Optical properties of Al-doped a-Si:H films

Abstract

The effect of doping on a-Si:H film optical constants was studied. Evaporated films were doped with Al for making p-type material. The optical energy gap E g decreased with increase in doping concentration. The Urbach parameter increased with doping. The refractive index decreased with increasing Al concentration showing a sharp rise in dispersion curve for p + samples. The absorption coefficient increased with doping and the absorption edge shifted to lower energies.

Structure and optical properties of aSiAl and aSiAlHx magnetron sputtered thin films

“…Further increase of the Al concentration resulted in a further reduction of the optical band gap to 0.6 eV, as derived from UV-Vis NIR data. This is in agreement with El-Naggar and Bakry 10 and Talukder et al, 18 who reported a continuous decrease in optical band gap upon doping aSi with Al. They attributed this decrease of the measured optical band gap to an increase in disorder of the material, owing to doping.…”

“…This indicates that our aSi:H x films contain more hydrogen than those in Ref. 10. Doping aSi with Al results in a p-doped material.…”

“…20 7 Ω/sq, which can be explained by the passivation of Al acceptors by hydrogen. Although more detailed and special studies are required to extract exact values of the sheet resistance (or resistivity) for highly resistive films, at this stage, it can be concluded that despite the high Al content, all amorphous hydrogenated Si films have sheet resistance values above 10 7 Ω/sq, which are within the typical range for a-Si:H (10 6 -10 8 Ω/sq), used for solar cells.…”

“…Some research effort has been focused on aSi heavily doped with Al (up to 8 at. %), 9,10 but no previous research has been reported on the formation of aSiAl compositions with increasing amounts of Al. In the present paper, we report on the formation of uniform (aSi 1−x Al x and aSi 1−x Al x :H y ) thin films which exhibit a significant flexibility in the band gap and other optoelectronic properties upon compositional tuning.…”

“…The increased disorder in turn leads to a redistribution of states, allowing thus a greater number of possible band to tail and tail to tail transitions. 10,19 When introducing hydrogen into aSi, the optical band gap increases to 1.7 eV (sample aSi:H x ). This is due to passivation of unsaturated bonds and reduced number of band gap states.…”