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Transmission electron microscopy study of high-dose iron-implanted sapphire
Materials Characterization 102 (2015) 19–23
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Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Transmission electron microscopy study of high-dose iron-implanted sapphire Y. Wang ⁎, X.P. Liu, G.W. Qin Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110004, China
a r t i c l e
i n f o
Article history: Received 3 November 2014 Received in revised form 14 February 2015 Accepted 17 February 2015 Available online 19 February 2015 Keywords: Sapphire Ion implantation Transmission electron microscopy Coincidence of reciprocal lattice point method
a b s t r a c t The microstructure of α-Al2O3 implanted with iron at energy of 50 keV to a dose of 1 × 1017 ions/cm2 at room temperature has been studied. The iron concentration profile shows a well-defined peak at ~50 nm from the surface, in agreement with the calculated value. The implanted Fe ions precipitate as the α-Fe particles during annealing in a reducing atmosphere. The voids around α-Fe particles and some large iron particles both have faceted outlines. Through the analysis of the composite selected-area electron diffraction pattern (SAED) and Moiré fringes, the orientation relationship (OR) between α-Fe particles and sapphire was determined to be (111)α-Fe//(0001)sapphire and [110]α-Fe//[1120]sapphire. Some deviation from this OR was detected in a few α-Fe particles, which is less than 3°. The coincidence of reciprocal lattice point (CRLP) method was utilized to confirm that this OR is preferred in the α-Fe/sapphire system. © 2015 Elsevier Inc. All rights reserved.
1. Introduction Implantation has shown considerable promise as a means of modifying the near-surface properties of a wide range of materials such as dielectric, semiconductor and ferroelectric. For the dielectric materials such as MgO and α-Al2O3, ion implantation has been used to create ferromagnetic nanoscale composites consisting of Fe, Ni or Co precipitate embedded in the dielectric host, which have several attractive characteristics for potential applications in magnetic data storage media [1–6]. For the semiconductor such as SiGe and In2O3, Fe and Co ions were implanted to prepare diluted magnetic semiconductors, for potential applications in spintronics with the purpose of exploiting the charge and spin properties of electrons [7–10]. For the ferroelectric materials such as BaTiO3 and SrTiO3, Fe ions were implanted to prepare multiferroic materials with magnetoelectric properties, which provide opportunities for potential applications in information technologies, radioelectronics, optoelectronics, and microwave electronics [11,12]. Among the dielectric hosts, sapphire (α-Al2O3) has received much attention due to its high chemical and thermal stability, high dielectric constant and resistance to oxidation. Many researchers have investigated the microstructures of sapphire implanted by Co [1,3,6], Pt [5] and Fe [4,13–15]. In Co-implanted sapphire, it has been found that the Co nanoparticles can be formed as different shapes depending on the implantation conditions: the particles in the specimen implanted at room temperature are spheroidal in shape and the particles in the specimen implanted at −100 °C are well faceted, with good crystal faces parallel to the hexagonal axes of the sapphire [3]. In Fe-implanted sapphire, a ⁎ Corresponding author. E-mail address: [email protected] (Y. Wang).
http://dx.doi.org/10.1016/j.matchar.2015.02.011 1044-5803/© 2015 Elsevier Inc. All rights reserved.
lot of effort has been devoted to study the morphology of α-Fe precipitate depending on the implantation and annealing conditions [4,13–15]. McHargue et al. have found that some α-Fe platelets several hundreds nanometers long by 25–35 nm thick are produced forming a discontinuous band parallel to the surface and the length of the particles increases with the dose of the implanted ions under the conditions implanting at energy of 160 keV to a dose of 2 × 1017 ions/cm2 at room temperature and annealing at 1100 °C for 1 h in a reducing atmosphere [4]. M. Ohkubo et al. have found that some α-Fe particles are clearly faceted and characterized by the habit plane of sapphire under the conditions implanting at energy of 400 keV to a dose of 1 × 1017 ions/cm2 at room temperature and annealing at 1200 °C for 1 h in a reducing atmosphere [15]. In Pt-implanted sapphire, Santala et al. discovered that the orientation relationships (OR) between Pt particles and sapphire depended on the degree of crystallinity of the sapphire matrix: the Pt particles precipitated from sapphire had a different orientation relationship from the Pt particles precipitated from a transition alumina phase that formed in partially amorphized sapphire [5]. Precipitate–matrix ORs are important as they can affect material properties and microstructural development during processing. The orientation relationship between precipitate and matrix can be predicted by the coincidence of reciprocal lattice point (CRLP) method, which has been proved to be successful in analyzing ORs of two adjoining crystals [16–19]. The CRLP method was first proposed by Ikuhara et al. [16], in which the overlap of reciprocal lattice points (RLPs) of two adjoining crystals was utilized to obtain a geometrically optimum OR between the two crystals. In the CRLP method each RLP, corresponding to the reciprocal lattice vector g, is represented by a sphere of radius r⁎ around the reciprocal lattice point. It is then hypothesized that the OR favored between two adjoining crystals is the one in which the sum of the
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Y. Wang et al. / Materials Characterization 102 (2015) 19–23
overlapping volumes is maximized. Given the primitive unit cell parameters of the two lattices and the radius r⁎, the sum of all overlapping volumes V(φ) is calculated as a function of rotation angle φ between the two axes that are originally assumed to be parallel to each other. The summation in overlapping volumes is carried out over all RLPs up to a maximum distance, R⁎, in the composite reciprocal lattice. Then, V(φ) is plotted versus φ. In all of the cases that have been calculated, the main peaks in the plots represent the preferential ORs [16]. This paper presents the results of a transmission electron microscopy (TEM) investigation in an iron-implanted sapphire. Special emphasis is placed on the morphology of α-Fe precipitate and the OR between αFe precipitate and sapphire, because both the morphology of α-Fe precipitate and orientation relationships are sensitive to the implantation conditions. By the coincidence of reciprocal lattice point (CRLP) method, the orientation relationships are analyzed and discussed in more detail than previously reported for this system.
