Abstract. The electrical conductivity studies of the Cu/ZnO/Si thin film heterostructures were carried out by the current-voltage (I-V) characteristics. It was found the I-V characteristics were asymmetric and showed weak rectifying properties. The most probable mechanism of electrical conductivity was determined. The concentration of trapping levels and the carrier mobility were calculated.

    1. Introduction

    Zinc oxide possesses an interesting combination of physical and chemical properties: a high melting point and thermal conductivity, photosensitivity [1], piezoelectric [2] and pyroelectric [3] effects, the adsorption of gases on the surface [4]. This material is widely used in microelectronics, chemistry and medicine [5]. ZnO crystals, thin films and diode structures based on it are the subjects for scientific studies and applications.

    An anisotropic crystal structure, a non-stoichiometric composition, semiconductor properties with a large band gap and other properties make this material not only interesting but also quite complex object for study. The dependence of the characteristics of ZnO thin film on the preparation conditions and various external factors is currently of great interest, for example in the semiconductor sensor technology. In connection with the aforementioned, the present work was to study the conductivity and dielectric properties of structure contained a thin layer of zinc oxide.

    1. Experiment

    The samples were Cu/ZnO/Si heterostructures. ZnO film was deposited on a p-type silicon substrate (100) by an atomic layer deposition. Argon (99.998%) used as purging and carrier gas. Process temperature was 150 OC. Diethylzinc  and deionized water were used as precursor. The pulse length for diethylzinc and water was 200 ms, purge time was 1 s . Figure 1 presents the results of structure and composition investigations of ZnO films.

    The thickness of the ZnO film on Si substrate is equal to 200 nm. Copper circular electrodes with diameter of 3 mm were sputtered on film surface through a shadow mask by a thermal evaporation. According to data obtained by scanning electron microscopy, the films have a solid structure.

    The conductivity measurements were carried out by the current-voltage (I-V) characteristics method using the immittance meter E7-20. This device allows applying the bias to the sample from -40 to +40 V in steps of 0.02 V in the range of -4 to +4 V and 0.2 V at higher voltages. The voltage applied to the film is accepted positive if a positive potential is applied to the top electrode, and vice versa.

     

         

    a

    b

    c

    Figure 1 SEM (a) and AFM (b) images, XRD spectra (c) of formed ZnO film.

    1. Results and discussion

    I-V characteristics were obtained at the room temperature to determine the conductivity mechanisms of the ZnO film deposited on Si. Figure 2 shows a typical dependence of electric current on a voltage applied to Cu/ZnO/Si heterosructure.

     

       

    a

    b

     

     

    Figure 2 Current-voltage characteristics of the ZnO film deposited on silicon: a) linear scale, b) semi-logarithmic scale.

     

    In a case of positive potential applied to the top electrode, the electric current increases nonlinearly with the voltage except for the initial voltage range from 0 to 5 V. Moreover, the I-V characteristics are asymmetric and show weak rectifying properties with a rectification coefficient K = 5 (at the U = 2.5 V) and the non-ideality coefficient of diode structure n = 5.55. On the basis of the linear section of the current-voltage characteristics, the electrical conductivity of ZnO layer is calculated according to the relation

    ,                                                                     (1)

    where I is the current, d is the layer thickness, U is the applied voltage, S is the top electrode area. The value of the ZnO layer conductivity is equal to 4.5×10-6 W-1m-1.

    Rectifying properties of the structure are associated with the presence of the potential barrier at the interface ZnO/Si. The asymmetry of the current-voltage characteristics of the barrier is typical for such structures. A dependence of current on voltage in these structures is caused by a change in the number of majority carriers taking part in charge transfer processes. A role of external voltage is to change the number of free carriers transferred from one to another parts of the barrier structure. However in this case, the barrier height is not uniform over the boundary of the zinc oxide due to the presence of surface states. Thus, current may leak through the lowered barrier areas. This explains the nonideal rectifying properties and the presence of the high reverse current.

    It should be noted that the forward and reverse trajectories of current-voltage characteristics are practically the same that indicates the absence of resistive switching effect in the present ZnO films.

    To explain the observed behavior of the current-voltage characteristics of several conduction mechanisms of film structures can be considered. It is well known [6,7] that the leakage current in dielectric or semiconductor films may be attributed to several conduction mechanisms including the Poole-Frenkel emission, the Schottky emission, the Fowler-Nordheim tunnelling and the space charge limited current (SCLC).

    In voltage region from 5 to 14 V, the current observed in the ZnO films increases in accordance with a quadratic law, i.e.  (Figure 3). Such a behavior can be ascribed to the SCLC mechanism. For the space charge limited current in the case when a single discrete trap level exists in the band gap, the current density is given by Eq. (2) [8]:

    ,                                                                (2)

    where j is the current density, e is the material dielectric permittivity, e0 is the vacuum dielectric permittivity, m is the carrier mobility.

     

    Figure 3 Dependence of the current on square voltage applied to Cu/ZnO/Si.

     

    As previously mentioned, the linear segment of the I-V characteristics preceded the quadratic region that corresponded to the SCLC model. At low voltage, the injection level is low and a concentration of injected carriers does not exceed a concentration of free equilibrium carriers. In this case, traps are not all filled, so the current obeys Ohm's law

    ,                                                                  (3)

    where n0 is the concentration of free equilibrium carriers, e is the elementary charge.

    The transition from linear to quadratic parts of I-V characteristics occurs when the concentration of injected carriers becomes comparable to the concentration of free equilibrium carriers. In this case, all traps are filled and do not affect the flow of space charge limited current. On one side, the transition voltage from linear to quadratic parts of I-V characteristics is given by

    .                                                                (4)

    On the other hand, the voltage is

    ,                                                               (5)

    where Nt is the concentration of trapping levels.

    Taking into account that dielectric permittivity of ZnO was equal to 8.5 and the transition occurred at the voltage of 5 V, we used the equation (5) and found the concentration of trapping levels Nt » 5.3×1018 cm-3. We determined n0 » 2×107 cm-3 from equation (4). We substituted the n0 concentration in the equation (3) and found μ » 190 cm2/(V×s).

    For the negative voltage part of I-V characteristics, the most probable mechanism of electrical conductivity is the Poole-Frenkel emission. The current dependence on the applied voltage modulus is shown in figure 4 in the Poole-Frenkel representation.

     

    Figure 4 Current–voltage characteristic of the structure on the base of ZnO according to the Poole-Frenkel mechanism.

     

    This dependence is nearly linear that gives evidence of the Poole-Frenkel emission contribution to the current. According to this mechanism, a field applied to the sample changes the shape of potential barriers for charge carriers between the atoms of the lattice. It leads to an increase in the number of free carriers in the sample due to overcoming the potential barrier.

     

    This work was supported by the Russian Science Foundation, grant no. 15-19-00138.

     

    References

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    • Heiland G and Ibach H 1966 Solid State Comm. 4 353
    • Shao C, Chang Y and LongY 2014 Sensors and Actuators B: Chemical 204667
    • Coleman V and Jagadish C 2006 Zinc Oxide Bulk, Thin Films and Nanostructures By Jagadish C and Pearton S (Elsiever) p. 1
    • Brazis R, Pipinys P, Rimeika A and Lapeika V 1990 Mater. Sci. Lett. 9 266
    • Solnyshkin A, Troshkin A,Bogomolov A, Raevski I, Sandjiev D and Shonov V 2009 Ferroelectrics 106 61
    • Kao K and Hwang W 1981 Electrical Transport in Solids. (Oxford: Pergamon Press)

     

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