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Structural and optical studies of Co and Ti implanted sapphire
Nuclear Instruments and Methods in Physics Research B 207 (2003) 55–62 www.elsevier.com/locate/nimb
Structural and optical studies of Co and Ti implanted sapphire E. Alves
a,*
, C. Marques a, R.C. da Silva a, T. Monteiro b, J. Soares b, C. McHargue c, L.C. Ononye d, L.F. Allard d
a
c
Instituto Tecnol ogico e Nuclear, Dep. Fisica, EN 10, 2686-953 Sacav em, Portugal b Dep. Fısica da Universidade de Aveiro, 3810-193 Aveiro, Portugal Center for Materials Processing, University of Tennessee, Knoxville 37996-0750, USA d Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
Abstract Single crystals of a-Al2 O3 with different orientations were implanted with several fluences of Ti and Co ions. For low fluences both Ti and Co ions are fully incorporated in Al lattice sites and remain stable up to annealing temperatures of 1000 °C. For fluences of 5 1016 cm2 the implanted region becomes completely disordered (amorphous) for samples implanted with Ti while for Co the same condition is achieved only for higher fluences (2 1017 Coþ /cm2 ). The recovery of the implantation damage is almost complete after annealing at 1000 °C in either oxidizing or reducing atmospheres for fluences below 5 1016 cm2 . For higher fluences annealing in a reducing ambient promotes the precipitation of crystalline metallic Co and Ti particles, as revealed by TEM and RBS. These precipitates retard the damage recovery. The presence of oxygen during annealing leads to the formation of mixed Co and Al oxides through the entire implanted region. Annealing at 1000 °C promotes the formation of a spinel phase (Al2 CoO4 ) and the blue or green coloration of sapphire, depending on the Co fluence. Moreover, narrow red emission lines were observed. For Ti the oxides concentrate at the surface and optical absorption measurements show the presence of a broad absorption band centered at 325 nm. This band is absent in the samples annealed in a reducing atmosphere. Photoluminescence measurements reveal the presence of an emission band centered near 840 nm. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 42.70.Hj; 61.72.Ww Keywords: Nanoparticles; a-Al2 O3 ; Optical properties
1. Introduction Sapphire has a large range of technological applications due to its great chemical stability and
*
Corresponding author. Tel.: +351-21-9946086; fax: +35121-9941525. E-mail address: [email protected] (E. Alves).
high electrical resistivity. In the past, most of the work done in sapphire aimed at the improvement of its mechanical properties using ion implantation [1–3]. This nonequilibrium doping process often leads to compositions and structures impossible to obtain by conventional processes. However the doping by ion implantation introduces lattice defects, and the metastable states thus formed can be relaxed through thermal annealing in a variety of atmospheres. The new microstructures developed
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00522-6
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in the implanted region are strongly dependent on the annealing conditions [4–6]. In addition to the tribological changes a broad range of new optical and magnetic properties can be produced in this way. These new synthesized composite materials have great potential applications ranging from quantum dot lasers [7] to nonlinear optical devices [7], and have received a great deal of recent interest [8,9]. Most of these properties are related with the precipitation of nanosized particles (metallic or semiconducting) dispersed in the sapphire matrix [10,11]. To control the formation of such nanoparticles in a-Al2 O3 it is necessary to study the interactions between the implanted species and implantation-induced defects, and the role of the annealing atmosphere. On the other hand, transition metal impurities are among the most important optically active ions in sapphire that give rise to luminescence bands from the infrared to the visible region [12–14]. In this work we studied the optical and structural modifications created in sapphire due to the implantation of Ti and Co ions, prior to and after annealing in oxidizing and reducing atmospheres.
2. Experimental details Sapphire single crystals with (0 0 0 1), (1 1 2 0) and (0 2 2 1) orientations were implanted at room temperature (RT) with 100 keV titanium ions or 150 keV cobalt ions with fluences in the range of 1 1015 cm2 to 5 1017 cm2 . Thermal annealings were carried out at 800 and 1000 °C for 1 and 3 h in reducing (p 6 107 mbar) and oxidizing atmospheres. After implantation and after each stage of thermal annealing, RBS/channeling and TEM studies were performed to study the structural changes induced by the annealings. A 2.0 MeV Heþ beam was used for RBS/channeling measurements and the backscattered particles were detected at 140° and 180° using silicon surface barrier detectors located in the standard IBM geometry, with resolutions of 13 and 18 keV, respectively. Detailed angular scans were done along the h0 0 0 1i, h1 1 2 0i and h0 2 2 1i axes to determine the lattice site location of implanted Co and Ti ions. Transmission
electron microscopy observations were performed on a Hitachi HF-2000 field emission electron microscope operating at 200 kV. Photoluminescence (PL) measurements were carried out with a 325 nm CW He–Cd laser with an excitation power density typically less than 0.6 W cm2 . A 325 nm band pass filter was used to attenuate lines other than the 325 nm laser line. The PL was measured between 20 K and RT using a closed cycle helium cryostat. The luminescence was dispersed by a Spex 1704 monochromator (1 m, 1200 mm1 ) and detected by a cooled Hamamatsu R928 photomultiplier. All the presented data are corrected to the spectral photomultiplier response. The optical absorption studies were carried with a Varian Cary 5G spectrophotometer in the UV–Vis–NIR range.
