ISSN 1068-3755, Surface Engineering and Applied Electrochemistry, 2016, Vol. 52, No. 5, pp. 456–468. © Allerton Press, Inc., 2016.
Direct Surface Relief Formation in Nanomultilayers Based on Chalcogenide Glasses: A Review1 E. Achimova Institute of Applied Physics, Academy of Sciences of Moldova, MD-2028, Chisinau, Republic of Moldova e-mail:
[email protected] Received July 27, 2015; in final form, March 01, 2016
Abstract– In this review the investigations of direct surface relief formations in nanomultilayers from chalcogenide glasses are summarized. Chalcogenide glasses are known to exhibit several photoinduced phenomena, both scalar and vectorial, which are connected with photoinduced structural transformations, defects creation, and atoms diffusion. Surface relief formation in chalcogenide glasses films has been intensively studied due to its applicability to reversibly form versatile patterns and diffractive optical elements. Both intensity and polarization holography have been employed to generate surface relief structures in chalcogenide glasses materials, including monolayers and multilayer structures. The research outlined here has not only led to better understanding of the material properties that affect the optical performance of chalcogenides structures, but also illustrated the momentum in the field that has led to the development of high-performance nanostructured devices. Keywords: chalcogenide glasses, nanomultilayers, direct surface relief formation, scalar and polarization holography, electron-beam recording, optical anisotropy DOI: 10.3103/S1068375516050021
INTRODUCTION Development of active and passive elements of photonics, in line with micro- and nanoelectronics, requires new materials, structures, and new methods for manufacturing those elements. Chalcogenide glasses (ChGs) are an important class of amorphous semiconductors used in this area. Applications of ChGs have mainly been based on their transparency to infrared light (passive) and sensitivity to different kinds of irradiation (active). The latter produce pronounced structural changes in ChGs that are the more important case, regarding the challenges of understanding their microscopic nature and applications as functional materials of photonics. A major constituent of ChGs is usually one or more of the chalcogen elements from group А6 of the periodic table (suphur, selenium and tellurium, but excluding oxygen) covalently bonded to the network formers such as As, Ge, Sb, Ga, Si or P. Photonic applications are essentially related to the changes of optical characteristics (optical transmission, reflection or refraction), which can result in a phase modulation together with the changes of dimensions and structure of the given element (lens, diffractive element, waveguide). 1 The article is published in the original.
ChGs are known to exhibit several photoinduced phenomena, both scalar (photodarkening, photorefraction, photodoping) and vectorial (photoinduced anisotropy, photoinduced gyrotropy, photoinduced light-scattering) that are connected with photoinduced structural transformations, defects creation and atoms diffusion. From the general point of view, the basic effects of irradiation on ChGs are similar the well-known irradiation-induced transformations that occur at both the initial level of the electron–hole excitation and further structural transformations, causing the same physical and/or chemical changes including optical (darkening and optical anisotropy), mechanical (softening), and geometrical effects (expansion or contraction). One of most important optically induced effects is the direct, one-step process of the surface relief (SR) formation, which was closely related to the induced mass-transport (in vertical or lateral directions) in a glassy material under non-uniform illumination. The direct light-induced fabrication of the SR by the lateral mass transport on ChG films has been widely studied for several selected compositions [1–4] and appropriate mechanisms have been recently proposed [5, 6]. Over the last decade, a steady progress regarding an ability to fabricate photonic nanocrystals from ChGs
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has led to a wide variety of different one-, two-, and three-dimensional periodic structures [7, 8]. They exhibit qualitatively novel and fascinating linear–optical, nonlinear–optical, and quantum–optical properties, thus providing a control of light propagation and light–matter interaction. Much research work has been focused on nanostructure formation suitable for micro/nano-electronics and photonics applications. Glasses are promising materials there for two reasons. First, the control of nanostructures could give the information about a glassy structure. Unlike crystalline materials, in which we can prepare atomically controlled surfaces, the amorphous structure is disordered at the atomic level. Second, the glass nanostructure may yield a wider variety than the crystalline one because bonding constraints of crystals do not exist in glasses [9]. Nanostructure processing in ChGs offers new possibilities for tailoring the electric, optical and thermal properties. These nanostructures are zero dimensional quantum dots, one dimensional chalcogenide molecules embedded in nanoscale channels, and two dimensional structures like ultrathin films and multilayers [10, 11]. The optical properties of thin films may largely depend on their geometrical structures; in this case, the desirable optical properties could be achieved by varying the thickness of films and the choice of materials. Nanomultilayered films are impressive because of the prominent photoinduced effects, since the photoinduced effects are related to structural changes which, in turn, are related to atomic movements or diffusion. Multilayer structures are simplest artificial nanostructures that can be rather easily fabricated with controlled geometrical parameters and investigated as thin films. It is essential, since the changes of the optical parameters (blue shift of the fundamental absorption edge, quantum states, luminescence) as well as of the conductivity, and melting temperature (stability), are characteristic for and usually examined in nanostructures. A few approaches are known for extending investigations of ChG layers towards the nanostructures, especially in the nanolayered, super lattice-like multilayer structures [12–14], but the problem of photostructural transformation dependence on the artificial nano-structuring is still not solved. In addition, all of these applications of ChGs are required a chemical developments to form an SR, i.e. cannot be fabricated by a one-step direct laser writing. The SR formation in ChG films has been intensively studied due to its applicability to reversibly form versatile patterns and holographic memories. Both intensity and polarization holography have been employed to generate SR structures in many ChG materials, including monolayers and multilayer structures. Additionally, the polarization direction of the exposed beams is important to generate an effective SR structure, especially for polarization holography
