ELECTRICAL AND OPTICAL-PROPERTIES OF IMPLANTED AMORPHOUS-SILICON

The amorphous silicon hydrogen alloys (a-Si:H) prepared by plasma-enhanced chemical-vapor deposition are implanted with various impurities, i.e., phosphorus, silicon and boron, followed by 250-degrees-C thermal annealing and hydrogen passivation. A critical phosphorus dosage of 5 X 10(15) cm-2 is found, beyond which the doping effect becomes evident and the electrical properities of the films are comparable to the n + a-Si:H deposited by glow-discharge decomposition of SiH4 and PH3. The silicon and boron implantation have less effect since the dosage is below 10(15) cm-2. It is found that the conductivity of the implanted sample after annealing is determined by three competing mechanisms, i.e., annihilation of the implant-induced defects, defect creation due to hydrogen evolution, and the impurity activation. From the IR spectra of the high dosage phosphorus-implanted samples, a broad peak due to PH(x) radicals located from 2100 to 3500 cm-1 is found. In addition, most of the voids are found to be filled by extra P ions during annealing, so very few hydrogen atoms can be driven into the film after hydrogen passivation. In photoluminescence spectra, the high-energy peak at 1.4 eV drops very quickly and the low-energy shoulder at 1.25 eV becomes evident after implantation. It is found that Brodsky's quantum-well model can be successfully applied to explain the observed results.