**3.3 Schottky-like metal/conducting oxide interfaces**

Prior to explaining the resistive switching effects, the realization about the fundamental of the electronic properties related to conducting oxides is very important. Also, the current and voltage (*I-V*) behavior of interface formed at conducting oxide and metal electrode plays a key role in device performance. However, the contact resistance of such interfaces are largely modified by two major effects. One of the two effects is the existence of unwanted chemical reaction of metal electrode with conducting oxide. Secondly, the distinct Fermi level of conducting oxide and metal electrode leads to generation of space charge layer. The high contact resistance of interface is predominantly due to the Schottky barrier. This forms space charge region where essentially the majority charge carriers are depleted. In many cases of memory devices, the different electrode metals not only induce considerable modification in the resistive switching properties but also the contact resistance is affected significantly [6, 15, 23]. In case of stratified M/Pr0.7Ca0.3MnO3/ SrRuO3 (M/PCMO/SRO) and M/SrTi0.99Nb0.01O3/Ag (M/Nb:STO/Ag) where M is top electrode metal, the change in *I-V* characteristics has been discussed utilizing different electrode materials. The authors have used Ti and Au with work functions as ~4.3 and 5.1 eV, respectively whereas SRO possesses the highest value of work functions as 5.3 eV [15]. In present case, PCMO and Nb:STO exhibit only Ohmic contacts with SRO and Ag acting as the bottom metal electrode. It is known that while PCMO semiconducting oxide is dominated by *p*-type behavior, Nb:STO depicts *n*-type conduction. One can realize that the contact resistance between M and *p*-type dominated PCMO oxide is the largest for M with the least work function. Therefore, for PCMO based memory cells, Ti having the lowest work function demonstrates rectifying I-V behavior i.e. hysteretic characteristics distinctive to

resistive switching properties. However, the contact resistance between M and *n*-type Nb:STO increases as the work function of M enhances. This indicates that Nb:STO based memory cells utilizing Au as the top electrode display hysteretic characteristics during *I-V* measurements. These observations are in good agreement with the fact that the rectifying behavior of *I-V* characteristics is governed by Schottky type barrier height formed at the interfaces. Therefore, an important and critical role of Schottky type barrier can be easily perceive in driving the resistive swathing effect in the fabricated memory cells. Considering the highly reactive nature of Ti top electrode material, it is also necessary to take the accounts of different chemical reactions occurring at the interface. For example, Ti being a more reactive metal can chemically react to the semiconducting oxide through the extraction of oxygen ions during film deposition and subsequent annealing procedure. Such events take place when the oxygen vacancies are injected *via* areas in close proximity to the interfaces. These oxygen vacancies are primarily of donor type and hence, capable of modifying the initial donor concentration of *n*-type Nb:STO semiconducting oxide. The considerable enhancement in the amount of donors ensures improved Ohmic contact conductance behavior. On the other hand, *p*-type PCMO oxide experiences the diminished conductivity which eventually results into an insulating type region of PCMO close to the interface. Such a phenomenon is similar to space charge effects which adds up to oxidize Ti metal. Thus, it evokes resistance at interface since the oxidation of Ti forming non-stoichiometric TiOx has low conductivity.

*I-V* as well as *C-V* behavior demonstrate a hysteretic type characteristics if the resistance switching is area scalable. This is well explained on the basis of Schottky depletion model through the mechanism of electronic trapping or detrapping [24]. In practice, different contributions from the metal electrode work function, electron affinity of the *n*-type Nb:STO semiconducting oxide and interface trap sates residing within low-*k* interfacial layer are taken into consideration while estimating the accurate Schottky barrier height [25]. Interestingly, a noticeable sign related to electronic trapping or detrapping of bandgap states can be perceived within the depletion region for memory cells where resistive switching characteristics are area scalable. Also, due to small read out currents, a shorter retention time (in the range of 102 –103 s) is measured for such cells which endorses the retention time and current variations obtained for only electronic switching. A voltage-induced unidirectional threshold resistive switching has been reported for Au/NiO/Nb:SrTiO3 devices fabricated by pulsed laser deposition. Interestingly, only positive voltage values demonstrate the forming process controlled threshold resistive switching behavior [26].

