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Deuteron irradiation induced changes in amorphous AsSe films
Nuclear Instruments and Methods in Physics Research B 229 (2005) 240–245 www.elsevier.com/locate/nimb
Deuteron irradiation induced changes in amorphous AsSe films I. Ivan a
a,* ,
S. Szegedi a, L. Daroczi b, I.A. Szabo b, S. Kokenyesi
a
Department of Experimental Physics, University of Debrecen, Bem te´r18/a, 4026 Debrecen, Hungary Department of Solid State Physics, University of Debrecen, P.O. Box 2, 4010 Debrecen, Hungary
b
Received 22 January 2004; received in revised form 25 November 2004 Available online 12 January 2005
Abstract Optical parameters of amorphous AsSe layers have been found to be sensitive to relatively low energy (40–180 keV) deuteron irradiation. The changes of the optical absorption edge and the refractive index behave almost similar to the known effects of laser illumination at comparable exposition energies, including the presence of irreversible and reversible components but the thermal erasure of the deuteron-induced changes differs in time constants and activation energies and results also in the effusion of implanted deuterons. The possible mechanism of changes is discussed, which implies the prevalent role of electron processes in structural transformations at the stage of irradiation. 2004 Elsevier B.V. All rights reserved. PACS: 71.55.Jv; 73.61.Jc Keywords: Chalcogenide glasses; Irradiation; Structural transformations; Optical parameters
1. Introduction Amorphous chalcogenide films are interesting first of all due to photo-induced changes of their optical properties that occur under laser illumination and can be used for optical recording and for fabrication of optoelectronic elements [1,2]. In well annealed (near the softening temperature Tg) films *
Corresponding author. Tel.: +36 52 415222; fax: +36 52 315087. E-mail address: ivani@delfin.klte.hu (I. Ivan).
these changes are known to be reversible with annealing. Similar reversible optical changes were found in these materials under irradiation by fast electrons and c-rays [3,4]. In the case of c-irradiation significant recovery of optical properties occurs even at room temperature. At the same time, information about the influence of ion implantation on different properties of amorphous chalcogenides is fairly limited. Only a few papers have been published concerning ion beam modification of these materials with heavy ions. Particularly, Dwiedi et al. [5] studied
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.12.112
I. Ivan et al. / Nucl. Instr. and Meth. in Phys. Res. B 229 (2005) 240–245
structural changes in Ge20Se80 films irradiated by 1 MeV Kr2+ ions. The solubility change of As2S3 caused by high fluence 6–19 keV Au+ implantation was reported by Stoycheva-Topalova et al. [6]. Kamboj et al. [7] observed a large decrease of the dc activation energy and the optical band gap in GeSe caused by 60 MeV 12C5+ bombardment. Tsvetkova et al. [8] reported irreversible darkening accompanied with film morphology changes in Ar+, N+ and O+ implanted As3Se2 films. These phenomena were discussed from several different viewpoints and it seems that some of these effects should not be treated as a result of radiation damage of solids in traditional meaning, but most probably there can be found some similarity with ion beam modification of polymers [9], all the more that chalcogenide glasses are often considered as inorganic polymers and their structure can be changed by laser illumination or other irradiation [10–13]. We have extended our investigations to the study of the influence of light ions, particularly deuterons, on the optical properties of AsSe, which is one of the best model-type chalcogenide materials for optical recording. After preliminary experimental results [14], which had shown the red shift of the optical absorption edge, here we present the results of the detailed investigations of these phenomena.
2. Experimental AsSe films with thicknesses 1 and 3 lm were deposited onto silica glass substrates and Si wafers by thermal evaporation of high-purity bulk glass in vacuum (the pressure of residual gases in the chamber was 2 · 105 mbar). The substrates were not specially heated or cooled. For transmission electron microscopy (TEM) measurements 80 nm thick films were deposited onto cleaved NaCl. The optical transmission spectra were measured by a HP 8453 spectrophotometer and used to calculate the absorption spectra, refractive index and the thickness of the samples using SwanepoelÕs method [15] before and after laser irradiation, ion bombardment and heat treatment.
