Instability effects in hydrogenated microcrystalline silicon thin films

Abstract

Instability effects on the dark conductivity (σDark) and photoconductivity (σphoto) of microcrystalline silicon films with different microstructures deposited on smooth and rough substrates, after exposure to different storage conditions have been investigated. Samples stored in vacuum are less affected by aging than films stored in air and sealed plastic bags filled with N2 gas. The aging is reversible after annealing at 450 K. Thin films deposited on smooth and rough substrates with the same thickness and silane concentrations (SC) during the same deposition run showed similar aging effects. Thick samples exhibited a decrease in σDark, while both decrease and increase in σDark occur in thin samples after aging. For each sample, σDark and σphoto presented the same type of aging, independent of thickness and type of substrate used. The applicability of the steady‐state photocarrier grating technique was successfully tested on the rough substrates. Light‐induced degradation studies confirm the dependence of the Staebler‐Wronski effect on the crystallinity of the material (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

New Findings and Interpretation on Atmospheric Adsorption Induced Instability in Microcrystalline Silicon Films

“…By atmospheric adsorption, the dark conductivity can be either decreased or increased. It has been postulated that the decrease (increase) is related to the electron acceptor (donor) such as oxygen (water) molecules adsorbed on the surface of the film, within cracks or at grain boundaries [1][2][3][4][5][6][7][8]. The adsorption ofelectron acceptor (donor) molecules will create upward (downward) band-bending and thereby establish depletion (accumulation) layer at the surface, which will result in a decrease (increase) in conductivity.…”

“…In TFTs, Negative bias-stress makes downward built-in band-bending due to the trapped holes within gate insulator. It has been generally accepted that increase in µc-Si:H is related with the physical adsorption of electron donor molecules such as water molecules during exposure to air [1][2][3][4][5][6]. The established adsorbate layer will then create anaccumulation layer or downward band-bending at and adjacent to the surface.…”

“…Since it is non-dissociative, the distribution of activation energy for adsorption is not expected. The atmospheric aging effect depends on the microstructure of the films [3,6,7]. The interaction of a molecule with a given surface will be dependent upon the physical and chemical environment of surface.…”

“…It was previously revealed that highly crystalline films with porous, large grain size and columns separated by deep crack show a significant adsorption effect [3]. [1,3,5,6]. Adsorbing run at fixed room temperature has led to a decrease in of about three orders of magnitude.…”

“…For some µc-Si:H films, decreases up to several orders of magnitude during the storage of films in the air, of which initial value may be restored by annealing in vacuum at around 200℃. The annealed value may remain stable when the film is kept in vacuum, whereas it changes again when put back into the air [1][2][3][4][5][6][7][8].…”