Transfer-free method for forming graphene layer

Abstract

The present invention relates to a transfer-free method for forming a graphene layer, in which a high-quality graphene layer having excellent crystallinity can be easily formed over a large area at low temperature by a transfer-free process so that it can be applied directly to a base substrate, which is used in a transparent electrode, a semiconductor device or the like, without requiring a separate transfer process, and to an electrical device comprising a graphene layer formed by the method. More specifically, the transfer-free method for forming a graphene layer comprises the steps of: depositing a Ti layer having a thickness of 3-20 m on a base substrate by sputtering; and growing graphene on the deposited Ti layer by chemical vapor deposition.

Claims

1. A method for manufacturing an electrical device, the method comprising the steps of: (A) modifying a surface of a base substrate by sputtering titanium (Ti) on the surface to form a Ti layer on the surface, wherein the Ti layer has a thickness of 3-20 nm; and (B) growing a graphene layer, by chemical vapor deposition, on the Ti layer formed on the surface, wherein the electrical device comprises the base substrate, the Ti layer, and the graphene layer.

2. The method of claim 1, wherein the base substrate is made of a material comprising glass, SiO.sub.2, a synthetic polymer containing an oxygen atom, or any combination thereof.

3. The method of claim 1, wherein the step (B) comprises growing graphene by chemical vapor deposition under a condition in which a temperature of the substrate is 150 C. to 900 C.

4. The method of claim 3, wherein the step (B) comprises growing graphene by chemical vapor deposition under a condition in which a temperature of the substrate is 150 C. to 400 C.

5. The method of claim 4, wherein a working pressure in the step (B) is 10 mTorr or lower.

6. The method of claim 1, further comprising, before growing the graphene layer, treating the Ti layer with hydrogen gas.

7. The method of claim 1, wherein a reactive gas which is used in the chemical vapor deposition comprises a mixture of hydrogen gas and one or more carbon sources selected from the group consisting of methane, ethane, propane, acetylene, methanol, ethanol and propanol.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the results of simulating the adsorption position of carbon on a titanium surface.

(2) FIG. 2 shows the results of simulating activation energy required for surface diffusion of carbon on a titanium surface.

(3) FIG. 3 shows the results of simulating thermodynamics of C6 ring formation and graphene layer formation.

(4) FIG. 4 shows the results of simulating graphene grown on a Ti (0001) substrate by perfect TiC lattice matching.

(5) FIG. 5 is a schematic view of a 3-zone furnace used in chemical vapor deposition in an example of the present invention.

(6) FIG. 6 is a Raman spectrum showing the characteristics of growth of graphene at varying temperatures according to an example of the present invention.

(7) FIG. 7 is a Raman spectrum showing the characteristics of growth of graphene under varying deposition conditions according to an example of the present invention.

(8) FIG. 8 depicts a Raman spectrum and graphs showing the characteristics of growth of graphene as a function of working pressures.

(9) FIG. 9 depicts a Raman spectrum and graphs showing the characteristics of growth of graphene as a function of hydrogen reduction time.

(10) FIG. 10 depicts a Raman spectrum and graphs showing the characteristics of growth of graphene as a function of the content of hydrogen gas in a reactive gas.

(11) FIG. 11 depicts a Raman spectrum and graphs showing the characteristics of growth of graphene as a function of the content of methane gas in a reactive gas.

DETAILED DESCRIPTION

(12) Hereinafter, the present invention will be described in further detail with reference to examples. It is to be understood, however, that these examples are for illustrative purposes only and are not intended to limit or change the scope of the present invention. In addition, those skilled in the art will appreciate that various modifications and alterations are possible based on this illustration, without departing from the scope and spirit of the invention.

EXAMPLES

Example 1

Density Functional Theory-based Simulation of Graphene Growth on Ti Surface

(13) The growth of graphene on a Ti (0001) crystal surface by the adsorption and surface diffusion of carbon was simulated based on the density functional theory (DFT).

(14) More specifically, the first-principle method based on the density functional theory, which is implemented in a VASP (Vienna Ab-initio Simulation Package) provided with projector-augmented-wave method (PAW) method [Phys. Rev. B 50, 1795317979 (1994)], was used [Phys. Rev. B 54, 1116911186 (1996)]. The exchange-correlation energy functional theory was represented by the generalized gradient approximation (GGA) in the PBE (Perdew-Burke-Ernzerhof) method [Phys. Rev. Lett. 77, 38653868 (1996)], and the kinetic energy cutoff was set at 400 eV. Electron density functional theory calculation for the junction of graphene and Ti was performed using 72 carbon atoms and three titanium layers arranged in a Ti (0001) direction with a 55 size, and calculation for determining the effect of oxygen was performed by adsorbing 37 oxygen atoms stepwise onto titanium to oxidize the titanium.