241
The irradiations were carried out with a 200 keV linear accelerator at the Institute of Experimental Physics of University of Debrecen. Optical measurements were performed on the 1 lm thick samples irradiated with 110 keV deuterons in the 2.9 · 1014–2 · 1016 ions/cm2 fluence range. The sample was mounted on sample holder that was turned at different angles to the beam line during the implantation, thus allowing to fill the layer with deuterons approximately homogeneously and to induce the same structural changes in the whole layer, not only in the certain sheet [14]. This was necessary for correct calculations of the optical parameters from the transmission spectra, as far as the calculation method we used [15] was developed for homogeneous films. The 2H(d, p)3H reaction was used for the measurement of the accumulation and the depth profile of the implanted deuterons in the film. The details of the measurement method can be found elsewhere [16]. For technical reasons these measurements were performed on 3 lm thick films. To measure the kinetics of the relaxation processes, the samples, which previously were irradiated with deuterons (with fluence 4 · 1015 ions/ cm2) or illuminated with He–Ne laser light (k = 0.63 lm, intensity at the surface of the sample P = 0.8 W/cm2), were placed to a pre-heated tube furnace and the transmission change during heat treatment was measured with a minimal probe beam intensity (Pprobe 0.008 W/cm2 at k = 0.63 lm), which could not induce any additional transmission changes in the sample. Atomic force microscope (AFM) and TEM measurements were also carried out to measure the thickness and microstructure changes. For thickness change measurements a standard TEM microgrid was put on the sample, which was then irradiated and the step height between the irradiated and non-irradiated regions was measured.
3. Results and discussion Recently it was established, that irradiation of amorphous chalcogenide thin films with deuterons to a fluence of 1015 ions/cm2 leads to a red shift of the optical absorption edge of the films, analogously
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to the exposition to light with photon energy larger or equal to the optical band gap of the materials [14]. It may be concluded that the two effects are quite similar. The total energy needed to cause equal transmission change is approximately the same. The magnitudes of the red shifts of the absorption edges as well as volume changes (DV/ V 0.8%), estimated from direct AFM measurements, do not differ strongly, either. TEM investigations (with magnifications up to 400,000) have shown that the microstructure of as deposited and implanted films are similar, no crystallization or microvoid creation have been detected. The plot of optical band gap and refractive index versus ion fluence for the AsSe films are shown in Fig. 1. The optical band gaps were obtained from the slope of the plots of (ahm)1/2 versus hm, where a is the absorption coefficient, h is the Planck constant and m is the light frequency. The kinetics of the refractive index and band gap change correlate rather well. We have observed similar behaviour of the refractive index and the optical band gap in As2S3 films irradiated with deuterons as well. It is interesting, that the observed increase in refractive index is accompanied by an increase in volume, while in general the increase of the refractive index is accompanied by density increase. Similar peculiarity is possible in the case of ion implantation of silicate glasses [17], but more often was observed during photo-induced structural changes in chalcogenide glasses [1].
Fig. 1. Dependencies of refractive index (n) at k = 1.06 lm and the optical bandgap (Eg) for AsSe on the ion fluence. Lines are only guides for eye.
In contrast with the irreversible character of heavy ion induced processes [8] we have observed the recovery of the optical properties of the films after annealing close to the softening temperature Tg, similarly to the thermal erasing of the optically induced changes. The transmission spectra of the samples after different treatments are shown in Fig. 2. One can see that due to deuteron irradiation the transmission spectra shifts towards longer wavelengths, but after annealing close to the glass softening temperature (Tg 460 K) it shifts back to shorter wavelengths (curve 3 in Fig. 2). The transmission spectrum of the sample annealed after irradiation is identical within the measurement error to that one of annealed without irradiation (curve 2). The darkening then can be induced once more by ion irradiation. A question arose if the observed changes of the optical properties can be attributed only to the changes of the local structural order of the amorphous network (as it is supposed in the models of photo-induced optical changes [1,2]) or to the incorporation of ions, too. For the study of ion accumulation during the irradiation a sample implanted with 70 keV ions at 45 beam incidence angle was used. The presence of deuterium in the sample have been confirmed by measurements of the 2H(d, p)3H reaction with different probe beam energies (see Fig. 3), which in turn gives us infor-
Fig. 2. Optical transmission spectra of AsSe films: (1) asdeposited, (2) annealed, (3) annealed after irradiation, (4) illuminated with He–Ne laser, (5) deuteron irradiated. Arrows denote the shifts of the spectrum during different treatments: (Q) annealing, (hm) laser treatment, (D+) irradiation with deuterons.
