strongly influence the crystalline and granular structure of Bi2O3 films. It is known ... thin bismuth films can be studied in limits of continuous (single-crystal) and.
ANALELE STIINTIFICE ALE UNIVERSITATII "AL.I.CUZA" IASI Tomul XLV - XLVI, s. Fizica Stării Condensate, 1999 – 2000, p. 104 – 112.
ON THE ELECTRONIC TRANSPORT PROPERTIES OF OXIDIZED BISMUTH THIN FILMS G. I. RUSU, L. LEONTIE, G. G. RUSU, MIHAELA GÎRTAN, IULIA SALAORU∗ KEYWORDS: semiconductors, bismuth trioxide, thin films, oxidation, electric properties In the present work some experimental investigations of electric properties of Bi2O3 thin films are presented. The bismuth trioxide films (d=0.10÷2.30 µm) were prepared by thermal oxidation, in air, of vacuum evaporated (onto glass substrates maintained at room temperature) Bi films. The structure inspection (XRD and polarising microscopy techniques) revealed that as prepared films are polycrystalline and multiphase. Depending on the oxidation conditions, α- or β-Bi2O3 forms preponderantly. The time, t0, for complete oxidation of bismuth films shows a parabolic dependence on the oxidation temperature, T0, with an activation energy Eac=1.45÷1.72 eV. For all investigated samples, either in thin films or in pellets, the temperature dependence of electrical conductivity, σ, indicates a typical semiconducting behaviour. From the lnσ=f(103/T) curves, the values of thermal activation energy of electrical conduction, Ea, were determined. In all cases, the Seebeck coefficient had negative values. The mechanism of electrical conduction in polycrystalline Bi2O3, with emphasis on role of the oxide layer at grain free surfaces, is discussed. The influence of the heat treatment on the oxidation process of Bi films is also studied.
INTRODUCTION In the last years, bismuth trioxide (Bi2O3) thin films have attracted the interest of many researchers because the values of some their characteristic parameters (energy gap, refractive index, dielectric permittivity, photoconductivity, etc.) are suitable for a large range of applications such as sensors, optical coatings, photovoltaic cells, microwave integrated circuits, etc. [1-5]. A variety of methods have been used for preparation of Bi2O3 films, but thermal oxidation of bismuth films is one of the most advantageous methods for obtaining of uniform and adherenced films [3, 5-7].
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Faculty of Physics “Al.I.Cuza” University, Iasi, 6600 Romania
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ON THE ELECTRONIC TRANSPORT PROPERTIES OF OXIDISED …
In a series of previous papers [8-11], we have studied some structural, optical and photoelectric properties of bismuth trioxide in thin films. In the present paper we report some experimental results relating to the electronic transport properties of this semiconducting oxide in thin films EXPERIMENTAL Bi2O3 films were prepared by thermal oxidation (in air) of bismuth (purity 99.999%) films, deposited onto glass substrates by thermal evaporation under vacuum [12, 13]. The substrate temperature during growth of bismuth films was 290 K. The deposition rate has been about 2 nm/s. The thickness of Bi and Bi2O3 films was measured with an interferometric method and for investigated samples ranged between 0.10 and 2.30 µm. Generally, the oxidation of bismuth films was performed by placing them in an open resistive furnace and maintaining them (in air) at a high temperature (700-750 K) for different times (10-60 min.). The heating and cooling rates may be modified between 2 K/min and 12 K/min. For each studied sample the oxidation conditions will be indicated. The adhesion of the bismuth trioxide films to the substrate was extremely good. The structural characteristics of the films were studied by X-ray diffractometry (XRD), metallographic microscopy and optical polarising microscopy. The temperature dependence of the electrical conductivity, σ, was studied on either thin films (using surface – type cells) or on the pellets. Pellets of about 10 mm in diameter and thick 0.5÷2.0 mm, were prepared by pressing polycrystalline powdered samples (average size of the crystallites ranged between 0.5 and 1.0 µm) at 10 t/cm2. Silver thin films (d=1.5÷2.5 µm) were used as electrodes. They were deposited (onto glass substrates or on both sides of the pellets) by thermal evaporation under vacuum. The ohmicity of the contacts was verified [14]. For measurement of σ a static electric field with intensity lower than 102 V/cm was used. Under these conditions non-ohmic effects were not observed [10, 14]. The experimental arrangement used for the study of temperature dependence of the electrical conductivity was similar to that described in [11, 12, 15]. The temperature dependence of Seebeck coefficient was determined by thermal-sonde method [16]. In the temperature range 350÷425 K, the Seebeck coefficient was found to be negative for all investigated samples. RESULTS AND DISCUSSION It was experimentally established [5, 6, 8-10] that the preparation method and preparation conditions (substrate temperature, deposition rate, substrate nature, etc) strongly influence the crystalline and granular structure of Bi2O3 films. It is known that Bi2O3 has four main crystalline modifications (phases). Generally, they are denoted as α-, β-, γ- and δ-Bi2O3. Only two of them, α-Bi2O3
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(monoclinic structure) and δ-Bi2O3 (face–centred cubic structure) are stable at normal atmospheric conditions. The β-Bi2O3 (tetragonal structure) and γ-Bi2O3 (body-centred cubic structure) are metastable modifications for T
