Method for preparing large-area transition metal dichalcogenide single-crystal films by performing vapor deposition on a single-crystal c-plane sapphire substrate with <10-10> surface steps

Abstract

The present invention discloses a method for preparing large-area transition metal dichalcogenide (TMDC) single-crystal films and the products obtained therefrom. The method comprises the steps of: (1) providing a single-crystal C-plane sapphire with surface steps along <1010> directions; and (2) taking the sapphire in step (1) as the substrate, generating unidirectionally arranged TMDC domains on the sapphire surface based on a vapor deposition method and keeping the domains continuously grow and merge into a large-area single-crystal film. The lateral size of the TMDC single-crystal films prepared by the method can reach inch level or above, and is limited only by the size of the substrate.

Claims

1. A method for preparing a transition metal dichalcogenide single-crystal films, characterized by comprising the steps of: (1) providing a single-crystal C-plane sapphire substrate with surface steps along <1010> orientation of the substrate, wherein the <1010> steps of the C-sapphire substrate are obtained by a major mis-cut towards <1120> crystallographic axis during machining process; and a minor mis-cut angle towards <1010> direction shall be less than 34.6% of the major mis-cut angle of the A-axis direction; and (2) taking the C-sapphire in step (1) as the substrate, generating unidirectionally arranged transition metal dichalcogenide domains on the surface of the substrate using a vapor deposition method and keeping the domains continuously grow and merge into a single-crystal film; wherein grains are aligned in an unidirectional manner and merged without gap between the grains into an entire single-crystal film up to length of two inch with grain boundary-free; the transition metal dichalcogenide is selected from the group consisting of molybdenum disulfide, tungsten disulfide, molybdenum diselenide and tungsten diselenide.

2. The method for preparing the transition metal dichalcogenide single-crystal films of claim 1, characterized in that in step (1), the surface atomic-level steps are oriented along the <1010> direction of the C-sapphire substrate within an allowable angular deviation of ±19.1°.

3. The method for preparing the transition metal dichalcogenide single-crystal films of claim 1, characterized in that in step (2), the vapor deposition methods include chemical vapor deposition method, molecular beam epitaxy method, pulsed laser deposition method, or magnetron sputtering method.

4. The method for preparing the transition metal dichalcogenide single-crystal films of claim 3, characterized in that in step (2), the transition metal dichalcogenide crystals prepared by the chemical vapor deposition method comprising: placing the sapphire in a vapor deposition chamber, loading the growth sources, setting a growth condition, and generating unidirectionally arranged transition metal dichalcogenide domains on the sapphire surface; and continuously introducing the growth sources to allow the transition metal dichalcogenide domains to gradually grow and merge into a transition metal dichalcogenide single-crystal film.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic of the step orientation on C/M sapphire and C/A sapphire and the corresponding epitaxial TMDC domains on both substrate;

(2) FIG. 2 is a photomicrograph of unidirectionally arranged molybdenum disulfide domains in Example 1;

(3) FIG. 3 is an SHG mapping of the unidirectional seamlessly-stitched molybdenum disulfide domains in Example 1;

(4) FIG. 4 is a TEM image of the unidirectionally arranged molybdenum disulfide grains in Example 1;

(5) FIG. 5 is a photograph of a wafer-level molybdenum disulfide single-crystal film grown in Example 2;

(6) FIG. 6 is an SHG mapping at any position of the wafer-level molybdenum disulfide single-crystal film grown in Example 2;

(7) FIG. 7 is a Raman spectrum of the molybdenum disulfide single crystal grown in Example 2;

(8) FIG. 8 is a photoluminescence spectrum of the molybdenum disulfide single crystal grown in Example 2;

(9) FIG. 9 is an SHG mapping image of a polycrystalline MoS.sub.2 film grown on a C/M sapphire substrate in Example 2;

(10) FIG. 10 is an optical photograph of an unidirectionally arranged MoS.sub.2 grown in Example 3;

(11) FIG. 11 is an atomic force microscope image of the unidirectional MoS.sub.2 domains grown in Example 3;

(12) FIG. 12 is an optical photograph of the unidirectionally arranged WSe.sub.2 domains obtained in Example 7;

(13) FIG. 13 is a corresponding atomic force microscope image of the unidirectional WSe.sub.2 grown in Example 7. It can be seen that one of the WSe.sub.2 edges parallels to the <1010> steps; and

(14) FIG. 14 is the Raman spectrum of the as-grown WSe.sub.2 grown in Example 7.

DETAILED DESCRIPTION OF THE INVENTION

(15) The technical solution of the present invention will be further described with reference to the accompanying drawings.

(16) The preparation method can be used for preparing TMDC single crystals such as molybdenum disulfide, tungsten disulfide, molybdenum diselenide, tungsten diselenide and the like. By changing the direction of the surface steps of common C-sapphire in the industry, the specific step direction can induce TMDC domains to form single-directional alignment, finally realizing accurate atomic-level stitching, and producing the large-area single-crystal TMDC films.