2. Experimental procedure High-purity α-Al2O3 single crystals having the c-axis normal to the surface were purchased from KMT Corporation. The wafers were polished to a mirror-like finish and were annealed at 1500 °C for 6 h in air to remove mechanical damage before ion implantation. Fe ions were implanted into (0001)-oriented sapphire wafers at energy of 50 keV to a dose of 1 × 1017 ions/cm2 at room temperature. The wafers were tilted 7° with respect to the ion beam to avoid channeling effects. The Fe-implanted samples were subsequently annealed under different temperature and time conditions (500 °C, 1000 °C and 1500 °C for temperature; one hour, three hours and six hours for time) in a reducing (4% hydrogen–argon mixture) atmosphere. Both plan-view and cross-sectional view specimens for TEM were thinned mechanically, and then thinned by ion milling. The plan-view specimen was thinned from the unimplanted side. The microscopy observation was carried out using a Tecnai F30 G2 field emission transmission electron microscope.
3. Results and discussion The bright-field image for a cross-sectioned specimen implanted to a dose of 1 × 1017 Fe ions/cm2 at room temperature is shown in Fig. 1. The corresponding selected-area electron diffraction pattern (SAED) confirms the crystalline nature of the implanted layer. Measurements of the iron content of this implanted region were made by X-ray EDS while operating in the scanning transmission electron microscopy (STEM) mode. The results are plotted in Fig. 2 as the ratio of iron to aluminum versus depth. The iron concentration profile shows a well-defined peak at ~ 50 nm from the surface, in agreement with the TRIM [20] calculated value of 51.6 nm, and a maximum Fe/Al atom fraction of ~21%. With annealing in a reducing atmosphere, the implanted Fe ions precipitate as the α-Fe particles. 1000 °C and 1 h is the critical point for α-Fe particle precipitation: less than this point, only a small quantity of particles precipitated; at this point, much more particles precipitated; more than this point, no further precipitation happened. Thus special emphasis was placed on the sample annealed under 1000 °C and 1 h condition. Fig. 3(a) shows the bright-field image of the plan-view specimen. It is shown that the α-Fe particles range in size from a few nanometers to greater than 20 nm. Some small α-Fe particles are attached to voids. McHargue et al. have proposed the mechanism for the formation of this association of a void with an iron particle: large numbers of Fe2+–oxygen vacancy complexes are present in the implanted samples and clustering of these vacancy–iron complexes would lead to the formation of iron particles and voids during annealing in a reducing atmosphere [4]. The large iron particles greater than 20 nm show an obvious hexagonal faceting. The SAED of the large iron particles greater than 20 nm is also shown in Fig. 3(a). In the diffraction pattern, there appear patterns from both α-Al2O3[0001] zone axis and α-Fe[111] zone axis, and also double-diffraction spots from α-Fe and α-Al2O3. It is evident that the orientation relationship between the large α-Fe particles and sapphire can be determined as: h i ð111Þα‐Fe ==ð0001Þsapphire and 110
α‐Fe
h i == 1120
sapphire
ð1Þ
and the sides of all the large α-Fe particles are parallel to (2110), (1120) and (1210) of sapphire. Fig. 3(b) shows the association of a void with a small α-Fe particle. As can be seen from the high-resolution image, the sides of the void are parallel to (2110), (1120) and (1210) of sapphire and the small αFe particle appears at the corner of the faceted void.
Fig. 1. Bright-field image and the corresponding SAED pattern of the cross-sectioned specimen implanted to a dose of 1 × 1017 Fe ions/cm2 at room temperature.
Fig. 2. Iron concentration profile obtained from X-ray EDS line scan across the implanted layer of the specimen shown in Fig. 1.
Y. Wang et al. / Materials Characterization 102 (2015) 19–23
Fig. 3. (a) Bright-field image and the corresponding SAED pattern of the plan-view specimen along α-Al2O3[0001] and α-Fe[111] zone axes. The diffraction spots from sapphire and α-Fe are marked by black stars and white arrows, respectively. (b) High-resolution image of the association of a void with a small α-Fe particle. (c) High-resolution image of a small iron particle about 4 nm formed in the sapphire matrix.
For the small iron particles just a few nanometers, it is difficult to determine the OR by the SAED. Moiré fringes which are formed by superposition of precipitate and matrix can also been utilized to determine the OR. Fig. 3(c) shows Moiré fringes inside a small iron particle about 4 nm. The Moiré fringes are parallel to (1120) of sapphire and the Moiré fringe spacing D is measured to be 1.4 nm. The plane spacing of α-Fe d2 can be calculated from Eq. (2) [21]: D¼
d1 d2 d1 −d2
ð2Þ
where D represents the Moiré fringe spacing and d1 represents the (1120) plane spacing of sapphire. The calculated value of d2 is 0.203 nm, which corresponds to the (110) plane spacing of α-Fe. Considering the Moiré fringes having a periodicity of 60° for rotation about [0001]sapphire, the OR between the small α-Fe particles and sapphire can be determined as the OR of Eq. (1), the same with the OR between the large α-Fe particles and sapphire. Fig. 4(a) shows the bright-field image of the cross-sectional view specimen. The cross-sectioned plane is parallel to (1120) of sapphire. As can been seen from the bright-field image, the small α-Fe particles are spheroidal in shape and the large α-Fe particles greater than 20 nm are clearly faceted. The SAED pattern of the large α-Fe particles greater than 20 nm is shown in Fig. 4(b). In the diffraction pattern, there appear patterns from both α-Al2O3[1120] zone axis and α-Fe[110] zone axis, and also double-diffraction spots from α-Fe and α-Al2O3. The OR between the α-Fe particles and sapphire can be determined using Eq. (1), the same
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Fig. 4. Bright-field image (a) and the corresponding SAED pattern (b) of cross-sectioned specimen, along α-Al2O3[1120] zone axis. (c) High-resolution image of three small α-Fe particles. (d) A schematic for the relationship between plane spacings, diffraction vectors and Moiré fringe spacing.
