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Ferroelectric lithium niobate. 2. Preparation of single domain crystals
Ji Phys. Chem. Solids
Pergamon
Press 1966. Vol. 27, pp. 989-996.
FERROELECTRIC PREPARATION K.NASSAU, Bell Telephone
Printed in Great Britain.
LITHIUM
NIOBATE.
OF SINGLE DOMAIN H. J. LEVINSTEIN Laboratories,
2.
CRYSTALS
and G. M. LOLPCONO
Incorporated
Murray
Hill, New Jersey
(Received 8 October 1965)
Abstract-Lithium niobate is shown to be an unusual ferroelectric material with the very high Curie temperature of 1210°C. Three techniques are presented for the preparation of single domain LiNbOs crystals. The first uses the presence of Moos or WOs in the melt. The other two techniques require the application of an electric field either during or after growth. Poling is complete at 1200°C with the application of as little as 0.2 V/cm for a period of 10 min. Other physical properties given include dielectric constant, resistivity, thermal expansion, and optical data. LiNbOs and LiTaOs differ from each other in several important respects.
1. INTRODUCTION
compensation
In the preceding paper,(l) the growth, domain structure, dislocations, and etching behavior of lithium niobate (LiNbOs) were discussed in some detail; ferroelectric domains have been shown to be present and to interfere with many of the potential uses of this material. Three techniques have been developed for their elimination; these depend on the addition of molybdenum or tungsten oxide during growth, or the application of an electric field either during or after growth. Lithium niobate is an unusual type of ferroelectric material in that it has the very high Curie temperature of 121O”C, and a field of less than 0.2 Vfcm is needed near this temperature for domain reversal.(s) A number of physical constants of LiNbOs have also been determined; the properties of lithium niobate and lithium tantalate appear to differ in several important respects, and these differences are summarized. 2. TECHNIQUE
1: MOLYBDENUM
:
(1 -x)LiNbOs+XMoOs
+ (Lil_,~,)(Nbl_,5+Mo,e+)Os,
where + denotes a vacancy in a lithium site. Under a series of very limited growth conditions, the multidomain appearance seen on polishing and etching(l) disappears completely. The single domain nature of such crystals has been confirmed by pyroelectric,(s) acoustic,@) phase matched harmonic generation,(sJQ difference frequency generation,(T) and tuneable optical parametric oscillation@) experiments. The following listing presents a particular set of conditions, which appears satisfactory, but not always sufficient, for the growth of single domain LiNbOs :
ADDITION
When molybdenum in the form of Moos is added at low concentrations to LiNbOs, about one twentieth of this amount appears in a crystal pulled from the melt as described previously.(l) It is assumed that the molybdenum replaces niobium, forming a lithium vacancy for charge 989
(1) 0.5 at. % Moos added to the melt, (2) growth direction 20-40” from the c-axis, e.g. perpendicular to (11*12), (3) negative dipole end of crystal(l) facing the melt, (4) flat melt-crystal interface, (5) seed free from twin boundaries, and very stable temperature so that twins will not form, (6) steady, slow pulling rate, e.g. l/2-3/4 in./hr.
990
K. NASSAU,
H. J. LEVI~STE~N
In addition to these necessary requirements there are several optional conditions which reduce cracking of the. crystals: (7) composition ranging from nominally stoichiometric LiNbOs to 2 at. 0/Oin either direction have been used; it is sometimes helpful to purify by first pulling single crystal material and then using this as feed material, and (8) use of platinum only in contact with the melt; both iridium and rhodium make it difficult or impossible to obtain colorless crystals. The crystals are annealed in oxygen at about 1100°C overnight and cooled slowly to room temperature. This removes the pale brown color which probably originates from oxygen vacancies and niobinm in a lower valence state.(Q) The flat interface of (4) has been achieved by growing at a low level in the crucible or by the use of an after heater as in the second technique. In crystals grown from a rhodium crucible O-030.1 y0 rhodium is found spectroscopically. Platinum and iridium are not detectable spectroscopically in crystals grown from these crucibles. A variety of additives have been tried, but tungsten is the only other ion that has shown any activity. The mechanism is not understood at present. One possibility is that the low distribution coefficient of about O-05 in the case of MO, and the non-turbulent stirring present, result in a layer of rejected MO** ions close to the growth interface, thus producing a small electric field and simulating the second technique. 3. TECHNIQUE 2: GROWTH IN AN ELECTRIC FIELI)
The application of a large electric field at the melt interface was at first considered to be impracticable due to the high resistance of the cooler seed-end of the crystal and the low resistance of the hot region near the growth interface. However, high voltages were found not to be necessary for single domain production. Resistance measurements made on single crystals at ternperat~~ near the melting point indicated resistivity values of a few hundred !&cm. The actual resistance (that of the seed, crystal, crucible, etc.) was found to range from 1000 to 5000 B at the start of crystaf growth. This depends on the length and diameter of the seed and depth in the crucible. As little ss l/Z v applied to a l/2 in. seed, producing a current of
and G. M. LXOACONO
l/2 II& has resulted in the complete removal of one type of antiparallel domain, yielding a single domain crystal. The pulling apparatus used for these experiments was a modified one as shown in Fig. 1. A rotating friction contact is used to apply the voltage to the insulated crystal holder. The circuit shown in Fig. 1 is designed to hold the current steady at approximately 24 or 5 mA even though the resistance of the crystal changes. In exploratory work a variable voltage supply capable of 1000 V and 100 mA was used. The electrical circuit is not grounded except through the crucible so as not to interfere with the temperature controlling system. The direction of the field can be changed by a reversing switch not shown; the voltage drop across the crystal plus melt waQ measured by a vacuum tube voltmeter. Table 1
Growth time without after-heater (hr) 0 112 :.
