Effect of oxidation on the conductivity of nanocrystalline PbTe:In films in an alternating electric field
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DIMENSIONAL SYSTEMS
Effect of Oxidation on the Conductivity of Nanocrystalline PbTe:In Films in an Alternating Electric Field A. A. Dobrovolskya^, T. A. Komissarovaa, Z. M. Dashevskyb, V. A. Kassiyanb, B. A. Akimova, L. I. Ryabovaa, and D. R. Khokhlova aMoscow
State University, Moscow, 119991 Russia ^e-mail: [email protected]; Fax: (495)9328876 bBen-Gurion University, 84105 Beer Sheba, Israel Submitted April 23, 2008; accepted for publication May 12, 2008
Abstract—Temperature and frequency dependences of components of complex impedance for nanocrystalline PbTe:In films in the temperature range of 4.2–300 K and the frequency range from 20 Hz to 1 MHz have been studied. The films were deposited onto a cooled glass substrate and then annealed in an oxygen atmosphere at temperatures of 300 and 350°C. The charge-carrier transport in the studied films is controlled by charge transport over inversion channels at the surface of grains and by transitions through barriers at the grain boundaries. Parameters (resistance and capacitance) corresponding to each of above-mentioned mechanisms were determined. Dominant contribution to conductance of the film annealed at 350°C is made by inversion channels. It is shown that the transport of charge carriers over inversion channels in the region of low temperatures is realized by hopping conductivity. PACS numbers: 73.63.Bd, 71.55.Ht, 72.20.Ee, 72.30.+q, 81.40.Rs DOI: 10.1134/S1063782609020249
1. INTRODUCTION Doping of lead telluride with indium provides pinning of the Fermi level at the position higher by 70 meV than the conduction-band bottom and initiation of long-term processes of relaxation of nonequilibrium electron distributions at temperatures below 25 K [1]. A system of impurity levels is formed in the energy spectrum of PbTe:In; this system is related to different charge states of the impurity. Stabilization of the Fermi level leads to a high homogeneity of electrical properties in the samples and to a high mobility of electrons (µ 105 cm2 V–1 s–1 at the liquid-helium temperature). The effect of stabilization of the Fermi level in nanocrystalline films based on PbTe:In gives rise to a high homogeneity of electrical characteristics for separate grains. However, properties of a grain in nanocrystalline structures do not always determine features of a system. A considerable contribution to conductivity can be made by surface effects and barriers formed at the boundaries of nanocrystallites. In particular, nanocrystalline PbTe:In films deposited on a cooled glass substrate feature the p-type conductivity [2]. At temperatures below 150 K, the effect of persistent photoconductivity is observed in these films. A relatively high temperature of appearance of persistent photoconductivity is caused by the presence of intercrystallite barriers giving rise to modulation of the band profile rather than by specific features of the PbTe:In energy spectrum. A study of the films’ impedance has shown that
the transport of charge carriers is controlled by the presence of two mechanisms:
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