Photovoltaics

Investigation of defect formation and electronic transport in microcrystalline silicon deposited by hot wire CVD

M. Stöger^a, A. Breymesser^b, V. Schlosser^b, M. Ramadori^b, V. Plunger^b, D. Peiro^c, C. Voz^c, J. Bertomeu^c, M. Nelhiebel^a, P. Schattschneider^a, J. Andreu^c

  ^aInstitut für Angewandte und Technische Physik, Technical University Wien, A-1040 Wien, Austria
  ^bInstitut für Materialphysik, Universität Wien, A-1090 Wien, Austria
  ^cDepartament de Física Aplicada i Electronica, Universitat de Barcelona, E-08028 Barcelona, Spain

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ABSTRACT:

We have investigated doped and undoped layers of microcrystalline silicon prepared by hot wire chemical vapour deposition optically, electrically and by means of transmission electron microscopy. Beside needlelike crystals grown perpendicular to the substrate's surface all of the layers contained a non-crystalline phase with a volume fraction between 4% and 25%. A high oxygen content of several per cent in the porous phase was detected by electron energy loss spectrometry. Deep level transient spec-troscopy of the crystals suggests that the concentration of electrically active defects is less than 1% of the undoped background concentration of typically 10¹⁷ cm^-3. Frequency dependent measurements of the conductance and capacitance perpendicular to the substrate surface showed that a hopping process takes place within the noncrystalline phase parallel to the conduction in the crystals. The parasitic contribution to the electrical circuit arising from the porous phase is believed to be an important loss mechanism in the output of a pin-structured photovoltaic solar cell deposited by hot wire CVD.

Keywords: microcrystalline silicon, transmission electron microscopy, deep level transient spectroscopy, nanopotentiometry.