The Meyer-Neldel Relation and Analysis of the Field-Effect in Amorphous Silicon
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THE MEYER-NELDEL RELATION AND ANALYSIS OF THE FIELD-EFFECT IN AMORPHOUS SILICON RUUD E.I. SCHROPP, JAN SNIJDER, JAN F. VERWEY Department of Applied Physics, Groningen State University, Nijenborgh 18, 9747 AG Groningen, The Netherlands. ABSTRACT The dependence of the conductance prefactor on the activation energy in accordance with the Meyer-Neldel relation has been observed in a-Si:H, by measuring the temperature dependence of the field-effect in a-Si:H thinfilm transistors. The Meyer-Neldel rule is for the first time properly taken into account in the analysis of the field-effect, thereby considering the non-uniform shift of the Fermi-level as induced by the gate bias. The analysis also yields the flat-band voltage, which is an important parameter in the density of states evaluation. The density of states is shown to be considerably overestimated in conventional analysis. INTRODUCTION The characteristics of amorphous silicon thin-film transistors (TFTs) are highly determined by the density of localized bandgap states. Several methods have been proposed to determine the density of states (DOS) distribution using the TFT structure, including the field-effect (FE) technique [1-5]. A problem that should not be overlooked in the analysis of conductance measurements is the anomalously large conductance prefactor and the exponential decrease of this prefactor with decreasing activation energy. The relation between the prefactor and the activation energy can be described by the characteristic slope A and is known as the Meyer-Neldel (MN)rule [6]. The MNrelation is generally regarded as an intrinsic property. Therefore, it has been argued that it should be taken into account in any model for the transport in a-Si:H [7]. Another problem in FE structures is, that the flat-band voltage VB has to be known exactly, as it has a strong effect on the results of the DOS calculation [8]. An incorrect flat-band voltage will generally lead to a wrong interpretation of physical bulk parameters. In this paper, we present measurements of the FE in undoped a-Si:H TFTs as a function of ambient temperature. In this way the dc conductivity can be investigated as a function of the effective Fermi-level in a controllable way, as there are no impurity related defects that are normally introduced when the Fermi-level is varied by doping.
We shall show, that the FE strictly obeys the MNrule. Therefore, we believe that the conventional analysis of the FE [1-5] is fundamentally incorrect. We present a FE analysis, where the MNrule is for the first time taken into account. Special care is taken to use the correct value of the slope A in the MNrelation, because the apparent value of A is unequal to the intrinsic bulk value, as will be pointed out later. It is shown, that the MNrule has a large effect on the DOS results. We shall discuss possible origins for the MNrule. EXPERIMENTAL
The measurements were performed on field-effect structures as shown in fig. 0 1.The gate insulator, 110 nm thick, was made by thermal oxidation at 1000 C of an n+-(1O0)
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