The Meyer-Neldel Rule in Conductivity of Microcrystalline Silicon
- PDF / 78,538 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 33 Downloads / 191 Views
The Meyer-Neldel Rule in Conductivity of Microcrystalline Silicon Sanjay K. Ram1, Satyendra Kumar1, and P. Roca i Cabarrocas2 1 2
Department of Physics, Indian Institute of Technology Kanpur, Kanpur-208016, India LPICM, UMR 7647 - CNRS - Ecole Polytechnique, 91128 Palaiseau Cedex, France
ABSTRACT The dark conductivity (σd) has been measured from 300 to 440K on undoped hydrogenated microcrystalline silicon (µc-Si :H) films having different thicknesses. The carrier transport is found to be thermally activated with single activation energy (Ea) in all the samples. The Ea increases as the film thickness decreases. At the same time logarithmic of dark conductivity prefactor (σo) is found to follow a linear relation with activation energy, known as the MeyerNeldel rule (MNR). Results are explained in terms of increased degree of disorder in thinner samples. Thus change in Ea with the film thickness is directly related to the density of localized states at the Fermi level in grain boundary (GB). Therefore varying the film thickness and, hence, the exponential density of states induces a statistical shift of Fermi level which gives rise to the observed MNR. INTRODUCTION The Meyer-Neldel rule (MNR) [1] or compensation law is universally observed in all heterogeneous systems involving thermally activated processes. MNR is frequently observed in the dark electrical conductivity (σd) of disordered materials:
σd=σo e –Ea / kT,
(1)
where prefactor (σo) correlates with the activation energy Ea as:
σ0=σ00 e GEa,
(2)
G and σ00 are called MNR parameters. Often G is denoted as EMN, the Meyer-Neldel characteristic energy. The Meyer-Neldel behavior is related to the disorder or inhomogeneous nature of the material [1,2]. However, the microscopic origin of the MNR and the physical meaning of G, are still a topic of discussion. Yelon and Movaghar proposed that multiphonons are the source of excitation energy [3]. On the other hand, Jackson suggested a multi-trapping process dominated by hydrogen diffusion as a reason for observed MNR in a-Si:H [2]. Further, various authors argue that the statistical shift of Fermi-level (EF) with temperature is the origin of MN behavior [4,5,6,7]. In a-Si:H the statistical shift model is frequently applied and various defect dynamics are explained on its basis [8]. Plasma-deposited µc-Si:H thin films are gaining importance for large area electronic devices such as solar cells, thin film transistors (TFT) for flat panel displays, and sensors. Large carrier mobility [9] and good stability are attractive features for TFTs [10]. However, plasma-deposited µc-Si:H is inherently a heterogeneous material consisting of micro and nano size crystallites, A21.4.1 Downloaded from https://www.cambridge.org/core. Univ of Michigan Law Library, on 02 Oct 2017 at 15:42:48, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-715-A21.4
amorphous tissues and voids. Understanding carrier transport in such a system is undoubtedly a challenging tas
Data Loading...
