Doping of amorphous silicon by alkali-ion implantations

The implantation of Na, K and Cs ions into films of a-Si prepared by the glow-discharge technique has been investigated. It is found that the range of control of conductivity that can be achieved with these ions in hot implantations is the same as that from substitutional doping by P atoms. The efficiency of Na-, K-and Cs-interstitial doping lies between that of P doping from the gas phase and from implantation. The electrical properties of Na-, K- and Cs-implanted specimens are thermally stable up to about 400°C, whereas those of Li-doped specimens showed rapidly increasing changes even after short annealing at 250°C. Preliminary thermoelectric power measurements confirm that the doped specimens are n-type and also that the changes in conductivity with implantation arise from a shift of the Fermi energy towards ε c rather than from hopping paths associated with introduced defects.