Write-erase Cycles Research Articles

Spatial and temporal light modulator of the photosemiconductor–liquid crystal type exhibiting texture and cholesteric–nematic transitions

A study was made of the operating characteristics of a spatial and temporal light modulator of the photosemiconductor–liquid crystal type exhibiting an orientational texture transition as well as a cholesteric–nematic phase transition. The photosemiconductor was a chalcogenide film of the selenium–arsenic type and the liquid crystal was a composite based on cyanbiphenyls, azoxy compounds, complex esters, and tolans. The modulator could be used for reversible recording, long-term storage, and positive and negative imaging without recourse to polarization optics or preliminary orientation of the liquid crystal. The maximum sensitivity was 1 μ W/cm2, the resolution was at least 65 pairs of lines/mm, and the minimum write–erase cycle time was 0.75 sec.

A high-density, high-performance EEPROM cell

The theory, design, and performance data of a new high density, high performanee EEPROM cell is described. The memory cell is fabricated using standard n-channel double-polysilicon processing together with thin-oxide technology, and has an area of 24 × 24 µm 2 using 4-µm design rules. The cell is of the floating gate type, and employs avalanche injection of electrons and holes from a common injector. The use of thin oxide (≃ 100 A) between the n+-p+injector region of the substrate and the floating gate of the memory transistor makes operation possible using voltages of less than 20 V. Write and erase times are 10 ms with an endurance to write-erase cycling of 105cycles. The power dissipation during writing and erasing is 10 mW.

Non-volatile memory properties of metal/SrTiO 3/SiO 2/Si structures

Non-volatile MIOS-type semiconductor memory elements were fabricated on silicon using electron-beam-evaporated SrTiO 3 as the second insulator. The charge storage properties were characterized for Au/SrTiO 3/SiO 2/Si structures. Our results show that a short-time post-deposition oxygen annealing is essential to anneal out the radiation damage resulting from electron beam deposition. The devices on n-type silicon substrates show fast switching for a positive applied pulse and a much lower switching speed (longer than 20 ms) for a negative pulse, which is believed to be caused by the minority carrier restriction. The devices show a logarithmic decay of the flat-band voltage as a function of time, with a rate of 0.4 V decade -1 for stored electrons and 0.5 V decade -1 for stored holes. The devices can survive 10 4 write-erase cycles of endurance testing. An inversion of the surface silicon layer is found for devices on p-type substrates subjected to high temperature oxygen annealing.

Degradation properties in metal-nitride-oxide-semiconductor structures

Degradation properties in metal-nitride-oxide-semiconductor (MNOS) structures are investigated using mainly p-channel MNOS transistors. A model is proposed on the basis of various experimental results, attributing the degradation to the passage of hole current through the SiO2 layer, followed by creation of hole traps in the SiO2 layer, and creation of interface states at the Si-SiO2 interface. A theoretical treatment of the enhancement of hole conduction in the degraded SiO2 layer of the p-channel thick-oxide MNOS transistor is performed, and the hole traps created in the SiO2 layer appear to be E′ centers when the experimental results are fitted to the theoretical calculations. The nature of the interface states created by write-erase (W/E) cycling is also discussed, comparing the experimental results using a p- and an n-channel MNOS transistor.

Photodichroic Silicate Glass Surface

The photodichroic silicate glass surface is a direct-read-after-write laser recording material which permits instant optical recording without processing of any kind. The required writing energy density at mega Hz rate and higher is 40 mj/cm2. An even better writing sensitivity is expected for a preconditioned glass surface which is yet to be demonstrated at mega Hz rates. The inherent resolution capability of this material is better than 2000 cycles/mm, which is consistent with the fine grain structure of the photodichroic silicate glass surface (grain size <0.02 pm). The photodichroic silicate glass surface can be pre-recorded with tracks of width 1µm or less. It is potentially a pinhole-free recording material due to the inherent homogeneity of the silicate glasses, and to the very small sizes of the photosensitive crystals, relative to the wavelength of light and the thickness of the photosensitive glass sur-face layer. The physical effect of recording is irreversible; yet the recorded spots or bits are erasable. The erased 1-bit can be distinguished easily from the 0-bit (of course, not distinguishable in the normal read mode), when necessary. There is also a mechanism to prevent erasing after recording and error corrections are completed. The writing is done with a polarized red light, and nondestructive read is done using a near infrared beam, with the recorded bits between crossed polarizers. The erasure is to re-expose with the write beam whose polarization direction has been rotated 45° with respect to the initial polarization exposure direction. In this manuscript, the physical effect of recording is discussed in detail, including the intensity dependent writing sensitivity, the threshold effect, and the relationships among observed spot size, contrast and the writing energy density. Recording in mega Hz rates as well as write-erase cycling are demonstrated.