6,500 25,000 145,000 -
with after-heater 3,200 10,600 17,500 66,000
As the crystal grows its resistance increases rapidly. Table 1 shows this effect during the growth of about 1 cm dia. crystals at a pulling rate of 314 in,lhr. The table also shows that the use of the after-heater shown in Fig. 1, held at an uncorrected optical pyrometer temperature of 105O”C, considerably reduced this increase in resistance. Since the electric field can only be active in the vicinity of the growth interface, it is convenient to control this at a constant value by maintaining a constant current; ideally the magnitude of the current should be adjusted to the cross-sectional area of the crystal, and the orientation of the c-axis with respect to the pulling direction. The results of a typical experiment with C-Z&J growing LiNbOs is illustrated in Figs. 2 and 3 ; the field designation indicates the electrical palarity connected to the crystal. The multidomain
FERROELECTRIC
LITHIUM
NIOBATE.
-PULLING
MECHANISM
.-CERAMIC
INSULATOR
2
991
8
R
I
---PLATINUM __..e....
MOLTEN
FIG. 1.
e
LiNbO,
Apparatus for the Czochralski growth of LiNbOa in an electric field.
regions have the same appearance previously described,(r) single domain regions showing no structure. Changing the magnitude of the current has no effect once the necessary minimum has been exceeded. Reversing the direction of the field, however, produces a sharp 180” domain
wall; the position and shape apparently corresponds to the growth interface. In crystals grown along the c-axis l/2 mA, or about 2 mA/cms, is the
FIG. 2. Appearance of a LiNbOa crystal grown under varying polarities and currents.
minimum current necessary. With the crystal electrically negative (with respect to the melt) up to 25 mA has been used, but above this value the current becomes unstable and difficult to control, perhaps due to localized resistive heating in the crystal. With the crystal positive the limit is approximately 10 mA. More current is required as the orientation departs from c-axis, as then only that part of the field resolved along the c-axis is active. Nevertheless, crystals up to 80 degrees from the c-axis have been grown single domain at 10 mA. The actual voltage drop required in the vicinity of the growth interface is apparently very low. With the crystal electrically negative with respect to the melt, the upper surface of the crystal when cut transversely is the positive dipole surface and is found to etch slowly(r) as expected. This is the surface which becomes positive in a pyroelectric experiment on cooling, and has been defined as the +c end of the crystal.(s)
992
K.
NASSAU,
H.
J.
LEVINSTEIN
In view of the results of the third technique (poling after growth) it is expected that the effect of the field would be significant only at the 1210°C isotherm in the crystal; it is neverthless possible that due to electrolytic action some structure is being imposed at the growth interface and survives through the paraelectric region. Impurities such as vanadium or molybdenum up to the 1 y0 level do not interfere with this process, but tungsten
and
G.
M.