with the observation result in the plan-view specimen. Fig. 4(c) shows the high-resolution image of three small α-Fe particles. Moiré fringes inside the α-Fe particles are formed by superposition of (1104) planes of α-Al2O3 and (1 10) planes of α-Fe. The analyses of the Moiré fringes were carried out in order to detect the misorientation of the small α-Fe particles with respect to the α-Al2O3 matrix. The relationship between plane spacings, diffraction vectors and Moiré fringe spacing is schematically shown in Fig. 4(d). The (1 10) plane spacing of α-Fe (d2) and the angle included between the (1 10) planes of α-Fe and the (1104) planes of sapphire (α) can be calculated from Eqs. (3) and (4): 1 1 2 cosβ 1 þ þ ¼ 2 D d1 D2 d21 d2
ð3Þ
sinβ sinα ¼ D d2
ð4Þ
where D represents the Moiré fringe spacing, d1 represents the (1104) plane spacing of sapphire and β represents the angle included between the Moiré fringes and the (1104) planes of sapphire. The measurements of D and β in the high-resolution image were carried out using the fast Fourier transform of the Moiré contrast region. The error was estimated to be approximately ±0.0500 nm for D and ±1° for β, having an influence in the calculated values of ±0.0025 nm for d2 and ±1° for α [22]. The calculated values d2 of the particles 1, 2 and 3 are 0.203 nm, 0.201 nm and 0.202 nm, respectively, which all lie within the error (±0.0025 nm) of the theoretical ( 1 1 0) plane spacing of α-Fe (0.2027 nm). The calculated values α of the particles 1, 2 and 3 are
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Y. Wang et al. / Materials Characterization 102 (2015) 19–23
2.67°, 1.37° and 2.52°, respectively. Comparing the theoretical value of α (3°) we can determine the relative rotations about [1120]sapphire between three α-Fe particles and sapphire matrix to be 0.33°, 1.63° and 0.48°, respectively. Considering the experimental error ±1° for α it can be concluded that particle 1 and particle 3 have the OR of Eq. (1) with sapphire on the whole and particle 2 has a deviation of less than 2° from the OR of Eq. (1) with sapphire. More than fifty α-Fe particles have been analyzed by this method. Most of the particles have the OR of Eq. (1) with sapphire considering the experimental error. Only a small quantity of the α-Fe particles have the deviation from the OR of Eq. (1) beyond the experimental error and the maximum deviation is less than 3°. The existence of particles deviating from the OR of Eq. (1) means that the α-Fe particles did not reach full equilibration under 1000 °C and 1 h annealing condition. The samples annealed at higher temperatures and longer times were investigated and no further equilibration happened, which indicates that the particles deviating from the OR of Eq. (1) have no relationship with annealing temperature and time. Through the analysis above, it has been confirmed that the α-Fe particles ranging in size from a few nanometers to greater than 20 nm have the same OR of Eq. (1) with sapphire under the experimental conditions implanting at energy of 50 keV to a dose of 1 × 1017 ions/cm2 at room temperature and annealing at 1000 °C for 1 h in a reducing atmosphere. Another OR: h i ð110Þα‐Fe ==ð0001Þsapphire and ½111α‐Fe == 51 40
sapphire
ð5Þ
has been observed by McHargue et al. [4] under the conditions implanting at energy of 160 keV to a dose of 4 × 1016 ions/cm2 at room temperature and annealing at 1500 °C for 1 h in a reducing atmosphere. The CRLP method is utilized to determine which OR is the preferential one in the α-Fe/sapphire system. Given the primitive unit cell parameters of α-Fe and α-Al2O3, the sum of all overlapping volumes V(φ) is calculated as a function of rotation angle φ between the two axes that are originally assumed to be parallel to each other. Fig. 5 shows the plot of V(φ) versus φ for rotation about [0001]sapphire//[111]α-Fe. The calculation was started from the orientation as shown in Fig. 5(a). The low-index directions of the two crystals are set to be parallel, i.e. [0001]sapphire//[111]α-Fe and [1010]sapphire//[110]α-Fe. The calculations were preformed for r⁎ = 0.05a⁎, and for all RLPs to within R⁎ = 3a⁎, where a⁎ is the reciprocal lattice parameter of the α-Fe phase. The calculated result is shown in Fig. 5(b). The computed plot of V(φ) versus φ is periodic in φ with a periodicity of 60°. Therefore, only the part of V(φ) corresponding to φ = 0–60° is shown in Fig. 5(b). Since the angle between [1010] and [1120] is 30°, the main peak occurring at φ = 30° corresponds to the OR of Eq. (1). The reciprocal lattice
