[en] Photoinduced characteristics of amorphous (a-) ZnSe thin films at 10 and 300 K have been investigated using real-time photoluminescence (PL) and X-ray diffraction. The structural phase of as-deposited film is evaluated to be predominantly amorphous with uniformly distributed nano-scale crystallites, and the optical energy gap and complex refractive index are approximately 2.928 eV and 3.04+i0.35 (at λ=325 nm), respectively. While the crystallite size is enlarged after illumination with HeCd laser at 300 K, a photodarkening effect without a change in crystallite size is observed in films illuminated at 10 K. That is, two types of temperature-dependent photoinduced changes are observed in a-ZnSe (i.e. amorphous-to-nanocrystalline transition at 300 K and amorphous-to-amorphous transition at 10 K). PL spectra of the photoinduced a-ZnSe measured at 10 K apparently show both the Stokes-shift and near band-edge broad peaks centered at ∼1.5 and ∼3.0 eV, respectively. In this work, we discuss a series of PL characteristics in a-ZnSe using a simple model based on valence-alternation pairs

[en] As a method to enhance the sensitivity (S) of an inorganic resist for focused-ion-beam (FIB), lithography, sub-0.1 μm patterning properties of a columnar structural α-Se₇₅Ge₂₅ resist have been investigated using 30 keV low-energy Ga⁺-FIB exposure and CF₄ reactive-ion etching (RIE). development. The Se₇₅Ge₂₅ thin films were 60 .deg. and 80 .deg. -obliquely deposited on Si substrate and parts of the films were annealed for several minutes at the glass transition temperature (T_g=∼220 .deg. C). Columnar structures with the angles of approximately 40 .deg. and 65 .deg. are observed in 60 .deg. and 80 .deg. -obliquely deposited films, respectively, and they disappear after annealing. Despite the disappearance of the columnar structures, a critical decrease in thickness is not observed. For the FIB exposures with a beam diameter of ∼0.1μm and around the threshold dose, the negative-type fine patterns with linewidth of about 0.06∼0.09 μm are fabricated successfully. Then, the imaging contrast (γ) is evaluated to be approximately 4.0 and the S to be ∼7.0x10¹⁴ ions/cm², which corresponds to about half that of normally (0 .deg. ) as -deposited resist

[en] In this work, several experimental methods were used to evaluate the phase transformation characteristics of Ge-Sb-Te pseudobinary thin films that were comprehensively utilized as phase change materials. Except for Ge₈Sb₂Te₁₁ (x = 8), the phase transition of the (GeTe)_x(Sb₂Te₃) (x = 0.5, 1, 2) thin films from the amorphous to the hexagonal structure via the fcc structure was confirmed by X-ray diffraction the measurements. Additionally, the X-ray photoelectron spectra analysis revealed that the Ge-Te bond was weakened for Ge₂Sb₂Te₅ (x = 2) and all of bonds were strengthened for Ge₈Sb₂Te₁₁ (x = 8) during the amorphous to crystalline transition. The optical energy gaps (E_OP ) were around 0.71 and 0.50 eV, and the slopes of the absorption in the extended region (B) were ∼5.1 x 10⁵ and ∼10 x 10⁵ cm^-1·V^-1 for the amorphous and the fcc crystalline structures, respectively. Finally, the speed of the amorphous-to-crystalline phase transition for the (GeTe)_x(Sb₂Te₃) (x = 0.5, 1, 2) films was characterized by using a nano-pulse scanner with a 658-nm laser diode (power: 1 ∼ 17 mW, pulse duration: 10 ∼ 460 ns).