I. Ivan et al. / Nucl. Instr. and Meth. in Phys. Res. B 229 (2005) 240–245
mation about the depth distribution of the implanted ions (the energy depth conversion of the depth profile was not performed because of the lack of proper data on stopping ranges of low energy deuterons in such materials, but in our case the fact of incorporation of the ions was important). Similar measurements have shown that deuterons had disappeared from the sample (nuclear reaction products were not detected at all) after 1 h annealing, while they were present for a long time in the implanted samples stored at room temperature (measurements during few weeks). This experiment let us conclude, that the implanted deuterons effuse out from the layer simultaneously with erasing of the optical changes due to the annealing. Such effusion of hydrogen during heat treatment also was observed in GexSe1x films prepared by PECVD method [18]. To determine the role of incorporated ions in the change of optical parameters of AsSe we have also measured optical transmission spectra of sample homogeneously implanted with deuterons and another one irradiated with the same fluence but higher energy (such that the stopping range of ions was larger then the film thickness). The transmission spectra of the two samples were practically identical (similar to curve 5 in Fig. 2). So, it seems that the deuterium incorporation as such has no measurable effect on the optical properties of the film none the less the deuterium con-
Fig. 3. The concentration of deuterium versus the probe beam energy in a sample previously implanted with 70 keV deuterons at 45 beam incidence angle, measured using the 2H(d, p)3H reaction.
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tent of the film implanted with 2 · 1016 ions/cm2 is as high as 0.5 at.%. Most probably, the deuterium incorporation could increase the band gap as in the case of plasma-deposited hydrogenated As2S3 films [19], but it must be rather small in comparison with the band gap decrease caused by the changes of the local structural order of the amorphous network. After the annealing the deuterium disappears from the film, so it could not influence the optical properties of the annealed films as well, although the last undergo the known irreversible changes after the first annealing [1]. So far we can conclude that deuteron irradiation leads to the increase of the refractive index (Dn up to 0.2) and decrease of optical band gap (DEg up to 0.18 eV) of the samples and that these changes are reversible. These observations give further support for some phenomenological similarity between deuteron and light induced optical changes. For first sight it is hard to find correlations between the effect caused by a 100–200 keV ion and a 2 eV photon. On the other side, Sarsembinov et al. [12] found correlations between the effect of 2 MeV electron irradiation and photo-darkening. In [13,20] both the Co60 c-irradiation and the light induced changes in As2S3 are treated as destruction-polymerization transformations, moreover they are attributed to the slightly different transformations of the charged defects. At the same time, one can think that the relatively low energy photons hardly can cause as serious structural damage as accelerated ions or other ionizing irradiation. However the term ‘‘damage’’ has qualitatively different meaning in crystalline and in disordered solids. Lee in his recent review [9], briefly considering the theory of ion beam modification of polymeric materials has shown that for polymeric materials both the nuclear and electronic processes could cause only scission or cross-linking of the polymer chains breaking bonds or on the contrary facilitating the formation of new bonds, as well as electron-, c- or even UVirradiation does. Moreover he has shown that the effect of all of these radiations can be treated in the scope of a universal theory, accounting the parameters (primarily the energy deposited per unit length) of the radiation and the material properties in each case. It is well known that, considering
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several phenomena, chalcogenide glasses can be treated as inorganic polymers [10,21] consisting of relatively small molecules, chain fragments or clusters, which define the medium range order in ˚ [22]. As far as these materials within 5.5–10.1 A the distribution of electron states and the electron spectrum of the material are defined by the structure, the size and the interaction of these clusters, photo-induced changes are usually connected with bond breaking or switching within these clusters or between them. These are often defined as destruction-polymerization structural transformations, although the detailed mechanism is still not clear. Therefore, the observed deuteron induced optical changes also may be treated using these concepts. Some indirect information about structural changes in chalcogenide glasses can be obtained from parameters of isothermal relaxation processes. With this aim we have measured the kinetics of relaxation processes in deuteron irradiated AsSe films during thermal annealing in the 125– 182 C temperature range (Fig. 4). For comparison, the relaxation kinetics of light induced changes also was measured by the same way as described in the experimental. Both in the case of light and deuteron induced transformations the temperature dependence of the relaxation times consists of two regions: high temperature region, corresponding to fast erasure processes and a low temperature one, correspond-
Fig. 4. Temperature dependences of relaxation times of light (1) and deuteron (2) irradiation induced transmission changes in AsSe film.
