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Photo-oxidation of As2Se3, Ag–As2Se3, and Cu–As2Se3 chalcogenide films
Journal of Non-Crystalline Solids 351 (2005) 3132–3138 www.elsevier.com/locate/jnoncrysol
Photo-oxidation of As2Se3, Ag–As2Se3, and Cu–As2Se3 chalcogenide films Kazuhiko Ogusu a,*, Yoshiaki Hosokawa a, Shinpei Maeda a, Makoto Minakata b, Hongpu Li a a
Department of Electrical and Electronic Engineering, Shizuoka University, 3-5-1, Johoku, Hamamatsu 432-8561, Japan b Research Institute of Electronics, Shizuoka University, 3-5-1, Johoku, Hamamatsu 432-8011, Japan Received 9 May 2005; received in revised form 27 July 2005
Abstract Three kinds of As-chalcogenide films made of As2Se3, Ag-doped As2Se3, and Cu-doped As2Se3 were prepared using thermal evaporation and photo-doping, and their surface was illuminated in air with a focused Ar laser beam of a wavelength of 0.5145 lm. The morphology of the film surface irradiated at different optical intensities was investigated by use of optical microscopy, atomic force microscopy, and scanning electron microscopy with energy dispersive spectroscopy. It has been found that two shapes of As2O3 microcrystals are formed on the As2Se3 film at room temperature and that the formation of such microcrystals never takes place at ambient temperature over about 35 °C. Therefore the observed distribution pattern of As2O3 microcrystals is like a ring in shape for intense illumination of the focused laser beam, which indicates a spatial profile of a temperature increase caused by absorption of the laser beam. A tentative model has been presented for explaining no generation of As2O3 microcrystals above about 35 °C. It has also been confirmed that the addition of metals Cu or Ag into As2Se3 films sufficiently eliminates or weakens such a photo-oxidation reaction. Ó 2005 Elsevier B.V. All rights reserved. PACS: 42.70.Gi; 81.05.Gc; 78.66.Jg; 68.60.Dv; 68.55.Jk
1. Introduction Chalcogenide glasses based on the chalcogen elements S, Se, and Te are very promising materials for use in fiber optics and integrated optics since they have many unique optical properties and exhibit a good transparency in the infrared region [1,2]. In particular, As2Se3 and closely related glasses possess a high thirdorder Kerr non-linearity greater than 1000 times that of fused silica [3,4] and an ultrafast time response because of non-resonant Kerr non-linearity. Recently, it
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0022-3093/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.07.034
has also been found that these glasses have large Raman gains nearly 800 times that of silica [5] and large Brillouin gains nearly 20 times that of silica [6]. These large optical non-linearities could be used for all-optical devices such as bistable optical devices and optical switches, parametric amplifiers, Raman (or Brillouin) amplifiers and Raman (or Brillouin) lasers. On the other hand, chalcogenide glasses exhibit numerous structural and physico-chemical changes when they are exposed to near band-gap light [7]. These photo-induced phenomena include photo-crystallization, photo-polymerization, photo-expansion, photodecomposition, photo-vaporization, photo-dissolution of metals (photo-doping) etc. In general, these changes are accompanied by changes in the optical constants
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of the material and shifts in the optical band-gap, i.e., photo-darkening or photo-bleaching. For example, the photo-darkening effect is actively utilized to fabricate Bragg gratings and optical waveguides for use in fiber optics and integrated optics [8–10]. However, such photo-induced phenomena can be sometimes detrimental for a variety of applications. It has already been found that the addition of metals such as Cu or Ag into As2S3 and As2Se3 glasses eliminates the photo-darkening effect [11,12]. In fact, we also encountered a few harmful photo-induced phenomena when we fabricated Bragg gratings in evaporated As2Se3 films with a focused optical beam from an Ar laser. In this paper, we investigate the changes in surface morphology of as-evaporated As2Se3 films when they are illuminated in air with a focused near band-gap beam from an Ar laser. We observed that numerous microcrystals of As2O3 were formed on the film surface after intense exposure as reported by various authors [13–19]. Although such a photo-oxidation reaction has been investigated in detail for a long time, there are still unknown phenomena. We especially reveal the reason why the As2O3 microcrystals formed on the film surface disappear gradually from the central portion of the laser focal area as the illumination intensity is increased. We also examine Cu(Ag)-doped As2Se3 glasses to find out materials in which the photo-oxidation reaction does not take place. It has been confirmed that the photo-oxidation reaction is absent in Cu-doped As2Se3 glasses as well as the photo-darkening and that the threshold optical intensity for the formation of As2O3 microcrystals in Ag-doped As2Se3 glasses is much higher than that for undoped As2Se3 glasses.
2. Experimental procedure Three kinds of chalcogenide films, i.e., As2Se3, Agdoped, and Cu-doped As2Se3 films were prepared to study the photo-structural changes of these glasses as follows: First, the As2Se3 films were deposited on a glass slide substrate by vacuum evaporation. The As2Se3 glasses used as an evaporation source were prepared from the conventional As and Se elements of 6N by the conventional melt-quenching method [20]. Next, the Ag-doped and Cu-doped As2Se3 films were prepared by the successive photo-doping technique [21]. An As2Se3 film was first deposited on a glass slide substrate by about 1 lm at a rate of 3 nm/s. Ag (or Cu) film was then evaporated on the top of the host film. In our experiment, the thickness of the Ag film was 10 nm and that of the Cu film was 5 nm. The thickness of these films was monitored during evaporation with a quartz crystal monitor and then was determined from the transmission spectrum using SwanepoelÕs method [22]. The photo-doping was carried out by illuminating
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the metal-coated film with light from a 500-W tungsten lamp through a Fresnel lens. Unlike Ref. [21], we did not use an IR cut filter to make use of thermally-induced diffusion in addition to the photo-diffusion. The concentration of the metal in the prepared sample was determined from measured metal and photo-doped layer thickness. The optical transmission spectra of the photo-doped As2Se3 films were recorded with a visible and near infrared spectrometer. We determined the film thickness d, refractive index n, and absorption coefficient a from the transmission spectrum using SwanepoelÕs method [22]. Moreover we determined the optical band-gap Eg from an intercept on the energy axis of the linear fit of the high-energy data in a plot of (ahm)1/2 versus hm. A single-frequency Ar laser operating at a wavelength of 0.5145 lm was used as a light source for illuminating the sample. The laser beam was focused on the surface of the sample through a lens of a focal length of 10 cm. In our case, the intensity profile is approximately expressed by [23] r 2 IðrÞ ¼ I 0 exp 2 ; ð1Þ a where a is the radius where the light intensity decreases to 1/e2 of the peak intensity I0. The beam radius a can be determined from the measured value of the size of beam incident before the lens [23] and the peak intensity I0 can be determined by measuring the incident power Pin = pa2I0/2. The beam radius a = 23.4 lm was used throughout our experiments and the peak intensity I0 was changed to study the photo-induced effects of these glasses. The illumination was carried out in air for 5 min at room temperature (23 °C). The morphology of the surface of illuminated films was investigated by use of laser scanning confocal microscopy (LSCM), atomic force microscopy (AFM), and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS).