(17) The lateral crystal size of the prepared TMDC single-crystal films is limited only by the size of the substrate, for example, by using a 2-inch sapphire substrate, a 2-inch TMDC single crystal can be prepared.

(18) The method specifically comprises the steps of:

(19) (1) Providing a C-plane sapphire single crystal with surface atomic-level steps directed along <1010> direction. Due to an unavoidable minor mis-cut towards <1010> orientation in the actual machining process, the final step direction is likely to deviate from the ideal <1010> direction. Through experimental and calculation, the maximum allowable angular deviation is ±19.1°.

(20) The machining of sapphire needs to first drill out an ingot from a larger cylinder, and then cut it into pieces. It is ensured that, during the whole machining process, the C-plane of the sapphire inclines towards the A-axis direction, and the M-direction does not incline, so that the sapphire wafer with the specific step direction in step (1) can be obtained. In practice, due to machining errors, there could be a minor mis-cut angle along the M-axis (α.sub.M) besides the major mis-cut angle along the A-axis (α.sub.A). It is ensured that α.sub.M shall not be more than 34.6% of α.sub.A. For example, the processing design value is that the α.sub.A=0.2 deg, and α.sub.M=0 deg. The measured result after processing shows that α.sub.A=0.2 deg, and α.sub.M=0.04 deg. 0.04/0.2 is 20% less than 34.6%, in an acceptable range. The arctan 0.346 is approximately equal to 19.1°, which is a threshold deviation.

(21) (2) Taking the sapphire single crystal cut in step (1) as a substrate, based on a vapor deposition method, such as a chemical vapor deposition method, a molecular beam epitaxy method, a pulse laser deposition method, a magnetron sputtering method and the like. Unidirectionally arranged TMDC domains are generated on the sapphire surface, and along with the reaction, the domains grow gradually and merge into a large-area TMDC single-crystal film.

(22) The process for preparing the TMDC crystals by adopting the chemical vapor deposition method comprises the steps of: placing the C/A sapphire substrate in a vapor deposition chamber, loading the TMDC growth sources, setting a growth condition, and generating unidirectionally arranged TMDC domains on the surface; and continuously introducing the growth sources to allow the TMDC domains to gradually grow and merge into a large-area TMDC single-crystal film.

Example 1

(23) Sulfur powder (S), molybdenum trioxide (MoO.sub.3), and a sapphire (C-plane, A-direction inclined 0.2 degrees, M-direction inclined 0°) substrate were placed in a first, second and third temperature zones of the three-temperature zone CVD system, respectively. Vacuum was drawn below 10 Pa and 100 standard milliliters per minute (sccm) of Ar was introduced. The substrate was warmed to 850° C., then S was heated to 150° C. to melt, then MoO.sub.3 was warmed to 530° C., and 5 sccm of oxygen was introduced. The oxygen was turned off after 30 minutes of reaction, and the MoO.sub.3 source was removed from the heating zone. The substrate was cooled to 300° C. in an Ar and S atmosphere and S heating was stopped. Followed by continue cooling to room temperature and the sample was removed.

(24) The optical micrograph is shown in FIG. 1, showing single arranged MoS.sub.2 triangular grains.

(25) Referring again to FIG. 2, SHG mapping of MoS.sub.2 grains arranged in the same direction shows that the strength of the interior of the two grains and their junction region is uniform, indicating that no grain boundaries exist.

(26) In the region where two MoS.sub.2 grains in the same direction merge, high-resolution imaging was carried out, and FIG. 3 is a high-resolution transmission electron microscope characterization of the sample, it can be seen that all the atoms are arranged in the same order from the inside of the grains on the two sides to the boundary region, no obvious distortion of the atomic direction is observed, nor 180° inversion of the atomic arrangement direction is observed, indicating that the grains arranged in the same direction can realize accurate atomic-level splicing, and merge into a complete large single crystal.

Example 2

(27) Sulfur powder (S), metallic molybdenum (Mo), and 2-inch sapphire substrate (C-plane, A-direction inclined 1 degree, M-direction inclined 0.05 degrees) were placed in the first, second and third temperature zones of the three-temperature zone CVD system, respectively. Vacuum was drawn below 10 Pa and 100 standard milliliters per minute (sccm) of Ar was introduced. The substrate was warmed to 900° C., then S was heated to 150° C. to melt, then Mo was warmed to 650° C., and 5 sccm of oxygen was introduced. The oxygen was turned off after 60 min of reaction. The substrate was cooled to 300° C. in an Ar and S atmosphere and S heating was stopped. Followed by continuously cooling to room temperature, and the sample was taken to obtain the continuously covered MoS.sub.2 single crystal 2-inch wafer.

(28) The physical photograph of the 2-inch single crystal sample is shown in FIG. 4. At any position of the sample, the SHG scanning test was used, as in FIG. 5, with no grain boundaries exist. FIGS. 6-7 are Raman and photoluminescence spectra, respectively, of the single crystal sample, from which it can be seen that the resulting sample is a monolayer of MoS.sub.2.