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where the polarization of the holographic beams is periodically modulated with a uniform intensity. Two main types of SR gratings (SRG) induced by holographic recording, due to the excitation light modulation with near band-gap light in ChGs, can be distinguished according to their formation mechanism and their properties [4]: (1) small scalar SRG induced by either volume expansion or shrinkage due to different responses of the material in the bright and dark zones of the interference pattern formed; (2) giant vectorial SRG induced by a lateral mass transport when the light polarization of the recording beams has a component along the light intensity gradient. From the fundamental point of view, photoinduced processes in ChGs are far from being fully understood according to the existing current opinion of specialists. Still polarization dependence of these processes is of both practical and theoretical importance. This brief review on most relevant scientific publications on ChG nanostructures based on the recording media is given in the context of its contribution towards the realization of the direct relief formation applicable for fabrication of photonic elements. SCALAR HOLOGRAPHY First experiments on optical recording in amorphous nanolayered structures and the analyses of possible structural changes, and differences between homogeneous and modulated layers were made by researchers from Uzhgorod State University in the late 1990s [15, 16]. Structural transformations are the base of optical recording which forms amplitude (∆α) and phase (∆n) optical reliefs [17, 18]. The phase relief may be complemented by the change of the thickness (∆d), i.e. by the geometrical SR formation directly during the optical recording or afterwards due to the selective chemical etching. The latter process is widely used nowadays for fabrication of molds, holographic gratings, integrated optical elements [19, 20]. For a long time the prevailing opinion was that the modification of the surface of ChG films due to the irradiation with light was possible only with their additional treatment of chemical etching [21]. The discovery of the effect of a photoinduced mass transport [1] under the influence of light allowed obtaining surface patterning purely by means of an optical method and even at relatively low intensities of light waves [22, 23]. That method typically uses a projection of the interference pattern that is formed by the interaction of two or more coherent plane waves with normal polarizations on the surface of a ChG film [24, 25]. The direct relief formation without etching is possible due to the effect of the light-stimulated volume expansion or contraction in chalcogenides [26–28], but usually the ∆d/d ≈ 1% and the spatial resolution is
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limited by diffraction. Essentially larger expansion effects were observed due to the light- and ion-stimulated inter diffusion in chalcogenide nanolayered films [29–31]. Remarkable results were achieved in a one-step direct optical recording-relief creation in Se/As2S3 nanomultilayers [29, 32, 33]. It was shown that the SR structures of submicron sizes could be fabricated in a direct, one-step process of recording by light or an ion beam on Se/As2S3 nanolayered films where the most efficient relief formation with a depth profile up to a few hundred nanometers and submicron spatial resolution can be performed due to the stimulated interdiffusion under the irradiation by relatively low energy H+ or D+ ions and laser lights (see Fig. 1). According to authors of [29] expectations the stimulated local expansion in nanolayered films was much more efficient for the SR formation in a one-step, direct process, than in separate homogeneous layers. The efficiency of the thickness change was higher in the case of a contact mask application than in a holographic recording: Δd/d ≈ 14%, as estimated by the atomic-force microscopy AFM (see Fig. 1). The total exposure during 30 min was near 600 J/cm2. A possible small expansion of As2S sublayers in these nanolayered films, as well as the contraction of Se sub-layers, compensate each other and have been neglected. The recording can be further developed in Bi/As2S3 nanomultilayers [34], which seems to be stable at normal environment conditions and can be used for direct optical recording-readout, but the solubility of Bi in As2S3 is rather low and slow [35], which that can result in crystallization effects in the intermixed layer and decrease the quality of there cording media. The Sb/As2S3 combination seems to be more attractive
because of a wider range of the Sb solubility and the amorphous state of the mixed layer. The role of the Sb incorporation due to the inter-diffusion or initial introduction to the evaporated glass, and optimized Sb/As2S3 nanomultilayers, were developed for in situ conditions in [36]: a one-step amplitude-phase optical recording with parameters enhanced in comparison with those of single homogeneous layers made of preliminary synthesized ternary (As2S3)xSb1 – x glasses or by vacuum thermal co-deposition and even with a model Se/As2S3 nanomultilayers structure. At the same time, the stimulated inter-diffusion in this structure does not result in a giant photo-expansion that can be attributed to the creation of Sb2S3 structural units due to the solid phase reaction and increased rigidity of the resulting structure of the mixed layer. An alternating deposition of two different compositions during thermal evaporation of initial materials from evaporators in a vacuum chamber is probably the simplest method of nanomultilayers fabrication used in laboratories [37–39]. The quality of the resulting multilayer structure depends on the stability of the modulation period, on the sharpness of interfaces and on the preservation of the given compositions and structure of the sub-layers through the whole multilayer. In [40], the inter diffusion in Se/As2S3 and Sb/As2S3 nanomultilayered films are studied by the X-ray photoelectron spectroscopy (XPS). Since the optical absorption measurements do not directly give any visualization of the mass transport of atoms during photoinduced diffusion, the XPS was used to study the atomic movements and also to analyze the new bonds formed between the components due to inter-diffusion. The XPS is considered to be a useful surface ana-
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Fig. 3. XPS Se3d spectra of as-prepared and irradiated (at different times) Se/As2S3 nanolayered films. Please note that with irradiation Se peaks shift to lower binding energy, which clearly indicating formation of As–Se bond [40].
