Hong-Bay Chung

Photoinduced transformations in amorphous Se75Ge25 thin film by XeCl excimer-laser exposure

The scalar photoinduced changes of the optical absorption edge and the optical constants in amorphous Se75Ge25 chalcogenide thin film have been studied using XeCl excimer laser light with a wavelength of 306 nm. The glass transition temperature (Tg) of this material is evaluated to be approximately 220 °C, at which the as-deposited film is initially annealed. The optical energy gaps Eop of as-deposited and initially annealed Se75Ge25 thin films are 1.90 and 1.84 eV, respectively. When initially annealed films are exposed to a 4.05 eV excimer laser (≫Eop) with energy densities of 100, 120 and 180 mJ/cm2, a photodarkening effect that exhibits a decrease in the optical transmittance (Top) and Eop and an increase of the refractive index n and the extinction coefficient k in the vicinity of the absorption edge (1.4–3.0 eV) is observed. In particular, in the case of a thin film exposed by 120 mJ/cm2, Eop and n+ik measured at 673.7 nm(1.84 eV), i.e., an absorption edge of initially annealed thin film, are 160 eV...

Low-energy focused-ion-beam exposure characteristics of an amorphous Se75Ge25 resist

Ion-beam exposure and patterning characteristics of an amorphous Se75Ge25 thin film which acts as a positive resist in focused-ion-beam (FIB) lithography have been studied using low-energy Ga+ ion sources below 30 keV. The a-Se75Ge25 resist exposed to 10 keV defocused-Ga+ ion beam with the dose of 5.0×1014 to 9.3×1015 ions/cm2 results in increasing the optical absorption, which was also observed in the film exposed to an optical dose of 4.5×1020 photons/cm2. The absorption edge shift of 0.3 eV for the resist exposed to a dose of 9.3×1015 ions/cm2 at 10 keV is about twice that of photo exposure. These large shifts could be estimated as due to an increase in disorder, considering a decrease in the slope of the Urbach tail and a broad pattern of x-ray diffraction. For the exposure of a 30 keV Ga+ ion beam and above a dose of 1.4×1015 ions/cm2, a 590-A-thick resist film is completely etched by dipping for 10 s in 1:1:3 HNO3:HCl:H2O solution (25 °C), and then the etching rate is about 60 A/s. As the incident e...

On Ag-doping in amorphous Sb2S3 thin film by HeNe and HeCd laser exposures and its optical characteristics

Abstract The metal photodoping effect and material characterization in the system of Ag (10 nm)/a-Sb2S3 (300 nm)/Si-substrate have been investigated using a HeCd laser (λ=325.0 nm ) and a HeNe laser (λ=632.8 nm ) . The Ag thin film is optically transparent for the HeCd laser light. Although an optical-bandgap shrinkage (negative ΔEop or darkening) due to the Ag-doping is observed in the all films exposed to each laser, the ΔEop dependency on photon dose and the Ag-doping profiles are distinctly different. While the maximum value of ΔEop in Sb2S3 films exposed to HeCd laser saturates at about −0.08 eV , the ΔEop in Sb2S3 films exposed to HeNe laser is not saturated and the maximum is a relatively large value (about −0.25 eV). The doping front (with a peak Ag concentration) on the Ag depth profiles always exists at an interface between the chalcogenide and semiconducting substrate. In the case of films exposed to HeNe laser, however, the Ag at the chalcogenide surface is completely depleted and the peak shape appears to be broadly distributed with the doping front as a symmetry center. On the other hand, the films exposed to HeCd laser exhibit a relatively uniform Ag distribution without a surface depletion and with a sharp peak shape. Of course, these differences in Ag-doping phenomena can be ascribed to the differences of optical energy for two types of lasers (above-Eop and sub-Eop exposures). However, we believe that it is also important whether the Ag layer absorbs optically an incident laser power or not, i.e., thermally.

Photoinduced dichroism and its low-temperature characteristics in obliquely deposited amorphous As–Ge–Se–S thin films

The photoinduced dichroism and its low-temperature characteristics in obliquely deposited amorphous As–Ge–Se–S thin films have been studied using a linearly polarized 632.8 nm He–Ne laser light and a control system capable of increasing the film temperature from 77 K. Our experimental results have been discussed on the basis of native valence-alternation pairs depended on the film fabrication. Although the absorption slopes in the extended region (hν>Eop) decrease with increasing deposition angle, the optical energy gap Eop remains the same as for 0° films, i.e., Eop=2.04 eV. A columnar structure with an inclination angle of approximately 70° is formed in 80° deposited films. While the value of saturated dichroism Dsat itself remains constant, independent of the intensity, it depends strongly on the deposition angle θ and the film temperature T, i.e., Dsat(θ,T). For example, Dsat(80°,77 K) is more than four times Dsat(80°,300 K), and the values of Dsat(0°,300 K) and Dsat(80°,300 K) are about 4.2% and 6.0%...

