Is Thermal and Light-Induced Annealing of Met Astable Defects in a-Si:H Driven by Electrons?
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HELENA GLESKOVA* AND S. WAGNER Department of Electrical Engineering, Princeton University, Princeton, NJ 08544
ABSTRACT We 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 na 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 oxranging from 0.39 to 0.76, but not the third set, where we varied the temperature.
INTRODUCTION The creation and removal of metastable dangling bonds in a-Si:H can be driven by heat, light, or charge carriers. Depending on the sample history, elevated temperature [1,2], illumination [3,4] or charge carriers [5,6] can create or remove metastable dangling bonds. The equilibrium defect density is represented by a balance between defect creation and annealing. While the defect creation and the thermal annealing of the defects have been characterized well [7], many open questions remain about the annealing induced by light or by charge carriers. All the external excitations - elevated temperature, illumination and, current injection - have one aspect in common: they raise the density of free carriers in a-Si:H. A two-carrier process is widely accepted for the defect creation [3]. Metastable defect recovery may be induced by a single carrier [8,9]. The annealing of metastable defects is dispersive. This dispersion can be introduced to the rate equation either via a distribution of annealing energies or by introducing a sublinear time dependence. The rate equation for the metastable defect density created by illumination can be written in the schematic form [10,11]: (dN) = t0'(LIG - LIA + TG -TA)() where the dispersion is expressed by t 1 "A,N is the defect density, and LIG, LIA, TG and TA stand for light-induced generation, light-induced annealing, thermal generation, and thermal annealing, respectively. The actual analytical forms of these four terms have changed over the past years [3,8,10-12]. It is accepted that near room temperature the thermal generation term TG is negligible compared to the others [1]. If one first builds a high defect density in the sample using strong illumination and then removes the defects under much weaker illumination, the LIG-term can be neglected early on during the annealing process. Therefore the annealing rate in the dark or under illumination can be written: 343 Mat. Res. Soc. Symp. Proc. Vol. 377 01995 Materials Research Society
t A(adN .=t
TA)
(2b)
We make the hypothesis that the rate of annealing depends on the density of free electrons n, the density of dangling bonds N, and a thermally-activated term f(T) which is dispersiv
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