lytical technique to study the chemical state and local environment of an atom. The chemical bonding is often realized through a correlation with chemical shifts in the XPS binding energies of the corresponding elements. When the X-ray photon impinges on a sample, photoionization takes place and an electron is expelled from the sample with a certain kinetic energy. The energy of the photo-electrons leaving the sample gives a spectrum with a series of peaks. The binding energies of the peaks are characteristic of each element. Since the XPS is a surface analytical technique, most of the signals come from the top 70–100 Å layer.
position in the Se/As2S3 multilayered system are analyzed, as these are the main factors influencing the efficiency of the relief formation via light induced intermixing. As a result of experiments, it was concluded that a new mechanism of optical recording, based on the light-stimulated inter-diffusion in multilayers, is established and applied for the SR formation. The modeling of this process has shown that it is very sensitive to the multilayer modulation period, especially in the nanoscale region. The focus of the research in [39] was to investigate the structural changes in the multilayer nanostructure (MN) As2S3–Se and to examine the relative contribution of As2S3 and Se layers to the nanostructuring by measuring the Raman spectra. Analyzing the Raman spectra and recording features of an MN, it was found that there are, at least, two processes influencing the recording. The first is photo-diffusion taking place at As2S3–Se interfaces, which is proved by the Raman spectra under illumination. The second one is the size restriction in an MN. The scaled samples had a modulation period covering one, two, and three molecular and cluster dimensions for As2S3 and Se, which is characteristic to the medium-range order in glasses. The MN As2S3–Se with a thickness of constituent layers around 2 medium range orders is the most promising having the maximal diffraction efficiency (30%) at direct recording (Fig. 4). The research of the influence of the structure of constituent amorphous layers on recording properties of nanomultilayers Ge5As37S58–Se by measuring the Raman spectra was carried out in [42]. The Raman spectra vs the laser power showed that the following
Atomic movements and new bond formations are observed in both Se/As2S3 and Sb/As2S3 nanomultilayered films during photoinduced diffusion. It was shown that in Se/As2S3 nanomultilayered films, some of the As–Se bonds started forming and there was a considerable decrease in the As–S bonds followed by an increase in the As–Se and S–Se bonds (Figs. 2, 3). The S–Se bond signals are very weak to record, maybe because of fewer bonds. Since it is energetically difficult to break an As–S bond to form an As–Se bond, it was assumed that the new bond formations are taking place by the bond rearrangement mechanism. In Sb/As2S3 nanomultilayered film, the homo polar bonds (As–As, S–S, Sb–Sb) are converted into heteropolar bonds (As–S, Sb–S). The authors [40] assumed that the energetically favored heteropolar bond formation takes place by a phonon assisted mechanism using the lone pair π electrons of S 02. In the paper [41], both the role of a multilayer period and the specific volume of the resulting com-
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Fig. 5. Differential Raman spectra of Se in dependence on exposure (for different values of laser power, exposure time is 132 s as compared to Raman spectra at 50 mW laser power [42].
