Modulated photocurrent measurements on pure and V-doped beta-rhombohedral boron

The modulated photocurrent method has been applied to pure and vanadium (V) -doped beta -rhombohedral boron (beta -B) with the goal of investigating th e difference in the distribution of electronic states in the band gap betwe en them, Excitation light intensity dependence of the amplitude and phase s hift of photocurrent shows that V-doped beta -B has a much larger trapping states density for photoexcited carriers than pure beta -B, With increasing temperature, the amplitude increases and decreases for pure and V-doped be ta -B, respectively, indicating that the conduction mechanism for photoexci ted carrier is completely different between the two samples. The unusual ne gative temperature dependence for V-doped beta -B is similar to that for Al -Pd-Re quasicrystal and the change of dependence from positive to negative is consistent with the approach to aluminum-based icosahedral quasicrystals in atomic structure and in transport properties by V-doping to beta -B, Th e modulated frequency dependence of the amplitude and phase shift cannot be explained by the usual photoconduction processes, which are indicating tha t the gap states distribution and photoconduction processes in these materi als are complicated. (C) 2000 Academic Press.