In this work, we investigated resistive switching behavior of CrO x thin films grown by using sput tering technique. Conventional I-V measurements obtained from Ag/CrOx/Pt/Ti/SiO2/Si structures depict the bipolar switching behavior, which is controlled by formation/dissolution processes of Ag conducting filaments through electrochemical redox reaction under external electric field driven. Conductive atomic force microscopy (C-AFM) technique provides the valuable mapping images of existing Ag filaments at low resistance state as well as the characteristics of filament distributions and diameters. This study also reveals that where the higher amplitude of topography is, the easier possibility of forming conducting filament paths is on CrO x surface films.
Resistance switching random access memory (RRAM) is a nonvolatile memory that works by changing resistance across the dielectric material layer referred to as a memristor. RRAM offers a variety of advantages such as fast write/erase speed, high integrated density, long retention and low power comsumption [
Mechanism of filamentary-controlled type relates to the formation and rupture processes of tiny conducting paths through the memristor structure corresponding to low resistance state (LRS) and high resistance state (HRS), respectively. The observation of the filament is done by modern techniques, consisting of scanning tunneling microscopy (STM) [
In this work, we have investigated the resistive switching effect of Ag/CrOx/Pt device prepared by using sputtering method. C-AFM technique is used to study the nanoscale electrical property as well as the switching mechanism of CrOx thin films. The surface mapping results show the distinct characteristics between LRS and HRS of device. Furthermore, the correlation between surface topography and capability of filament formation has been discussed in detail.
The 100-nm-thick chromium oxide films were deposited by using the DC sputtering technique at room temperature, from metallic Cr target (99.95%) on Pt/Ti/SiO2/Si commercial substrate. The deposition process was executed under the total pressure
Microstructure and surface morphologies of the films were obtained by D8 Advance (Bruker) X-ray diffractometer (XRD) with Cu Ka radiation (l = 0.154 nm) and scanning electron microscopy (SEM). Current-voltage (I-V) measurements were carried out on a probe station using a semiconductor characterization system (Keithley 4200 SCS). The voltage profile for the I?V measurement was 0 V à −Vmax à 0 V à +Vmax à 0 V. Topography images and current images during C-AFM scanning were detected by Veeco Dimension 3100 AFM system with DC sample bias.
partial pressure (6%), so the structure of film remains primarily amorphous or poor cystallinity. This result is similar to that of other studies about chromium oxide thin films [
corner of
The direct observation of resistive switching mechanism of CrOx film has been justified by C-AFM. In our work, 10-nm-thick CrOxA/Ag/glass structure was prepared for the test. Here, Ag layer plays a role as a bottom electrode while Pt-coated tip of AFM system plays a role of top electrode (
After completing the scanning with the down direction, the process is executed again with the up direction. For comparison, the latter scanning process performs only on the below half part of the mapping images. At this time, negative voltage of −1 V is applied on the Ag bottom electrode. It shows that conductive regions are completely vanished and previous bright pots has been totally disappeared (the below half part of
In order to specify the characteristics of conducting filaments, the mapping images are investigated on the 500 nm × 500 nm area of CrOx surface film at very slow speed. Topography image of CrOx film surface firstly records and shows nanometer scale roughness (rms value of 1 - 2 nm) as seen in
smooth morphology can provide significant advantages for C-AFM technique in which the measurement is tested in contact mode. The high rms value or rough surface may affect unexpectedly the tip during scanning. Figures 5(b)-(e) represented the 2D and 3D current mapping images of LRS and HRS of device corresponding postive and negative bias voltage, respectively. In this canning region, it is realized that a few of conductive filaments appears randomly in almost insulator area at LRS. Current mapping values are high fluctuation from hundreds of picoampere to several nano-ampere. These values are the same as in some other studies [
In addition, cross-section data at AB, CD and EF lines shows the close relation between surface morphology and leakage current value, as seen in
In summary, we propose the nonvolative memory device based on chromium oxide thin film and study behavior of resistive switching by using C-AFM technique. Current mapping images confirm that the electrochemical growth and dissolution of silver metallic filaments control the LRS and HLR of memory devices. Experimental results also suggest that the high amplitude of topography leading a high possibility of filament formation on the film surface. However, high fluctuation of filaments is still the crucial challenge to develop performance of memory devices based on resistive switching. Our work would be useful in understanding characteristics of filament paths in transition metal oxide thin films.
This work was funded by National Foundation of Science and Technology Development of Vietnam (NAFOSTED- 103.02-2012.50). The authors gratefully acknowledge Prof. Jaichan Lee (Sungkyunkwan University, Republic of Korea) for supporting C-AFM analysis and Prof. Taekjib Choi (Sejong University, Republic of Korea) for valuable discussion.
Kim NgocPham,Kieu Hanh ThiTa,Lien Thuong ThiNguyen,Vinh CaoTran,Bach ThangPhan, (2016) Surface Mapping of Resistive Switching CrOx Thin Films. Advances in Materials Physics and Chemistry,06,21-27. doi: 10.4236/ampc.2016.63003