Change in The Spectral Shape of the Subgap Absorption in a-SI:H by Photodegradation at 4.2K and its Relation to the Mobility-Lifetime Product

We have observed a pronounced change in the shape of subgap absorption spectrum α(hν) of a-Si:H due to photodegradation at T=4.2K, in contrast to light exposure at T=300K. During the 4.2K exposure, α(hν) at 1.1≤(h‥)≤1.6eV increases with time as (α(t)-α(0)) ∝t 0.3 . However, for hν≤1eV the absorption α(hν) stays constant or decreases with exposure at 4.2K. Subsequent anneal causes the subgap absorption at the higher photon energies to decrease monotonically while a (hν ≤ 1.1 eV) increases first and goes through a maximum at T≈350K. This unexpected increase in a(hν) during annealing occurs at the anneal temperatures where the degraded mobility-lifetime product recovers. We suggest that the nonmonotonic recovery of a (hν≤ 1.1eV) during annealing is associated with preferential anneal of deeper defects or with structural relaxations around the defects during anneal causing a shift in their energy levels.