Defect Equilibration and Intrinsic Stress in Undoped Hydrogenated Amorphous Silicon

Abstract

Relaxation data for the thermal equilibrium defect densities in undoped a-Si:H are obtained by time-of-flight (TOF) measurement in the temperature range of 160° to 250° C. The internal stress in the material is also measured. The mobility-lifetime product of electrons (µτ) increases from 0.50×10-7 to its equilibrium value of 2.24×10-7 cm2/V during the 160° C annealing. The equilibrium value of µτ is equivalent to the spin density (N s) of 1.12×1015 cm-3. The N s curves have a minimun value just before their equilibrium. The time dependence of the N s relaxation follows a two-term stretched exponential form which corresponds to two metastable states, and each relaxation time is activated with activation energies of 1.10 to 1.20 eV. The thermal equilibrium N s increases with temperature with an activation energy of 0.20 to 0.30 eV. The data for the second annealing at 160° C after the first long annealings at 200° and 250° C also follows the two-term stretched exponential form derived from the first annealing data. The result suggests the presence of a multivalley energy configuration diagram at metastable states. The drift mobility of electrons (µ) increases slightly compared with the µτ changes, and no stress change is observed during the various annealing steps. It is concluded that the structural change is much smaller than the change in metastable-state densities during annealing in the temperature range of 160° to 250° C.