Electrical Switching Studies Research Articles

Electronic and optical band gap of Ge–Te–Sn nanostructure

Amorphous thin film Ge15Te85−xSnx (1 ≤ x ≤ 5) and Ge17Te83−xSnx (1 x 4) switching devices have been deposited in sandwich geometry using a flash evaporation technique, with aluminum as the top and bot- tom electrodes. Electrical switching studies indicate that these films exhibit memory type electrical switching behavior. The switching fields for both the series of samples have been found to decrease with increase in Sn con- centration, which confirms that the metallicity effect on switching fields/voltages, commonly seen in bulk glassy chalcogenides, is valid in amorphous chalcogenide thin films also. In addition, there is no manifestation of rigid- ity percolation in the composition dependence of switching fields of Ge15Te85−xSnx and Ge17Te83−xSnx amorphous thin film samples. The observed composition dependence of switching fields of amorphous Ge15Te85−xSnx and Ge17Te83−xSnx thin films has been understood on the basis of Chemically Ordered Network model. The optical band gap for these samples, calculated from the absorption spectra, has been found to exhibit a decreasing trend with in- creasing Sn concentration, which is consistent with the com- position dependence of switching fields.

Influence of Cu Doping in Si–Te-Based Chalcogenide Glasses and Thin Films: Electrical Switching, Morphological and Raman Studies

To understand the electrical switching behavior of Si15Te $_{85-{x}}$ Cu x ( $1\le {x} \le10$ ) series, ${I}$ – ${V}$ characterization has been performed on bulk as well as amorphous thin films of the as-prepared samples. Both the bulk glasses and amorphous thin films are found to manifest memory-type switching behavior. The threshold voltages of thin-film devices are found to be much lower than the bulk counterparts and hence could find application for phase change memory (PCM). The composition analyses of both have divulged the existence of intermediate phase (IP) in the composition range of $2 \le {x} \le6$ . To examine the probability of the given glass for PCM application, Set–Reset studies have been performed on the glasses with a triangular pulse of 6 mA for set operation and rectangular pulse of 12 mA for the reset operation. The study has revealed a continuous repetition of few Set–Reset cycle by the Si–Te–Cu series. Raman studies carried out on the bulk glasses report the occurrence of blue shift over the composition in a regular manner. Further, SEM studies have been carried out on Si–Te–Cu samples to understand the morphological changes that would have occurred during switching. Additionally, thickness dependence of threshold voltage of representative Si15Te80Cu5 and Si15Te76Cu9 glasses has been carried out to reveal the relationship between threshold voltage and thickness.

Electrical switching and crystalline peak studies on Si20Te80−xSnx (1 ≤ x ≤ 7) chalcogenide bulk glasses

The Si20Te80-xSnx (1 ≤ x ≤ 7) glasses are synthesized using melt quenching technique. Result of X-ray diffraction (XRD) exhibits the non-crystallinity of the prepared samples. Depending on applied on state current, the synthesized Si20Te80-xSnx (1 ≤ x ≤ 7) samples show both threshold and memory type of behavior. The composition dependence of threshold voltage (Vth) displays a decrease in the Vth values. Further the composition and morphological studies are carried out using Energy Dispersive X-ray analysis (EDAX) and Electron microscopy (SEM). The differential scanning calorimetry (DSC) is used for crystalline peak studies. Using DSC, the crystallization temperature (Tc) of representative glass samples are found. The crystalline peak studies are carried out by annealing representative samples at their respective Tc for two hours to find different phases present in the samples. The SET-RESET studies are performed to find the applicability of the given material in phase change memory (PCM) applications.

Electrical Switching and Optical Bandgap Studies on Quaternary Ag-Doped Ge–Te–In Thin Films

Amorphous Ge15Te80– x In5Ag x ( $6\le {x} \le24$ ) thin films prepared in sandwich geometry exhibit memory switching behavior unlike the bulk sample that has shown both threshold switching for less current (1–2 mA) and memory switching for current greater than 2 mA. As anticipated, the threshold voltage of Ge15Te80– x In5Ag x ( $6\le {x} \le24$ ) thin films is found to be lower as contrast to those of bulk counterparts. The compositional dependence of amorphous system shows an extensive plateau in the range of $\textsf {6} \le {x} \le \textsf {12}$ which literally stands for the intermediate phase after which there is a drastic increase in the threshold field. Shifted rigidity percolation threshold has also been confirmed from the compositional dependence of threshold field of amorphous Ge15Te80– x In5Ag x thin films. In addition, the optical bandgap of a-Ge15Te80– x In5Ag x thin-film sample has been reckoned considering absorption spectra, and the compositional dependence has been described based on average bond energy of the system.

