Charge transport in dielectrics by tunneling between traps
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ONIC PROPERTIES OF SOLID
Charge Transport in Dielectrics by Tunneling between Traps K. A. Nasyrova,* and V. A. Gritsenkob a
Institute of Automation and Electrometry, Siberian Branch, Russian Academy of Science, pr. Akademika Koptyuga 1, Novosibirsk, 630090 Russia *email: [email protected] b A.V. Rzhanov Institute of Semiconductor Physics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Lavrent’eva 13, Novosibirsk, 630090 Russia Received October 26, 2010
Abstract—The theory of charge transport in dielectrics by tunneling between traps is developed. In contrast to the Frenkel model, traps in silicon nitride are characterized by two energies, optical and thermal ones, and ionization occurs by the multiphonon mechanism. The theory predicts that tunneling between such traps is thermally stimulated: the halfdifference of the optical and thermal energies plays the role of the activation energy. This theory successfully explains the experimental current–voltage characteristics of siliconenriched silicon nitride. Such silicon nitride contains a large number of traps whose nature is associated with excess silicon. Charge transport in this material occurs by tunneling between adjacent traps. DOI: 10.1134/S1063776111040200
1. INTRODUCTION Except for thermal oxide SiO2 on silicon, all other dielectrics, i.e., oxides (SiOx, Al2O3, Ta2O5, Nb2O5, HfO2, ZrO2, TiO2) and nitrides (Si3N4, SiNx, Ge3N4, BN), are characterized by high trap densities, Nt > 1019 cm–3. Currently, the concept is generally accepted that the conduction of dielectrics with traps is con trolled by Coulomb trap ionization via the Frenkel mechanism [1, 2]. The Frenkel effect consists in a decrease in the trap Coulomb potential in an electric field [3, 4] and facilitation of trap ionization. The Frenkel effect is involved to interpret the conduction of siliconenriched silicon oxide SiOx < 2, which was used as an insulator before the advent of silicon tech nology [1]. In silicon devices, two key insulators are silicon oxide SiO2 and silicon nitride Si3N4. The latter has the memory effect, i.e., the capability to localize electrons and holes injected into it with a giant con finement time in a localized state (ten years at 85°C). Currently, the effect of electron and hole localization in silicon nitride is used for developing nextgenera tion flash memory [5, 6]. The storage time in flash memory devices based on silicon nitride is controlled by trap ionization. Therefore, in recent years, the elec tron and hole transport mechanism in Si3N4 became a subject of detailed studies [7–13]. If the charge transport in silicon nitride is limited by the Frenkel effect [14], linearization of the cur rent–voltage characteristics in logJ – F coordinates is considered as an attribute of the Frenkel effect. Here J is the current density, F = V/d is the average electric field, V is the voltage applied to an insulator, and d is the insulator thickness. In a series of studies [8–12] by the present authors, they attempted to quantitatively
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