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channeling and PAC studies of hafnium-doped LiNbO3
Nuclear Instruments and Methods in Physics Research B45 (1990) 495-498 ~o~h-Holl~~d
COMBINED L. REBOUTA
~S/C~NNELING
AND PAC STUDIES
495
OF ~FNIUM-DOPE
LiNbO,
and J.C. SOARES
Centro de Fisica Nuclear da Vniversidade
de Lisboa, Au. Prof Cama Pinto 2, 1699 Lisboa, Portugal
M.F. DA SILVA Departamento
de Fisica, Institute
J.A. SANZ-GARCIA, Departamento
de Cikias
E. DIkGUEZ
de Fisica Aplicada
C-Iv,
e Engenharia
Nucleares,
LNETI,
2685 Sacavtkz,
Portugal
and F. AGULL&L~PEZ
V&e&dad
Aut&oma
de Madrid,
Canto Blanco, 28049 Madrid
Spain
The lattice location of hafnium in lithium niobate single crystals has been studied, combining results of both the FWS/channeling and the perturbed angular correlation techniques. Studies on surface m~fications of LiNbO, crystals by hafnium implantation, foliowed by heat treatments, are also reported. The results are compared with those obtained with crystals doped with hafnium during growth.
LiNbO, is a relevant material to prepare a number of electro-optic (bulk and waveguide) devices. Its photorefractive behaviour is potentially useful for optoelectronic applications [1,2]. Studies of the lattice location and valence state of active impurities introduced in this material during the growth or by implantation are essential for a full understanding and optimization of the photorefractive effect 131.During the past few years, an increasing number of studies including optical absorption and luminiscence, Mossbauer effect, EPR and ENDOR techniques have been reported [4]. Appleton et al. [5] used ion implantation and ion beam mixing for introducing dopants into LiNbO,. The analysis was carried out with RBS/channeling, secondary-ion mass spectrometry and optical spectroscopy. However, RBS/ channeling and perturbed angular correlation (PAC) experiments, being powerful tools for studies of lattice location of impurities and characterization of defects, are scarce. In the sequence of a recent programme initiated on lattice location studies of Eu- and Nd-doped LiNbO, [6], crystals doped with I-If during growth have been prepared for RBS/chatmeling and PAC studies. The results of these studies are reported in the present work. The combination of both techniques allows not only to characterize the lattice location of the hafnium probe but also the charge distribution of the near surrounding of the probe. The data obtained are compared with those obtained using LiNbO, crystals implanted with radioactive hafnium. The surface damage and its re0168-583X/90/$03.50 0 Elsevier Science Publishers B.V. (North-Halland)
covery during annealing treatments are studied with both techniques. The data indicate that Hf does neither occupy niobium sites nor the free octahedral sites.
2. Ex~rimen~l details Congruent LiNbO, crystals were grown by the Czochralski method from grade I JohnsonMathey powder. Plates were cut both perpen~~~~ly and parallel to the c-axis and polished with 0.3 pm of alumina powder. For the channeling experiments the samples were hafnium-doped during growth, with 1% mol HfO, in the melt, and by ion implantation, using a raOHfi beam with an energy of 150 keV and a dose of about 2.7 X 1Or4 cm -2. For the PAC measurements the radioactive “iHf activity was produced by neutron irradiation of the hafnium doped as grown crystal and by implantation using a radioactive “‘Hf beam at 80 keV with a dose of about 1 x 1013 crnm2, Lithium niobate crystals have trigonal symmetry; their structure below the ferroelectric Curie temperature (To = 1480 K) is shown in fig. 1. For the orientation of the samples, hexagonal axes of reference were used and the LiNbO, structure has a c/u ratio of about 2.693. The lattice location measurements were carried out using the Rutherford backscattering/channeling facility at LNETI, Sacadm [7]. The energy of the He’ analysing beam was 1.6 MeV, with typical currents of 1 nA in order to minimize the radiation damage and the dectronic pileup. VI. ION CHANNELING
L. Rebouta et al. / Combined RBS/channeling
_--___
and PAC of Hf-doped LiNbO,
Li
Vacancy
__-Q,oe_
Nb
___-__
Li
C
a1
0.231
d-
Vacancy 0
_--__
aI
Nb 0
Fig. 1. Structure of LiNbO, below the ferroelectric Curie temperature(Tc = 1480 K). The distances are given in nm.