ing to slow erasure. According to the known relaxation law in irradiated chalcogenide layer [21]: Eai si ¼ Bi exp ; ð1Þ RT where si is the relaxation time of the preliminary induced optical absorption change, Eai is the activation energy of relaxation, and Bi is a constant, which depends on the size of the relaxing structural units and R = 8.31 J/K. The activation energies in the high temperature region are Ea1 = 126 ± 15 kJ/mol and 5.98 ± 0.08 kJ/mol, in the low temperature region Ea2 = 62 ± 2 kJ/mol and 29.9 ± 0.5 kJ/mol for the light and deuteron induced changes respectively. An order of magnitude lower relaxation times and the significantly lower high temperature activation energy in the case of deuteron induced changes, which characterizes the shift of the structural units (fragments of the chain-layer structure) at the conditions close to the viscous flow in the vicinity of glass softening temperature, both suggest that the structural units, acting in structural transformations, are smaller in the deuteron-irradiation produced amorphous metastable state in comparison with the laser illumination produced one. In other phrases, the amorphous network becomes much less rigid and chemically ordered in case of deuteron irradiation. It can be attributed to the scission of chain-like structural units caused by ion irradiation as well as to the possible acting of deuterons as a network modifier [10]. This assumption is also supported by the fact, that the temperature at which the fast erasure starts and which is connected to Tg [21] is about 10 K lower for deuteron-induced changes. However, for the low temperature activation energies, which characterize structure relaxation through configurational transformations [10,21], the situation is different: in spite of lower relaxation times the activation energy is higher for the deuteron irradiated samples than for light irradiated ones. It can be caused by several reasons connected with changes of molecular as well as defect structure, such as filling up the ÔmicrovacanciesÕ with deuterons, formation of covalent As–D or Se–D bonds or hydrogen bonds between neighbouring clusters etc. To reveal it, further investigations like
I. Ivan et al. / Nucl. Instr. and Meth. in Phys. Res. B 229 (2005) 240–245
Raman-scattering, infrared spectroscopy, focused on the changes in the bond structure are in way. According to SRIM/TRIM [14,24] calculations 98–99% of a 100–200 keV deuteronÕs (or protonÕs) energy is transferred to the electron subsystem such as in the case of electron and c-irradiation. Further it is transferred to the lattice due to electron–phonon coupling. It is generally believed that electron–phonon coupling processes have a definitive role in the photo-stimulated phenomena in chalcogenide glasses and thin films [23]. Only about 1% of the ionÕs total energy is transferred to the lattice directly through nuclear collisions, mainly in the end of the ions trajectory. It suggests that the optical changes also can be attributed predominantly to the electronic processes, tightly connected with structural transformations. 4. Conclusions Significant changes of the optical absorption edge (DEg up to 0.18 eV) and the refractive index (Dn up to 0.2) can be induced in AsSe layers by 40–180 keV deuteron ion irradiation in the 2.9 · 1014–2 · 1016 ions/cm2 fluence range. In many aspects these are similar to the light induced changes of optical parameters of AsSe films, and stable at room temperature but can be erased by thermal annealing close to the glass softening temperature. The mechanism proposed for this process consists of rearrangements of structural units during the exposition or thermal annealing. The differences in the relaxation processes are connected to the presence of deuterons that due to high diffusion mobility at elevated temperatures can leave the implanted layer, which in turn allows the full recovery of the ion implantation induced changes. Acknowledgements Authors would like to thank Prof. J. Csikai for fruitful discussions and the support of the OTKA Grants T37509 and T046758.
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References [1] K. Tanaka, Rev. Sol. St. Sci. 4 (1990) 641. [2] A. Kikineshi, A. Mishak, in: Physics and Applications of Non-Crystalline Semiconductors in Optoelectronics, NATO ASI Series 36, 1997, p. 249. [3] V.L. Averyanov, B.T. Kolomiets, V.M. Lyubin, S.I. Nesterov, V.P. Shilo, Sov. J. Tech. Phys. 49 (1979) 865. [4] V. Balitska, J. Filipecki, O. Shpotyuk, J. Swiatek, M. Vakiv, J. Non-Cryst. Solids 287 (2001) 216. [5] P.K. Dwiedi, S.K. Tripathy, A. Pradhan, V.N. Kulkarni, S.C. Agarval, J. Non-Cryst. Solids 266–269 (2000) 924. [6] R. Stoycheva-Topalova, J. Assa, A. Buroff, T. Tzvetkov, S. Nesheva, D. Drandarov, R. Vichev, Vacuum 51 (1998) 277. [7] M.S. Kamboj, G. Kaur, R. Thangaray, D.K. Avasthi, J. Phys. D 35 (2002) 477. [8] T. Tsvetkova, S. Balabanov, B. Amov, A. Djakov, I.H. Wilson, Nucl. Instr. and Meth. B 80–81 (1993) 1264. [9] E.H. Lee, Nucl. Instr. and Meth. B 151 (1999) 29. [10] D.C. Sanditov, G.M. Bartenev, Physical Properties of Disordered Structures, Nauka, Novosibirsk, 1982 (in Russian). [11] A. Kikineshi, T. Melnichenko, in: Proc. 15th Int. Congress on Glass 1B, Leningrad, 1989, p. 66. [12] S.S. Sarsembinov, E.E. Abdulgafarov, M.A. Tumanov, N.A. Rogachev, J. Non-Cryst. Solids 35–36 (1980) 877. [13] V.O. Balitska, O.I. Shpotyuk, J. Non-Cryst. Solids 227– 230 (1998) 723. [14] S. Ko¨ke´nyesi, J. Csikai, P. Raics, I. Szabo, S. Szegedi, A. Vitez, J. Non-Cryst. Solids 326–327 (2003) 209. [15] R. Swanepoel, J. Phys. E: Sci. Instrum. 