LOIACONO
4. TECHNIQUE
3: POLING
Poling, i.e. the electric reversal of the polarization direction of a ferroelectric, has usually been carried out on thin slices of material with high fields applied at temperatures near room temperature.(ls) These conditions do not produce any poling on LiNbOs. The reverse conditions, namely low fields at very high temperatures, were tried and did convert multidomain lithium niobate
ION
D ENT
PLATINUM
tb)
FIG. 4. Arrangements for poling LiNbOs crystals; crystals as in (a), short crystals as in (b).
does seem to prevent single domain growth with an electric field applied. The addition of impurities may be desirable to adjust the refractive indexes so that phase matched harmonic generation without double refraction(s) can occur at a convenient temperature. Interaction is often observed at the platinumcrystal contact when the crystal is held electrically positive. X-ray diffraction examination of the tan product shows this to consist of NbaOs, apparently produced by electrolytic action. Cracking of the seed is often a result of this attack, and it seems preferable to grow with the crystal negative, since this also permits stability at higher currents.
long
to single domain crystals. Fields of O-2-5 V/cm have been used for this purpose at temperatures from 1090 to 1210°C. At such high temperatures it is important to avoid alternate parallel paths through the various furnace components. The two arrangements shown in Fig. 4 have been used successfully. Long thin crystals are hung from platinum wires anchored in grooves cut into the crystal. Platinum legs may also be used at one end of the crystal only. The wires which also carry the current are kept under tension to support the crystal in a horizontal tube furnace; contact with any part of the furnace is permitted at only one point. Short wide crystals
FIG. 3. Polished LiNbOs crystal showing transition to single domain growth when the field is turned on ( x 100).
FERROELECTRIC
LITHIUM
NIOBATE.
2
993
are painted on both ends with platinum pasteol) and placed on a platinum foil electrode in a vertical furnace. Contact with the upper face is made by a plunger consisting of platinum foil at the end of a platinum wire passing through a ceramic tube. This slides freely in a quartz guide tube, neither touching any part of the furnace. Some platinum diffuses into the crystal, particularly at the positive electrode, but sanding away a few mils from each electrode area appears to remove this. Even at 1200°C poling takes about 10 min, and longer periods are necessary at lower temperatures. If the process is interrupted before completion, it can be seen that one type of polarity domains are growing at the expense of the other type; accordingly the process occurring appears to be one of sidewise domain wall motion.(ls) Alternatively, the crystal may be poled by heating above 121O”C, and then cooling to below this temperature with the electric field turned on. If such a single domain crystal is reheated above 1210°C without an electric field, a multidomain crystal results. Below 1210°C however even extended heating does not affect the domain structure in the absence of afield, indicating 1210°C to be the Curie temperature, with an estimated uncertainty of + 10°C. Experiments on the variation of the resistivity with temperature, differential thermal analysis, and the thermal expansion and high temperature X-ray workos) all show no evidence of a phase change between room temperature and the Curie temperature. Preliminary pyroelectric measurements indicate that the spontaneous polarization decreases monotonically with increasing temperature,(s) substantiating the assumption previously made.(5)
The thermal expansion coefficient a in the a-axis direction is 16.7 x 10-s per “C over the range room temperature up to 800°C; the c-axis coefficient is 2 x 10-s per “C over the range room temperature to 6OO”C.(ls) The density is 4.64 g/cm3 at 25”C, and the Moh’s hardness about 5. The material is brittle and a reproducible Knoop diamond penetration hardness at loads as low as 5 g could not be obtained due to cracking. Lithium niobate is optically uniaxial negative, with ns = 2.2967 and ne = 2.2082 at 6000 A, giving a birefringence of 0*0885.(s) The optical nonlinear coefficients and the indices of refraction from 0.42 to 4.00~ have been reported,(s) as have been the temperature variation of the birefringence and the dispersion.@) The optical transmission curve of LiNbOs is given in Fig. 5. When allowance is made for reflection losses at the crystal surfaces, the transmission becomes lOOo/o within experimental accuracy in the central region.
5. MISCELLANEOUS PROPERTIES A number of additional crystalline properties have been determined in this and related work on LiNbOs, and are summarized below. The resistivity p of a c-axis slice was measured and found to vary from 5 x 10s n cm at 400°C to 140 fi cm at 1200°C. Expressing the temperature in degrees Kelvin, the approximate resistivity in fi cm over this temperature range is given by the expression
The melting point has been given as 1253’%,(s) 125O”C,(ls) and 117O”C,(14)the value of 1260 + 10°C was obtained in the present work. Piezoelectric parameters have been reported by WARNER;~~) coupling constants as high as 57 o/o have been obtained. The velocity of sound in LiNbOs is 7.5 x 105 cm/set longitudinal and 3.7 x 105 cm/set transverse; the elastic Q is about 105.(4) The dielectric constant (K) and loss tangent (dissipation factor) variation with crystal orientation, temperature, and frequency was determined. For l-100 MC/S a Model 33A-Sll admittance
7150 logp = 7 -2.823
FIG. 5. Optical transmission curve of LiNbOs.
