Morphology, Phonon Confinement and Properties of a-Si:H Films
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Morphology, Phonon Confinement and Properties of a-Si:H Films Valeri Ligatchev School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798 ABSTRACT It is usually assumed that optical and electrical properties of amorphous semiconductors are determined by the short-range-order (SRO) parameters of atomic structure. The SRO parameters behavior is considered here as governed by morphology geometrical parameters (MGP) variations. Generalized Skettrup model is developed for a quantitative description of the MGP influence on the density of electron states distribution N(E). Common N(E) relation for the power, exponential and the ‘defect’ regions of the dependence is derived both for adiabatic (optical) and non-adiabatic (thermal) electron excitation processes. The simulated (by the N(E) convolution) spectral dependence of the opti-cal absorption coefficient contains the Tauc, Urbach and the ‘defect’ parts. The optical gap and the Urbach tail slope energies are typical for the ‘device quality’ a-Si:H films at the average morphology geometrical parameters values of order of 1 µm. Nearly linear the optical gap versus the Urbach tail slope energies dependence is obtained at the mor-phology geometrical parameters and temperature changes. Good agreement of experi-mental and the simulation results is achieved for a-Si:H films, prepared both by the RF sputtering and by the silane decomposition. INTRODUCTION It is usually assumed that optical and electrical properties of amorphous semiconductors are determined by the short (2 – 10 c) range order (SRO) parameters of the atomic structure [1]. Meanwhile, the SRO structural parameters of a ‘poor-quality’ amorphous silicon (a-Si) and of a ‘device-quality’ a-Si:H films are practically indistinguishable [2, 3]. The SRO parameters of the a-Si:H films are not strongly dependent on deposition method and conditions, while photoelectrical properties of ‘glow-discharged’ a-Si:H material are usually much better then the similar properties of ‘sputtered’ a-Si:H films [4, 5]. Hence, existence of some additional parameters of a spatial atomic distribution in the amorphous media can be supposed. Morphology formation is considered in this paper as a process which really determines the a-Si:H macroscopic properties and a quantitative model is proposed for the analysis of morphology geometrical parameters (MGP) effect on the a-Si:H electronic spectrum as well as on their optical and electrical properties. THE MODEL DESCRIPTION The Skettrup model [6] has been taken as the base for a quantitative simulation of the electron states distribution density N(E) within the a-Si:H gap. The original model was created under the assumption of the adiabatic (Frank-Condon) approach validity. Initial (at the absence of disorder) energies (‘positions’) of electron levels in the Skettrup model belong to the whole volume of the material. Energies of the electron levels can be perturbed (shifted) due to an electron-phonon interaction at the non-zero temperature T. A9
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