Optical property modification of ruby and sapphire by N-ion implantation

Optical property modification of ruby and sapphire by N-ion implantation

Surface & Coatings Technology 196 (2005) 108 – 112 www.elsevier.com/locate/surfcoat Optical property modification of ruby and sapphire by N-ion impla...

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Surface & Coatings Technology 196 (2005) 108 – 112 www.elsevier.com/locate/surfcoat

Optical property modification of ruby and sapphire by N-ion implantation C. Chaiwonga, L.D. Yua,*, K. Schinarakisb, T. Vilaithonga a

Fast Neutron Research Facility, Department of Physics, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand b Department of Physics, University of Wuppertal, 42097 Wuppertal, Germany Available online 22 September 2004

Abstract Effects of N-ion implantation on surface modification of synthetic single crystalline ruby (Cr-doped Al2O3) and sapphire (a-Al2O3) were studied. N ions at energy 120 keV were implanted to samples with doses ranging from 11016 to 11018 ions/cm2. Temperature studies ruled out the effect of ion-beam heating and, therefore, the gemological modification was the direct result of the implanted ions and radiation damage. Optical measurements showed that the optical absorption of the ion-implanted ruby uniformly increased without observable characteristic changes in any wavelength bands, whereas for the high-dose ion-implanted sapphire, optical absorption in the short wavelength region became stronger. The refractive indices however showed the same decreasing trend in the higher dose ion-implanted samples for both types of crystals. N-ion implantation also caused blistering of the surface, as studied by scanning electron microscopy (SEM). The mediumdose (51017 ions/cm2) ion-implanted surfaces started to show drastic blistering, and the high-dose (11018 ions/cm2) ion implantation further resulted in amorphization of the ruby and sapphire surfaces. According to these data, the optical property changes are then attributed to the gem-material surface modification. D 2004 Elsevier B.V. All rights reserved. Keywords: Optical property; Ruby; Sapphire; N-ion implantation; Temperature; Optical absorption; Refractive index; Gemological modification

1. Introduction Ion implantation is being explored for gemstone enhancement in ruby and sapphire at Chiang Mai University. Various ion species have been employed to implant natural local corundum. Changes in color from the appearance of the ionimplanted gemstones are then observed. For example, rubies with the original color of red/purple showed purer red color and vivid saturation with improved clarity and luster after Oion implantation; sapphire with the original color of blue/ green turned into those having a more vivid blue color and less green after N-ion implantation [1]. Various causes of the gemological modification with respect to color change have been studied and proposed [2]. An example of such causes is heat treatment, which is a popular conventional technique to enhance the colors of gemstones and has been widely investigated [3–5]. However, the effects of ion implantation

on color change in gemstones have not been concluded. Here we describe and experimental observation of the optical property changes in synthetic corundum when bombarded with gaseous ions. A number of experiments have been widely carried out to study the effects of ion implantation on the structure and many physical properties, including the optical property, of sapphire (a-Al2O3) (e.g. Refs. [6–11]). The previous studies mainly stemmed from the interests in potential applications of the ion-implanted crystal on optics, optoelectronics and tooling, rather than on gemology. In this work, we performed some studies on N-ion implantation in synthetic ruby and sapphire, attempting to obtain evidence, which may point to ways in which changes in the optical properties could constructively be induced to natural gemstones.

2. Experiments * Corresponding author. Tel.: +66 53 943379; fax: +66 53 222776. E-mail address: [email protected] (L.D. Yu). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.115

Samples of commercially available synthetic sapphire (single crystalline a-Al2O3) and ruby (single crystalline a-

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Al2O3 doped with 1.2% Cr), having a dimension of 772 mm3 and the c-axis oriented perpendicular to surface normal, were used in this study. Atomic nitrogen ions accelerated at 120 keV with a beam current of about 10 AA were implanted into the samples in the direction normal to the surface plane at room temperature. The implanted doses were 0.1, 0.5, 1, 5, 101017 ions/cm2. The samples were held by glass-ceramic pins to minimize sputtering contamination from the sample holder. The operating pressure in the target chamber was maintained at about 104 Pa by a turbo-molecular pump with high cleanness. No cooling was applied to the samples during ion implantation. An infrared temperature sensor was used to measure the temperature of the sample surface through a CaF2 window. A critical-angle refractometer was used to measure the refractive index (range: 1.40–1.79) of the crystal surface layer [4]. A spectrophotometer (SpectraPro-300i, Acton Research, Acton, MA) was used to measure the optical absorption in the wavelength range of 300–700 nm. The sample surface was examined using scanning electron microscopy (SEM).