994
K. NASSAU,
H.
J. LEVINSTEIN
bridge, and for 100 kc/s a Model 74C-58 capacitante bridge, both from the Boonton Electronics Corp. were used. In Fig. 6 is shown the dielectric constant variation with temperature at 100 kc/set for a c-axis slice [cut parallel to (00-l)] and an a-axis slice [cut parallel to (10-O)]. The c-axis curve shows the expected high temperature increase as the Curie temperature is approached.
100
zoo
300
400
and G. M. LOIACONO possibility of oxygenvacancymovements. Investigation is continuing on this matter. It has always been assumed that lithium niobate and lithium tantalate have the same structure and have similar properties. Several properties are so different however, that in the absence of detailed work this near identity must remain an open question; some of these differences are summarized
500
600
700
TEMPERATURE
PC)
FIG. 6. Dielectric constant variation of LiNbOs temperature at lo6 c/s.
A plot of the reciprocal of the susceptibility (x = K- 1) vs. temp.@) for the c-axis data gives a straight line from 300 to 95O”C, with an extrapolated Curie-Weiss intercept of about 1080°C. If this extrapolation, admitted a long one, were valid, it would imply a first order phase change at the Curie temperature of 1210°C. Dielectric loss tangent data for Fig. 6 were as follows: tan6
400°C
SOO’C 600°C 7OO’C 8OO’C 900°C
c-axis a-axis
0.001 0.0006
0.016 0.01
0.12 0.1
1.0 0.8
5 8
11 25
The variation ofthe dielectric constant at various temperatures and the loss tangent at 25°C with frequency is presented in Figs. 7 and 8. At least one strong dispersion and associated loss appear at about 107 c/s; this does not correspond to an acoustic mode for the size samples used (O-3 x O-3 x O-1 in. approximately). Loss peaks such as this have been attributed to the movement of charged particles or to second phase effects.(s2) There appears to be no obvious mechanism available in LiNBOs to account for the loss except for the
with
in Table 2. The electro-optic experiments on LiTaOs of Ref. 18 were performed with multidomain crystals; recent observation@) on single domain crystals of LiTaOs do show the presence of a significant electro-optic effect. Some of the electro-optic constants rif of LiTaOs are similar in magnitude, others are smaller by orders of magnitude than those of LiNbOs. Table 2 Property Curie temp. Melting temp. Birefringence at 0.6 p
LiNbOs
LiTaOs
Reference
1210°c 126O’C
665°C 1650°C
16, this work this work, 13
+O.OW
5, 17
-0.0885
Recent work by SHAPIRO et aZ,(23) on LiTaOsLiNbOs system reports a continuous isomorphous solid solution series, identified as being ilmenite structure on the basis of powder diffraction patterns only. Curie temperatures were determined from ceramic dielectric constant measurements in the O-8Oo/o LiNbOs range; extrapolation
FERROELECTRIC
LITHIUM
NIOBATE.
2
995
C-AXIS
a-AXIS
rd
(CYCLES,&DND)
FIG. 7. Dielectric constant variation of LiNbOs frequency, temperature, and orientation.
to pure LiNbOa is said to give a value above their melting point of 1170°C; from the diagram given, a value of about 1210°C may be estimated in good agreement with this work. 0'25,
0 10’
I
106 FREQUENCY
10’ (CYCLES/SECOND)
,
7
ti FREQUENCY
100
FIG. 8. Loss tangent variation of LiNbOs with frequency at 25°C.
with
A detailed X-ray diffraction determination crystal structure of LiNbOs is described following paper.
of the in the
authors wish to thank G. D. Bon, and G. E. PETERSON,whose enthusiasm provided the initial stimulus for much of this investigation. Helpful discussion with R. C. MILLER and R. A. LAUDISE were also appreciated. The assistance of C. D. CAPIO in many of the measurements is gratefully acknowledged, as is also the permission given to use unpublished data by the various workers cited as well as Miss D. M. DODD and R. L. BARNS.
Acknowledgements--The
REFERENCES 1. NASSAUK., LEVINSTEINH. J. and LOIACONOG. M., J. Phys. Chem. Solids 27, 983 (1966). 2. NASSAU K., LEVINSTEINH. J. and LOIACONOG. M., Appl. Phys. Lett. 6, 69 (1965). 3. SAVAGEA. and MILLER R. C. (unpublished observations). 4. SPENCER E. G., LENZO P. V. and NASSAUK., Appl.
