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Ion beam induced modification of nanolayers containing vanadium and silica
Surface & Coatings Technology 196 (2005) 113 – 117 www.elsevier.com/locate/surfcoat
Ion beam induced modification of nanolayers containing vanadium and silica G. Savignya, J.R. Currie Jr.b, D. Ilaa, R.L. Zimmermana,*, C.I. Muntelea, R. Muc, A. Uedac a
Center for Irradiation of Materials, Alabama A&M University, P.O. Box 144, Normal, AL 35762-1447, USA b George C. Marshall Space Flight Center, AL 35812, USA c Chemical Physics Laboratory, Center for Photonic Materials and Devices, Fisk University, Nashville, TN, USA Available online 27 October 2004
Abstract Layers of vanadium monoxide (VO) and pure silica were alternately deposited on a silica substrate. Similarly, layers of V2O3 and pure silica were alternately deposited on a silica substrate. The interfaces between the codeposited layers and pure silica layers were at no time exposed to air. Each sample was bombarded by 5.0 MeV Si beam at fluences between 1015 and 1017 ions/cm2 at room temperature. Optical properties were measured using Optical Absorption Photospectrometry (OAP). The elemental distribution was measured using Rutherford Backscattering Spectrometry (RBS) at each step of Si bombardment. A different piece of each layered sample was annealed at temperatures between 100 and 1100 8C steps, for 1 h in argon environment. OAP and RBS were observed after each temperature. The dramatic apperance of an optical absortion resonance at 410 nm after heat treatment to 600 8C indicates that vanadium nanocrystals form in silica at that temperature. An ion bomdarment induced mixing coefficient is derived and experimentally evaluated for vanadium in silica. It is observed to increase with the accumulated fluence and multilayered samples. Keywords: Ion beam mixing; Nanolayers; Vanadium nanocrystals; Nano technology; Ion beam modification
1. Introduction The versatility of vanadium makes this element suited for a very large number of applications. Indeed, the relatively large number of valence states characteristic of this element allows for a number of different oxides. VO2 and V2O3 have been the most thoroughly studied for the remarkable electrical and optical properties owing to the induced crystallographic phase transition that they experience around 70 8C [1,2]. The purpose of the present investigation is to make and characterize layers of vanadium oxides in a pure silica lattice. Nanolayers may be produced by coimplantation of several elements [3,5] or by sequential deposition, and they may be modified by ion bombardment as well as by thermal annealing. Here, we focused our effort on ion bombardment * Corresponding author. Tel.: +1 256 372 5854; fax: +1 256 372 5868. E-mail address: [email protected] (R.L. Zimmerman). doi:10.1016/j.surfcoat.2004.08.194
and thermal treatment to modify sequentially deposited layers of VO and V2O3 in pure SiO2.
2. Experimental procedures VO/SiO2 and V2O3/SiO2 multilayer structures were deposited on a pure silica substrate (Suprasil 300) with less than 0.01% of impurity under high vacuum conditions. Sputtering of the substrate surface was performed prior to thin film deposition using argon ions at 100 eV. In order to monitor the deposition, a quartz microbalance was used during the evaporation process. A Fabry–Perrot interferometer was employed to determine the final thickness of the different layers. The samples were cut into small squares of 55 mm. Thermal annealing was carried out at temperatures between 100 and 1100 8C with 100 8C steps, for 1 h in a nonreactive environment (argon). During the cooling phase, the argon flow was kept until the temperature went below 50 8C. The silica crucibles used during the thermal
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annealing were meticulously cleaned beforehand in order to avoid any contamination of the samples. The samples were analyzed by Optical Absorption Photospectrometry (OAP) after each heat treatment. Optical absorption measurements were performed at room temperature using a spectrophotometer capable of absorption measurements in the UV-visible range (190 to 900 nm).The samples were then analyzed using Rutherford Backscattering Spectrometry (RBS) in order to determine the elemental distribution of the different layers. The RBS spectra were acquired with 2.1 MeV He2+ incident particles. Ion bombardment of the multilayer samples with silicon ions was performed at 5 MeV (at room temperature and fluences between 11015 and 11017/cm2). Optical measurements were done after each bombardment. The bombarded multilayer samples were compared with equally bomdarded silica substrates. In order to make accurate measurements when doing ion implantation or ion bombardment, one has to be