Statistical signature of electrobreakdown in graphene nanojunctions

The resistive switching (RS) behavior of resistive random access memory (RRAM) based on oxygen vacancy (VO) conduction is significantly affected by the interface properties between metal electrode and oxide layer, yet the modulation between the RS behavior and the physico-chemical properties of the interface is still not very clear. In this study, the correlative role of Ta/HfO2 interface with the RS behavior in HfO2-based RRAM is explored at atomic level. First-principles thermodynamic calculations show that the strong interaction between three-fold oxygen vacancies (VO3) leads to a formation of VO3-based conductive filament (CF) along direction perpendicular to the interface. Four-fold oxygen vacancies (VO4) make a major contribution to the re-formation and growth of CFs during the set process by diffusing into the residual filaments. The results of electronic properties further indicate that as the number of VOs perpendicular to the interface increases, the charge redistribution between O and Ta atoms at the interface is significantly increased, and more electron clouds are gathered around VOs. This is the underlying mechanism of forming a conductive channel. This study reveals the important regulation mechanism of the interface characteristics between metal electrode and oxide layer in RRAM on the formation and growth of VO-based CFs.

The resistance switching (RS) behaviors of La-doped SrTiO3 ceramics before and after the electrical forming process (E-forming) have been investigated in order to clarify the RS mechanism. The formation processes for the double Schottky barriers (DSBs), such as reoxidation and E-forming, are important to realize stable RS properties, and Joule heating strongly influences the RS behaviors after the E-forming. Thermally stimulated current measurements clearly indicated the contribution of defects related to oxygen vacancies to the current conduction through DSBs and RS phenomena both before and after the E-forming. Experimental results suggest that the electron trapping in the defects and/or the defect migration induced by the voltage application and Joule heating change the space-charge distribution near the grain boundaries, resulting in the change in the resistance state.