[en] In this study, the electrical, optical, and structural properties of tungsten (W)-doped Ge_8Sb_2Te_1_1 thin films were investigated. Previously, GeSbTe alloys were doped with various materials in an attempt to improve the thermal stability. Ge_8Sb_2Te_1_1 and W-doped Ge_8Sb_2Te_1_1 films with a thickness of 200 nm were fabricated by using an RF magnetron reactive co-sputtering system at room temperature on Si (p-type, 100) and glass substrate. The fabricated thin films were annealed in a furnace in the ∼ 0 - 400 ℃ temperature range. The optical properties were analyzed using a UV-Vis-IR spectrophotometer, and by using Beer’s Law equation, the optical-energy band gap (E_o_p), slope B"1"/"2, and slope 1/F were calculated. For the crystalline materials, an increase in the slope B"1"/"2 and 1/F was observed, exhibiting a good effect on the thermal stability in the amorphous state after the phase change. The structural properties were analyzed by X-ray diffraction, and the result showed that the W-doped Ge_8Sb_2Te_1_1 had a face-centered-cubic (fcc) crystalline structure increased crystallization temperature (T_c). An increase in the T_c increased the thermal stability in the amorphous state. The electrical properties were analyzed using a 4-point probe, exhibiting an increase in the sheet resistance (R_s) in the amorphous and the crystalline states indicating a reduced programming current in the memory device.

[en] In this work, we evaluated the structural, electrical and optical properties of carbon-doped Ge8Sb2Te11 thin films. In a previous work, GeSbTe alloys were doped with different materials in an attempt to improve the thermal stability. Ge8Sb2Te11 and carbon-doped Ge8Sb2Te11 films of 250 nm in thickness were deposited on p-type Si (100) and glass substrates by using a RF magnetron reactive co-sputtering system at room temperature. The fabricated films were annealed in a furnance in the 0 ∼ 400 ◦C temperature range. The structural properties were analyzed by using X-ray diffraction (XRD), and the result showed that the carbon-doped Ge8Sb2Te11 had a face-centeredcubic (fcc) crystalline structure and an increased crystallization temperature (Tc). An increase in the Tc leads to thermal stability in the amorphous state. The optical properties were analyzed by using an UV-Vis-IR spectrophotometer, and the result showed an increase in the optical-energy band gap (Eop) in the crystalline materials and an increase in the Eop difference (ΔEop), which is a good effect for reducing the noise in the memory device. The electrical properties were analyzed by using a 4-point probe, which showed an increase in the sheet resistance (Rs) in the amorphous state and the crystalline state, which means a reduced programming current in the memory device.

[en] Preparation of Cu(In,Ga)Se2 (CIGS) thin films has continued to face problems related to the selenization of sputtered Cu-In-Ga precursors when using H2Se vapor in that the materials are highly toxic and the facilities extremely costly. Another obstacle facing the production of CIGS thin films has been the required annealing temperature, as it relates to the decomposition temperature of a typical flexible polymer substrate. A novel laser-annealing process for CIGS thin films, which does not involve the selenization process and which can be performed at a lower temperature, has been proposed. Following sputtering with a Cu0.9In0.7Ga0.3Se2 target, the laser-annealing of the CIGS thin film was performed using a continuous 532-nm Nd:YAG laser with an annealing time of 200 - 1000 s at a laser optical power of 2.75 W. CIGS chalcopyrite (112), (220/204), and (312/116) phases, with some weak diffraction peaks corresponding to the Cu-Se- or the In-Se-related phases, were successfully obtained for all the CIGS thin films that had been laser-annealed at 2.75 W. The lattice parameters, the d-spacing, the tetragonal distortion parameter, and the strain led to the crystallinity being worse and grain size being smaller at 600 s while better crystallinity was obtained at 200 and 800 s, which was closely related to the deviations from molecularity and stoichiometry, which were greatest at 600 s while the values exhibited near-stoichiometric compositions at 200 and 800 s. The band gaps of the laser-annealed CIGS thin films were within a range of 1.765 - 1.977 eV and depended on the internal stress. The mean absorbance of the laser-annealed CIGS thin films was within a range of 1.598 - 1.900, suggesting that approximately 97.47 - 98.74% of the incident photons in the visible spectral region were absorbed by this 400-nm film. The conductivity types exhibited the same deviations (Δm > 0 and Δs < 0) in all the laser-annealed CIGS thin films. After laser-annealing, the resistivity fell abruptly to a range of 3.551 × 10−2 - 1.022 × 10−1 Ω·cm. The carrier concentration was on the order of 1019 - 1021 cm−3, and the carrier mobility was 5.7 × 10−2 - 5.7 × 100 cm2/V·s.