3. Results First, we present the experimental results for as-evaporated As2Se3 chalcogenide films on a glass slide substrate. Fig. 1 shows typical optical microscope photographs of the sample surface irradiated for 5 min at room temperature, where the peak optical intensity I0 is changed from 0 to 1750 W/cm2. In this case, the peak intensity I0 = 4.7 and 1750 W/cm2 corresponds to the incident power Pin = 40 lW and 15 mW, respectively. The thickness and band-gap wavelength of the As2Se3 film used for this observation are 1.3 and 0.705 lm, respectively. This figure shows that the surface morphology changes remarkably with increasing incident optical power onto the sample. First, the photo-darkening takes place for peak intensities below 3.4 W/cm2 although its photograph is not given in
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Fig. 1. Typical optical microscope photographs of the surface of an As2Se3 film after it was illuminated with an Ar laser beam having different optical intensities I0. The intensity profile of the illuminating laser beam is also shown for convenience.
Fig. 1 (because it is not clear). Next, the formation of small particles starts to occur at a peak intensity of 3.4 W/cm2, and the number of particles is increased and the region where the particles are found is spread as the illumination intensity is increased (Fig. 1(a) and (b)). However, the particles disappear around the central part of their distribution pattern at peak intensities above 35 W/cm2 and hence the density distribution becomes a ring in shape (we call hereafter this phenomenon ring-formation) (Fig. 1(c) and (d)). As the illumination intensity is further increased, thermal phenomena such as melting and boiling take place because of a temperature increase in the sample due to absorption of the laser beam. The surface of the sample starts to sink at a peak intensity of 810 W/cm2, which corresponds to the beginning of melting of the As2Se3 film (we call hereafter this phenomenon dip-formation). The depth of the dip is increased with increasing illumination intensity. Finally, a further increase in illumination intensity leads to a damage of the As2Se3 film, i.e., a uniform removal of the film material As2Se3 from the substrate as shown in Fig. 1(e). This damage is usually called laser-induced damage or laser damage. The threshold peak intensity for the laser damage is 1550 W/cm2. When the As2Se3 film is irradiated by such an intense laser beam, a small portion of the film in the laser focal area is heated to temperature equal or beyond the boiling point. We investigated the composition of the small particles formed by intense exposure as shown in Fig. 1 by use of SEM with EDS. Fig. 2 shows the FE-SEM image of the sample surface where numerous microcrystals were formed. It is found that the size of these microcrystals is less than 0.5 lm. The height of the microcrystals ranges from 0.2 to 0.5 lm, which consists with observations with LSCM and AFM. We can also find two kinds
Fig. 2. FE-SEM image of the surface of an As2Se3 film illuminated at the optical intensity I0 = 45 W/cm2. We can see two kinds of microcrystals; trigonal-prism-like crystals and octahedron-like crystals.
of microcrystal structure, i.e., a trigonal-prism-like crystal and an octahedron-like crystal. Fig. 3 shows the results of EDS analysis. The composition (in at.%) of the microcrystals on spots S1–S3 and the composition of the illuminated film among microcrystals on spots S4–S6 were analyzed. Our EDS spectra cannot give an exact identification since the spatial resolution is about 1 lm in width and a few lm in depth, and is not high enough to analyze each microcrystal individually. From the EDS analysis on spots S4–S6, the average composition of the illuminated film without microcrystals is O: 2.4 at.%, As: 37.8 at.%, and Se: 59.8 at.%. This result indicates that the composition of the film is approximately the stoichiometric composition of As2Se3 glass, but is slightly enriched in Se. EDS result on spot S1 is the most reliable since the microcrystal on spot S1 is
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the largest of the three. The microcrystals have a high content of O, indicating that a new phase of As2O3 is formed on the film surface after illuminated with band-gap light. This phenomenon has already been reported by several research groups [13–19] and is caused by both photo-decomposition and photo-oxidation. On the other hand, we could not find microcrystals of Se or As in contrast to the case of As-S films [13,14]. This fact agrees with the result given in Ref. [13]. Generally, As2O3 exists in two crystalline polymorphs: arsenolite (cubic) and claudetite (monoclinic). In Fig. 2, octahedron microcrystals are undoubtedly arsenolite, which have frequently been observed in the previous works. On the other hand, flat triangular microcrystals have not yet been observed to date. From the relation between crystal systems and crystal shapes, the triangular microcrystals are presumably not claudetite but a twinned crystal of the spinel-twin type, which corresponds to a macle of diamonds. The size of these microcrystals can be sufficiently controlled by changing the irradiation time as reported in Ref. [19]. To investigate the origin of the above-mentioned ring-formation, we carried out the EDS analysis of the central portion of the film illuminated by the intense laser beam, which corresponds to the center of the ring-shape distribution of As2O3 microcrystals. The composition of the film at that position is O: 1.1 at.%, As: 38.1 at.%, and Se: 60.8 at.%. We conclude the chemical composition of the central portion of the illuminated film without
As2O3 microcrystals is the same as that of the illuminated film among As2O3 microcrystals. Next, we present the experimental results for Agdoped, Cu-doped, and PMMA-coated As2Se3 films that were fabricated to examine their durability for the photo-oxidation reaction. Fig. 4 shows the transmission spectra of As2Se3, Ag3.1(As0.4Se0.6)96.9, and Cu2.4(As0.4Se0.6)97.6 films (where we hereafter describe the last two chalcogenide glasses simply as Ag–As2Se3 and Cu–As2Se3). The band-gap wavelength of As2Se3, Ag– As2Se3, and Cu–As2Se3 chalcogenide films is 0.705, 0.744, and 0.776 lm, respectively and the thickness of these three films is 1.3 lm. The penetration depth of the incident Ar laser light into these films is less than 0.1 lm. We performed the same experiment as one done in Fig. 1. In the present case, we pay attention to the characteristic peak optical intensity I0 at which the following four phenomena begin; the formation of As2O3 microcrystals, the formation of ring-shape distribution of As2O3 microcrystals, the dip-formation at the central portion, and laser damage. The obtained results are summarized in Table 1. The threshold peak intensity for the formation of As2O3 microcrystals in the Ag– As2Se3 films is increased by about 30 times that of the As2Se3 films. Of course, the photo-darkening effect takes place in the Ag–As2Se3 films below the threshold peak intensity. The threshold peak intensity for the ring-formation in the Ag–As2Se3 films is also six times as high as that of the As2Se3 films. Regarding the beginning of dip-formation and laser damage, we could not find a distinct difference between Ag–As2Se3 and undoped As2Se3 films. This means that the melting point and boiling point of these two materials equal each other. On the other hand, the photo-darkening effect and the formation of As2O3 microcrystals did not occur in the Cu– As2Se3 films. It is found that the threshold intensities for the dip-formation and laser damage in the Cu– As2Se3 films are higher than those of the As2Se3 and 100 80 Transmission (%)
Fig. 3. SEM image and EDS analysis of an As2Se3 film illuminated at the optical intensity I0 = 116 W/cm2. Two kinds of regions are analyzed with EDS; spots S1–S3 with a microcrystal and spots S4– S6 without microcrystals.