3. Results and discussion 3.1. Co implantation Detailed angular scans along the main crystallographic directions show that for low concentrations (fluences of 1015 Coþ /cm2 ) the Co ions are incorporated into Al lattice, assuming the trivalent charge state (Co3þ ) in order to maintain the charge neutrality [15]. Amorphization of the implanted region is only achieved for fluences higher than 5 1016 Coþ /cm2 as can be concluded from the aligned RBS spectra shown in Fig. 1. The complete overlap between the random and aligned spectra through the entire implanted region (first 215 nm) was observed for the sample implanted with 2 1017 Coþ /cm2 . The damage starts to anneal at 800 °C whether the annealing was performed in oxidizing or reducing atmosphere. The damage recovery is almost complete after annealing at 1000 °C in samples where the amorphization threshold was not reached. Fig. 2 shows the random and h0 2 2 1i aligned spectra of the sample implanted with 5 1016 Coþ / cm2 , where two buried damage regions limiting the Co profile are visible in the aligned spectra. The broadening of Co towards the surface is also evident and more pronounced for the sample annealed in oxidizing ambient. These fine details are
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 207 (2003) 55–62
Fig. 1. Random and h0 0 0 1i aligned RBS spectra showing the damage evolution for sapphire samples implanted with 1 1015 and 5 1016 Coþ /cm2 .
Fig. 2. Random and h0 2 2 1i aligned RBS spectra showing the annealing behavior for a sample implanted with 5 1016 Coþ / cm2 (a) after annealing for 1 h at 1000 °C in an oxidizing ambient and (b) after annealing for 1 h at 1000 °C in a reducing ambient. The arrows in the aligned spectrum indicate the presence of two damage region around the Co profile.
well evidenced due to the increased depth resolution achieved for this large tilt direction (52° with
57
respected to the surface normal). In contrast, samples implanted with higher fluences develop completely different microstructures after annealing at 1000 °C, depending on the annealing atmosphere (Fig. 3). When the annealing is performed in oxidizing ambient the Co diffuses through the entire damage region creating a box like profile. The simulation of the random spectrum (solid line) indicates the formation of a spinel compound Al2 CoO4 . This compound was also identified by XRD measurements and it is obvious that the presence of free oxygen makes possible the formation of this new structure. A completely different behavior is observed when the annealing occurs in reducing atmosphere. The strong narrowing of the Co profile (Fig. 3(b)) suggests the formation of a Co rich structure. The angular scans through the h1 1 2 0i, h0 0 0 1i and h0 2 2 1i directions indicate that these precipitates are partially coherent with the sapphire. These results are shown in Fig. 4 and it is evident that
Fig. 3. Random and h0 2 2 1i aligned RBS spectra showing the annealing behavior for a sample implanted with 2 1017 Coþ / cm2 (a) after annealing for 1 h at 1000 °C in an oxidizing ambient and (b) after annealing for 1 h at 1000 °C in a reducing ambient. The continuous line represents results simulated using the computer code RUMP.
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Fig. 5. Optical absorption spectra (dashed lines) of sapphire implanted with Coþ annealed for 1 h at 1000 °C in vacuum: (a) sample implanted with 5 1016 ions/cm2 ; (b) sample implanted with 2 1017 ions/cm2 . Full line: RT PL spectra of samples (a) and (b) obtained under He–Cd excitation.
Fig. 4. Angular scans through the h1 1 2 0i, h0 0 0 1i and h0 2 2 1i axis for a sample implanted with 2 1017 Coþ /cm2 after annealing for 1 h at 1000 °C in a reducing ambient.
