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

Bulk Ge₁₇Te_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 Ge₁₇Te_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 Ge₁₇Te_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 Ge₁₇Te_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 Ge₁₇Te_83−xTl_x glasses have been found to decrease with sample thickness almost linearly. The set-reset studies indicate that the Ge₁₇Te₈₁Tl₂ 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 Ge₁₈Te_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 Ge₁₈Te_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, Bi₂Te₃ 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.     

Determination of the contribution of defect creation and charge trapping to the degradation of a-Si:H/SiN TFTs at room temperature and low voltages

In this paper is presented a qualitative investigation upon the mechanisms that cause the shift of the electrical parameters in thin film transistors (TFT). The transistors are made of amorphous hydrogenated silicon (a-Si:H) as active semiconductor layer and substoichiometric silicon nitride (SiN) as gate insulator. The work refers to the degradation at room temperature and low positive and negative gate voltage stresses. The electrical parameters: threshold voltage (V_th), subthreshold swing (S) and flat-band voltage (V_fb) have been determined from the dependence of the drain current on gate voltage (I_d–V_g) and the dependence of the capacitance on the gate voltage (C–V_g) measurements as function of stress time and bias.     

Thermal switching due to a local inhomogeneity existing between the electrode and the amorphous semiconductor

Switching transients in sandwich-type amorphous switching devices with electrodes of large area were studied experimentally. The results were analyzed in terms of a modified thermal switching model; the current concentration in the inhomogenous region existing at the electrode-semiconductor interface causes a thermal switching. The reciprocal delay time 1/t_D is proportional to the square of an applied voltage V²_p, indicating that the switching is caused by a certain kind of thermal process. The slope on the characteristic, however, is much larger than the theoretical value, in spite of using the resistivvity obtained from the current-voltage characteristic in the regime just before switching. These results must be attributed to a localized small region with a resistivity much lower tthan the bulk resisitivity. Some examples of asymmetric switching characteristics due to a local inhomogeneity are also shown.     

High field electrical switching behavior of Ge10Se90−xTlx glasses

Electrical switching studies on bulk Ge₁₀Se_90−xTl_x (15 ? x ? 34) glasses have been undertaken to examine the type of switching, composition and thickness dependence of switching voltages. Unlike Ge-Se-Tl thin films which exhibit memory switching, the bulk Ge₁₀Se_90−xTl_x glasses are found to exhibit threshold type switching with fluctuations seen in their current-voltage (I-V) characteristics. Further, it is observed that the switching voltages (V_T) of Ge₁₀Se_90−xTl_x glasses decrease with the increase in the Tl concentration. An effort has been made to understand the observed composition dependence on the basis of nature of bonding of Tl atoms and a decrease in the chemical disorder with composition. In addition, the network connectivity and metallicity factors also contribute for the observed decrease in the switching voltages of Ge₁₀Se_90−xTl_x glasses with Tl addition. It is also interesting to note that the composition dependence of switching voltages of Ge₁₀Se_90−xTl_x glasses exhibit a small cusp around the composition x = 22, which is understood on the basis of a thermally reversing window in this system in the composition range 22 ? x ? 30. The thickness dependence of switching voltages has been found to provide an insight about the type of switching mechanism involved in these samples.     

Variations in the mechanical properties of glass induced by high-pressure phase change

The elastic property of As₂Se₃ glass has been determined under high pressure from the data of the sound wave velocities and density. The sound velocities in As₂Se₃ glass have been measured under hydrostatic pressure up to 20 kb by means of ultrasonic interferometry. Sound velocities for both the longitudinal and shear mode, bulk modulus and shear modulus were found to increase linearly with pressure up to 8 kb. Above 9 kb the pressure derivatives diminished. The acoustic Grüneisen parameter shows an abrupt decrease at about 9 kb.Comparisons were made among the observed values of the volume ratio V/V₀, the isothermal bulk modulus K_T and the pressure derivative K′_T, and predictions based upon the Murnaghan equation or the lattice theory equation of state for solids. It is concluded that the abrupt changes in the parameters are probably due to a phase transition in glass at 9–10 kb.The clear resolution of the bulk modulus on both sides of the transition enables one to compute all the mechanical properties of the high-pressure phase, using the law of corresponding states. The decrease of K′_T from 7.86 to 6 in the high-pressure phase change is consistent with the change of density from 4.60 to 4.75.A modified equation of state has been proposed to describe the elastic property of glass. This equation includes a variable parameter C which reflects the volume change in glass due to bond angle change with pressure. By this equation the pressure dependence of C can also explain the elastic anomaly of silica glass as well as the variation of elastic constants of As₂Se₃ glass across the phase transition pressure.     