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60 40 As2Se3 Cu2.4(As0.4Se0.6)97.6 Ag3.1(As0.4Se0.6)96.9
20 0 400
600
800 1000 1200 Wavelength (nm)
1400
Fig. 4. Transmission spectra of As2Se3, Ag3.1(As0.4Se0.6)96.9, and Cu2.4(As0.4Se0.6)97.6 films.
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Table 1 Comparison of the presence of photo-darkening effect and threshold optical intensities I0 (in W/cm2) corresponding to the onset of four phenomena (formation of As2O3 microcrystals, ring-formation of As2O3 microcrystals, dip-formation, and laser damage) among four examined materials Sample
Photo-darkening
As2O3 microcrystals
Ring-formation
Dip-formation
Laser damage
As2Se3 Ag3.1(As0.4Se0.6)96.9 Cu2.4(As0.4Se0.6)97.6 PMMA-coated As2Se3
Yes Yes No Yes
3.4 115 No No
35 210 No No
815 930 1160 –
1550 1530 2300 2300
Ag–As2Se3 films. In the case of the PMMA-coated As2Se3 films, it is confirmed that the formation of As2O3 microcrystals never takes place since the sample surface is out of touch with the atmosphere. Of course, the photo-darkening effect can occur since the coating film is transparent. Although the threshold intensity for the laser damage is higher than that of As2O3 films, this is attributed to an increase in the reflectance of the sample by the coating film. On the other hand, we could not measure the threshold intensity for the dipformation.
4. Discussion Before discussing the reason why the ring-formation of As2O3 microcrystals takes place on the surface of As2Se3 films at optical intensities over a critical value, we first summarize the mechanism of the formation of As2O3 microcrystals proposed so far [13,14]. The chemical bonds associated with As atoms in the glass network are broken by irradiation with above band-gap light, the liberated As atoms can diffuse freely in the film and rejoin with each other in local regions forming small As crystals. The As species are oxidized to As2O3 in the presence of oxygen and water vapor. The photo-decomposition and oxidation reaction can be expressed as follows: hm
As2 Se3 ! xAs þ As2x Se3 H2 O
4As þ 3O2 ! 2As2 O3
ð2Þ ð3Þ
where the water appears to enter the oxidation reaction as a catalyst. Next, we discuss the mechanism of the ring-formation of As2O3 microcrystals. In order to obtain more information on the ring-formation, we have to perform additional experiments. From optical microscope photographs as shown in Fig. 1, we first determine the critical illumination intensity at which the microcrystals of As2O3 disappear, i.e., the optical intensity at the inner fringe of the ring pattern of As2O3 microcrystals. Fig. 5 shows the critical illumination intensity Ic as a function of peak illumination intensity I0. The critical illumination intensity is not a constant but decreases with increasing peak illumination intensity. In Fig. 5,
Fig. 5. Critical illumination intensity Ic as a function of peak illumination intensity I0. Here we define the lowest illumination intensity at which As2O3 microcrystals disappear as the critical illumination intensity.
