Holographic and e-Beam Image Recording in Ge5As37S58–Se Nanomultilayer Structures
© Stronski et al. 2016
Received: 13 October 2015
Accepted: 5 January 2016
Published: 27 January 2016
Processes of e-beam and holographic recording of surface relief structures using Ge5As37S58–Se multilayer nanostructures as registering media were studied in this paper. Optical properties of Ge5As37S58, Se layers, and Ge5As37S58–Se multilayer nanostructures were investigated. Spectral dependencies of refractive index were analyzed within the frames of single oscillator model. Values of optical band gaps for Ge5As37S58, Se layers, and Ge5As37S58–Se multilayer nanostructures were obtained from Tauc dependencies. Using e-beam and holographic recording, diffraction gratings were fabricated in Ge5As37S58–Se multilayer nanostructures. Images of Ukraine and Moldova state emblems were obtained by e-beam recording. Image size consisted of 512 × 512 pixels (size of 1 pixel was ~2 μm). Ge5As37S58–Se multilayer nanostructures are perspective for the direct recording of holographic diffraction gratings and other optical elements.
KeywordsMultilayer nanostructures Holographic recording e-beam recording Diffraction gratings Chalcogenide glasses
Thin films based on chalcogenide glasses have evolved as light-sensitive materials for high-density recording media application due to their optical and structural properties. The light sensitivity effect of thin chalcogenide films was discovered 50 years ago . Chalcogenide glasses and films are also sensitive to the electron or ion beams, X-rays [2–5], and perhaps photo-stimulated or stimulated by electron or ion beams; X-rays’ change of their properties is the most interesting phenomena exhibited by these materials.
In this work, the experimental results showing the surface relief formation in Ge5As37S58–Se nanomultilayer structures under e-beam or holographic exposure are presented.
Selective etching after exposure enables to obtain surface reliefs and to use media such as high-resolution inorganic resists [2–7]. Using of thin layers of chalcogenide glasses as high-resolution media and selective etching after exposure enables fabrication of high-quality holographic diffraction gratings and other optical elements [6–11].
Multilayer nanostructures on the base of chalcogenide glasses and the possibility to use them as registering media were proposed in . Such media do not require the step of selective etching for the formation of the surface relief [12–16]. Surface relief in such media is formed directly during exposure process. Absence of the selective etching step is the advantage of such media because often USED etchants are toxic, and during selective etching process, it is necessary to control many parameters (temperature, concentration of etchant, etc.). Thus, the development of one-step method for the fabrication of surface reliefs is considered perspective for the fabrication of planar optical elements.
Chalcogenide glasses of Ge–As–S composition are characterized by high values of refractive index, and their nonlinear optical properties are two orders higher than characteristic of quartz glasses [17, 18]. Earlier, we demonstrated the possibility to use Ge5As37S58–Se multilayer nanostructures for the fabrication of surface reliefs . In present work, results of direct recording (without the selective etching step) of holographic diffraction grattings and images by e-beam exposure using Ge5As37S58–Se multilayer nanostructures as recording media are presented.
Overlapping part of the sample consists of alternating Se and Ge5As37S58 nanolayers; two wide rings are overlapping in the central part of the substrate forming the Ge5As37S58–Se multilayer nanostructure. Consequently, external and inner rings of the layers are Ge5As37S58 and Se layers. Ge5As37S58 and Se were deposited through mask windows. The substrate with deposited multilayer structure and Ge5As37S58 and Se layers was used for the composition control and also for AFM thickness measurement and estimation of modulation period N (common thickness of one Ge5As37S58 nanolayer and one Se nanolayer) of multilayer nanostructure. Overlapping part of the samples (see Fig. 2.) contains alternating nanolayers of Ge5As37S58 with thickness of 16 nm and Se nanolayers with thickness of 14 nm. The total number of nanolayers was 200, modulation period N ~ 30 nm. In order to prevent crystallization of Se layers which are structurally unstable under heating and/or exposure by light, e-beams, etc., heating of layers was minimized by substrate rotation and lowered evaporator temperature.