Phase-change characteristics of chalcogenide Ge1Se1Te2 thin films for use in nonvolatile memories

In the present work, the authors show that Ge1Se1Te2 thin films provide a promising alternative for phase-change random access memory (PRAM) applications to overcome the problems of conventional Ge2Sb2Te5 PRAM devices. 100‐nm-thick chalcogenide Ge1Se1Te2 thin films were prepared by evaporating a stoichiometric bulk target, and Ge1Se1Te2 thin-film PRAM devices with a 20‐μm-sized memory cell have been fabricated. The devices exhibited a successful switching between an amorphous and a crystalline phase by applying a 50ns, 7.3V set pulse and a 30ns, 7.4V reset pulse with a switching dynamic range (the ratio of Rhigh to Rlow) as high as 103. For a static-mode switching operation, two different resistance states in Ge1Se1Te2 thin films have been observed at low voltages, depending on the two different crystalline states of the film. The first phase-transition temperature of Ge1Se1Te2 thin film is found to be 110°C, which is clearly lower than that of Ge2Sb2Te5 films from the temperature-dependent conductivity m...

The electrical erasing of holographic grating in amorphous chalcogenide thin films.

We investigated the electrical erasing of information recorded by a holographic method in an amorphous As40Ge10Se15S35 chalcogenide thin film. Even though the amplitude of photoinduced birefringence (Δn) strongly depends on applied DC voltage (VDC), the kinetics exhibits a tendency. For VDC=0–3 V, Δn increases with increasing induction time and eventually saturates to its maximum (Δnmax) even though its saturation time and amplitude are not the same at different VDCs. For VDC>4, Δn increases rapidly at the start of the induction process (within several seconds), reaches Δnmax and then decreases very slowly. In addition, phase gratings in amorphous As40Ge10Se15S35 films were formed by the holographic method and diffraction efficiency (η) was monitored in real time. Immediately after reaches its maximum (ηmax), a short pulse was applied to the sample which cause a marked decrease in diffraction efficiency (η=0.25%→η10-3).

Non-volatile switching characteristics in wet-deposited Ag2Se/GeSe double layers for resistive random access memory applications

The resistance switching characteristics of resistive random access memory (ReRAM) based on amorphous GeSe thin films have been demonstrated by using Al/Ag2Se/GeSe/Pt structure. The Ag2Se layer that serves as the metal ionic source of GeSe switching medium was formed by a very simple wet-deposition process in AgNO3 solution. The X-ray diffraction and Auger electron spectroscopy analysis revealed the existence of the Ag2Se layer. As a result, an extremely low-voltage set/reset operation of less than 0.2 V, a high on-off resistance ratio larger than 103, good endurance characteristics, and excellent long-term reliability were achieved by the proposed Ag2Se/GeSe bilayered ReRAMs using the wet-deposition process.

Electrical Switching Studies of Amorphous Ge1Se1Te2 Thin Film for a High-Performance Nonvolatile Phase-Change Memory

In the present work, we studied the changes in the electrical conductivity occurring in a new composition of amorphous Ge1Se1Te thin film for a high-performance nonvolatile phase-change memory. A phase-change random access memory (PRAM) device without an access transistor is successfully fabricated with the Ge1Se1Te2 phase-change resistor, which has a much higher electrical resistivity than that of conventional Ge2Sb2Te5, and its electric resistivity can be varied by the factor of 105, related to the degree of crystallization. Chalcogenide Ge1Se1Te2 thin film of 100 nm thickness was formed by vacuum deposition at 1.5×10-5 Torr. The static mode switching characteristics are tested for the 100 µm-sized Ge1Se1Te2 PRAM device. In the first sweep, the amorphous Ge1Se1Te2 thin film showed a high resistance state in the low voltage region. However, when it reached the threshold voltage, Vth, the electrical resistance of the device was drastically reduced through the formation of an electrically conducting path. The results of pulsed mode switching of the 20 µm-sized Ge1Se1Te2 PRAM devices show that the reset process of the device was accomplished with an 80 ns–8.6 V pulse and the set process of the device was accomplished with a 200 ns–4.3 V pulse.

Polarization dependence of holographic grating in chalcogenide film

The authors have investigated the polarization dependence of holographic gratings in chalcogenide Se/sub 75/Ge/sub 25/ thin film. They observed the vectoral effect, which depends on the polarization states. The high diffraction efficiency was induced by the photoinduced phenomena for the vertical polarization state and by wet etching with NaOH.

Holographic Grating Formation by Wet Etching of Amorphous As40Ge10Se15S35 Thin Film

Amorphous As40Ge10Se15S35 thin films have been studied with the aim of identifying optimum etching conditions that can be used to produce holographic grating structures for use as diffractive optical elements. We have fabricated holographic gratings using 632.8 nm HeNe laser light. In addition, the fabricated gratings are developed with a method involving wet etching a using NaOH etching solution with one of three concentrations (0.26 N, 0.33 N, 0.40 N). The formed grating profiles were observed by atomic force microscopy which showed that the expected grating profile could be achieved by controlling the etching time. The maximum diffraction efficiency of a grating formed by two-beam interference without the etching process is 0.88%. The diffraction efficiency increases with time by selective etching. When the formed grating is etched using 0.26 N NaOH, the highest diffraction efficiency for these gratings was attained and the value was about 5.05%. NaOH solutions used in these experiments are suitable for the fabrication of relief gratings in As40Ge10Se15S35 layers with Λ=1.82 µm. Better results will be obtained by optimizing the etching conditions (concentration, etching time, temp, etc.).