it should be noted that the ring-formation of As2O3 microcrystals takes place at peak intensities I0 above 35 W/cm2. The origin of the ring-formation seems to be concerned not with optical energy itself but with thermal phenomena induced by it. One possibility is the sublimation (vaporization) or melting of As2O3 microcrystals that is caused by a temperature raise due to absorption of optical energy. This can be regarded as the same mechanism that Janai and Rudman have already proposed for the photo-vaporization observed in amorphous As2S3 films [24,25]. At a temperature of about 200 °C at which they did experiment, the photooxidation reaction products, As2O3 and free S, vaporize as gaseous As4O6 and S2 because of their high volatility, which is much greater than that of As2S3. We must here examine thermal data on As2O3 crystals for discussion. The melting point of As2O3 is 275 °C (for arsenolite) and 313 °C (for claudetite) and the boiling point is 465 °C. Although data on the sublimation point are scattered in literatures, it appears that cubic As2O3 begins to sublime at about 135 °C. To examine the possibility of sublimation or melting of As2O3 crystals, after we created As2O3 microcrystals by illumination of a peak intensity of 35 W/cm2 (Fig. 1(b)), we illuminated the same position of the sample at a peak intensity of 230 W/cm2. Note that, at the latter intense irradiation, the ring-formation sufficiently takes place as shown in Fig. 1(d). However, the pattern
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of As2O3 microcrystals formed in advance was never changed by the second illumination. It is necessary to obtain information on a local temperature increase in the sample due to optical absorption for our discussion. We must estimate the value of the temperature raise using an assumption because it is difficult to measure it directly. We assume that the temperature raise DT at the center potion of the illuminated sample is proportional to the peak illumination intensity I0 and that the laser damage (or melting) of As2Se3 films takes place at a boiling point of about 700 °C (or a melting point of 375 °C [26]), respectively. Under such assumptions, we can get the temperature raise DT = 15 °C for a peak intensity of 35 W/cm2, which is the optical intensity for the beginning of the ring-formation. The temperature at the central portion of the sample is finally estimated to be 38 °C, which is much lower than a sublimation point of 135 °C. Therefore, we tried to form As2O3 microcrystals with exposure of an Ar laser beam while keeping an As2Se3 film at about 35 °C. Although we illuminated the sample with two values of illumination intensity I0 = 35 and 230 W/cm2 for 5 min, we could not obtain As2O3 microcrystals. Although we also heated the As2O3 microcrystals formed in advance up to about 35 °C, these microcrystals did not disappear. We moreover carried out the same experiments at two ambient temperatures of 40 and 50 °C and obtained the same results as those at about 35 °C. From these experimental results, we conclude that the As2O3 microcrystals do not disappear through sublimation but from the beginning they are never grown on the film surface at temperatures above about 35 °C. We belive that the light-enhanced vaporization of As2O3, which was pointed out by Janai and Rudman, never takes place since the vapor pressure of As2O3 at 35 °C is seven orders of magnitude lower than that at 200 °C [27]. Therefore it is adequate to consider that the photo-induced As species are oxidized not to As2O3 but to a volatile product at ambient temperature above about 35 °C. Although it is difficult to obtain direct evidence, we can assume the presence of ortho-arsenic H3AsO4 as a volatile oxidation product, according to the following reaction: 4As þ 5O2 þ 6H2 O ! 4H3 AsO4 "
ð4Þ
The melting point of H3AsO4 is 35.5 °C and it is likely that H3AsO4 volatilizes near such a melting temperature before its crystal grows. In fact, the presence of this material has already been confirmed in a test for UV light and moisture durability of As2Se3 fibers [16] and this seems to support our assumptions indirectly. On the other hand, our tentative model can qualitatively explain the dependence of the critical illumination intensity for the ring-formation on the peak illumination intensity shown in Fig. 5. Compared with the intensity profile of the illuminating laser beam, the temperature
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raise distribution in the sample spreads because of the thermal diffusion. Therefore the experimental result shown in Fig. 5 is not curious. Finally, we discuss the photo-darkening and photooxidation of Ag–As2Se3 and Cu–As2Se3 glasses. Although the addition of Ag or Cu into As2Se3 glasses are very effective for suppressing the generation of photoinduced oxidation reaction, there exists a distinct difference in the photo-induced effects between these two doped-glasses as shown in Table 1. That is, both the photo-darkening effect and photo-oxidation reaction never occur in Cu–As2Se3 glasses, whereas they take place in Ag–As2Se3 glasses. Taylor et al. [11,12] have maintained that the incorporation of metals such as Cu or Ag into amorphous As2S3 and amorphous As2Se3 eliminates photo-darkening effect and its effect appears to be strongly corrected with the twofold coordination of the chalcogen atoms. They have also explained that the coordination number of chalcogen atoms increases from two to four when the metal concentrations are high enough. However their statements seem to be not correct in the case of Ag-doped As2Se3 glasses. For example, according to the extend X-ray absorption fine structure experiments of (Ag2Se)y(As2Se3)1y glasses [28], the coordination number of the Se atom varies from two to three depending on Ag content, i.e., y. Since the Ag or Cu content treated in this paper is not high, the doped metal elements may be regarded as glass network modifiers. However we belive that there exists a difference in the glass structure between Ag-doped and Cu-doped As2Se3 glasses even if the metal content is low since we have already found a distinct difference in the optical properties between them [21]. We now estimate the temperature at which the ringformation begins on the surface of Ag–Se2Se3 films in the same manner as one described above. Assuming that the melting takes places at a peak illumination intensity of 930 W/cm2 and that the melting point is 364 °C [26], the onset temperature of the ring-formation is calculated to be 100 °C. This calculated temperature agrees nearly with a sublimation temperature of cubic As2O3 of 135 °C. The ring-formation of Ag–As2Se3 films is presumably done by the sublimation of As2O3 in contrast to the case of undoped As2Se3 films. In this paper, we selected only Ag and Cu as dopants into As2Se3 glasses since these two metals can be easily photo-doped. However the addition of other materials is effective for the elimination of photo-oxidation reaction if there exist glass formation regions in the glass system after the addition and the chemical bonds associated with As atoms are modified by it. Let us consider the doping of Au since its element belongs to the 1B group as well as Cu and Ag elements do. Since there are no published reliable data on glass formation in the Au–As–Se system, it is difficult for us to obtain homogeneous glasses in the indicated systems [29]. If Au is
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added into As2Se3 glasses by some way, we will obtain glasses in which small Au particles are embedded. Therefore we do not think the doping of Au into As2Se3 glasses is effective for the present purpose.
5. Conclusions We investigated the photo-darkening effect and the photo-oxidation reaction of three kinds of As-chalcogenide films, i.e., as-evaporated As2Se3, Ag-doped As2Se3, and Cu-doped As2Se3 films from the applicationÕs point of view. In this paper, a focused near band-gap light from an Ar laser was used as the light source and the illumination intensity was changed from zero to the threshold for laser-induced damage. It has been found that the As2O3 microcrystals formed on the surface of As2Se3 films by the photo-oxidation reaction are gradually absent from the central portion of the laser focal area as the illumination intensity is increased. The reason for the generation of this phenomenon is that the illuminated sample is locally heated due to the optical absorption and the formation of As2O3 microcrystals then stops at ambient temperature above 35 °C because of the creation of a volatile oxidation product. It has confirmed that the addition of metals such as Ag or Cu into As2Se3 glasses is useful for suppressing the generation of harmful photo-oxidation reaction. In the case of Cu-doped As2Se3 glasses, both the photo-darkening effect and photo-oxidation reaction never take place. Therefore, we have to use Ag-doped As2Se3 glasses when the photo-darkening effect is utilized, for example, to fabricate Bragg gratings or optical waveguides.