structural changes in Se nanolayers take place by laser illumination: (a) the band at 251 cm–1 diminishes, which points on the reduction of Se8 rings quantity, (b) the band at 235 cm–1 heightens, which is associated with enlargement of a small fraction of “pure” helical chains (Fig. 5). The Raman spectra of the Ge5As37S58–Se nanomultilayers structure contain all bands characteristic of Se and Ge5As37S58 films. Similar to the Raman spectra of pure Se films, for differential Raman spectra in dependence on exposure: the longer the exposure, the higher the intensity of the 235 cm–1 band and the lower the intensity of the 251 cm–1 band. Concerning the recording features, the direct one step grating SR formation in Ge5As37S58–Se nanomultilayer structures is shown with a depth of the grating relief about ~100 nm at the total structure thickness 1760 nm and the modulation period of layers 17 nm. In the Ge5As37S58–Se nanomultilayers, the diffraction efficiency of 18% in absolute value was obtained at the λ = 0.65 μm illumination wavelength. The s–s polarization of the chosen recording laser beams provides the SR grating formation due to the expansion/shrinking of the volume of the nanomultilayered structure. Based on abovementioned works the following conclusion can be made. Two mechanisms contribute in the process of the SR formation, i.e., photoinduced volume changes in ChGs which generate the surface patterning by photoexpansion or photocontraction, and the lateral mass transport phenomena under the optical gradient force. The latter has attracted much attention due to an opportunity of a direct formation of SRGs in contrast to the well-known two-step method exploiting the phenomenon of photoinduced changes in the dissolution rate of ChG. Despite the
significance of SRGs formation in ChG there is a limited number of investigations on surface deformations of nanomultilayered ChG. Note that a conventional holography is exploiting the s–s polarization of recording beams only when the intensity distribution pattern is recorded on the sample surface excluding polarization states of beams. For the s–s polarization scheme, no mass transport phenomenon was detected for all of the ChG compositions and the SRG is mainly formed through the photoinduced volume changes [43]. The mass transport is a so-called vector effect because it depends on the direction of the electric field vector of light and is particularly intriguing because the starting structure of the no-irradiated films is amorphous and isotropic. When two arbitrarily polarized light waves interfere, they create a mixed, spatial light-field modulation, consisting of polarization modulation and intensity modulation. The polarization modulation enables a vector hologram recording whereas the intensity modulation enables a scalar hologram recording. Note also that a possible contribution of the polarization state of writing beams was not considered in earlier studies of direct recording in nanomultilayered ChG. Due to the vector character of the mass transport effect it is meaningful to apply vector holographic scheme for SGR recording on the nanomultilayers based on ChG. POLARIZATION HOLOGRAPHY In contrast to the conventional holographic process, in which intensity variations in an interference pattern between an object beam and are ference beam are recorded, polarization holography employs beams with two different polarizations for recording the
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information [44]. In this case, the polarization state of the resultant beam is recorded on a suitable medium. Polarization holographic storage has several unique properties: (1) It is possible to achieve, theoretically, a 100% diffraction efficiency. (2) The diffracted beams have unique polarization properties, depending on the polarization of the recording and read-out beams. (3) It is possible to fabricate polarization-sensitive optical elements. (4) The optical elements fabricated with polarization holography are achromatic, allowing their use at all wavelengths. As mentioned above, in the case where the light polarization of the recording beams has a component along the light intensity gradient, the giant vectorial SRG induced by the lateral mass transport can be formed in ChGs. Experiments have shown that only few compositions of ChG demonstrate both types of SRG. For example, vectorial SRG have been observed only for the Se-rich films in the binary As-Se system (As20Se80) [2] and for the compositions close to the As40S60 in As–S glasses [1] while scalar SRGs are common to all glass compositions of As–S and As–Se glasses. Most polarization-sensitive media are sensitive to a polarization azimuth. Then, birefringence and dichroism are induced in these media. There are several reports on photoinduced anisotropy in ChGs [45–47]. The photosensitivity of the ChG materials depends on the polarization of the optical field and anisotropy is induced by the illumination of polarized light [48]. The experiments demonstrated that photoinduced structural transformations, expressed in photodarkening, are characteristic for the nanodimensioned glassy As2S3 films just as they exist in thick chalcogenide films. Linearly polarized light was shown to excite the linear dichroism in the nanodimensioned glassy AsS3 films. The photoinduced anisotropy is expected to be characteristic also for nanodimensioned films of various ChGs with different properties. The main features of the phenomenon of a lightinduced anisotropy in ChGs are: —Linearly polarized light induces dichroism and birefringence of a certain sign, which is usually saturated during tens of seconds or several minutes of irradiation depending on the light intensity. —The anisotropy can be destroyed after heating the an anisotropic sample to a temperature somewhat lower than the glass transition temperature and can be created repeatedly by a subsequent irradiation of the cooled sample with linearly polarized light. —The anisotropy is erased with irradiation with nonpolarized light. —The anisotropy axis can be reoriented repeatedly by means of irradiation with linearly polarized light
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with an orthogonally oriented electrical vector without any fatigue effect. A possibility to observe a photo-induced anisotropy at energies much larger than the exciting photon energy indicates that, by irradiation of ChGs with linearly polarized light, the main interatomic covalent bonds can be oriented and reoriented [49]. All previous experimental data, obtained by different researchers, demonstrated the phenomenon of photoinduced optical anisotropy. In [49] it is success-fully demonstrated that the photoinduced optical anisotropy is accompanied by the photoinduced anisotropy of photoconductivity. When the As50Se50 sample with two parallel electrodes is irradiated by a non-polarized He–Ne laser beam, the appearance and subsequent saturation of photocurrent are detected. The following irradiation by linearly polarized light with an electric vector E either parallel, Ex, or orthogonal, Ey, to the electrodes results in the appearance of an anisotropy in the photocurrent (Fig. 6). To the best of our knowledge, this is the first reported observation of the photoinduced electrical anisotropy in ChGs. Analyzing the obtained results, it was concluded that the observed anisotropy of photoconductivity is due to the anisotropy of the carriers mobility. The obtained data indicate that the microanisotropic species, responsible for optical anisotropy, affect not only the light absorption process but also the transport of the nonequilibrium charge carriers. While normally these microanisotropic species are oriented randomly, which results in anisotropic photoconductivity, irradiation with linearly polarized light results in the alignment of these species and an anisotropic photoconductivity [49]. Kwak and colleagues [50] reported the first recording of both scalar and vector holographic gratings in amorphous As2S3 thin films. Linearly polarized argon-ion laser beams at 514 nm are used to record dif-
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Fig. 8. Temporal behaviors of diffracted intensities of vector (β = π/2) holograms in amorphous As2S3 thin film. Dashed curves represent the theory and solid ones—the experiment [50].