calculations are in good agreement with the experimental observations in this work. Fig. 6 shows the plot of V(φ) versus φ for rotation about [0001]sapphire//[110]α-Fe. The calculation was started from the orientation as shown in Fig. 6(a). The low-index directions of the two crystals are set to be parallel, i.e. [0001]sapphire//[110]α-Fe and [10 1 0] sapphire //[1 1 0] α-Fe . The calculated result is shown in Fig. 6(b). Since the angle between [1010] and [51 40] is 10.89°, the peak occurring at φ = 10.9° corresponds to the OR of Eq. (5) observed by McHargue et al. [4]. Comparing the peak value V(φ) (9.13 × 10 − 4 ) corresponding to the OR of Eq. (5) with the peak value V(φ) (2.3 × 10 − 3 ) corresponding to the OR of Eq. (1), it can be seen that the OR of Eq. (1), i.e. (111) α-Fe //(0001) sapphire and [110]α-Fe//[1120]sapphire, is preferred in the α-Fe/sapphire system and the OR of (110)α-Fe//(0001)sapphire and [111]α-Fe//[5140]sapphire is one of the secondary preferred orientation relationships. The existence of two different orientation relationships in the α-Fe/ sapphire system can be attributed to different implantation conditions. In McHargue's experiment [4], the implanting energy is 160 keV, which is much higher than the implanting energy (50 keV) in our experiment. The higher implanting energy can cause α-Al2O3 targets to be amorphized more severely. Amorphized α-Al2O3 can recrystallize and form some transition phases of Al2O3, such as θ-Al2O3 [5] and γ-Al2O3 [23], at the beginning of annealing. Since θ-Al2O3 and γ-Al2O3 have different space-groups and lattice constants from α-Al2O3, Fe particles precipitated from α-Al2O3 may have a different orientation relationship from Fe particles precipitated from θ-Al2O3 or γ-Al2O3 considering the coincidence of reciprocal lattice point (CRLP) theory discussed above. After α-Fe precipitates nuclear, the transition Al2O3 phases convert to sapphire by growth of the sapphire into the metastable transition Al2O3 phases and disappear finally [23], however α-Fe precipitates' orientation does not change [5]. In the end, α-Fe particles precipitated from sapphire have the OR of Eq. (1) in our experiment with low implanting energy, while α-Fe particles precipitated from a transition Al2O3 phase have the OR of Eq. (5) in McHargue's experiment with high implanting energy.
4. Conclusions Fe ions were implanted into α-Al2O3 single crystals (sapphire) at energy of 50 keV to a dose of 1 × 1017 ions/cm2 at room temperature. The microstructure in the implanted region was studied by transmission electron microscopy. The iron concentration profile obtained from X-ray EDS line scan shows a well-defined peak at ~50 nm from the surface, in agreement with the TRIM calculated value. With annealing in a reducing
Fig. 5. (a) Assumed OR from which the calculation started. (b) Plot of V(φ) versus φ for rotation about [0001]sapphire//[111]α-Fe. Note that the main peak at φ = 30° corresponds to the preferential OR of Eq. (1).
Y. Wang et al. / Materials Characterization 102 (2015) 19–23
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Fig. 6. (a) Assumed OR from which the calculation started. (b) Plot of V(φ) versus φ for rotation about [0001]sapphire//[110]α-Fe. Note that the peak at φ = 10.9° corresponds to the OR of Eq. (5).
atmosphere, the implanted Fe ions precipitate as the α-Fe particles. Some α-Fe particles are attached to voids. The large iron particles greater than 20 nm and some voids clearly have faceted outlines. Through the analysis of the composite SAED pattern and Moiré fringes, the orientation relationship between α-Fe particles and sapphire was determined to be (111)α-Fe//(0001)sapphire and [110]α-Fe//[1120]sapphire. This OR is predicted precisely by the coincidence of reciprocal lattice point (CRLP) method. The other OR of (110)α-Fe//(0001)sapphire and [111]α-Fe//[5140]sapphire reported before is confirmed by the same method to be one of the secondary preferred orientation relationships in the α-Fe/sapphire system. The existence of two different orientation relationships can be attributed to different implantation conditions. The α-Fe particles precipitated from α-Al2O3 have a different orientation relationship from the α-Fe particles precipitated from a transition Al2O3 phase. Acknowledgments This research was supported by the Fundamental Research Funds for the Central Universities (N130402003). References [1] E. Cattaruzza, F. Gonella, G. Mattei, P. Mazzoldi, D. Gatteschi, C. Sangregorio, M. Falconieri, G. Salvetti, G. Battaglin, Cobalt nanoclusters in silica glass: nonlinear optical and magnetic properties, Appl. Phys. Lett. 73 (1998) 1176. [2] S. Zhu, X. Xiang, X.T. Zu, L.M. Wang, Magnetic nano-particles of Ni in MgO single crystals by ion implantation, Nucl. Instrum. Methods B 242 (2006) 114–117. [3] A. Meldrum, L.A. Boatner, K. Sorge, Microstructure and magnetic properties of Co nanoparticles in ion-implanted Al2O3, Nucl. Instrum. Methods B 207 (2003) 36–44. [4] C.J. McHargue, S.X. Ren, P.S. Sklad, L.F. Allard, J. Hunn, Preparation of nanometer-size dispersions of iron in sapphire by ion implantation and annealing, Nucl. Instrum. Methods B 116 (1996) 173–177. [5] M.K. Santala, V. Radmilovic, R. Giulian, M.C. Ridgway, A.M. Glaesera, R. Gronsky, Precipitate orientation relationships in Pt-implanted sapphire, Scr. Mater. 62 (2010) 187–190. [6] Z. Werner, M. Pisarek, M. Barlak, R. Ratajczak, W. Starosta, J. Piekoszewski, W. Szymczyk, R. Grötzschel, Chemical effects in Zr- and Co-implanted sapphire, Vacuum 83 (2009) 57–60.