Acknowledgement The authors would like to thank Dr M. Kitao for his helpful advice.
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Photo-oxidation of As2Se3, Ag–As2Se3, and Cu–As2Se3 chalcogenide films Kazuhiko Ogusu a,*, Yoshiaki Hosokawa a, Shinpei Maeda a, Makoto Minakata b, Hongpu Li a a
Department of Electrical and Electronic Engineering, Shizuoka University, 3-5-1, Johoku, Hamamatsu 432-8561, Japan b Research Institute of Electronics, Shizuoka University, 3-5-1, Johoku, Hamamatsu 432-8011, Japan Received 9 May 2005; received in revised form 27 July 2005
Abstract Three kinds of As-chalcogenide films made of As2Se3, Ag-doped As2Se3, and Cu-doped As2Se3 were prepared using thermal evaporation and photo-doping, and their surface was illuminated in air with a focused Ar laser beam of a wavelength of 0.5145 lm. The morphology of the film surface irradiated at different optical intensities was investigated by use of optical microscopy, atomic force microscopy, and scanning electron microscopy with energy dispersive spectroscopy. It has been found that two shapes of As2O3 microcrystals are formed on the As2Se3 film at room temperature and that the formation of such microcrystals never takes place at ambient temperature over about 35 °C. Therefore the observed distribution pattern of As2O3 microcrystals is like a ring in shape for intense illumination of the focused laser beam, which indicates a spatial profile of a temperature increase caused by absorption of the laser beam. A tentative model has been presented for explaining no generation of As2O3 microcrystals above about 35 °C. It has also been confirmed that the addition of metals Cu or Ag into As2Se3 films sufficiently eliminates or weakens such a photo-oxidation reaction. Ó 2005 Elsevier B.V. All rights reserved. PACS: 42.70.Gi; 81.05.Gc; 78.66.Jg; 68.60.Dv; 68.55.Jk
1. Introduction Chalcogenide glasses based on the chalcogen elements S, Se, and Te are very promising materials for use in fiber optics and integrated optics since they have many unique optical properties and exhibit a good transparency in the infrared region [1,2]. In particular, As2Se3 and closely related glasses possess a high thirdorder Kerr non-linearity greater than 1000 times that of fused silica [3,4] and an ultrafast time response because of non-resonant Kerr non-linearity. Recently, it
*
Corresponding author. Tel./fax: +81 53 478 1089. E-mail address: [email protected] (K. Ogusu).
0022-3093/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.07.034
has also been found that these glasses have large Raman gains nearly 800 times that of silica [5] and large Brillouin gains nearly 20 times that of silica [6]. These large optical non-linearities could be used for all-optical devices such as bistable optical devices and optical switches, parametric amplifiers, Raman (or Brillouin) amplifiers and Raman (or Brillouin) lasers. On the other hand, chalcogenide glasses exhibit numerous structural and physico-chemical changes when they are exposed to near band-gap light [7]. These photo-induced phenomena include photo-crystallization, photo-polymerization, photo-expansion, photodecomposition, photo-vaporization, photo-dissolution of metals (photo-doping) etc. In general, these changes are accompanied by changes in the optical constants
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of the material and shifts in the optical band-gap, i.e., photo-darkening or photo-bleaching. For example, the photo-darkening effect is actively utilized to fabricate Bragg gratings and optical waveguides for use in fiber optics and integrated optics [8–10]. However, such photo-induced phenomena can be sometimes detrimental for a variety of applications. It has already been found that the addition of metals such as Cu or Ag into As2S3 and As2Se3 glasses eliminates the photo-darkening effect [11,12]. In fact, we also encountered a few harmful photo-induced phenomena when we fabricated Bragg gratings in evaporated As2Se3 films with a focused optical beam from an Ar laser. In this paper, we investigate the changes in surface morphology of as-evaporated As2Se3 films when they are illuminated in air with a focused near band-gap beam from an Ar laser. We observed that numerous microcrystals of As2O3 were formed on the film surface after intense exposure as reported by various authors [13–19]. Although such a photo-oxidation reaction has been investigated in detail for a long time, there are still unknown phenomena. We especially reveal the reason why the As2O3 microcrystals formed on the film surface disappear gradually from the central portion of the laser focal area as the illumination intensity is increased. We also examine Cu(Ag)-doped As2Se3 glasses to find out materials in which the photo-oxidation reaction does not take place. It has been confirmed that the photo-oxidation reaction is absent in Cu-doped As2Se3 glasses as well as the photo-darkening and that the threshold optical intensity for the formation of As2O3 microcrystals in Ag-doped As2Se3 glasses is much higher than that for undoped As2Se3 glasses.
2. Experimental procedure Three kinds of chalcogenide films, i.e., As2Se3, Agdoped, and Cu-doped As2Se3 films were prepared to study the photo-structural changes of these glasses as follows: First, the As2Se3 films were deposited on a glass slide substrate by vacuum evaporation. The As2Se3 glasses used as an evaporation source were prepared from the conventional As and Se elements of 6N by the conventional melt-quenching method [20]. Next, the Ag-doped and Cu-doped As2Se3 films were prepared by the successive photo-doping technique [21]. An As2Se3 film was first deposited on a glass slide substrate by about 1 lm at a rate of 3 nm/s. Ag (or Cu) film was then evaporated on the top of the host film. In our experiment, the thickness of the Ag film was 10 nm and that of the Cu film was 5 nm. The thickness of these films was monitored during evaporation with a quartz crystal monitor and then was determined from the transmission spectrum using SwanepoelÕs method [22]. The photo-doping was carried out by illuminating
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the metal-coated film with light from a 500-W tungsten lamp through a Fresnel lens. Unlike Ref. [21], we did not use an IR cut filter to make use of thermally-induced diffusion in addition to the photo-diffusion. The concentration of the metal in the prepared sample was determined from measured metal and photo-doped layer thickness. The optical transmission spectra of the photo-doped As2Se3 films were recorded with a visible and near infrared spectrometer. We determined the film thickness d, refractive index n, and absorption coefficient a from the transmission spectrum using SwanepoelÕs method [22]. Moreover we determined the optical band-gap Eg from an intercept on the energy axis of the linear fit of the high-energy data in a plot of (ahm)1/2 versus hm. A single-frequency Ar laser operating at a wavelength of 0.5145 lm was used as a light source for illuminating the sample. The laser beam was focused on the surface of the sample through a lens of a focal length of 10 cm. In our case, the intensity profile is approximately expressed by [23] r 2 IðrÞ ¼ I 0 exp 2 ; ð1Þ a where a is the radius where the light intensity decreases to 1/e2 of the peak intensity I0. The beam radius a can be determined from the measured value of the size of beam incident before the lens [23] and the peak intensity I0 can be determined by measuring the incident power Pin = pa2I0/2. The beam radius a = 23.4 lm was used throughout our experiments and the peak intensity I0 was changed to study the photo-induced effects of these glasses. The illumination was carried out in air for 5 min at room temperature (23 °C). The morphology of the surface of illuminated films was investigated by use of laser scanning confocal microscopy (LSCM), atomic force microscopy (AFM), and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS).
