Direct surface relief formation in As-S(Se) layers M. Trunova, P. Lytvynb , P.M. Nagyc, Cs. Cserhatid, I. Charnovichd, S. Kokenyesi*d a
b
Uzhgorod National University, Pidhirna 46, Uzhgorod, Ukraine 88000 V.Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, pr. Nauki 41, Kiev 03028, Ukraine c Central Institute of Chemical Researches, HAS, Pusztaseri ut 59-67, P.O. Box 17, Budapest, Hungary d University of Debrecen, Institute of Physics, Bem Sq.18/a, Debrecen, Hungary ABSTRACT
The process of holographic recording based on a direct formation of periodic surface relief in AsxSe1-x (0 ≤ x ≤ 0.5) and As2S3 layers was investigated by in situ AFM depth profiling and compared with data on diffraction efficiency η of the similar relief holographic gratings, measured in a reflection mode. It is established, that the time (exposure) dependence of η has at least two components, which are connected with different components of the surface deformation Δd and relief formation up to the giant, Δd/d >10% changes in the best As0.2Se0.8 or As2S3 compositions. Correlation is found between light and e-beam induced surface deformations during recording in similar compositions. Applications for prototyping phase-modulated optoelectronic elements are considered. Keywords: Amorphous chalcogenide layers, surface relief recording, optoelectronic element.
1. INTRODUCTION Chalcogenide glasses and amorphous layers of As2S3, AsSe, GeAsSe and similar compositions reveal well known photoinduced structural transformation effects within the amorphous phase and related changes of optical parameters (absorption coefficient Δαλ, refractive index Δn, reflectivity ΔR) [1,2] as well as of chemical stability, which in turn provide applications of these materials as memory elements, high-resolution inorganic photo-resist, integrated structures. Besides the high performance diffraction grating fabrication [3]. other optical elements like lens arrays, waveguides for VIS and especially for IR spectral range can be produced this way too. Direct, one step formation of geometrical and optical relief without the etching step is very attractive for simple prototyping, [4,5]. The possibility of such technology follows from the photo-induced volume (thickness) change, which is inherent in many compositions as reversible one (usually near 1% [1,6]), but larger effects are also known [7 ,8]. The mechanisms of volume changes (expansion or just contraction even in the same composition at different authors) are not completely explained or even controversial, since the composition, technology, treatments have essential influence on this effect as well as on the photo-induced transformations on whole. In addition, peculiar effects of photo-plasticity [9,10] may accompany the above mentioned transformations, but these are not obviously interconnected since compositions like As0.2Se0.8 reveal large deformation but small photo-darkening under the influence of He-Ne laser illumination [11]. Stimulated darkening or expansion also appears when chalcogenide layer is irradiated by e-beam [12,13], but up to now these different effects are not enough correlated with composition and experimental conditions. Examples of different possibilities for recording in thin homogeneous AsSe-type chalcogenide layers are presented in Fig.1. The simplest compositions which reveal almost all of the above-mentioned effects are glasses from AsxSe1-x system (0< x ≤ 0.6) and As2S3. As it will be mentioned later, a-Se layers also reveals this effects, but are rather unstable due to the low softening temperatures and crystallization. Compositions with excess of S in As-S system are also less stable, while As-Se compositions even with small As addition are not crystallizing at normal conditions. Stable As-Se layers can be easily prepared by thermal evaporation in vacuum, they are sensitive to the most common red laser illumination. In As-S system the As2S3 composition is similarly stable, easy in technology, but sensitive to the green laser illumination. *
[email protected], phone 3652415222, fax 3652315087 Optical Components and Materials VII, edited by Shibin Jiang, Michel J. F. Digonnet, John W. Glesener, J. Christopher Dries, Proc. of SPIE Vol. 7598, 75981H · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.837604 Proc. of SPIE Vol. 7598 75981H-1
Figure 1. Recording in amorphous chalcogenide layers: optical amplitude-phase relief (a), surface relief after the selective etching of the previous sample (b), surface relief recorded in a direct process (c).
In our present paper a particular emphasis is given to the development of gratings with deep relief as well as to the comparison with separate line (ridge) formation under the influence of the focused e-beam. This comparison may give us some new insight to the mechanism of photo-induced processes in chalcogenide glasses and help us to select compositions with the best recording parameters, i.e. with the highest deformation under the same exposition.
2. EXPERIMENTAL Bulk glasses were prepared by the conventional technology of synthesis in evacuated quartz ampoules from high purity As, S and Se materials. Homogeneous films with total thickness d between 0.5-2.0 μm were fabricated by physical vapor deposition on silica glass substrates. Film thickness was measured in direct way by an Ambios XP-1 nanoprofilometer. Optical recording of surface (lateral and vertical) structures (and corresponding optical amplitude-phase relief) was performed on a special small integrated experimental stage, which consists of the sample holder, laser and the beamsplitter-coupler prism element. It enables us to create periodical, close to the sinusoidal distribution of the coherent light intensity in the spot of approximately 3 mm in the investigated chalcogenide layer, which is placed on the top of the device. So in this device illumination during the recording was performed from the bottom (substrate) side of the sample at λ=0.63 or λ=0.53 μm, with an average power densities near 0.8 W.cm-2. The readout of the diffraction efficiency in the reflecting mode ( λ=0.44 μm, well above the optical absorption edge Eg0 for the given compositions of semiconductor glasses) was done from the top side, and compared with in situ surface relief measurements. The last were performed on the same integrated stage, which was placed into the AFM ( Nanoscope Dimension 3100 ). Scanning was made in a non-contact mode, the time dependence of the grating development was monitored. Light induced surface transformation (relief formation) was compared with the development of lines (ridges) during ebeam irradiation in the Hitachi S-4300 scanning electron microscope. The accelerating voltages were chosen between 10 -20 kV, currents were 5-10 nA. Samples were covered with 50 nm gold layer to avoid surface charging, but some experiments were made on a clean surface too. All experiments were made at room temperatures, at a normal ambient conditions (except of e-beam recording). Besides the in situ AFM measurements of the relief during or after the opticalrecording, SEM pictures were made in situ after the e-beam recording, but the line profiles were measured also ex situ by Ambios XP profilometer. The presence of photo-darkening in the layers after the homogeneous illumination with same expositions as at holographic recording was estimated from optical transmission spectra, which were measured by Shimadzu UV-VIS spectrophotometer.
