Remote epitaxy and stripping method of GaAs solar cell based on graphene / large-angle beveled Ge (001) substrate

Abstract

The invention relates to the technical field of semiconductor photoelectric device manufacturing, and discloses a remote epitaxy and stripping method of a GaAs solar cell based on a graphene / large-angle beveled Ge (001) substrate. The core of the method is as follows: a Ge (001) substrate which is beveled at a large angle of 10-15 degrees along a [111] crystal orientation is used as a template, and single crystal graphene is directly grown through chemical vapor deposition, so that a complex transfer step is omitted; then, growing a GaAs nucleating layer through a migration enhancement epitaxial technology, and cooperatively solving the defect problem caused by large-angle beveling and lattice mismatch in combination with an InGaAs / GaAs strain buffer layer, so as to ensure the quality of an epitaxial layer; and finally, carrying out mechanical stripping by utilizing Van der Waals force of graphene, and transferring to a flexible substrate. According to the method, peelability and high-quality epitaxy are unified, the technological process is simple, pollution is avoided, the cost is low, and a reliable scheme is provided for industrialization of the high-performance flexible GaAs solar cell.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor optoelectronic device manufacturing technology, specifically relating to a method for preparing flexible thin-film solar cells, and particularly a remote epitaxy and stripping method for GaAs (gallium arsenide) solar cells based on graphene / large-angle obliquely cut Ge(001) substrates. Background Technology

[0002] III-V compound solar cells, especially GaAs cells, are ideal power sources for spacecraft and high-end portable devices due to their highest photoelectric conversion efficiency, excellent high-temperature resistance, and radiation resistance. To meet the modern application requirements for lightweight, flexible, and high-power-density solar cells, the development of flexible thin-film GaAs technology is crucial. The core of this technology lies in how to peel off a high-quality epitaxial layer from the growth substrate and transfer it onto a flexible substrate.

[0003] Currently, mainstream technologies face significant challenges. One approach is wet exfoliation, which takes several hours, is prone to introducing defects, and poses safety hazards. Another approach is remote van der Waals epitaxy and exfoliation based on two-dimensional materials. This technology typically requires growing graphene on copper foil first, followed by multiple complex steps including polymer spin-coating, copper foil etching, transfer to the target substrate, and polymer removal to obtain the graphene / target substrate. This process involves numerous steps, has a low yield, and inevitably introduces polymer residues and metal ion contamination, severely degrading the quality of the subsequently epitaxial GaAs functional layer. This results in device performance far below theoretical values, high costs, and has become a bottleneck for industrialization.

[0004] Therefore, there is an urgent need in the field for a method to fabricate flexible GaAs solar cells that can fundamentally avoid pollution, simplify the process, and at the same time ensure the quality of the epitaxial layer crystals. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a remote epitaxy and stripping method for GaAs solar cells based on graphene / large-angle oblique-cut Ge(001) substrates that has fewer process steps, lower cost, no pollution, and can ensure high quality of epitaxial layers.

[0006] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: A remote epitaxy and lift-off method for GaAs solar cells based on graphene / large-angle obliquely cut Ge(001) substrates includes the following steps: S1. Substrate Preparation: A Ge(001) wafer is offset by 10° to 15° relative to the (001) crystal plane normal along the

[111] direction. To reduce surface energy, germanium atoms on the wafer surface bond together to form germanium-germanium dimer chains, providing catalytic conditions for graphene growth and effectively reducing the activation energy of the reaction. On the flat Ge(001) surface, these dimers have two equivalent orientations (perpendicular to each other), resulting in multi-domain growth of graphene. The large-angle offset leads to the formation of high-density, narrow-width atomic steps on the substrate surface. On the narrow platform, there is not enough space for the other vertically oriented dimer chains to nucleate and grow, ultimately achieving the epitaxial growth of single-crystal graphene.

[0007] S2. Direct Graphene Growth: A beveled substrate is placed in a chemical vapor deposition apparatus, and a carbon source gas (such as methane) is introduced at a high temperature (e.g., 916°C) to directly grow a single layer of single-crystal graphene. This step completely eliminates the graphene transfer process.

[0008] S3. GaAs nucleation layer growth: A substrate with graphene growth is placed in an epitaxial device (such as MBE), and a GaAs nucleation layer is grown using migration-enhanced epitaxy. This technique forcibly separates the "nucleation" and "migration" processes of atoms through a "growth-interruption" pulse cycle, effectively suppressing island growth that is prone to occur on stepped substrates, and laying the foundation for subsequent layered growth.