while along the h1 1 2 0i axis we have a perfect overlap between the Co and Al curves, along the c-axis there is a small misalignment (0.15°). When we look along the tilted h0 2 2 1i axis the difference between the minima of Co and Al curves is 0.8°, which is compatible with the formation of Co precipitates. Similar results were found for Er in sapphire [16]. The formation of different structures is also revealed by the optical properties of the samples. In fact a metallic coloration is observed on the samples annealed in reducing ambient while the samples annealed in oxidizing ambient display a blue/green coloration depending on the Co concentration. Fig. 5 shows a comparison between the absorption and PL spectra observed at RT for samples implanted with fluences of 5 1016 and 2 1017 cm2 . The strongest absorption band occurs mainly in the
ultraviolet region. The PL observed at RT under He–Cd (325 nm) excitation shows two main luminescence regions in the UV-blue and in the red. Since no narrow absorption bands were observed even at low temperatures we can conclude that the optical centers, where the observed luminescence originates, cannot be attributed to Co2þ and Co3þ in sapphire [17,18]. Also the observed luminescence occurs at higher energies than these absorption bands, which excludes the hypothesis that the lu-
Fig. 6. Random and h0 0 0 1i aligned RBS spectra showing the damage evolution for sapphire samples implanted with 1 1015 and 5 1016 Tiþ /cm2 .
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Fig. 7. Micrograph of cross-sectioned sample implanted with 1 1017 Tiþ /cm2 annealed 1 h at 1000 °C in reducing atmosphere.
minescence originates on these ions. Fluorescence imaging results suggest that the luminescent centers
are related with the precipitates occurring in the near surface region.
Fig. 8. Random and aligned RBS spectra showing the annealing behavior for a sample implanted with 1 1017 Tiþ /cm2 (a) after implantation (b) after annealing for 3 h at 1000 °C in an oxidizing ambient and (c,d) after annealing for 3 h at 1000 °C in a reducing atmosphere.
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3.2. Ti implantation According to previous lattice site location studies [19] the complete overlap of the Al and Ti curves along the principal axial directions indicates the complete substitutionality of the implanted ions in Al sites of the a-Al2 O3 lattice (for low implanted ion concentrations). The increase of the damage with the fluence of the implanted ions is shown in Fig. 6. The overlap of the random and aligned spectra is observed after an implantation fluence of 5 1016 Tiþ /cm2 . This lower value for the amorphization threshold found for Ti, despite its lower mass compared with Co, clearly indicate that the chemical nature of the implanted ions plays a role in the amorphization process [20]. The behavior of the sample implanted with 1 1017 Tiþ /cm2 during isochronal (60 min) annealings in air revealed the segregation of Ti towards the surface while annealing in vacuum lead to the narrowing of the profile around the range (Rp) of the implanted ions [19]. The TEM photographs shown in Fig. 7 show the presence of a buried band of metallic Ti precipitates for the samples annealed in reducing atmosphere, parallel to the (0 0 0 1) surface. This kind of orientation was also observed for Fe nanocrystals in a-Al2 O3 obtained using the same procedure [21]. The surface region behind the Ti profile shows no trace of channeling, suggesting a polycrystalline phase. The presence of oxygen during the annealing allows the complete recrystallization of the damage region accompanied by the segregation of Ti to the surface where it oxidizes. This agrees with XPS analysis which indicates that titanium at the surface is in the Ti3þ oxidation state, characteristic of Ti2 O3 . Despite the observed differences the Ti ions are completely random whether we use a reducing or oxidizing annealing atmosphere. Longer annealing (180 min) at 1000 °C in oxygen ambient results in a decrease in the amount of Ti (Fig. 8(b)). The same annealing in vacuum produces some out-diffusion of Ti to the surface, which is clearly seen in the spectra obtained for the tilted direction (Fig. 8(c)). It is evident that for longer annealing the Ti precipitates start to crystallize and a channeling effect is visible in the aligned spectra along both the
Fig. 9. Angular scans through the h0 0 0 1i and h0 2 2 1i axis for a sample implanted with 1 1017 Tiþ /cm2 after annealing for 3 h at 1000 °C in a reducing ambient.
h0 0 0 1i and h0 2 2 1i directions. The complete angular scans (Fig. 9), show in this case that there is an alignment along the c-axis, while some shift and broadening is observed for the h0 2 2 1i axis. The optical properties of the samples are also dependent on the annealing atmosphere. The samples annealed in vacuum do not show any luminescence in the near infrared region [22]. On the contrary, the results shown in Fig. 10 compare
Fig. 10. RT optical absorption spectra (dashed line) and PL spectra (full line) of sapphire sample implanted with 1 1017 Tiþ /cm2 and annealed 1 h at 1000 °C in air.