Network topological thresholds in gallium doped As–Te glasses – Electrical and thermal investigations

Electrical switching and differential scanning calorimetric studies are undertaken on bulk As₂₀Te_80−xGa_x glasses, to elucidate the network topological thresholds. It is found that these glasses exhibit a single glass transition (T_g) and two crystallization reactions (T_c1 & T_c2) upon heating. It is also found that there is only a marginal change in T_g with the addition of up to about 10% of Ga; around this composition an increase is seen in T_g which culminates in a local maximum around x = 15. The decrease exhibited in T_g beyond this composition, leads to a local minimum at x = 17.5. Further, the As₂₀Te_80−xGa_x glasses are found to exhibit memory type electrical switching. The switching voltages (V_T) increase with the increase in gallium content and a local maximum is seen in V_T around x = 15. V_T is found to decrease with x thereafter, exhibiting a local minimum around x = 17.5. The composition dependence of T_c1 is found to be very similar to that of V_T of As₂₀Te_80−xGa_x glasses. Based on the present results, it is proposed that the composition x = 15 and x = 17.5 correspond to the rigidity percolation and chemical thresholds, respectively, of As₂₀Te_80−xGa_x glasses.     

The composition dependence of electrical switching behavior of Ge7Se93−xSbx glasses

Bulk Ge₇Se_93?xSb_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 Ge₇Se_93?xSb_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 Ge₇Se_93?xSb_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 Ge₇Se_93?xSb_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.     

Change of short-range order with temperature and composition in liquid GexSe1−x as shown by density measurements

Density measurements were performed on melts of the binary chalcogenide system Ge_xSe_1?x (0 ? x ? 0.5) up to 1000°C. The isotherm of molar volumes V_m at 750°C shows a relative maximum near GeSe₂. Molar volumes of the system behave linearly between GeSe and Se at 1000°C. V_m's of melts between x = 0.30 and x = 0.35 decrease at high temperatures on heating. The anomalous density behaviour of the melts clearly shows a change of short-range order from a less to a more densely packed structure, caused by the development of a pσ-bonding system. Within the composition range $\text{0 ? x ?}\text{1}\text{3}$ the short-range order at lower temperatures is determined mainly by $\text{GeSe}{}_{\mathrm{<mtext>4</mtext><mtext>2</mtext>}}$ tetrahedra linked directly corner-to-corner or via Se atoms. At higher temperatures pσ bonds arise more and more, even in melts rich in selenium. Within the composition range $\text{1}\text{3}\text{? x ? 0.5}$ the short-range order is mainly determined by a distorted octahedral configuration, even at lower temperatures. From the short-range orders of melts and from crystalline structures of GeSe₂ and GeSe, the tendency of glass formation from the melt is discussed in detail.     

Extended elastic model for flow in metallic glasses

We report that both shear and bulk moduli, not only shear modulus, are critical parameters involved in both homogeneous and inhomogeneous flows in metallic glass. The flow activation energy (?F) of various glasses when scaled with average molar volume V_m, which is defined as flow activation energy density ρ_E = ?F/V_m, can be expressed as: . The extended elastic model is suggestive for understanding the glass transition and deformation in metallic glasses.     

Spectroscopy,oxidation-reduction behaviour and DC conductivity of a vanadium-containing barium aluminoborate glass

The vanadium(IV)-vanadium(V) equilibrium in a 37.5BaO, 5.0Al₂O₃, 57.5B₂O3 mol% + X mol% V₂O₅ (where X = 0.25?32.5) glass system has been studied as functions of temperature, partial pressure of oxygen and total vanadium concentration of the melt. The vanadium(V)/vanadium(IV) ratio in the melt increased with increasing partial pressure of oxygen, lowering temperature of melting, and with increasing total vanadium content of the melt. With X ? 10, the vanadium(V)/vanadium(IV) ratio became almost independent of the total vanadium content of the melt.With this knowledge of oxidation-reduction behaviour, a series of glasses containing 2.8?32.5 mol% V₂O₅ (at about 4 mol% intervals) and having a constant vanadium(IV)/vanadium(V) ratio (0.17) were prepared. Density, electronic absorption spectrum (both d-d and charge transfer transitions), and ESR of these glasses were measured. Optical and ESR spectra of these glasses indicated the vanadium(IV) to be present as vanadyl ion, VO²⁺; g| decreased monotonically with increasing vanadium content of these glasses, whereas gβ remained unchanged. The charge transfer transition energy due to vanadium(V) decreased, and the extinction coefficient increased by orders of magnitude with increasing vanadium content of the glass; the most striking changes occurred at X ≈ 10 mol%. DC conductivity of these glasses was measured at different temperatures; a plot of log (?/T) versus 1/T produced straight lines. The slope of these lines remained almost constant (39 ± 1 kcal/mol) for the glasses containing up to about 10 mol% V₂O₅; with further increase of V₂O₅ the slope decreased sharply.It has been concluded that the abrupt changes in properties like partial molar volume of V₂O₅, charge transfer spectrum of vanadium(V), activation energy of polaron hopping — all of which occurred around X ≈ 10 mol% — is due to a major change in the nature of vanadate groups rather than vanadium(IV) in these glasses.     

