AMBIPOLAR DIFFUSION OF ELECTRON-HOLE PAIRS IN BETA-RHOMBOHEDRAL BORON

In beta-rhombohedral boron the electronic transport is essentially determined by deep electron traps of different bonding energies. The ambipolar diffusion of optically excited electron-hole pairs on diffusion paths of up to 5.37 mm length was investigated. The diffusion length at stationary conditions is 21 mm and the mean diffusion velocity is 5.8 x 10(-6) m s-1. From these data a lifetime of 3620 s results. By Fick's second law the diffusion coefficient was determined to increase from about 5 x 10(-3) to 10(1) cm2 s-1 with the diffusion velocity increasing from 10(-5) to 10(-1) m s-1. The Einstein relation yields the corresponding mobilities, which increase accordingly from about O.2 to about 50 cm2 V-1 s-1. These mobilities are mean values for electrons and holes. The relaxation of thc excited state of the sample into the thermal equilibrium is determined by five relaxation times (6.6, 50, 514, 5650, 14700 s), which are immediately related to the thermal excitation of the electrons from the traps with the bond energies 0.1 8, 0.37, 0.58, 0.77, and 0.89 eV.