Hologram writing in lithium niobate: Beam coupling and the transport length in the bulk photovoltaic effect

Holograms were written in a short-circuited antireflection-coated crystal of iron-doped lithium niobate, using essentially uniform illumination of the whole crystal to minimize light-envelope space-charge field effects. The results were compared with the predictions of a computer model which allows for space-charge feedback and for the modification of the writing light pattern by interaction with the hologram being written, but which assumes short transport lengths of electrons (on the scale of the hologram grating spacing) for diffusion, drift, and the bulk photovoltaic effect. The measured ’’virtual’’ field Ev to which the bulk photovoltaic effect is equivalent (for these assumptions) was about 45 kV/cm so that the bulk photovoltaic effect, rather than diffusion, dominated the writing process under the above conditions. The development of the diffraction efficiency was consistent with the computed data but the beam coupling (transfer of energy between the writing beams) was much greater than predicted. The observed beam coupling could occur with the drift-equivalent process of the bulk photovoltaic effect if the transport distance in this effect was on the order of 24 nm, which is larger than has been generally considered probable. In repeated write-erase experiments the beam coupling was variable, more so than the diffraction efficiency. The variability is, we believe, due to parasitic holograms such as are involved in the scattering process observed with these crystals and also to residual space-charge fields from repeated write-erase cycles. If the transport length is indeed long, the term καI used to describe the bulk photovoltaic effect (where κ is a parameter specific to a dopant, α is the absorption coefficient, and I is the intensity) is inapplicable to hologram writing conditions since it implies short transport length. A new representation for this bulk photovoltaic effect allowing for an arbitrary transport length is proposed. This representation is shown to introduce a significant phase shift in the refractive-index pattern relative to the light pattern and can thus account for the beam coupling observed.

Metal-nitride-oxide-semiconductor transistor characterization using implanted MNOS capacitors

MNOS capacitors were fabricated using ion implantation to replace the conventional diffusion step required to provide a source of carriers for inversion layer formation during write-erase cycling. The electrical characteristics of the implanted capacitors closely approximate those of the diffused capacitors. A single implant at 25 keV of 5 × 10 14 ions cm -2 using shadow mask metal dots for masking and a 500 °C sinter operation for activation of the implant was shown to be optimum for an Si 3N 4 thickness of 475 Å. No deleterious effects were observed to result from the implantation step.

Discharge of m.n.o.s. structures

The discharge behaviour of m.n.o.s. structures with no applied voltage is investigated at temperatures from −155° C to 120° C for times from 10 to 104s. The decay rate is practically independent of the temperature up to 80° C but increases significantly at 120° C. The discharge curves are explained by direct tunnelling from traps, combined with the thermal excitation of these traps. The time dependence of the charge centroid is ascribed to a redistribution of the stored charge in the nitride. A correlation between the decay rate increase after write-erase cycling with the increased nitride conductivity is found.

Amorphous GdCo Thin Films for High Density Thermomagnetic Recording

Experiments on the stability of small bits written in amorphous GdCo thin films were performed. It was confirmed that the bits written in films which are inhomogeneous along their thickness are stable and this stability did not decrease after many write-erase cycles with a laser beam. It can be concluded that amorphous GdCo thin films which are inhomogeneous along their thickness and have a compensation temperature near ambient are suitable for the high-density thermomagnetic recording application.