Electrical switching and morphological studies on Si15Te84Cu1 thin film

The electrical switching studies of Si based ternary material with composition Si15Te84Cu1 has been presented in this paper. The material has been prepared in both bulk and thin film device form. The bulk sample is prepared by melt quenching technique. The thin film device is prepared in sandwich geometry on a glass substrate with copper as top and bottom electrodes. The material Si15Te84Cu1 is coated by flash evaporation technique and the electrodes by sputtering technique in DC mode. To carry out SET studies, electric field is employed to the device and it has been found that the thin film device exhibits memory type of switching. The essential instruments used in the fabrication of amorphous switching devices are high temperature furnace and deposition units like flash evaporation unit, sputtering unit. Keithley source-measure unit along with two probe station succoured the study of electrical switching behaviour of the device. A morphological study has been carried out with Scanning Electron Microscopy and it has revealed some structural changes, which have occurred during switching.

Electrical switching and thermal behavior of ternary Si15Te85-xBix (0 ≤ x ≤ 2) chalcogenide glasses

Bulk semiconducting Si15Te85-xBix(0 ≤ x ≤ 2) chalcogenide glasses have been prepared using a well established melt-quenching technique. Electrical switching studies have been undertaken on Si15Te85-xBix(0 ≤ x ≤ 2) chalcogenide glasses. The results indicate that these samples exhibit memory type electrical switching behavior. It has been observed that the switching voltage VT of the glasses decreases with the addition of Bi. In addition, thermal stability and OFF state resistivity of the samples have been found to decrease with the increase in Bi concentration and are related to the observed decrease in switching voltages. The switching voltage (VT) has been found to increase with the thickness of the sample and decrease with increase in temperature confirming the thermal origin of the memory switching process. Further, scanning electron microscopy (SEM) studies reveal the formation of a crystalline channel indicating the conducting path between the two electrodes in the switched region

Crystallization kinetics of Sn doped Ge20Te80−xSnx (0 ≤ x ≤ 4) chalcogenide glassy alloys

Chalcogenide semiconductors have evolved as multifunctional materials due to their fascinating thermal, optical, electrical and mechanical properties. In this report, Ge20Te80−xSnx (0 ≤ x ≤ 4) glassy alloys are systematically studied in order to understand the effect of variation of Sn content on the thermal parameters such as glass transition (Tg), onset crystallization (Tc), peak crystallization (Tp), melting temperature (Tm), activation energy of glass transition (Eg), and crystallization (Ec). The values of Eg are calculated from the variation of Tg with the heating rate (α), according to Kissinger and Moynihan model, while the values of Ec are calculated from the variation of Tp with the heating rate (α), according to Kissinger, Takhor, Augis-Bennett and Ozawa model. Thermal stability and glass forming ability (GFA) are discussed for understanding the applicability of the synthesized materials in phase change memory (PCM) applications. Thermal parameters are correlated with the electrical switching studies to get an insight into the phase change mechanism. The results of the calculated thermal parameters reveal that the GFA of the synthesized Ge20Te80−xSnx (0 ≤ x ≤ 4) glassy alloys has a synchronous relationship with their thermal properties studied through differential scanning calorimetry, indicating their potential for phase-change memory device applications.

SET and RESET states of As2Se3 doped GeTe4 bulk glasses probed by Raman spectroscopy

Te based chalcogenide glasses are being explored for non-volatile memory applications. We report the electrical switching studies on bulk Ge0.20Te0.80 glass alloyed with As0.40Se0.60 in different proportions. The addition of As0.40Se0.60 increases the electrical resistivity and the threshold voltage of (As0.4Se0.6)x(Ge0.2Te0.8)1−x glasses in the composition range 0 ≤ x ≤ 0.40. Glasses in the range 0 ≤ x ≤ 0.10 are found to exhibit memory switching, whereas glasses with x ≥ 0.15 exhibit threshold switching. The memory (SET) state can be brought back to RESET state by passing a current of 3–5 mA. Particularly, the glass with x = 0.10 is identified to be very stable, and it can be cycled between the SET and RESET state consistently with 3 mA current. The Raman spectra of SET and RESET states indicate that the SET sample has GeTe and Te crystalline units while the structure of RESET state is analogous to the as-quenched glass. Interestingly, the variation in the local structure is minimal for SET and RESET states for x = 0.10. And also the resistance of the SET state is relatively high. So they can be set to RESET state with less current and can be explored for low power phase change memory applications.

