Resistive Switching Properties of Sol–Gel-Derived V-Doped SrTiO3 Thin Films

V-doped and undoped SrTiO₃ (V:STO and STO) thin films on Pt/Ti/SiO₂/Si substrates were synthesized using a sol–gel method to form metal–insulator–metal (MIM) structures. Coexistence of the bipolar and unipolar resistive switching (BRS and URS) modes in Pt/STO/Pt and Pt/V:STO/Pt structures was observed as a irreversible transition from BRS to URS on adjustment of the compliance current (I _comp). Both states were stable and reproducible over 60 cycles, and the maximum operating voltage of the Pt/STO/Pt was reduced from 10 V to 2 V by doping with V. Linear fitting of current–voltage curves suggests that space-charge-limited leakage was the limiting leakage mechanism for these two devices. Based on these results, a switching mechanism based on filament theory is proposed to explain both resistive switching modes.     

Single-parameter model for the post-breakdown conduction characteristics of HoTiOx-based MIM capacitors

The post-breakdown conduction characteristics of holmium titanium oxide (HoTiO_x)-based metal–insulator–metal capacitors fabricated by the atomic layer deposition technique on Si substrates were investigated. Diode-like and power-law models were fitted to the experimental current–voltage (I–V) curves and the results assessed with the aim of detecting any possible correlation among the model parameters. It was found that the number of parameters involved can be reduced in both cases and that for the power-law model a single parameter is solely required to approximate the I–V curves in a wide current range (from 10⁻¹¹ to 10⁻⁴ A). This property, which has also been observed in a variety of material systems, was used to simulate the bipolar switching behavior exhibited by the I–V characteristics. The connection with the physics of electron transport through atom-sized constrictions is discussed.     

Engineering the current–voltage characteristics of metal–insulator–metal diodes using double-insulator tunnel barriers

The femtosecond-fast transport in metal–insulator–metal (MIM) tunnel diodes makes them attractive for applications such as ultra-high frequency rectenna detectors and solar cells, and mixers. These applications impose severe requirements on the diode current–voltage I(V) characteristics. For example, rectennas operating at terahertz or higher frequencies require diodes to have low resistance and adequate nonlinearity. To analyze and design MIM diodes with the desired characteristics, we developed a simulator based on the transfer-matrix method, and verified its accuracy by comparing simulated I(V) characteristics with those measured in MIM diodes that we fabricated by sputtering, and also with simulations based on the quantum transmitting boundary method. Single-insulator low-resistance diodes are not sufficiently nonlinear for efficient rectennas. Multi-insulator diodes can be engineered to provide both low resistance and substantial nonlinearity. The improved performance of multi-insulator diodes can result from either resonant tunneling or a step change in tunneling distance with voltage, either of which can be made to dominate by the appropriate choice of insulators and barrier thicknesses. The stability of the interfaces in the MIIM diodes is confirmed through a thermodynamic analysis.     

Anomalous CV curves of Al/SRO/Si structure after electrical stress

The electronic transport property of silicon-rich oxide (SRO) films under electrical stress was studied in Al/SRO/Si structures using capacitance versus voltage (C–V) and current versus voltage (I–V) measurements. The measured C–V curves show an unexpected vertical shift either upward or downward. Analysis of the data shows that these upward and downward shifts can be almost totally attributed to a sudden change of the capacitance of the SRO layer. I–V measurements also present some abrupt jumps in current. A simple model was proposed to explain the experimental results taking into account the change of the transport properties of the SRO layer. It is proposed that under electrical stress the SRO film can be switched between two states: low resistive and high resistive states. These conductive states are produced by the creation and, or annihilation of neutral traps, or defects, in the SRO layer. If defects are created, they will “connect” adjacent excess Si dots already existing in the material, then percolation paths would be formed and electrons could move through these paths resulting in a low resistive state. On the other hand, if the defects are annihilated by electrical stress, the percolation paths would be eliminated and the SRO layer turns to a high resistive state. Then, the switching between the on and off states of the SRO films under electrical stress would produce the anomalous shifts observed in the measured C–V and I–V curves.     