(15) 1) Simulation of Adsorption Positions of Carbon Atoms on Titanium Surface

(16) First, the binding energy (E.sub.bind) of carbon atoms adsorbed on a titanium surface was simulated at each adsorption position, and the results of the simulation are shown in FIG. 1. In FIGS. 1-4, the ash color(larger sphere) represents titanium atoms, and the yellow color(smaller sphere) represents carbon atoms. The binding energy is represented by the difference between the bottom-state energy of carbon atoms adsorbed on the titanium surface and the sum of the bottom-state energies of the titanium surface and the carbon atoms. As can be seen in FIG. 1, the binding energy at the FCC point at which the lower titanium layer is present while three titanium atoms are gathered in a triangular form is 8.22 eV/C atom, indicating that the FCC point is the most preferable point for adsorption of carbon atoms. This suggests that carbon atoms can be located at the titanium FCC point when graphene is produced on the titanium surface, and that TiC atoms are morphologically well matched.

(17) 2) Simulation of Graphene Growth on Titanium Surface by Surface Diffusion of Carbon

(18) The surface diffusion of carbon on the titanium surface was simulated by DFT calculation. FIG. 2 shows the results of calculating activation energy required for the surface diffusion of carbon on the titanium surface, and indicates that activation energy between 0.26 eV and 0.56 eV is required depending on the position to which carbon is diffused. Table 1 below shows the results of simulating the surface diffusion rates of carbon at varying vapor deposition temperatures when carbon is diffused with activation energy of 0.56 eV. Table 1 shows that the surface diffusion rate of carbon at 300K is 3.9110.sup.2/sec, indicating that carbon can be diffused to the titanium surface even at low temperatures when the amount of carbon is sufficient.

(19) TABLE-US-00001 TABLE 1 Temperature 300K 400K 500K Diffusion rate (times/sec) 3.91 10.sup.2 8.80 10.sup.4 2.27 10.sup.6 Movement distance (/sec) 132 nm 29.7 m 767 m Movement distance (/hour) 475.2 m 106.9 m 2761.2 mm

(20) FIG. 3 shows the results of simulating thermodynamics of C6 ring formation and graphene layer formation by DFT calculation. Assuming that a sufficient amount of carbon is present and a sufficient amount of time is given for diffusion, carbon atoms are adsorbed onto the FCC point of the titanium layer, and then form a C6 ring by surface diffusion. The formation energy (E.sub.form) of the C6 ring is a total of 1.66 eV, indicating that the C6 ring is in a stable state. Then, the C6 ring acts as a nucleus for growth, and the formation energy gradually decreases as graphene grows, suggesting that growth to large-area graphene is possible through continuous supply and diffusion of carbon. FIG. 4 shows simulation results indicating that when a sufficient amount of Carbon and a sufficient amount of time are given, graphene is grown on a Ti (0001) substrate by perfect Ti-C lattice matching.

Example 2

Growth of Graphene Layer on Titanium Layer

(21) 1) Deposition of Ti by Sputtering

(22) Ti was deposited on a base substrate by sputtering to modify the surface of the substrate. Specifically, a SiO.sub.2(250 nm)/Si substrate or a 100 m-thick PET (polyethylene terephthalate) substrate as a base substrate was washed, and N.sub.2 gas was used to remove foreign matter from the surface. Then, using a 2 inch-diameter Ti metal target (purity: 99.99%), Ti was deposited at room temperature by a DC sputtering method. Herein, the working pressure was maintained at 0.13 Pa, and 20 sccm (standard cc/min) of Ar gas was used as the sputtering gas. In addition, a DC power of 20 W was applied to the Ti target, and a Ti layer having a thickness of 10 nm was deposited at a rate of 1.5 nm/min.

(23) 2) Evaluation of Characteristics of Graphene at Varying Graphene Growth Temperatures

(24) Using a 3-zone furnace shown in FIG. 5, a graphene layer was formed on the Ti(10 nm)/SiO.sub.2(250 nm)/Si substrate, prepared in .sup.1), by rapid thermal chemical vapor deposition (T-CVD). Because the 3-zone furnace has a heating device at the inside side through which a reactive gas is introduced, C atoms are produced by thermal decomposition of the reactive gas. Because only the inlet of the furnace is heated, the temperature of the furnace decreases as it moves away from the inlet, and thus the reaction temperature can be controlled depending on the position of the substrate. For example, a substrate having high heat resistance, such as a Si or glass substrate, may be located in a high-temperature or middle-temperature zone, and a flexible polymer substrate having low thermal stability may be located in a low-temperature zone, thereby controlling the reaction temperature at which Carbon atoms are deposited on the substrate.

