Electrical Conductivity and Structural Order of p-Type Amorphous Silicon Thin Films
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Electrical Conductivity and Structural Order of p-Type Amorphous Silicon Thin Films K. Shrestha, D. Whitfield, V. C. Lopes, A. J. Syllaios, and C.L. Littler Department of Physics, University of North Texas, Denton, TX 75203, USA ABSTRACT The dependence of dark conductivity and room temperature Raman spectra on boron and hydrogen incorporation in thin films of hydrogenated amorphous silicon (a-Si:H) prepared by plasma enhanced chemical vapor deposition was investigated. It was found that the dominant conductivity is Mott variable range hopping conduction. However, at lower temperatures, EfrosShklosvkii hopping conduction is observed and contributes to the total conductivity. For structural characterization, transverse optical (TO) and transverse acoustic (TA) modes of the Raman spectra were studied to relate changes in short- and mid-range order to the effects of boron and hydrogen incorporation. With an increase of hydrogen incorporation and/or substrate temperature, both short and mid-range order improve, whereas the addition of boron results in the degradation of the short range order. The line width and frequency of the Raman TO Raman peak correlate with electrical measurements and suggest that this technique can be used for nondestructive characterization of a-Si:H. INTRODUCTION In disordered materials such as amorphous silicon, electrical transport is via localized states near the band edges. The temperature dependent conductivity, σ(T), generally follows the hopping conduction model, σ(Τ) = σo exp[-(To/T)p] Eq. (1) where T0 is the characteristic temperature, T is the material temperature, and σo is the conductivity prefactor. The value of exponent, p, depends on the details of the density of states (DOS), as affected by long range Coulomb interaction among the charge carriers. A strong Coulomb interaction results in a gap in the density of states, generally referred to as a ‘Coulomb Gap’. In such a case, p = ½ and the conductivity is described by the Efros-Shklovskii Variable Range Hopping (ES-VRH) model [1]. If the Coulomb interaction is negligible, the conductivity follows the Mott VRH model (M-VRH) [2] and p = ¼. In most materials the Coulomb gap is small, and Efros-Shklovskii type conductivity is observed only at low temperature. For intermediate temperatures, there is a crossover from Efros-Shklovskii to Mott conduction [3], and at high temperatures, both the Mott and Efros-Shklovskii transition to Nearest Neighbor Hopping (NNH) conduction [1,5] where p =1. The importance of understanding such conduction is evident since a variety of materials exhibit such conduction, including disordered semiconductors such as a-Si:H, VOx, TiO2, InxOy, amorphous HgCdTe, organic semiconductors, nanomaterials such as PbSe nanocrystals, and CdSe quantum dots. In addition, hopping conduction is seen in non-traditional conductors, such as hydrogenated graphene, carbon nanotubes, and even biomaterials such as DNA. The electrical conduction mechanisms and optical properties of boron doped hydrogenated amorphous silicon thin fil
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