METASTABLE EFFECTS INDUCED BY THERMAL QUENCHING IN UNDOPED AMORPHOUS-SILICON

Thermal equilibrium processes induced by thermal quenching have been studied in undoped r.f. glow-discharge a-Si:H films deposited under various conditions. After thermal quenching, d.c. and a.c. conductivity measurements were made on samples in coplanar and sandwich (n+-i-n+) configurations. During a study of the time dependence of isothermal relaxation at different temperatures T, other experimental techniques were also employed, namely the constant photo-current method, photoconductivity and space-charge-limited current measurements. Fundamental parameters such as sub-bandgap absorption coefficient alpha(hv,T,t), defect density N(s)(T,t) and exponent gamma(T,t) in sigma-ph proportional G-gamma(T,t) have been deduced. The equilibrium temperature of the films is found to be 180-200-degrees-C. The inverse equilibration time is thermally activated with an energy of 1.4-1.8 eV. The defect density increases with temperature with an activation energy of about 0.12 eV. Experimental results are analysed using a distribution of dangling-bond levels D0 centered at E(D) = 0.9 to 1 eV below the conduction band edge. After quenching it is shown that, compared to a slow cool: (a) the D0 distribution is broadened, (b) the peak of the distribution is shifted towards the valence band edge. We observe a good correlation between the isothermal relaxations of the dark conductivity, the photoconductivity and the defect density. Finally the results are discussed in terms of dispersive diffusion of hydrogen.