Electronic properties of doped glow-discharge amorphous germanium

For glow-discharge a-Ge:H doped with phosphorus and boron the temperature dependence of conductivity and thermoelectric power has been investigated. For both dopants a maximum shift of the Fermi energy E_F to the mobility edges of about 0.3 eV is obtained. The difference of the conductivity and thermoelectric power activation energies E_Q = E_δ-E_s lies around 0.16 eV and varies only little with doping; it rises somewhat by hydrogen effusion at temperatures above 300°C. The spin density of undoped a-Ge:H is approximately two orders of magnitude larger than for a-Si:H;it increases by irradiation with 3 MeV electrons and by hydrogen effusion by a factor of 4-5. Annealing at temperatures above 300°C leads to a positive sign of the thermoelectric power indicating that defect states created by hydrogen effusion pull the Fermi level below midgap.