Fast relaxation processes in glasses as revealed by depolarized light scattering

Applying tandem-Fabry-Perot interferometry together with a double monochromator, depolarized light scattering spectra were measured in order to investigate the fast relaxation processes and vibrations in molecular, ionic and polymeric glasses in the 1–5000 GHz range covering temperatures from the glass transition temperature T_g down to some 10 K. In addition to the boson peak, the spectra reveal quasi-elastic contributions that we attribute to (i) a nearly constant loss in the frequency range below ≅10 GHz and (ii) a power-law contribution with positive exponent α at higher frequencies. In the majority of glasses the latter may be attributed to thermally activated dynamics in asymmetric double well potentials as previously found for the light scattering spectra in silica. Following the Gilroy–Phillips model the exponent α shows a master curve as a function of T/V₀ for the various glasses, where V₀ specifies the width of the exponential distribution of barriers g(V), i.e., g(V) ∝ exp(−V/V₀). The parameter V₀ is found to be ∼T_g/2 in most cases. The relative strength of the dynamics in asymmetric double well potentials and the nearly constant loss contribution is different in the glasses studied.     

Structure and properties of Ni60(Nb100−xTax)34Sn6 bulk metallic glass alloys

Refractory bulk metallic glasses and bulk metallic glass composites are formed in quaternary Ni-Nb-Ta-Sn alloy system. Alloys of composition Ni₆₀(Nb_100−xTa_x)₃₄Sn₆ (x = 20, 40, 60, 80) alloys were prepared by injection-casting the molten alloys into copper molds. Glassy alloys are formed in the thickness of half mm strips. With thicker strips (e.g., 1 mm), Nb₂O₅ and Ni₃Sn phases and the amorphous phase form an in situ composite. Glass transition temperatures, crystallization temperatures, and ΔT_x, defined as T_x1 − T_g (T_x1: first crystallization temperature, T_g: glass transition temperature) of the alloys increase dramatically with increasing Ta contents. These refractory bulk amorphous alloys exhibit high Young’s modulus (155-170 GPa), shear modulus (56-63 GPa), and estimated yield strength (3-3.6 GPa).     

Some features relevant to switching processes in the amorphous semiconductor Si12Ge10As30Te48

The current-voltage characteristics of the amorphous semiconductor glass Si₁₂Ge₁₀As₃₀Te₄₈ have been measured under various experimenal conditions. The experimental results show that the resistance of the device in the ‘off’ state and the threshold voltage for the onset of switching action decrease with increasing the maximum current (I_p) passing through the device in the ‘on’ state, and that the threshold input power to set in the switching action is practically independent of temperature. The stability and the consistency of the device depend on the magnitude of I_p. When I_p is increased to a certain value the glass within and near the current filament between the electrodes become softened, and when it reaches a critical value the device is changed from its ‘switching-on’ state to a ‘memory’ state. All the results are in good agreement with the model that the filament formed to cause switching consists of a mixture of crystalline domains and amorphous domains with phase separations.     

Small polaron conduction in V2O5P2O5 glasses

Dc conductivity measurements have been made between 90 and 520 K on three bulk samples of V₂O₅P₂O₅ glass. Heat treatment is found to result in a reduction of the activation energy at a given temperature and this is most noticeable at low temperatures. The behaviour at low temperatures can be described using Mott's variable range hopping arguments, and at high temperatures by non-adiabatic small polaron hopping between nearest neighbours. At intermediate temperatures a simple model is used in which excitations by optical and acoustic phonons are considered to make independent contributions to the jump frequency. Mott's theory is extended to the polaron case for $\text{T>}\text{1}\text{4}\text{?}$ and is shown to be in good agreement with results. Values for $\text{r}{}_{\mathrm{<mtext>p</mtext>}}\text{(～2.8}\text{A}\text{?}\text{)}$ the polaron radius and $\text{α(～3.5}\text{A}\text{?}{}^{\mathrm{?1}}\text{)}$ the electron decay constant are shown to be consistent with the model for small polarons. A method is suggested for obtaining α and N(E_F) from the ac conductivity and the slope of 1nσ versus $\text{1}\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ at low temperatures. Values of N(E) are obtained which correlate with those obtained by the previous analysis. This implies that the disorder energy separating adjacent sites Δ₀ is large (～0.4 eV) in these materials.     