fraction gratings and a He–Ne laser at 633 nm is used for the read-out. The intersection angle between the recording beams was 15ο, corresponding to a grating spacing of 2 lin/mm. They observed that the amplitude modulated gratings gave an efficiency of approximately 20% in 180 s, and pure polarization gratings produced a monotonic steady-state efficiency of 0.2% (Figs. 7, 8). The fabrication of polarization gratings is explained on the base of the induced nonlinear optical polarization in amorphous media.
The report [52] presents the studies of direct holographic recording of the SR gratings on amorphous As2S3 films by the 532 nm laser light. It is shown that it is possible to raise the efficiency of holographic recording by extra illumination of the sample by incoherent laser light during the recording process. In some cases even more than hundred times better recording efficiency can be reached. It is possible to obtain much stronger diffraction than without extra illumination, or, alternatively, much less time is needed to reach the same diffraction efficiency as without extra illumination. The produced SR is very stable at the room temperature and together with the real time SR depth control (by controlling the diffraction efficiency measurements) can find practical application in optics (holography, lithography, antireflection coatings, sun batteries), electronics (micro and nano matrix structures), nanotechnology (topdown approaches) and in many other sectors. Although elemental amorphous Se (a-Se) is a model glass-former in chalcogenide science, few systematic studies of SRG formation in a-Se have been carried out up to now, while investigations of other photo induced effects abound. The authors of [4] report on two distinct mechanisms of the SRG holographically recorded in a-Se depending on the polarization of writing beams (Fig. 9). In the s-s scheme of recording, scalar SRGs appear through the photo induced volume contraction. For p-p polarized beams, a lateral mass-transport takes places altering the mechanism of the SRG formation and, in turn, generates giant non-saturated SRGs. Consequently, polarized light induces effects of the SRG formation by the holographic recording in amorphous chalcogenides. Much experimental and theoretical work has been performed to clarify the mechanism of this phenomenon. Models have been proposed to describe the for-
The effect of the photo induced anisotropy and its application to vector hologram recording is reviewed focusing on amorphous chalcogenides in [51]. Vector holographic grating recording in amorphous As–S–Se films is experimentally studied and analyzed in comparison with scalar recording. It is holographically established that a linearly polarized 632.8 nm light produces photoinduced anisotropy. The chalcogen related D+, D– center reorientation and generation mechanism is proposed, which is used to explain the observed peculiarities of vector recording in comparison with scalar recording based on photo induced structural changes: a much lower diffraction efficiency (4 × 10–3% vs 4%), a much larger specific recording energy (6.4 kJ/(cm2 %) vs 20 J/(cm2 %)), a difference in the spatial frequency response, an instability (vector hologram lifetime of about two days vs practically permanent scalar holograms), the absence of the hologram self-enhancement (present in scalar recording), nearly perfect reversibility. It is also experimentally found that vector holograms in amorphous As–S–Se films indeed reconstruct the signal wave polarization but only in the minus first diffraction order. It is also shown that photoinduced anisotropy also contributes to the scalar hologram recording in amorphous chalcogenides stimulating it by means of sub and gap readout light and enabling a sub and gap recording.