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with the observed optically active centre, indicating that this band is related to a different defect. As shown by RBS and TEM analysis Ti oxides form at the specimen surface under our synthesis conditions. It is known that deep emission bands are observed by PL in TiO2 with band maxima located between 850 and 870 nm [25,26]. These optical centers show a different behavior with time and temperature from that observed in our Ti implanted samples, which we tentatively assign to a new Ti-related defect. 4. Conclusions Fig. 11. 20 K PL spectra of a sapphire sample implanted with 1 1017 Tiþ /cm2 annealed 1 h at 1000 °C in air and excited with He–Cd laser. Inset: high resolution spectra of the high energy side.
the absorption and PL spectra measured at RT for samples annealed in air. The absorption spectrum shows a dominant band at 325 nm and no doublehumped blue–green band is observed as expected from Ti3þ absorption [17,23]. The PL observed under excitation at the maximum of the absorption band reveals a broad band in the near infrared region peaked near 840 nm (region where our detector begins to cut off). At 20 K and under He–Cd (325 nm) excitation a structured luminescence emission band in the low wavelength region occurs as shown in Fig. 11. Weak zero-phonon lines at 782.4, 782.6 and 782.9 nm are observed as indicated in the inset of the figure. The integrated intensity of the broad emission increases four times in the temperature range of 20–220 K. Above 220 K the intensity decreases and the value at 270 K is nearly identical to the one observed at low temperature. The nature of the optical centre responsible for this emission cannot be related with the Ti3þ luminescence band [24] as its characteristics differ from those observed in absorption and PL spectra [17,22]. No double-humped absorption on the blue–green region was found and the observed PL is shifted to lower energies when compared with Ti3þ emission. The typical zero-phonon lines and vibronic structure associated with Ti3þ band and the intensity dependence on temperature of Ti3þ (even when excited in the maxima of a charge transfer band at 270 nm) [24] cannot be correlated
For low doses (up to 1 1015 cm2 ) all the Ti and Co ions are incorporated into Al substitutional sites and remain stable up to a temperature of 1000 °C. A fluence of 5 1016 Tiþ /cm2 creates a continuous amorphous layer through the implanted region while for Co, higher fluences are necessary (2 1017 Coþ /cm2 ). Ambient atmosphere plays an important role in the annealing process of the implantation damage and final microstructure. For fluences of 2 1017 Coþ /cm2 , annealing in oxidizing atmosphere leads to the formation of a Al2 CoO4 spinel while a reducing atmosphere induces the precipitation of metallic Co. The optical absorption band in all the Co doped samples occurs mainly in the ultraviolet region. The PL observed at RT under He–Cd (325 nm) excitation shows two main luminescence regions in the UVblue and in the red. In the Ti implanted samples the formation of metallic precipitates was also observed in the samples annealed in vacuum, and the annealing in oxidizing ambient leads to the formation of Ti oxides at the surface. The absorption spectra show a dominant band at 325 nm and PL observed under excitation at the maximum of the absorption band reveals a broad band in the near infrared region peaked near 840 nm. The samples annealed in vacuum do not show any luminescence in the near infrared region after annealing. Acknowledgements We acknowledge our Technician Jorge Rocha for the implantations and FCT for its support
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through the PRAXIS XXI under the contract C/ CTM/12067/1998.
References [1] C.W. White, C.J. McHargue, P.S. Sklad, L.A. Boatner, G.C. Farlow, Mater. Sci. Rep. 4 (1989) 41. [2] M.E. OÔHern, C.J. McHargue, C.W. White, G.C. Farlow, Nucl. Instr. and Meth. B 46 (1990) 171. [3] S.M.M. Ramos, B. Canut, L. Gea, P. Thevenard, M. Bauer, Y. Maheo, Ph. Kapsa, J.L. Loubet, J. Mater. Res. 7 (1) (1992) 178. [4] C.J. McHargue, J.D. Hunn, E. Alves, M.F. da Silva, J.C. Soares, Nucl. Instr. and Meth. B 166–167 (2000) 188. [5] G.C. Farlow, C.W. White, C.J. McHargue, B.R. Appleton, Mater. Res. Symp. Proc. 27 (1984) 395. [6] C.J. McHargue, G.C. Farlow, C.W. White, J.M. Williams, B.R. Appleton, H. Naramoto, Mater. Sci. Eng. 69 (1985) 123. [7] A. Meldrum, R.F. Haglund Jr., L.A. Boatner, C.W. White, Adv. Mater. 13 (19) (2001) 1431. [8] G.N. van den Hoven, E. Snoeks, A. Polman, J.W.M. van Uffelen, Y.S. Oei, M.K. Smit, Appl. Phys. Lett. 62 (1993) 3065. [9] A. Polman, J. Appl. Phys. 82 (1997) 1. [10] C.W. White, J.D. Budai, S.P. Withrow, J.G. Zhu, E. Sonder, R.A. Zhur, A. Meldrum, D.M. Hembree, D.O. Henderson, S. Prawer, Nucl. Instr. and Meth. B 141 (1998) 228. [11] E.M. Hunt, J.M. Hampikian, D.B. Poker, N.D. Evans, Surf. Coat. Technol. 103–104 (1998) 409.