3. Results First, we present the experimental results for as-evaporated As2Se3 chalcogenide films on a glass slide substrate. Fig. 1 shows typical optical microscope photographs of the sample surface irradiated for 5 min at room temperature, where the peak optical intensity I0 is changed from 0 to 1750 W/cm2. In this case, the peak intensity I0 = 4.7 and 1750 W/cm2 corresponds to the incident power Pin = 40 lW and 15 mW, respectively. The thickness and band-gap wavelength of the As2Se3 film used for this observation are 1.3 and 0.705 lm, respectively. This figure shows that the surface morphology changes remarkably with increasing incident optical power onto the sample. First, the photo-darkening takes place for peak intensities below 3.4 W/cm2 although its photograph is not given in
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Fig. 1. Typical optical microscope photographs of the surface of an As2Se3 film after it was illuminated with an Ar laser beam having different optical intensities I0. The intensity profile of the illuminating laser beam is also shown for convenience.
Fig. 1 (because it is not clear). Next, the formation of small particles starts to occur at a peak intensity of 3.4 W/cm2, and the number of particles is increased and the region where the particles are found is spread as the illumination intensity is increased (Fig. 1(a) and (b)). However, the particles disappear around the central part of their distribution pattern at peak intensities above 35 W/cm2 and hence the density distribution becomes a ring in shape (we call hereafter this phenomenon ring-formation) (Fig. 1(c) and (d)). As the illumination intensity is further increased, thermal phenomena such as melting and boiling take place because of a temperature increase in the sample due to absorption of the laser beam. The surface of the sample starts to sink at a peak intensity of 810 W/cm2, which corresponds to the beginning of melting of the As2Se3 film (we call hereafter this phenomenon dip-formation). The depth of the dip is increased with increasing illumination intensity. Finally, a further increase in illumination intensity leads to a damage of the As2Se3 film, i.e., a uniform removal of the film material As2Se3 from the substrate as shown in Fig. 1(e). This damage is usually called laser-induced damage or laser damage. The threshold peak intensity for the laser damage is 1550 W/cm2. When the As2Se3 film is irradiated by such an intense laser beam, a small portion of the film in the laser focal area is heated to temperature equal or beyond the boiling point. We investigated the composition of the small particles formed by intense exposure as shown in Fig. 1 by use of SEM with EDS. Fig. 2 shows the FE-SEM image of the sample surface where numerous microcrystals were formed. It is found that the size of these microcrystals is less than 0.5 lm. The height of the microcrystals ranges from 0.2 to 0.5 lm, which consists with observations with LSCM and AFM. We can also find two kinds
Fig. 2. FE-SEM image of the surface of an As2Se3 film illuminated at the optical intensity I0 = 45 W/cm2. We can see two kinds of microcrystals; trigonal-prism-like crystals and octahedron-like crystals.
of microcrystal structure, i.e., a trigonal-prism-like crystal and an octahedron-like crystal. Fig. 3 shows the results of EDS analysis. The composition (in at.%) of the microcrystals on spots S1–S3 and the composition of the illuminated film among microcrystals on spots S4–S6 were analyzed. Our EDS spectra cannot give an exact identification since the spatial resolution is about 1 lm in width and a few lm in depth, and is not high enough to analyze each microcrystal individually. From the EDS analysis on spots S4–S6, the average composition of the illuminated film without microcrystals is O: 2.4 at.%, As: 37.8 at.%, and Se: 59.8 at.%. This result indicates that the composition of the film is approximately the stoichiometric composition of As2Se3 glass, but is slightly enriched in Se. EDS result on spot S1 is the most reliable since the microcrystal on spot S1 is
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the largest of the three. The microcrystals have a high content of O, indicating that a new phase of As2O3 is formed on the film surface after illuminated with band-gap light. This phenomenon has already been reported by several research groups [13–19] and is caused by both photo-decomposition and photo-oxidation. On the other hand, we could not find microcrystals of Se or As in contrast to the case of As-S films [13,14]. This fact agrees with the result given in Ref. [13]. Generally, As2O3 exists in two crystalline polymorphs: arsenolite (cubic) and claudetite (monoclinic). In Fig. 2, octahedron microcrystals are undoubtedly arsenolite, which have frequently been observed in the previous works. On the other hand, flat triangular microcrystals have not yet been observed to date. From the relation between crystal systems and crystal shapes, the triangular microcrystals are presumably not claudetite but a twinned crystal of the spinel-twin type, which corresponds to a macle of diamonds. The size of these microcrystals can be sufficiently controlled by changing the irradiation time as reported in Ref. [19]. To investigate the origin of the above-mentioned ring-formation, we carried out the EDS analysis of the central portion of the film illuminated by the intense laser beam, which corresponds to the center of the ring-shape distribution of As2O3 microcrystals. The composition of the film at that position is O: 1.1 at.%, As: 38.1 at.%, and Se: 60.8 at.%. We conclude the chemical composition of the central portion of the illuminated film without
As2O3 microcrystals is the same as that of the illuminated film among As2O3 microcrystals. Next, we present the experimental results for Agdoped, Cu-doped, and PMMA-coated As2Se3 films that were fabricated to examine their durability for the photo-oxidation reaction. Fig. 4 shows the transmission spectra of As2Se3, Ag3.1(As0.4Se0.6)96.9, and Cu2.4(As0.4Se0.6)97.6 films (where we hereafter describe the last two chalcogenide glasses simply as Ag–As2Se3 and Cu–As2Se3). The band-gap wavelength of As2Se3, Ag– As2Se3, and Cu–As2Se3 chalcogenide films is 0.705, 0.744, and 0.776 lm, respectively and the thickness of these three films is 1.3 lm. The penetration depth of the incident Ar laser light into these films is less than 0.1 lm. We performed the same experiment as one done in Fig. 1. In the present case, we pay attention to the characteristic peak optical intensity I0 at which the following four phenomena begin; the formation of As2O3 microcrystals, the formation of ring-shape distribution of As2O3 microcrystals, the dip-formation at the central portion, and laser damage. The obtained results are summarized in Table 1. The threshold peak intensity for the formation of As2O3 microcrystals in the Ag– As2Se3 films is increased by about 30 times that of the As2Se3 films. Of course, the photo-darkening effect takes place in the Ag–As2Se3 films below the threshold peak intensity. The threshold peak intensity for the ring-formation in the Ag–As2Se3 films is also six times as high as that of the As2Se3 films. Regarding the beginning of dip-formation and laser damage, we could not find a distinct difference between Ag–As2Se3 and undoped As2Se3 films. This means that the melting point and boiling point of these two materials equal each other. On the other hand, the photo-darkening effect and the formation of As2O3 microcrystals did not occur in the Cu– As2Se3 films. It is found that the threshold intensities for the dip-formation and laser damage in the Cu– As2Se3 films are higher than those of the As2Se3 and 100 80 Transmission (%)
Fig. 3. SEM image and EDS analysis of an As2Se3 film illuminated at the optical intensity I0 = 116 W/cm2. Two kinds of regions are analyzed with EDS; spots S1–S3 with a microcrystal and spots S4– S6 without microcrystals.
