On the Physical Nature of Traps in Polycrystalline Sb ₂ O ₃

Abstract

IntroductionTwo types of carrier traps are observed in several organic and anorganic compounds : traps whit discrete energy level in forbidden band of the crystal and traps whit a quasi -continuous distribution of energy levels having the maximum density at the density at the edge of the conduction or valence band. Both types of traps have been found and extensively investigated in molecular organic crystal. Simultaneous presence of both types of carrier traps have been observed also in a number of polycrystalline molecular layers obtained by vacuum evaporation.Carrier traps are experimentally determined either by the space -charge -limited current (SCLC) method [1,2], by the thermostimulated current (TSC) method , or by photoemission. These methods give detailed information about the energetic and kinetic parameters of the traps , but practically no information about their possible physical nature. An important stage in the analysis of the generation and transport phenomena of charge carriers is represented by studying the physical processes occurring at metal -dielectric contact.This paper deals with the physical phenomena occurring at the contact of Sb 2 O 3 layers with various metals. A new approach is proposed to consider the formation of localized energy level in Sb 2 O 3 polycrystalline films.In order to investigate the effects of voltage generation of charge carriers, thin films samples were obtained by thermal evaporation in vacuum of Sb 2 O 3 , at a pressure of 10 -5 torr, sandwiched between metallic electrodes. Silver and aluminium were used. The thickness of dielectric films varied from 0.1 to 4 µm. X-ray analysis evinced the polycrystalline structure of thin films containing the αSb 2 O 3 phase[3]. Results and DiscussionA complete study of current as a function of voltage , thickness and temperature was performed.The current voltage characteristics of structures Ag-Sb 2 O 3 -Ag (Fig. 1) and Al-Sb 2 O 3 -Al (Fig.2) are perfectly symmetric with respect to the polarity of applied voltage. The I -U characteristics indicates lower electrical conductivity of thin films of Sb 2 O 3 . This is due to weak intermolecular interaction forces of the Van der Waals type among Sb 2 O 3 molecules. At room temperature , the density of free charge carriers is extremely low . which prevents electrical conduction.When low voltage (U < 1V) is applied the contacts Ag -Sb 2 O 3 and Al -Sb 2 O 3 are ohmic. This is because , giving the low concentration of intrinsec carriers , the conductivity of Interface Controlled Materials. Edited by M. Rühle and H. Gleiter

Structural characterization of polycrystalline Sb2O3 thin films prepared by thermal vacuum evaporation technique

“…This is in good agreement with SEM analysis. All thin films exhibited semiconducting behaviors with the conductivity range 10 À7 -10 À9 O À1 / cm [12,13,15]. The conductivity of the films was found to increase from 1.54 Â 10 À9 to 1.06 Â 10 À7 O À1 /cm when the substrate temperature increases from 300 to 573 K. This increase may be attributed to the structural characteristics of films.…”

“…The UV-VIS-NIR transmission spectrum studies showed that the bandgap of Sb 2 O 3 thin films (3.62 eV) is much larger than that in orthorhombic crystal of Sb 2 O 3 (3.30 eV) [9,10]. In a series of previous papers [11][12][13][14][15] we have studied the electrical and optical properties of Sb 2 O 3 thin films prepared by thermal evaporation method. However, no detailed study is reported of the structural properties of these thin films.…”