Optical, photo-acoustic and electrical switching studies of amorphous GeS2 thin films

This paper reports optical, photo-acoustic and electrical switching investigations of GeS2 amorphous thin films of different thicknesses, deposited on glass substrates in vacuum. The Tauc parameter (B 1/2) and Urbach energy (E U) have been determined from the transmittance spectra, to understand the changes in structural disorder; it is found that B 1/2 increases whereas E U decreases as the thickness of the films increases. Based on the results, it is suggested that bond re-arrangement, i.e. transformation from homopolar bonds to heteropolar bonds, takes place with increase in thickness. The thermal diffusivity values of GeS2 thin films also show the presence of a chemically ordered network in the GeS2 thin films. Further, it is found that these films exhibit memory-type electrical switching. The observed variation in the switching voltages has been understood on the basis of increase in chemical order.

Multi-resistance states in the electrical switching behavior of amorphous Si15Te75Ge10 thin films: Possibility of multi-bit storage

Electrical switching studies on amorphous Si15Te75Ge10 thin film devices reveal the existence of two distinct, stable low-resistance, SET states, achieved by varying the electrical input to the device. The multiple resistance levels can be attributed to multi-stage crystallization, as observed from temperature dependant resistance studies. The devices are tested for their ability to be RESET with minimal resistance degradation; further, they exhibit a minimal drift in the SET resistance value even after several months of switching.

Electrical switching in amorphous Si–Te–Ge thin films: Impact of input energy on crystallization process and switching parameters

Electrical switching studies on amorphous Si15Te74Ge11 thin film devices show interesting changes in the switching behavior with changes in the input energy supplied; the input energy determines the extent of crystallization in the active volume, which is reflected in the value of SET resistances. This in turn, determines the trend exhibited by switching voltage (Vt) for different input conditions. The results obtained are analyzed on the basis of the amount of Joule heat generated, which determines the temperature of the active volume. Depending on the final temperature, devices are rendered either in the intermediate state with a resistance of 5∗102Ω or the ON state with a resistance of 5∗101Ω.

Manifestation of intermediate phase in mechanical properties: Nano-indentation studies on Ge–Te–Si bulk chalcogenide glasses

Nano-indentation studies have been undertaken on bulk Ge15Te85−xSix glasses (0≤x≤9), to estimate hardness, H and elastic modulus, E. It is found that E and H increase initially with the increase in the atomic percent of Si. Further, a plateau is seen in the composition dependence of E and H in the composition range 2≤x≤6. It is also seen that the addition of up to 2at% Si increases the density ρ of the glass considerably; however, further additions of Si lead to a near linear reduction in ρ, in the range 2≤x≤6. Beyond x=6, ρ increases again with Si content. The variation of molar volume Vm brings out a more fascinating picture. A plateau is seen in the intermediate phase suggesting that the molecular structure of the glasses is adapting to keep the count of constraints fixed in this particular phase. The observed variations in mechanical properties are associated with the Boolchand’s intermediate phase in the present glassy system, in the composition range 2≤x≤6, suggested earlier from calorimetric and electrical switching studies. The present results reveal rather directly the existence of the intermediate phase in elastic and plastic properties of chalcogenide glasses.

Thermodynamic, Raman and electrical switching studies on Si15Te85-xAgx (4 ≤ x ≤ 20) glasses

We have investigated thermal properties of bulk Si15Te85-xAgx (4 ≤ x ≤ 20) glasses in detail, through alternating differential scanning calorimetry experiments. The composition dependence of thermal parameters reveal the signatures of rigidity percolation and chemical threshold at compositions x = 12 and x = 19, respectively. The stability and glass forming ability of these glasses have also been determined using the data obtained from different thermodynamic quantities and it is found that the Si15Te85-xAgx glasses in the region 12 ≤ x ≤ 17 are more stable when compared to other glasses of the same series. Further, the blueshift observed in Raman spectroscopy investigations, in the composition range 12 ≤ x ≤ 13, support the occurrence of stiffness threshold in this composition range. All Si15Te85-xAgx (4 ≤ x ≤ 20) glasses are found to exhibit memory type switching (for sample thickness 0.25 mm) in the input current range 3–9 mA. The effect of rigidity percolation and chemical thresholds on switching voltages are observed at x = 12 and 19, respectively.