able to determine precisely the integrated beam dose. Several minor processes prevent the integrated beam dose to be known exactly and thus their effects need to be minimized. Those minor processes are: high-energy electrons, including Auger electrons, photons, X-rays and excited sputtered atoms [4]. The use of a faraday cup system in a high vacuum reduces the contribution of these processes by eliminating ion-neutral collisions. The experimental setup that was used for the ion bombardment experiments consists of an electrostatically shielded cup surrounding the target holder so that only a small fraction of any emitted secondary electrons can escape [4]. For isotropic emission, this fraction is given by A/2pl 2, where A is the opening area and l is the opening-to-target distance. In addition, at the entrance opening, a negative 300-V bias potential is placed to prevent any low-energy electrons from leaving or entering the cup. The collimator, at ground potential, prevents the ion beam from hitting the aperture of the faraday cup. The collimator and the secondary electron repelling plate, as well as the faraday cup holder, were all made using aluminum. Small pieces of insulating ceramic were used to insulate the electron repelling plate from ground. The electron repelling plate was brought to a 300 V potential by connecting to an external battery. The faraday cup and the charge-collecting electrode was cut from a thin copper sheet and linked to an external integrator. The samples were fixed on the electrode by using carbon tape.
number and distribution of Si ions implanted. The RBS simulation software RUMP, together with the software SRIM, showed the fluence to be (9.9F0.2)1016/cm2.
3. Results and discussion Ion beam induced diffusion may be treated as a statistical process along the silicon ion track. The interaction of the fast silicon ion is almost entirely with the electrons in the target material. Many electrons are ejected and move far from the parent atoms, leaving a cylindrical region severely strained by coulomb forces. The subsequent displacement of the positively ionized target atoms has lateral and axial components near the cylindrical axis, near the silicon projectile track, with little or no memory of the direction of the projectile. The result is different from thermal diffusion because the displacements occur only near the fast ion track. However, as the fluence is increased (beyond about 1014 ions/cm2), such that area of the target is penetrated many times, the position of the target atoms will be governed by statistical processes analogous to thermally induced diffusion. Specifically, the vanadium atoms in the target will move opposite the gradient of the vanadium concentration (Fick’s law), and the vanadium concentration C will increase with added fast ion bombardment fluence only where the gradient in the x direction has a positive curvature, in accordance with the classical diffusion equation: dC d2 C ¼D 2 df dx where the left differential is relative to the fluence f, not the time. The ion induced mixing coefficient D has dimensions of m2 per fluence unit. Fig. 1 shows the RBS spectrum of a vanadium layer deposited sequentially between deposited SiO2 layers on a
2.1. Ion bombardment calibration The reliability of the charge collecting arrangement was confirmed by observing the RBS yield from a sample implanted with silicon to a fluence calculated from the integrated charge. A glassy polymeric carbon substrate was bombarded with an accumulated charge of 1 MeV Si+ ions corresponding to a fluence of 11017/cm2. An RBS analysis was next performed on the bombarded sample to determine
Fig. 1. RBS spectra of a multilayered sample untreated and bombarded with different 5 MeV Si fluences. The ion-induced mixing coefficient may be obtained from these spectra (see text), first by observing the decrease in amplitude of the signal from the nearly periodic layers, and then by observing the diffusion from the layers into the silica substrate.
G. Savigny et al. / Surface & Coatings Technology 196 (2005) 113–117
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concentration. The dimension x is measured from a maximum in the spectrum. The diffusion equation is satisfied when with the following value of the function: 2
Fn ð f Þ ¼ Con exn Df The higher frequency components of the spectrum disappear quickly as fluence is accumulated just as for thermal diffusion. Moreover, the experimental RBS data in Fig. 1 clearly shows that the amplitude of the fundamental decreases with increasing fluence, about 20% after silicon ion bombardment to a fluence of 1016 ions/cm2. Thus, the coefficient in the exponential is x20 Df ¼ 0:22: Fig. 2. Optical spectra V2O3 multilayered sample after ion bombardment at fluences ranging from 11015 to 11017/cm2.
