The direct growth of two-dimensional (2D) materials on three-dimensional (3D) substrates represents a critical advancement in semiconductor technology, enabling seamless integration of atomically thin layers with conventional bulk materials. This approach eliminates the need for transfer processes, which often introduce defects, contaminants, or interfacial inconsistencies. Key examples include graphene grown on silicon carbide (SiC) and transition metal dichalcogenides (TMDCs) such as MoS2 or WS2 synthesized on gallium nitride (GaN). The success of these systems hinges on understanding interfacial bonding mechanisms and strain management to ensure optimal electronic, optical, and mechanical performance.

Graphene growth on SiC is one of the most studied systems in this category. The process typically involves high-temperature sublimation of silicon from the SiC surface, leaving behind a carbon-rich layer that reorganizes into graphene. The SiC substrate provides a crystalline template that influences the orientation and quality of the graphene. The interface between graphene and SiC is characterized by a buffer layer, which exhibits covalent bonding with the substrate while the overlying graphene retains its sp2 hybridized structure. This buffer layer plays a crucial role in determining the electronic properties of the system, as it introduces a bandgap in the graphene due to substrate interactions. The lattice mismatch between graphene and SiC is approximately 20%, leading to significant strain. However, the formation of a moiré superlattice helps accommodate this mismatch, periodically modulating the electronic structure and creating localized electronic states.

Strain management in graphene-on-SiC systems is achieved through controlled growth conditions, including temperature, pressure, and substrate orientation. For instance, growth on the silicon-terminated (0001) face of SiC results in different strain profiles compared to the carbon-terminated (000-1) face. Post-growth annealing can further modify strain distribution, influencing carrier mobility and defect density. The ability to tailor these parameters is essential for applications in high-frequency electronics and quantum devices, where uniformity and low defect concentrations are paramount.

Similarly, the direct growth of TMDCs on GaN substrates offers a pathway to integrate 2D semiconductors with wide-bandgap materials for optoelectronic applications. GaN, with its high thermal stability and compatibility with existing III-V semiconductor processes, serves as an ideal platform. The synthesis of TMDCs like MoS2 on GaN is typically performed via chemical vapor deposition (CVD), where molybdenum and sulfur precursors react to form a monolayer or few-layer film. The interfacial bonding in this system is primarily van der Waals in nature, as the chalcogen-terminated surface of TMDCs does not form strong covalent bonds with GaN. This weak interaction minimizes interfacial defects but requires careful optimization to ensure adhesion and thermal stability.

The lattice mismatch between MoS2 and GaN is around 4%, which is relatively small compared to other heterostructures. However, even this minor mismatch can induce strain, affecting the bandgap and excitonic properties of the TMDC. Strain manifests as localized variations in photoluminescence intensity and peak position, which can be mapped using spectroscopic techniques. To mitigate strain, growth parameters such as precursor flux, temperature, and substrate pretreatment are adjusted. For example, sulfurization of pre-deposited molybdenum layers at controlled temperatures can yield more uniform films with reduced strain gradients. Additionally, the use of patterned or nanostructured GaN substrates can guide the growth of TMDCs, promoting alignment and reducing defects.

Interfacial charge transfer is another critical consideration in these systems. In graphene-on-SiC, the buffer layer induces n-type doping due to electron transfer from the substrate. In TMDCs on GaN, the direction and magnitude of charge transfer depend on the work function difference between the materials. For instance, GaN’s polarization fields can influence the carrier distribution in overlying TMDCs, modifying their transport properties. This effect is particularly relevant for devices such as photodetectors or light-emitting diodes, where interfacial charge dynamics directly impact performance.

Thermal expansion mismatch between 2D materials and 3D substrates also poses challenges. For example, the thermal expansion coefficient of SiC is significantly lower than that of graphene, leading to compressive strain in the graphene during cooling from growth temperatures. This strain can cause wrinkling or delamination if not properly managed. Strategies to address this include graded cooling protocols or the introduction of intermediate layers to absorb strain. In TMDC-GaN systems, the difference in thermal expansion is less pronounced, but thermal cycling during device operation can still induce mechanical stress over time.

The scalability of direct growth methods is a key advantage for industrial applications. Graphene-on-SiC wafers can be produced at wafer scale, making them suitable for integration into existing semiconductor fabrication lines. Similarly, CVD growth of TMDCs on GaN can be adapted to large-area substrates, enabling high-throughput manufacturing. The absence of transfer steps reduces contamination and improves yield, which is critical for commercial adoption.

Despite these advances, challenges remain in achieving perfect interfaces and uniform strain distribution. Defects such as grain boundaries, vacancies, or substrate steps can propagate into the 2D material, degrading its properties. Advanced characterization techniques, such as scanning transmission electron microscopy (STEM) or Raman mapping, are essential for identifying and mitigating these defects. Furthermore, the development of in-situ monitoring tools during growth can provide real-time feedback for process optimization.

Looking ahead, the direct growth of 2D materials on 3D substrates will continue to evolve, driven by demands for higher performance and new functionalities. Innovations in precursor chemistry, substrate engineering, and strain-tuning techniques will further enhance the quality and versatility of these heterostructures. As the field progresses, the integration of 2D materials with conventional semiconductors will unlock new possibilities in electronics, photonics, and quantum technologies, bridging the gap between atomic-scale precision and macroscopic device applications.