A method for controlling van der Waals epitaxy and remote epitaxy growth modes

Abstract

The present invention discloses a device and method for controlling van der Waals epitaxy and remote epitaxy growth mode, using gallium nitride-graphene substrate, by changing the gate bias applied to the graphene layer, to control the selection of epitaxial growth mode. If a forward bias is applied to the gate of the graphene layer, the main carrier type in the graphene is electrons, which suppresses the polarity of gallium nitride, and the epitaxial growth mode selects the traditional van der Waals epitaxy, and only thin films other than two-dimensional van der Waals gallium nitride single crystal can be grown on the graphene. On the contrary, if no external voltage is applied to the gate of the graphene layer or a reverse bias is applied, remote epitaxy is selected, and a two-dimensional van der Waals gallium nitride single crystal thin film will be grown on the graphene. The present invention avoids the problem of multiple substrate manufacturing during the production process due to the different graphene thicknesses required for the two epitaxial modes. Not only can high-quality gallium nitride materials be produced, but also time and manufacturing costs are significantly saved.

Description

Technical Field

[0001] The present invention relates to the technical field of low-dimensional thin film single crystal semiconductor epitaxial growth, and in particular to a device and method for controlling van der Waals epitaxy and remote epitaxial growth modes. Background Art

[0002] In recent years, semiconductors have penetrated into every aspect of our lives. Compared with the first and second generation semiconductor materials, the third generation wide bandgap semiconductor material gallium nitride (GaN) has the characteristics of high saturated electron drift velocity, high thermal conductivity, high critical breakdown voltage, and other characteristics. It has superior performance such as high frequency, high voltage resistance, and high temperature resistance. It is the preferred material for high-power devices in extreme environments. It can better meet the needs of 5G technology, new energy vehicles, and military detection. At the same time, it has broad market prospects in the field of high-end optoelectronics and power electronic devices, and has become the focus of global semiconductor technology and industry competition.

[0003] However, the performance of GaN-based ultraviolet optoelectronic devices is still not ideal, mainly because most GaN-based devices are prepared on heterogeneous substrates (such as silicon, silicon carbide, sapphire, etc.). Problems such as lattice mismatch and thermal mismatch between the GaN epitaxial layer and the substrate will cause lattice distortion in the epitaxial layer, thereby forming a higher dislocation density, mosaic crystal structure, biaxial stress and wafer warpage, which seriously affect the performance and service life of GaN-based devices. In order to reduce epitaxial defects and threading dislocations, various heteroepitaxial growths for highly mismatched materials have been developed, such as low-temperature buffer layers, metamorphic buffer layers, etc. Despite these advances, compared with homogeneous epitaxial layers, it is still a challenge to obtain good crystal quality for heteroepitaxial layers with high lattice mismatch. The two growth methods (van der Waals epitaxy and remote epitaxy) involved in the present invention are mainly used for the growth of van der Waals layered materials, which can solve the key problems such as thermal mismatch and lattice mismatch caused by the above-mentioned traditional heterogeneous substrate growth method that seriously affect the performance of materials and devices.

[0004] Van der Waals epitaxy was discovered by Atsushi Koma's group at the University of Tokyo in the mid-1980s during the epitaxial growth of selenium (Se) on a tellurium (Te) substrate. It is a method for performing epitaxy of two-dimensional thin film materials on a two-dimensional substrate material with passivated dangling bonds on the surface. Because the manufacturing process of graphene is now well mastered, graphene materials are usually used as two-dimensional substrate materials. In the traditional epitaxial process, there are dangling bonds on the surface of the substrate, and covalent chemical bonding occurs between the adsorbed atomic layer, and the traditional two-dimensional thin film material is grown. In van der Waals epitaxy, the adsorbed atomic layer is bound to the graphene surface by van der Waals forces without transferring or sharing electrons. This van der Waals bond is different from an ionic bond or a covalent bond. It is not a chemical bond between two materials, but originates from the dipole interaction between atoms and is very weak, so van der Waals epitaxy can allow the growth of heteroepitaxial films with a lattice mismatch greater than 60%. Two-dimensional layered epitaxial materials grown on substrates such as graphene through van der Waals epitaxy and bonded by van der Waals forces are called van der Waals thin film materials.