silica substrate. Fig. 1 also shows the spectrum of the same sample after bombardment with 5 MeV silicon ions to fluences of 1016, 31016 and 1017 ions/cm2. The silicon ions stop far beyond the vanadium layers. For the fluence 11016 ions/cm2, the vanadium layers are significantly diffused. The sinusoidal-like spectrum, already smoothed by instrumental resolution, is further reduced by bombardment. Representing the spectrum with the periodic function made up of the sum of a series of contributors Cn ¼ Fn ð f Þcosðxn xÞ; where x n is the spatial frequency of the one dimensional structure and F n ( f) is the amplitude of each component, expected to decrease with fluence f as vanadium is induced by ion bomdarment to diffuse out of regions of greater
The spatial frequency x 0=8107 rad/m is obtained from the interferometrically determined 79-nm spacing between vanadium layers. We calculate an approximate numerical value of the ion induced mixing coefficient D ¼ 0:35 1036 m2 m2 : As the fluence is increased to 31016 and 1017 silicon ions/cm2, the layer structure is essentially homogenized and acts as an exhaustible source for vanadium that diffuses into the silica substrate. Assuming an error function Erf (x 2/2r 2) to represent the profile of vanadium concentration near the interface with the substrate, Fig. 1 compares the observed concentration profiles after bombardment with the ones calculated. The argument 2r 2 of the error function is expressed in (channel)+2 in Fig. 2. Conversion of channels to nm yields r which increases from 35 to 78 nm as the fluence increases from 310p16ffiffiffiffiffiffi to 1017 silicon ions/m2. Using the relation r ¼ Df , the ion-induced mixing coefficient is determined to be 4.01036 m2 m2 at 31016
Fig. 3. Optical spectra VO multilayered sample after ion bombardment at fluences ranging from 11015 to 11017/cm2.
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Fig. 4. Optical spectra of V2O3 multilayered sample after heat treatment from 100 to 1000 8C.
Si ions/cm2 and 6.210 36 m2 m2 at 11017 Si ions/cm2. This fluence dependence of the mixing coefficient may be a consequence of accumulating damage in the substrate. The much lower mixing coefficient in the layers may indicate the crystalline nature of the deposited layers of silica. Values of the mixing coefficient for low fluence should scale linearly to other ions and energies in accordance with the linear electronic energy transferred by those ions to the target. We are accustomed to the thermal diffusion coefficient that has dimesions of m2/s. The distance an atom diffuses by random movements involves the square root of the diffusion
coefficient and the time. The ion-induced mixing coeficient D has dimensions of m2 per fluence unit, or m2 m2, certainly less familiar. We attempt to interpret D physically by first changing meters to nanometers, an appropriate atomic dimesion, such that the dimension may be written nm3 nm, and multiply the numerical value by 1036 nm4/m4. Thus, D is the distance in nanometers that one vanadium atom moves (or half the distance that two vanadium atom move) when one silicon projectile passes through a volume of one nm3 of SiO2. After many such movements assumed to be in random direction, the atoms are moved to a distance determined by the square root of the diffusion constant and
Fig. 5. Optical spectra of V2O3 multilayered sample after heat treatment from 100 to 1000 8C. Formation of vanadium metal nanocrystal after heat treatment to 600 8C is believed to cause the dramatic appearance of an absortion resonance at 410 nm.
G. Savigny et al. / Surface & Coatings Technology 196 (2005) 113–117
pffiffiffiffiffiffi the accumulated fluence r ¼ Df , similar to thermal diffusion. The numerical values 0.35, 4.0 and 6.2 nm3 nm for the ion-induced mixing constant show how hugely effective is ion-induced mixing. The optical spectra of VO and V2O3 layers (Figs. 2 and 3) show enhanced absorption near 300 nm where vanadium metal nanoclusters are expected to absorb light. Figs. 4 and 5 shows the OAP results after annealing from 100 to 1100 8C with 100 8C steps. Both graphs display a similar behavior. The major change appears at a temperature between 500 and 600 8C where there is a shift. Indeed, the peak at around 300 nm keeps growing in intensity until 500 8C and then shifts to 410 nm at 600 8C for both oxides. This leads us to think that VO and V2O3 both form the same vanadium oxide after heat treatment at 600 8C. At temperatures above 600 8C, the absorbance decreases and above 1000 8C, the samples are very much alike the substrate. The layers have been completely diffused and no longer reveal any characteristic absorption peak. Ion bombardment increases the optical absorption in the silica substrates. The effect of this damage has been effectively subtracted from the spectra shown by using equally bombarded silica substrates as a reference. The incremental absorption due to bombardment of the surface layers is presented in the figures.