Diffraction gratings of Ukrainian and Moldavian state emblems were recorded by e-beam exposure using scanning electron microscope Tesla BS 300 with programmable exposure control unit. The accelerating voltage was 25 kV, and the size of the electron spot at this voltage was about 300 nm. Size of images consisted of 512 × 512 pixels. Morphology and surface relief of the obtained images were studied by AFM microscopy. Distance between pixels consisted of 3 μm. Size of pixels was about 2 μm and profile depth ~300 nm.
Results and Discussion
B is the slope of the Tauc edge which reflects some disorder of the samples. Usually, this constant depends on the width of the localized states in the band gap, a fact explained with the homopolar bonds’ presence in the chalcogenide glasses. Thus, Tauc plots of (αhν)1/2 versus (hν) should be linear and extrapolate to values of the optical gap, E g.
Parameters of single oscillator model and values of optical band gap for Ge5As37S58, Se films, and Ge5As37S58–Se multilayer nanostructures
E d, еВ
E 0, еВ
E g, еВ
The thicknesses of constituent Ge5As37S58 and Se nanolayers were 16 and 14 nm, respectively, and are sufficiently smaller than the light wavelength. Also, transmission curve of Ge5As37S58–Se multilayer nanostructure is a typical interference curve for films with high optical quality and uniform thickness (see Fig. 3). Thus, in the analysis of optical transmission spectra of nanomultilayer structure, it was possible to use the “effective optical medium” model: the layers with small optical band gap E g value determine the optical absorption at the average absorption edge E g, and the “barrier” layers with larger E g are transparent.
Obtained spectral dependencies of refractive index of Ge5As37S58, Se layers, and Ge5As37S58–Se multilayer nanostructures were analyzed within the frames of single oscillator model. Parameters of the model (dispersion energy, position of the effective oscillator) were obtained.
Parameters of single oscillator model dispersion energy and effective oscillator position and also values of optical band gap obtained with the use of Tauc dependence αhν = const(hν − E g)2, where hν is the light quantum energy, α absorption coefficient, and static refractive index, n 0, are presented in Table 1.
It is necessary to note that values of optical band gaps (see Table 1) of Se layers and Ge5As37S58–Se multilayer nanostructure are close. The Wemple–DiDomenico model in the range of the low-frequency optical dielectric response of glasses can be used as a tool to probe the building blocks conforming a glass and allows quantitative analysis in combination with Raman spectroscopy .
Mechanism of recording in chalcogenide multilayer nanostructures is connected with stimulated by light (ion, electron beams) interdiffusion processes in nanolayers . The proposed model for low-intensity recording radiation enabled to calculate evolution of the recorded reliefs during holographic recording of gratings. Further improvement of this model was done in , where heating of multilayer nanostructure during recording process by high-intensity light and respective non-linearities of the recording processes were taken into account. Also, it is noted that the thickness change as a result of interdiffusion processes in alternating nanolayers can reach ~5 % values and more. In mechanisms of surface relief formation, it is also necessary to take into account peculiarities of possible surface relief formation in constituent layers of multilayer structure (Se and chalcogenide layer of other composition) . Here, it is necessary to note that mechanisms, processes of mass transfer during surface relief formation, and kinetics of holographic gratings recording in chalcogenide layers and in nanomultilayer structures on the base of chalcogenide glasses are polarization sensitive [32–35].
Under exposure by ion- or e-beams of thin layers of chalcogenide, vitreous semiconductors structural transformations in films are observed [2–5, 36–38]. Shifts of absorption edge or surface deformation under e-beam exposure were observed. With the use of consequent selective etching thin layers of chalcogenide, glasses can be applied in high-resolution lithography processes. It was noted in [36–38] that the effects of stimulated by e-beam thickness increase in separate components of multilayer nanostructure (for example, Se and As2S3) are not additive in respect to thickness increase in the same conditions for Se–As2S3 multilayer nanostructure. The sum of the thickness increase in separate layers consists of only 30 % from the total measured thickness increase in Se–As2S3 multilayer nanostructures. In multilayer nanostructures, it is necessary to take into account also the presence of other processes, for example, formation of As–S–Se. It is supposed that stimulated by light or e-beams processes activate the mentioned processes in such media.
Obtained results show that with the use of Ge5As37S58–Se multilayer nanostructures, it is possible to realize direct recording of holographic optical elements (diffraction gratings) and also direct recording by e-beam exposure of diffraction gratings and other surface relief structures.
The research was supported by the project FP–7 SECURE–R21.
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