Diamond surfaces exhibit unique electronic properties that are highly dependent on their termination. The most common terminations include hydrogen, oxygen, and fluorine, each imparting distinct characteristics to the surface. These terminations influence charge transfer, surface conductivity, and device performance, making them critical for applications such as field-effect transistors (FETs) and sensors. Understanding the mechanisms behind these effects is essential for optimizing diamond-based electronic devices.

Hydrogen termination of diamond surfaces is particularly significant due to its ability to induce p-type surface conductivity without intentional doping. When a diamond surface is hydrogen-terminated, the C-H dipoles create a negative electron affinity, promoting electron transfer from the diamond valence band to adsorbates such as atmospheric water molecules. This transfer leaves behind holes near the surface, forming a conductive layer. The conductivity of hydrogen-terminated diamond surfaces typically ranges between 10^-5 and 10^-3 S per square, depending on environmental conditions and surface preparation. The hole mobility in this layer can exceed 100 cm²/Vs, making it suitable for high-performance electronic devices. However, this conductivity is highly sensitive to environmental exposure, as adsorbates can be displaced or altered over time, leading to instability.

Oxygen termination, in contrast, results in a positive electron affinity and suppresses surface conductivity. Oxygen-terminated diamond surfaces are insulating due to the absence of charge transfer mechanisms that generate mobile carriers. The C-O bonds create a high work function, making oxygen termination useful for applications requiring insulating barriers or surface passivation. Oxygen termination is often achieved through plasma treatment or wet chemical oxidation, and it exhibits greater stability under ambient conditions compared to hydrogen termination. However, oxygen-terminated surfaces may still undergo gradual changes due to interactions with atmospheric species.

Fluorine termination offers an intermediate case, where the strong electronegativity of fluorine modifies the surface electronic structure without inducing significant conductivity. Fluorine-terminated diamond surfaces exhibit negative electron affinity similar to hydrogen termination but do not generate a conductive hole layer. Instead, the C-F bonds create a chemically inert and stable surface, resistant to oxidation and environmental degradation. This makes fluorine termination advantageous for applications requiring long-term stability in harsh environments. The lack of surface conductivity, however, limits its use in active electronic devices unless combined with other functionalization strategies.

Passivation techniques are critical for maintaining the desired surface properties of diamond. Hydrogen termination, while useful for conductivity, is prone to degradation when exposed to air, high temperatures, or UV radiation. To enhance stability, researchers have explored methods such as encapsulation with dielectric layers or controlled atmospheric storage. For instance, alumina or hafnia overlayers can protect hydrogen-terminated surfaces while preserving their conductive properties. Oxygen and fluorine terminations inherently offer better stability but may require periodic re-treatment if exposed to reactive environments.

The impact of surface termination extends directly to device performance, particularly in FETs and sensors. Hydrogen-terminated diamond FETs leverage the inherent p-type surface conductivity to achieve high hole mobility and low noise, making them suitable for high-frequency and high-power applications. The threshold voltage and on/off ratio of these devices are heavily influenced by the quality and stability of the hydrogen termination. Environmental exposure can lead to threshold voltage shifts and reduced carrier density, necessitating careful passivation strategies.

In sensor applications, surface termination dictates sensitivity and selectivity. Hydrogen-terminated diamond surfaces are effective for detecting reducing gases due to their hole accumulation layer, which responds to electron-donating adsorbates. Oxygen-terminated surfaces, on the other hand, are more suited for detecting oxidizing gases, as their high work function facilitates interactions with electron-accepting species. Fluorine termination, while less common in sensors, provides a chemically inert platform that can be functionalized with specific receptors for targeted sensing.

The stability of these terminations under operational conditions is a key consideration. Hydrogen-terminated surfaces may degrade over time in ambient air, leading to increased sheet resistance and reduced device performance. Encapsulation or controlled environments can mitigate this, but long-term stability remains a challenge. Oxygen-terminated surfaces are more robust but may still undergo slow changes due to contamination or surface reactions. Fluorine termination offers the highest stability but requires specialized fabrication techniques.

In summary, diamond surface termination plays a pivotal role in determining electronic properties and device functionality. Hydrogen termination enables high surface conductivity but demands careful passivation to maintain stability. Oxygen termination provides insulating properties with good environmental resistance, while fluorine termination offers exceptional inertness at the cost of conductivity. Each termination has distinct advantages and trade-offs, influencing their suitability for FETs, sensors, and other electronic applications. Future advancements in surface engineering and passivation will further enhance the reliability and performance of diamond-based devices.