Electrical Mobility and Carrier Lifetime in Single-Crystal, Isotopically Pure Type IIa Synthetic Diamond

ABSTRACTSingle-crystal type IIa diamonds were synthesized by applying high temperatures (1200°;C) and high pressures (52000 atm) to powdered polycrystalline diamond grown using conventional CVD techniques. These samples were isotopically pure, consisting of approximately 99.93% 12C, compared to roughly 98.96% in natural IIa diamonds. In addition, the dislocation density of the synthetic samples is significantly lower than in natural IIa diamonds, as indicated by birefringence measurements. Electrical properties of these samples were measured using transient photoconductivity, where a 3 ps pulse of ultraviolet light (6.1 eV) was used to excite free electron-hole pairs. Compared to natural IIa diamonds, the lifetime at low fields (200 V/cm) and low excitation densities (1015 cm-3) in the synthetic sample was significantly longer (1 ns in the synthetic sample vs. 200 to 300 ps in natural diamond). At higher fields, a much longer decay component, exceeding 10 ns, was observed in the synthetic sample. Combined electron and hole mobilities were around 2500 cm2/V-s in the synthetic diamond, compared to 3000 to 4000 cm2/V-s in the best natural samples. At a field of 2 kV/cm, the drift distance in the synthetic sample was over 50 μm, considerably longer than that of natural diamond (10 μm). This is due primarily to the much longer carrier lifetimes. The longer lifetimes in the synthetic sample demonstrate that the properties of the best natural diamonds can be exceeded and are encouraging for the development of sensitive diamond radiation detectors. These longer lifetimes are likely due to the higher quality and lower defect density in the synthetic samples, rather than the isotopic purity.