Electrical Properties and Current Transport Mechanisms of the Au/n-GaN Schottky Structure with Solution- Processed High-k BaTiO3 Interlayer

The electrical properties and current transport mechanisms of Au/BaTiO₃ (BTO)/n-GaN metal–insulator–semiconductor (MIS) structures have been investigated by current–voltage (I–V) and capacitance–voltage (C–V) measurements at room temperature. Experimental results reveal that the MIS structure has a higher rectification ratio with low reverse leakage current compared with the Au/n-GaN metal–semiconductor (MS) structure. The calculated barrier height of the Au/BTO/n-GaN MIS structure [0.87 eV (I–V)/1.02 eV (C–V)] increases compared with the Au/n-GaN MS structure [0.73 eV (I–V)/0.96 eV (C–V)]. The series resistance is extracted using Cheung’s functions, and the values are in good agreement with each other. Furthermore, the energy distribution of the interface state density is estimated from the forward-bias I–V data. It is noteworthy that the interface state density of the MIS structure is lower than that of the MS structure. In both MS and MIS structures under forward-bias conditions, ohmic and space-charge-limited conduction mechanisms are identified at lower and higher voltages, respectively. Investigations reveal that Poole–Frenkel emission dominates the reverse leakage current in both Au/n-GaN and Au/BTO/n-GaN structures.     

Organic memristive devices based on silver nanoparticles and DNA

A novel organic memory device ‘Al/silver nanoparticles-deoxyribonucleic acid-cetyltrimethylammonium Bromide/ITO’ (Al/Ag NPs–DNA–CTMA/ITO) was fabricated. The measured I–V curve of the device exhibits unipolar switching. The conductivity and the memristive characteristics are significantly improved by the introduction of Ag nanoparticles, but with a poor stability. Better stability is achieved by annealing the device, which also changes the switching characteristic from unipolar to bipolar. As the annealing temperature is raised, the switching voltage first decreases and then increases, while the current I_RESET first increases and then decreases. The range of the optimal annealing temperature is from 383 K to 403 K and the maximum ON/OFF current ratio (I_ON/I_OFF) can reach 10⁴. The switching voltage, the current, and I_ON/I_OFF all increase with the applied voltage amplitude, and V_SET and I_ON/I_OFF obey a quadratic and Boltzmann relationship, respectively.     

Temperature dependent of the current–voltage (I–V) characteristics of TaSi2/n-Si structure

Tantalum silicide (TaSi₂) thin films were deposited on n-type silicon single crystal substrates using a dual electron-gun system and with Ta and Si targets. The electrical transport properties of the TaSi₂/n-Si structures were investigated by temperature-dependent current–voltage (I–V) measurements. The temperature-dependent I–V characteristics revealed that the forward conduction was determined by thermionic-emission and space-charge-limited current mechanisms at low and high voltage respectively. On the other hand, the reverse current is limited by the carrier generation process.     

Multilevel characteristics for bipolar resistive random access memory based on hafnium doped SiO2 switching layer

We fabricated TiN/Hf:SiO₂/Pt memory cell with the small size of 1×1 um² by lithography and sputtering technology, which demonstrated excellent bipolar resistive switching (RS) characteristics. The device presents good endurance and outstanding uniformity. The coefficient of variation of V_set, V_reset, R_on and R_off were found to be 5.05%, 4.78%, 4.18%, and 15.78%, respectively. For the device with hafnium doped SiO₂ switching layer, multilevel storage capability can be successfully obtained by varying either the stop voltage or the compliance current in the SET process. In addition, the impact of forming current on the RS properties was studied. We found that the ratio of On/Off current for the device increased with the decrease of the forming current, which would be beneficial for the design of low power device. Possible RS mechanisms aiming to explain the impact of forming current on the RS characteristics and multilevel storage were also deduced.     

Electrical switching in single crystal VO2

I–V characteristics of single crystal vanadium dioxide has been measured using a constant current source in the ambient temperature region 220–325°K. The temperature of the crystal surface has also been measured. It is observed that the switching voltage (V_th) increases but the current at switching (I_th) decreases with decreasing temperature, giving a temperature independent threshold power (V_th × I_th). At switching, the temperature of the crystal surface increases only by 3–6°K above ambient for different ambient temperatures. These results can be qualitatively explained by assuming that a filament (channel) is formed before switching. The switching occurs when the temperature of this filament of finite width approaches the semiconductor-metal transition temperature. The initial width of the channel at switching decreases with decreasing temperature and at a given ambient temperature the channel width increases with increasing current in the post breakdown region.     