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Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Transmission electron microscopy study of high-dose iron-implanted sapphire Y. Wang ⁎, X.P. Liu, G.W. Qin Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110004, China
a r t i c l e
i n f o
Article history: Received 3 November 2014 Received in revised form 14 February 2015 Accepted 17 February 2015 Available online 19 February 2015 Keywords: Sapphire Ion implantation Transmission electron microscopy Coincidence of reciprocal lattice point method
a b s t r a c t The microstructure of α-Al2O3 implanted with iron at energy of 50 keV to a dose of 1 × 1017 ions/cm2 at room temperature has been studied. The iron concentration profile shows a well-defined peak at ~50 nm from the surface, in agreement with the calculated value. The implanted Fe ions precipitate as the α-Fe particles during annealing in a reducing atmosphere. The voids around α-Fe particles and some large iron particles both have faceted outlines. Through the analysis of the composite selected-area electron diffraction pattern (SAED) and Moiré fringes, the orientation relationship (OR) between α-Fe particles and sapphire was determined to be (111)α-Fe//(0001)sapphire and [110]α-Fe//[1120]sapphire. Some deviation from this OR was detected in a few α-Fe particles, which is less than 3°. The coincidence of reciprocal lattice point (CRLP) method was utilized to confirm that this OR is preferred in the α-Fe/sapphire system. © 2015 Elsevier Inc. All rights reserved.
1. Introduction Implantation has shown considerable promise as a means of modifying the near-surface properties of a wide range of materials such as dielectric, semiconductor and ferroelectric. For the dielectric materials such as MgO and α-Al2O3, ion implantation has been used to create ferromagnetic nanoscale composites consisting of Fe, Ni or Co precipitate embedded in the dielectric host, which have several attractive characteristics for potential applications in magnetic data storage media [1–6]. For the semiconductor such as SiGe and In2O3, Fe and Co ions were implanted to prepare diluted magnetic semiconductors, for potential applications in spintronics with the purpose of exploiting the charge and spin properties of electrons [7–10]. For the ferroelectric materials such as BaTiO3 and SrTiO3, Fe ions were implanted to prepare multiferroic materials with magnetoelectric properties, which provide opportunities for potential applications in information technologies, radioelectronics, optoelectronics, and microwave electronics [11,12]. Among the dielectric hosts, sapphire (α-Al2O3) has received much attention due to its high chemical and thermal stability, high dielectric constant and resistance to oxidation. Many researchers have investigated the microstructures of sapphire implanted by Co [1,3,6], Pt [5] and Fe [4,13–15]. In Co-implanted sapphire, it has been found that the Co nanoparticles can be formed as different shapes depending on the implantation conditions: the particles in the specimen implanted at room temperature are spheroidal in shape and the particles in the specimen implanted at −100 °C are well faceted, with good crystal faces parallel to the hexagonal axes of the sapphire [3]. In Fe-implanted sapphire, a ⁎ Corresponding author. E-mail address: [email protected] (Y. Wang).
http://dx.doi.org/10.1016/j.matchar.2015.02.011 1044-5803/© 2015 Elsevier Inc. All rights reserved.
lot of effort has been devoted to study the morphology of α-Fe precipitate depending on the implantation and annealing conditions [4,13–15]. McHargue et al. have found that some α-Fe platelets several hundreds nanometers long by 25–35 nm thick are produced forming a discontinuous band parallel to the surface and the length of the particles increases with the dose of the implanted ions under the conditions implanting at energy of 160 keV to a dose of 2 × 1017 ions/cm2 at room temperature and annealing at 1100 °C for 1 h in a reducing atmosphere [4]. M. Ohkubo et al. have found that some α-Fe particles are clearly faceted and characterized by the habit plane of sapphire under the conditions implanting at energy of 400 keV to a dose of 1 × 1017 ions/cm2 at room temperature and annealing at 1200 °C for 1 h in a reducing atmosphere [15]. In Pt-implanted sapphire, Santala et al. discovered that the orientation relationships (OR) between Pt particles and sapphire depended on the degree of crystallinity of the sapphire matrix: the Pt particles precipitated from sapphire had a different orientation relationship from the Pt particles precipitated from a transition alumina phase that formed in partially amorphized sapphire [5]. Precipitate–matrix ORs are important as they can affect material properties and microstructural development during processing. The orientation relationship between precipitate and matrix can be predicted by the coincidence of reciprocal lattice point (CRLP) method, which has been proved to be successful in analyzing ORs of two adjoining crystals [16–19]. The CRLP method was first proposed by Ikuhara et al. [16], in which the overlap of reciprocal lattice points (RLPs) of two adjoining crystals was utilized to obtain a geometrically optimum OR between the two crystals. In the CRLP method each RLP, corresponding to the reciprocal lattice vector g, is represented by a sphere of radius r⁎ around the reciprocal lattice point. It is then hypothesized that the OR favored between two adjoining crystals is the one in which the sum of the
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overlapping volumes is maximized. Given the primitive unit cell parameters of the two lattices and the radius r⁎, the sum of all overlapping volumes V(φ) is calculated as a function of rotation angle φ between the two axes that are originally assumed to be parallel to each other. The summation in overlapping volumes is carried out over all RLPs up to a maximum distance, R⁎, in the composite reciprocal lattice. Then, V(φ) is plotted versus φ. In all of the cases that have been calculated, the main peaks in the plots represent the preferential ORs [16]. This paper presents the results of a transmission electron microscopy (TEM) investigation in an iron-implanted sapphire. Special emphasis is placed on the morphology of α-Fe precipitate and the OR between αFe precipitate and sapphire, because both the morphology of α-Fe precipitate and orientation relationships are sensitive to the implantation conditions. By the coincidence of reciprocal lattice point (CRLP) method, the orientation relationships are analyzed and discussed in more detail than previously reported for this system.
