Diamond semiconductors have long been recognized for their exceptional properties, including high thermal conductivity, wide bandgap (5.47 eV), and remarkable breakdown voltage. However, the development of n-type diamond has been a significant challenge due to the difficulty in introducing shallow donors. Among the potential dopants, phosphorus and nitrogen have been the most studied, each presenting unique advantages and limitations.

Phosphorus doping in diamond introduces a donor level at approximately 0.6 eV below the conduction band minimum, making it one of the shallowest known n-type dopants in diamond. Nitrogen, by contrast, creates a much deeper donor level at 1.7 eV, severely limiting its effectiveness at room temperature due to low ionization efficiency. The activation energy difference between these two dopants is critical, as it directly impacts carrier concentration and conductivity. Phosphorus-doped diamond exhibits higher electron mobility compared to nitrogen-doped diamond, but both suffer from relatively low values—typically in the range of 100–500 cm²/Vs for P-doped diamond and significantly lower for N-doped diamond. These mobility limitations arise from ionized impurity scattering and defects introduced during growth.

The growth of n-type diamond is primarily achieved using microwave plasma chemical vapor deposition (MPCVD). This technique allows precise control over dopant incorporation by introducing phosphine (PH₃) or nitrogen gas (N₂) into the hydrogen-methane plasma. A critical factor in achieving n-type conductivity is hydrogen termination of the diamond surface. Hydrogen passivates defects and dangling bonds, reducing trap states that could otherwise capture free electrons. However, excessive hydrogen incorporation can lead to defect complexes that degrade electronic properties.

One of the major challenges in n-type diamond doping is achieving high conductivity. Even with phosphorus, which has the shallowest donor level, the ionization ratio remains low at room temperature due to the large activation energy. Post-growth treatments such as annealing can improve dopant activation, but excessive temperatures may cause dopant diffusion or structural degradation. Nitrogen doping is even more problematic—its deep donor level means that less than 1% of nitrogen atoms contribute free electrons at room temperature, resulting in very low conductivity.

The stability of doped layers is another concern. Phosphorus-doped diamond shows better thermal stability than nitrogen-doped diamond, but both can suffer from dopant segregation or compensation by unintentional impurities such as boron. Compensation effects are particularly detrimental in n-type diamond because they reduce the net carrier concentration. Careful optimization of growth conditions, including temperature, pressure, and gas phase composition, is necessary to minimize these effects.

Sulfur has also been explored as an alternative n-type dopant, but its donor level is even deeper than nitrogen, at around 2.0 eV, making it impractical for room-temperature applications. Additionally, sulfur incorporation is difficult to control, often leading to high defect densities. Other potential dopants, such as lithium or oxygen, have been investigated but face similar challenges with deep levels or poor solubility.

The differences between phosphorus and nitrogen doping extend beyond electronic properties. Phosphorus-doped diamond tends to grow with smoother surfaces and lower defect densities compared to nitrogen-doped diamond, which often exhibits higher dislocation densities and non-uniform dopant distribution. This makes phosphorus more suitable for electronic applications despite its higher cost and more complex incorporation process.

In summary, n-type diamond semiconductors remain a challenging but promising area of research. Phosphorus doping offers the best balance between donor depth and material quality, while nitrogen doping is limited by its deep level. Sulfur and other potential dopants are not yet viable alternatives due to their even deeper levels and incorporation difficulties. Advances in MPCVD growth techniques and surface passivation methods will be crucial for improving n-type conductivity and enabling practical applications of diamond-based electronics.