Rainer Waser is a physical chemist. Throughout his life, he has worked at the frontiers among several disciplines from natural to engineering sciences by pursuing the search for fundamental understanding of certain phenomena, shaping the technology of our current world. Since the early age he has always shown a singular aptitude for scientific disciplines, and his life has been characterized by outstanding achievements and granted awards in many fields. After a pre-doctoral stay in the Electrochemistry Group of the University of Southampton, he has successfully accomplished his doctorate in electrochemistry at the TH Darmstadt under the supervision of prof Konrad Weil setting up a complete computer-controlled measuring station – from the circuit design to the circuit board production to the measuring programs in Z80 assembler. After his doctorate in 1984, he went to the Philips research laboratory in Aachen to investigate the causes of long-term degradation of the insulation resistance in perovskite ceramics under DC voltage and temperature stress in the electronic ceramics group. He succeeded in elucidating the phenomenon of long-term degradation as an ion transport process that leads to the formation of a pn junction in a dielectric. For this work, together with Prof. Härdtl at the University of Karlsruhe and his PhD student, Tudor Baiatu, he received the American Ceramic Society award from for the best paper of the decade furthermore was also rewarded ranking second in the European finals in the Philips European Contest for Young Scientists and Inventors with a scientific-theoretical paper on the sociology of music during his chemistry studies. After his appointment to the Institute of Materials in Electrical Engineering (IWE-2) at RWTH Aachen University in 1992, Rainer Waser initially worked in the areas of dielectric and ferroelectric materials and was invited to join the large DARPA project on high-epsilon materials for DRAMs as the only non-U.S citizen participant. He has therefore also received an award from the IEEE in 2000 for working in these areas. In 1997, Rainer Waser took over the management of the currently named “Institute for Electronic Materials” (PGI-7) at Forschungszentrum Jülich. In 2001 was appointed to lead the organization of the IFF Spring School named “New Materials in Information Technology” being also the title of his well-known textbook “Nanoelectronics and Information Technology”. He always has directed his research group on promising resistive switching in the following years. Through a first publication in 2006[1] and especially his publication together with Masakazu Aono in 2007,[2] he established an explanation for resistive switching, which has been known since the 1960s. He was able to show that an oxygen ion movement and a coupled electrochemical redox process on the nanometre scale in the crystal lattice of the oxides is responsible for the switching property. Together with Matthias Wuttig, he succeeded in acquiring the SFB 917 “Nanoswitches”, in which redox-based and phase-change-based switching has been studied since 2011. Rainer Waser's work was awarded the DFG's prestigious Leibniz Prize in 2014. On the occasion of the 60th birthdays of the IWE and himself, he organised a symposium “Crossing Borders and the Craziness in Science” in September 2015 – as well as a costume party “60 Years of Craziness” at Kasteel Vaalsbroek, inspired by the Tomorrowland festival in Belgium. During a sabbatical period, at Stanford University in 2016, which indeed it turned a very fruitful year, Rainer Waser and his collaborators acquired the structural change project “NEUROTEC – Neuro-inspired artificial intelligence technologies for the electronics of the future in the Rhenish Revier” in 2019. Together with neighbouring institutes at FZJ and RWTH and selected industrial companies in the region, the project is concerned with researching and developing the basic technologies for neuromorphic computing based on resistive/memristive devices and other alternative concepts to conventional von Neumann computing technology. Rainer Waser has always relied on a strong group of scientists from different disciplines, each driven by the will to achieve goals together. He has certainly achieved this interdisciplinary ideal state to a very large extent and this explains the success of our working group. The special issue dedicated to the 65th anniversary of Rainer is focused on the fundamental understanding of the operation principles and physical mechanism in memristive devices. It includes as well the materials perspective and nicely demonstrates how a combinatory approaches benefit our knowledge to improve the memristive performance and widen the application horizon for resistive switching random access memories. Leading groups have contributed to this special issue presenting works on electrochemical metallization memories (ECM, CBRAM or PMC), valence change memories (VCM or OxRAM), ferroelectric memories (FeRAM), and phase-change memories (PCM). The presented works go far beyond purely memristive characterizations and functions and discuss nanoionics effects, biomimicking properties, combination of materials (e.g. bilayer structures, 2D, organic-inorganic devices) and processes, networking and functional optimizations. The demonstration of the vital importance of multidisciplinary studies on the material properties, microscopic processes, combined with structural and chemical analysis, electrical measurements and modelling is the highlighted of this special issue. ECM/CBRAM devices are based on redox reactions and transport of cations, typically Ag or Cu (but as well others). However, the variability in their characteristics is restricting their reliable operation. In aelm.202100209 Kim et al. demonstrate that this stochasticity can be significantly reduced by using cone-type devices. This shape of the cells helps on focusing the electric field and achieving stable (multiple) resistive states, being of highest importance for the synaptic functions. Zhuo and co-authors is presenting a dynamic compact model of diffusive and drift memristors in aelm.202100696. Using Ag:SiO2 switching film the experiment and model were found nicely coinciding. Spike-timing dependent plasticity of artificial neurons based on these two types of memristors are used to demonstrate the validity of the theoretical model. 2D materials are especially attractive for thin film flexible and/or transparent electronics. Using Ag cation migration Farronato et al. created a 3D transistor based on 2D MoS2 material in aelm.202101161. Advantageous is the short channel length allowing for fast operation. A NAND flash type structure based on these memtransistors is showcased, where the individual memory devices can be selected for write and read operations. Lanza et al. reported in aelm.202100580 on 150 nm × 150 nm Au/Ag/h-BN/Au memristors. The 2D h-BN has high in-plane conductivity and is ensuring low operation temperature (≈310 K) and thus, minimizing thermal effects and resulting in less stable filaments, whereas at high current compliances the formed low resistive states are stable. Lateral ECM/CBRAM devices are also frequently discussed but their development is challenging due to the typically large distance between the electrodes and related high operating voltages. However, in aelm.202100897 Chamele and co-authors reported on approach allowing to overcome this issue. Using an approach of low temperature oxidation of Cu, deposited on WO3 they created a bilayer system of Cu2O/Cu-WO3 where the copper oxide provides a low resistive path for electrons, enabling filament formation over large distances. Thus, they succeeded in bridging a 14 µm long channels below time of 3 seconds and using only 2 V bias. VCM devices have different advantages compared to ECM, however the mechanism and control over the processes is comparably challenging. The research on clarifying further details on the mechanism and the efforts for improving the devices is an ongoing challenge. TaOx is currently one of the most favoured materials for memristive cells. In the