Indentation response of glass with temperature

The mechanical response of different glasses to a Vickers indentor has been investigated between room temperature and T_g+50 °C. The permanent deformation, from which hardness is estimated, as well as the brittle fracture characteristics, allowing for an evaluation of the fracture toughness, were measured and analysed. Comparison between a standard float glass and advanced glasses such as chalcogenide (with mainly covalent bonding) and metallic glasses was made to get a more general insight into high temperature indentation behaviour. As temperature increases, the glass response becomes more and more time-dependent, and in the vicinity of T_g the permanent deformation was observed to increase rapidly for all glasses. Further, while the standard float glass showed an enhanced apparent toughness at elevated temperatures due to a brittle to ductile transition, almost no change in apparent toughness was revealed in the GeAsSe glass emphasizing the time-dependent response of glass at elevated temperature.     

Dielectric behaviour in a vanadium phosphate glass

The dielectric constant and conductivity of 80% V₂O₅: 20% P₂O₅ glass has been measured in the frequency range 10² to 10⁹Hz and in the temperature range 80 to 350°K. It is shown that the dielectric behaviour over these ranges is described by a Debye type relaxation process with distribution of relaxation times. A method is proposed to determine the width of distribution from the data at fixed frequencies and different temperatures. The width of distribution increases at frequencies ω > 10/τ, which leads to an a.c. conductivity at these frequencies almost linearly proportional to frequency and independent of temperature. The estimated value of the static dielectric constant of about 30 was found to decrease with temperature while the infinite frequency dielectric constant of 10 was independent of temperature. The carrier concentration calculated from the dielectric relaxation time and the d.c. conductivity through a thermal diffusion model shows reasonable agreement with direct measurement using electron paramagnetic resonance.     

The application of small polaron theory to transition metal oxide glasses

A review of the experimental data on a range of transition metal oxide glasses shows a strong relationship between activation energy and site spacing for glasses based on V₂O₅. The behaviour confirms that transport in amorphous V₂O₅ and vanadate glasses is by adiabatic hopping with an appreciable polaron bandwidth, while non-adiabatic hopping between isolated centres is apropriate for glasses based on most other transition metal oxides. The behaviour of vanadium is linked to the retention of a well-defined VO^5? structural unit in the glass network and to optimisation of the packing of this unit within the glass.     

Effect of γ-irradiation on non-linear I-V behaviour and thermoelectric measurements in amorphous semiconducting AsSeTe system

An amorphous semiconducting compound of the composition As₂SeTe₂ is studied for its non-linear current-voltage characteristics, conductivity and thermoelectric measurements. It has been observed that γ-irradiation of the sample gives a lower threshold voltage and greater thermoelectric power. The threshold voltage shifts to a lower voltage value for the irradiated sample. The non-linear I-V behaviour and p-type nature of unirradiated and γ-irradiated As₂SeTe₂ are discussed on the basis of charged defect states existing and also created after γ-irradiation in the material. The thermoelectric power for unirradiated and γ-irradiated As₂SeTe₂ is discussed on the basis of a quantity, related to conductivity and thermoelectric power which explains to a great extent the effect observed experimentally.     

Thermally stimulated depolarisation current in sodium-potassium borosilicate glass

The thermally stimulated depolarisation current (TSDC) has been measured in sodium-potassium borosilicate glass of composition 15% Na₂O, 13.64% K₂O, 19.47%B₂O₃ 51.89% SiO₂. Gold and aluminium electrodes were used for these measurements and TSDC curves of NaK borosilicate glass were obtained by varying the polarising temperature T_p and polarising voltage V_p in a systematic manner. The observed peaks were remarkably reproducible. Partial heating experiments were performed to investigate the activation energy of the thermally released carriers. By an analysis of the current peaks, the relaxation time τ(T) and hence the activation energy of relaxation of the carriers were determined. Results of dc measurements on the same samples are presented for comparison.