] The method for preparation of sample 1: after preparation of the starting surface characterised by a (33) surface reconstruction, as described above: annealing of the sample at a temperature 1500 C. in a stream of silicon atoms resulting in a nominal silicon growth rate of approx. 1 /min for a time t=10 minutes.

EXAMPLE 2

[0054] The method for preparation of sample 2: after preparation of the starting surface characterised by a (33) surface reconstruction, as described above: annealing of the sample at a temperature 1500 C. in a stream of silicon atoms resulting in a nominal silicon growth rate of approx. 25 /min for a time t=10 minutes.

EXAMPLE 3

[0055] The method for preparation of sample 3: after preparation of the starting surface characterised by a (33) surface reconstruction, as described above: annealing of the sample at a temperature 1200 C. in a stream of silicon atoms resulting in a nominal silicon growth rate of approx. 1 /min for a time t=10 minutes.

EXAMPLE 4

[0056] The method for preparation of sample 4: after preparation of the starting surface characterised by a (33) surface reconstruction, as described above: annealing of the sample at a temperature 1700 C. in a stream of silicon atoms resulting in a nominal silicon growth rate of approx. 4 /min for a time t=6 minutes.

[0057] The quality and structure of the graphene samples obtained in Examples 1-4 were analysed.

[0058] Tests 1

[0059] The quality and structure of the graphene samples obtained in accordance with Examples 1-4 (samples 1-4, respectively) were tested by low energy electron diffraction at room temperature. The results in the form of diffraction patterns for electron energy equal to 156 eV are shown in FIG. 7.

[0060] Tests 2

[0061] The quality and structure of the graphene samples obtained in accordance with Examples 1-2 (samples 1 and 2, respectively) were tested by angle-resolved photoemission spectroscopy at room temperature. The results in the form of diffraction patterns for electron energy equal to 156 eV are shown in FIG. 8.

[0062] In each of the embodiments, 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.

[0063] Tests 3

[0064] The graphene sample obtained in Example 4 was analysed by angle-resolved photoemission spectroscopy at liquid nitrogen temperature (T=78 K). The results in the form of an ARPES spectrum are shown in FIGS. 9A and B.

[0065] 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.

[0066] Tests 4

[0067] 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 FIGS. 10A and B

[0068] In the microscopic image, 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.sup.2 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.

[0069] Summary

[0070] The presented method for graphene synthesis take advantage of the physical phenomena described in the Patent Application No. US 20110223094 A1. In both cases, the high quality of graphene is obtained by a reduction of the sublimation rate of silicon atoms from the silicon carbide surface by introducing additional external silicon atoms to the atmosphere in the nearest vicinity of the silicon carbide surface. However, these methods differ in numerous important aspects.

[0071] Firstly, in the Patent Application No. US 20110223094 A1, the graphitisation process of the silicon carbide surface is carried out in a vacuum chamber ensuring a baseline pressure of the order of 110.sup.6 Torr (see FIG. 1A in the cited document). Thus, the described method suffers from the same significant purity limitation as the method described in the U.S. Pat. No. 9,150,417 B2, in which the sublimation rate of silicon atoms is reduced by an application of buffer gases. The baseline pressure of the order of 110.sup.5-110.sup.6 Torr corresponds to an exposure of the surface to unknown impurities of the order of 1.sup.10 L/s even before the beginning of annealing. It is a better value than that obtained while using buffer gases, however it corresponds still to a high degree of the sample contamination. During annealing at so high temperatures, the pressure in the chamber deteriorates by at least one order of magnitude, thus the exposure to impurities amounts to at least 10.sup.100 L/s.

[0072] On the other hand, 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. 110.sup.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 510.sup.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 A1).

[0073] Moreover, the method described in the Patent Application No. US 20110223094 A1 does not plan a precise control over the value of the stream (partial pressure) of silicon atoms in the direct vicinity of the sample, apart from increasing the distance between the crystals.

[0074] Meanwhile, 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. Thank to precise control over the process parameters, the obtained results are significantly different than those obtained by the methods known earlier.

[0075] Also, the state of art does not provide the preliminary preparation of the silicon carbide surface.

[0076] On the other hand, in the preferred embodiment of the method according to the invention, a preparation of the starting surface of silicon carbide subjected to graphitisation is carried out by annealing it at temperatures of 300 C.-900 C. under ultra-high vacuum, followed by annealing it at a temperature of 900-1100 C. in a stream of silicon atoms, corresponding to a nominal rate of silicon growth of 0.5-2.5 /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 ((33) 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.

[0077] Also, the known method excludes a direct control over temperature of the silicon carbide surface.

[0078] On the other hand, 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.

[0079] The lack of a direct measurement of temperature on the surface in the method known from the state of art leads to a lack of repeatability of the results and a lower credibility of the results reported by the authors of the cited documentsall direct methods for temperature measurement such by thermocouples, or a pyrometer directed onto the rear surface of an annealed SiC crystal results in a very high uncertainty and possible temperature oscillations during the process, impairing the quality of the graphene being synthesised.