3. RESULTS AND DISCUSSION
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It was established that pure a-Se, all As-Se compositions possess photo-darkening under red laser illumination, as well as As2S3 under green laser illumination, since the last has larger optical gap in comparison with selenides, which can be penetrated by the red light (λ ≈ 600 nm) in the best way. The photo-stimulated darkening (red shift of the optical absorption edge and corresponding decrease of the transmission τλ at the given wavelength relative to the initial one τ0λ) is small, τλ/τ0λ ≈ 0.1-0.2 in a-Se and As-Se layers with As concentration up to 30 at%. In AsSe, As2Se3 at as well as in As2S3.layers this decrease is larger, up to 0.5-0.8 and the Δn change is also larger, up to 0.1 i.e. about 3-5 % of this parameter in a non-exposed layer. At the same time the highest reliefs were obtained in AsxSe1-x layers with x= 0.1 - 0.2 and in As2S3. Relief parameters for pure amorphous Se films were comparable with glass composition at x=0 (excursion amplitudes for gratings were above 50 nm), but photo-crystallization decreases the quality in a short time. It can be concluded that the two effects – photo-darkening and expansion – are not directly related. Thus the best recording conditions in our experiments were determined from the height of the surface geometrical relief and from the diffraction efficiency of the recorded grating if applicable. In our case of short wavelength reflection mode readout of the surface grating, the efficiency can reach 20% and it can be further improved by the deposition of the reflective coating.
Figure 2. Time dependence of the surface holographic grating efficiency in As0.2Se0.8 layer (recording at λ=635 nm, readout in reflection mode at 440 nm)
Comparing this data with a few percent of diffraction efficiency (measured usually in a transmission mode) in AsSe layers, where the surface relief is not so developed but the change of n is larger it may be concluded, that the depth modulation is the main parameter which determine the efficiency of holographic recording in the examined As-Se compositions. Two components of the time dependent relief formation were clearly separated during the η measurement in the reflection mode (see Figure 2) and direct scanning of the surface in SPM during the recording (Figure 3). The “bands” from the left to right in Figure 3,b correspond to the small but fast initial increase of η (Figure 2): after each 1second of exposition 40 seconds in darkness were used for measurement. It should be mentioned that the first “jump” is a common feature for recording in all compositions and so can be connected with the first step of non-equilibrium charge carrier generation and their redistribution due to the differences in mobility, that results generation of defects, change of the free volume. The next part of slow increase of η up to the saturation obviously is connected with a photoplastic effect [14], i.e. decrease of viscosity of glass under illumination, which serve the basis for directed transport of defects, molecular
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fragments between the illuminated and dark regions. Do not touching the details of rather complex and still not clear mechanism of different photo-induced transformations in chalcogenide glasses (see the comprehensive review [14], it can be concluded that similar efficient recording in the same compositions can be performed by e-beam excitation, presuming the common stage of charge and defect generation.
a)
b)
Figure 3. Development of the surface holographic grating profile in As0.2Se0.8 layer (recording at λ=635 nm): a) excursion amplitude vs. exposition, b) in situ AFM profiling.
This assumption was supported by separate investigations of surface relief formation during e-beam recording of simple lines (ridges) by scanning focused e-beam (10 nm spot) on the surface. The width and the height of the lines depend on the accelerating voltage, current and exposition time, i.e. on the energy delivered by the given number of electrons and on the concentration, distribution of the generated defects. Large penetration depth of electrons (> 2 μm) and lateral scattering (more than 2 μm) causes essential broadening of the recorded lines (see Figure 4). It can be reduced by decreasing the exposition.
Figure 4. SPM profiles of e-beam (20 kV, 7 nA) recorded lines in As0.2Se0.8 layer with exposition 10 s (A), 30s (B) and 60 s (C).
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Optically, as well as e-beam recorded reliefs were stable (more than one year storage under normal ambient conditions) but they can be partially erased by annealing the layer at temperatures below the softening temperature of the given composition. Prototipes of ssurface Fresnel lenses were recorded by e-beam lithography on the selected glass compositions, as well as surface coupling gratings to the chalcogenide waveguides were fabricated by optical holography.
4. CONCLUSIONS We have realized direct, one-step optical end e-beam recording of surface reliefs with amplitude excursion up to 400 nm in homogeneous, 0.5 – 2.0 μm thick chalcogenide layers. Common features of the processes consist in the presence of non-linear increase of the relief depth up to the saturation, long term stability and partial erase ability. It is assumed that the mechanism of the recoding process is tightly connected to the irradiation-induced charge and defect creation which is combined with the decrease of viscosity and results in plastic deformation of non-homogeneously irradiated surface. Optimized materials for laser recording in the red spectral range are AsxSe1-x glasses with x between 0.1-0.2 μm and As2S3 for the green spectral range.
ACKNOWLEDGEMENTS Support of OTKA grant K67685 as well as of Hungarian-Ukrainian cooperation grant UA-17/08 is acknowledged.
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