The Effects of the Implantation of Oxygen, Nitrogen and Carbon Impurities on the Density of States in the Mobility Gap of Hydrogenated Amorphous Silicon.
Abstract
The effects of impurities on the properties of semiconductors has long been a subject of considerable interest. For amorphous semiconductors, it was generally thought that the effects of impurities would be suppressed by the large density of gap states normally present in these materials, and also by the ability of the amorphous network to adjust to the bonding requirements of the impurity. However, hydrogenated amorphous silicon (a-Si:H) was found to have a low density of gap states, because the hydrogen passivates dangling bonds which would be otherwise present in the material. As a result, a-Si:H has proved to be sensitive to impurities that allow it to be doped n-type or p-type, or to those that adversely affect its electronic properties by the addition of deep states to the gap. The most common of impurities found in a-Si:H are oxygen, nitrogen, and carbon. An investigation of the effects of implanting these impurities on the density of states in the mobility gap has been carried out. The experimental techniques employed included a variety of capacitance techniques such as DLTS (deep level transient spectroscopy) and transient photocapacitance, as well as FTIR (Fourier transform infrared spectroscopy) for evaluating changes in bonding that might have occurred as a result of the ion implantation. After implantation with oxygen we observed a broadening of both the conduction and valence band tails, as well as a shift of the Fermi level towards midgap by roughly 100 meV in two out of the three films studied. We also observed the addition of a distinct defect band approximately 0.7 eV below the conduction band mobility edge in films implanted with doses greater than 10 ^{12}cm^{-2 }. After nitrogen and carbon implantation with doses greater than 10^{13} cm^{-2}, we observed a shift of the Fermi level towards midgap by 50 meV to 300 meV, depending on dose, but did not observe the addition of any distinct defect band to the upper half of the gap.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1988
- Bibcode:
- 1988PhDT.......108M
- Keywords:
-
- Physics: Condensed Matter