Role of H in hot-wire deposited a-Si:H films revisited: optical characterization and modeling

Device quality hydrogenated amorphous silicon (a-Si:H) thin films were grown by hot wire chemical vapor deposition (HWCVD) on glass (Corning 7059) using silane under high hydrogen dilution as a function of substrate temperature (TS) ranging 50–515 °C. As a consequence of variation of TS, the hydrogen concentration [CH] varied from 20.0% to 0.2%. They are optically examined ex situ using Raman spectroscopy (RS) and spectroscopic phase modulated ellipsometry (SPME) from near IR to near UV (1.5–5.0 eV) obtaining their vibration frequencies and pseudo-dielectric function, respectively for analyzing network disorder. The ellipsometry raw data (〈εr(E)〉, 〈εi(E)〉) were modeled using Bruggeman effective medium theory (BEMT) and the dispersion relations for the amorphous semiconductors comprising a two-layer model consisting of a top surface roughness layer (dS) containing an effective medium mix of 50% a-Si:H and 50% voids and a single “bulk” layer (dB) of a-Si:H to simulate the data reasonably well. We performed these simulations by non-linear least-square regression analysis based on Marquardt–Levenberg algorithm and it was possible to estimate the true dielectric function of a-Si:H thin films and the energy band gap (Eg), besides film thickness (dSE), bulk void fraction, surface roughness layer (dS), and the confidence limits (χ2). Moreover, it is shown that the Tauc–Lorentz (TL) model fits the ellipsometry data much better than Forouhi and Bloomer (FB) and help elucidate the layered structure of a-Si:H thin films. We also compared the optical band gap (Eg) determined using ellipsometry modeling and the Tauc gap (ET) using conventional approach. We discuss the possible physical meaning and the variation of the deduced parameters in terms of role of TS (T-role) or of hydrogen (H-role) in the dispersion model abovementioned. The bandgap was found to decrease systematically with increasing TS, reconfirming the role of hydrogen as alloy in conjunction with network relaxation, which is in agreement with excitation dependent Raman spectroscopy (i.e. strongly versus weakly absorbing) results interpreted in terms of the variation of three “order parameters” relating the short- and medium-range order with respect to substrate temperature. This is since, previous Raman scattering studies in various hydrogenated amorphous silicon (a-Si:H) materials resulted in contradicting conclusions as to the role of hydrogen on the atomic-network order. Hereby, we elucidated these contradictions using the abovementioned optical probes by establishing that the surface and the bulk of HW a-Si:H films behave differently due to their different hydrogen concentrations. Micro-Raman spectroscopy (RS) and atomic force microscopy (AFM) were used to validate the simulations also. Further consequences of these findings are also discussed in terms of other optical and structural properties. These analyses led to a correlation between the films’ microstructure (or network disorder) and their electronic properties for several technological applications, in general, and for solar cells applications, in particular.