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The Role of Lattice Relaxation in the Competition Between Optical and Thermal Transitions from Gap States in Hydroacenated Amorphous Silicon

  • A. V. Gelatos (a1), J. D. Cohen (a1) and J. P. Harbison (a2)

Abstract

We have carried out a detailed study of the competition between optical and thermal transitions from mobility gap states in hydrogenated amorphous silicon films. By employing transient photocapacitance and capacitance DLTS techniques on n-type doped films, we have observed the dangling bond optical transitions:D → D0 + e and D0 → D+ + e, and the thermal transitions: D → D0 + e and D0 → D0 + h. From the transition energies we have determined that the degree of lattice relaxation for dangling bonds does not exceed 0.1 eV and that the value Ueff of the D/D0 splitting is 0.3 ± 0.1 eV. Voltage pulse photocapacitance studies on undoped films revealed the existence of the optical transition D/D0 + e. In addition, in all films we have observed a photocapacitance signal due to optical transitions from the valence band tail states (over the energy range 1.4 to 1.9 eV) at temperatures as high as 440 K, in spite of the expectation that such shallow photoexcited holes would immediately be thermally emitted yielding no photocapacitance signal. We discuss possible explanations for this result by considering two different trapping models.

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