Control of impurity incorporation into CVD diamond synthesized with 𝑡𝑒𝑟𝑡-butylphosphine for quantum applications

Nitrogen-vacancy (NV) centers in phosphorus-doped diamond have potential applications in quantum technologies because their electron spin has the longest spin coherence time among those in solid systems at room temperature (RT) and because their negative charge state is more stable than that in undoped diamond. However, the conventional phosphorus source, phosphine, is toxic and explosive; a safer precursor is therefore preferred. We consequently attempted to synthesize phosphorus-doped diamonds using 𝑡𝑒𝑟𝑡-butylphosphine (TBP), which has lower toxicity and explosivity than phosphine. However, controlling phosphorus incorporation during doping with TBP is difficult because it condenses easily and remains in the synthesis reactor due to its low boiling point. Here, we report controlling the incorporation of impurities by optimizing the synthesis conditions, such as the synthesis pressure. Comparing diamonds synthesized at 15 and 50 kPa reveals that the phosphorus concentration was reduced by 93% and the nitrogen concentration by 72% in the diamond synthesized at 50 kPa. For diamonds prepared under optimized conditions in our synthesis reactor, a long spin coherence time (𝘛₂ = 2.23 ms) for NV centers with almost perfect selective alignment of the NV axes was achieved. The 𝘛₂ was comparable to the longest reported length for NV centers in a sample synthesized with phosphine, indicating that TBP is a promising phosphorus source for obtaining NV centers with excellent properties. We also achieved high electron mobility [580 cm²/(V s) at RT] in diamonds synthesized with TBP; this value is comparable to that for diamonds synthesized with phosphine.