[0005] Remote epitaxy is a special new van der Waals epitaxy technology that occurs by inserting a two-dimensional graphene layer between the van der Waals epitaxial film layer to be grown and the traditional substrate. Since the thin graphene layer below the critical thickness is transparent to the Coulomb interaction between the atoms of the upper epitaxial layer to be grown and the surface of the traditional substrate below, the substrate polarity below the graphene layer can control the growth of the upper epitaxial layer through the graphene layer, thereby achieving the growth of a two-dimensional van der Waals film with the same crystal structure and material properties as the substrate. Previously, Kim, Y et al. have demonstrated the remote nucleation effect from substrate to graphene by growing a wafer-level (001) GaAs single crystal film on a single-layer graphene-coated (001) gallium arsenide (GaAs) wafer. This remote epitaxy method has also been proven to be used to grow two-dimensional van der Waals GaN materials on a GaN substrate with a graphene coating, which can eliminate the influence of lattice mismatch, is the basis for the development of new devices, and is the fundamental guarantee for achieving high-performance devices.

[0006] Both epitaxy methods use graphene materials. Graphene has extremely high carrier mobility and bipolar electric field effect, and is a zero-bandgap semi-metallic material. The so-called bipolar electric field effect means that the type (electron or hole) and density of carriers in graphene can be controlled by an external electric field. In remote epitaxy, the thickness of the graphene material must be very thin to be "transparent", and the properties of the epitaxially grown van der Waals layer are the same as those of the substrate under the graphene layer. In van der Waals epitaxy, the graphene layer must be very thick to suppress the polarity of the substrate, and the lattice structure of the epitaxially grown van der Waals layered material is determined by the graphene layer and has nothing to do with the properties of the substrate layer. In both of the above epitaxial growth methods, the weak van der Waals bonding of the graphene material helps to peel off the epitaxially grown film from the substrate to obtain a perfect van der Waals layered material, which can then be transferred to any substrate of interest to the user.

[0007] The latest research shows that the critical thickness of the epitaxial growth of two-dimensional van der Waals GaN thin film material on the above-mentioned GaN-graphene substrate is the thickness of the double-layer graphene. When the thickness of the graphene layer on the GaN substrate is less than the critical thickness, the polarity of GaN can penetrate the potential field of graphene and affect the growth of the epitaxial layer. At this time, it is suitable to select the remote epitaxy technology. When the thickness is greater than the critical thickness, the polarity of GaN is not enough to penetrate the graphene layer, and only the traditional van der Waals epitaxy mode can be used to grow other van der Waals films that are not related to the GaN substrate. Although van der Waals epitaxy and homoepitaxial technology can both be used to grow two-dimensional van der Waals films, it can be seen from the different roles played by graphene in them that the crystal structure and material properties of the films grown by these two technologies are different. In traditional technology, if it is necessary to switch between van der Waals epitaxy and remote epitaxy, it is necessary to change the thickness of the graphene layer on the GaN substrate, that is, it is necessary to make different substrates, and the process cost and time cost of thin film material preparation will increase sharply. Summary of the invention

[0008] The present invention provides a device and method for controlling the growth mode of van der Waals epitaxy and remote epitaxy. At present, there is no method that can freely select remote epitaxy or van der Waals epitaxy technology on the same substrate. The present invention proposes to change the polarity of the graphene layer by changing the bias voltage applied to the graphene layer, so as to selectively grow van der Waals GaN-based epitaxial thin film layers with different crystal structures and properties.

[0009] The technical solution adopted by the present invention is:

[0010] A device for controlling van der Waals epitaxy and remote epitaxial growth modes comprises a gallium nitride substrate, a single-layer graphene layer, a gate, a source, a drain, an external gate bias, and an external source-drain voltage; the source is connected to one end of the single-layer graphene layer, and the drain is connected to the other end of the single-layer graphene layer; the graphene layer is connected above the gallium nitride substrate, and the gate is connected below the gallium nitride substrate; the external gate bias is a voltage applied between the source and the gate; and the external source-drain voltage is a voltage applied between the source and the drain.

[0011] A method for preparing a device for controlling van der Waals epitaxy and remote epitaxial growth modes comprises the following steps:

[0012] Step 1, growing graphene by chemical vapor deposition (CVD), and obtaining a graphene layer on a gallium nitride substrate by a roll-to-roll peeling transfer technique;

[0013] Step 2, depositing metal contacts on the bottom layer of the GaN substrate to form a gate;

[0014] Step 3, depositing metal contacts on both sides of the graphene layer to form source and drain electrodes;

[0015] Step 4, applying a source-drain voltage between the source and the drain, and applying a gate bias voltage between the source and the gate.