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mation of SR, where volumetric internal pressures, interaction among dipoles, anisotropic diffusion, or optical gradient forces were considered to be the driving forces for deformation. Those efforts have not led to a unified model that captures all experimental observations [53]. ELECTRON-BEAM RECORDING Electron beam (e-beam)-induced surface patterning of ChGs has attracted considerably less attention and so far successful attempts to obtain SR on ChGs thin films have been made on the base of a well-known two-step method exploiting the phenomenon of photo-induced changes in the dissolution rate of ChGs (see reviews [54, 55] as an example). A limited number of thorough experimental studies devoted to the direct formation of SR by an e-beam exposure have been performed. Those studies carried out have been mainly focused on the selected homogeneous (stoichiometric) compositions such as Ge0.2Se0.8, As0.4S0.6, As0.4Se0.6 films [56–60] or nanostructured chalcogenide layers [12, 61]. For the first time, in [58] it was demonstrated that the surface deformation occurs in insulating chalcogenide glasses when subjected to an e-beam exposure. The mechanism is considered to be a thermal, i.e., a combination of electrostatic force and enhanced fluidity, which are caused by injected electrons and generated carriers. The phenomenon is considered promising for the direct production of microscopic optical elements such as relief-type gratings. In comparing the e-beam effect observations with the giant photo expansion, several differences were noticed despite a common fact that both give expanded regions. First, in the photo-expansion, no depressed region appears, while e-beams give rise to the peripheral depression. Second, the thickness dependence is different. The e-beam effect becomes smaller in samples thinner than ~5 mm, which can be related to the penetration depth of incident electrons. On the other hand, for the
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photo effect the threshold thickness is ~100 mm, which has been related to the self-focusing effect of sub-band gap light. The surface variations caused by an e-beam were found in [56] to depend exponentially on an electron dose. The magnitude of the surface contractions depends on the film thickness, suggesting that the whole exposed area contributes to the observed surface changes. The SR depth for As2S3 samples with different thicknesses is small. Figure 10 shows the magnitude D of the measured surface contraction as a function of the electron dose. The values of D are displayed for the As2S3 films with thicknesses of 4.7 μm and 11 μm. The values of D for the 1.3 μm thick films were so small that it was difficult to measure them reliably due to the noise coming from other sources. As an explanation of the phenomena observed in [56] the following is proposed. The As2S3 films are formed from two dimensional networks. In those networks atoms are bound with covalent bonds. Networks instead are bound together only by weak Van der Waals forces. A photon with the bandgap energy may break one of those covalent bonds in a two-dimensional network. This broken bond may form a new covalent bond but this time between adjacent two-dimensional networks. This kind of crosslinking bonds causes stress fields inside As2S3 films, hence inducing surface changes by the observed photon. It is suggested that surface variations induced by electrons have a similar mechanism. One electron has the energy sufficient to break more than one covalent bond. These broken bonds form new crosslinking bonds between twodimensional networks causing a necessary stress field to induce the relief structure. Comparison of the mechanisms of photo- and electron-induced structural transformations in S-rich As35S65 ChG thin films, done in a later work [59],
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Fig. 11. AFM picture and its cross-section for e-beam irradiated spots on Se/As2S3 [61].
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X, μm
104.75
Fig. 12. E-beam fabricated scattering lens on Se/As2S3 MN and its profile measured by AFM [61].
revealed that light irradiation from a halogen lamp does not change the chemical composition of the surface. However, it causes bond-switching and ordering of the structure, which results in the gradual disappearance of “wrong” homo polar As–As bonds. A similar process occurs even within the volume of the exposed region but it is relatively slow. The diminishing concentration of As–As bonds on the surface is not significantly influenced by the oxygen environment
20
nm 50 25
10 0
0
10
20
0
Fig. 13. AFM picture of e-beam fabricated Fresnel lens on Se/As2S3 MN [61].
during illumination. A low-do see-beam irradiation activates the surface, which contributes to the oxidation of As atoms in “wrong” As–As bonds, thereby decreasing the S concentration in the very top parts of thin ChG films. Electron irradiation with high doses destroys the structure drastically. On a subsequent exposure to air, the under-coordinated As atoms react with oxygen and decrease the S/As ratio dramatically. An analysis of the vibration binding spectra shows an anomalous increase of the ~10 eV band associated with non-bonding As 4s electrons after light- and low dose e-beam irradiation. It also confirms the destruction of the surface during a high-dose electron irradiation. The lack of a clear explanation of the nature of the peculiar local light-induced expansion, which is especially large (Δd/d ≥10%) in Se/As2S3-type MN, the need of improvement of the lateral resolution towards the nanometer scale, and an ability to fabricate different surface structures have encouraged researchers [61] to explore the use of e-beams for one-step recording. Simple dots were recorded by light or an e-beam to determine and compare the basic properties: bleaching or darkening, and expansion or contraction. The volume change can be easily visualized by the AFM or even by optical microscopy after the recording (see Fig. 11). The induced change in transmission was measured only after laser illumination, as an in situ optical measurement was not possible during e-beam recording and also because of a small size of the spot irradiated by an e-beam. Two of the three main parameters of interest (transmission, refractive index, and thickness) showed maximum for Sb/As2S3 MN with a modulation period Λ ≈ 4 nm: the spectral range of the effective change in transmission extended from 500 to 1000 nm and the refractive index decreased by Δn ≈ –0.3. By contrast, for Se/As2S3 MN, the changes were observed only from 500 to 600 nm, and n increased with Δn ≈ 0.12 at the saturation point. It is important to mention that the appropriate changes in separate Se or As2S3 layers (with the total thickness equal to the sum of the thicknesses of sub-layers in MN) were smaller, and there was no bleaching, neither any expansion was observed in Sb/As2S3 MN. On the base of the investigations reported in [61], an opportunity is shown to plan and create certain optical elements. For example, a lens as a sum of e-beam recorded circles with a step-wise decreasing n can be created, as it is presented in Fig. 12. Another example is that of a Fresnel lens shown in Fig. 13. The resolution theoretically equals the diameter of the e-beam, but in practice it depends on the scattering of electrons in the layer. According to [61] estimate from the Monte Carlo modelling, the effective diameter of the written spot or width of the line at the surface (and in the whole depth of the layer) is ~0.6–1.0 μm. It can be reduced by decreasing the film thickness and by using lower electron energies.
