Effect of grain boundaries on carrier lifetime in chemical-vapor-depositeddiamond film

Using the reflective measurements, the effect of grain boundaries on the li fetime of photogenerated carriers was investigated for chemical-vapor-depos ited (CVD) diamond films. To investigate the effect of grain boundaries on the carrier dynamics, photons having energy lower than the band gap energy of a single crystal of diamond were used for pumping. For a 4.8 eV photon, the measured lifetime was several tens of picoseconds, and that was consist ent with photoconductive current measurements. However, a dramatic decrease of the carrier lifetime was observed in the case of 3.2 eV irradiation. Th e variation of the lifetime inside the single grain was measured by a micro scopic pump-probe method. The carrier lifetime near the grain boundary decr eased from 5 to 8 ps at the center to 0.35-0.5 ps. This decreased lifetime and the carrier generation efficiency with lower energy photon had a negati ve correlation. To explain this mechanism, we considered the decrease in li fetime to be related to the density of the imperfection or mid-band gap sta tes inside the single-CVD-polycrystalline grain. (C) 2000 American Institut e of Physics. [S0003-6951(00)04036-5].