Bulk and interface Gap states in a-Si:H: A comparative study of field effect and capacitance measurements on codeposited samples

Abstract

Pseudogap density of states (DOS) estimates from field effect, Schottky diode and M-I-S tunnel diode admittance measurements on several sets of identically prepared, codeposited a-Si:H samples are reported. Additional measurements of composition, optical absorption, electronic transport, photoconductivity and photoluminescence yield consistent parameters from sample to sample. The field effect-derived DOS is shown to depend strongly on the properties of the film/substrate interface: equivalent bulk DOS estimates range from 5 × 10 16 to greater than 10 19 eV −1cm −3 at the Fermi level. Schottky diode admittance data provide DOS values of 1 × 10 16 eV −1 cm −3 for all the samples measured, consistent with the reproducibility of the other bulk parameters. The M-I-S data indicate that the dominant contribution to the M-I-S space-charge layer is due to interfacial, rather than bulk gap states. Because of the rather large and unpredictable density of interface states (10 11 – 10 12 eV −1cm −2, field effect is found unsuitable for characterizing bulk DOS, while Schottky diode admittance experiments give reasonably consistent estimates for bulk DOS at the Fermi level.