[12] J.C. McCallum, L.D. Morphet, Nucl. Instr. and Meth. B 148 (1999) 726. [13] P.D. Townsend, P.J. Chandler, L. Zhang, Optical Effects of Ion Implantation, Cambridge University Press, 1994, p. 127. [14] H.H. Tippins, Phys. Rev. B 1 (1970) 126. [15] C. Marques, M.M. Cruz, R.C. da Silva, E. Alves, Nucl. Instr. and Meth. B 175–177 (2001) 500. [16] E. Alves, R.C. Silva, M.F. da Silva, J.C. Soares, Nucl. Instr. and Meth. B 139 (1998) 313. [17] D.S. McClure, J. Chem. Phys. 36 (1962) 2757. [18] R. Muller, Hs.H. Gunthard, J. Chem. Phys. 44 (1966) 365. [19] C. Marques, E. Alves, C. McHargue, L.C. Ononye, T. Monteiro, J. Soares, L.F. Allard, Nucl. Instr. and Meth. B 191 (2002) 644. [20] E. Alves, M.F. da Silva, J.G. Marques, J.C. Soares, K. Freitag, Nucl. Instr. and Meth. B 141 (1998) 353. [21] E. Alves, C. McHargue, R.C. Silva, C. Jesus, O. Conde, M.F. da Silva, J.C. Soares, Surf. Coat. Technol. 128–129 (2000) 434. [22] S. Garcia-Revilla, F. Rodriguez, R. Valiente, M. Pollnau, J. Phys.: Condens. Matter 14 (2002) 447. [23] B.F. Gachter, J.A. Koningstein, J. Chem. Phys. 60 (1974) 2003. [24] W.C. Wong, D.S. McClure, S.A. Basun, M.R. Kokta, Phys. Rev. B 51 (1995) 5682. [25] L. Grabner, S.E. Stokowski, W.S. Brower Jr., Phys. Rev. B 21 (1970) 590. [26] A.K. Ghosh, R.B. Lauer, R.R. Addiss Jr., Phys. Rev. B 8 (1973) 4842.
Structural and optical studies of Co and Ti implanted sapphire E. Alves
a,*
, C. Marques a, R.C. da Silva a, T. Monteiro b, J. Soares b, C. McHargue c, L.C. Ononye d, L.F. Allard d
a
c
Instituto Tecnol ogico e Nuclear, Dep. Fisica, EN 10, 2686-953 Sacav em, Portugal b Dep. Fısica da Universidade de Aveiro, 3810-193 Aveiro, Portugal Center for Materials Processing, University of Tennessee, Knoxville 37996-0750, USA d Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
Abstract Single crystals of a-Al2 O3 with different orientations were implanted with several fluences of Ti and Co ions. For low fluences both Ti and Co ions are fully incorporated in Al lattice sites and remain stable up to annealing temperatures of 1000 °C. For fluences of 5 1016 cm2 the implanted region becomes completely disordered (amorphous) for samples implanted with Ti while for Co the same condition is achieved only for higher fluences (2 1017 Coþ /cm2 ). The recovery of the implantation damage is almost complete after annealing at 1000 °C in either oxidizing or reducing atmospheres for fluences below 5 1016 cm2 . For higher fluences annealing in a reducing ambient promotes the precipitation of crystalline metallic Co and Ti particles, as revealed by TEM and RBS. These precipitates retard the damage recovery. The presence of oxygen during annealing leads to the formation of mixed Co and Al oxides through the entire implanted region. Annealing at 1000 °C promotes the formation of a spinel phase (Al2 CoO4 ) and the blue or green coloration of sapphire, depending on the Co fluence. Moreover, narrow red emission lines were observed. For Ti the oxides concentrate at the surface and optical absorption measurements show the presence of a broad absorption band centered at 325 nm. This band is absent in the samples annealed in a reducing atmosphere. Photoluminescence measurements reveal the presence of an emission band centered near 840 nm. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 42.70.Hj; 61.72.Ww Keywords: Nanoparticles; a-Al2 O3 ; Optical properties
1. Introduction Sapphire has a large range of technological applications due to its great chemical stability and
*
Corresponding author. Tel.: +351-21-9946086; fax: +35121-9941525. E-mail address: [email protected] (E. Alves).