Studies on electrical switching behavior and optical band gap of amorphous Ge–Te–Sn thin films

Amorphous thin film Ge15Te85-xSnx (1 <= x <= 5) and Ge17Te83-xSnx (1 <= x <= 4) switching devices have been deposited in sandwich geometry using a flash evaporation technique, with aluminum as the top and bottom electrodes. Electrical switching studies indicate that these films exhibit memory type electrical switching behavior. The switching fields for both the series of samples have been found to decrease with increase in Sn concentration, which confirms that the metallicity effect on switching fields/voltages, commonly seen in bulk glassy chalcogenides, is valid in amorphous chalcogenide thin films also. In addition, there is no manifestation of rigidity percolation in the composition dependence of switching fields of Ge15Te85-xSnx and Ge17Te83-xSnx amorphous thin film samples. The observed composition dependence of switching fields of amorphous Ge15Te85-xSnx and Ge17Te83-xSnx thin films has been understood on the basis of Chemically Ordered Network model. The optical band gap for these samples, calculated from the absorption spectra, has been found to exhibit a decreasing trend with increasing Sn concentration, which is consistent with the composition dependence of switching fields.

Thermodynamic, kinetic and electrical switching studies on Si 15Te 85− xIn x glasses: Observation of Boolchand intermediate phase

An interesting topic for quite some time is an intermediate phase observed in chalcogenide glasses, which is related to network connectivity and rigidity. This phenomenon is exhibited by Si–Te–In glasses also. It has been addressed here by carrying out detailed thermal investigations by using Alternating Differential Scanning Calorimetry technique. An effort has also been made to determine the stability of these glasses using the data obtained from different thermodynamic quantities and crystallization kinetics of these glasses. Electrical switching behavior by recording I–V characteristics and variation of switching voltages with indium composition have been studied in these glasses for phase change memory applications.

Synthesis and Phase Transitional Behavior of Dimer-like Optically Active Liquid Crystals

The results of our detailed studies pertaining to a relatively new class of low molar mass mesogens are presented. Four homologous series of optically active dimer-like mesogens, comprising cholesterol as the conventional pro-mesogenic core covalently tethered to a nonmesogenic salicylaldimine segment through a flexible spacer of varying length and parity, have been synthesized and evaluated for their thermal, electrical switching, electrochemical, and gelation properties. The thermal behavior, being the prime focus of this study, has been characterized by optical, calorimetric, X-ray diffraction, and electrical switching studies. In each series, the length of the even/odd-parity spacer is held constant, while the length of the terminal N-alkyl tail attached to nonmesogenic salicylaldimine core has been varied to gain insight into the fundamental correlation between structure and phase transitional properties. These compounds, with a few exceptions, exhibit liquid crystal phase(s); the identified phases are chiral nematic (N*), twist grain boundary (TGB), chiral smectic A (SmA), and chiral smectic C (SmC*) phases. The selective reflection property of the N* phase and ferroelectric behavior of the SmC* phase have been ascertained for some selected members. In general, the phase behavior shows a dependence on the length and parity of the central spacer as well as the length of the terminal tail. The odd-even effect has been prominently found in the clearing temperatures; the even-parity dimer-like compounds belonging to three different series exhibit higher values when compared to members of an odd-parity series. Thus, our study demonstrates that these new class of low molar mass materials behave analogous to liquid crystal dimers. Electrochemical behavior and gelation ability have been demonstrated for some selected materials.

Electrical switching studies on Ge–Te–Tl chalcogenide glasses: Effect of thallium on the composition dependence of switching voltages

Bulk Ge 17Te 83−xTl x glasses (0 ≤ x ≤ 13), have been found to exhibit memory type electrical switching. The switching voltages (also known as threshold voltage — V th) of Ge 17Te 83−xTl x glasses are found to decrease with increasing thallium content. The rate of decrease of V th is greater at lower concentrations and V th falls at a slower rate for higher thallium concentrations (x ≥ 6). The addition of thallium to the Ge–Te network fragments the covalent network and introduces ionic nature to it; the reduction in network connectivity leads to the decrease in switching voltages with thallium content. The decrease in the glass transition temperatures of Ge 17Te 83−xTl x glasses with increasing thallium concentration supports the idea of decrease in network connectivity with Tl addition. The more metallic nature of Tl also contributes to the observed reduction in the switching voltages of Ge 17Te 83−xTl x glasses with Tl content. Further, there is an interesting correlation seen between the threshold voltage V th and the average bond energy, as a function of Tl content. In addition, the switching voltages of Ge 17Te 83−xTl x glasses have been found to decrease with sample thickness almost linearly. The set–reset studies indicate that the Ge 17Te 81Tl 2 sample can be switched for more than 10 cycles, whereas other glasses could not be reset beyond two switching cycles.