Electrically-driven metal–insulator transition of vanadium dioxide thin films in a metal–oxide-insulator–metal device structure

We report the successful growth of vanadium dioxide (VO₂) films on SiO₂ buffered metal electrode and the fabrication of metal–oxide-insulator–metal (MOIM) junction. The VO₂ film has an abrupt thermal-induced metal–insulator transition (MIT) with a change of resistance of 2 orders of magnitude. The electrically-driven MIT (E-MIT) switching characteristics have been investigated by applying perpendicular voltage to VO₂ based MOIM device at particular temperatures, sharp jumps in electric current were observed in the I–V characteristics under a low threshold voltage of 1.6 V. The Ohmic behavior, non-Ohmic super-linear one, and metallic regime are sequentially observed in the MOIM device with the increase of voltage. It is expected to be of significance in exploring ultrafast electronic devices incorporating correlated oxides based MOIM structure.     

Inhomogeneous Ni/Ge Schottky barriers due to variation in Fermi-level pinning

To achieve high performance Ge nMOSFETs it is necessary to reduce the metal/semiconductor Schottky barrier heights at the source and drain. Ni/Ge and NiGe/Ge Schottky barriers are fabricated by electrodeposition using n-type Ge substrates. Current (I)–voltage (V) and capacitance (C)–voltage (V) and low temperature I–V measurements are presented. A high-quality Schottky barrier with extremely low reverse leakage current is revealed. The results are shown to fit an inhomogeneous barrier model for thermionic emission over a Schottky barrier. A mean value of 0.57 eV and a standard deviation of 52 meV is obtained for the Schottky barrier height at room temperature. A likely explanation for the distribution of the Schottky barrier height is the spatial variation of the metal induced gap states at the Ge surface due to a variation in interfacial oxide thickness, which de-pins the Fermi level.     

Development of Ag/ZnO/FTO thin film memristor using aqueous chemical route

The present paper report the development of the Ag/ZnO/FTO memristor device using a simple aqueous chemical route. The structural, electrical, morphological and optical properties of Ag/ZnO/FTO memristor device are characterized using X-Ray diffraction (XRD), semiconductor characterization unit, field emission scanning electron microscopy (FESEM), and UV-Vis-NIR spectrophotometer respectively. The fabricated memristor device shows bipolar resistive switching behavior within low operating voltage (±0.88 V). The hysteresis loop in the I–V plane is the fingerprint characteristics of memristor and same has been seen in developed device. Furthermore, the effect of temperature on ZnO based memristor device is investigated using the thermal reaction model. The results suggested that, temperature of conductive filaments increases rapidly and affected the memory window of memristor.     

A prognostic approach for non-punch through and field stop IGBTs

Development of prognostic approaches for insulated gate bipolar transistors (IGBTs) is of interest in order to improve availability, reduce downtime, and prevent failures of power electronics. In this study, a prognostic approach was developed to identify anomalous behavior in non-punch through (NPT) and field stop (FS) IGBTs and predict their remaining useful life. NPT and FS IGBTs were subjected to electrical–thermal stresses until their failure. X-ray analysis performed before and after the stress tests revealed degradation in the die attach. The gate–emitter voltage (V_GE), collector–emitter voltage (V_CE), collector–emitter current (I_CE), and case temperature were monitored in situ during the experiment. The on-state collector–emitter voltage (V_CE(ON)) increased and the on-state collector–emitter current (I_CE(ON)) decreased during the test. A Mahalanobis distance (MD) approach was implemented using the V_CE(ON) and I_CE(ON) parameters for anomaly detection. Upon anomaly detection, the particle filter algorithm was triggered to predict the remaining useful life of the IGBT. The system model for the particle filter was obtained by a least squares regression of the V_CE(ON) at the mean test temperature. The failure threshold was defined as a 20% increase in V_CE(ON). The particle filter approach, developed using the system model based on the V_CE(ON), was demonstrated to provide mean time to failure estimates of IGBT remaining useful life with an error of approximately 20% at the time of anomaly detection.     

