Ag Photodoping Research Articles

Chalcogenide Photonic Crystals Fabricated by Soft Imprint-Assisted Photodoping of Silver.

This work presents a low-cost, large-scale nanofabrication approach that combines imprint lithography and silver doping (IL-SD) to pattern chalcogenide glass (ChG) films for realizing IR devices. The IL-SD method involves controled photodoping of silver (Ag) atoms into ChG films and selective removing of undoped ChG. For photodoping of Ag, an Ag-coated elastomer stamp is brought in contact with the ChG film and exposed to ultraviolet light, and subsequently, the Ag atoms are photo-dissolved into the ChG film following the nanopatterns on the elastomer stamp. Due to the high wet-etching selectivity of the undoped ChG to Ag-doped one, the ChG film can be precisely patterned with a spatial resolution on the order of a few tens of nanometers. Also, by controling the lateral diffusion of Ag atoms during ultraviolet exposure, it is possible to adjust the size of the final patterns formed in the ChG film. As an application demonstration of the IL-SD process, the As2 S3 -based near-infrared photonic crystals (PhCs) in the wavelength range and flexible midinfrared PhCs are formed, and their optical resonances are investigated. The IL-SD process enables the low-cost fabrication of ChG nanostructures on different substrate materials and gives a great promise to realize various IR devices.

Preparation, properties, and photodoping behavior of GeS2-, Ga2S3-, and Sb2S3-based glasses with excess sulfur and CsCl

Abstract

Effect of Ag photo-doping on structural, optical and phase change properties of GeTe chalcogenide films

This paper reports the effect of Ag photo-doping on optical, electrical and structural properties of GeTe:Ag thin films. The absence of sharp diffraction peak confirms the amorphous nature of as-deposited and photo-doped films. The decrease in reflectance of GeTe:Ag bilayer films with photo-doping reaction time has been observed. The RAMAN spectrum showed the characteristic Raman bands for GeTe4 (127cm−1), long chain interactions of Te–Te chains (145cm−1) and a broad peak for Ge–Ge vibrations (275cm−1) without appreciable change in their position/shape with photodoped Ag concentration. The electrical resistivity measurement shows that photo-doping of Ag led to sharp amorphous-crystalline phase transition along with an increase in transition temperature and resistivity value in both amorphous as well as crystalline state. The annealing of photo-doped GeTe:Ag samples showed an enhancement in the crystallinity of GeTe phase without any segregation of Ag phases in annealed samples. The preferential formation of GeTe (200) phase upon crystallization has been observed for GeTe:Ag films. Different optical parameters have been calculated for photo-doped and annealed samples and are discussed in conjunction with the modification of network structure of GeTe with inclusion of photo-doped Ag content.

Observation of Ag photodoping phenomena in GeS2 chalcogenide glass films by spectroscopic ellipsometry and atomic force microscopy

The anomalous diffusion process of Ag into a GeS2 film induced by the illumination of light with the photon energy near or larger than the optical gap of a GeS2 film was examined in-situ by the spectroscopic ellipsometry in the wavelength region between 450 and 850nm. The physical parameters such as the thickness and the refractive indices n and κ for each layer were derived from the ellipsometric parameters measured at respective wavelength using the simulation assuming a three layers (Ag, Ag: GeS2 and GeS2 layers) model. Two steps of doping process called the first and second stage were derived through the analyses of the optical constants, which were different from the results for As2S3 films. Especially, the first stage progressed very fast in GeS2 films in the three layers model. The in-situ observations using an atomic force microscope were also carried out to understand the doping process through the change of the surface morphology during doping of Ag and the results which supported the ellipsometric behaviors were obtained. Other simulation model assuming optical constants considering effective medium approximation for the doped layer during doping was discussed for the analysis of ellipsometric data and the difference of the doping behavior between two models was discussed.

Optical properties and phase transition in photodoped amorphous Ge–Sb–Te:Ag thin films

Photodoping of Ag into amorphous Ge2Sb2Te5 thin films has been carried out by illuminating thermally evaporated Ge–Sb–Te:Ag bilayers with a halogen lamp in the inert atmosphere at room temperature. Amorphous nature of the as-deposited and photodoped films was confirmed by an X-ray diffraction study. The composition of Ge2Sb2Te5 thin films and the amount of Ag photodiffused have been gathered from electron probe micro-analyzer having a wavelength dispersive spectrometer. Amorphous-to-crystalline transition of the films was induced by thermal annealing and the structural phases were identified by X-ray diffraction. The crystalline temperature of the films was evaluated by temperature dependent sheet resistance measurements. The effects of photodoped Ag concentration on the optical properties of the Ge2Sb2Te5 films have been examined by transmission and reflection spectrometry. It was found that the optical band gap, the sheet resistance and the phase transition temperature increase while the absorption coefficient and extinction coefficient decrease upon incorporation of Ag in Ge2Sb2Te5 system. A correlation between the optical band gap and the electronegativity of the alloys indicates that the optical band gap increases with the decrease of electronegativity.