Interface states and memory decay in MNOS capacitors

A method for measuring the density and energy distribution of interface states at the silicon-silicon dioxide interface in MNOS capacitors is described. Application of this method to capacitors fabricated on n-type substrates obtains a logarithmic dependence of the decay rate for stored holes on the interface state density generated during write-erase cycling. The slope of this relationship is strongly dependent on sample preparation. No dependence of the decay rate of electrons on interface state density was observed.

Measurement of the effect of write-erase cycling on noise in MNOS memory transistors

p-channel MNOS memory transistors with 25-Å oxide, 550-Å nitride are investigated. Changes in memory characteristics such as high-conductivity and low-conductivity threshold voltages are examined as a function of the number of write-erase cycles taking the amplitude and the duration of the pulses as parameters. Simultaneous measurements of the channel carrier mobility and low-frequency noise spectrum give new information. Changes in noise properties after cycling depend on the amplitude and duration of the write-erase pulses. Beyond about 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> write-erase cycles the memory window decays, the channel carrier mobility drops, while the noise exhibits a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/f^{2}</tex> spectrum in the lowest frequency region. Experimental results are interpreted by the formation, during cycling, of a high density of hole traps distributed in energy and located inside the insulators near the oxide-nitride interface.

Electrically erasable buried-gate nonvolatile read-only memory

An electrically erasable buried (floating) gate memory is described. The memory is programmed by electron injection by junction avalanche. An internal voltage multiplication scheme using varactor bootstrapping is used which makes nearly 40 V available at the memory cell for programming, yet requires input voltages no higher than 25 V. Erasure takes place by modified Poole-Frenkel conduction in a Si 3 N 4 film of 700-A thickness which overlays the buried gate. Standard silicon gate p-MOS processing is used with only minor modifications. Memory retention is excellent and is extrapolated to many years even at 150°C. Above 298 K, the time required for the charge to decay to one-half its initial value is given by \log t_{1/2} = \frac{5254}{T}-\frac{771}{T}√V_{E}(s) where T (K) is the temperature and V E is the erase voltage. The endurance of the buried-gate memory is approximately 10 K write-erase cycles and is limited by electron trapping in the insulator. A fully decoded 1024-bit memory chip was designed and fabricated.

Electron-beam damage in cathodochromic sodalite

In recent years sodalite, Na8Al6Si6O24X2, where X is Cl, Br, or I, has received wide attention as a cathodochromic material. The utility of sodalite, however, is severely restricted because of damage produced after repeat write-erase cycles. This damage was investigated and found to be of two kinds. The first kind was considerable loss of potential color centers, i.e., after a certain number of write-erase cycles the material would not color up to the original optical density. The second kind was build up of permanent coloration which could not be removed on exposure to bleaching light but could be removed by heat. Electron microprobe, electron spectroscopy for chemical analysis, thermogravimetric analysis, x rays, and a variety of other analytical tools were used to understand the causes of damage. Considerable loss of sodium and a smaller loss of halogen ions on continued electron-beam irradiation was found to be closely associated with the damage. Possibility of heat generated by the electron beam was discounted as the source of damage.

Sensitivity and fatigue of LiTaO3 for holographic recording

The sensitivity of LiTaO3 crystals to hologram formation has been observed to vary with impurity concentration. For a writing wavelength of 488.0 nm and power density of 1.1 W/cm2 the sensitivity varied from a value comparable to the most sensitive doped LiNbO3 for an impure crystal to a value more than 5 orders of magnitude smaller for a purer crystal. Fatigue effects were observed upon write-erase cycling. These effects were dependent upon writing and erasure polarization and power density and could be minimized by proper choice of optical parameters.

Laser writing and erasing on chalcogenide films

Optical switching from the mostly crystalline to the amorphous state has been studied in TeGeAs alloy films using ∼ 5-nsec pulses from a nitrogen laser pumped dye laser. These ``reverse-mode'' written spots were switched (erased) by both optical and thermal means. Microscopic examination of the films, thermally crystallized before optical writing, indicates the existence of a two-phase system consisting of Te crystallites in a background of amorphous Ge-rich material. A similar condition is again found after a write-erase cycle has occurred. Computer calculations on the thermal history of the crystalline film resulting from a 5-nsec pulse indicate initial cooling rates of the order of 1010°C/sec.