High-resolution transmission electron microscopy study on bipolar resistive switching behavior in TiO2 thin films

We fabricated TiO₂ thin films the by sol–gel process. Successful I–V curves can be obtained in the Cu/TiO₂/ATO structure device in which TiO₂ thin film was calcined at 300 °C. The bipolar resistive switching behavior was observed and the ratio of R_off/R_on can be increased to 10⁴. The switching voltage changes from 4.8 to 3.5 V when the current compliance drops from 10 to 0.1 mA. We also investigated the microstructure by HRTEM technology.     

Impact of Bi Deficiencies on Ferroelectric Resistive Switching Characteristics Observed at p‐Type Schottky‐Like Pt/Bi1–δFeO3 Interfaces

This work reports a resistive switching effect observed at rectifying Pt/Bi_1–δFeO₃ interfaces and the impact of Bi deficiencies on its characteristics. Since Bi deficiencies provide hole carriers in BiFeO₃, Bi‐deficient Bi_1–δFeO₃ films act as a p‐type semiconductor. As the Bi deficiency increased, a leakage current at Pt/Bi_1–δFeO₃ interfaces tended to increase, and finally, rectifying and hysteretic current–voltage (I–V) characteristics were observed. In I–V characteristics measured at a voltage‐sweep frequency of 1 kHz, positive and negative current peaks originating from ferroelectric displacement current were observed under forward and reverse bias prior to set and reset switching processes, respectively, suggesting that polarization reversal is involved in the resistive switching effect. The resistive switching measurements in a pulse‐voltage mode revealed that the switching speed and switching ratio can be improved by controlling the Bi deficiency. The resistive switching devices showed endurance of >10⁵ cycles and data retention of >10⁵ s at room temperature. Moreover, unlike conventional resistive switching devices made of metal oxides, no forming process is needed to obtain a stable resistive switching effect in the ferroelectric resistive switching devices. These results demonstrate promising prospects for application of the ferroelectric resistive switching effect at Pt/Bi_1–δFeO₃ interfaces to nonvolatile memory.     

Investigation of electrical characteristics of memory cells based on self-forming conducting nanostructures in a form of the TiN-SiO2-W open sandwich structure

The results of experimental investigations of electroforming and quasi-static current-voltage (I-V) characteristics of formed TiN-SiO₂-W open sandwich structures in comparison to the Si-SiO₂-W structures are presented. It is shown that similar memory and switching effects, which are associated with self-forming the conducting nanostructures on the surface of the open end of the dielectric film (silicon dioxide) about 20 nm thick, are observed in them. However, the features of the structures with the lower TiN electrode are the noticeably larger current, the lower threshold switching voltage from the low-conducting state into the high-conducting state, and a flatter I–V characteristic at voltages below the threshold one. These features can be explained by the decrease in the spreading resistance from the conducting structure, which is formed during electroforming, into the material of the lower electrode for titanium nitride compared to silicon (their resistivity values differ by a factor of 4) and the lower potential barrier at the TiN-SiO₂ interface compared with the Si-SiO₂ interface. The cell of energy-independent electrically reprogrammable memory with the TiN-SiO₂-W structure possesses the better technical characteristics and manufacturability.     

Transport and storage properties of CrSi2/Si junctions made using the CAPVD technique

