In conclusion, I have studied SiOx based resistive switching memories, whose active regions were conned into few nanometer wide graphene gaps. Since the formation of the switching region is electric eld driven, it can be anticipated that the intrinsic resistive switching in the SiOx layer also takes place within a similarly short length scale. The small dimensions of the switching region was conrmed by the low electroforming voltages. The devices still show fast switching, high endurance, high OFF/ON ratio and multilevel programming. The resistive switching capability of the SiOx memristive system has not been demonstrated yet in the sub-10nm size.
The time resolved measurement revealed that the device operation is governed by multiple physical time scales. The resistance transitions do not happen right after the bias voltage reaches the set or reset region. The device stays in the initial state for a certain period of time (τset, τreset) and after that the resistance changes abruptly. The timescale of the transitions between the two resistance states are below our experimental resolution (50ns). The modest variation in the set/reset voltage induces exponential change in the set/reset time, referring to voltage driven mechanisms.
Besides the above mentioned switching times, another fundamental time scale, the dead time was also identied. After switching OFF the device, it can not be set to ON state again as long as the dead time has not passed. The detailed study of dead time revealed that unlike the set and reset time, its length does not depend on the driving conditions. However, increasing the temperature by 50K the dead time decreased almost two order of magnitudes. These results suggest that instead of voltage driven mechanism, thermally assisted structural rearrangements may take place during the dead time. This phenomenon has fundamental technological impact since it combines the positive properties of unipolar and bipolar switches, using unipolar voltage pulses both resistance states can be achieved at zero bias.
Summary 7
The major conclusions of this Ph.D. work are summarized in the following thesis statements.
1. I developed a novel measurement setup for the controlled electrical thinning and breakdown of nanofabricated junctions. This included the development of a new high vacuum sample holder, the assembly of an optimized measurement setup and the development of a versatile measurement program. This measure-ment system enables us to reduce the active region of nanofabricated devices well below the resolution of present lithographic techniques. The specialty of this system is the pulsed breakdown technique, which allows us to expose the device to much shorter voltage intervals than in real-time feedback-controlled systems. Applying this setup I could establish a few nanometers wide gaps in nanofabricated Ag wires. I have extended this method on single-layer chemi-cal vapor deposited graphene nanostripes achieving nanogaps with measurable tunnel current with a yield over (≈ 98 %). The wafer scale growth of CVD graphene enabled the simultaneous fabrication of a large ensemble of devices on a single chip. The statistical analysis of hundreds of devices reveals typical gap sizes between 0.3nm and 2.2nm [13].
2. I studied the resistive switching phenomena of nanofabricated Ag-Ag2S-Ag memristors [2]. I have demonstrated that the resistive switching can be es-tablished using a simplied sample design lacking the conventionally employed inert electrode. In this design a simple lithographic step is sucient to fab-ricate the base structure, which is an asymmetrically shaped Ag nanowire.
I have established the ultrasmall resistive switching region by the controlled
electromigration of the Ag nanowires, and the in-situ sulfurization of the such created nanogaps. I have demonstrated that these devices exhibit the con-ventional switching characteristics of nanometer-scale Ag2S memristors, such that the direction of the switching is governed by the inhomogeneity of the local electric eld due to the geometrical asymmetry of the device. In simi-lar devices I have also demonstrated stable room temperature atomic switching phenomenon, indicating that the surrounding Ag2S matrix stabilizes the atomic switching process.
3. I analyzed the inuence of the environmental conditions on the electrical break-down of graphene nanostripes [3]. The systematic study of the breakbreak-down power as the function of pulse length and pressure revealed two fundamentally dierent breakdown processes. I have found, that in high vacuum a signi-cantly higher power was needed to achieve the breakdown than in atmospheric pressure air. Using a thermal model I rescaled the breakdown power to the maximal local temperature of the graphene stripe. Assuming thermally acti-vated processes I estimated the activation energies of the physical mechanisms involved in the breakdown. The signicantly dierent activation energies are consistent with oxidation in air and sublimation in high vacuum. Using two dierent substrates (SiO2 and Si3N4), I found that the oxygen content of the SiO2 substrate does not play role in the breakdown process.