4. Conclusion The ion-induced mixing coefficient was formally defined and measured as a function of fluence for vanadium in silica. Values of 0.35, 4.0 and 6.21036 m2 m2 at fluences
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of 1, 3 and 101016 of 5 MeV silicon ions/cm2 were determined and show that mixing is enhanced as bombardment damage accumulates. The new optical absorption peak at 410 nm that appears only after heat treatment of the vanadium oxide layers to 600 8C may indicate that the vanadium atoms that diffuse into silica not only form nanocrystal, but increase the refractive index in their neighborhood.
Acknowledgements This project was supported by the Center For Irradiation of materials at Alabama A&M University and the Department of Defense. The authors are grateful to Professor Lawrence Holland for exploiting the parallel between thermal diffusion and ion induced mixing.
References [1] L. Gea, S. Honda, L.A. Boatner, T.E. Haynes, B.C. Sales, F.A. Modine, A. Meldrum, J.D. Budai, L. Beckers, Mater. Res. Soc. Symp. Proc. 501 (1998). [2] D.H. Thanh, P.D. Long, V.T. Bich, N.N. Dinh, Commun. Phys. 8 (3) (1998) 152. [3] Al Meldrum, Richard F. Haglund Jr., L.A. Boatner, C.W. White, Adv. Mater. (19) (2001). [4] Handbook of modern ion beam materials analysis—J.R. Tesmer, M. Nastasi, J.C. Barbour, C.J. Maggiore, J.W. Maye; Materials Research Society. [5] A. Meldrum, L.A. Boatner, C.W. White, Nucl. Instrum. Methods Phys. Res., B 178 (2001) 7.
Ion beam induced modification of nanolayers containing vanadium and silica G. Savignya, J.R. Currie Jr.b, D. Ilaa, R.L. Zimmermana,*, C.I. Muntelea, R. Muc, A. Uedac a
Center for Irradiation of Materials, Alabama A&M University, P.O. Box 144, Normal, AL 35762-1447, USA b George C. Marshall Space Flight Center, AL 35812, USA c Chemical Physics Laboratory, Center for Photonic Materials and Devices, Fisk University, Nashville, TN, USA Available online 27 October 2004
Abstract Layers of vanadium monoxide (VO) and pure silica were alternately deposited on a silica substrate. Similarly, layers of V2O3 and pure silica were alternately deposited on a silica substrate. The interfaces between the codeposited layers and pure silica layers were at no time exposed to air. Each sample was bombarded by 5.0 MeV Si beam at fluences between 1015 and 1017 ions/cm2 at room temperature. Optical properties were measured using Optical Absorption Photospectrometry (OAP). The elemental distribution was measured using Rutherford Backscattering Spectrometry (RBS) at each step of Si bombardment. A different piece of each layered sample was annealed at temperatures between 100 and 1100 8C steps, for 1 h in argon environment. OAP and RBS were observed after each temperature. The dramatic apperance of an optical absortion resonance at 410 nm after heat treatment to 600 8C indicates that vanadium nanocrystals form in silica at that temperature. An ion bomdarment induced mixing coefficient is derived and experimentally evaluated for vanadium in silica. It is observed to increase with the accumulated fluence and multilayered samples. Keywords: Ion beam mixing; Nanolayers; Vanadium nanocrystals; Nano technology; Ion beam modification
1. Introduction The versatility of vanadium makes this element suited for a very large number of applications. Indeed, the relatively large number of valence states characteristic of this element allows for a number of different oxides. VO2 and V2O3 have been the most thoroughly studied for the remarkable electrical and optical properties owing to the induced crystallographic phase transition that they experience around 70 8C [1,2]. The purpose of the present investigation is to make and characterize layers of vanadium oxides in a pure silica lattice. Nanolayers may be produced by coimplantation of several elements [3,5] or by sequential deposition, and they may be modified by ion bombardment as well as by thermal annealing. Here, we focused our effort on ion bombardment * Corresponding author. Tel.: +1 256 372 5854; fax: +1 256 372 5868. E-mail address: [email protected] (R.L. Zimmerman). doi:10.1016/j.surfcoat.2004.08.194
and thermal treatment to modify sequentially deposited layers of VO and V2O3 in pure SiO2.