[en] Highlights: • TeO_2.33/SiO₂ 1D PCs were prepared by using sputtering technique. • Photonic band structures in the PCs with and without a defect layer were analyzed. • Measured transmittance spectra are in good agreement with simulated results. • The defect mode moved towards the longer wavelength after laser exposure. - Abstract: The aim of this work is to experimentally investigate one-dimensional (1D) photonic crystals (PCs) and the photo-induced effect of their defect layer. A radio frequency (RF) magnetron sputtering technique was used to fabricate ten-pair 1D PCs with and without a single defect layer, utilizing TeO_2.33 and SiO₂ with different refractive indices. The photonic band structures in the PCs were also analyzed, and the measured transmittance (T) spectra were compared with the simulated results. The 1D PC without a defect layer had a forbidden band gap in the wavelength range of 1203–1421 nm. The 1D PC with defect layer generated a sharp peak within a photonic band gap (PBG) at the central wavelength of 1291 nm. After exposure to a He-Cd laser of λ = 325 nm, the resonant T peak shows red-shift because of photodarkening effect caused by the behavior of valence-alternation pairs (VAPs) in amorphous chalcogenides.

[en] Arrays of two-dimensional multi-paired photonic crystals (PCs) have been fabricated by a multiple-exposure nanosphere lithography (MENSL) method utilizing a self-assembled nanosphere as a lens mask and an expanded He–Cd laser. The nanospheres were self-assembled on a photoresist. The masked PR was then multi-exposed with changing rotation angle (θ) and tilt angle (γ). Scanning electron microscopy reveals that MENSL is a useful tool for fabricating multi-paired PCs with various lattice structures.

[en] We have investigated the optical and amorphous-to-crystalline transition properties in four-types of chalcogenide thin films; Ge₂Sb₂Te₅, Ge₈Sb₂Te₁₁, Ag-Ge₂Sb₂Te₅ and Ag-Ge₈Sb₂Te₁₁. Crystallization was caused by nano-pulse illumination (λ = 658 nm) with power (P) of 1-17 mW and pulse duration (t) of 10-460 ns, and the morphologies of crystallized spots were observed by SEM and microscope. It was found that the crystallized spot nearby linearly increases in size with increasing the illuminating energy (E = P · t) and eventually ablated out by over illumination. Changes in the optical transmittance of as-deposited and annealed films were measured using a UV-vis-IR spectrophotometer. In addition, a speed of amorphous-to-crystalline transition was evaluated by detecting the reflection response signals for the nano-pulse scanning. Conclusively, the Ge₈Sb₂Te₁₁ film has a faster crystallization speed than the Ge₂Sb₂Te₅ film despite its higher crystallization temperature. The crystallization speed was largely improved by adding Ag in Ge₂Sb₂Te₅ film but not in Ge₈Sb₂Te₁₁ film. To explain these results, we considered a heat confinement by electron hopping.

[en] In this work, we report evaluation of the atomic-scale phase transformation characteristics in one of the most comprehensively utilized phase change materials today, Ge₂Sb₂Te₅ thin film. The phase transformation of Ge₂Sb₂Te₅ thin film from amorphous to hexagonal structure via fcc structure was confirmed by XRD measurements. The approximate values of optical energy gap are 0.72 and 0.50 eV, with slopes (B^1/2) in the extended absorption region of 5.3 x 10⁵ and 10 x 10⁵ cm^-1·eV^-1 for the amorphous and fcc-crystalline structures, respectively. In addition, X-ray photoelectron spectroscopy analysis revealed strengthening of the Te-Te bond as well as weakening of the Ge-Te bond during the amorphous-to-crystalline transition. This trend was also observed in extended X-ray absorption fine structure analysis where the Ge metallic bond lengths in the amorphous, fcc, and hexagonal structures were 0.262, 0.280, and 0.290 nm