Collective phonon interaction and annealing of metastable defects in amorphous semiconductors

Abstract

In hydrogenated amorphous semiconductors, metastable defects (MSDs) are induced by recombination of non-equilibrium carriers. The isothermal annealing of MSD follows a stretched-exponential time dependence with a dispersion parameter β proportional to the temperature: β=T/T*. In this study, we observe that the experimental T* values are proportional to the lattice optical phonon frequency for a wide collection of hydrogenated amorphous alloys, including undoped a-Ge : H, a-Si : H, a-SiC_0.13 : H and a-SiN_1.6 : H and n-doped a-Si : H. This result indicates an intrinsic origin for the metastability. We propose a new interpretation of the available data based on a multiphonon-assisted decay of the metastable defects. A large barrier energy E _act requires a collective interaction of a large number n=E_act/hω_o of localized optical phonons. The number ω(n) of accessible vibrational configurations for a set of n phonons within a finite medium-range interaction volume, is defined as the density of n-phonon collective excitation states; it is an exponential function of E_act/hω_o . This thermodynamic model predicts the observed Meyer-Neldel behaviour for the characteristic decay time of the MSD. In an alternative kinetic phonon-assisted hopping model, the electron-lattice interaction energy can be deduced from isothermal decay kinetic data. Combining both models, we find that the estimated localization length of optical phonons (L _op=0.9 + 0.2 nm) is slightly higher for a-Si : H than for the alloys or a-Ge : H, which may indicate some influence of the rigidity of the lattice on the optical phonon localization.