2. Experimental procedures VO/SiO2 and V2O3/SiO2 multilayer structures were deposited on a pure silica substrate (Suprasil 300) with less than 0.01% of impurity under high vacuum conditions. Sputtering of the substrate surface was performed prior to thin film deposition using argon ions at 100 eV. In order to monitor the deposition, a quartz microbalance was used during the evaporation process. A Fabry–Perrot interferometer was employed to determine the final thickness of the different layers. The samples were cut into small squares of 55 mm. Thermal annealing was carried out at temperatures between 100 and 1100 8C with 100 8C steps, for 1 h in a nonreactive environment (argon). During the cooling phase, the argon flow was kept until the temperature went below 50 8C. The silica crucibles used during the thermal
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G. Savigny et al. / Surface & Coatings Technology 196 (2005) 113–117
annealing were meticulously cleaned beforehand in order to avoid any contamination of the samples. The samples were analyzed by Optical Absorption Photospectrometry (OAP) after each heat treatment. Optical absorption measurements were performed at room temperature using a spectrophotometer capable of absorption measurements in the UV-visible range (190 to 900 nm).The samples were then analyzed using Rutherford Backscattering Spectrometry (RBS) in order to determine the elemental distribution of the different layers. The RBS spectra were acquired with 2.1 MeV He2+ incident particles. Ion bombardment of the multilayer samples with silicon ions was performed at 5 MeV (at room temperature and fluences between 11015 and 11017/cm2). Optical measurements were done after each bombardment. The bombarded multilayer samples were compared with equally bomdarded silica substrates. In order to make accurate measurements when doing ion implantation or ion bombardment, one has to be able to determine precisely the integrated beam dose. Several minor processes prevent the integrated beam dose to be known exactly and thus their effects need to be minimized. Those minor processes are: high-energy electrons, including Auger electrons, photons, X-rays and excited sputtered atoms [4]. The use of a faraday cup system in a high vacuum reduces the contribution of these processes by eliminating ion-neutral collisions. The experimental setup that was used for the ion bombardment experiments consists of an electrostatically shielded cup surrounding the target holder so that only a small fraction of any emitted secondary electrons can escape [4]. For isotropic emission, this fraction is given by A/2pl 2, where A is the opening area and l is the opening-to-target distance. In addition, at the entrance opening, a negative 300-V bias potential is placed to prevent any low-energy electrons from leaving or entering the cup. The collimator, at ground potential, prevents the ion beam from hitting the aperture of the faraday cup. The collimator and the secondary electron repelling plate, as well as the faraday cup holder, were all made using aluminum. Small pieces of insulating ceramic were used to insulate the electron repelling plate from ground. The electron repelling plate was brought to a 300 V potential by connecting to an external battery. The faraday cup and the charge-collecting electrode was cut from a thin copper sheet and linked to an external integrator. The samples were fixed on the electrode by using carbon tape.
number and distribution of Si ions implanted. The RBS simulation software RUMP, together with the software SRIM, showed the fluence to be (9.9F0.2)1016/cm2.