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60 40 As2Se3 Cu2.4(As0.4Se0.6)97.6 Ag3.1(As0.4Se0.6)96.9
20 0 400
600
800 1000 1200 Wavelength (nm)
1400
Fig. 4. Transmission spectra of As2Se3, Ag3.1(As0.4Se0.6)96.9, and Cu2.4(As0.4Se0.6)97.6 films.
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Table 1 Comparison of the presence of photo-darkening effect and threshold optical intensities I0 (in W/cm2) corresponding to the onset of four phenomena (formation of As2O3 microcrystals, ring-formation of As2O3 microcrystals, dip-formation, and laser damage) among four examined materials Sample
Photo-darkening
As2O3 microcrystals
Ring-formation
Dip-formation
Laser damage
As2Se3 Ag3.1(As0.4Se0.6)96.9 Cu2.4(As0.4Se0.6)97.6 PMMA-coated As2Se3
Yes Yes No Yes
3.4 115 No No
35 210 No No
815 930 1160 –
1550 1530 2300 2300
Ag–As2Se3 films. In the case of the PMMA-coated As2Se3 films, it is confirmed that the formation of As2O3 microcrystals never takes place since the sample surface is out of touch with the atmosphere. Of course, the photo-darkening effect can occur since the coating film is transparent. Although the threshold intensity for the laser damage is higher than that of As2O3 films, this is attributed to an increase in the reflectance of the sample by the coating film. On the other hand, we could not measure the threshold intensity for the dipformation.
4. Discussion Before discussing the reason why the ring-formation of As2O3 microcrystals takes place on the surface of As2Se3 films at optical intensities over a critical value, we first summarize the mechanism of the formation of As2O3 microcrystals proposed so far [13,14]. The chemical bonds associated with As atoms in the glass network are broken by irradiation with above band-gap light, the liberated As atoms can diffuse freely in the film and rejoin with each other in local regions forming small As crystals. The As species are oxidized to As2O3 in the presence of oxygen and water vapor. The photo-decomposition and oxidation reaction can be expressed as follows: hm
As2 Se3 ! xAs þ As2x Se3 H2 O
4As þ 3O2 ! 2As2 O3
ð2Þ ð3Þ
where the water appears to enter the oxidation reaction as a catalyst. Next, we discuss the mechanism of the ring-formation of As2O3 microcrystals. In order to obtain more information on the ring-formation, we have to perform additional experiments. From optical microscope photographs as shown in Fig. 1, we first determine the critical illumination intensity at which the microcrystals of As2O3 disappear, i.e., the optical intensity at the inner fringe of the ring pattern of As2O3 microcrystals. Fig. 5 shows the critical illumination intensity Ic as a function of peak illumination intensity I0. The critical illumination intensity is not a constant but decreases with increasing peak illumination intensity. In Fig. 5,
Fig. 5. Critical illumination intensity Ic as a function of peak illumination intensity I0. Here we define the lowest illumination intensity at which As2O3 microcrystals disappear as the critical illumination intensity.
