Crystal growth and transport properties of CuAlO 2 single crystal

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Crystal Growth and Transport Properties of CuAlO2 Single Crystal1 R. Brahimi, G. Rekhila, M. Trari*, and Y. Bessekhouad Laboratory of Storage and Valorization of Renewable Energies, Faculty of Chemistry (USTHB) BP 32 Algiers, 16111 Algeria * email: [email protected] Received April 18, 2013

Abstract—The transport properties of the delafossite CuAlO2 single crystal, grown by the flux method, are confined in ∞[AlO2] layers extending in the (001) plans. The dielectric properties are measured up to 490 K in the frequency range (102–105 Hz). The small variation of the dielectric loss tan(δ) is attributed to the wide space charge region. The linear plot log (conductivity) vs. 1000/T follows an Arrhenius type law and the results are discussed in terms of electron hopping among localized states. The activation energy (0.18 eV) gives an effective mass of 16m0 indicating that the levels in the vicinity of the Fermi level are strongly localized. Hence, the increase of the conductivity (σ) results from a thermal activation of the mobility (μ300 K = 1.2 × 10–5 cm–2 V–1 s–1). The sign of hole like small polarons is that of p type carriers originating from oxygen inter calation. The thermopower is little temperature dependent and characteristic of non degenerate conductivity with a low holes concentration and a large concentration of surface states within the gap region. DOI: 10.1134/S1063774514070062

INTRODUCTION The thermoelectric materials with low lattice ther mal variation are developed for various applications [1]. The electrical conductivity of semiconductors is gov erned by the quantity of dopants which generate charge carriers. The interest with the delafossite family A+M3+O2, where M denotes a trivalentmetal, is due to the modification of their transport properties, ranging from metals to semiconductors (in this case, from A = Pt, Pd to A = Cu, Ag) [2]. Moreover, the versality of the structure allows doping with the heterovalent ions with n and ptype specimen in the Msublattice. Special attention has been paid to the anisotropy in delafossites and some authors have reported that the mobility of p type specimen is much larger than n type one by three orders of magnitude [3]. This is due to the difference of coordinations between Cu+ and M3+ ions. In ∞[MO2] layers, the ions M3+ are octahedrally coordinated and the electron hops over oxygen octa hedra and must overcome a further potential barrier (see structure below). On the contrary, Cu+ is bonded to six copper ions in the basal plans in direct interac tion, with no potential barrier for the electron transfer. The electrical conductivity is tailored by oxygen inser tion in the copper layers and the oxide is a valence compensated system with internal redox process. CuMO2 could be used for making capacitors [4], where it is for the fact that they are poor dielectric. Their permittivity is not high due to the partial anionic conductivity. Among the congeners, CuAlO2 is a 1 The article is published in the original.