2. Experimental procedure High-purity α-Al2O3 single crystals having the c-axis normal to the surface were purchased from KMT Corporation. The wafers were polished to a mirror-like finish and were annealed at 1500 °C for 6 h in air to remove mechanical damage before ion implantation. Fe ions were implanted into (0001)-oriented sapphire wafers at energy of 50 keV to a dose of 1 × 1017 ions/cm2 at room temperature. The wafers were tilted 7° with respect to the ion beam to avoid channeling effects. The Fe-implanted samples were subsequently annealed under different temperature and time conditions (500 °C, 1000 °C and 1500 °C for temperature; one hour, three hours and six hours for time) in a reducing (4% hydrogen–argon mixture) atmosphere. Both plan-view and cross-sectional view specimens for TEM were thinned mechanically, and then thinned by ion milling. The plan-view specimen was thinned from the unimplanted side. The microscopy observation was carried out using a Tecnai F30 G2 field emission transmission electron microscope.
3. Results and discussion The bright-field image for a cross-sectioned specimen implanted to a dose of 1 × 1017 Fe ions/cm2 at room temperature is shown in Fig. 1. The corresponding selected-area electron diffraction pattern (SAED) confirms the crystalline nature of the implanted layer. Measurements of the iron content of this implanted region were made by X-ray EDS while operating in the scanning transmission electron microscopy (STEM) mode. The results are plotted in Fig. 2 as the ratio of iron to aluminum versus depth. The iron concentration profile shows a well-defined peak at ~ 50 nm from the surface, in agreement with the TRIM [20] calculated value of 51.6 nm, and a maximum Fe/Al atom fraction of ~21%. With annealing in a reducing atmosphere, the implanted Fe ions precipitate as the α-Fe particles. 1000 °C and 1 h is the critical point for α-Fe particle precipitation: less than this point, only a small quantity of particles precipitated; at this point, much more particles precipitated; more than this point, no further precipitation happened. Thus special emphasis was placed on the sample annealed under 1000 °C and 1 h condition. Fig. 3(a) shows the bright-field image of the plan-view specimen. It is shown that the α-Fe particles range in size from a few nanometers to greater than 20 nm. Some small α-Fe particles are attached to voids. McHargue et al. have proposed the mechanism for the formation of this association of a void with an iron particle: large numbers of Fe2+–oxygen vacancy complexes are present in the implanted samples and clustering of these vacancy–iron complexes would lead to the formation of iron particles and voids during annealing in a reducing atmosphere [4]. The large iron particles greater than 20 nm show an obvious hexagonal faceting. The SAED of the large iron particles greater than 20 nm is also shown in Fig. 3(a). In the diffraction pattern, there appear patterns from both α-Al2O3[0001] zone axis and α-Fe[111] zone axis, and also double-diffraction spots from α-Fe and α-Al2O3. It is evident that the orientation relationship between the large α-Fe particles and sapphire can be determined as: h i ð111Þα‐Fe ==ð0001Þsapphire and 110
α‐Fe
h i == 1120
sapphire
ð1Þ
and the sides of all the large α-Fe particles are parallel to (2110), (1120) and (1210) of sapphire. Fig. 3(b) shows the association of a void with a small α-Fe particle. As can be seen from the high-resolution image, the sides of the void are parallel to (2110), (1120) and (1210) of sapphire and the small αFe particle appears at the corner of the faceted void.
Fig. 1. Bright-field image and the corresponding SAED pattern of the cross-sectioned specimen implanted to a dose of 1 × 1017 Fe ions/cm2 at room temperature.
Fig. 2. Iron concentration profile obtained from X-ray EDS line scan across the implanted layer of the specimen shown in Fig. 1.
Y. Wang et al. / Materials Characterization 102 (2015) 19–23
Fig. 3. (a) Bright-field image and the corresponding SAED pattern of the plan-view specimen along α-Al2O3[0001] and α-Fe[111] zone axes. The diffraction spots from sapphire and α-Fe are marked by black stars and white arrows, respectively. (b) High-resolution image of the association of a void with a small α-Fe particle. (c) High-resolution image of a small iron particle about 4 nm formed in the sapphire matrix.
For the small iron particles just a few nanometers, it is difficult to determine the OR by the SAED. Moiré fringes which are formed by superposition of precipitate and matrix can also been utilized to determine the OR. Fig. 3(c) shows Moiré fringes inside a small iron particle about 4 nm. The Moiré fringes are parallel to (1120) of sapphire and the Moiré fringe spacing D is measured to be 1.4 nm. The plane spacing of α-Fe d2 can be calculated from Eq. (2) [21]: D¼
d1 d2 d1 −d2
ð2Þ
where D represents the Moiré fringe spacing and d1 represents the (1120) plane spacing of sapphire. The calculated value of d2 is 0.203 nm, which corresponds to the (110) plane spacing of α-Fe. Considering the Moiré fringes having a periodicity of 60° for rotation about [0001]sapphire, the OR between the small α-Fe particles and sapphire can be determined as the OR of Eq. (1), the same with the OR between the large α-Fe particles and sapphire. Fig. 4(a) shows the bright-field image of the cross-sectional view specimen. The cross-sectioned plane is parallel to (1120) of sapphire. As can been seen from the bright-field image, the small α-Fe particles are spheroidal in shape and the large α-Fe particles greater than 20 nm are clearly faceted. The SAED pattern of the large α-Fe particles greater than 20 nm is shown in Fig. 4(b). In the diffraction pattern, there appear patterns from both α-Al2O3[1120] zone axis and α-Fe[110] zone axis, and also double-diffraction spots from α-Fe and α-Al2O3. The OR between the α-Fe particles and sapphire can be determined using Eq. (1), the same
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Fig. 4. Bright-field image (a) and the corresponding SAED pattern (b) of cross-sectioned specimen, along α-Al2O3[1120] zone axis. (c) High-resolution image of three small α-Fe particles. (d) A schematic for the relationship between plane spacings, diffraction vectors and Moiré fringe spacing.
