Advances in Phosphorus Doping of Diamond via Pulsed Deposition Techniques

Diamond as a wide bandgap semiconductor can enable devices with superior properties in particular for high power, high frequency and high radiation operations. With high carrier mobilities, a high breakdown voltage and high temperature/thermal conductivity capability diamond electronics would be ideally suited for high demanding applications. While intrinsic and boron-doped (p-type) epitaxial layers can be prepared on a wide range of surface orientations, phosphorus-doped (n-type) diamond is preferentially prepared on the (111) surface with a higher doping efficiency than on the (100) crystallographic plane. Reliable and repeatable doping with phosphorus in this material system presents a significant challenge. This work demonstrates an approach for the controlled doping of diamond by microwave plasma-enhanced chemical vapor deposition (MPCVD), utilizing real-time and in-situ characterization by residual gas analysis (RGA) and contactless temperature measurements by optical pyrometry.
In a novel growth approach, deposition is performed by a pulsed technique, i.e., oscillations are effected through regular interruptions in the methane supply while the supply of hydrogen and the dopant (200 ppm trimethylphosphine in hydrogen) are kept constant. It was observed that cycling of the methane supply resulted in temperature fluctuations at the surface of the diamond substrate. Residual gas analysis during the growth communicated oscillations in the CH and CH₃ concentrations while the concentration of the PH doping indicator was observed to be constant. Secondary ion mass spectroscopy (SIMS) of the corresponding epilayer indicated a phosphorus doping profile with oscillations in the phosphorus concentration that correlated with the CH/CH₃ pulses where consecutive sharp doping gradients (50 nm/decade) peaked at a phosphorus concentration of approximately 2 × 10¹⁸ cm^-3. Optical microscopy of the phosphorus-doped epilayer displayed a smooth surface. With an adjustment in the pulse duration (duty cycle) and the methane flow rate prominent changes in the phosphorus incorporation were effected: a continuous phosphorus doping profile emerged and a significant increase in the phosphorus doping concentration of about 3 x 10¹⁹ cm^-3 was achieved. An investigation of the growth surface by optical microscopy presented a step-bunch-like morphology. The results of the phosphorus doping will be discussed in relation to the pulse-induced variations in the growth rate and growth/doping dynamics.

This research was support by the NSF through grant DMR-2003567 and the U.S. Department of Energy, Office of Science, Basic Energy Sciences through ULTRA, an Energy Frontier Research Center under Award #DE-SC0021230.