high electrical resistivity. In the past, most of the work done in sapphire aimed at the improvement of its mechanical properties using ion implantation [1–3]. This nonequilibrium doping process often leads to compositions and structures impossible to obtain by conventional processes. However the doping by ion implantation introduces lattice defects, and the metastable states thus formed can be relaxed through thermal annealing in a variety of atmospheres. The new microstructures developed
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00522-6
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in the implanted region are strongly dependent on the annealing conditions [4–6]. In addition to the tribological changes a broad range of new optical and magnetic properties can be produced in this way. These new synthesized composite materials have great potential applications ranging from quantum dot lasers [7] to nonlinear optical devices [7], and have received a great deal of recent interest [8,9]. Most of these properties are related with the precipitation of nanosized particles (metallic or semiconducting) dispersed in the sapphire matrix [10,11]. To control the formation of such nanoparticles in a-Al2 O3 it is necessary to study the interactions between the implanted species and implantation-induced defects, and the role of the annealing atmosphere. On the other hand, transition metal impurities are among the most important optically active ions in sapphire that give rise to luminescence bands from the infrared to the visible region [12–14]. In this work we studied the optical and structural modifications created in sapphire due to the implantation of Ti and Co ions, prior to and after annealing in oxidizing and reducing atmospheres.
2. Experimental details Sapphire single crystals with (0 0 0 1), (1 1 2 0) and (0 2 2 1) orientations were implanted at room temperature (RT) with 100 keV titanium ions or 150 keV cobalt ions with fluences in the range of 1 1015 cm2 to 5 1017 cm2 . Thermal annealings were carried out at 800 and 1000 °C for 1 and 3 h in reducing (p 6 107 mbar) and oxidizing atmospheres. After implantation and after each stage of thermal annealing, RBS/channeling and TEM studies were performed to study the structural changes induced by the annealings. A 2.0 MeV Heþ beam was used for RBS/channeling measurements and the backscattered particles were detected at 140° and 180° using silicon surface barrier detectors located in the standard IBM geometry, with resolutions of 13 and 18 keV, respectively. Detailed angular scans were done along the h0 0 0 1i, h1 1 2 0i and h0 2 2 1i axes to determine the lattice site location of implanted Co and Ti ions. Transmission
electron microscopy observations were performed on a Hitachi HF-2000 field emission electron microscope operating at 200 kV. Photoluminescence (PL) measurements were carried out with a 325 nm CW He–Cd laser with an excitation power density typically less than 0.6 W cm2 . A 325 nm band pass filter was used to attenuate lines other than the 325 nm laser line. The PL was measured between 20 K and RT using a closed cycle helium cryostat. The luminescence was dispersed by a Spex 1704 monochromator (1 m, 1200 mm1 ) and detected by a cooled Hamamatsu R928 photomultiplier. All the presented data are corrected to the spectral photomultiplier response. The optical absorption studies were carried with a Varian Cary 5G spectrophotometer in the UV–Vis–NIR range.
3. Results and discussion 3.1. Co implantation Detailed angular scans along the main crystallographic directions show that for low concentrations (fluences of 1015 Coþ /cm2 ) the Co ions are incorporated into Al lattice, assuming the trivalent charge state (Co3þ ) in order to maintain the charge neutrality [15]. Amorphization of the implanted region is only achieved for fluences higher than 5 1016 Coþ /cm2 as can be concluded from the aligned RBS spectra shown in Fig. 1. The complete overlap between the random and aligned spectra through the entire implanted region (first 215 nm) was observed for the sample implanted with 2 1017 Coþ /cm2 . The damage starts to anneal at 800 °C whether the annealing was performed in oxidizing or reducing atmosphere. The damage recovery is almost complete after annealing at 1000 °C in samples where the amorphization threshold was not reached. Fig. 2 shows the random and h0 2 2 1i aligned spectra of the sample implanted with 5 1016 Coþ / cm2 , where two buried damage regions limiting the Co profile are visible in the aligned spectra. The broadening of Co towards the surface is also evident and more pronounced for the sample annealed in oxidizing ambient. These fine details are
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 207 (2003) 55–62
Fig. 1. Random and h0 0 0 1i aligned RBS spectra showing the damage evolution for sapphire samples implanted with 1 1015 and 5 1016 Coþ /cm2 .
Fig. 2. Random and h0 2 2 1i aligned RBS spectra showing the annealing behavior for a sample implanted with 5 1016 Coþ / cm2 (a) after annealing for 1 h at 1000 °C in an oxidizing ambient and (b) after annealing for 1 h at 1000 °C in a reducing ambient. The arrows in the aligned spectrum indicate the presence of two damage region around the Co profile.
