The sub‐band‐gap optical absorption spectra of high‐quality hydrogenated amorphous silicon (a‐Si:H) films are shown to be dominated by surface and interface state absorption when measured by photothermal deflection spectroscopy (PDS), while spectra determined using the constant photocurrent method (CPM) are not. For bulk defect states (both as‐deposited and light‐induced), the integrated subgap absorption is approximately twice as large for PDS as for CPM. Similarly, the conversion factor relating integrated subgap absorption with neutral dangling bond density is twice as large for CPM as PDS. This factor of 2 results from CPM seeing only transitions from below midgap into the conduction band while PDS sees transitions from the valence band into states above midgap as well.

The first experimental evidence for a thermal-equilibrium defect density in undoped hydrogenated amorphous silicon, a metastable material, is presented. The defect density is in thermal equilibrium at temperatures above 200°C; the defects measured at room temperature are largely those frozen in during cooling from the equilibrium regime. Rapid quenching reversibly increases defect density. The defect density can be reversibly decreased by shunting of the defect-generation process during cooling. This latter technique has important implications for $a$-Si:H-based solar cells.

The effect of hydrogen dilution on the optical, transport, and structural properties of amorphous and microcrystalline silicon thin films deposited by hot-wire (HW) chemical vapor deposition and radio-frequency (rf) plasma-enhanced chemical vapor deposition using substrate temperatures ${\mathrm{(T}}_{\mathrm{sub}})$ of 100 and 25 °C is reported. Microcrystalline silicon $\mathrm{(\mu c-}\mathrm{Si:H})$ is obtained using HW with a large crystalline fraction and a crystallite size of ∼30 nm for hydrogen dilutions above 85% independently of $T_{\mathrm{sub}}.$ The deposition of $\mathrm{\mu c-}\mathrm{Si:H}$ by rf, with a crystallite size of ∼8 nm, requires increasing the hydrogen dilution and shows decreasing crystalline fraction as $T_{\mathrm{sub}}$ is decreased. The photoconductivity, defect density, and structure factor of the amorphous silicon films $\mathrm{(a-}\mathrm{Si:H})$ are strongly improved by the use of hydrogen dilution in the $T_{\mathrm{sub}}$ range studied. $\mathrm{a-}\mathrm{Si:H}$ films with a photoconductivity-to-dark conductivity ratio above ${10}^5,$ a deep defect density below ${10}^{17} {\mathrm{cm}}^{\text{−3}},$ an Urbach energy below 60 meV and a structure factor below 0.1 were obtained for rf films down to 25 °C (at growth rates ∼0.1–0.4 Å/s) and for HW films down to 100 °C (at growth rates ∼10 Å/s), using the appropriate hydrogen dilution. In the low $T_{\mathrm{sub}}$ range studied, the growth mechanism, film properties, and the amorphous to microcrystalline silicon transition depend on the flux of atomic hydrogen available. The properties of the films are compared to those of samples produced at 175 and 250 °C in the same reactors.

The optoelectronic and structural properties of hydrogenated amorphous silicon-carbon alloys (a-SiC:H) are studied over the entire compositional range of carbon content. The films are prepared using low-power electron-cyclotron resonance (ECR) plasma-enhanced chemical vapor deposition. The carbon content was varied by using different methane (or ethylene-)-to-silane gas phase ratios and by introducing the methane (or ethylene) either remotely into the plasma stream or directly through the ECR source, together with the excitation gas (hydrogen). Regardless of the deposition conditions and source gases used, the optical, structural and transport properties of the a-SiC:H alloys followed simple universal dependencies related to changes in the density of states associated with their structural disorder. The deep defect density from photothermal deflection spectroscopy, the ESR spin density, the steady state and the transient photoluminescence, the dark and photoconductivity, the temperature of the hydrogen evolution peaks and the bonding from infrared spectroscopy are correlated to the Urbach tail energy, the B factor of the Tauc plot and $E_{04}$ (defined as the energy at which the absorption coefficient is equal to ${10}^4$ cm⁻¹). Silicon-rich and carbon-rich regions with very different properties, corresponding approximately to carbon fractions below and above 0.5, respectively, can be distinguished. The properties of the ECR a-SiC:H alloys are compared with those of alloys deposited by rf glow discharge.