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Yttrium oxide thin films: chemistry- stoichiometry-strain and microstructure
Crystal Engineering 5 (2002) 169–175 www.elsevier.com/locate/cryseng
Yttrium oxide thin films: chemistrystoichiometry-strain and microstructure F. Paumier a, R.J. Gaboriaud a,∗, A.R. Kaul b a
Universite´ de Poitiers-LMP-UMR6630 CNRS - SP2MI-BP 30179 86962-Chasseneuil-Futuroscope cedex, France b Chemistry Department, Moscow State University, Moscow, Russia
Abstract Yttrium oxide thin films were in-situ deposited by ion beam sputtering on Si, MgO and SrTiO3 substrates. These Y2O3 thin films were investigated mainly by means of x-ray diffraction. The strained state of the oxide layers was studied by the sin2ψ method as a function of the deposition parameters as well as the post annealing treatments. An in situ study of the kinetics of the internal strain relaxation process was performed as a function of temperature. The Arhenius plot of relaxation rate gives the activation energy of this strain relaxation process, which is 1.3 eV. The results obtained in this work were interpreted in terms of crystal chemistry and the stoichiometry-microstructure relationship. 2003 Elsevier Science Ltd. All rights reserved. Keywords: Yttrium oxide; Thin films; Microstructure; Strain
1. Introduction The aggressive scaling of complementary metal-oxide-semiconductor technology promotes a broad search for high dielectric constant (high k) materials to replace SiO2 gate oxides (k=3.9) which thickness is near the quantum tunnelling limit, which is in the 1–1.5 nm range. The technology road map for semiconductors predicts how thin gate oxide must be to keep pace with the rate at which lateral dimension are being reduced in complementary metal-oxide-semiconductor (CMOS) devices. [1] Extensive researches have been carried out in recent years to find extrinsic high
∗
Corresponding author. Tel.: 33-05-49-49-66-62; fax: 66-92. E-mail address: [email protected] (R.J. Gaboriaud).
1463-0184/02/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1463-0184(02)00026-6
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F. Paumier et al. / Crystal Engineering 5 (2002) 169–175
dielectric constant materials which should satisfy stringent requirements such as high breakdown voltage, low leakage current and high thermal stability. Therefore a number of metal oxides such as Al2O3, Ta2O5, HfO2, Gd2O3, CeO2, ZrO2 and PrO2 have been studied. Among those oxides yttrium oxide Y2O3 exhibits attractive features for electronic applications as part of metal-oxide-semiconductors heterostructures used in the MOS transistor [2–5]. Y2O3 should be a suitable material for a Metal/Extrinsic Oxide/Semiconductor (MEOS) structure because of several particularly relevant physical properties such as a wide band gap (5.5 ev), a high k value (12–18), high thermal stability up to 2300 °C, chemical compatibility with silicon, and a low lattice mismatch between the Y2O3 and silicon lattice parameters [6–8]. This paper is therefore devoted to the study of the relationships between the chemistry, the stoichiometry, the internal stress, and the nanostructure of thin films of yttrium oxide deposited by ion beam sputtering on Si, MgO and SrTiO3.
2. Experiments Yttrium oxide thin films were deposited at 700 °C by both ion beam sputtering and laser ablation. The sputtering is carried out with an RF ion source which delivers an 1.2 keV argon beam with an intensity of 60 mA. The argon beam sputters a watercooled 10 cm diameter Y2O3 target. The 2.10-8 torr background pressure increased to 10-4 torr during the deposition process. The oxygen partial pressure (PO2) estimated from the oxygen flow introduced in the sputtering chamber was 3.10-5 Torr. The laser deposition is performed with a pulsed Nd:YAG laser using a frequency of 5 Hz and a pulse duration of 5 ns. The target was a sintered pellet of Y2O3 . This insitu growth was achieved under an oxygen partial pressure of 3.10-5 Torr. The post annealing treatments of the as-deposited thin films of yttrium oxide were performed in a quartz tube furnace in air or under a vacuum of roughly 10-6 Torr. The X-ray diffraction study of the thin films was performed with a 4-circle Siefert goniometer using the CuKa radiation (0.15406 nm). The internal stresses in the thin films were measured by the X-ray sin2ψ method using seven Bragg reflections. The kinetic study of the internal strain relaxation was realised by X-ray in situ experiments as a function of temperature.