Electrical switching and thermal studies on bulk Ge–Te–Bi glasses

Bulk, melt quenched Ge 18Te 82-xBi x glasses (1 ≤ x ≤ 4) have been found to exhibit memory type electrical switching behavior, which is in agreement with the lower thermal diffusivity values of Ge–Te–Bi samples. A linear variation in switching voltages (V th) has been found in these samples with increase in thickness which is consistent with the memory type electrical switching. Also, the switching voltages have been found to decrease with an increase in temperature which happens due to the decrease in the activation energy for crystallization at higher temperatures. Further, V th of Ge 18Te 82-xBi x glasses have been found to decrease with the increase in Bi content, indicating that in the Ge–Te–Bi system, the resistivity of the additive has a stronger role to play in the composition dependence of V th, in comparison with the network connectivity and rigidity factors. In addition, the composition dependence of crystallization activation energy has been found to show a decrease with an increase in Bi content, which is consistent with the observed decrease in the switching voltages. X-ray diffraction studies on thermally crystallized samples reveal the presence of hexagonal Te, GeTe, Bi 2Te 3 phases, suggesting that bismuth is not taking part in network formation to a greater extent, as reflected in the variation of switching voltages with the addition of Bi. SEM studies on switched and un-switched regions of Ge–Te–Bi samples indicate that there are morphological changes in the switched region, which can be attributed to the formation of the crystalline channel between two electrodes during switching.

Thermal and electrical switching studies on Ge 20Se 80 − xBi x (1 ≤ x ≤ 13) ternary chalcogenide glassy system

Alternating Differential Scanning Calorimetric (ADSC) and electrical switching studies have been undertaken on Ge 20Se 80 − xBi x glasses (1 ≤ x ≤ 13), to understand the effect of topological thresholds on thermal properties and electrical switching behavior. It is found that the compositional dependence of glass transition temperature (T g), crystallization temperature (T c1) and thermal stability (∆T) of Ge 20Se 80 − xBi x glasses show anomalies at a composition x = 5, the rigidity percolation/stiffness threshold of the system. Further, unusual variations are also observed in different thermal properties, such as T g, T c1, ∆T, ∆C p and ∆H NR, at the composition x = 10, which indicates the occurrence of chemical threshold in these glasses at this composition. Electrical switching studies indicate that Ge 20Se 80 − xBi x glasses with 5 ≤ x ≤ 11 exhibit threshold switching behavior and those with x = 12 and 13 show memory switching. A sharp decrease has been noticed in the switching voltages with bismuth concentration, which is due to the more metallic nature of bismuth and the presence of Bi + ions. Further, a saturation is seen in the decrease in V T around x = 6, which is related to bismuth phase percolation at higher concentrations of Bi.

The composition dependence of electrical switching behavior of Ge 7Se 93−xSb x glasses

Bulk Ge 7Se 93− x Sb x (21 ⩽ x ⩽ 32) glasses are prepared by melt quenching method and electrical switching studies have been undertaken on these samples to elucidate the type of switching and the composition and thickness dependence of switching voltages. On the basis of the compressibility and atomic radii, it has been previously observed that Se-based glasses exhibit memory switching behavior. However, the present results indicate that Ge 7Se 93− x Sb x glasses exhibit threshold type electrical switching with high switching voltages. Further, these samples are found to show fluctuations in the current–voltage ( I– V) characteristics. The observed threshold behavior of Ge 7Se 93− x Sb x glasses has been understood on the basis of larger atomic radii and lesser compressibilities of Sb and Ge. Further, the high switching voltages and fluctuations in the I– V characteristics of Ge–Se–Sb samples can be attributed to the high resistance of the samples and the difference in thermal conductivities of different structural units constituting the local structure of these glasses. The switching voltages of Ge 7Se 93− x Sb x glasses have been found to decrease with the increase in the Sb concentration. The observed composition dependence of switching voltages has been understood on the basis of higher metallicity of the Sb additive and also in the light of the Chemically Ordered Network (CON) model. Further, the thickness dependence of switching voltages has been studied to reassert the mechanism of switching.