16 (1983) 1214. [16] J. Csikai, S. Szegedi, L. Ola´h, S.M. Ibrahim, A.M. ElMegrab, N.I. Molla, M.M. Rahman, R.U. Miah, F. Habbani, I. Shaddad, Nucl. Instr. and Meth. A 397 (1997) 75. [17] G.W. Arnold, P. Mazzoldi, Ion Beam Modification of Glasses in Beam Modification of Materials, Vol. 2, Elsevier, Amsterdam, 1987. [18] E. Sleeck, L. Tichy, P. Nagels, R. Callerts, J. Non-Cryst. Solids 198–200 (1996) 723. [19] H. Fritzche, V. Smid, H. Ugur, P.J. Gaczi, Journal de Physique 42 (1981) C4-699. [20] O.I. Shpotyuk, J. Kasperczyk, I.V Kityk, J. Non-Cryst. Solids 215 (1997) 218. [21] A. Kikineshi, Opt. Memory Neural Networks 4 (1995) 177. [22] R. Holomb, V. Mitsa, Solid State Commun. 129 (2004) 655. [23] R.A. Street, Phys. Rev. B 17 (1978) 3984. [24] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids , Pergamon, New York, 1985 (1996), Code SRIM/TRIM downloaded from www.srim.org.
Deuteron irradiation induced changes in amorphous AsSe films I. Ivan a
a,* ,
S. Szegedi a, L. Daroczi b, I.A. Szabo b, S. Kokenyesi
a
Department of Experimental Physics, University of Debrecen, Bem te´r18/a, 4026 Debrecen, Hungary Department of Solid State Physics, University of Debrecen, P.O. Box 2, 4010 Debrecen, Hungary
b
Received 22 January 2004; received in revised form 25 November 2004 Available online 12 January 2005
Abstract Optical parameters of amorphous AsSe layers have been found to be sensitive to relatively low energy (40–180 keV) deuteron irradiation. The changes of the optical absorption edge and the refractive index behave almost similar to the known effects of laser illumination at comparable exposition energies, including the presence of irreversible and reversible components but the thermal erasure of the deuteron-induced changes differs in time constants and activation energies and results also in the effusion of implanted deuterons. The possible mechanism of changes is discussed, which implies the prevalent role of electron processes in structural transformations at the stage of irradiation. 2004 Elsevier B.V. All rights reserved. PACS: 71.55.Jv; 73.61.Jc Keywords: Chalcogenide glasses; Irradiation; Structural transformations; Optical parameters
1. Introduction Amorphous chalcogenide films are interesting first of all due to photo-induced changes of their optical properties that occur under laser illumination and can be used for optical recording and for fabrication of optoelectronic elements [1,2]. In well annealed (near the softening temperature Tg) films *
Corresponding author. Tel.: +36 52 415222; fax: +36 52 315087. E-mail address: ivani@delfin.klte.hu (I. Ivan).
these changes are known to be reversible with annealing. Similar reversible optical changes were found in these materials under irradiation by fast electrons and c-rays [3,4]. In the case of c-irradiation significant recovery of optical properties occurs even at room temperature. At the same time, information about the influence of ion implantation on different properties of amorphous chalcogenides is fairly limited. Only a few papers have been published concerning ion beam modification of these materials with heavy ions. Particularly, Dwiedi et al. [5] studied
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.12.112
I. Ivan et al. / Nucl. Instr. and Meth. in Phys. Res. B 229 (2005) 240–245
structural changes in Ge20Se80 films irradiated by 1 MeV Kr2+ ions. The solubility change of As2S3 caused by high fluence 6–19 keV Au+ implantation was reported by Stoycheva-Topalova et al. [6]. Kamboj et al. [7] observed a large decrease of the dc activation energy and the optical band gap in GeSe caused by 60 MeV 12C5+ bombardment. Tsvetkova et al. [8] reported irreversible darkening accompanied with film morphology changes in Ar+, N+ and O+ implanted As3Se2 films. These phenomena were discussed from several different viewpoints and it seems that some of these effects should not be treated as a result of radiation damage of solids in traditional meaning, but most probably there can be found some similarity with ion beam modification of polymers [9], all the more that chalcogenide glasses are often considered as inorganic polymers and their structure can be changed by laser illumination or other irradiation [10–13]. We have extended our investigations to the study of the influence of light ions, particularly deuterons, on the optical properties of AsSe, which is one of the best model-type chalcogenide materials for optical recording. After preliminary experimental results [14], which had shown the red shift of the optical absorption edge, here we present the results of the detailed investigations of these phenomena.
