The n-type doping of diamond: Present status and pending questions

Diamond rises much research interest because of its high potential for high-temperature, high-power and high-frequency electronic applications. The microwave plasma-assisted chemical vapor deposition (MPCVD) is the most used technique to grow high-purity diamond. In order to use diamond in electronic devices, control of its n-type doping is necessary. The range of impurities that should fit into the diamond lattice is limited and the equilibrium solubility of dopants in bulk diamond is often low. With a covalent radius ∼1.4 times larger than carbon, phosphorus is currently the shallower substitutional n-type dopant in diamond. Nevertheless, it has a donor level relatively deep (EC−0.6 eV) inducing a very low conductivity at 300 K. The substitutional dopants may not be the only solution for the n-type doping of diamond. Recently, high n-type conductivity has been obtained by deuteration of boron-doped monocrystalline diamond epilayers. In this review, we present the state of the art of growing monocrystalline phosphorus-doped diamond epilayers and their electronic properties. Comparison will be made with the n-type doping obtained by the conversion process. Then, we address several pending questions concerning the n-type doping of diamond.