Masakuni Suzuki

Nonvolatile memory based on reversible phase transition phenomena in telluride glasses

The phase transitions from amorphous to crystalline states, and vice versa, by applying electrical pulses were studied for sandwich structures of metal/GexTe100-x thin film/metal. A systematic study of the crystallization temperature in GexT100-x(5\leqslant x \leqslant 30) thin films has been carried out. In some compositions, more than 104 repetitions of amorphous to crystalline states, and vice versa, were attained by the application of electric pulses. A model is proposed to explain the results observed.

Submicron Nonvolatile Memory Cell Based on Reversible Phase Transition in Chalcogenide Glasses

Electrically rewritable nonvolatile memories using chalcogenide semiconductors were studied. The memory cell size was changed from 0.3 to 1.5 µm using a focused ion beam. This material can be used for nonvolatile random access memory. Reversible phase transition between the amorphous and crystalline states, which is accompanied by a considerable change in electrical resistivity, is exploited to store bits of information. The currents for write/erase were decreased with reducing memory cell size. In the memory cell of 0.6 µm, more than 104 repetition cycles of the phase transition were attained by the electric pulses. The voltages for the crystallization and amorphization processes were 2 V and 2.2 V, respectively.

Some properties of Sb2Te3−xSex for nonvolatile memory based on phase transition

Composition dependence of properties of Sb2Te3−xSex in the range 0⩽x<3 were studied using differential thermal analysis and X-ray diffraction. Sb2Te3−xSex form solid solution for 0<x⩽1.25 and 2.75⩽x<3. A systematic study of crystallization temperature in Sb2Te3−xSex (0⩽x⩽2.75) thin films prepared by flash evaporation was carried out. In preliminary experiments for some compositions, more than 103 repetitions between amorphous and crystalline states were attained by the application of electric pulses.

Nonvolatile Memory Based on Phase Change in Se–Sb–Te Glass

The phase transitions from the amorphous to crystalline states, and vice versa, of Se–Sb–Te films by applying electrical pulses have been studied. This material can be used as nonvolatile memory. The reversible phase transition between the amorphous and crystalline states, which is accompanied by a considerable change in electrical resistivity, is exploited as a means to store bits of information. The nonvolatile memory cells are composed of a simple sandwich structure (metal/chalcogenide thin film/metal). More than 104 write/erase cycles were attained by applying electric pulses. In this case, the voltage and pulse width of crystallization and amorphization processes were 2.4 V, 2.0 µs, 2 V and 0.1 µs, respectively. The melting point of the Se–Sb–Te system is lower than that of the Ge–Sb–Te system, so that the current density for the amorphization process can be decreased to about 30 mA/µm2.

Impurity effects of some metals on electrical properties of amorphous As2Se1Te2 films

Amorphous As2Se1Te2 (a-As2Se1Te2) films, into which Cu or Cd ions were doped by thermal diffusion below the glass transition temperature exhibited the increase of conductivity by several orders of magnitude. In addition, these samples exhibited enhanced photoconductivity and the activation type conduction. From the measurements of the thermoelectric power, it was found that Cu doped samples were p-type and Cd doped samples were n-type. The impurities was also studied by SIMS.

Influence of Mn Impurity in Te-As-Ge-Si Glasses

In order to investigate the roles and an incorporation scheme of impurities in amorphous semiconductors, electrical, optical and ESR measurements are carried out for Te48As30Ge10Si12 glasses doped with various amounts of Mn. A decrease of the electrical activation energy Ea and broadening of the optical absorption edge are caused by the introduction of Mn, while the optical gap does not change appreciably. An ESR signal with g=4.3 due to a small amount of Mn incorporated in the amorphous network is observed. The ESR center is explained by the double acceptor characteristic of Mn on the basis of the g-value and the magnitude of the hyperfine structure constant. It is concluded that the decrease of Ea is caused by the shift of the Fermi level EF due to holes supplied by the double acceptors. From the linear relation between the shift of EF and the center density of the ESR signal with g=4.3, the upper limit of the density of localized states around EF is estimated to be 1.5×1020 cm-3 eV-1.

Photoelectrochemical properties of rhodamine-C18 sensitized p-CuSCN photoelectrochemical cell (PEC)

Abstract Surface states in p-type CuI thiocyanate (CuSCN) were detected from I−V characteristics, diffuse reflectance spectra, and photocurrent action spectra. The p-CuSCN films are sensitized by rhodamine with octadecyl-alkyl chain, and the sensitized photocurrent is observed with the visible light illumination. In spite of the surface states in p-CuSCN, the maximum photocurrent quantum efficiency (gfmax) at λ = 560 nm, in 1 × 10−4 M KI + I2 solution, pH = 6, reached ∼ 8.6%, where the surface dye concentration of photocathode Cu/p-CuSCN/Dye was 1.1 × 1014 molecules cm−2. Photocathodes were biased at −0.25 V versus AgCl/Ag to give a zero dark current. From the variation of φ values with the reduction potential of electron acceptors, the cathodic sensitization mechanism presented is further confirmed.

Composition Dependences of Electrical and Optical Properties of AsxTe90-xGe10 and As35Te55Ge10-xSix Glasses

The activation energy for conduction, the optical energy gap and the slope of the exponential tail of the optical absorption were measured for AsxTe90-xGe10 and As35Te55Ge10-xSix glasses and their composition dependences were studied. It was found that there exist some correlations among the compositional variations of the activation energy for conduction, of the slope of the exponential tail of the optical absorption and of the ESR signal with g=2.00 from doped Mn as a probe. The activation energy for conduction is found not to change in step, with the optical energy gap determined by the bond strength but to be affected by the shift of the mobility edge due to a change in the randomness of the amorphous structure or the shift of the Fermi level caused by a change in the density and the distribution of the localized gap states.

The reversible photostructural change studied by ESR of Mn2+ in As2Se3 films

Abstract The reversible photostructural change is studied from a microscopic point of view using ESR of Mn 2+ doped in As 2 Se 3 films. It is observed that the hyperfine structure constant changes reversibly corresponding to the reversible optical absorption edge shift. The result is explained in terms of increasing randomness by photoirradiation.

Chalcogenide Amorphous Semiconductor Diodes

Thin film diodes with Al/a-As2Se2Te/Sb structures were fabricated by vacuum deposition. Al ions were found to diffuse into a-As2Se2Te films below the glass transition temperature. Measurements of ac conductance and capacitance of the diodes revealed that they consist of two layers of different resistivity which correspond to undoped and Al-doped layers. The upward shift of the Fermi level due to Al doping was inferred from the variation of the forward and backward current with time. Accordingly, the p-n junction formed between the undoped p-type layer and the Al doped n--layer is responsible for the rectification.