Manipulation and observation of Ag photodoping patterns in GeS_2 amorphous films by a dual functional laser scanning microfabrication/microscope system

The photodoping phenomenon of Ag is one of the light-induced phenomena in GeS(2) amorphous chalcogenide films. It has potential as a process for fabricating photonic structures, such as waveguides and micro-optics, but its fabrication method is still under research. A dual functional laser scanning system integrating microfabrication and microscope systems was developed. In situ nanoscale fabrication by a UV laser effective for photodoping, and observation by a VIS wavelength laser, which does not affect the material, were demonstrated under same setup in one system. Several fine doped patterns were fabricated and the optical performances were evaluated. These results give the feasibility of forming various photonic structures.

비정질 GeSe 박막으로의 은-광도핑에 대한 기판의존성

The dependence of substrate on the Ag photodoping phenomenon into amonhous <TEX>$({\alpha}-)$</TEX> GeSe thin film has been investigated using holographic method. A 442 nm HeCd laser was utilized as a light source for the holographic exposure and a 632.8 nm HeNe laser to measure the variation of diffraction efficiency <TEX>$(\eta)$</TEX> in real time. The films (Ag and <TEX>${\alpha}-GeSe$</TEX>) were thermally deposited on the substrates, i.e. p-type Si(100), n-type Si(100) and slide glass. The sample structures prepared were two types: type I (Ag/<TEX>${\alpha}$</TEX>-SeGe/substrate) and type II (<TEX>${\alpha}$</TEX>-SeGe/Ag/substrate). The <TEX>$\eta$</TEX> kinetics comprised to be three steps in which <TEX>$\eta$</TEX> initially increases, is saturated to be maximized <TEX>$(\eta_M)$</TEX>, and then decreases relatively gradually. For the same substrate, the <TEX>$\eta_M$</TEX> values of the type II were higher than those of type I. In addition, the type II exhibited the highest <TEX>$\eta_M$</TEX> for p-type Si substrate, while that in type I was observed for n-type Si substrate. These tendency is explained by the diffusion of minority carrier in the films and the change of magnitude and direction in internal fields generated at the film interfaces. Atomic-force-microscope (AFM) was used to observe relief-type grating patterns.

Behavior of Ag photodoping in sulfide bulk glasses

The relationship between Ag photodoping behavior and glass structure has been studied using various sulfide bulk glasses. The transmission measurement revealed that the photodoping rate increased with an increase of S concentration in Ge–S glasses and GeS2–SbS2 glasses, while the disappearance of Ag film was not be observed in Ge40S60 and Ga2S3-based glasses, which contain modifier ions. The correlation between photodoping rate and S–S bond content in a host glass was confirmed. From the point of view of the behavior of Ag photodoping, sulfide glasses were classified into two groups: (1) a covalent glasses containing S–S bonds and (2) an ionic glasses containing modified ions.

Effect of Ag photodoping on deep-level emission spectra of ZnO epitaxial films

Effect of Ag photodoping into ZnO films grown by molecular beam epitaxy has been examined via the real-time measurement of photoluminescence (PL) spectra using a HeCd laser at room temperature. While an asymmetric deep-level (green-yellow) broadband (D) is observed in PL of as-grown ZnO, it appears to change to two distinct bands (at 2.29 and 2.50 eV) in the illuminated Ag/ZnO. From the result of a Gaussian fitting, the yellow band at 2.29 eV is evaluated to relate with photodoped Ag atoms. With increasing the laser illumination time (photodoping), the PL intensity of broadband (ID) decreases and contrarily, the intensity of the near band edge emission observed at 3.297 eV (INB) shows a tendency to increase. We believe that the Ag can be photodoped into ZnO at 300 K and it contributes to the suppression of deep-level band, which results in an increase of INB.

