Influence of disorder on electrical transport properties in hydrogenated amorphous silicon

F. VENTOSINOS; N. BUDINI; J. A. SCHMIDT

La Falda, Cordoba

Workshop; Mini School on Disordered Systems 2008 and 6th International Workshop on Disordered Systems; 2008

Fa.M.A.F.

The three main characteristics of amorphous semiconductors are the short range order, the long range disorder and the correlation defects. The short range order results in the preservation of the general electronic properties of amorphous silicon (a-Si) compared to crystalline silicon. However, the abrupt band edges of the crystal are replaced by a broadened tail of states extending into the forbidden gap, which originate from the deviations of the bond length and bond angle arising from the long range structural disorder. The width of the tails depends on the degree of disorder and on the bonding character of the states. Disorder also reduces the carrier mobility because of frequent scattering, and leads to the localization of the wavefunction in states that lie at the band edges. Multiple trapping and release events between the localized states at the band tails and the extended states at the bands characterize the electronic transport in a-Si. When the semiconductor is illuminated, after a brief transient a steady-state non-equilibrium situation is attained. The ambipolar diffusion length defines the distance travelled by the carriers between generation and recombination, and is an important parameter that defines the potential applications of a-Si in devices. In this work we explore through numerical simulations the influence of disorder on the ambipolar diffusion length. We present results that evaluate the dependence of the transport properties on the width of the conduction and valence band tails. We also report measurements of the ambipolar diffusion length on samples with different degrees of disorder.