(25) In order to confirm the growth of graphene on the Ti layer, CH.sub.4:H.sub.2 gas (50:50 sccm) was supplied as a reactive gas, and CH.sub.4 gas was decomposed at a process temperature of 1000 C. at a heating rate of 5 C./min. To remove an oxide layer from the surface of Ti, 50 sccm of H.sub.2 gas was supplied during the heating process. The deposition pressure was maintained at 200 mTorr, and deposition was carried out for 2 hours at each of 900, 800, 400 and 150 C. by controlling the position of the substrate.

(26) In order to confirm that graphene was grown on the substrate produced by the above-described method, the Raman spectrum was measured, and the results of the measurement are shown in FIG. 6. As shown in FIG. 6, not at a low temperature of 150 C., but at a temperature of 400 C. or higher, the G peak and the 2D peak appeared, indicating that graphene could grow on the Ti layer at high temperatures even in the absence of a catalytic nickel or copper metal. Particularly, graphene was produced even at 400 C., suggesting that graphene can grow at low temperatures. The sharp G peak appearing at about 1581 cm.sup.1 indicates that graphene grown on the Ti layer has excellent crystallinity, and the sharp 2D peak appearing at about 2704 cm.sup.1 without shift indicates that Carbon deposited on the Ti layer by the above-described method grew into graphene rather than graphite.

Example 3

Low-Temperature Growth of Graphene and Evaluation of Characteristics Thereof

(27) In order to grow graphene on a polymer substrate having low thermal stability, such as a flexible substrate, growth at low temperature is necessary. Thus, the growth characteristics of graphene under varying deposition conditions were evaluated, thereby confirming the possibility of growing graphene at low temperatures.

(28) In order to grow graphene on a transparent flexible PET substrate, graphene was prepared on a Ti(10 nm)/PET substrate (fabricated according to the method described in Example 2-1) by T-CVD (thermal chemical vapor deposition) in the same manner as described in Example 2. CH.sub.4 gas was decomposed at a process temperature of 1100 C. at a heating rate of 5 C./min, and 50 sccm of H.sub.2 gas was supplied during the heating process to remove an oxide layer from the surface of Ti. During the reaction, a deposition temperature of 150 C. and a deposition pressure of 300 mTorr were maintained, and graphene was grown at a reactive gas composition and flow rate of CH.sub.4:H.sub.2=60:150, 100:200 or 200:100 sccm.

(29) FIG. 7 is the Raman spectrum of C deposited on the Ti layer by the above-described method. From FIG. 7, it can be seen that no graphene was produced under the conditions of growth temperature of 150 C. and flow rate of CH.sub.4:H.sub.2=60:150 sccm. However, as the ratio and flow rate of CH.sub.4 increased, formation of the G and 2D peaks was observed under a condition of CH.sub.4:H.sub.2=100:200 sccm, even though the shift of the peaks appeared. Thus, under this condition, graphene was grown on the substrate, even though the crystallinity or quality of the graphene decreased. Graphene grown under a condition of CH.sub.4:H.sub.2=200:100 sccm (with an increased CH.sub.4 ratio and flow rate) showed a sharp G peak at 1582 cm.sup.1, indicating that the graphene has excellent crystallinity. In addition, the 2D peak appearing at 2704 cm.sup.1 without shift indicates that Carbon thermally decomposed by the above-described method was grown into graphene rather than graphite on the Ti layer of the PET polymer substrate at low temperature.

(30) Thus, in order to enable a graphene layer having excellent crystallinity even at low temperature, the effect of each deposition condition on the growth of graphene was examined.

(31) 1) Evaluation of Growth Characteristics of Graphene at Varying Working Pressures

(32) Using the Ti(10 nm)/SiO.sub.2/Si substrate fabricated according to the method described in Example 2-1), graphene was grown by the same method as described in Example 2-2). CH.sub.4 gas was decomposed at a process temperature of 1100 C. at a heating rate of 5 C./min, and 10 sccm of H.sub.2 gas was supplied at 750 C. for 240 minutes during the heating process to remove an oxide layer from the surface of Ti. During the reaction, the deposition temperature was maintained at 150 C., and graphene was deposited for 2 hours while a reactive gas was supplied at a composition and flow rate of CH.sub.4:H.sub.2=1:10 sccm. Graphene was grown at a working pressure of each of 5, 10 and 50 mTorr.