[0016] A method for controlling the van der Waals epitaxy and remote epitaxy growth modes using the above device, on the same substrate, by changing the gate bias applied to the graphene layer to achieve the selection of the van der Waals epitaxy and remote epitaxy growth modes.

[0017] Furthermore, in step 4, when the applied gate bias is a forward bias, the epitaxial growth mode selected is van der Waals epitaxy, and a two-dimensional van der Waals single crystal thin film independent of the gallium nitride substrate is epitaxially grown; when the applied gate bias is zero or a reverse bias, the epitaxial growth mode selected is remote epitaxy, and a two-dimensional van der Waals gallium nitride single crystal thin film is epitaxially grown.

[0018] Furthermore, in step 4, when the gate bias voltage and the source-drain voltage are both zero, the epitaxial growth mode selected is remote epitaxy, and at this time, a more perfect two-dimensional van der Waals gallium nitride single crystal thin film is epitaxially grown.

[0019] Furthermore, the GaN substrate in step 1 can be replaced by other GaN-based single crystal materials such as aluminum gallium nitride (AlGaN), aluminum nitride (AlN), etc. After replacement, the properties of the two-dimensional van der Waals thin film grown by remote epitaxy technology are determined by the corresponding replacement substrate.

[0020] Furthermore, the preparation of the graphene layer in step 2 can not only use chemical vapor deposition, but also utilize a self-limiting graphitization process to sublimate Si atoms in a silicon carbide (SiC) wafer, or utilize pyrolytic graphite to separate graphene, or other preparation methods, which do not affect the effectiveness of the method for controlling the two-dimensional van der Waals thin film growth mode proposed in this application.

[0021] Furthermore, in step 5, the source and drain should be as far away from the center of the GaN substrate as possible, because the temperature at the center of GaN growth is relatively high. At the same time, the source and drain power supplies need to be protected to prevent failure at high temperatures.

[0022] By adopting the above technical solution, the present invention has the following beneficial effects:

[0023] (1) On the same substrate, the van der Waals epitaxy and remote epitaxial growth modes can be freely selected to obtain epitaxial layers with different crystal structures and properties. When GaN materials and other two-dimensional materials are needed at the same time, the substrate can be reused, saving time and production costs.

[0024] (2) Due to the extremely weak van der Waals bonding of the two-dimensional material graphene, the epitaxial layer produced by van der Waals epitaxy and remote epitaxy can be easily peeled off from the graphene layer using the two-dimensional auxiliary layer transfer technology. At the same time, the thickness of the peeled epitaxial layer can be precisely controlled, the peeling speed is fast, and a pure substrate is left after peeling.

[0025] (3) In the GaN-graphene substrate, since graphene has good heat dissipation properties, the single-layer graphene transferred on the GaN substrate can reduce the substrate temperature during the subsequent epitaxial growth process. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the structure of the device for controlling the van der Waals epitaxy and remote epitaxial growth modes of the present invention.

[0027] Figure 2 The figure is a flow chart of the method for controlling the van der Waals epitaxy and remote epitaxial growth modes of the present invention.

[0028] Figure 3 Schematic diagram for selecting the remote epitaxial growth mode.

[0029] Figure 4 Schematic diagram of selecting the van der Waals epitaxial growth mode. DETAILED DESCRIPTION

[0030] In order to make the contents of the present invention more clearly understood, the present invention is further described in detail below based on specific embodiments in conjunction with the accompanying drawings.

[0031] Embodiment 1:

[0032] See Figure 1 The device for controlling the van der Waals epitaxy and remote epitaxial growth mode of this embodiment includes a self-supporting gallium nitride (GaN) substrate 1, a single-layer graphene layer 2, a gate 3, a source 4, and a drain 5.

[0033] The single-layer graphene layer 2 is on the self-supporting GaN substrate 1 to form a GaN-graphene substrate.