2. Experimental AsSe films with thicknesses 1 and 3 lm were deposited onto silica glass substrates and Si wafers by thermal evaporation of high-purity bulk glass in vacuum (the pressure of residual gases in the chamber was 2 · 105 mbar). The substrates were not specially heated or cooled. For transmission electron microscopy (TEM) measurements 80 nm thick films were deposited onto cleaved NaCl. The optical transmission spectra were measured by a HP 8453 spectrophotometer and used to calculate the absorption spectra, refractive index and the thickness of the samples using SwanepoelÕs method [15] before and after laser irradiation, ion bombardment and heat treatment.
241
The irradiations were carried out with a 200 keV linear accelerator at the Institute of Experimental Physics of University of Debrecen. Optical measurements were performed on the 1 lm thick samples irradiated with 110 keV deuterons in the 2.9 · 1014–2 · 1016 ions/cm2 fluence range. The sample was mounted on sample holder that was turned at different angles to the beam line during the implantation, thus allowing to fill the layer with deuterons approximately homogeneously and to induce the same structural changes in the whole layer, not only in the certain sheet [14]. This was necessary for correct calculations of the optical parameters from the transmission spectra, as far as the calculation method we used [15] was developed for homogeneous films. The 2H(d, p)3H reaction was used for the measurement of the accumulation and the depth profile of the implanted deuterons in the film. The details of the measurement method can be found elsewhere [16]. For technical reasons these measurements were performed on 3 lm thick films. To measure the kinetics of the relaxation processes, the samples, which previously were irradiated with deuterons (with fluence 4 · 1015 ions/ cm2) or illuminated with He–Ne laser light (k = 0.63 lm, intensity at the surface of the sample P = 0.8 W/cm2), were placed to a pre-heated tube furnace and the transmission change during heat treatment was measured with a minimal probe beam intensity (Pprobe 0.008 W/cm2 at k = 0.63 lm), which could not induce any additional transmission changes in the sample. Atomic force microscope (AFM) and TEM measurements were also carried out to measure the thickness and microstructure changes. For thickness change measurements a standard TEM microgrid was put on the sample, which was then irradiated and the step height between the irradiated and non-irradiated regions was measured.
3. Results and discussion Recently it was established, that irradiation of amorphous chalcogenide thin films with deuterons to a fluence of 1015 ions/cm2 leads to a red shift of the optical absorption edge of the films, analogously
242
I. Ivan et al. / Nucl. Instr. and Meth. in Phys. Res. B 229 (2005) 240–245
to the exposition to light with photon energy larger or equal to the optical band gap of the materials [14]. It may be concluded that the two effects are quite similar. The total energy needed to cause equal transmission change is approximately the same. The magnitudes of the red shifts of the absorption edges as well as volume changes (DV/ V 0.8%), estimated from direct AFM measurements, do not differ strongly, either. TEM investigations (with magnifications up to 400,000) have shown that the microstructure of as deposited and implanted films are similar, no crystallization or microvoid creation have been detected. The plot of optical band gap and refractive index versus ion fluence for the AsSe films are shown in Fig. 1. The optical band gaps were obtained from the slope of the plots of (ahm)1/2 versus hm, where a is the absorption coefficient, h is the Planck constant and m is the light frequency. The kinetics of the refractive index and band gap change correlate rather well. We have observed similar behaviour of the refractive index and the optical band gap in As2S3 films irradiated with deuterons as well. It is interesting, that the observed increase in refractive index is accompanied by an increase in volume, while in general the increase of the refractive index is accompanied by density increase. Similar peculiarity is possible in the case of ion implantation of silicate glasses [17], but more often was observed during photo-induced structural changes in chalcogenide glasses [1].
Fig. 1. Dependencies of refractive index (n) at k = 1.06 lm and the optical bandgap (Eg) for AsSe on the ion fluence. Lines are only guides for eye.