work aelm.202100936 Heisig et al. have studied by spectromicroscopy the electronic structure of the conductive filaments formed in Ta2O5 devices. They found the filament is composed by reduced oxide phase with approximately 20% oxygen vacancies and found no metallic phase. The role of the thermodiffusion has been discussed, based on finite element simulations. The authors showed that thermal diffusion is not of primary importance for the for the forming process but might be supportive by accelerating ion mobility and stabilizing the filament. Sugawara and co-authors are studying in aelm.202100758 TaOx-based VCN devices by low frequency noise spectroscopy. Based on the temperature dependence of the noise spectra it was concluded that multiple trap levels exists over a large range of resistance values, confirming that these devices are particularly advantageous when used for analogue applications. S. Yoo et al. have demonstrated in aelm.202101025 how resistive switching behaviour can be tuned by controlling the internal ionic dynamics. They have showed by systematic engineering of the material properties and device structure they can modulate the temperature as a second state variable. The temperature is influencing the ion motion and provides and is a base for implementation of timing and rate-based rules such as spike timing dependent plasticity. The tunable STDP characteristics have different time constants and combined with a post synaptic neuron can capture the temporal correlation in the input streaming events. Cheng et al. have demonstrated in aelm.202100669 that memristive devices can also mimic the biological function of the astrocyte cells in brain. The authors named the devices based on yttria stabilized zirconia astrocyte memristors. They are characterized with lower forming and SET voltages and have endurance of over 1011 cycles. Remarkable is that that the nonlinearity in the current-voltage characteristics can be depressed and as well reversed by applying refresh operation in analogy to biological astrocytes. This new functionality is of significant advantage for the future development of the brain inspired computing. In their work on Ti/TiOx/Pd memristive devices aelm.202100827, Hu et al. have shown the importance of the morphology of the bottom electrode on determining the type of resistive switching. In case of presence of hillocks formed during the lift-off process the electric field is locally enhanced, resulting in a filamentary type switching with non-volatile characteristics. In case the formation of spikes is suppressed (flat bottom electrode) the devices are switching in a volatile manner due to the internal ion dynamics modulated by the concertation gradients and formed Schottky barrier. Ti oxides (as well as many other oxides) can be prepared not only by sputtering approach but also electrochemically. In aelm.202200215 Tominov et al. have studied the preparation and properties of anodically prepared TiOx nanostructures. Depending on the anodization conditions different compositions (oxidations states) have been detected by X-ray photoelectron spectroscopy. The device properties and characteristics were found to strongly depend on the preparation conditions and related distribution of different Ti-species. The work demonstrates the potential of electrochemical anodization in preparing memristive devices with targeted properties. In aelm.202200378 the authors propose electrochemical memory with a vertical structure demonstrating small size, scalable channel volume and low programming energy. Using simulations with advanced algorithms the suitability of these memories for high density arrays applications has been confirmed. The theoretical study has also suggested that ECRAM can be further improved by balancing the weight update characteristics. The switching phenomena in oxide memristors is also studied in a combinatory modelling research presented by Ascoli et al. in aelm.202200182. Special focus was set on the fading memory. The work presents experimental verification for the fading memory, the importance of the RESET process and continues with a thorough theoretical analysis on a system level. The presented models help to better understand the mechanism of the emergence of history effects and to identify the crucial factors to tune this non-linear phenomenon. VCM memristive devices can also be used as a content addressable memory (CAM) as demonstrated in aelm.202101198. Non-volatile analogue CAM has been recently presented, however CAM blocks cannot be easily inserted within larger adaptive systems. Pedretti et al. reports on differentiable CAM where utilizing non-volatile memories with analogue outputs enables learning and tuning of the memory operation and performance. One advantage of memristive technologies is that there is no restriction in respect using different materials. Both organic and inorganic materials are widely used. In aelm.202100426 Tuchman and co-authors offered a hybrid stack composed of porous inorganic matrix, permeated by an ionic liquid. This advantageous combination allows for over 109 switching cycles and switching with energy as low as 2.7 pJ. The fabrication process the authors are suggesting allows for control over the channel length and gate dimensions. The suggested material combination is indicative for the potential of using this approach with various different matrixes and liquid electrolytes in the future. In aelm.202200323 the authors reported as well on hybrid structure, however combining two inorganic materials. Using HfO2-based OxRAM (VCM) and Ge-Se-Sb-N ovonic threshold switch back-end selector is proposed for high-density binarized spiking neural networks synaptic weight hardware implementation. Optimization of the network and training procedures ensured high operation efficiency. The critical and essential component in the stack is the ovonic memory where extensive experimental studies combined with a novel Monte Carlo model is allowing for in-depth understanding of the field accelerated switching dynamics with nucleation as rate limiting factor. Crystallization is important kinetic limitation for phase change memories as well. Müller et al. discuss in aelm.202100974 in detail the crystallization kinetics of chalcogenides. Apart from purely phase change memories chalcogenides show due the phase transition variety of optical changes/properties highly relevant to memory storage, neuromorphic computing and as well – optical switches. The authors compare the properties of Sb2Se3 with these of well-known phase change materials such as GeSb2Te4 and Ge2Sb2Te5. The differences in the kinetics and resulting properties are attributed to differences in the bonding in the crystalline state. Ferroelectric memories belong to the “memristor family”. Often such devices are sowing multiple functionalities combining more physical effects. In aelm.202100662 Jiang et al. reported on significant stabilization of the remanent polarization in HfZrO4 devices. To achieve this the authors have inserted an ZrO2 layer. This is prolonging the wake-up period and the fatigue process is postponed. The device endurance is over 1010 cycles pointing out reliable prospective for applications. The memristive phenomena in general is based on both ionic and electronic effects where these effects are typically interrelated. Nevertheless the main focus of the developing electronics has been set on purely electronic effects and materials. In aelm.202100645 Terabe and co-authors are reviewing and highlighting the importance of the nanoionics and its importance for complementing electronic components. The potential of the ionic nano architectonics is demonstrated on the example of several impressing applications based on local control of ionic motion and electrochemical reactions. In conclusion, the special issue on the occasion of the 65th anniversary of Rainer Waser is including most recent developments in the field of memristive devices and applications related to the underlying physical phenomena, chemical reactions structural changes, electrochemical phenomena and materials science.