Wet-etching selectivity of Ag-photodoped AsGeSeS thin films and the fabrication of a planar corrugated one-dimensional photonic crystal by a holographic method

Characteristics of Ag photodoping into an amorphous As40Ge10Se15S35 thin film and its wet-etching selectivity containing edge-sharpening effect have been investigated. A planar corrugated one-dimensional photonic crystal (1D PC) has also been fabricated by a holographic method based on the Ag photodoping. Ag-doped films by an illumination of HeCd laser at 77 K (cooled Ag-doped films) exhibit a relatively uniformly distributed Ag profile over the entire depth of the chalcogenide and a strong resistance against an alkaline development solution. That is, the cooled Ag-doped film acts as a negative-type resist for wet-etching development using a solution of NaOH (selectivity &amp;gt;99%). In addition, its sensitivity and contrast, obtained from the exposure response curve, are very good to be approximately 3.3x1015 photons/cm2 and 6.46, respectively. Contrarily, Ag-doped films at 300 K (normally Ag-doped films) exhibit a Gaussian-formed Ag distribution, which has a peak at the interface (about 500 nm depth) between chalcogenide and substrate and a depletion at a depth of 0–400 nm. In particular, an edge-sharpening effect is observed in both developed films and this is discussed considering the model of photoinduced chemical modifications. This effect is thought to play an important role in fabricating a fine planar 1D PC structure by the holographic method. The fabricated 1D PC consisted of a space (s) of 350 nm and a linewidth (l) of 230 nm. For the holographic method based on Ag photodoping, a fabrication criterion for controlling the ratio s/l and the period Λ(=s+l) that are the important parameters in 1D PC structures is also proposed.

Silver photodoping into Al–ZnSe for application to white light emitters

Room-temperature Ag photodoping into crystalline Al-doped ZnSe grown by molecular-beam epitaxy (MBE) has been achieved. A promising approach to apply to a patterned white light emitter has also been proposed. An Al–ZnSe with Ag islands was illuminated with a HeCd laser at 300 K and annealed in the MBE chamber. Then, their photoluminescence characteristics were measured in real time. The self-activated (SA) emission, observed in as-grown Al–ZnSe, undergoes a considerable change in intensity and band energy position through Ag photodoping and subsequent annealing. The resultant emission bands centered at 2.145 eV and 2.250 eV are assigned as a donor-to-acceptor pair transition related with Ag impurities rather than SA centers. In addition, chromaticity can be controlled by an adjustment of photoirradiated Ag-doped area.

Photoluminescence Properties of Selectively Ag-Photodoped Al–ZnSe at Room Temperature

Selective Ag photodoping at room temperature (RT) has been succesfully performed on Al-doped n-type ZnSe grown by molecular beam epitaxy (MBE). Some 6-nm-thick Ag islands with a square shape (2 mm×2 mm) are deposited on the as-grown Al–ZnSe, which in turn is illuminated with a 325 nm HeCd laser at RT and the photoluminescence (PL) characteristics are measured in real time. The self-activated (SA) emission shown in Ag-undoped Al–ZnSe shifts to a shorter wavelength ΔE(I2)∼+60 meV) via a series of processes of Ag photodoping and subsequent annealing in the MBE chamber. The emission bands centered at 2.245 eV and 2.146 eV distinctly show a blue shift with the increase of the PL-excitation power, which can be assigned to a donor-to-acceptor (DA) pair transition. That is, the SA bands in Al–ZnSe appear to be converted into DA bands by Ag photodoping. The coloring is controlled to some degree by adjustment of the percentage of the Ag-doped area included in the PL-excitation spot. For example, a 40% spectrum, which corresponds to one obtained from an excitation spot consisting of an Ag doped area of ∼40% and an Ag undoped area of ∼60%, appears yellowish-white to the naked eye. We believe that it is possible to realize a patterned ZnSe-based white-light emitter with Ag islands of micrometer size using the conventional UV-lithography and lift-off processes and RT–Ag photodoping.

Ag photodoping into crystalline ZnSe

We have investigated the possibility of Ag photodoping into crystalline (c-) ZnSe grown by molecular-beam epitaxy. A Ag/c-ZnSe sample was illuminated with a 325 nm HeCd laser at 10 K and its photoluminescence (PL) characteristics were measured in real time. Depending on the illumination time, there are considerable changes in the intensities of the deep-level peaks (2.059 and 2.345 eV) as well as the near-band-edge peak (2.798 eV). The emission band at 2.059 eV is almost independent of PL-excitation power, while the green luminescence band at 2.345 eV exhibits distinctly a power-dependent blueshift with increasing excitation power, which can be assigned as a donor-to-acceptor pair transition (AgiAgZn). In this letter, a model to explain Ag photodoping into c-ZnSe is proposed on the basis of the PL results. We believe that Ag, as an activator for luminescence, can be photodoped into c-ZnSe even though the extent and kinetics of the Ag doping are different from those of the amorphous chalcogenide.

Photoinduced and thermally-induced effects in mesoscopic scale for amorphous GeS film

For virgin, illuminated and annealed GeS 2 films, the Ag photodoping, dissolution in a KOH solution and volume change have been examined to reveal the photo-and thermally-induced effects in mesoscopic scale (10–100 Å). The results can be explained by two ideas: (1) decrease of the structural disorder in atomic scale causes increase of the disorder in mesoscopic scale; (2) the mesoscale disorder remains for a long time at room temperature but reduces rapidly at the annealing temperature.

