| Measurements of low frequency (<10 kHz) coplanar conductance fluctuations in n-type doped hydrogenated amorphous silicon (a-Si:H) thin films are reported. 1/f noise from the chromium/a-Si:H contact region is the dominant noise source in a large number of films and is weakly non-Gaussian, non-linear, and has higher noise magnitudes. Contact noise was identified in part using a new measurement technique of conductance fluctuations that differentiates between material bulk fluctuations and contact fluctuations and is a variation of the well-known four-probe technique used to eliminate the influence of the DC contact resistance from the sample resistance measurement. These contact noise properties indicate that a non-uniform a-Si:H/metal interface influences the conductance fluctuations. Noise from the bulk of the a-Si:H films studied here is always Gaussian, linear, stationary, and has noise magnitudes consistent with a Hooge value of 2 × 10-5—2 × 10-4. Under certain etching treatments the noise reflects the kinetics of surface defect states, with a noise power spectrum in the form of a broadened Lorentzian associated with surface damage by CF4 reactive ion etching. The Lorentzian spectral feature is explained by trapping-detrapping from surface states induced by the RIE etch, which cause fluctuations in the depletion width of the space charge region near the film surface, and is a measure of the degree of band bending and Fermi energy at the thin film surface. Surface etches using ion milling or wet chemicals remove the Lorentzian spectral feature and only a 1/f spectral form is observed. The bulk 1/f noise power spectrum exhibits a significant dependence on temperature and light exposure. In all aspects the conductance fluctuations of the a-Si:H films studied here are similar to that of other semiconductor materials and indicate that the noise mechanisms most likely have the usual property of local independent microscopic fluctuators rather than modulating current filaments. |