it should be noted that the ring-formation of As2O3 microcrystals takes place at peak intensities I0 above 35 W/cm2. The origin of the ring-formation seems to be concerned not with optical energy itself but with thermal phenomena induced by it. One possibility is the sublimation (vaporization) or melting of As2O3 microcrystals that is caused by a temperature raise due to absorption of optical energy. This can be regarded as the same mechanism that Janai and Rudman have already proposed for the photo-vaporization observed in amorphous As2S3 films [24,25]. At a temperature of about 200 °C at which they did experiment, the photooxidation reaction products, As2O3 and free S, vaporize as gaseous As4O6 and S2 because of their high volatility, which is much greater than that of As2S3. We must here examine thermal data on As2O3 crystals for discussion. The melting point of As2O3 is 275 °C (for arsenolite) and 313 °C (for claudetite) and the boiling point is 465 °C. Although data on the sublimation point are scattered in literatures, it appears that cubic As2O3 begins to sublime at about 135 °C. To examine the possibility of sublimation or melting of As2O3 crystals, after we created As2O3 microcrystals by illumination of a peak intensity of 35 W/cm2 (Fig. 1(b)), we illuminated the same position of the sample at a peak intensity of 230 W/cm2. Note that, at the latter intense irradiation, the ring-formation sufficiently takes place as shown in Fig. 1(d). However, the pattern
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of As2O3 microcrystals formed in advance was never changed by the second illumination. It is necessary to obtain information on a local temperature increase in the sample due to optical absorption for our discussion. We must estimate the value of the temperature raise using an assumption because it is difficult to measure it directly. We assume that the temperature raise DT at the center potion of the illuminated sample is proportional to the peak illumination intensity I0 and that the laser damage (or melting) of As2Se3 films takes place at a boiling point of about 700 °C (or a melting point of 375 °C [26]), respectively. Under such assumptions, we can get the temperature raise DT = 15 °C for a peak intensity of 35 W/cm2, which is the optical intensity for the beginning of the ring-formation. The temperature at the central portion of the sample is finally estimated to be 38 °C, which is much lower than a sublimation point of 135 °C. Therefore, we tried to form As2O3 microcrystals with exposure of an Ar laser beam while keeping an As2Se3 film at about 35 °C. Although we illuminated the sample with two values of illumination intensity I0 = 35 and 230 W/cm2 for 5 min, we could not obtain As2O3 microcrystals. Although we also heated the As2O3 microcrystals formed in advance up to about 35 °C, these microcrystals did not disappear. We moreover carried out the same experiments at two ambient temperatures of 40 and 50 °C and obtained the same results as those at about 35 °C. From these experimental results, we conclude that the As2O3 microcrystals do not disappear through sublimation but from the beginning they are never grown on the film surface at temperatures above about 35 °C. We belive that the light-enhanced vaporization of As2O3, which was pointed out by Janai and Rudman, never takes place since the vapor pressure of As2O3 at 35 °C is seven orders of magnitude lower than that at 200 °C [27]. Therefore it is adequate to consider that the photo-induced As species are oxidized not to As2O3 but to a volatile product at ambient temperature above about 35 °C. Although it is difficult to obtain direct evidence, we can assume the presence of ortho-arsenic H3AsO4 as a volatile oxidation product, according to the following reaction: 4As þ 5O2 þ 6H2 O ! 4H3 AsO4 "
ð4Þ
The melting point of H3AsO4 is 35.5 °C and it is likely that H3AsO4 volatilizes near such a melting temperature before its crystal grows. In fact, the presence of this material has already been confirmed in a test for UV light and moisture durability of As2Se3 fibers [16] and this seems to support our assumptions indirectly. On the other hand, our tentative model can qualitatively explain the dependence of the critical illumination intensity for the ring-formation on the peak illumination intensity shown in Fig. 5. Compared with the intensity profile of the illuminating laser beam, the temperature
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raise distribution in the sample spreads because of the thermal diffusion. Therefore the experimental result shown in Fig. 5 is not curious. Finally, we discuss the photo-darkening and photooxidation of Ag–As2Se3 and Cu–As2Se3 glasses. Although the addition of Ag or Cu into As2Se3 glasses are very effective for suppressing the generation of photoinduced oxidation reaction, there exists a distinct difference in the photo-induced effects between these two doped-glasses as shown in Table 1. That is, both the photo-darkening effect and photo-oxidation reaction never occur in Cu–As2Se3 glasses, whereas they take place in Ag–As2Se3 glasses. Taylor et al. [11,12] have maintained that the incorporation of metals such as Cu or Ag into amorphous As2S3 and amorphous As2Se3 eliminates photo-darkening effect and its effect appears to be strongly corrected with the twofold coordination of the chalcogen atoms. They have also explained that the coordination number of chalcogen atoms increases from two to four when the metal concentrations are high enough. However their statements seem to be not correct in the case of Ag-doped As2Se3 glasses. For example, according to the extend X-ray absorption fine structure experiments of (Ag2Se)y(As2Se3)1y glasses [28], the coordination number of the Se atom varies from two to three depending on Ag content, i.e., y. Since the Ag or Cu content treated in this paper is not high, the doped metal elements may be regarded as glass network modifiers. However we belive that there exists a difference in the glass structure between Ag-doped and Cu-doped As2Se3 glasses even if the metal content is low since we have already found a distinct difference in the optical properties between them [21]. We now estimate the temperature at which the ringformation begins on the surface of Ag–Se2Se3 films in the same manner as one described above. Assuming that the melting takes places at a peak illumination intensity of 930 W/cm2 and that the melting point is 364 °C [26], the onset temperature of the ring-formation is calculated to be 100 °C. This calculated temperature agrees nearly with a sublimation temperature of cubic As2O3 of 135 °C. The ring-formation of Ag–As2Se3 films is presumably done by the sublimation of As2O3 in contrast to the case of undoped As2Se3 films. In this paper, we selected only Ag and Cu as dopants into As2Se3 glasses since these two metals can be easily photo-doped. However the addition of other materials is effective for the elimination of photo-oxidation reaction if there exist glass formation regions in the glass system after the addition and the chemical bonds associated with As atoms are modified by it. Let us consider the doping of Au since its element belongs to the 1B group as well as Cu and Ag elements do. Since there are no published reliable data on glass formation in the Au–As–Se system, it is difficult for us to obtain homogeneous glasses in the indicated systems [29]. If Au is
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added into As2Se3 glasses by some way, we will obtain glasses in which small Au particles are embedded. Therefore we do not think the doping of Au into As2Se3 glasses is effective for the present purpose.
5. Conclusions We investigated the photo-darkening effect and the photo-oxidation reaction of three kinds of As-chalcogenide films, i.e., as-evaporated As2Se3, Ag-doped As2Se3, and Cu-doped As2Se3 films from the applicationÕs point of view. In this paper, a focused near band-gap light from an Ar laser was used as the light source and the illumination intensity was changed from zero to the threshold for laser-induced damage. It has been found that the As2O3 microcrystals formed on the surface of As2Se3 films by the photo-oxidation reaction are gradually absent from the central portion of the laser focal area as the illumination intensity is increased. The reason for the generation of this phenomenon is that the illuminated sample is locally heated due to the optical absorption and the formation of As2O3 microcrystals then stops at ambient temperature above 35 °C because of the creation of a volatile oxidation product. It has confirmed that the addition of metals such as Ag or Cu into As2Se3 glasses is useful for suppressing the generation of harmful photo-oxidation reaction. In the case of Cu-doped As2Se3 glasses, both the photo-darkening effect and photo-oxidation reaction never take place. Therefore, we have to use Ag-doped As2Se3 glasses when the photo-darkening effect is utilized, for example, to fabricate Bragg gratings or optical waveguides.
Acknowledgement The authors would like to thank Dr M. Kitao for his helpful advice.
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