(29) The MoS.sub.2 crystal was grown on a common sapphire substrate using the same process as in the present example, and the SHG scanning pattern of the obtained sample is shown in FIG. 8. It can be seen that a MoS.sub.2 polycrystalline film is grown on the common sapphire substrate, and there is an obvious grain boundary.

Example 3

(30) Sulfur powder (S), molybdenum trioxide (MoO.sub.3), and a sapphire substrate (C-plane, A-direction inclined 0.2 degrees, M-direction inclined 0.04 degrees, step deviation <1010> direction 11 degrees) were placed in the first, second and third temperature zones of the three-temperature zone CVD system, respectively. Vacuum was drawn below 10 Pa and 100 standard milliliters per minute (sccm) of Ar was introduced. The substrate was warmed to 930° C., then S was heated to 160° C. to melt, then MoO.sub.3 was warmed to 530° C., and 3 sccm of oxygen was introduced. After 30 minutes of reaction, the oxygen was turned off and the MoO.sub.3 source was removed from the heating zone. The substrate was cooled to 300° C. in an Ar and S atmosphere and S heating was stopped. Followed by continue cooling to room temperature and the sample was removed.

(31) FIG. 9 is an optical photograph of the unidirectionally arranged MoS.sub.2 obtained in this example. FIG. 10 is a corresponding atomic force microscope image. It can be seen that although the steps deviate by 11 degrees, MoS.sub.2 remains well unidirectional.

Example 4

(32) Sulfur powder (S), tungsten trioxide (WO.sub.3) and a sapphire substrate (C plane, A direction inclined 0.5 degrees) were placed in the first, second and third temperature zones of the three-temperature zone CVD system, respectively. Vacuum was drawn below 10 Pa and 100 sccm Ar+10 sccm H.sub.2 were introduced. The substrate was warmed to 850° C., then S was heated to 150° C. to melt, then WO.sub.3 was warmed to 1050° C. to start the reaction. H.sub.2 was turned off after 60 min of reaction. The substrate was cooled to 300° C. in an Ar and S atmosphere and S heating was stopped. Followed by continuously cooling to room temperature, and the sample was taken to obtain the continuously covered WS.sub.2 single crystal wafer.

Example 5

(33) Selenium powder (S), tungsten trioxide (WO.sub.3) and a sapphire substrate (C plane, A direction inclined 1.5 degrees) were placed in the first, second and third temperature zones of the three-temperature zone CVD system, respectively. Vacuum was drawn below 10 Pa and 100 sccm Ar+10 sccm H.sub.2 were introduced. The substrate was warmed to 900° C., then S was heated to 250° C. to melt, then WO.sub.3 was warmed to 1050° C. to start the reaction. H.sub.2 was turned off after 60 min of reaction. The substrate was cooled to 300° C. in an Ar and Se atmosphere and Se heating was stopped. Followed by continuously cooling to room temperature, and the sample was taken to obtain the continuously covered WSe.sub.2 single crystal wafer.

Example 6

(34) Selenium powder (S), molybdenum trioxide (MoO.sub.3) and a sapphire substrate (C plane, A direction inclined 1.5 degrees) were placed in the first, second and third temperature zones of the three-temperature zone CVD system, respectively. Vacuum was drawn below 10 Pa and 100 sccm Ar+10 sccm H.sub.2 were introduced. The substrate was warmed to 870° C., then Se was heated to 250° C. to melt, then MoO.sub.3 was warmed to 530° C. to start the reaction. H.sub.2 was turned off after 60 min of reaction. The substrate was cooled to 300° C. in an Ar and Se atmosphere and Se heating was stopped. Followed by continuously cooling to room temperature, and the sample was taken to obtain the continuously covered MoSe.sub.2 single crystal wafer.

Example 7

(35) Selenium powder (S), tungsten trioxide (WO.sub.3) and a sapphire substrate (C plane, A direction inclined 1.0 degrees) were placed in the first, second and third temperature zones of the three-temperature zone CVD system, respectively. Vacuum was drawn below 10 Pa and 100 sccm Ar+5 sccm H.sub.2 were introduced. The substrate was warmed to 850° C., then Se was heated to 250° C. to melt, then WO.sub.3 was warmed to 870° C. to start the reaction. H.sub.2 was turned off after 30 min of reaction. The substrate was cooled to 300° C. in an Ar and Se atmosphere and Se heating was stopped. Followed by continuously cooling to room temperature, and the sample was taken to obtain the unidirectional aligned WSe.sub.2 domains.

(36) FIG. 12 is an optical photograph of the unidirectionally arranged WSe.sub.2 obtained in this example.

(37) FIG. 13 is a corresponding atomic force microscope image. It can be seen that one of the WSe.sub.2 edges parallel to the <1010> steps.

(38) FIG. 14 is the Raman spectrum of the as-grown WSe.sub.2.