K. D. Tsendin

Physics of switching and memory effects in chalcogenide glassy semiconductors

Switching and memory effects in chalcogenide glassy semiconductors have been known for nearly fifty years. However, the physics of these effects remains unclear. Recent interest in this problem is caused by active developments of a new generation nonvolatile memory based on the chalcogenide glass-crystal phase transition. In this paper, we review the main experimental features of switching and memory effects, review and analyze the models of the switching effect. Consider the main characteristics of phase-change memory cells made of various materials. On these grounds, the main advantages of modern phase-change memory cells are presented in comparison with first-generation memory elements.

Nonlinearity of current-voltage characteristics of chalcogenide glassy semiconductors, caused by multiphonon tunnel ionization of negative-U-centers

The behavior of negative-U-centers in high electric fields was considered. It was shown that multiphonon tunnel ionization of negative-U-centers in chalcogenide glassy semiconductors results in a significant increase in the number of conduction electrons, hence, to highly nonlinear current-voltage characteristics. This nonlinearity mechanism well explains the experimentally observed current-voltage characteristic of chalcogenide glassy semiconductors currently used as memory cells.

Room-temperature electroluminescence of Er-doped hydrogenated amorphous silicon

Abstract We have observed room-temperature erbium-ion electroluminescence in erbium-doped amorphous silicon. Electrical conduction through the structure is controlled by thermally activated ionization of deep D − defects in an electric field and the reverse process of capture of mobile electrons by D 0 states. Defect-related Auger excitation (DRAE) is responsible for excitation of erbium ions located close to dangling-bond defects. Our experimental data are consistent with the mechanisms proposed.

Instabilities with S-and N-shaped current-voltage characteristics and phase transitions in chalcogenide vitreous semiconductors and polymers

The conditions necessary for electric current instabilities to arise in strong electric fields are considered for disordered media, such as chalcogenide vitreous semiconductors and organic polymers. The thermal and electronic-thermal mechanisms of instabilities with S-and N-shaped current-voltage characteristics are discussed. The influence of phase transitions from one conduction state to another conduction state on the development of electrical instabilities is analyzed.

Photoluminescence and composition of amorphous As2Se3 films modified with Er(thd)3 complex compound

The photoluminescence and composition of amorphous As2Se3 films modified with an Er(thd)3 complex compound have been studied. A band centered at 1.54 μm, characteristic of photoluminescence from Er embedded in amorphous matrices, has been revealed at room temperature. The composition of thin amorphous As2Se3 films modified with an Er(thd)3 complex compound has been examined by methods of nuclear microanalysis: Rutherford backscattering and nuclear resonant reactions. Dependences of the concentrations of Er ions, oxygen, and carbon on the growth conditions of the films are obtained. It is shown that the Er concentration in a thin film varies nonlinearly as the relative concentration of the starting complex compound increases. In addition, the increase in the Er content of a film is accompanied by a simultaneous rise in the content of such light elements as oxygen and carbon. Comparative analysis of the nuclear microanalysis data and IR spectra demonstrates that, in modification of As2Se3 with the Er(thd)3 complex compound by the given method, the nearest environment of Er in the complex compound is partly preserved.

Current-voltage characteristics of thin Ge2Sb2Te5 films taken using a measuring circuit with a current source

The measuring circuit with a voltage generator, which is traditionally used for studying electric properties of glasslike semiconductors, is compared with an alternative version using a current generator. The results obtained in the latter case reflect the interrelation between the memory formation effect and changes in the electrical parameters more comprehensively. A new electric parameter, viz. hold voltage Uhold, which has not been described earlier, is introduced. The relation between Uhold and the memory formation process during the phase transition is established. The effect of emergence of oscillations in the current-limiting regime in the conduction channel in the film is detected.

Nuclear microanalysis of amorphous As2Se3 films modified with Ln(thd)3 (Ln = Eu, Tb)

The compositions of amorphous As2Se3 thin films modified with the Ln(thd)3 (Ln = Eu, Tb) complexes have been determined by nuclear microanalysis using deuterons provided by an electrostatic accelerator: Rutherford backscattering spectroscopy and nuclear reaction analysis. The rare-earth, oxygen, and carbon contents have been determined as functions of the initial concentration of the complex. The films have been also characterized by IR spectroscopy. Analysis of the nuclear microanalysis and IR spectroscopy results suggests that, in the modified films, the rare-earth atoms retain their local environment, in contrast to thin films of amorphous hydrogenated silicon.

Controlling the U−-center density in Se-As chalcogenide-glass semiconductors by doping with metals and halogens

Temperature dependences of the drift mobilities of electrons and holes are investigated in chalcogenide-glass semiconductors with composition Se95As5, both without impurities and with the impurities Ag and Br. The data obtained indicate that the localized states that control the transport of charge carriers are U−-centers, and that the change in the magnitude of the drift mobility after doping is caused by a change in the concentration of these centers. Estimates of the concentrations of positive and negatively charged intrinsic defects show that their values are similar, equalling ∼1016 cm−3 in impurity-free glasses with the composition Se95As5 and lying in the range 1013–1017 cm−3 when these glasses are doped with Ag, Br, and Cl. It is established that halogen impurities change the concentration of U−-centers most strongly (by two to three orders of magnitude). Analysis of the data obtained shows that the percentage of electrically active Br and Cl impurity atoms is 1%, while for Ag atoms it is 10−2%.

Current instability with an S-shaped I-V characteristic in films of a metal-polymer complex of polyamide acid with Tb+2

The manifestation of current instability as an S-shaped I-V characteristic was detected in thin films of a metal-polymer complex of polyamide acid with Tb+2. The current instability characteristics are comparable to those of the switching effect in chalcogenide glasses. The origin of the current instability in the metalpolymer complex may be related to electrothermal switching. The threshold voltage and current in films ∼0.1 µm thick are 3–7 V and (2−1)×10−2 A/cm2, respectively.

Temperature dependence of the hole concentration in the model of a hole metal with negative-U centers

The temperature dependence of the hole concentration is studied within the model of a hole metal with negative-U centers. Different versions of the model with a simplified representation of allowed bands and a quadratic valence-band dispersion law are considered. It is found that the temperature dependence of the hole concentration can acquire different forms, and the regions of the model parameters corresponding to these forms are determined. The results obtained are used in the discussion of the relation between normal to superconducting properties of high-temperature superconductors.