4. I investigated the resistive switching phenomena of graphene-SiOx-graphene devices [4]. The intrinsic resistive switching in the SiOx layer was conned under a few nanometers wide graphene gap, resulting in a yet unexplored, sub-10nm size-scale switching region of SiOx. My detailed electrical charac-terizations revealed that these ultrasmall devices exhibit a signicantly smaller electroforming voltage than conventional SiOx switches, such that the further benecial properties of larger devices, like fast switching speed, excellent en-durance and data retention are maintained. I have performed detailed time resolved measurements to identify the physical timescales governing the device operation. I have demonstrated, that the switching is not a gradual transition:
the device keeps its initial state for a certain period of time after the voltage is applied, and nally an abrupt resistance change is observed, which is faster than the ≈ 50ns temporal resolution of the measurements. I demonstrated that a modest, linear decrease of the set/reset voltage induces an exponential slowdown of the set/reset operation. I have also identied another fundamen-tal time-scale, the dead time. I found, that after switching OFF the device, it cannot be set to the ON state again as long as the dead time has not passed,
even if the driving signal is sucient for a set transition. The detailed study of the dead time revealed that its length does not depend on the driving con-ditions, however it could be decreased signicantly by a modest increase of temperature, indicating a thermally activated rearrangement of the switching region.
Acknowledgments 8
Throughout the years of my PhD work I have received support from many people to whom I would like to express my gratitude. First of all, I am very grateful to my supervisor András Halbritter for giving me the opportunity to do my PhD in his group and for coordinating my work. The discussions and his ideas were always inspiring. A special thanks goes to Péter Makk for helping me with various problems and for the many discussions and advice since my BSc studies.
I also express my gratitude to Michel Calame for inviting me to his research group and for the fruitful collaboration and discussions. I acknowledge Cornelia Nef for her help during my stay in Basel and our cowork for developing the high yield graphene nanogap fabrication technique. I thank Maria El Abbassi for sharing our research experiences and for our work in respect of graphene breakdown mechanisms. I also thank them for fabricating and sending me the graphene samples.
I am also grateful to Miklós Csontos who gave a lot of advice and helped in the interpretation and discussion of the results. I thanks Mészáros Gábor for building me the multi-channel current amplier. Further I want to express my gratitude to the people with whom I worked together in our group, Ágnes Gubicza, Botond Sánta, András Magyarkuti and special thanks for Zoltán Balogh with whom I started my work together and we helped each other a lot.
I am very grateful to János Volk for providing me the possibility to use the nanofabrication facilities of MFA for the careful reading of my thesis. I also thank István Lukács for his invaluable help and advice in device fabrication.
I am also pleased to thank my friends and colleagues: Szabolcs Csonka, Zoltán Scherübl, Ádám Butykai, Dávid Szaller, Gerg® Fülöp and Bálint Fülöp. They all contributed to lively working days and many discussions about physics and other
topics.
I am also grateful to the support from the mechanical workshop. Béla Horváth, Sándor Bacsa and Krisztián Németh for manufacturing the sample holders and for the lots of ideas. Not to forget Edit Honti, Tímea Varga and Edina Beck from the secretary, who make all administrative matters run smoothly.
Finally, I thank my family for all the support they provided me over all the years of my studies.
Publications related to the thesis statements
[1] C. Nef, L. Pósa, P. Makk, W. Fu, A. Halbritter, C. Schönenberger, and M.
Calame, High-yield fabrication of nm-size gaps in monolayer CVD graphene;
Nanoscale, 6, 72497254 (2014)
[2] A. Gubicza, D. Manrique, L. Pósa, C. Lambert, G. Mihály, M. Csontos, and A.
Halbritter, Asymmetry-induced resistive switching in Ag-Ag2S-Ag memristors enabling a simplied atomic-scale memory design; Scientic Reports, 6, (2016) [3] M. El Abbassi1, L. Pósa1, P. Makk, C. Nef, K. Thodkar, A. Halbritter, and M. Calame, From electroburning to sublimation: substrate and environmental eects in the electrical breakdown process of monolayer graphene; Nanoscale, 9, 1731217317 (2017)
[4] L. Pósa, M. El Abbassi, P. Makk, B. Sánta, C. Nef, M. Csontos, M. Calame, and A. Halbritter, Multiple Physical Time Scales and Dead Time Rule in Few-Nanometers Sized GrapheneSiOx-Graphene Memristors; Nano Letters, 17, 67836789 (2017)
1
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