[0034] The source 4 and drain 5 are placed on both sides of the single-layer graphene layer 2, and the gate 3 is set on the bottom layer of the GaN substrate 1. A gate bias 6 is applied between the gate 3 and the source 4, and a source-drain voltage 7 is applied between the source 4 and the drain 5. The source 4 and drain 5 should be as far away from the center of the GaN substrate 1 as possible, because the growth temperature at the center is relatively high. Even if they are far away, protection measures need to be taken for the 4 and 5 power supplies to prevent failure at high temperatures.

[0035] A method for manufacturing a device for controlling van der Waals epitaxy and remote epitaxial growth modes comprises the following steps:

[0036] Step 1: Purchase a commercial GaN product as the self-supporting GaN substrate 1 of the device of the present invention.

[0037] Step 2, the growth of graphene adopts chemical vapor deposition method, and copper foil with catalytic growth effect is selected as the substrate for growing graphene. After fully emptying the residual gas in the quartz tube, it is heated to 1000°C in a mixed gas of 10sccm H2 and 100sccm Ar, and the heating time is 50min. The copper foil is annealed at 1000°C for 30min to increase its single crystal size in preparation for graphene growth. Then, 1sccm of CH4 is introduced for 10min to form graphene nucleation sites on the copper foil, and then a flow rate of 3sccm of CH4 is introduced to grow graphene for 40min. The growth of graphene will terminate through a self-limiting process to obtain a single-layer graphene film. Finally, the copper foil is quickly cooled to room temperature. The single-layer graphene layer 2 is transferred to the GaN substrate 1 by a "roll-to-roll" transfer method to obtain a GaN-graphene substrate.

[0038] Step 3, depositing contact metal Ti / Au on both sides of the single-layer graphene layer 2 and the bottom of the GaN substrate 1 by electron beam evaporation to obtain good ohmic contact and form the source 4, drain 5 and gate 3 of the device.

[0039] Step 4, applying a source-drain voltage 7 between the source 4 and the drain 5;

[0040] Step 5, applying a gate bias 6 between the source 4 and the gate 3, and controlling the selection of the epitaxial mode by changing the positive and negative of the applied gate bias 6.

[0041] In the step 1, the self-supporting GaN substrate 1 is a 2-inch bulk single crystal with a thickness of 600 nm.

[0042] The thermal release tape in step 2 is composed of a unique adhesive that has relatively strong adhesion at room temperature. However, its adhesion is rapidly lost at 90–120°C. Taking advantage of this feature, the steps of the "roll-to-roll" transfer method are as follows: after growing graphene on the copper foil, the copper foil and graphene are passed through two rollers together, and soft pressure (~0.2MPa) is applied between the two rollers so that the graphene film grown on the copper foil is adhered to the thermal release tape. After etching the copper foil in a container filled with copper etchant, the transferred graphene film on the tape is rinsed with deionized water to remove the residual etchant, and then the graphene film on the thermal release tape is inserted between the rollers together with the GaN substrate, and the rollers are heated to about 90-120°C, with a transfer rate of about 150-200mm min -1 , the graphene film is transferred from the tape to the GaN substrate to form a GaN-graphene substrate.

[0043] The heat release tape can be purchased directly.

[0044] Compared with the traditional graphene transfer method that requires the use of PMMA, the "roll-to-roll" transfer method used in step 2 avoids the problem of heavy metal impurities and PMMA residues that lead to reduced electrical properties of graphene. At the same time, the "roll-to-roll" transfer method has a simple process, does not require the use of a large amount of organic solvents, does not involve high-temperature treatment, and can also obtain large-area graphene with high cleanliness and flatness.

[0045] In the steps 4 and 5, when the applied source-drain voltage 7 and the gate bias 6 are both zero, the remote epitaxy mode is selected, and the two-dimensional van der Waals GaN single crystal film grown at this time is the most perfect. When the applied source-drain voltage 7 is -1V, considering the influence of the GaN substrate thickness and dielectric constant, if a 237V forward gate bias 6 is applied, the epitaxial growth mode is van der Waals epitaxy; if a -237V reverse gate bias 6 or no gate bias 6 is applied, remote epitaxy is selected.

[0046] In step 5, although the required external gate voltage is as high as 237V, after the first layer of van der Waals material is grown, the high gate voltage is no longer required, and the time is very short, so it will not damage the substrate. In addition, if the external gate voltage is still too high, the required gate voltage can be reduced by reducing the thickness of the GaN substrate.