3. Results and discussion The temperatures of the samples during ion implantation always reached a steady state after a short period of rising and the steady-state temperatures never exceeded 100 8C under various experimental conditions, as shown in Fig. 1a. The temperature distribution in a solid slab was theoretically calculated using the model of the 1D heat transportation equation that contains a heat source term with thermal-emission boundary conditions:   2   dT x; t dT ¼a þ c x; t dt dx2  T x; 0 ¼ Ta k

  dT ¼  e T 4 0  Ta4 dx x¼0

k

  dT ¼ e T 4 L  Ta4 ; dx x¼L

where c(x,t) is the source term due to ion-implantation energy deposition, e is the emissivity of the target material, T 4 reflects the Stefan–Boltzmann Law of the thermal emission, Ta is the ambient temperature (300 K), and a=k/ qC with k the thermal conductivity, q the mass density, and C the specific heat. The source term c(x,t) is simplified to be directly corresponding to the distribution of the created vacancies or collision events, which can be derived from

Fig. 1. (a) A typical example of the temperature of a-Al2O3 measured during 120-keV N-ion implantation. (b) The calculated temperature distribution during ion implantation of a-Al2O3 obtained for 1D heat equation containing a heat source term with the thermal emission boundary conditions.

theoretical calculation or simulation. The equation was solved using the finite difference method with the Newton– Raphson-algorithm linearization [12] for the steady state c(x). The calculated results are very close to the measured data, as shown in Fig. 1b. It was found in some cases that ion beams could result in strong localized heating in solids, such as from the thermal spikes [13], leading to phase transformation and defect (such as color centers) formation, which might cause optical property modifications [14]. However, rapid quenching of the localized heating (in an order of 1013 s) [13] and short operation time as well as surface thermal emission may restrict the ion beam heating effect. From temperature measurements in this study, it was concluded that ion-beaminduced heating is insignificant and the notion that ion implantation is a heat source for heat treatment of gemstones can be ruled out. Therefore, any gemological modification in this experiment is only attributed to implanted ions and radiation damage. The results of the measured optical absorption are shown in Fig. 2. Compared with the absorption of the unimplanted

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sample, the absorption of the implanted rubies uniformly increases in intensity with increasing dose in the whole measurement range without observable changes in the characteristics in any wavelength band; while for the implanted sapphire, the absorption becomes increasingly stronger with increasing dose only in the ultraviolet to violet region. There have been a number of studies on the ionimplantation-induced absorption change of sapphire. It is noticed that in most of the cases, particularly before postannealing, the absorption increased or appeared in the short wavelength range (~200–360 nm) and increased with increasing dose [15–17,8]. Discussions made about this behavior were based on several reasons, for example, the presence of F-center [16], change of electronic state of implanted ions [17], etc. We have attributed the cause of optical absorption in sapphire to the V-type center and Ftype center by comparing the measured absorption band with the known ones associated with specific lattice defects [16]. Under irradiation, three types of hole centers connected with anion vacancies (V, V2 and VOH) can be created, all of which have the absorption bands centered

Fig. 2. Ion dose dependence of optical absorption for (a) ruby and (b) sapphire.