with the observation result in the plan-view specimen. Fig. 4(c) shows the high-resolution image of three small α-Fe particles. Moiré fringes inside the α-Fe particles are formed by superposition of (1104) planes of α-Al2O3 and (1 10) planes of α-Fe. The analyses of the Moiré fringes were carried out in order to detect the misorientation of the small α-Fe particles with respect to the α-Al2O3 matrix. The relationship between plane spacings, diffraction vectors and Moiré fringe spacing is schematically shown in Fig. 4(d). The (1 10) plane spacing of α-Fe (d2) and the angle included between the (1 10) planes of α-Fe and the (1104) planes of sapphire (α) can be calculated from Eqs. (3) and (4): 1 1 2 cosβ 1 þ þ ¼ 2 D d1 D2 d21 d2
ð3Þ
sinβ sinα ¼ D d2
ð4Þ
where D represents the Moiré fringe spacing, d1 represents the (1104) plane spacing of sapphire and β represents the angle included between the Moiré fringes and the (1104) planes of sapphire. The measurements of D and β in the high-resolution image were carried out using the fast Fourier transform of the Moiré contrast region. The error was estimated to be approximately ±0.0500 nm for D and ±1° for β, having an influence in the calculated values of ±0.0025 nm for d2 and ±1° for α [22]. The calculated values d2 of the particles 1, 2 and 3 are 0.203 nm, 0.201 nm and 0.202 nm, respectively, which all lie within the error (±0.0025 nm) of the theoretical ( 1 1 0) plane spacing of α-Fe (0.2027 nm). The calculated values α of the particles 1, 2 and 3 are
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2.67°, 1.37° and 2.52°, respectively. Comparing the theoretical value of α (3°) we can determine the relative rotations about [1120]sapphire between three α-Fe particles and sapphire matrix to be 0.33°, 1.63° and 0.48°, respectively. Considering the experimental error ±1° for α it can be concluded that particle 1 and particle 3 have the OR of Eq. (1) with sapphire on the whole and particle 2 has a deviation of less than 2° from the OR of Eq. (1) with sapphire. More than fifty α-Fe particles have been analyzed by this method. Most of the particles have the OR of Eq. (1) with sapphire considering the experimental error. Only a small quantity of the α-Fe particles have the deviation from the OR of Eq. (1) beyond the experimental error and the maximum deviation is less than 3°. The existence of particles deviating from the OR of Eq. (1) means that the α-Fe particles did not reach full equilibration under 1000 °C and 1 h annealing condition. The samples annealed at higher temperatures and longer times were investigated and no further equilibration happened, which indicates that the particles deviating from the OR of Eq. (1) have no relationship with annealing temperature and time. Through the analysis above, it has been confirmed that the α-Fe particles ranging in size from a few nanometers to greater than 20 nm have the same OR of Eq. (1) with sapphire under the experimental conditions implanting at energy of 50 keV to a dose of 1 × 1017 ions/cm2 at room temperature and annealing at 1000 °C for 1 h in a reducing atmosphere. Another OR: h i ð110Þα‐Fe ==ð0001Þsapphire and ½111α‐Fe == 51 40
sapphire
ð5Þ
has been observed by McHargue et al. [4] under the conditions implanting at energy of 160 keV to a dose of 4 × 1016 ions/cm2 at room temperature and annealing at 1500 °C for 1 h in a reducing atmosphere. The CRLP method is utilized to determine which OR is the preferential one in the α-Fe/sapphire system. Given the primitive unit cell parameters of α-Fe and α-Al2O3, the sum of all overlapping volumes V(φ) is calculated as a function of rotation angle φ between the two axes that are originally assumed to be parallel to each other. Fig. 5 shows the plot of V(φ) versus φ for rotation about [0001]sapphire//[111]α-Fe. The calculation was started from the orientation as shown in Fig. 5(a). The low-index directions of the two crystals are set to be parallel, i.e. [0001]sapphire//[111]α-Fe and [1010]sapphire//[110]α-Fe. The calculations were preformed for r⁎ = 0.05a⁎, and for all RLPs to within R⁎ = 3a⁎, where a⁎ is the reciprocal lattice parameter of the α-Fe phase. The calculated result is shown in Fig. 5(b). The computed plot of V(φ) versus φ is periodic in φ with a periodicity of 60°. Therefore, only the part of V(φ) corresponding to φ = 0–60° is shown in Fig. 5(b). Since the angle between [1010] and [1120] is 30°, the main peak occurring at φ = 30° corresponds to the OR of Eq. (1). The reciprocal lattice
