DOPING OF AMORPHOUS SEMICONDUCTOR ALLOYS
There has been intense research on amorphous semiconductor materials due to their potential applications in the electronic industry. So far, amorphous silicon has been the most investigated because, when prepared under well controlled deposition conditions, it shows excellent photoelectronic response and very good sensitivity to doping which has not been matched by any other amorphous semiconductor. The use of amorphous silicon in the fabrication of solar cells at low price, prompted enthusiastic efforts on the optimization of the material performance, but soon it was discovered that good quality amorphous silicon had a relatively wide gap which was negatively reflected on the solar cell efficiency. Alloying silicon with elements such as carbon or nitrogen was found to increase the optical gap, whereas alloying with germanium reduced it. The changes of the optical gap seemed to be dependent on the relative concentrations of the atoms present in the alloy. Although changes in the gap were possible, the performance of the new materials still had to be tested. These tests involve the study of the density of states in the band gap which shows its signature in the optical and electrical properties. However the understanding of the material physical properties is primarily based on its structural characterization. Previous studies showed that compensation is fundamental in order to obtain a material with low density of states in the gap. Doping should then be possible. In the present work most of the studies involve a-SiGe alloys which is believed to be a very good candidate for solar cell applications. The effect of hydrogen and oxygen on the structural, electrical and optical properties of a-SiGe is investigated. Changes in the optical gap and conductivity are correlated with the hydrogen and oxygen content. Although some degree of compensation exists with oxygen incorporation, an increase of the density of states is most likely to occur due to the poor photoconductivity response of the material. Doping of a-SiGe:H was achieved and it was confirmed by the conductivity, optical gap and thermoelectric power measurements. The effect of nitrogen on the a-SiGe alloys was also studied. Nitrogen seems to have a stronger influence on the optical gap than in the conductivity. High nitrogen content is seen to give rise to very high resistivity material, although the values lie quite below those reported for silicon. ^
Engineering, Materials Science
JORGE M PEREIRA,
"DOPING OF AMORPHOUS SEMICONDUCTOR ALLOYS"
Dissertations and Master's Theses (Campus Access).