Electro-optical modulation on resistive switching behavior in Ag/BaTiO3/LaNiO3 device

In recent years, nanoionics-based memristive devices with resistive switching behaviors have garnered significant attention as promising candidates for next-generation nonvolatile memory known as resistance random access memory (RRAM) due to their potential high scalability and low-power consumption. In addition, memristive devices are also promising for logic and neuromorphic computations. The memristive devices typically have a simple stacked structure of metal-insulator-metal (MIM), where the resistance can be reversed changed through local ion migration and electrochemical reaction after an initial electroforming process. Nanoionics–based memristive devices can be classified as cation migration cells and anion migration cells. When an electrochemically active electrode such as one made of Ag or Cu is used in a MIM device, resistive switching occurs through formation and annihilation of a metallic atom bridge resulting from the migration of highly mobile cations such as Ag+ and Cu+ ions. This type of nanoionics-based device is referred to as an atomic switch or electrochemical metallization systems.[1] Resistive switching behavior can also be observed for a MIM device without an electrochemically active electrode. In this case, the migration of anions, usually oxygen ions plays a crucial role. Local oxygen ion migration, which is better described as the migration of oxygen vacancy (V O ‥) and a resultant change in the electronic barrier at the metal electrode/TMO interface, is associated with the resistive switching behavior. In general, the cation migration cell and anion migration cell both need electroforming process to obtain resistive switching behaviors. For the cation migration cells, the formation of Ag or Cu bridges in the forming process has been directly observed by TEM or in-situ TEM.[2]In contrast, the mechanism of the electroforming process in the anion migration cell is still uncertain due to the lack of direct characterization. In this study, the electroforming process and resistive switching behaviors of the anion migration cell of Pt/WO3-x/Pt have been investigated as a function of temperature. As shown in Figure 1a, electroforming process with soft-breakdown was triggered as the sweep voltage was raised to about 6.0 V at 296 K and 396 K. But, at 110 K, no soft-breakdown was observed even by increasing the sweep voltage to 20 V, which indicates that the devices cannot be electroformed at ultralow temperature. In contrast, the device shows resistive switching behaviors at ultralow temperature after being electroformed at room temperature, see Figure 1b. In order to real-time observe the structure changes during electroforming process, we prepared a Pt/WO3-x/Pt devices that operated inside a TEM, see the inset of Figure 1g. As reported in Figure 1c to 1f, the initial amorphous WO3-x layer gradually crystallized by applying voltage weeping. And, crystallized nanoparticles of Magnéli phase were observed, as shown in Figure 1g. Actually, the current level of in situ measurements is much smaller than that in the conventional electroforming processes. This may be the reason why no conductive filament composed of the Magnéli phases was observed. The crystallization and formation of the conductive filament in WO3-x layer have been observed by cutting the electroformed device in our previous work.[3] Based on above results, it is proposed that the Joule heating effect plays a key role in the electroforming process. Under ultralow temperature, the current level of the device is too low to generate enough heating effect to introduce the filament formation. Moreover, the heat dissipates much quickly at ultralow temperature. Therefore electroforming process could not be triggered at ultralow temperature. In contrast, the resistive switching behavior obtained after electroforming process mainly occurs in the nanogap region between the electrode and the conductive filaments, where the V O ‥ migration under external electric field determines the resistive switching process. So, the resistive switching behavior can be observed at low temperature. Based on these results and discussions, it is reasonable to conclude that Joule heating effect domains the electroforming process, instead of the resistive switching processes. Terabe, K.; Hasegawa, T.; Nakayama, T.; Aono, M., Nature 2005, 433, 47-50. Yang, Y.; Gao, P.; Gaba, S.; Chang, T.; Pan, X.; Lu, W., Nat.Commun. 2012, 3, 732. Tan, Z. H.; Yang, R.; Terabe, K.; Yin, X. B.; Zhang, X. D.; Guo, X., Adv. Mater. 2016, 28, 377-384. Figure 1

