Hydrogenated amorphous silicon characterization from steady state photoconductive measurements

Abstract

We propose to use steady state measurements and a teaching-learning-based optimization (TLBO) algorithm to get the complete set of material transport parameters of disordered semiconductors, taking undoped hydrogenated amorphous silicon as an example. First, the steady-state conductivity under illumination and the ambipolar diffusion length (L amb) are measured for several temperatures and generation rates. The steady-state photocarrier grating (SSPG) technique is used for the evaluation of L amb. Then, the TLBO algorithm is used for the obtainment of the material parameters that best satisfy the charge neutrality and the continuity equations. The use of this algorithm allowed us to get an excellent estimation of the valence band tail slope, as compared to the one obtained from measurements of the absorption coefficient by Fourier transform photocurrent spectroscopy and transmittance/reflectance. The dangling bonds and the conduction band tail parameters were also found to be in very good agreement with those measured from high frequency modulated photocurrent experiments (MPC). Numerical simulations show that the capture coefficients of the band tails and defects states can also be estimated, although with less precision than the DOS parameters.

Increasing the scope and precision of the steady-state photocarrier grating technique by measuring the photocurrents at several voltages

“…To deduce equation ( 52), we used equation (49), which was obtained assuming that the electron capture coefficient is independent of the localized state's energy. This condition is not verified in some a-Si:H-like materials [7,27,29], such as in the one we are simulating, where the electron capture coefficients of the deep states are higher than those from the band tail states (see table 1). Consequently, we observe an increase of ε c r n / (q µ n ) with temperature.…”

“…Replacing equation (29) in equation ( 25) or (26), and using equation ( 28) we obtain the following quadratic equation for , ,…”

“…(41) Another much simpler equation for L 2 d is obtained by combining equations ( 26), ( 28) and (29), and making the approximations τ p /τ r p ∼ 1 and…”

“…Above room temperature, the overestimation does not reach 41%, but at the lowest temperature, it exceeds 200%. Some papers [6,16,29,30] make the approximation γ n + γ p ∼ 2 in order to determine the minority carrier diffusion length from the ambipolar diffusion length, L 2 p = L 2 amb / (γ n + γ p ) ≃ L 2 amb /2, which is an extra but generally a lower source of errors [11].…”

“…This one was the procedure used for extracting the points presented in figure 14. A simpler and more popular version of this approach is obtained by approximating γ n + γ p by 2 in the previous formula to obtain L 2 p directly from L 2 D [6,11,16,29,30]. With this extra approximation, the L 2 p values are sometimes higher than those obtained from the multi-voltage approach.…”