In contrast with the irreversible character of heavy ion induced processes [8] we have observed the recovery of the optical properties of the films after annealing close to the softening temperature Tg, similarly to the thermal erasing of the optically induced changes. The transmission spectra of the samples after different treatments are shown in Fig. 2. One can see that due to deuteron irradiation the transmission spectra shifts towards longer wavelengths, but after annealing close to the glass softening temperature (Tg 460 K) it shifts back to shorter wavelengths (curve 3 in Fig. 2). The transmission spectrum of the sample annealed after irradiation is identical within the measurement error to that one of annealed without irradiation (curve 2). The darkening then can be induced once more by ion irradiation. A question arose if the observed changes of the optical properties can be attributed only to the changes of the local structural order of the amorphous network (as it is supposed in the models of photo-induced optical changes [1,2]) or to the incorporation of ions, too. For the study of ion accumulation during the irradiation a sample implanted with 70 keV ions at 45 beam incidence angle was used. The presence of deuterium in the sample have been confirmed by measurements of the 2H(d, p)3H reaction with different probe beam energies (see Fig. 3), which in turn gives us infor-
Fig. 2. Optical transmission spectra of AsSe films: (1) asdeposited, (2) annealed, (3) annealed after irradiation, (4) illuminated with He–Ne laser, (5) deuteron irradiated. Arrows denote the shifts of the spectrum during different treatments: (Q) annealing, (hm) laser treatment, (D+) irradiation with deuterons.
I. Ivan et al. / Nucl. Instr. and Meth. in Phys. Res. B 229 (2005) 240–245
mation about the depth distribution of the implanted ions (the energy depth conversion of the depth profile was not performed because of the lack of proper data on stopping ranges of low energy deuterons in such materials, but in our case the fact of incorporation of the ions was important). Similar measurements have shown that deuterons had disappeared from the sample (nuclear reaction products were not detected at all) after 1 h annealing, while they were present for a long time in the implanted samples stored at room temperature (measurements during few weeks). This experiment let us conclude, that the implanted deuterons effuse out from the layer simultaneously with erasing of the optical changes due to the annealing. Such effusion of hydrogen during heat treatment also was observed in GexSe1x films prepared by PECVD method [18]. To determine the role of incorporated ions in the change of optical parameters of AsSe we have also measured optical transmission spectra of sample homogeneously implanted with deuterons and another one irradiated with the same fluence but higher energy (such that the stopping range of ions was larger then the film thickness). The transmission spectra of the two samples were practically identical (similar to curve 5 in Fig. 2). So, it seems that the deuterium incorporation as such has no measurable effect on the optical properties of the film none the less the deuterium con-
Fig. 3. The concentration of deuterium versus the probe beam energy in a sample previously implanted with 70 keV deuterons at 45 beam incidence angle, measured using the 2H(d, p)3H reaction.
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tent of the film implanted with 2 · 1016 ions/cm2 is as high as 0.5 at.%. Most probably, the deuterium incorporation could increase the band gap as in the case of plasma-deposited hydrogenated As2S3 films [19], but it must be rather small in comparison with the band gap decrease caused by the changes of the local structural order of the amorphous network. After the annealing the deuterium disappears from the film, so it could not influence the optical properties of the annealed films as well, although the last undergo the known irreversible changes after the first annealing [1]. So far we can conclude that deuteron irradiation leads to the increase of the refractive index (Dn up to 0.2) and decrease of optical band gap (DEg up to 0.18 eV) of the samples and that these changes are reversible. These observations give further support for some phenomenological similarity between deuteron and light induced optical changes. For first sight it is hard to find correlations between the effect caused by a 100–200 keV ion and a 2 eV photon. On the other side, Sarsembinov et al. [12] found correlations between the effect of 2 MeV electron irradiation and photo-darkening. In [13,20] both the Co60 c-irradiation and the light induced changes in As2S3 are treated as destruction-polymerization transformations, moreover they are attributed to the slightly different transformations of the charged defects. At the same time, one can think that the relatively low energy photons hardly can cause as serious structural damage as accelerated ions or other ionizing irradiation. However the term ‘‘damage’’ has qualitatively different meaning in crystalline and in disordered solids. Lee in his recent review [9], briefly considering the theory of ion beam modification of polymeric materials has shown that for polymeric materials both the nuclear and electronic processes could cause only scission or cross-linking of the polymer chains breaking bonds or on the contrary facilitating the formation of new bonds, as well as electron-, c- or even UVirradiation does. Moreover he has shown that the effect of all of these radiations can be treated in the scope of a universal theory, accounting the parameters (primarily the energy deposited per unit length) of the radiation and the material properties in each case. It is well known that, considering
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several phenomena, chalcogenide glasses can be treated as inorganic polymers [10,21] consisting of relatively small molecules, chain fragments or clusters, which define the medium range order in ˚ [22]. As far as these materials within 5.5–10.1 A the distribution of electron states and the electron spectrum of the material are defined by the structure, the size and the interaction of these clusters, photo-induced changes are usually connected with bond breaking or switching within these clusters or between them. These are often defined as destruction-polymerization structural transformations, although the detailed mechanism is still not clear. Therefore, the observed deuteron induced optical changes also may be treated using these concepts. Some indirect information about structural changes in chalcogenide glasses can be obtained from parameters of isothermal relaxation processes. With this aim we have measured the kinetics of relaxation processes in deuteron irradiated AsSe films during thermal annealing in the 125– 182 C temperature range (Fig. 4). For comparison, the relaxation kinetics of light induced changes also was measured by the same way as described in the experimental. Both in the case of light and deuteron induced transformations the temperature dependence of the relaxation times consists of two regions: high temperature region, corresponding to fast erasure processes and a low temperature one, correspond-
Fig. 4. Temperature dependences of relaxation times of light (1) and deuteron (2) irradiation induced transmission changes in AsSe film.
