Contact engineering for diamond semiconductors is a critical aspect of device performance, particularly for high-power, high-frequency, and high-temperature applications. Diamond’s exceptional properties, including its wide bandgap (5.47 eV), high thermal conductivity (2200 W/m·K), and high breakdown field (10 MV/cm), make it an ideal candidate for extreme environment electronics. However, the formation of reliable electrical contacts—both ohmic and Schottky—poses significant challenges due to diamond’s chemical inertness and high electron affinity.

Metal selection for contacts on diamond semiconductors is driven by the need to achieve low contact resistivity, thermal stability, and controlled Schottky barrier heights. Titanium (Ti), gold (Au), and molybdenum (Mo) are among the most studied metals due to their distinct interfacial behaviors with diamond.

Titanium is widely used for ohmic contacts on hydrogen-terminated diamond (H-diamond). When deposited on H-diamond, Ti forms a carbide layer (TiC) at the interface upon annealing, which reduces the Schottky barrier height and facilitates ohmic behavior. The annealing temperature plays a crucial role; temperatures between 400°C and 500°C are typically optimal for TiC formation. Excessive annealing (>600°C) can lead to excessive interfacial reactions, increasing contact resistivity. For oxygen-terminated diamond (O-diamond), Ti forms a higher Schottky barrier (~1.7 eV), making it less favorable for ohmic contacts but suitable for Schottky diodes.

Gold is often employed as a Schottky contact due to its high work function (~5.1 eV), which aligns well with the electron affinity of O-diamond (~1.7 eV), resulting in a Schottky barrier height of ~1.7–1.9 eV. However, Au does not form strong chemical bonds with diamond, leading to poor adhesion and thermal instability at elevated temperatures. To mitigate this, Au is frequently used in bilayer or multilayer structures with adhesion-promoting metals like Ti or Mo.

Molybdenum exhibits intermediate behavior, forming a Schottky barrier height of ~1.3–1.5 eV on O-diamond. Unlike Ti, Mo does not readily form carbides, but it does exhibit good thermal stability up to 600°C, making it suitable for high-temperature applications. Mo contacts maintain stable electrical characteristics even after prolonged annealing, though the absence of carbide formation can result in higher contact resistivity compared to Ti-based contacts.

Annealing effects are pivotal in contact engineering. For Ti contacts, annealing induces TiC formation, which lowers the effective barrier height by creating an intermediate layer with a modified electronic structure. The optimal annealing window for Ti/H-diamond is 400–500°C, where contact resistivities as low as 10^-4 Ω·cm² have been reported. Beyond this range, excessive diffusion of carbon into the metal layer degrades performance. For Au contacts, annealing above 400°C can cause intermixing and increased leakage currents, while Mo contacts show minimal degradation up to 600°C.

Interfacial reactions must be carefully controlled to prevent undesirable phases. In Ti/diamond systems, the formation of a uniform TiC layer is essential for low-resistance ohmic contacts. Non-stoichiometric TiCx (x < 1) can introduce defect states that increase contact resistivity. For Mo/diamond interfaces, the absence of carbide formation means that the contact properties are dominated by metal-induced gap states (MIGS), which can lead to Fermi-level pinning and less tunable Schottky barriers.

Contact resistivity measurements are typically performed using transmission line model (TLM) structures for ohmic contacts and current-voltage (I-V) or capacitance-voltage (C-V) characterization for Schottky contacts. For Ti/H-diamond ohmic contacts, specific contact resistivities in the range of 10^-4 to 10^-5 Ω·cm² are achievable with optimized annealing. Au and Mo Schottky contacts exhibit ideality factors between 1.1 and 1.3, indicating high-quality interfaces with minimal defect-assisted tunneling.

High-temperature stability is a major challenge for diamond contacts. While diamond itself can withstand extreme temperatures (>500°C), most metal contacts degrade due to interdiffusion, oxidation, or delamination. Ti contacts suffer from excessive carbide growth and carbon diffusion at temperatures above 600°C, while Au contacts fail due to agglomeration and poor adhesion. Mo demonstrates better stability, but prolonged exposure to temperatures above 700°C can still lead to increased resistivity. To enhance thermal stability, refractory metals like tungsten (W) or multilayer schemes incorporating diffusion barriers (e.g., TiN) are being investigated.

Another challenge is the control of Schottky barrier heights for device applications. Unlike conventional semiconductors, diamond lacks shallow dopants, making Fermi-level tuning difficult. Surface termination plays a critical role; hydrogen termination induces a surface conduction layer with low sheet resistance (~10 kΩ/sq), while oxygen termination results in a more insulating surface. Metal selection must account for these terminations to achieve the desired contact behavior.

In summary, contact engineering for diamond semiconductors requires careful consideration of metal selection, annealing conditions, and interfacial chemistry. Ti is optimal for ohmic contacts on H-diamond due to carbide formation, while Au and Mo are better suited for Schottky applications. Thermal stability remains a key challenge, necessitating further research into refractory metals and diffusion barriers. Advances in contact engineering will be essential for unlocking the full potential of diamond-based electronics in high-power and high-temperature environments.