Resistive random access memory based on gallium oxide thin films for self-powered pressure sensor systems

The volatile and nonvolatile resistance switching (RS) characteristics can be, respectively, used for the selector and memristor, which have received much attention. Thus, it is essential to find a simple and effective method to control the specific RS behavior of NbOx materials due to the co-occurrence of memory RS and threshold RS behaviors. Here, the NbOx film with a thickness of 100 nm was prepared by magnetron sputtering at 80 °C. The tungsten steel tip/NbOx/Pt device exhibited the co-existence of memory RS and threshold RS behaviors. By properly regulating compliance current (Icc), the specified memory and threshold RS behaviors were observed: the memory RS behavior occurred with an Icc of 5 mA, the threshold RS behavior occurred with an Icc of 10 mA, and integrated one selector-one-RRAM (resistive random access memory) (1S1R) RS behavior occurred with an Icc of 50 mA. Moreover, individual RS behavior showed good performance, e.g., good stability of memory RS, good repeatability and concentrated voltage distribution of threshold RS. The memory RS behavior occurred mainly due to the formation and fracture of oxygen vacancy conductive filaments (CFs). Meanwhile, mediated by local Joule heating, thermally induced conductivity change was responsible for the threshold RS behavior. Under an Icc of 50 mA, the oxygen vacancy CFs and a thermally induced conductivity change triggered the 1S1R RS behavior, which significantly suppressed the leakage current in RRAM 3D integrated structures. This work provides an efficient and convenient method for modulating and obtaining the desired RS behavior and better understanding the conversion mechanism between them.

Biodiesel production from Chlorella Vulgaris microalgal-derived oil via electrochemical and thermal processes

The advent of memristors and the continuing research and development in the field of brain-inspired computing could allow realization of a veritable “thinking machine”. In this study, ZnO-based memristors were fabricated using a radio frequency magnetron sputtering method. The ZnO oxide layer was prepared by incorporating silver nanocrystals (NCs). Several synaptic functions, i.e. nonlinear transmission characteristics, short-term potentiation, long-term potentiation/depression, and pair-pulse facilitation, were imitated in the memristor successfully. Furthermore, the transition from synaptic behaviors to bipolar resistive switching behaviors of the device was also observed under repeated stimulus. It is speculated that the switching mechanism is due to the formation and rupture of the conductive Ag filaments and the corresponding electrochemical metallization. The experimental results demonstrate that the Ag/Ag:ZnO/Pt memristor with resistive switching and several synaptic behaviors has a potential application in neuromorphic computing and data storage systems.

Multi-factors-regulated multi-level down-scalable and robust memristors

Scaling-down characteristics of nanoscale diamond-like carbon based resistive switching memories

We present resistive switching (RS) behavior of few-layer hexagonal boron nitride (h-BN) mediated by defects and interfacial charge transfer. Few-layer h-BN is grown by metal-organic chemical vapor deposition and used as active RS medium in Ti/h-BN/Au structure, exhibiting clear bipolar RS behavior and fast switching characteristics about ∼25 ns without an initial electroforming process. Systematic investigation on microstructural and chemical characteristics of the h-BN reveals that there are structural defects such as homoelemental B-B bonds at grain boundaries and nitrogen vacancies, which can provide preferential pathways for the penetration of Tix+ ions through the h-BN film. In addition, the interfacial charge transfer from Ti to the h-BN is observed by in situ X-ray photoelectron spectroscopy. We suggest that the attractive Coulomb interaction between positively charged Tix+ ions and the negatively charged h-BN surface as a result of the interfacial charge transfer facilitates the migration of Tix+ ions at the Ti/h-BN interface, leading to the facile formation of conductive filaments. We believe that these findings can improve our understanding of the fundamental mechanisms involved in RS behavior of h-BN and contribute a significant step for the future development of h-BN-based nonvolatile memory applications.