Further, Schottky barrier at the interface is altered not only *via* the impact ionization but also depend onto the movement of oxygen vacancies driven applied electric field [15]. In another report, for single crystals of self-doped SrTiO3 (STO), the authors have discussed the effect of electrode engineering with variable work function, device geometry and measurement configurations upon the resistive switching. Additionally, the various metal electrode combinations such as Ti and Pt has been exploited to analyze and manipulate the electrical transport i.e. Ohmic or Schottky type across junctions. It has been concluded that the observed resistive switching behavior is greatly influenced by changing the amount of oxygen vacancies only at or near the interface under an effective applied electrical bias [27]. For most of the switching binary or complex transition metal oxides (i.e. solid electrolytes), it is well known that current transport is the collective manifestation of electronic charge carriers as well as the mobile ions and related ionic defects. There exists cationic interstitials, delocalized electrons and oxygen vacancies in hypostoichiometric oxides whereas oxygen interstitials, delocalized holes and cationic vacancies are the major charge carriers in hyper-stoichiometric oxides.

### *Effect of Surface Variations on Resistive Switching DOI: http://dx.doi.org/10.5772/intechopen.97562*

In general, both the electron affinity of metal oxide materials and metal electrode work function define the Schottky barrier height. This implies the fact that the metal work function is believed to be directly related to the Schottky barrier. Nevertheless, in practice, the Schottky barrier height is considerably affected by the formation of metal electrode/oxide interfacial layer providing an additional capacitor i.e. insulator type thin slab. In this context, Cowley and Sze revealed that formation of such capacitive layers evoke a definite and noteworthy drop in voltage which ultimately alters the ideal height of Schottky barrier.

Moreover, the formation of interface between metal electrode and oxide layer is also described in terms of the Helmholtz plane and diffuse double layers [28]. It has been thoroughly discussed that there exists an intrinsic electrochemical potential difference at the interface of metal electrode and oxide which induces the transfer of electrons among oxide layer and metal. Such an event produces dipole layer at the interface due to the movement of electrons ensuing space charge effect. Under applied electrical bias, electronic charge carriers, ions and related ionic defects take part in screening the electric field via the diffuse double layer in the oxide region. However, there is a little screening of electric field at the metal electrode side because of sufficiently high enough concentration of electrons. It is evident that the screening length is extended much deeper inside the oxide layer than that of metal electrode owing to large difference in the concentration of charge carriers and electrons. While the first screening of electric field is caused by the ions residing over the Helmholtz plane, the second screening is primarily due to the electronic charge carriers, ions and related ionic defects present in the diffuse double layer [28].

For the most frequently employed metal oxides such as Ta2O5, HfO2, and SiO2 based memristive devices with compatible metal electrodes like Ta, Hf, and Ti have been extensively studied. For example, HfOx/AlOy-based homeothermic devices depict low-power and homogeneous RS behavior useful synaptic applications [29]. In vanadium-based devices, Lin *et al.* demonstrate excellent RS characteristics through interface where localized transition occurs [30]. Hsieh *et al.* discussed mitigation of critical issue of short-term relaxation in HfOx/CeOx derived RRAM devices [31]. In case of bilayer structure devices employing Ni/SiNx/HfO2/p++-Si stacks, a self-rectifying has been shown to improve the sneak-path current emerging in the crossbar arrays fabrication process. Such bilayered devices provided enhanced rectification ratio usually >104 . It has been revealed that during negative bias, the formation of large Schottky barrier of HfO2 facilitate the reduction in current [32]. The progressive and consistent investigations have shown that interfacial layer exists inherently between the metal electrode and oxide. However, the oxygen affinity of metal as well as chemical and thermodynamic strength of the oxide layer determines the degree of oxidation of metal electrodes. Thus, the performance of fabricated device is highly dependent onto the extent of interfacial layer altering the initial crystal structure of oxide layer [33].
