Boron-doped diamond has emerged as a significant p-type semiconductor due to its exceptional properties, including high thermal conductivity, chemical stability, and wide bandgap. The incorporation of boron into diamond introduces acceptor levels, enabling hole conduction and making it suitable for high-power, high-temperature, and harsh-environment applications. This article explores the doping process, electronic properties, synthesis methods, and challenges associated with boron-doped diamond, while comparing it with other p-type wide-bandgap semiconductors.

The doping of diamond with boron introduces acceptor states within the bandgap, located approximately 0.37 eV above the valence band. This energy level is shallow enough to allow thermal excitation of holes into the valence band at room temperature, resulting in p-type conductivity. The hole mobility in high-quality boron-doped diamond can exceed 2000 cm²/V·s, a value significantly higher than many other wide-bandgap semiconductors. The high mobility is attributed to diamond's strong covalent bonding and low phonon scattering rates, which facilitate efficient charge transport.

Two primary methods are employed for synthesizing boron-doped diamond: chemical vapor deposition (CVD) and high-pressure high-temperature (HPHT) growth. In CVD, a mixture of hydrocarbon gas, hydrogen, and a boron-containing precursor, such as diborane or trimethylboron, is introduced into a reaction chamber. The gas phase is activated using plasma or hot filament techniques, leading to the deposition of boron-doped diamond films on substrates. The boron concentration can be precisely controlled by adjusting the gas-phase ratio of the boron precursor to the carbon source. CVD allows for the growth of high-purity, single-crystal diamond with tailored boron doping levels.

HPHT synthesis involves dissolving carbon and boron in a metal catalyst, typically iron, nickel, or cobalt, under extreme pressures (5-6 GPa) and temperatures (1300-1600°C). The dissolved carbon precipitates as diamond, with boron atoms incorporated into the lattice. HPHT-grown boron-doped diamond often exhibits higher crystalline quality but may contain metallic impurities from the catalyst. Both methods enable the production of boron-doped diamond with doping concentrations ranging from 10¹⁶ to 10²¹ cm⁻³.

The electrical conductivity of boron-doped diamond is highly dependent on the boron concentration. At low doping levels (below 10¹⁸ cm⁻³), the material behaves as a semiconductor with activated hole conduction. As the boron concentration increases, the acceptor states form an impurity band, leading to degenerate conduction and metallic behavior at concentrations above 10²⁰ cm⁻³. The relationship between conductivity and boron concentration follows a non-linear trend due to compensation effects and the formation of boron-boron pairs or clusters at high doping levels.

Optical properties of boron-doped diamond are also influenced by boron incorporation. At low concentrations, the material remains transparent in the visible spectrum, with absorption occurring primarily in the infrared due to electronic transitions from the valence band to the acceptor level. Heavy boron doping introduces additional absorption features, including a broad band centered around 2800 cm⁻¹, attributed to electronic transitions within the impurity band. The optical bandgap of diamond remains largely unaffected by boron doping, preserving its wide-bandgap character.

Challenges in boron-doped diamond include compensation effects and defect formation. Compensation arises when unintentional impurities, such as nitrogen or vacancies, neutralize the acceptor states, reducing the effective hole concentration. Nitrogen, a common impurity in diamond, acts as a deep donor and can compensate boron acceptors, particularly in HPHT-grown material. Defects such as dislocations and grain boundaries in polycrystalline CVD diamond can also trap holes, degrading mobility and conductivity. Optimizing growth conditions and post-growth treatments, such as annealing, can mitigate these issues.

Boron-doped diamond exhibits several advantages over other p-type wide-bandgap semiconductors. Compared to gallium nitride (GaN) and silicon carbide (SiC), diamond has a higher breakdown field and thermal conductivity, making it ideal for high-power devices. The acceptor level of boron in diamond (0.37 eV) is shallower than that of magnesium in GaN (0.16 eV above the valence band) or aluminum in SiC (0.23 eV), resulting in higher hole concentrations at room temperature. Additionally, diamond's chemical inertness and radiation hardness surpass those of other wide-bandgap materials, enabling operation in extreme environments.

Despite its advantages, boron-doped diamond faces limitations, including the difficulty of achieving uniform doping over large areas and the lack of a compatible n-type dopant with similar performance. The high cost of high-quality diamond substrates further restricts widespread adoption. However, ongoing research into growth techniques and defect engineering continues to improve the material's performance and feasibility for practical applications.

In summary, boron-doped diamond stands out as a high-performance p-type semiconductor with unique properties tailored for demanding applications. Its synthesis via CVD and HPHT methods allows precise control over doping levels, while its superior electronic and thermal characteristics make it a promising candidate for next-generation semiconductor technologies. Addressing challenges such as compensation and defect formation will be crucial for unlocking its full potential in the field of wide-bandgap semiconductors.