3. Experimental results 3.1. X-ray investigation of the Y2O3 thin films Y2O3 thin films were in situ grown by ion beam sputtering on different substrates: [001] Si wafer, [001] MgO, and [001] SrTiO3 single crystals. The out-plane θ-2θ Bragg reflections corresponding to the different samples of Y2O3 thin films are shown
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Fig. 1. (a) θ-2θ diagram of both the as-deposited Y2O3 thin film deposited on Si and the thin films annealed at 700 and 900 °C during 2 h under vacuum and air atmosphere. (b) Measurements of the internal stresses in the thin film by the sin2ψ method.
in Figs. 1–4. Y2O3 films deposited on Si are textured and exhibit a [222] growth direction. The X-ray spectra performed with the as-deposited and annealed films of Y2O3 on Si are depicted in Fig. 1a. This as-deposited sample shows a non-symmetric [222] peak with its vertex at a Bragg angle of 2θ = 28.2° corresponding to a (222) d-spacing of 0.316 nm compared to a bulk material (222) d-spacing of 0.306 nm. The Bragg reflections of sample annealed at 700 and 900 °C are symmetric and centred at 2θ = 29.13° which is very close to the bulk material value. Y2O3 deposited on MgO exhibits an epitaxial relationship with the substrate forming four variants. The growth direction is [111]. The corresponding (222) Bragg peaks are shown in Fig. 2a. Again the as-deposited film exhibits a non-symmetric (222) peak. The film annealed at 700 °C (1 h, air) exhibits an intense symmetric peak centred on the bulk material value. Y2O3 deposited on SrTiO3 also exhibits an epitaxial relationship with the substrate with one variant corresponding to the [004] growth direction. The asdeposited film exhibits a non-symmetric (400) peak. The films annealed at 700 °C show a symmetric peak as shown in Fig. 3a.
Fig. 2. (a) θ-2θ diagram of both the as-deposited Y2O3 thin film deposited on MgO and the annealed thin films at 700 °C during 1 h in air atmosphere. (b) Measurements of the internal stresses in the thin film by the sin2ψ method.
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Fig. 3. (a) θ-2θ diagram of both the as-deposited Y2O3 thin film deposited on SrTiO3 and the annealed thin films at 700 °C during 1 h in air atmosphere. (b) Measurements of the internal stresses in the thin film by the sin2ψ method.
Fig. 4. (a) θ-2θ diagram of the as-deposited Y2O3 thin films grown on Si by both ion beam sputtering and by laser ablation. (b) Measurements of the internal stresses in the thin film by the sin2ψ method.
3.2. Measurement of the internal stress in the Y2O3 thin films The X-ray results described above lead to the notion of internal stress within the thin film of oxide. The stress present in the different films depicted in Figs. (1a, 2a and 3a) were measured by the well known sin2ψ method [9]. This method is based on the assumption of both a linear elasticity for isotropic material and an in-plane biaxial isotropic stress. This analysis is plotted as ln(ad/ao) as a function of sin2ψ where ad and ao are the strain and strain free lattice spacing respectively, and ψ is the angle between the normal of the diffracting plane and the normal of the surface of the sample. This should be a linear relationship between ln(ad/ao) and sin2ψ. The slope of the corresponding straight line indicates the stress and the intercept leads to the so-called strain-stress free lattice parameter ao. The analysis in terms of stress is depicted in the Figs. 1b, 2b and 3b. In all the cases the stress present in the asdeposited samples is a compressive stress whith a value near 5GPa. In each case the annealing treatment gives a very important relaxation of the stress with an improvement of the crystallinity of the films.
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The X-ray analysis performed with the samples deposited by both sputtering and laser ablation are depicted in Fig. 4a,b. Obviously these films are in different states. The as-deposited sample obtained by laser ablation exhibits a symmetric (222) Bragg peak. Nevertheless both films are under compressive stress. 3.3. Kinetics of the stress relaxation in the Y2O3 thin films The kinetic of the stress relaxation as a function of temperature has been studied by in situ X-ray measurements between 260 and 350 °C. For each temperature a θ2θ spectrum is recorded each 5 min leading to 60 X-ray spectra. The (222) Bragg peak angles are transformed into the corresponding (222) d-spacings and plotted as a function of time (Fig. 5). Obviously the stress relaxation is very fast at 350 °C and rather low at 260 °C. For a given (222) d-spacing in the direction perpendicular to the surface of the substrate the slope of the different curves (222) d-spacing as a function of time (Fig. 5) gives the strain rate v defined as v = δ[d(222)] / δt. This strain rate corresponds to the stress relaxation rate. An Arhenius analysis ln(v) = f(1 / T) has been performed and is depicted in Fig. 6. The experimental data fit a linear regression analysis with a very good accuracy. The slope of the lines which are obtained for different (222) d-spacing gives the activation energy of the relaxation process equal to 1.3 ev. This result can be compared to the activation energy of the oxygen diffusion in the Y2O3 crystal which is near 1 eV. Activation energy for the diffusion of yttrium is around 5 eV.