[0009] S4. In x Ga1₋ x As / GaAs strain buffer layer growth: 5-10 cycles of In are grown on the nucleation layer. x Ga1₋ x As / GaAs strain buffer layer. This buffer layer utilizes the stress generated by lattice mismatch to induce and annihilate penetrating dislocations, acting as a highly efficient "dislocation filter" to actively remove defects extending from the interface and chamfered steps.

[0010] S5. Cell functional layer growth: GaAs solar cell functional layer is epitaxially grown on the strain buffer layer, preferably with an inverted structure.

[0011] S6. Device fabrication and release: A patterned insulating reflective layer and back electrode are fabricated, bonded to a flexible polymer substrate, and finally, the entire functional structure is released non-destructively by applying mechanical force and utilizing the weak van der Waals forces between the graphene layers.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Pioneering Process Integration and Simplification: This invention creatively integrates four key technical elements—"large-angle oblique cutting of Ge substrates," "direct graphene growth," "MEE nucleation," and "strain buffer layer"—into a coherent process flow completed in a vacuum or controlled environment. This eliminates all the complex and contamination-prone steps related to graphene transfer in traditional methods, representing a significant technological leap forward in the field.

[0013] 2. Synergistic Solution to Coupling Technical Challenges: This invention is not a simple superposition of features, but rather solves the coupling challenges of "peelability" and "high-quality epitaxy" simultaneously through the synergistic effect of each step. A large-angle beveled substrate is key to achieving direct single-crystal growth of graphene, but traditionally it is unfavorable for epitaxy. This invention, through a specially designed combination of "MEE nucleation layer + strain buffer layer," unexpectedly overcomes this inherent bias, transforming the disadvantage into a favorable platform for achieving high-quality, peelable epitaxy.

[0014] 3. Significantly Improved Epitaxial Layer Quality and Device Performance: By avoiding transfer contamination and employing an effective defect suppression strategy, the GaAs epitaxial layer crystal quality prepared by this invention is significantly superior to that of traditional transfer methods. The metal back electrode with a complex pyramid structure enhances light reflection, further increasing the current density. Examples show that the flexible GaAs solar cells thus fabricated exhibit higher conversion efficiency and excellent mechanical flexibility.

[0015] 4. Broad prospects for industrial application: This invention provides a reliable, economical, and high-performance technical path, paving the way for the large-scale commercial production of flexible GaAs solar cells. Attached Figure Description

[0016] Figure 1 : A schematic diagram of a large-angle obliquely cut Ge(001) substrate based on single-crystal graphene provided by the present invention.

[0017] Figure 2 : Schematic diagram of the structure of the MEE GaAs nucleation layer and InGaAs / GaAs strain buffer layer grown on the graphene / Ge substrate in this invention.

[0018] Figure 3 This invention provides a schematic diagram of the process for the van der Waals mechanical peeling of flexible batteries. Detailed Implementation

[0019] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. It should be noted that the following embodiments are illustrative and not intended to limit the scope of protection of the present invention. Example

[0020] 1. Substrate preparation: A 4-inch Ge(001) substrate was selected. Using crystal orientation and cutting technology, the substrate was precisely offset by 10° along the

[111] direction relative to the (001) crystal plane normal.

[0021] 2. Direct Graphene Growth: A beveled substrate is placed into a horizontal CVD furnace. The system is evacuated to a vacuum level of 5 × 10⁻⁻⁻⁶. 6 After mbar loading, high-purity argon (Ar, 99.9999% purity) and hydrogen (H2, 99.9999% purity) were introduced. The temperature was raised to 916℃ and stabilized for 1 hour. Subsequently, methane (CH4, 99.99% purity) was introduced for growth for 30 minutes. After growth, the substrate was cooled to room temperature in an Ar and H2 mixed gas atmosphere to obtain a Ge substrate with a surface covered by single-crystal graphene.

[0022] 3. GaAs nucleation layer growth: Transfer the above substrate into a molecular beam epitaxy (MBE) apparatus. Migration-enhanced epitaxy is employed: each cycle includes 2 seconds of Ga beam on, 2 seconds of interruption, 2 seconds of As beam on, and 2 seconds of interruption. Repeat 10 cycles. The As / Ga beam equivalent pressure ratio is set to 25.