1.0
0.5
The PAC measurements were carried CFNUL 4-detector angular correlation With this technique a frequency ve is information on the electric field gradient
eQV,,
Q “=-X-=3
2 1(21T
out using the apparatus [8]. measured and is derived:
1) ’
00
-2.0
0.0
2.0 -2.0 angle(degrees)
0.0
2.0
Fig. 2. Angular dependence of the normalized backscattering yield of 1.6 MeV 4He + ions from the Hf and Nb of a Hf-doped LiNbO, crystal. The lines were drawn to guide the eyes.
where e is the electron charge, Q the quadrupole moment of the intermediate state of the used cascade with spin I, V,, the electric field gradien; at the site of the nucleus, h the Planck constant and l/T the frequency derived with Fourier analysis.
3. Results and discussion Fig. 2 shows the angular scans for several axes and one planar direction. From the results of the (0001) c-direction it is clear that the hafnium site presents substitutionality, compared with the Nb string. This means that Hf could occupy either the Nb site or a site shadowed by the Nb ion in the c-direction. On the other hand, in the (liO2) planar direction, Hf presents a flux peak near the middle of the channel. This excludes the possibility of Hf occupying the Nb site. This conclusion is also supported by the two .other axial scans (1120) and (Okl), presented in the second row of fig. 2. Fig. 3 presents projections on planes normal to the scanned crystal axes and one plane shown in fig. 2. It is
Fig. 3. Projections on planes normal to the indicated directions.
As irradiated
Fig. 4. (a) PAC spectrnm of ‘*lTa in Hf-doped LiNbO, measured at TM = 294 K directly after neutron irradiation; (b) the correspondingFourier transform.
seen that the lithium site in the (O&U) direction is slightly displaced from the Nb string and, in fact, the almost substitutionality observed in this direction leads to the conclusion that Hf must be located near the lithium strings. From this data the free octahedral site is clearly excluded, since in that case we should observe instead a flux peak in the middle of the channel. Taking into account the structure observed in the (Oill), (Olfl), (0221) and (02%) axial directions, two Hf fractions occupying two different regular sites displaced from the lithium site coot be excluded. These RBS/channeling results are in agreement with the preliminary PAC data presented in fig_ 4. From the Fourier analysis it is clear that, at least, two main frequencies are needed to describe the data. The frequencies ve, = X19(6) X IO3 MHz and peZ= 1.57(7) X lo3 MHz describe the data well. Further experiments with the PAC technique are in progress, in order to measure the orientation of the occupied sites.
Figs. 5 and 6 show the preliminary results of the RBS/channeling and PAC experiments for LiNbO, samples implanted with “‘Hf and r*‘Hf ions, respectively. The first conclusion to be made is that after the ~pl~tation the Hf ions do not occupy regular sites. The ~S/cha~e~ng spectra show that a complete amo~~ation, like that observed by Appleton et al. [5] for lighter ions and higher doses, has been produced with the 2.7 X 1014 Hf’/cm’ dose. Although the fluence of the sample implanted with radioactive ‘**Hf was an order of magnitude lower, microscopically the PAC results show the same amorphization. These results have been the starting point for further studies of the annealing of the damaged unplanted layer. Fig. 7 shows PAC data of the amealing at 498 K of the implanted probe which are fitted with the same two frequencies observed with the probe described above. The start of the recrystallization was observed at 400 K. Measurements taken with the crystal oriented with the c-direction perpendicular and in the plane of the detectors support the axial structure for the field gradient
3’“‘““““““““‘““““i
TW=294 K As implanted ,o.oo
:
E
I)epth(-nm) Fig. 5. Random spectra for 1.6 LiNbO, single Hf+ ions/cm*.
and (O&l)-aligned Rutherford backscattering MeV helium (4He+) ions obtained from a crystal implanted at 150 keV with 2.7~10’~ The energy scale was converted into a depth scale.
_0.15~‘,~‘,,,,,,,,,,,‘,‘,,‘,,,,.’,, 0 2o t(ns)40
60
Fig. 6. PAC spectrum of “ITa in Hf implanted in LiNbO, at 80 keV with a dose of 1 X lOI ions/cm*, measured as implanted at Thl = 294 K. VI. ION ~HA~ELING
498
0
0
500
250 w(MHz)
Fig. 7. (a) PAC spectrum of “*Ta in Hf implanted in LiNbO, at 80 keV with a dose of 1 x 1013 ions/cm’, measured at r, with the (0001) axis perpendicular to the detector plane; (b) the corresponding Fourier transform.
along the (0001)
direction, in excellent agreement with the ch~nel~g data. These results indicate that the recrystallization, only observed at 770 K with the RBS/channeling measurements, microscopically starts at much lower temperatures. Therefore, this technique is not sensitive enough to detect the first steps of this process. The other interesting result is related with the frequencies observed by the PAC technique which support the conclusion that after ion ~pl~tation the annealing leads the Hf+ ions to the same regular site as that occupied by the Hf when introduced during the growth process.