Fig. 5. Kinetics of the strain/stress relaxation in the [222] growth direction as a function of the annealing temperature in the range 260–350 °C (for details see text).
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Fig. 6. Arhenius plot ln (v) = f(1 / T) for Y2O3 deposited on Si. The slope of the straight lines gives the value (1.3 eV) of the activation energy of the relaxation process.
4. Discussion The investigations by X-ray experiments of the Y2O3 thins films deposited on Si indicate a textured structure in the [111] crystallographic direction which is the growth direction. This result can be explained by two arguments [8,10,11]: -A layer of amorphous SiO2 sandwiched between the Si wafer substrate and the deposited oxide—or the (111) plane has the lowest surface energy of the Y2O3 crystalline structure which is the cubic-C type (bixbyite) structure of the rare earth oxides. The corresponding x-ray spectrum depicted in Fig. 1a shows a non-symmetric (222) Bragg peak which can be interpreted either by a (222) out-plane d-spacing distribution due to a gradient in the oxygen stoichiometry or to the convolution of two peaks coming from a stressed and relaxed structure within the thin film of the yttrium oxide. Another possibility is the presence of a monoclinic phase mixed with the cubic phase of Y2O3. This hypothesis is presently under study. Figs. 2 and 3 exhibit Y2O3 films deposited on MgO and SrTiO3 respectively. An epitaxy relationships are observed in both cases even though the growth direction is [222] in the first case (MgO) and [004] in the later case (SrTiO3). It is worth noting that the Y2O3 thin films deposited on Si by laser ablation exhibit a symmetric (222) Bragg peak. Obviously the non-symmetric shape of the Bragg peak obtained with the films deposited by ion beam sputtering is mainly due to the particular deposition conditions of this technique. The as-deposited films are under compressive stress with a value around 5 GPa which decreases almost to zero after an annealing treatment. In order to obtain further insights concerning the kinetics of this relaxation process in situ X-ray measurements have been done as a function of temperature. The variation of the (222) d-spacing as a function of the annealing time for different temperatures are
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plotted in Fig. 5. The activation energy of the stress relaxation process gives a value of 1.3 eV which seems to indicate that rate controlling step in this stress relaxation is the oxygen diffusion. Oxygen diffusion in Y2O3 has an activation energy close to 1 ev. It is worthy of mention that this relaxation process is very fast even at 350 °C, a rather low temperature with respect to the temperature of its crystallographic stability which is 2325 °C. Obviously the defective Y2O3 anion network fluoritetype structure is of prime importance for this very high mobility of the oxygen ions and consequently for this unusually fast process of relaxation which decreases the stress from 5 GPa to zero in a few minutes at temperatures as low as 350 °C. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
S.A. Chambers, Materials Today April (2002) 34. S. Zang, R. Xiao, J. Appl. Phys 83 (1998) 87. J.J. Chambers, G.N. Pearson, J. Appl. Phys 90 (2001) 918. K.J. Hubbard, D.G. Schlom, J. Mater. Res 11 (1996) 2757. R.J. Gaboriaud, F. Pailloux, P. Guerin, F. Paumier, Thin Sol. Films 400 (2001) 106. V. Swamy, N.A. Dubrovinskaya, L.S. Dubrovinsky, J. Mater. Res 14 (1999) 2456. A. Huignard, A. Aron, P. Haschehoung, B. Viana, J. Thery, A. Laurent, J. Perriere, J. Mater. Chem 10 (2000) 549. R.J. Gaboriaud, F. Pailloux, P. Guerin, F. Paumier, J. Phys. D: Appl. Phys 33 (2000) 2884. G. Mader, Chem. Scripta 26A (1986) 23. S.L. Jones, D. Kumar, R.K. Singh, P.H. Holoway, Appl. Phys. Letters 71 (1997) 404. M.H. Cho, D.H. Ko, K. Jeong, S.W. Wangbo, C.N. Wang, S.C. Choi, S.J. Cho, J. Appl. Phys 85 (1999) 2909.
