Annealing Of Metastable Defects Research Articles

Mechanism of Two-Level Hopping Current Fluctuations in Mesoscopic a-Si Based Structures

Two-level fluctuations (random telegraph noise) of transverse hopping current have been investigated in mesoscopic nanostructures fabricated on a submicrometer area of a thin amorphous silicon layer. An electric field and light-induced change of the system lifetimes in the high and low conductive states is observed. The results are explained by formation and annealing of metastable defects in the atomic two-level hydrogen-related systems.

Effect of illumination time on the annealing of optically created metastable defects in p-type a-Si:H

The effect of illumination time on the relaxation of the concentration of optically created metastable defects in films of boron-doped a-Si:H in the temperature range 360–400 K is discussed. The concentration relaxes according to a stretched exponential law ~exp[\t-(t/σr)β. In the region of temperatures under study and for illumination times of 0.1–7.0 s, the coefficient β is equal to 0.55–0.65; furthermore, the temperature dependence of the effective time τr was activated, with an activation energy Ea of 0.97–1.07 eV. As illumination increased, a weak increase in Ea and β is observed. The quantity τr increases as the illumination time increases, in accordance with a law that is close to logarithmic. The experimental results obtained are compared with the existing microscopic models for the formation and annealing of metastable defects in a-Si:H films.

Pulsed-light-induced annealing of metastable defects in hydrogenated amorphous silicon.

Annealing of the metastable defects in hydrogenated amorphous silicon (a-Si:H) under pulsed light illumination is reported. It is found that light pulse increases the rate of annealing compared to annealing in the dark, as observed under continuous wave (cw) light. Annealing under cw light with an intensity similar to pulsed light has also been investigated for comparison. Results show that the rate of annealing under pulsed light is lower than under cw light. These results suggest that the rate of the metastable defect creation and the saturation value of the metastable defect density by pulsed light may be higher than by cw light.

Is Thermal and Light-Induced Annealing of Met Astable Defects in a-Si:H Driven by Electrons?

ABSTRACTWe report results of a search for a unifying rate law for the annealing of metastable defects in hydrogenated amorphous silicon (a-Si:H). We tested the hypothesis that defect-annealing by both heating or illumination is driven by the density of free electrons. This hypothesis is formulated via the rate equation - dN/dt = A nα N f (T), where N is the defect density, t the time, A a constant, n the free electron density, and f (T) a function of temperature derived from a distribution of annealing energies. The model fits two sets of data, with light-intensity and electrical conductivity as the independent variables, reasonably well, with a ranging from 0.39 to 0.76, but not the third set, where we varied the temperature.

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-SiC0.13 : H and a-SiN1.6 : 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=Eact/hωo 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...

Rate equation for metastable defects in hydrogenated amorphous silicon: The form of the light-induced annealing term.

Experimental evidence for light-induced annealing of defects in a-Si:H has recently been reported. However the experimental data about the removal of the defects under illumination are so limited that considerable uncertainty remains about the form of the light-induced term in the defect rate equation and its thermal activation energy. We here analyze the kinetics of light-induced creation and annealing of metastable defects in terms of a dispersive model. Our analysis suggests that the light-induced annealing term is proportional to the product of defect density and the carrier density. The activation energy of the light-induced annealing prefactor is about 0.45 eV.

Explanation of Light-Enhanced Annealing of Defects in Amorphous Silicon

ABSTRACTSeveral recent measurements have shown that annealing of metastable defects in a- Si:H can be accelerated by the presence of light. This is the opposite of the usual light-induced defect generation, and no existing rate equation explains it while maintaining the necessary symmetry of generation and recovery processes, and consistency with the stretched-exponential transients that best describe observed generation and anneal behavior. This paper shows that this light-enhanced annealing (LEA) can be explained readily by the usual rate equation leading to stretched exponentials with no other terms by allowing a variation of coefficients with temperature or light intensity. This equation then leads to good simulations of observed LEA. Interpretation of these results in terms of distributional changes is presented, and an experimental test is proposed.

Comparison of Dark and Light-Induced Annealing of Metastable Defects in a-Si:H

ABSTRACTWe propose a rate law that unifies the dark and light-induced annealing of metastable defects in hydrogenated Amorphous silicon (a-Si:H). Its form is dN/dt ∼ - C Naf (T), where N is the defect density, C the concentration of free electrons (holes), f (T) a dispersive function, t the time, and T the temperature. Special dark and light-annealing experiments, designed to keep the free electron density constant, were conducted to eliminate C from the rate law. We find that f (T) for the dark annealing process differs from that for light-annealing. We calculated the thermal activation energies for the decay of N to 1/e of its initial value. They are 1.56±0.2 eV for dark annealing and 0.91±0.2 eV for light-induced annealing.

Accumulation and annealing of implantation damage in a-Si:H

Hydrogenated amorphous silicon (a-Si : H) films have been irradiated with H +, B +, P+, and Ar ÷ ion beams. The accumulation and the annealing of irradiation-induced defects has been investigated through a series of electronic transport and PDS measurements. We find that for all projectiles damage accumulation is dominated by atomic displacement collisions with the damage saturating for energy transfers in excess of about 10 eV/target atom. Annealing at elevated temperatures causes the conductivity of doped and irradiated a-Si:H films to increase according to stretched exponential decay curves. All annealing parameters derivable from such fits scale with the energy originally dissipated into atomic displacement collisions. For energy transfers up to 10 eV/target atom the activation energy for annealing increases up to a saturation value and, at the same time, an increasing fraction of the irradiation-induced defects becomes stable against annealing at moderate temperatures (T, < 250 ° C). We discuss these results with respect to damage accumulation data in crystalline silicon (c-Si) and with regard to the annealing of metastable defects in a-Si : H.

Defect relaxation in hydrogenated amorphous silicon

Annealing of metastable defects in hydrogenated amorphous silicon, a-Si:H, follows a stretched exponential form and obeys a Meyer-Neldel rule. Two different models can explain this behavior showing that annealing kinetics alone are not a test of the models. I show additional experiments that distinguish the models.

Annealing of metastable defects in hydrogenated amorphous silicon.

The annealing process of light-induced metastable dangling-bond states in hydrogenated amorphous silicon is studied using the analysis presented in the preceding paper for electron-spin-resonance transients. The annealing kinetics are monomolecular, with a thermally activated decay rate R=${\ensuremath{\nu}}_{0}$exp(-${E}_{a}$/${\mathrm{kT}}_{A}$). The decay curves of the metastable defects in pure, UHV-deposited a-Si:H and in a-Si:N,H a-Si:C,H and a-Si:O,H with impurity contents between 1 and 20 at.?are analyzed in terms of a well-defined prefactor, ${\ensuremath{\nu}}_{0}$, and a broad distribution of thermal activation energies, N(${E}_{a}$). Both ${\ensuremath{\nu}}_{0}$ and N(${E}_{a}$), depend in a characteristic way on the density and the chemical nature of the impurities present in a sample. Moreover, a strong correlation between ${\ensuremath{\nu}}_{0}$ and N(${E}_{a}$) similar to the Meyer-Neldel rule for the conductivity in a-Si:H is observed. Implications of the experimental results for the intrinsic or extrinsic nature of the metastable defects and possible annealing mechanisms are discussed.