4. Conclusions RBS/channeling results combined with PAC measurements, applied to Hf-doped LiNbO, crystals specially grown for these experiments, show to be a powerful tool for basic materials science studies of compounds presenting a very complex structure. Further N~/ch~ne~ng experiments are needed in order to accurately determine the precise location of the Hf+ ion related to the lithium or the oxygen sites. Thanks are due to Prof. E. Bodenstedt and Dr. K. Freitag for the hafnium implantations and the neutron
= 498 K
irra~ation using the KFA, Julich, FRG facility. This work has been partially supported by the Volkswagenwerk Foundation and JNICT (L.R.) and by the Portuguese-Spanish bilateral agreement no. 7.38/42. Grant no. PR 84-072’7 from CAYCIT is also acknowledged.
References [1] P. Gxmter and J.P. Huignard, eds., Photore~acti~e Materials and Their Applications I (Springer, Berlin, 1988). [2] C. Flytzenis and J.L. Ondar, eds., Nonlinear Optics: Materials and Devices (Springer, Berlin, 1986). [3] P. Gunter, Phys. Rev. 93 (1982) 199. [4] M.D. Glinchuk, V.G. Grachev, G.I. Malovichko and I.P. Bykov, Proc. First Eur. Conf. on Application of Polar Dielectrics, to be published in Ferroelectrics. [5] B.R. Appleton, G.M. Beardsley, G.C. Farlow and W.H. Christie, J. Mater. Res. 1 (1986) 104. [6] L. Rebouta, J.C. Soares, M.F. Da Silva, J.A. Sanz-Garcia, E. Dieguez and F. Agullo-Lopez, Appl. Phys. Lett. 55 (1989) 120. [7] M.R. Da Silva, AA. Melo, J.C. Soares, E.J. Alves, M.F. Da Silva, P.W. Winand and R. Vianden, Portugal Phys. 14 (1983) 175. [S] M.M. Cruz, A.A. Melo, J.C. Soares, M.F. Da Silva, K. Freitag and R. Vianden, Nucl. Instr. and Meth. B19/20 (1987) 200.
COMBINED L. REBOUTA
~S/C~NNELING
AND PAC STUDIES
495
OF ~FNIUM-DOPE
LiNbO,
and J.C. SOARES
Centro de Fisica Nuclear da Vniversidade
de Lisboa, Au. Prof Cama Pinto 2, 1699 Lisboa, Portugal
M.F. DA SILVA Departamento
de Fisica, Institute
J.A. SANZ-GARCIA, Departamento
de Cikias
E. DIkGUEZ
de Fisica Aplicada
C-Iv,
e Engenharia
Nucleares,
LNETI,
2685 Sacavtkz,
Portugal
and F. AGULL&L~PEZ
V&e&dad
Aut&oma
de Madrid,
Canto Blanco, 28049 Madrid
Spain
The lattice location of hafnium in lithium niobate single crystals has been studied, combining results of both the FWS/channeling and the perturbed angular correlation techniques. Studies on surface m~fications of LiNbO, crystals by hafnium implantation, foliowed by heat treatments, are also reported. The results are compared with those obtained with crystals doped with hafnium during growth.
LiNbO, is a relevant material to prepare a number of electro-optic (bulk and waveguide) devices. Its photorefractive behaviour is potentially useful for optoelectronic applications [1,2]. Studies of the lattice location and valence state of active impurities introduced in this material during the growth or by implantation are essential for a full understanding and optimization of the photorefractive effect 131.During the past few years, an increasing number of studies including optical absorption and luminiscence, Mossbauer effect, EPR and ENDOR techniques have been reported [4]. Appleton et al. [5] used ion implantation and ion beam mixing for introducing dopants into LiNbO,. The analysis was carried out with RBS/channeling, secondary-ion mass spectrometry and optical spectroscopy. However, RBS/ channeling and perturbed angular correlation (PAC) experiments, being powerful tools for studies of lattice location of impurities and characterization of defects, are scarce. In the sequence of a recent programme initiated on lattice location studies of Eu- and Nd-doped LiNbO, [6], crystals doped with I-If during growth have been prepared for RBS/chatmeling and PAC studies. The results of these studies are reported in the present work. The combination of both techniques allows not only to characterize the lattice location of the hafnium probe but also the charge distribution of the near surrounding of the probe. The data obtained are compared with those obtained using LiNbO, crystals implanted with radioactive hafnium. The surface damage and its re0168-583X/90/$03.50 0 Elsevier Science Publishers B.V. (North-Halland)
covery during annealing treatments are studied with both techniques. The data indicate that Hf does neither occupy niobium sites nor the free octahedral sites.
