Bombardment Conductivity and Photoconductivity in Rhombic Sulphur

Using pure single crystals of rhombic sulphur, of volume of few mm 3 , conduction counting occurs for α-particle bombardment but only in a region close to the negative electrode, indicating a barrier effect. If the applied voltage is increased there is an increase in the counting rate proportional to V n where n = 0.7, but no increase in the maximum pulse height. Creation of a space charge barrier is essential before counting can begin, after which the counting rate decays with time but the crystal can be reactivated by light or other ionizing radiations. Examination was made of the primary photoconductivity between 4400 Å and 9000 Å, down to 130°K and the photoconductive efficiency was found to be about 1.5 μAW -1 . Comparing these results with von Hippel's interpretation for the alkali halides, a mean value of N = 10 13 cm -3 was obtained for the donor density. Secondary currents were also observed and breakdown pulses were attributed to field emission at the electrodes. The free path w per unit field was estimated as gt-or-equal, slanted 5 × 10 -8 cm, and the lower limit of the mobility μ e of the electrons, which were the charge carriers, as about 10 -1 cm 2 v -1 sec -1 . The energy per ion pair was found experimentally to be about 32 ev which agrees with the value deduced from independent data on the electronic band structure of sulphur. The thickness of the barrier layer was approximately 10 -2 cm with barrier field strength of the order of 10 5 v cm -1 . The conductivity pulse magnitude depends upon the position of the α-particle track inside the barrier and the excitation mechanism for conductivity pulses in a molecular crystal like sulphur differs considerably from that of a covalent crystal like diamond.