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(a)
132.2 40 μm 30
40 μm 30 20
20 10
10 –200
10
20
30
(b)
500 400 300 200 100 0 –100 –200
Height, nm
nm 659.2
465
40 μm
2
4 6 8 10 Surface position, μm
12
14
Fig. 14. Lines produced by e-beam for As0.2Se0.8 composition: (a) AFM image and (b) cross-section of one of lines. U = 20 kV, I = 7 nA, 60 s of exposure [63].
The authors in [61] came to the conclusion that an efficient irreversible amplitude-phase optical and geometrical surface patterning can be realized by a direct, one-step e-beam recording much more efficiently in Se/As2S3 and Sb/As2S3 nanomultilayers than in equivalent single homogeneous layers. To sum up, the experiments about the response of amorphous thin films with different compositions to an e-beam revealed that different kinds of a surface deformation (ridges or grooves) take place by the e-beam irradiation depending on the composition and the electron current dose [56–61]. The researchers in [62] reported the influence of an e-beam on the surface stability of a non-annealed As0.2Se0.8 film and found that the formation of ridges and depressions near the ridges was induced via the e-beam accelerated lateral mass transport. The aim of [63] was to extend their previous results to the surface e-beam-induced patterning of amorphous films of AscSe1 − c system, for0.2 < c < 0.5. This system has been chosen for the following reasons: (i) it allows the synthesis of stable glasses over a wide range of compositions including Se-enriched glasses and possesses reproducible physical properties; (ii) there was no information available about the response of amorphous As-Se thin films with different compositions to e-beam irradiation, especially in c < 0.3 range; (iii) these materials are a suitable object for modelling the network glass structure that shows an unusual rigidity percolation in the glass structure at the mean coordination number r = 2.29. This value is significantly lower than the mean-field rigidity percolation transition value of 2.40 for the observed onset of rigidity in the most well-known system of ChGs (r is defined as the average number of covalent bonds per atom). With the generally accepted point of view that a large-scale response of ChGs to the external perturbation should be expected for an under-constrained floppy system, this rigidity transformation promises maximum in the trend of thee-beam-induced relief
formation in Se-reach glasses. In such a case, the contribution of the ChG structure to the e-beam response could be revealed and discussed. Films of all compositions gave rise to surface patterning under e-beam irradiation, as is shown in Fig. 14a, as an example. It has been found that the irradiated regions look as the ridges and depressions on the periphery (Fig. 14b). For a quantitative estimation, the height of the lines from the bottom of the depressions near the ridges to the top of the central peak has been measured (see Fig. 14b, as an example). In this experiment, the material moves towards the ebeam and the volume of depressions is close to the volume of ridges. The e-beam-induced formation of ridges was revealed for all investigated compositions irrespective of the time of exposure. Note that both the ridges and depressions near the ridges grow with the exposure time. It means that the relief is induced by the e-beamaccelerated lateral mass transport. The authors in [63] also noticed that there was no significant change in the height or shape of any of the lines after storing the samples in air at ambient conditions for six months. They found that the extent of the e-beam-induced local surface deformations i.e. the height of the SR at the same exposition decreases with increasing the As content in AscSe1 − c amorphous chalcogenide layers. For a long-time exposure (300 s) in the Se-enriched films (As0.2Se0.8 composition), a giant, up to the film thickness, SR was obtained. It was established through the AFM measurements that the time (exposure) dependence of the SR consists of, at least, two stages that occur with different rates and give different contributions in the profile variation. The first stage is detected immediately after the beginning of an e-beam irradiation and may be connected with the local volume change, which results in relatively small deformations, whereas the second stage is much slower, however, resulting in giant changes of the profile. These changes are caused by the lateral mass transport
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200 μm
(a)
y: 47 μm
μm 0.30 0 x: 48 μm
(b)
y: 49 μm x: 49 μm
Fig. 15. Results of e-beam pixel recording of Ukrainian (a) and Moldovan (b) state emblems using Ge5As37S58–Se MN. AFM images of recorded emblems fragments are shown on the right sides of figures.