near 413 nm [18]. It has also been found that the electroncenters (F2, F2+ and F22+) which contribute absorption bands near 303, 357 and 450 nm [19] can overlap the V-band [20]. However, for ruby there must be other reasons to cause the difference in absorption from that for sapphire. This was found to be related to the ion-implantation-caused special surface modification of ruby as discussed below. The measured refractive indices from the ion-implanted surface layers of ruby and sapphire are almost the same as those of the unimplanted surface for the lower doses (0.1, 0.5, 11017 ions/cm2). However, the refractive indices could not be measured for the higher dose (5, 101017 ions/ cm2) samples. The refractive index of the Si-oxide film has been reported to decrease from 1.46 down to 1.29 after Fion or high-dose F+C-ion implantation [21]. The refractive index of our high-dose N-ion implanted Al-oxide seems to behave similarly due to the ion-implantation produced damage on the surface, particularly on the high-dose implanted surface. It was found from scanning electron microscopy (Fig. 3) that the surface of both ruby and sapphire started to show severe blistering for the dose equal to or higher than 51017 ions/cm2 (Fig. 3a and b), whereas no blistering was observed for the cases of lower doses. At the highest dose (11018 ions/cm2) blistering could hardly be observed on ruby except very few small spots (Fig. 3c), while it was clearly seen on the sapphire surface (Fig. 3d). Blistering of gaseous ion implanted a-Al2O3 single crystals has been reported [22,23] and found to be dependent on ion species, dose and temperature. Generally, for a particular ion species, the higher the temperature, the higher the dose required to blister. Blistering is attributed to bubble formation, which is due to the concentration of implanted gas ion species being higher than the solid solubility of the ion species in the crystalline substrate [24], and featured by growth and agglomeration of the bubbles as the ion dose and substrate temperature increase [25]. The formation of bubbles of H, He and O-ion implanted sapphire has been described [26,27]. Our result shows that for N-ion implantation at room temperature, drastic blistering starts at the dose of about 51017 ions/cm2. For the high dose (11018 ions/cm2), it is likely that the ruby surface is already amorphized while the sapphire surface is at the beginning of amorphization. So blisters are difficult to form as is often the case for noncrystalline structure. Amorphization of ion implanted sapphire surface has been widely studied [6,15,28,29] and critical doses for specific ion species at various temperatures have been obtained. A simple way to identify amorphization of Al2O3 is testing the surface hardness as a function of ion dose, and an apparent hardness decrease would indicate amorphization occurring at the corresponding dose [30]. Our hardness testing result, as shown in Fig. 4, indicates that the dose of 11018 ions/cm2 is the critical dose of amorphization for room temperature N-ion implantation.

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Fig. 3. SEM photographs of the high-dose N-ion implanted ruby and sapphire surface. (a) Ruby, 51017 ions/cm2; (b) sapphire, 51017 ions/cm2; (c) ruby, 11018 ions/cm2, inside a selected small spot; (d) sapphire, 11018 ions/cm2.

The decrease in hardness of the sample implanted with 51017 ions/cm2 was supposed to be due to the surface blistering, as observed, rather than amorphization. It was reported that simultaneous Al, O, and He-ion implantation of alumina at room temperature for the doses that led to the produced damage energy of 3.1 keV/atom did not yet induce amorphization [31]. The damage energy (h D) [13] has been calculated to be about 11 keV/atom in this study, 12 keV/atom in O-ion implantation [27] and 8 keV/atom in Nb-ion implantation [15], for amorphization at room temperature. The changes in the optical properties of gemstone materials investigated are therefore related to the surface

modifications in terms of blistering and amorphization by ion beams. These surface modifications cause a general increase in optical absorption, which increases with increasing ion dose. Up to the highest dose, the absorption prominently increases in ruby since the surface was totally amorphized. The amorphization turns the nearsurface crystalline structure of the materials to a more glassy structure. This was thought to be the main cause that reduces the refractive index to the value below our measurement limit, 1.40, and thus the indices could not be measured in the cases of the high-dose samples. The immeasurability might also be due to diffusion of light at the surface since it was greatly roughened by the ion beam.

4. Conclusion

Fig. 4. Microhardness testing result: relative Knoop hardness (HK, tested under a load of 10 g) as a function of N-ion dose. HK0 is the Knoop hardness number of the unimplanted surface.

In this study, the effects of N-ion implantation on synthetic single crystalline ruby and sapphire were investigated. Changes in optical properties such as optical absorption and refractive index of gemstones induced by ion implantation are proposed to be parts of the reasons for gemological modification in appearance of the materials by ion beams. These changes are caused not by heat treatment from the ion beam heating effect but by implanted ions and induced surface modification such as blistering and amorphization. The surface blistering and further amorphization at high doses lead to an increase in the optical absorption, and the amorphization at high doses results in a drastic decrease in the refractive indices of the gem materials.

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Acknowledgements We wish to thank S. Davydov for helpful discussions. We also thank P. Vichaisirimongkol, B. Panchaisri and T. Thibprasit for their assistance. This work was supported by the National Metal and Materials Technology Center.

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