Experimental evidence of energy-controlled switching in amorphous semiconductors

The switching delay time and transition time, and threshold voltage for the onset of switching in the amorphous semiconductor Si₁₂Ge₁₀As₃₀Te₄₈ have been measured under various conditions using rectangular voltage pulses. The results show that both the threshold voltage and the delay time decrease, but the transition time increases with increasing temperature; and that these switching properties are strongly dependent on the width and the repetition rate of applied pulses. It is proposed that the delay time is associated with the time required for the formation of a filament to cause switching, and that the transition time is associated with the transit time of a carrier across the switching filament. All the experimental phenomena indicate clearly that the switching process is energy-controlled.     

Switching in amorphous semiconductors

This article presents experimental data on switching and other related properties of (1?x)As₂Se₃·xSb₂Se₃ materials. These data indicate that the threshold switching mechanism is electronic in thin devices (<10 μm) and thermal in thick devices. In addition, it raises some questions about the crystallization/phase separation model for memory switching.     

Voids in amorphous semiconductors

Small angle X-ray scattering (SAXS) has been used to investigate density fluctuations occurring in amorphous semiconductors prepared by sputtering, evaporation and electrodeposition. Correlations of the total number of dangling bonds determined by SAXS, optical absorption, and ESR signals have been made. The density deficits from the density of the FC-2 crystal are in some cases accounted for by the voids. It is argued that models based on domains 10–15 Å in size are not supported by the SAXS data.     

Avalanche ionization in amorphous semiconductors

Using the results of a previous treatment of hot electrons in amorphous semiconductors, the avalance ionization coefficient, α, has been calculated as a function of the electric field. The Shockley hot-electron model was used. Because there is no conservation of crystal momentum, the threshold for electron-hole pair production is lower than it would be in a crystal of the same energy gap, and above the threshold the probability per unit time for pair production is quadratic in the energy. The results for α has been used to calculate the current-voltage relation for avalanche injection, and to study the avalanche process as a mechanism for switching.     

Optical properties of amorphous semiconductors

The density of states and the dielectric constant of electrons in crystalline solids can be discussed easily by using the band structure scheme. This is due to the fact that a crystal is defined by both a short range order and a long range order of atoms. If the long range order is relaxed this procedure no longer works and one has to calculate the measurable quantities directly, which involves some configurational averaging process.     

Electrical switching in chalcogenide liquid semiconductors

Reversible electrical switching from a higher resistance off-state to a lower resistance on-state has been observed in several chalcogenide semiconductor alloys both in the solid and liquid phases. Experiments conducted between 100 and 500°C showed that the threshold switching voltage decreases with increasing ambient temperature, and that the on-state resistance remains relatively constant up to the melting region but generally increases thereafter. The alloy Se_0.77Te_0.13S_0.10 shows a preserved on-state which can be maintained using 60 Hz ac in both the solid and liquid phases. The on-state can be reversed to the off-state by decreasing the applied field, and hence is not a conventional memory state. Below the melting region (250°C) the on-state I–V curve has a distinct knee or barrier voltage above which the current increases steeply. Above the melting range the knee becomes less clearly defined and occurs at a lower voltage. Below 345°C the above alloy has an activation energy of 0.37 eV. At this temperature an abrupt slope change or a discontuinity exists in the temperature dependence of off-state resistance, threshold field, threshold power and on-state resistance.     

Pressure-induced structural transformations in amorphous semiconductors

Pressure-induced amorphous-to-crystalline transformations in Ge, As and Se have been studied thermodynamically and microscopically. A thermodynamic analysis based on experimental data suggests that the structural transformations in Ge and Se can be regarded as a phase transition and a structural relaxation, and that the transformation in As may be relaxational. There is a correlation between the transformation pressure and the covalent bond energy, but such a correlation cannot be seen in the thermal crystallization temperature.     

Thermoelectric power of amorphous compound semiconductors

Thermoelectric power measurements as a function of temperature have been made on four V–VI amorphous chalcogenides, ten IV–VI amorphous chalcogenides of the Ge_xTe_1?x system, and five amorphous IV–V materials. In the IV–V and the intermediate composition IV–VI materials, the charge transport cannot be described on the basis of conduction at only one energy level. The former exhibit characteristics of conduction both in extended states and in localized states at the Fermi level. Transport in the V–VI materials can be formally described in terms of conduction at one level in either the chaotic band or the small polaron models, but the use of a two-channel model with transport simultaneously in both p-type extended and localized states seems the most promising.     