2. Ex~rimen~l details Congruent LiNbO, crystals were grown by the Czochralski method from grade I JohnsonMathey powder. Plates were cut both perpen~~~~ly and parallel to the c-axis and polished with 0.3 pm of alumina powder. For the channeling experiments the samples were hafnium-doped during growth, with 1% mol HfO, in the melt, and by ion implantation, using a raOHfi beam with an energy of 150 keV and a dose of about 2.7 X 1Or4 cm -2. For the PAC measurements the radioactive “iHf activity was produced by neutron irradiation of the hafnium doped as grown crystal and by implantation using a radioactive “‘Hf beam at 80 keV with a dose of about 1 x 1013 crnm2, Lithium niobate crystals have trigonal symmetry; their structure below the ferroelectric Curie temperature (To = 1480 K) is shown in fig. 1. For the orientation of the samples, hexagonal axes of reference were used and the LiNbO, structure has a c/u ratio of about 2.693. The lattice location measurements were carried out using the Rutherford backscattering/channeling facility at LNETI, Sacadm [7]. The energy of the He’ analysing beam was 1.6 MeV, with typical currents of 1 nA in order to minimize the radiation damage and the dectronic pileup. VI. ION CHANNELING
L. Rebouta et al. / Combined RBS/channeling
_--___
and PAC of Hf-doped LiNbO,
Li
Vacancy
__-Q,oe_
Nb
___-__
Li
C
a1
0.231
d-
Vacancy 0
_--__
aI
Nb 0
Fig. 1. Structure of LiNbO, below the ferroelectric Curie temperature(Tc = 1480 K). The distances are given in nm.
1.0
0.5
The PAC measurements were carried CFNUL 4-detector angular correlation With this technique a frequency ve is information on the electric field gradient
eQV,,
Q “=-X-=3
2 1(21T
out using the apparatus [8]. measured and is derived:
1) ’
00
-2.0
0.0
2.0 -2.0 angle(degrees)
0.0
2.0
Fig. 2. Angular dependence of the normalized backscattering yield of 1.6 MeV 4He + ions from the Hf and Nb of a Hf-doped LiNbO, crystal. The lines were drawn to guide the eyes.
where e is the electron charge, Q the quadrupole moment of the intermediate state of the used cascade with spin I, V,, the electric field gradien; at the site of the nucleus, h the Planck constant and l/T the frequency derived with Fourier analysis.
3. Results and discussion Fig. 2 shows the angular scans for several axes and one planar direction. From the results of the (0001) c-direction it is clear that the hafnium site presents substitutionality, compared with the Nb string. This means that Hf could occupy either the Nb site or a site shadowed by the Nb ion in the c-direction. On the other hand, in the (liO2) planar direction, Hf presents a flux peak near the middle of the channel. This excludes the possibility of Hf occupying the Nb site. This conclusion is also supported by the two .other axial scans (1120) and (Okl), presented in the second row of fig. 2. Fig. 3 presents projections on planes normal to the scanned crystal axes and one plane shown in fig. 2. It is
Fig. 3. Projections on planes normal to the indicated directions.
As irradiated
Fig. 4. (a) PAC spectrnm of ‘*lTa in Hf-doped LiNbO, measured at TM = 294 K directly after neutron irradiation; (b) the correspondingFourier transform.
seen that the lithium site in the (O&U) direction is slightly displaced from the Nb string and, in fact, the almost substitutionality observed in this direction leads to the conclusion that Hf must be located near the lithium strings. From this data the free octahedral site is clearly excluded, since in that case we should observe instead a flux peak in the middle of the channel. Taking into account the structure observed in the (Oill), (Olfl), (0221) and (02%) axial directions, two Hf fractions occupying two different regular sites displaced from the lithium site coot be excluded. These RBS/channeling results are in agreement with the preliminary PAC data presented in fig_ 4. From the Fourier analysis it is clear that, at least, two main frequencies are needed to describe the data. The frequencies ve, = X19(6) X IO3 MHz and peZ= 1.57(7) X lo3 MHz describe the data well. Further experiments with the PAC technique are in progress, in order to measure the orientation of the occupied sites.
