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A5.6.1

Transport and Meyer-Neldel Rule in Microcrystalline Silicon Films Steve Reynolds, Vlad Smirnov1, Friedhelm Finger, Charlie Main2 and Reinhard Carius Forschungszentrum Jülich, Institute for Photovoltaics, D-52425 Jülich, Germany. 1 School of Computing and Creative Technologies, University of Abertay Dundee, Bell Street, Dundee DD1 1HG, U.K. 2 University of Dundee, Division of Electronic Engineering and Physics, Nethergate, Dundee DD1 4HN, U.K. ABSTRACT Changes in the electrical conductivity of thin (< 300 nm) silicon films following prolonged exposure to atmosphere, are reported. Both reversible (by annealing at 150 ºC under vacuum) and irreversible (annealing-resistant) effects are found to occur, which are larger in thinner films. The conductivity prefactor and thermal activation energy obey the Meyer-Neldel rule, although detailed behaviour depends on film thickness and microstructure. Irreversible changes may result from oxidation of thinner, more porous films, with water and/or oxygen adsorption and desorption responsible for reversible changes. The need to identify and account for these effects when discussing and formulating transport mechanisms in these materials is underlined.

INTRODUCTION It has been known for some time that the properties of amorphous [1] and microcrystalline [2] silicon films may be influenced by exposure to atmospheric gases. More recently, reversible and irreversible changes have been studied in detail in microcrystalline silicon thin films [3] and solar cells [4] as a function of composition and microstructure. In films, reversible changes in dark conductivity σ can occur typically on a time-scale of minutes or hours at room temperature, and as they may be removed by annealing in vacuum at moderate temperatures, probably involve the physical adsorption of atmospheric components such as water vapour and oxygen. Over a longer exposure of days or weeks however, the original σ cannot be restored by annealing [2] and a permanent modification of the film, termed post-deposition oxidation [5-8], or irreversible ageing, occurs. Typical ageing behaviour is illustrated in Figure 1. It follows that a given film may enter a range of metastable conductivity states, which display an Arrhenius-like activation:

σ (T ) = σ 0 exp(− Eσ / kT ) ,

(1)

where σ0 is the intercept at 1/T = 0, Eσ is the conductivity activation energy, k is Boltzmann’s constant and T the experimental temperature. We have found that Eσ and σ0 for these states are linked by the Meyer-Neldel rule (MNR):

log( σ 0 ) = log( σ 00 ) + E σ / E MNR ,

(2)

A5.6.2

-5

10

-6

90 nm film

C

-1

σ (S cm )

10

-7

10

B

-8

10

A

-9

10

-10

10

as deposited 40 days room air

A'

-11

10

2,2

2,4

2,6 2,8 3,0 -1 1000/T (K )

3,2

3,4

Figure 1. Typical conductivity behavior. After exposure to atmosphere, σ rises from A′ to A. Heating under vacuum is represented by path A-B-C. Desorption occurs at B. On cooling under vacuum, conductivity returns to A′. where σ00 is a constant and EMNR is the Meyer-Neldel characteristic ener