Direct observation of atom movement in photoexcited ionic-electronic conductors

An analysis of the topological, optical, and chemical characteristics of pattern-photoexposed ionic-electronic conductor ${\mathrm{Ag}}_{2}$Se surfaces has revealed a very interesting phenomenon: the formation of hills 100--500 \AA{} in height on the exposed ${\mathrm{Ag}}_{2}$Se/Ge-Se surface area. The hills are a result of pressure caused by the production of Ag metal clusters at the photoexposed ${\mathrm{Ag}}_{2}$Se surface, which results in a four-layer structure of (Ag metal)/ ${\mathrm{Ag}}_{2}$Se/(Ag-doped Ge-Se)/Ge-Se. The Ag hill formation plays a more dominant role in Ag lateral diffusion than Ag photodoping does.

Infrared absorption of Ag- and Cu-photodoped chalcogenide films

Abstract Infrared reflectance and transmittance spectra of chalcogenide glass layers (As 2 S 3 , GeS 2 , GeSe 2 ), non-doped and photodoped by Ag and Cu, were measured. The spectra were analyzed in terms of a molecular model for optic-mode vibrational frequencies. With Ag photodoping of As 2 S 3 the main band (312 cm −1 ) shifts to higher frequencies. The area under the ωe 2 ( ω ) contour is unchanged ( e 2 -imaginary part of the dielectric permeability). The ratio of areas under such contours for photodoped layers to undoped ones is 0.87 ± 0.3 for GeS 2 and 0.80 ± 0.03 for GeSe 2 . In the photodoped As 2 S 3 and GeS 2 layer spectra a broad weak band at low frequencies (200 cm −1 ) is observed. This band is affected by the presence of metal-chalcogen bonds. Variations in the IR-spectra with photodoping by Ag or Cu are consistent with the supposition about normal covalent and coordinative bond formation between metal additives and chalcogen atoms. It results in the creation of the two types of impurity centers.

A soft-x-ray core absorption study on the electronic states in Ag- and Cu-photodissolved amorphous using synchrotron radiation

Conduction-band electronic states of amorphous photodissolved by Ag and Cu metals have been examined by means of soft-x-ray core absorption spectroscopy. Both the Ge and the Se spectra of the photodoped samples are similar to the corresponding spectra of undoped in contrast with the Se 3d core absorption spectra, which suggests that the prominent change in the conduction band density of states by Ag or Cu photodoping is caused through the Se 4p contributions. The core absorption spectrum has a clear peak which would originate from the Cu - Se covalent bonds. These results do not contradict the structural model described by Fischer-Colbrie et al.

Photodoping of Ag in AgAsS glasses

The photodoping rate of Ag in AsAsS bulk glasses has been studied as a function of composition. The rate is evaluated from the resistance change of Ag layers. In the Ag:AsxS100−x system, the rate shows a maximum at ≅30. In the Ag:(Ag2S)x(As2S3)1−x system, with an increase in the Ag content, the rate increases first, then decreases, and it becomes approximately zero in the glass containing ⪆25 at.% Ag. These results are compared with thermal diffusion phenomena and are discussed in the light of the Owen's idea for photodoping.

The Ag photodoping mechanism in As2S3

We have measured the number of photons/cm2 over the energy range 1.95 to 3.35 eV, required to form a negative-relief image in 2000 Å films of As2S3 using 100 Å films of evaporated Ag, Ag2Te, Ag2Se, and AgBr. We have observed sensitivity below the band gap of As2S3 in all cases. We have observed an increase in sensitivity by several orders of magnitude with increase of photon energy between 1.95 and 3.35 eV. The sensitivity decreases from Ag to the semimetals Ag2Te and Ag2Se to the wide band gap AgBr by the ratio 1:50:2500 at 2.4 eV. Assuming an internal photoemission model as proposed by Goldschmidt and Rudman, we extract barrier heights of 1.71, 2.10, 2.22 and 2.42 eV for Ag, Ag2Te, Ag2Se, and AgBr.

Composition dependence of Ag photodoping into amorphous Ge-S films

Photodoping kinetic studies of thin sample of Ag/GexS100−x (21≤x≤45) were made through optical transmission measurements. The initial photodoping rate increases with increasing S content, whereas the total amount of dissolved Ag shows a maximum around the stoichiometric composition GeS2. The results are similar to those of the Ag/Ge-Se system but different from the Ag/As-S system. The composition dependencies are qualitatively explained by the structure of Ge-S bulk glass and the glass-forming ability of Ag-Ge-S bulk glass. In this study it was also found that aging effect is considerable within several hours after sample preparation and a reciprocity law holds between the initial rate and illumination power.