Yttrium oxide thin films: chemistrystoichiometry-strain and microstructure F. Paumier a, R.J. Gaboriaud a,∗, A.R. Kaul b a
Universite´ de Poitiers-LMP-UMR6630 CNRS - SP2MI-BP 30179 86962-Chasseneuil-Futuroscope cedex, France b Chemistry Department, Moscow State University, Moscow, Russia
Abstract Yttrium oxide thin films were in-situ deposited by ion beam sputtering on Si, MgO and SrTiO3 substrates. These Y2O3 thin films were investigated mainly by means of x-ray diffraction. The strained state of the oxide layers was studied by the sin2ψ method as a function of the deposition parameters as well as the post annealing treatments. An in situ study of the kinetics of the internal strain relaxation process was performed as a function of temperature. The Arhenius plot of relaxation rate gives the activation energy of this strain relaxation process, which is 1.3 eV. The results obtained in this work were interpreted in terms of crystal chemistry and the stoichiometry-microstructure relationship. 2003 Elsevier Science Ltd. All rights reserved. Keywords: Yttrium oxide; Thin films; Microstructure; Strain
1. Introduction The aggressive scaling of complementary metal-oxide-semiconductor technology promotes a broad search for high dielectric constant (high k) materials to replace SiO2 gate oxides (k=3.9) which thickness is near the quantum tunnelling limit, which is in the 1–1.5 nm range. The technology road map for semiconductors predicts how thin gate oxide must be to keep pace with the rate at which lateral dimension are being reduced in complementary metal-oxide-semiconductor (CMOS) devices. [1] Extensive researches have been carried out in recent years to find extrinsic high
∗
Corresponding author. Tel.: 33-05-49-49-66-62; fax: 66-92. E-mail address: [email protected] (R.J. Gaboriaud).
1463-0184/02/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1463-0184(02)00026-6
170
F. Paumier et al. / Crystal Engineering 5 (2002) 169–175
dielectric constant materials which should satisfy stringent requirements such as high breakdown voltage, low leakage current and high thermal stability. Therefore a number of metal oxides such as Al2O3, Ta2O5, HfO2, Gd2O3, CeO2, ZrO2 and PrO2 have been studied. Among those oxides yttrium oxide Y2O3 exhibits attractive features for electronic applications as part of metal-oxide-semiconductors heterostructures used in the MOS transistor [2–5]. Y2O3 should be a suitable material for a Metal/Extrinsic Oxide/Semiconductor (MEOS) structure because of several particularly relevant physical properties such as a wide band gap (5.5 ev), a high k value (12–18), high thermal stability up to 2300 °C, chemical compatibility with silicon, and a low lattice mismatch between the Y2O3 and silicon lattice parameters [6–8]. This paper is therefore devoted to the study of the relationships between the chemistry, the stoichiometry, the internal stress, and the nanostructure of thin films of yttrium oxide deposited by ion beam sputtering on Si, MgO and SrTiO3.
2. Experiments Yttrium oxide thin films were deposited at 700 °C by both ion beam sputtering and laser ablation. The sputtering is carried out with an RF ion source which delivers an 1.2 keV argon beam with an intensity of 60 mA. The argon beam sputters a watercooled 10 cm diameter Y2O3 target. The 2.10-8 torr background pressure increased to 10-4 torr during the deposition process. The oxygen partial pressure (PO2) estimated from the oxygen flow introduced in the sputtering chamber was 3.10-5 Torr. The laser deposition is performed with a pulsed Nd:YAG laser using a frequency of 5 Hz and a pulse duration of 5 ns. The target was a sintered pellet of Y2O3 . This insitu growth was achieved under an oxygen partial pressure of 3.10-5 Torr. The post annealing treatments of the as-deposited thin films of yttrium oxide were performed in a quartz tube furnace in air or under a vacuum of roughly 10-6 Torr. The X-ray diffraction study of the thin films was performed with a 4-circle Siefert goniometer using the CuKa radiation (0.15406 nm). The internal stresses in the thin films were measured by the X-ray sin2ψ method using seven Bragg reflections. The kinetic study of the internal strain relaxation was realised by X-ray in situ experiments as a function of temperature.
