Measurement of band tail widths in hydrogenated amorphous silicon

We have measured, separately, the width of the tail states of the conduction band and of the valence band in amorphous silicon, by `photomodulated infrared absorptionʹ. In this technique, we populate the tail states of the amorphous materials by irradiation of visible or near-UV light, and observe the resulting increase of infrared absorption. The method is made very sensitive by using a modulation of the irradiation light and a lock-in detection of the IR absorption. We have developed a model that describes the infrared absorption of the photocarriers. For n-type material, the measured signal corresponds to optical transitions from the populated tail states of the conduction band to higher conduction band states, allowing for a determination of the width of the tail of the conduction band. Similarly p-type material allows for valence-band tail probing. A value of 28 meV for the width of the conduction band tail in a-Si:H (to be compared to the value of 55 meV for the Urbach edge) is deduced from our results.