induced by an e-beam. The driving force of the mass transport is mainly defined by the lateral steady-state electric field caused by steady-state distributions of electrons and holes generated by an e-beam. It was concluded that both of the optical- and e-beam-stimulated SRs in As-Se system correlate with the rigidity percolation range and the maximum photoplasticity, which are not directly connected to the known photo darkening effect since it is minimal for those compositions. More important is the defect structure, which enhances the diffusion processes in the gradients of the excited electron–hole pairs and the localized electrons in the case of the e-beam excitation. Processes of the e-beam and holographic recording of the SR structures with the use of Ge5As37S58–Se MNs as registering media were studied in [64]. In that work, the experimental results showing the SR formation in Ge5As37S58–Se nanomultilayer structures under the e-beam exposure are presented. Diffraction gratings, Ukrainian and Moldovan state emblems were recorded by the e-beam exposure using a scanningelectron microscope Tesla BS 300 with the programmable exposure control unit. A acceleration voltage of 25 kV, and a specimen current of 8–10 nA were used to irradiate the samples. In Fig. 15a the results of e-beam
recording of the Ukrainian state emblem with the use of Ge5As37S58–Se MN are shown. Image sizes of the Ukrainian state emblem were 512 × 512 pixels (pixel size ~2 μm). An AFM image of the recorded emblem fragment is shown on the right side of Fig. 15a. It is necessary to note that under the given recording conditions, pixels height is up to 200–300 nm. Figure 15b shows the results of the e-beam recording of the state emblem of the Republic of Moldova, using the Ge5As37S58–Se MN. An AFM image of the recorded emblem fragment is shown on the right side of Fig. 15b. From these results it is possible to conclude that the surface pattern formation on chalcogenide materials with electrons is possible and the recording mechanisms, achievable effects at low or higher exposures, are approximately similar to the light induced ones. CONCLUSIONS ChGs offer a unique set of properties amongoptical glasses that make them an excellent choice for nanosized devices. Nanoengineering of ChGs, namely the creation of different nanolayered structures, offers new opportunities for tuning the basic optical parameters and the stimulated structural changes, which can
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be used for the development of special photosensitive media or direct one-step amplitude-phase optical/ebeam recording and fabrication of the SR structures. The research outlined here has not only led to better understanding of the material properties that affect the optical performance of chalcogenides structures, but also has illustrated the momentum in the field that has led to the development of high-performance nanostructured devices The main challenge for these applications is to identify glass nanostructures that have the required stability at a high optical performance – together with adequate recording methods – from among the diversity of ChG compositions currently available. REFERENCES 1. Trunov, M.L., Lytvyn, P.M., Nagy, P.M. and Dyachynska, O.M., Appl. Phys. Lett., 2010, vol. 96, p. 111908. 2. Trunov, M.L., Lytvyn, P.M. and Dyachyns’ka, O.M., Appl. Phys. Lett., 2010, vol. 97, p. 031905. 3. Trunov, M.L., Lytvyn, P.M., Yannopoulos, S.N., Szabo, I.A. and Kokenyesi, S., Appl. Phys. Lett., 2011, vol. 99, p. 051906. 4. Voynarovych, I., Schroeter, S., Poehlmann, R., and Vlcek, M., J. Phys. D: Appl. Phys., 2015, vol. 48, p. 265106. 5. Kaganovskii, Yu., Beke, D.L., Charnovych, S.S., Kokenyesi, S. and Trunov, M.L., J. Appl. Phys., 2011, vol. 110, p. 063502. 6. Kaganovskii, Yu., Trunov, M.L., Beke, D.L. and Kokenyesi, S., Mater. Lett., 2012, vol. 66, p. 159. 7. Feigel, A., Veinger, M., Sfez, B., Arsh, A., Klebanov, M., and Lyubin, V., Appl. Phys. Lett., 2003, vol. 83, p. 4480. 8. Wong, S., Deubel, M., Perez-Willard, F., John, S., Ozin, G.A., Wegener, M., and von Freymann, G., Adv. Mater., 2006, vol. 18, p. 265. 9. Tanaka, K., Chalcogenide glasses, in Encyclopedia of Materials: Science and Technology, Buschow, K.H.J., Eds., Amsterdam: Elsevier, 2001, pp. 1123–1131. 10. Tanaka, K., J. Non-Cryst. Solids, 2003, vol. 21, pp. 326–327. 11. Kurioz, Y., Klebanov, M., Lyubin, V., Eisenberg, N., et al., Mol. Cryst. Liq. Cryst., 2008, vol. 489, pp. 94– 104. 12. Takats, V., Nemec, P., Miller, A.C., Jain, H., et al., Opt. Mater., 2010, vol. 32, pp. 677–679. 13. Dikova, J., Vlaeva, I., Babeva, Tz., Yovcheva, T., et al., Opt. Laser Eng., 2012, vol. 50, pp. 838–843. 14. Saito, I., Masuzawa, T., Kudo, Y., Pittner, S., et al., J. Non-Cryst. Solids, 2013, vol. 378, pp. 96–100. 15. Kikineski, A., Mishak, A., Palyok, V., and Shiplyak, M., Nanostruct. Mater., 1999, vol. 12, pp. 417–420. 16. Palyok, V., Mishak, A., Szabo, I., Beke, D.L., et al., Appl. Phys. A: Mater. Sci. Process., 1999, vol. 68, pp. 489–492. 17. Popescu, M., Andries, A., Ciumas, V., Iovu, M., Sutov, S., and Tciuleanu, D., in Fizica Sticelor Calcogenice, Bucureşti, S., Ed., Chisinau: Ştiinţa, 1996.
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