[0023] 4. Strain buffer layer growth: The sample was transferred to an MOCVD device. An eight-cycle strain buffer layer was grown at 650℃ and 50 mbar pressure, with each cycle consisting of 5 nm In atoms. 0.1 Ga 0.9 Composed of As and 5nm GaAs.

[0024] 5. Cell functional layer growth: In the same MOCVD equipment, the inverted structure solar cell functional layers are grown: from bottom to top, they are GaInP top cell, GaAs middle cell, and InGaAs bottom cell.

[0025] 6. Back Electrode Fabrication and Flexible Substrate Lamination: An insulating reflective layer was fabricated using inkjet printing. The pyramidal periodic structure of the insulating reflective layer was replicated using nanoimprint lithography and then cured. A 3500nm thick Cu back electrode was deposited on the battery surface using magnetron sputtering. This Cu layer replicated the underlying pyramidal insulating reflective layer structure, forming a microstructured light-reflecting back electrode. Subsequently, a PET flexible substrate was laminated onto the back electrode using a thermoimprint lithography process.

[0026] 7. Van der Waals peeling: Fix the sample, clamp the edge of the PET with a jig, and slowly and vertically apply a pulling force until the entire battery structure is completely peeled off from the graphene interface.

[0027] 8. Previous process completion: The front gate electrode is prepared by photolithography, electron beam evaporation (AuGeNi / Au) and lift-off process, followed by evaporation of MgF2 / ZnS double antireflection film.

[0028] Effect verification: To demonstrate the superiority of the present invention, comparative tests were conducted.

[0029] Comparative example: Graphene grown on copper foil was transferred to a regular Ge(001) substrate using a conventional transfer method, followed by the same epitaxial growth and device fabrication process.

[0030] result: Crystal quality: High-resolution X-ray diffraction tests showed that the full width at half maximum (FWHM) of the (004) crystal plane diffraction peak of the GaAs epitaxial layer grown in the present invention was 25 arcseconds, while that of the comparative sample was 45 arcseconds, indicating that the crystal quality of the epitaxial layer of the present invention is significantly better.

[0031] Surface contamination: X-ray photoelectron spectroscopy analysis showed that no polymer residues (such as PMMA characteristic peaks) were detected at the graphene / Ge interface of the embodiments of the present invention, while obvious carbon contamination peaks were found at the interface of the comparative sample.

[0032] Device efficiency: The flexible GaAs solar cell prepared by this invention achieves a conversion efficiency of 28.5% under AM1.5G spectrum, while the efficiency of the comparative cell is only 25.1%.

[0033] The above results demonstrate that the present invention, through a unique technical approach, successfully and simultaneously simplifies the process flow, eliminates contamination, and improves the quality of the epitaxial layer, achieving unexpected technical effects.

[0034] This invention proposes a process route with fewer steps and lower cost. Single-crystal graphene is directly grown on Ge(001) wafers cut at a large angle (10-15°) along the

[111] crystal direction, simplifying the process and solving the contamination problem of traditional transfer methods. When cutting germanium crystals along the (001) crystal plane to obtain Ge(001) wafers, the covalent bonds inside the crystal are broken, resulting in each germanium atom on the surface having two unpaired electrons, i.e., "two free bonds." This is a high-energy unstable state. To reduce the surface energy, the germanium atoms on the surface will move closer to each other, causing their free bonds to form new chemical bonds, forming germanium-germanium dimer chains, providing catalytic conditions for graphene growth and effectively reducing the reaction activation energy. The formation energy of graphene along the dimer long chain direction is lower than that perpendicular to the dimer direction. When the cut angle is small, the step is wide, allowing sufficient space for the carbon precursor to diffuse and grow along the perpendicular dimer direction, making it impossible to control the orientation of graphene domains and obtain single-crystal graphene wafers. When the bevel angle is greater than 10 degrees, the substrate steps become narrow and steep, greatly restricting the nucleation of graphene in the vertical direction, thus allowing for the acquisition of perfect single-crystal graphene. Simultaneously, to ensure the quality of subsequent GaAs epitaxial processes, migration-enhanced epitaxy (MEE) is used, employing a 10-pulse cycle of "Ga growth-interruption-As growth" to grow the GaAs nucleation layer. The first deposited layer is a Ga atomic layer, physically eliminating the possibility of As appearing as the first atomic layer, thereby fundamentally eliminating the nucleation source for antiphase domains in Ge-based epitaxial GaAs. The high bevel angle substrate has a high step density and narrow mesa, making the potential barrier for forming new islands on the narrow mesa actually lower than the barrier for atoms reaching the step, thus making island nucleation a more probable event. MEE completely separates the competing processes of "nucleation" and "migration" in time and greatly reduces the spatial density of adsorbed atoms, thereby forcibly suppressing the probability of atomic collision island formation and creating favorable conditions for layered growth. Above the nucleation layer, In is grown for 5-10 cycles. 0.1 Ga 0.9 As / GaAs strain buffer layer. This layer acts as a highly efficient dislocation filter, actively removing most of the penetrating dislocations extending from the interface and chamfered steps. This invention integrates high-quality epitaxy and non-destructive exfoliation technology into a simple and controllable process, paving the way for its large-scale industrialization.