3. Results and discussion Ion beam induced diffusion may be treated as a statistical process along the silicon ion track. The interaction of the fast silicon ion is almost entirely with the electrons in the target material. Many electrons are ejected and move far from the parent atoms, leaving a cylindrical region severely strained by coulomb forces. The subsequent displacement of the positively ionized target atoms has lateral and axial components near the cylindrical axis, near the silicon projectile track, with little or no memory of the direction of the projectile. The result is different from thermal diffusion because the displacements occur only near the fast ion track. However, as the fluence is increased (beyond about 1014 ions/cm2), such that area of the target is penetrated many times, the position of the target atoms will be governed by statistical processes analogous to thermally induced diffusion. Specifically, the vanadium atoms in the target will move opposite the gradient of the vanadium concentration (Fick’s law), and the vanadium concentration C will increase with added fast ion bombardment fluence only where the gradient in the x direction has a positive curvature, in accordance with the classical diffusion equation: dC d2 C ¼D 2 df dx where the left differential is relative to the fluence f, not the time. The ion induced mixing coefficient D has dimensions of m2 per fluence unit. Fig. 1 shows the RBS spectrum of a vanadium layer deposited sequentially between deposited SiO2 layers on a
2.1. Ion bombardment calibration The reliability of the charge collecting arrangement was confirmed by observing the RBS yield from a sample implanted with silicon to a fluence calculated from the integrated charge. A glassy polymeric carbon substrate was bombarded with an accumulated charge of 1 MeV Si+ ions corresponding to a fluence of 11017/cm2. An RBS analysis was next performed on the bombarded sample to determine
Fig. 1. RBS spectra of a multilayered sample untreated and bombarded with different 5 MeV Si fluences. The ion-induced mixing coefficient may be obtained from these spectra (see text), first by observing the decrease in amplitude of the signal from the nearly periodic layers, and then by observing the diffusion from the layers into the silica substrate.
G. Savigny et al. / Surface & Coatings Technology 196 (2005) 113–117
115
concentration. The dimension x is measured from a maximum in the spectrum. The diffusion equation is satisfied when with the following value of the function: 2
Fn ð f Þ ¼ Con exn Df The higher frequency components of the spectrum disappear quickly as fluence is accumulated just as for thermal diffusion. Moreover, the experimental RBS data in Fig. 1 clearly shows that the amplitude of the fundamental decreases with increasing fluence, about 20% after silicon ion bombardment to a fluence of 1016 ions/cm2. Thus, the coefficient in the exponential is x20 Df ¼ 0:22: Fig. 2. Optical spectra V2O3 multilayered sample after ion bombardment at fluences ranging from 11015 to 11017/cm2.
silica substrate. Fig. 1 also shows the spectrum of the same sample after bombardment with 5 MeV silicon ions to fluences of 1016, 31016 and 1017 ions/cm2. The silicon ions stop far beyond the vanadium layers. For the fluence 11016 ions/cm2, the vanadium layers are significantly diffused. The sinusoidal-like spectrum, already smoothed by instrumental resolution, is further reduced by bombardment. Representing the spectrum with the periodic function made up of the sum of a series of contributors Cn ¼ Fn ð f Þcosðxn xÞ; where x n is the spatial frequency of the one dimensional structure and F n ( f) is the amplitude of each component, expected to decrease with fluence f as vanadium is induced by ion bomdarment to diffuse out of regions of greater
The spatial frequency x 0=8107 rad/m is obtained from the interferometrically determined 79-nm spacing between vanadium layers. We calculate an approximate numerical value of the ion induced mixing coefficient D ¼ 0:35 1036 m2 m2 : As the fluence is increased to 31016 and 1017 silicon ions/cm2, the layer structure is essentially homogenized and acts as an exhaustible source for vanadium that diffuses into the silica substrate. Assuming an error function Erf (x 2/2r 2) to represent the profile of vanadium concentration near the interface with the substrate, Fig. 1 compares the observed concentration profiles after bombardment with the ones calculated. The argument 2r 2 of the error function is expressed in (channel)+2 in Fig. 2. Conversion of channels to nm yields r which increases from 35 to 78 nm as the fluence increases from 310p16ffiffiffiffiffiffi to 1017 silicon ions/m2. Using the relation r ¼ Df , the ion-induced mixing coefficient is determined to be 4.01036 m2 m2 at 31016
Fig. 3. Optical spectra VO multilayered sample after ion bombardment at fluences ranging from 11015 to 11017/cm2.
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Fig. 4. Optical spectra of V2O3 multilayered sample after heat treatment from 100 to 1000 8C.