ing to slow erasure. According to the known relaxation law in irradiated chalcogenide layer [21]: Eai si ¼ Bi exp ; ð1Þ RT where si is the relaxation time of the preliminary induced optical absorption change, Eai is the activation energy of relaxation, and Bi is a constant, which depends on the size of the relaxing structural units and R = 8.31 J/K. The activation energies in the high temperature region are Ea1 = 126 ± 15 kJ/mol and 5.98 ± 0.08 kJ/mol, in the low temperature region Ea2 = 62 ± 2 kJ/mol and 29.9 ± 0.5 kJ/mol for the light and deuteron induced changes respectively. An order of magnitude lower relaxation times and the significantly lower high temperature activation energy in the case of deuteron induced changes, which characterizes the shift of the structural units (fragments of the chain-layer structure) at the conditions close to the viscous flow in the vicinity of glass softening temperature, both suggest that the structural units, acting in structural transformations, are smaller in the deuteron-irradiation produced amorphous metastable state in comparison with the laser illumination produced one. In other phrases, the amorphous network becomes much less rigid and chemically ordered in case of deuteron irradiation. It can be attributed to the scission of chain-like structural units caused by ion irradiation as well as to the possible acting of deuterons as a network modifier [10]. This assumption is also supported by the fact, that the temperature at which the fast erasure starts and which is connected to Tg [21] is about 10 K lower for deuteron-induced changes. However, for the low temperature activation energies, which characterize structure relaxation through configurational transformations [10,21], the situation is different: in spite of lower relaxation times the activation energy is higher for the deuteron irradiated samples than for light irradiated ones. It can be caused by several reasons connected with changes of molecular as well as defect structure, such as filling up the ÔmicrovacanciesÕ with deuterons, formation of covalent As–D or Se–D bonds or hydrogen bonds between neighbouring clusters etc. To reveal it, further investigations like
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Raman-scattering, infrared spectroscopy, focused on the changes in the bond structure are in way. According to SRIM/TRIM [14,24] calculations 98–99% of a 100–200 keV deuteronÕs (or protonÕs) energy is transferred to the electron subsystem such as in the case of electron and c-irradiation. Further it is transferred to the lattice due to electron–phonon coupling. It is generally believed that electron–phonon coupling processes have a definitive role in the photo-stimulated phenomena in chalcogenide glasses and thin films [23]. Only about 1% of the ionÕs total energy is transferred to the lattice directly through nuclear collisions, mainly in the end of the ions trajectory. It suggests that the optical changes also can be attributed predominantly to the electronic processes, tightly connected with structural transformations. 4. Conclusions Significant changes of the optical absorption edge (DEg up to 0.18 eV) and the refractive index (Dn up to 0.2) can be induced in AsSe layers by 40–180 keV deuteron ion irradiation in the 2.9 · 1014–2 · 1016 ions/cm2 fluence range. In many aspects these are similar to the light induced changes of optical parameters of AsSe films, and stable at room temperature but can be erased by thermal annealing close to the glass softening temperature. The mechanism proposed for this process consists of rearrangements of structural units during the exposition or thermal annealing. The differences in the relaxation processes are connected to the presence of deuterons that due to high diffusion mobility at elevated temperatures can leave the implanted layer, which in turn allows the full recovery of the ion implantation induced changes. Acknowledgements Authors would like to thank Prof. J. Csikai for fruitful discussions and the support of the OTKA Grants T37509 and T046758.
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