p-CrSi₂/n-crystSi and p-CrSi₂/p-crystSi hetero junctions produced by cathodic arc physical vapor deposition were worked out by means of capacitance–voltage–temperature (C–V–T) and current–voltage–temperature (I–V–T) measurements to investigate storage and transport properties. Former measurement on p-CrSi₂/n-crystSi structure confirmed an abrupt type junction together with a building voltage at the proximity of 0.7 V. Though a fairly well rectification ratio (10³ at ±2 V) was realized by I–V measurement, it became deteriorated with the increase in ambient temperature. From temperature dependence of I–V variations, distinct conduction mechanisms were identified. In forward (reverse) direction trap assisted single-multistep tunneling recombination (generation) and space-charge limited current flow that corresponded to low and high bias voltage regions, respectively, were identified. Moreover, an activation energy (E_A) determined from the slopes of I–V–T curves as 0.22 and 0.26 eV was interpreted as the energy position of a chromium–boron (Cr–B) complex-type point defect residing in n/p doped c-Si semiconductor in CrSi₂/n–c-Si and CrSi₂/p–c-Si junctions. The retrieved E_A was in agreement with the recent DLTS measurement. Based on the experimental observations, schematic current path was built to interpret I–V/C–V behaviors. The model was successful in explaining the decrease in measured capacitance under large forward bias voltage reported for the first time by us for the present CrSi₂/Si junctions.     

Resistive switching characteristics of MIM structures based on oxygen-variable ultra-thin HfO2 and fabricated at low temperature

We report the resistive switching characteristics of Metal-Insulator-Metal (MIM) structures fabricated at low temperature and having different concentrations of oxygen vacancies in the insulator. The oxygen modulation in HfO₂ is promoted by a very simple variation of standard thermal Atomic-Layer Deposition (ALD), so that different exposure times to H₂O during each half-cycle of the hafnium oxide deposition are used (being Tetrakis Dimethylamino Hafnium–TDMAH the other precursor). We show the correlation of the stoichiometry with the forming voltage, conduction mechanisms and resistance windows of memory devices. All structures present a bipolar operation mode in which the resistive switching mechanism is related to the migration of oxygen vacancies inside the dielectric. These MIM devices have a simple structure, low power consumption and they are fabricated using a very low thermal budget of only 250 °C, thus enabling their integration at the Back-End of Line (BEOL) stage of an integrated circuit in order to increase the density of memory arrays in at least one order of magnitude.     

Characteristics of metal/tunnel-oxide/n/p+ silicon switching devices—II: Two-dimensional effects in oxide-isolated structures

Effects of oxide isolation on the two-terminal D.C. characteristics of metal/tunnel-oxide/n/p⁺ silicon switching devices have been studied.Recent experimental results have shown that the switching characteristics are strongly dependent on area, and area-to-perimeter ratio of the device. To carry out a systematic investigation of this phenomenon, the devices in this study were isolated using V-grooves of various areas. For a given tunnel-oxide thickness and area, it was found that the magnitude of the switching voltage and holding current of the device increased with isolation area, whereas the switching current remained essentially constant. Furthermore, it is shown that the switching current is almost completely determined by the characteristics of the tunnel-oxide; in particular, the minority carrier concentration at the SiSiO₂ interface. Physical arguments are presented which adequately explain the observed trends. It is also experimentally shown that both switching current and holding current decrease as the tunnel-oxide thickness is increased.A simple two-dimensional model for the oxide-isolated MISS device is derived which effectively explains the above area-related phenomena. In agreement with experimental results, the model predicts that for a given tunnel-oxide thickness and area, an increase in switching voltage magnitude and holding current will result as the isolated p⁺-n junction area is increased. Calculations based on this model are shown to be in good agreement with experimental data.     

Electrical parameters of a DC sputtered Mo/n-type 6H-SiC Schottky barrier diode

A Mo/n-type 6H-SiC/Ni Schottky barrier diode (SBD) was fabricated by sputtering Mo metal on n-type 6H-SiC semiconductor. Before the formation of Mo/n-type 6H-SiC SBD, an ohmic contact was formed by thermal evaporation of Ni on n-type 6H-SiC and annealing at 950 °C for 10 min. It was seen that the structure had excellent rectification. The electrical parameters were extracted using its current–voltage (I–V) and capacitance–voltage (C–V) measurements carried out at room temperature. Very high (1.10 eV) barrier height and 1.635 ideality factor values were reported for Mo/n-type 6H-SiC using ln I–V plot. The barrier height and series resistance values of the diode were also calculated as 1.413 eV and 69 Ω from Norde׳s functions, respectively. Furthermore, 1.938 eV barrier height value of Mo/n-type 6H-SiC SBD calculated from C–V measurements was larger than the one obtained from I–V data.