calculations are in good agreement with the experimental observations in this work. Fig. 6 shows the plot of V(φ) versus φ for rotation about [0001]sapphire//[110]α-Fe. The calculation was started from the orientation as shown in Fig. 6(a). The low-index directions of the two crystals are set to be parallel, i.e. [0001]sapphire//[110]α-Fe and [10 1 0] sapphire //[1 1 0] α-Fe . The calculated result is shown in Fig. 6(b). Since the angle between [1010] and [51 40] is 10.89°, the peak occurring at φ = 10.9° corresponds to the OR of Eq. (5) observed by McHargue et al. [4]. Comparing the peak value V(φ) (9.13 × 10 − 4 ) corresponding to the OR of Eq. (5) with the peak value V(φ) (2.3 × 10 − 3 ) corresponding to the OR of Eq. (1), it can be seen that the OR of Eq. (1), i.e. (111) α-Fe //(0001) sapphire and [110]α-Fe//[1120]sapphire, is preferred in the α-Fe/sapphire system and the OR of (110)α-Fe//(0001)sapphire and [111]α-Fe//[5140]sapphire is one of the secondary preferred orientation relationships. The existence of two different orientation relationships in the α-Fe/ sapphire system can be attributed to different implantation conditions. In McHargue's experiment [4], the implanting energy is 160 keV, which is much higher than the implanting energy (50 keV) in our experiment. The higher implanting energy can cause α-Al2O3 targets to be amorphized more severely. Amorphized α-Al2O3 can recrystallize and form some transition phases of Al2O3, such as θ-Al2O3 [5] and γ-Al2O3 [23], at the beginning of annealing. Since θ-Al2O3 and γ-Al2O3 have different space-groups and lattice constants from α-Al2O3, Fe particles precipitated from α-Al2O3 may have a different orientation relationship from Fe particles precipitated from θ-Al2O3 or γ-Al2O3 considering the coincidence of reciprocal lattice point (CRLP) theory discussed above. After α-Fe precipitates nuclear, the transition Al2O3 phases convert to sapphire by growth of the sapphire into the metastable transition Al2O3 phases and disappear finally [23], however α-Fe precipitates' orientation does not change [5]. In the end, α-Fe particles precipitated from sapphire have the OR of Eq. (1) in our experiment with low implanting energy, while α-Fe particles precipitated from a transition Al2O3 phase have the OR of Eq. (5) in McHargue's experiment with high implanting energy.
4. Conclusions Fe ions were implanted into α-Al2O3 single crystals (sapphire) at energy of 50 keV to a dose of 1 × 1017 ions/cm2 at room temperature. The microstructure in the implanted region was studied by transmission electron microscopy. The iron concentration profile obtained from X-ray EDS line scan shows a well-defined peak at ~50 nm from the surface, in agreement with the TRIM calculated value. With annealing in a reducing
Fig. 5. (a) Assumed OR from which the calculation started. (b) Plot of V(φ) versus φ for rotation about [0001]sapphire//[111]α-Fe. Note that the main peak at φ = 30° corresponds to the preferential OR of Eq. (1).
Y. Wang et al. / Materials Characterization 102 (2015) 19–23
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Fig. 6. (a) Assumed OR from which the calculation started. (b) Plot of V(φ) versus φ for rotation about [0001]sapphire//[110]α-Fe. Note that the peak at φ = 10.9° corresponds to the OR of Eq. (5).
atmosphere, the implanted Fe ions precipitate as the α-Fe particles. Some α-Fe particles are attached to voids. The large iron particles greater than 20 nm and some voids clearly have faceted outlines. Through the analysis of the composite SAED pattern and Moiré fringes, the orientation relationship between α-Fe particles and sapphire was determined to be (111)α-Fe//(0001)sapphire and [110]α-Fe//[1120]sapphire. This OR is predicted precisely by the coincidence of reciprocal lattice point (CRLP) method. The other OR of (110)α-Fe//(0001)sapphire and [111]α-Fe//[5140]sapphire reported before is confirmed by the same method to be one of the secondary preferred orientation relationships in the α-Fe/sapphire system. The existence of two different orientation relationships can be attributed to different implantation conditions. The α-Fe particles precipitated from α-Al2O3 have a different orientation relationship from the α-Fe particles precipitated from a transition Al2O3 phase. Acknowledgments This research was supported by the Fundamental Research Funds for the Central Universities (N130402003). References [1] E. Cattaruzza, F. Gonella, G. Mattei, P. Mazzoldi, D. Gatteschi, C. Sangregorio, M. Falconieri, G. Salvetti, G. Battaglin, Cobalt nanoclusters in silica glass: nonlinear optical and magnetic properties, Appl. Phys. Lett. 73 (1998) 1176. [2] S. Zhu, X. Xiang, X.T. Zu, L.M. Wang, Magnetic nano-particles of Ni in MgO single crystals by ion implantation, Nucl. Instrum. Methods B 242 (2006) 114–117. [3] A. Meldrum, L.A. Boatner, K. Sorge, Microstructure and magnetic properties of Co nanoparticles in ion-implanted Al2O3, Nucl. Instrum. Methods B 207 (2003) 36–44. [4] C.J. McHargue, S.X. Ren, P.S. Sklad, L.F. Allard, J. Hunn, Preparation of nanometer-size dispersions of iron in sapphire by ion implantation and annealing, Nucl. Instrum. Methods B 116 (1996) 173–177. [5] M.K. Santala, V. Radmilovic, R. Giulian, M.C. Ridgway, A.M. Glaesera, R. Gronsky, Precipitate orientation relationships in Pt-implanted sapphire, Scr. Mater. 62 (2010) 187–190. [6] Z. Werner, M. Pisarek, M. Barlak, R. Ratajczak, W. Starosta, J. Piekoszewski, W. Szymczyk, R. Grötzschel, Chemical effects in Zr- and Co-implanted sapphire, Vacuum 83 (2009) 57–60.
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