well evidenced due to the increased depth resolution achieved for this large tilt direction (52° with
57
respected to the surface normal). In contrast, samples implanted with higher fluences develop completely different microstructures after annealing at 1000 °C, depending on the annealing atmosphere (Fig. 3). When the annealing is performed in oxidizing ambient the Co diffuses through the entire damage region creating a box like profile. The simulation of the random spectrum (solid line) indicates the formation of a spinel compound Al2 CoO4 . This compound was also identified by XRD measurements and it is obvious that the presence of free oxygen makes possible the formation of this new structure. A completely different behavior is observed when the annealing occurs in reducing atmosphere. The strong narrowing of the Co profile (Fig. 3(b)) suggests the formation of a Co rich structure. The angular scans through the h1 1 2 0i, h0 0 0 1i and h0 2 2 1i directions indicate that these precipitates are partially coherent with the sapphire. These results are shown in Fig. 4 and it is evident that
Fig. 3. Random and h0 2 2 1i aligned RBS spectra showing the annealing behavior for a sample implanted with 2 1017 Coþ / cm2 (a) after annealing for 1 h at 1000 °C in an oxidizing ambient and (b) after annealing for 1 h at 1000 °C in a reducing ambient. The continuous line represents results simulated using the computer code RUMP.
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Fig. 5. Optical absorption spectra (dashed lines) of sapphire implanted with Coþ annealed for 1 h at 1000 °C in vacuum: (a) sample implanted with 5 1016 ions/cm2 ; (b) sample implanted with 2 1017 ions/cm2 . Full line: RT PL spectra of samples (a) and (b) obtained under He–Cd excitation.
Fig. 4. Angular scans through the h1 1 2 0i, h0 0 0 1i and h0 2 2 1i axis for a sample implanted with 2 1017 Coþ /cm2 after annealing for 1 h at 1000 °C in a reducing ambient.
while along the h1 1 2 0i axis we have a perfect overlap between the Co and Al curves, along the c-axis there is a small misalignment (0.15°). When we look along the tilted h0 2 2 1i axis the difference between the minima of Co and Al curves is 0.8°, which is compatible with the formation of Co precipitates. Similar results were found for Er in sapphire [16]. The formation of different structures is also revealed by the optical properties of the samples. In fact a metallic coloration is observed on the samples annealed in reducing ambient while the samples annealed in oxidizing ambient display a blue/green coloration depending on the Co concentration. Fig. 5 shows a comparison between the absorption and PL spectra observed at RT for samples implanted with fluences of 5 1016 and 2 1017 cm2 . The strongest absorption band occurs mainly in the
ultraviolet region. The PL observed at RT under He–Cd (325 nm) excitation shows two main luminescence regions in the UV-blue and in the red. Since no narrow absorption bands were observed even at low temperatures we can conclude that the optical centers, where the observed luminescence originates, cannot be attributed to Co2þ and Co3þ in sapphire [17,18]. Also the observed luminescence occurs at higher energies than these absorption bands, which excludes the hypothesis that the lu-
Fig. 6. Random and h0 0 0 1i aligned RBS spectra showing the damage evolution for sapphire samples implanted with 1 1015 and 5 1016 Tiþ /cm2 .
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Fig. 7. Micrograph of cross-sectioned sample implanted with 1 1017 Tiþ /cm2 annealed 1 h at 1000 °C in reducing atmosphere.
minescence originates on these ions. Fluorescence imaging results suggest that the luminescent centers
are related with the precipitates occurring in the near surface region.
Fig. 8. Random and aligned RBS spectra showing the annealing behavior for a sample implanted with 1 1017 Tiþ /cm2 (a) after implantation (b) after annealing for 3 h at 1000 °C in an oxidizing ambient and (c,d) after annealing for 3 h at 1000 °C in a reducing atmosphere.
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3.2. Ti implantation According to previous lattice site location studies [19] the complete overlap of the Al and Ti curves along the principal axial directions indicates the complete substitutionality of the implanted ions in Al sites of the a-Al2 O3 lattice (for low implanted ion concentrations). The increase of the damage with the fluence of the implanted ions is shown in Fig. 6. The overlap of the random and aligned spectra is observed after an implantation fluence of 5 1016 Tiþ /cm2 . This lower value for the amorphization threshold found for Ti, despite its lower mass compared with Co, clearly indicate that the chemical nature of the implanted ions plays a role in the amorphization process [20]. The behavior of the sample implanted with 1 1017 Tiþ /cm2 during isochronal (60 min) annealings in air revealed the segregation of Ti towards the surface while annealing in vacuum lead to the narrowing of the profile around the range (Rp) of the implanted ions [19]. The TEM photographs shown in Fig. 7 show the presence of a buried band of metallic Ti precipitates for the samples annealed in reducing atmosphere, parallel to the (0 0 0 1) surface. This kind of orientation was also observed for Fe nanocrystals in a-Al2 O3 obtained using the same procedure [21]. The surface region behind the Ti profile shows no trace of channeling, suggesting a polycrystalline phase. The presence of oxygen during the annealing allows the complete recrystallization of the damage region accompanied by the segregation of Ti to the surface where it oxidizes. This agrees with XPS analysis which indicates that titanium at the surface is in the Ti3þ oxidation state, characteristic of Ti2 O3 . Despite the observed differences the Ti ions are completely random whether we use a reducing or oxidizing annealing atmosphere. Longer annealing (180 min) at 1000 °C in oxygen ambient results in a decrease in the amount of Ti (Fig. 8(b)). The same annealing in vacuum produces some out-diffusion of Ti to the surface, which is clearly seen in the spectra obtained for the tilted direction (Fig. 8(c)). It is evident that for longer annealing the Ti precipitates start to crystallize and a channeling effect is visible in the aligned spectra along both the
Fig. 9. Angular scans through the h0 0 0 1i and h0 2 2 1i axis for a sample implanted with 1 1017 Tiþ /cm2 after annealing for 3 h at 1000 °C in a reducing ambient.