Claims

1. A method for remote epitaxy and lift-off of GaAs solar cells based on graphene / large-angle obliquely cut Ge(001) substrates, characterized in that, Includes the following steps: S1. Substrate preparation: Provide a Ge(001) wafer and offset it by 10° to 15° along the [111] direction relative to the (001) crystal plane normal; S2. Direct growth of graphene: On the surface of the obliquely cut Ge(001) wafer, a single layer of single-crystal graphene is directly grown by chemical vapor deposition to form a graphene / Ge composite substrate. S3. Nucleation layer growth: A GaAs nucleation layer is grown on the graphene / Ge composite substrate using migration-enhanced epitaxy. S4. Buffer layer growth: On the GaAs nucleation layer, In layers consisting of 5-10 cycles are grown. x Ga1₋ x A strain buffer layer composed of alternating As and GaAs layers, wherein 0 <x≤0.2; S5. Functional layer growth: On the strain buffer layer, a GaAs solar cell functional layer is epitaxially grown; S6. Device fabrication: A patterned back insulating reflective layer and a back electrode are fabricated on the functional layer of the solar cell. After being bonded to a flexible polymer substrate, the overall structure of the solar cell functional layer, the back electrode, and the flexible polymer substrate is peeled off from the graphene / Ge composite substrate by applying mechanical force.

2. The remote epitaxy and lift-off method for GaAs solar cells based on graphene / large-angle obliquely cut Ge(001) substrates according to claim 1, characterized in that, The migration-enhanced epitaxy technique in step S3 employs a pulsed cycle, with each cycle consisting of: starting Ga source growth for 2 seconds, interrupting growth for 2 seconds, starting As source growth for 2 seconds, and interrupting growth for 2 seconds. The cycle is repeated 10 times, and the beam equivalent pressure ratio of As / Ga is 25.

3. The remote epitaxy and lift-off method for GaAs solar cells based on graphene / large-angle obliquely cut Ge(001) substrates according to claim 1, characterized in that, In step S4, the single-period structure of the strain buffer layer is a 5nm thick In... 0.1 Ga 0.9 An As layer and a 5nm thick GaAs layer.

4. The remote epitaxy and lift-off method for GaAs solar cells based on graphene / large-angle obliquely cut Ge(001) substrates according to claim 1, characterized in that, In step S5, the GaAs solar cell functional layer has an inverted structure with its bandgap decreasing from bottom to top, and includes GaInP sub-cells, GaAs sub-cells, and InGaAs sub-cells in sequence.

5. The remote epitaxy and lift-off method for GaAs solar cells based on graphene / large-angle obliquely cut Ge(001) substrates according to claim 1, characterized in that, In step S2, the growth temperature of chemical vapor deposition is 916℃, and the carbon source is methane.

6. The remote epitaxy and lift-off method for GaAs solar cells based on graphene / large-angle obliquely cut Ge(001) substrates according to claim 1, characterized in that, The back electrode in step S6 is prepared by magnetron sputtering a copper metal layer on an insulating reflective layer with a pyramidal structure. The copper metal layer replicates the pyramidal structure and has a thickness of 2500–4500 nm.

7. The remote epitaxy and lift-off method for GaAs solar cells based on graphene / large-angle obliquely cut Ge(001) substrates according to claim 1, characterized in that, The flexible polymer substrate in step S6 is PET, PEN, or PI.

8. A flexible GaAs solar cell, characterized in that, The GaAs solar cell based on graphene / large-angle oblique-cut Ge(001) substrate, prepared by any one of claims 1 to 7, is obtained by remote epitaxy and lift-off method, and the lift-off interface between the epitaxial layer and the flexible substrate is free of polymer residue and metal ion contamination.

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