Figs. 5 and 6 show the preliminary results of the RBS/channeling and PAC experiments for LiNbO, samples implanted with “‘Hf and r*‘Hf ions, respectively. The first conclusion to be made is that after the ~pl~tation the Hf ions do not occupy regular sites. The ~S/cha~e~ng spectra show that a complete amo~~ation, like that observed by Appleton et al. [5] for lighter ions and higher doses, has been produced with the 2.7 X 1014 Hf’/cm’ dose. Although the fluence of the sample implanted with radioactive ‘**Hf was an order of magnitude lower, microscopically the PAC results show the same amorphization. These results have been the starting point for further studies of the annealing of the damaged unplanted layer. Fig. 7 shows PAC data of the amealing at 498 K of the implanted probe which are fitted with the same two frequencies observed with the probe described above. The start of the recrystallization was observed at 400 K. Measurements taken with the crystal oriented with the c-direction perpendicular and in the plane of the detectors support the axial structure for the field gradient
3’“‘““““““““‘““““i
TW=294 K As implanted ,o.oo
:
E
I)epth(-nm) Fig. 5. Random spectra for 1.6 LiNbO, single Hf+ ions/cm*.
and (O&l)-aligned Rutherford backscattering MeV helium (4He+) ions obtained from a crystal implanted at 150 keV with 2.7~10’~ The energy scale was converted into a depth scale.
_0.15~‘,~‘,,,,,,,,,,,‘,‘,,‘,,,,.’,, 0 2o t(ns)40
60
Fig. 6. PAC spectrum of “ITa in Hf implanted in LiNbO, at 80 keV with a dose of 1 X lOI ions/cm*, measured as implanted at Thl = 294 K. VI. ION ~HA~ELING
498
0
0
500
250 w(MHz)
Fig. 7. (a) PAC spectrum of “*Ta in Hf implanted in LiNbO, at 80 keV with a dose of 1 x 1013 ions/cm’, measured at r, with the (0001) axis perpendicular to the detector plane; (b) the corresponding Fourier transform.
along the (0001)
direction, in excellent agreement with the ch~nel~g data. These results indicate that the recrystallization, only observed at 770 K with the RBS/channeling measurements, microscopically starts at much lower temperatures. Therefore, this technique is not sensitive enough to detect the first steps of this process. The other interesting result is related with the frequencies observed by the PAC technique which support the conclusion that after ion ~pl~tation the annealing leads the Hf+ ions to the same regular site as that occupied by the Hf when introduced during the growth process.
4. Conclusions RBS/channeling results combined with PAC measurements, applied to Hf-doped LiNbO, crystals specially grown for these experiments, show to be a powerful tool for basic materials science studies of compounds presenting a very complex structure. Further N~/ch~ne~ng experiments are needed in order to accurately determine the precise location of the Hf+ ion related to the lithium or the oxygen sites. Thanks are due to Prof. E. Bodenstedt and Dr. K. Freitag for the hafnium implantations and the neutron
= 498 K
irra~ation using the KFA, Julich, FRG facility. This work has been partially supported by the Volkswagenwerk Foundation and JNICT (L.R.) and by the Portuguese-Spanish bilateral agreement no. 7.38/42. Grant no. PR 84-072’7 from CAYCIT is also acknowledged.
References [1] P. Gxmter and J.P. Huignard, eds., Photore~acti~e Materials and Their Applications I (Springer, Berlin, 1988). [2] C. Flytzenis and J.L. Ondar, eds., Nonlinear Optics: Materials and Devices (Springer, Berlin, 1986). [3] P. Gunter, Phys. Rev. 93 (1982) 199. [4] M.D. Glinchuk, V.G. Grachev, G.I. Malovichko and I.P. Bykov, Proc. First Eur. Conf. on Application of Polar Dielectrics, to be published in Ferroelectrics. [5] B.R. Appleton, G.M. Beardsley, G.C. Farlow and W.H. Christie, J. Mater. Res. 1 (1986) 104. [6] L. Rebouta, J.C. Soares, M.F. Da Silva, J.A. Sanz-Garcia, E. Dieguez and F. Agullo-Lopez, Appl. Phys. Lett. 55 (1989) 120. [7] M.R. Da Silva, AA. Melo, J.C. Soares, E.J. Alves, M.F. Da Silva, P.W. Winand and R. Vianden, Portugal Phys. 14 (1983) 175. [S] M.M. Cruz, A.A. Melo, J.C. Soares, M.F. Da Silva, K. Freitag and R. Vianden, Nucl. Instr. and Meth. B19/20 (1987) 200.