Temperature variation of the mobility gap in non-polar amorphous semiconductors

For a non-polar amorphous semiconductor such as a-Si, we derive an explixit formula for (?E_g/?T)_V, the derivative with temperature of the mobility gap E_g at constant volume V. Within the framework of second-order perturbation theory for the electron-phonon (eφ) interaction, many of our physical assumptions are fundamentally different from those that apply to the crystal phase. The principal ingredients of our model are: (1) the random-phase-model (RPM); (2) the principle of non-conservation of particle momentum in the eφ interaction; and (3) the deformation potential approximation. Narrowing of E_g is found with increasing values of the temperature T. At very low T, we have $\text{(?E}{}_{\mathrm{<mtext>g</mtext>}}\text{/?T)}{}_{\mathrm{<mtext>V</mtext>}}\text{≌ ? ¢A · c}{}_{\mathrm{V}}\text{(T)}$, where c_V(T) is the average lattice specific heat per mode at constant volume and ¢A is a positive dimensionless quantity in the model. By contrast with low-temperature behavior of the crystal, this result implies that the mobility gap at constant volume dynamically responds to the phonomic “gas” of the disordered lattice. The high-T limit yields behavior quite similar to that of the crystal phase. We find $\text{(?E}{}_{\mathrm{<mtext>g</mtext>}}\text{/?T)}{}_{\mathrm{V}}\text{≌ ? x ¢A · k}{}_{\mathrm{<mtext>B</mtext>}}$, where k_B is Boltzmann's constant and the parameter x, expected to be confined to the interval $\text{1}\text{2}\text{? x ? 1}$, measures the admixture of the optical-phonon and acoustical-phonon coupling strengths.     

On the character of local electric fields in amorphous semiconductors

On the basis of the agreement of calculated and measured parameters of some amorphous semiconductors, an argument is stated in favour of the central-symmetric type of random local electric fields. A possible mechanism of the transport of electrons is discussed.     

The structure of tetrahedrally coordinated amorphous semiconductors

The diffracted X-ray intensities, F(k)'s, and radial distribution functions of Ge, some III–V compounds, and Ge_0.5Sn_0.5 are presented. It is shown that the radial distribution functions of the III–V compounds are similar to those of Ge. If the continuous random network, as developed for amorphous Ge, is also applicable to the III–V's, then several bonds between like atoms (wrong bonds) must be present. It is argued that the energy of wrong bonds may not be sufficiently high to prohibit their formation.     

Conducting filaments and switching phenomena in chalcogenide semiconductors

The sizes of the conducting filaments formed after switching operations in chalcogenide semiconductors have been measured as functions of input power in the on-state and specimen thickness using a scanning electron microscope. The experimental results show that the threshold switching involves double injection from both metallic contacts in the sandwich structure, and the combination of the electrothermal and the electronic processes. The conducting filament responsible for the switching consists of two permanent portions, one started from the anode and the other from the cathode, which have undergone a permanent change in material composition and structure after even one switching operation; and one temporary portion between the ends of the two permanent portions, which has not undergone any change in material composition or structure after many switching operations. The size of each portion depends strongly on the current level in the on-state, at which the filament is formed.     

Electronic-thermal switching and memory in chalcogenide glassy semiconductors

A constant value of the activation energy of direct current conductivity in chalcogenide glassy semiconductors is a strong evidence of the existence of negative-U centers in these materials. Multiphonon tunnel ionization of negative-U centers in strong electric fields is the cause of current-voltage characteristic nonlinearity. Taking into account the Joule heating allows to obtain electronic-thermal instability and then form an S-shaped current-voltage characteristic. The model described in this paper is in good agreement with the available experimental data on chalcogenide glassy semiconductors.     

Conduction and switching phenomena in covalent alloy semiconductors

High field breakdown often leads (i) to a regenerative structural change in the material, or (ii) without any material change, to a conducting state which is maintained only above a certain holding current value. It is suggested that breakdown can be triggered by different processes depending on the temperature and the electrode configuration. The observed high field and non-equilibrium phenomena will be discussed in relation with the several proposed breakdown mechanisms. The material characteristics yielding the different switching effects will be discussed.     

Photoluminescence of geminate and non-geminate pairs in amorphous semiconductors

Rates of radiative recombination of geminate and non-geminate pairs and the corresponding lifetimes are calculated at thermal equilibrium. It is shown that two kinds of geminate recombinations occur in amorphous semiconductors: (I) through the recombination of the usual geminate pairs (non-excitonic) and (II) through the recombination of those excitons that have relaxed to the tail states. The latter geminate pairs can exist in singlet and triplet spin states, only singlet is considered here, whereas former ones do not have spin dependence. Examples of recent observations and their comparison with the theoretical results are presented.