(33) FIG. 8 depicts the Raman spectrum of the graphene layer formed by the above-described method, and also depicts graphs showing the growth area of graphene and peak characteristics observed on the Raman spectrum. From FIG. 8, it can be seen that, as the working pressure decreases, the growth area of graphene increases, and a graphene layer having better crystallinity is obtained.

(34) 2) Evaluation of Growth Characteristics as a Function of Pretreatment Time of Ti Layer

(35) Graphene was grown in the same manner as described in 1) above, except that the time of treatment with H.sub.2 gas for removing an oxide layer from the surface of Ti was controlled to each of 60, 120, 180 and 240 minutes at 750 C. The working pressure was maintained at 5 mTorr.

(36) FIG. 9 depicts the Raman spectra of the graphene layers formed using varying hydrogen reduction times, and also depicts graphs showing the growth area of graphene and peak characteristics observed on the Raman spectrum. From FIG. 9, it can be seen that, as the hydrogen reduction time increases, the growth area of graphene greatly increases, and as the hydrogen reduction time decreases, the signal of the Raman spectrum becomes weaker, but as the hydrogen reduction time increases, the intensity of the signal increases, a clear spectrum can be obtained, suggesting that the crystallinity of graphene increases.

(37) 3) Evaluation of Growth Characteristics of Graphene as a Function of Composition of Reactive Gas

(38) (1) Evaluation of Growth Characteristics of Graphene with Increase in Content of Hydrogen in Reactive Gas

(39) Graphene was grown in the same manner as described in 1) above, except that the composition and flow rate of the reactive gas were controlled to CH.sub.4:H.sub.2=1:10, 1:20, 1:30 and 1:40 sccm. The working pressure was maintained at 10 mTorr.

(40) FIG. 10 depicts a Raman spectrum showing the characteristics of graphene formed while increasing the content of hydrogen in the reactive gas, and also depicts graphs showing the growth area of graph and peak characteristics observed on the Raman spectrum. As can be seen in FIG. 10, when only the content of hydrogen increased while the content of the carbon source CH.sub.4 was fixed, the growth rate of graphene gradually decreased, and the growth area of graphene decreased rapidly at CH.sub.4:H.sub.2=1:40 sccm. In addition, it could be seen that the Raman signal of the produced graphene decreased as the content of hydrogen increased, and graphene formed under a condition of CH.sub.4:H.sub.2=1:40 sccm showed a weak signal on the Raman spectrum.

(41) (2) Evaluation of Growth Characteristics of Graphene with Increase in Content of Carbon Source in Reactive Gas

(42) Graphene was grown in the same manner as described in 1) above, except that the composition and flow rate of the reactive gas were controlled to CH.sub.4:H.sub.2=1:10, 3:10, 5:10 and 10:10 sccm. The working pressure was maintained at 10 mTorr.

(43) FIG. 11 depicts a Raman spectrum showing the characteristics of graphene formed while increasing the content of the carbon source methane gas in the reactive gas, and also depicts graphs showing the growth area of graph and peak characteristics observed on the Raman spectrum. From FIG. 11, it could be seen that, as the content of CH.sub.4 increased, the growth area of graphene greatly decreased, and the Raman characteristics of the formed graphene also decreased rapidly.

(44) As described above, according to the graphene layer formation method of the present invention, graphene can be formed directly on a base substrate by modifying the surface of the base substrate with a Ti layer without changing the transparency and electrical properties of the base substrate, indicating that a separate transfer process is not required, thereby minimizing mechanical defects of graphene. Thus, the method of the present invention can be used to fabricate an electrical device comprising a good-quality graphene layer.

(45) In addition, according to the present invention, graphene can be grown on a substrate even at a low substrate temperature of 400 C. or lower, particularly 150 C., and thus a graphene layer can be formed directly on a flexible substrate made of a polymer such as PET, which serves as a base substrate. Furthermore, during the growth of graphene, the graphene forms a strong bond with the substrate by the bonding between oxygen from the graphene and the Ti layer and the bonding between the Ti layer and oxygen from the substrate, and thus has excellent durability. Therefore, the graphene layer formed according to the present invention can be more advantageously used in flexible electrical devices that have recently attracted attention.