3. Experimental results 3.1. X-ray investigation of the Y2O3 thin films Y2O3 thin films were in situ grown by ion beam sputtering on different substrates: [001] Si wafer, [001] MgO, and [001] SrTiO3 single crystals. The out-plane θ-2θ Bragg reflections corresponding to the different samples of Y2O3 thin films are shown
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Fig. 1. (a) θ-2θ diagram of both the as-deposited Y2O3 thin film deposited on Si and the thin films annealed at 700 and 900 °C during 2 h under vacuum and air atmosphere. (b) Measurements of the internal stresses in the thin film by the sin2ψ method.
in Figs. 1–4. Y2O3 films deposited on Si are textured and exhibit a [222] growth direction. The X-ray spectra performed with the as-deposited and annealed films of Y2O3 on Si are depicted in Fig. 1a. This as-deposited sample shows a non-symmetric [222] peak with its vertex at a Bragg angle of 2θ = 28.2° corresponding to a (222) d-spacing of 0.316 nm compared to a bulk material (222) d-spacing of 0.306 nm. The Bragg reflections of sample annealed at 700 and 900 °C are symmetric and centred at 2θ = 29.13° which is very close to the bulk material value. Y2O3 deposited on MgO exhibits an epitaxial relationship with the substrate forming four variants. The growth direction is [111]. The corresponding (222) Bragg peaks are shown in Fig. 2a. Again the as-deposited film exhibits a non-symmetric (222) peak. The film annealed at 700 °C (1 h, air) exhibits an intense symmetric peak centred on the bulk material value. Y2O3 deposited on SrTiO3 also exhibits an epitaxial relationship with the substrate with one variant corresponding to the [004] growth direction. The asdeposited film exhibits a non-symmetric (400) peak. The films annealed at 700 °C show a symmetric peak as shown in Fig. 3a.
Fig. 2. (a) θ-2θ diagram of both the as-deposited Y2O3 thin film deposited on MgO and the annealed thin films at 700 °C during 1 h in air atmosphere. (b) Measurements of the internal stresses in the thin film by the sin2ψ method.
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Fig. 3. (a) θ-2θ diagram of both the as-deposited Y2O3 thin film deposited on SrTiO3 and the annealed thin films at 700 °C during 1 h in air atmosphere. (b) Measurements of the internal stresses in the thin film by the sin2ψ method.
Fig. 4. (a) θ-2θ diagram of the as-deposited Y2O3 thin films grown on Si by both ion beam sputtering and by laser ablation. (b) Measurements of the internal stresses in the thin film by the sin2ψ method.
3.2. Measurement of the internal stress in the Y2O3 thin films The X-ray results described above lead to the notion of internal stress within the thin film of oxide. The stress present in the different films depicted in Figs. (1a, 2a and 3a) were measured by the well known sin2ψ method [9]. This method is based on the assumption of both a linear elasticity for isotropic material and an in-plane biaxial isotropic stress. This analysis is plotted as ln(ad/ao) as a function of sin2ψ where ad and ao are the strain and strain free lattice spacing respectively, and ψ is the angle between the normal of the diffracting plane and the normal of the surface of the sample. This should be a linear relationship between ln(ad/ao) and sin2ψ. The slope of the corresponding straight line indicates the stress and the intercept leads to the so-called strain-stress free lattice parameter ao. The analysis in terms of stress is depicted in the Figs. 1b, 2b and 3b. In all the cases the stress present in the asdeposited samples is a compressive stress whith a value near 5GPa. In each case the annealing treatment gives a very important relaxation of the stress with an improvement of the crystallinity of the films.
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The X-ray analysis performed with the samples deposited by both sputtering and laser ablation are depicted in Fig. 4a,b. Obviously these films are in different states. The as-deposited sample obtained by laser ablation exhibits a symmetric (222) Bragg peak. Nevertheless both films are under compressive stress. 3.3. Kinetics of the stress relaxation in the Y2O3 thin films The kinetic of the stress relaxation as a function of temperature has been studied by in situ X-ray measurements between 260 and 350 °C. For each temperature a θ2θ spectrum is recorded each 5 min leading to 60 X-ray spectra. The (222) Bragg peak angles are transformed into the corresponding (222) d-spacings and plotted as a function of time (Fig. 5). Obviously the stress relaxation is very fast at 350 °C and rather low at 260 °C. For a given (222) d-spacing in the direction perpendicular to the surface of the substrate the slope of the different curves (222) d-spacing as a function of time (Fig. 5) gives the strain rate v defined as v = δ[d(222)] / δt. This strain rate corresponds to the stress relaxation rate. An Arhenius analysis ln(v) = f(1 / T) has been performed and is depicted in Fig. 6. The experimental data fit a linear regression analysis with a very good accuracy. The slope of the lines which are obtained for different (222) d-spacing gives the activation energy of the relaxation process equal to 1.3 ev. This result can be compared to the activation energy of the oxygen diffusion in the Y2O3 crystal which is near 1 eV. Activation energy for the diffusion of yttrium is around 5 eV.