h0 0 0 1i and h0 2 2 1i directions. The complete angular scans (Fig. 9), show in this case that there is an alignment along the c-axis, while some shift and broadening is observed for the h0 2 2 1i axis. The optical properties of the samples are also dependent on the annealing atmosphere. The samples annealed in vacuum do not show any luminescence in the near infrared region [22]. On the contrary, the results shown in Fig. 10 compare
Fig. 10. RT optical absorption spectra (dashed line) and PL spectra (full line) of sapphire sample implanted with 1 1017 Tiþ /cm2 and annealed 1 h at 1000 °C in air.
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with the observed optically active centre, indicating that this band is related to a different defect. As shown by RBS and TEM analysis Ti oxides form at the specimen surface under our synthesis conditions. It is known that deep emission bands are observed by PL in TiO2 with band maxima located between 850 and 870 nm [25,26]. These optical centers show a different behavior with time and temperature from that observed in our Ti implanted samples, which we tentatively assign to a new Ti-related defect. 4. Conclusions Fig. 11. 20 K PL spectra of a sapphire sample implanted with 1 1017 Tiþ /cm2 annealed 1 h at 1000 °C in air and excited with He–Cd laser. Inset: high resolution spectra of the high energy side.
the absorption and PL spectra measured at RT for samples annealed in air. The absorption spectrum shows a dominant band at 325 nm and no doublehumped blue–green band is observed as expected from Ti3þ absorption [17,23]. The PL observed under excitation at the maximum of the absorption band reveals a broad band in the near infrared region peaked near 840 nm (region where our detector begins to cut off). At 20 K and under He–Cd (325 nm) excitation a structured luminescence emission band in the low wavelength region occurs as shown in Fig. 11. Weak zero-phonon lines at 782.4, 782.6 and 782.9 nm are observed as indicated in the inset of the figure. The integrated intensity of the broad emission increases four times in the temperature range of 20–220 K. Above 220 K the intensity decreases and the value at 270 K is nearly identical to the one observed at low temperature. The nature of the optical centre responsible for this emission cannot be related with the Ti3þ luminescence band [24] as its characteristics differ from those observed in absorption and PL spectra [17,22]. No double-humped absorption on the blue–green region was found and the observed PL is shifted to lower energies when compared with Ti3þ emission. The typical zero-phonon lines and vibronic structure associated with Ti3þ band and the intensity dependence on temperature of Ti3þ (even when excited in the maxima of a charge transfer band at 270 nm) [24] cannot be correlated
For low doses (up to 1 1015 cm2 ) all the Ti and Co ions are incorporated into Al substitutional sites and remain stable up to a temperature of 1000 °C. A fluence of 5 1016 Tiþ /cm2 creates a continuous amorphous layer through the implanted region while for Co, higher fluences are necessary (2 1017 Coþ /cm2 ). Ambient atmosphere plays an important role in the annealing process of the implantation damage and final microstructure. For fluences of 2 1017 Coþ /cm2 , annealing in oxidizing atmosphere leads to the formation of a Al2 CoO4 spinel while a reducing atmosphere induces the precipitation of metallic Co. The optical absorption band in all the Co doped samples occurs mainly in the ultraviolet region. The PL observed at RT under He–Cd (325 nm) excitation shows two main luminescence regions in the UVblue and in the red. In the Ti implanted samples the formation of metallic precipitates was also observed in the samples annealed in vacuum, and the annealing in oxidizing ambient leads to the formation of Ti oxides at the surface. The absorption spectra show a dominant band at 325 nm and PL observed under excitation at the maximum of the absorption band reveals a broad band in the near infrared region peaked near 840 nm. The samples annealed in vacuum do not show any luminescence in the near infrared region after annealing. Acknowledgements We acknowledge our Technician Jorge Rocha for the implantations and FCT for its support
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through the PRAXIS XXI under the contract C/ CTM/12067/1998.
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