Si ions/cm2 and 6.210 36 m2 m2 at 11017 Si ions/cm2. This fluence dependence of the mixing coefficient may be a consequence of accumulating damage in the substrate. The much lower mixing coefficient in the layers may indicate the crystalline nature of the deposited layers of silica. Values of the mixing coefficient for low fluence should scale linearly to other ions and energies in accordance with the linear electronic energy transferred by those ions to the target. We are accustomed to the thermal diffusion coefficient that has dimesions of m2/s. The distance an atom diffuses by random movements involves the square root of the diffusion
coefficient and the time. The ion-induced mixing coeficient D has dimensions of m2 per fluence unit, or m2 m2, certainly less familiar. We attempt to interpret D physically by first changing meters to nanometers, an appropriate atomic dimesion, such that the dimension may be written nm3 nm, and multiply the numerical value by 1036 nm4/m4. Thus, D is the distance in nanometers that one vanadium atom moves (or half the distance that two vanadium atom move) when one silicon projectile passes through a volume of one nm3 of SiO2. After many such movements assumed to be in random direction, the atoms are moved to a distance determined by the square root of the diffusion constant and
Fig. 5. Optical spectra of V2O3 multilayered sample after heat treatment from 100 to 1000 8C. Formation of vanadium metal nanocrystal after heat treatment to 600 8C is believed to cause the dramatic appearance of an absortion resonance at 410 nm.
G. Savigny et al. / Surface & Coatings Technology 196 (2005) 113–117
pffiffiffiffiffiffi the accumulated fluence r ¼ Df , similar to thermal diffusion. The numerical values 0.35, 4.0 and 6.2 nm3 nm for the ion-induced mixing constant show how hugely effective is ion-induced mixing. The optical spectra of VO and V2O3 layers (Figs. 2 and 3) show enhanced absorption near 300 nm where vanadium metal nanoclusters are expected to absorb light. Figs. 4 and 5 shows the OAP results after annealing from 100 to 1100 8C with 100 8C steps. Both graphs display a similar behavior. The major change appears at a temperature between 500 and 600 8C where there is a shift. Indeed, the peak at around 300 nm keeps growing in intensity until 500 8C and then shifts to 410 nm at 600 8C for both oxides. This leads us to think that VO and V2O3 both form the same vanadium oxide after heat treatment at 600 8C. At temperatures above 600 8C, the absorbance decreases and above 1000 8C, the samples are very much alike the substrate. The layers have been completely diffused and no longer reveal any characteristic absorption peak. Ion bombardment increases the optical absorption in the silica substrates. The effect of this damage has been effectively subtracted from the spectra shown by using equally bombarded silica substrates as a reference. The incremental absorption due to bombardment of the surface layers is presented in the figures.
4. Conclusion The ion-induced mixing coefficient was formally defined and measured as a function of fluence for vanadium in silica. Values of 0.35, 4.0 and 6.21036 m2 m2 at fluences
117
of 1, 3 and 101016 of 5 MeV silicon ions/cm2 were determined and show that mixing is enhanced as bombardment damage accumulates. The new optical absorption peak at 410 nm that appears only after heat treatment of the vanadium oxide layers to 600 8C may indicate that the vanadium atoms that diffuse into silica not only form nanocrystal, but increase the refractive index in their neighborhood.
Acknowledgements This project was supported by the Center For Irradiation of materials at Alabama A&M University and the Department of Defense. The authors are grateful to Professor Lawrence Holland for exploiting the parallel between thermal diffusion and ion induced mixing.
References [1] L. Gea, S. Honda, L.A. Boatner, T.E. Haynes, B.C. Sales, F.A. Modine, A. Meldrum, J.D. Budai, L. Beckers, Mater. Res. Soc. Symp. Proc. 501 (1998). [2] D.H. Thanh, P.D. Long, V.T. Bich, N.N. Dinh, Commun. Phys. 8 (3) (1998) 152. [3] Al Meldrum, Richard F. Haglund Jr., L.A. Boatner, C.W. White, Adv. Mater. (19) (2001). [4] Handbook of modern ion beam materials analysis—J.R. Tesmer, M. Nastasi, J.C. Barbour, C.J. Maggiore, J.W. Maye; Materials Research Society. [5] A. Meldrum, L.A. Boatner, C.W. White, Nucl. Instrum. Methods Phys. Res., B 178 (2001) 7.