[0047] See Figure 2 The principle of the method for controlling the van der Waals epitaxy and remote epitaxial growth mode of this embodiment is:

[0048] In the above-mentioned device for controlling the selection of van der Waals epitaxy and remote epitaxial growth modes, for the gate bias 6 applied between the gate 3 and the source 4, when the applied gate bias 6 is changed, since the GaN substrate 1 is not conductive, it is equivalent to just adding a gate electric field to the single-layer graphene layer 2, and no loop will be formed. For the Ga-surface GaN substrate 1, the top layer is Ga ions, and the Ga ions at this time are cations. When a reverse gate bias is applied, the inversion layer of the graphene layer 2 is a hole, which enhances the polarity of the GaN substrate, and the epitaxial growth mode that occurs is remote epitaxy (such as Figure 3 ), the generated epitaxial layer is a two-dimensional van der Waals gallium nitride single crystal film 8 with the same lattice structure as the GaN substrate. At the same time, if the external gate bias 6 and the source-drain voltage 7 are both zero, since the critical thickness of the epitaxial growth of the two-dimensional van der Waals GaN thin film material on the GaN-graphene substrate is the thickness of the double-layer graphene, and the thickness of the graphene layer used in the present invention is a single layer, if no gate bias is applied to the graphene layer above the GaN substrate, the polarity of the GaN substrate below the graphene layer can control the growth of the upper epitaxial layer through the graphene layer, and remote epitaxy is still selected, and at this time, the effect of remote epitaxial growth of the two-dimensional van der Waals gallium nitride single crystal film 8 is the best. If a forward gate bias is applied, the inversion layer of the graphene is electrons, the polarity of the graphene is enhanced, and the potential field of the substrate is shielded. At this time, van der Waals epitaxy (such as Figure 4 ), the lattice structure of the epitaxially grown two-dimensional van der Waals single crystal thin film 9 which is independent of the gallium nitride substrate is determined by the graphene layer 2.

[0049] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A device for controlling van der Waals epitaxy and remote epitaxial growth modes, characterized in that: The device comprises: a gallium nitride substrate (1), a single-layer graphene layer (2), a gate (3), a source (4), a drain (5), an external gate bias (6), and an external source-drain voltage (7); the source (4) is connected to one end of the single-layer graphene layer (2), and the drain (5) is connected to the other end of the single-layer graphene layer (2); the graphene layer (2) is connected above the gallium nitride substrate (1), and the gate (3) is connected below the gallium nitride substrate (1); the external gate bias (6) is a voltage applied between the source (4) and the gate (3); and the external source-drain voltage (7) is a voltage applied between the source (4) and the drain (5); On the same substrate (1), the selection of the van der Waals epitaxy and remote epitaxy growth modes is achieved by changing the gate bias (6) applied to the graphene layer (2); The polarity of the graphene layer can be changed by changing the bias voltage applied to the graphene layer.

2. The method for preparing the device according to claim 1, characterized in that: The following steps are involved: Step 1, growing graphene by chemical vapor deposition (CVD), and obtaining a graphene layer (2) on a gallium nitride substrate (1) by a roll-to-roll peeling transfer technique; Step 2, depositing a metal contact on the bottom layer of the gallium nitride substrate (1) to form a gate (3); Step 3, depositing metal contacts on both sides of the single-layer graphene layer (2) to form a source electrode (4) and a drain electrode (5); Step 4: applying a source-drain voltage (7) between the source (4) and the drain (5), and applying a gate bias voltage (6) between the source (4) and the gate (3).

3. The preparation method according to claim 2, characterized in that: In step 4, when the externally applied gate bias (6) is a forward bias, the epitaxial growth mode selected is van der Waals epitaxy, and a two-dimensional van der Waals single crystal thin film (9) independent of the gallium nitride substrate (1) is epitaxially grown; when the externally applied gate bias (6) is zero or a reverse bias, the epitaxial growth mode selected is remote epitaxy, and a two-dimensional van der Waals gallium nitride single crystal thin film (8) is epitaxially grown.

4. The preparation method according to claim 3, characterized in that: In step 4, when the gate bias voltage (6) and the source-drain voltage (7) are both zero, the epitaxial growth mode selected is remote epitaxy, and at this time, a two-dimensional van der Waals gallium nitride single crystal thin film (8) is epitaxially grown.

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