Metal–semiconductor interfaces play a crucial role not only for regulating the electronic conduction mechanism but also in determining new functionalities in nanosized devices. In this work, we reported the investigation of the junction properties of single ZnO nanowires (NWs) asymmetrically contacted by means of a Pt electrochemically inert and a Cu electrochemically active electrode. At low applied voltages, these devices operate as diodes where the conduction mechanism was found to be dominated by the Schottky barrier at the Cu/ZnO interface. Junction parameters such as the Schottky barrier height, the ideality factor and the series resistance have been analyzed according to the thermionic emission theory. Different methods for parameter retrieval from I–V–T measurements are discussed and compared. A potential fluctuation model is considered in order to account for barrier inhomogeneities, revealing the presence of two Gaussian distribution of barrier heights. On the other hand, new device features arise from electrochemical dissolution and migration of Cu ions along the NW when high electric fields are implied. These electrochemical processes are underlaying the resistive switching and memristive behavior observed in single ZnO NWs, as suggested also by direct observation of Cu nanoclusters along the nanostructures after the switching events.

Electromechanical delay (EMD) represents a series of complex processes of converting an electrical stimulus to a mechanical response. To quantify the contribution of electrochemical and mechanical processes of EMD in the human biceps brachii muscle over a wide range of elbow joint angles, we determined the onset of muscle contraction and the beginning of force development by recording acceleration of skin surface over the muscle and elbow flexion force, respectively. Ten healthy male volunteers underwent two experimental sessions, in which submaximal paired-pulse stimuli were applied percutaneously to the resting biceps brachii muscle at 10 different elbow joint angles from 40° to 130° (0° represents full extension). The electrical stimulation induced repeatable contractions, in which the test-retest reliability of time parameters was sufficiently high (intraclass correlation coefficient=0.84-0.88). The time for electrochemical process ranged between 3.1±0.8 and 3.6±0.9 ms and was independent of elbow joint angle (P=0.64). The time for mechanical process and the total duration of EMD, however, were significantly greater at elbow flexion positions than at 40°, the most extended position in this study (P<0.05). Regression analysis revealed that at elbow flexion positions, the time for mechanical process increased significantly with decreasing the muscle-tendon length of the biceps brachii calculated from a musculoskeletal model (R=0.54, P<0.001). These results suggest that, in the human biceps brachii muscle, the prolongation of EMD at short muscle-tendon length is not attributed to the impairment of the electrochemical process of muscle contraction but to the increased slack within the muscle-tendon unit.

Ultra-fast electronic and thermal processes for the energy deposition mechanism during femtosecond laser ablation of Si have been identified by means of atomic force microscopy and Raman scattering techniques. For this purpose, Si targets were exposed with 800-nm, 25-fs Ti:sapphire laser pulses for different laser fluencies in air and under UHV (ultra high vacuum) conditions. Various nano- and microstructures on the surface of the irradiated samples are revealed by a detailed surface topography analysis. Ultra-fast electronic processes are dominant in the lower-fluence regime. Therefore, by starting from the ablation threshold three different fluence regimes have been chosen: a lower-fluence regime (0.06–0.5 J cm−2 single-shot irradiation under UHV condition and 0.25–2.5 J cm−2 single-shot irradiation in ambient condition), a moderate-fluence regime (0.25–1.5 J cm−2 multiple-shot irradiation), and a higher-fluence regime (2.5–3.5 J cm−2 multiple-shot irradiation). Around the ablation threshold fluence, most significant features identified at the Si surface are nanohillock-like structures. The appearance of these nanohillocks is regarded as typical features for fast electronic processes (correlated with existence of hot electrons) and is explained on the basis of Coulomb explosion. The growth of these typical features (nanohillocks) by femtosecond laser irradiation is an element of novelty. At moderate irradiation fluence, a ring-shaped ablation with larger bumps and periodic surface structures is observed and is considered as a footprint of ultra-fast melting. Further increase in the laser fluence, i.e. a higher-fluence regime, resulted in strong enhancement of the thermal process with the appearance of larger islands. The change in surface topography provides an innovative clue to differentiate between ultra-fast electronic processes, i.e. Coulomb explosion (sub-100 fs) at a lower-fluence regime and ultra-fast melting (hundreds of fs) at a moderate-fluence regime, and slow thermal processes (ps time scale) at a higher-fluence regime. These fast electronic and thermal processes are well correlated to structural and crystallographic alterations, inferred from Raman spectroscopy.