Fig. 5. Kinetics of the strain/stress relaxation in the [222] growth direction as a function of the annealing temperature in the range 260–350 °C (for details see text).
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Fig. 6. Arhenius plot ln (v) = f(1 / T) for Y2O3 deposited on Si. The slope of the straight lines gives the value (1.3 eV) of the activation energy of the relaxation process.
4. Discussion The investigations by X-ray experiments of the Y2O3 thins films deposited on Si indicate a textured structure in the [111] crystallographic direction which is the growth direction. This result can be explained by two arguments [8,10,11]: -A layer of amorphous SiO2 sandwiched between the Si wafer substrate and the deposited oxide—or the (111) plane has the lowest surface energy of the Y2O3 crystalline structure which is the cubic-C type (bixbyite) structure of the rare earth oxides. The corresponding x-ray spectrum depicted in Fig. 1a shows a non-symmetric (222) Bragg peak which can be interpreted either by a (222) out-plane d-spacing distribution due to a gradient in the oxygen stoichiometry or to the convolution of two peaks coming from a stressed and relaxed structure within the thin film of the yttrium oxide. Another possibility is the presence of a monoclinic phase mixed with the cubic phase of Y2O3. This hypothesis is presently under study. Figs. 2 and 3 exhibit Y2O3 films deposited on MgO and SrTiO3 respectively. An epitaxy relationships are observed in both cases even though the growth direction is [222] in the first case (MgO) and [004] in the later case (SrTiO3). It is worth noting that the Y2O3 thin films deposited on Si by laser ablation exhibit a symmetric (222) Bragg peak. Obviously the non-symmetric shape of the Bragg peak obtained with the films deposited by ion beam sputtering is mainly due to the particular deposition conditions of this technique. The as-deposited films are under compressive stress with a value around 5 GPa which decreases almost to zero after an annealing treatment. In order to obtain further insights concerning the kinetics of this relaxation process in situ X-ray measurements have been done as a function of temperature. The variation of the (222) d-spacing as a function of the annealing time for different temperatures are
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175
plotted in Fig. 5. The activation energy of the stress relaxation process gives a value of 1.3 eV which seems to indicate that rate controlling step in this stress relaxation is the oxygen diffusion. Oxygen diffusion in Y2O3 has an activation energy close to 1 ev. It is worthy of mention that this relaxation process is very fast even at 350 °C, a rather low temperature with respect to the temperature of its crystallographic stability which is 2325 °C. Obviously the defective Y2O3 anion network fluoritetype structure is of prime importance for this very high mobility of the oxygen ions and consequently for this unusually fast process of relaxation which decreases the stress from 5 GPa to zero in a few minutes at temperatures as low as 350 °C. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
S.A. Chambers, Materials Today April (2002) 34. S. Zang, R. Xiao, J. Appl. Phys 83 (1998) 87. J.J. Chambers, G.N. Pearson, J. Appl. Phys 90 (2001) 918. K.J. Hubbard, D.G. Schlom, J. Mater. Res 11 (1996) 2757. R.J. Gaboriaud, F. Pailloux, P. Guerin, F. Paumier, Thin Sol. Films 400 (2001) 106. V. Swamy, N.A. Dubrovinskaya, L.S. Dubrovinsky, J. Mater. Res 14 (1999) 2456. A. Huignard, A. Aron, P. Haschehoung, B. Viana, J. Thery, A. Laurent, J. Perriere, J. Mater. Chem 10 (2000) 549. R.J. Gaboriaud, F. Pailloux, P. Guerin, F. Paumier, J. Phys. D: Appl. Phys 33 (2000) 2884. G. Mader, Chem. Scripta 26A (1986) 23. S.L. Jones, D. Kumar, R.K. Singh, P.H. Holoway, Appl. Phys. Letters 71 (1997) 404. M.H. Cho, D.H. Ko, K. Jeong, S.W. Wangbo, C.N. Wang, S.C. Choi, S.J. Cho, J. Appl. Phys 85 (1999) 2909.