Phys. Lett. 6, 67 (19651. 5. BOG G. D., ~&LLA R. d., NASSAUK., BOND W. L. and SAVAGEA., Appl. Phys. Lett. 5,234 (1964).
996
K. NASSAU,
H. J. LEVINSTEIN
6. MILLER R. C., Bovn G. D. and SAVAGE A., Appl. Phys. Lett. 6, 77 (1965). 7. A~HKIN A. and Bon, G. D. (unpublished observations) . 8. GIORDMAINEJ. A. and MILLER R. C., Phys. Rev. Lett. 14, 973 (1965). 9 REISMAN A. and HOLTZBERC F., J. Am. Chem. Sot. Sot. 80, 6503 (1958). 10. JONA J. and SHIRANEG., Ferroelectric crystals, p. 171. MacMillan, New York (1962). 11. Platinum Paste #6082 Hanovia Liquid Gold Cornpany, East Newark, New Jersey. 12. AEXAHAMSS. C., LEVINSTEINH. J. and REIIDY J. M., J. Phys. Chem. Solids 27, 1019 (1966). 13. BALLMANA. A., J. Am. &ram. Sot. 48,112 (1965). 14. FEDULOV S. A., SHAPIRO 2. I. and LAIJYZHENSKII P. B., Kristallografya 10, 268 (1965). 15. WARNER A. W., Proc. of the 19th Annual Symposium on Frequency Control, Atlantic City, New Jersey, 19 April 1965 (to be published). Yu N. 16. SHAPIROZ. I., FEDULOVS. A. and VENEVTSEV Fiz. Tverd. Tela 6, 316 (1964).
and
G.
M.
LOIACONO
17. BOND W. L., J. Appl. Phys. 36, 1674 (1965). 18. PETERSONG. E.. BALLMANA. A.. LENZO P. V. and BRIDENHAUCHP. M., Appl. Ph&. Lett. 5,62 (1964) J. M. and BERNSTEINJ. L., 19. ABRAHAMSS. C., REDDY J. Phys. Chem. Solids 27, 997 (1966). 20. LENU) P. V., SPENCERE. G. and BALLMAN A. A. (to be published). to Solid State Physics; 21. KITTEL C., Introduction p. 199, 2nd edition. John Wiley, New York (1956). Sot. 47, 539 22. See e.g. FLOYD J. R., J. Am. Gram. (1965). 23. SHAPIRO Z. I., FEDULOV S. A., VENEVTSEVYu. N. and RIGERMANL. G., Izv. Akad. Nauk. SSSR. Ser. Fiz. 29, 1047 (1965).
Note added in proof-Note 1 added in proof to Paper 5 of this series(rs) should also be considered part of this paper.
Pergamon
Press 1966. Vol. 27, pp. 989-996.
FERROELECTRIC PREPARATION K.NASSAU, Bell Telephone
Printed in Great Britain.
LITHIUM
NIOBATE.
OF SINGLE DOMAIN H. J. LEVINSTEIN Laboratories,
2.
CRYSTALS
and G. M. LOLPCONO
Incorporated
Murray
Hill, New Jersey
(Received 8 October 1965)
Abstract-Lithium niobate is shown to be an unusual ferroelectric material with the very high Curie temperature of 1210°C. Three techniques are presented for the preparation of single domain LiNbOs crystals. The first uses the presence of Moos or WOs in the melt. The other two techniques require the application of an electric field either during or after growth. Poling is complete at 1200°C with the application of as little as 0.2 V/cm for a period of 10 min. Other physical properties given include dielectric constant, resistivity, thermal expansion, and optical data. LiNbOs and LiTaOs differ from each other in several important respects.
1. INTRODUCTION
compensation
In the preceding paper,(l) the growth, domain structure, dislocations, and etching behavior of lithium niobate (LiNbOs) were discussed in some detail; ferroelectric domains have been shown to be present and to interfere with many of the potential uses of this material. Three techniques have been developed for their elimination; these depend on the addition of molybdenum or tungsten oxide during growth, or the application of an electric field either during or after growth. Lithium niobate is an unusual type of ferroelectric material in that it has the very high Curie temperature of 121O”C, and a field of less than 0.2 Vfcm is needed near this temperature for domain reversal.(s) A number of physical constants of LiNbOs have also been determined; the properties of lithium niobate and lithium tantalate appear to differ in several important respects, and these differences are summarized. 2. TECHNIQUE
1: MOLYBDENUM
:
(1 -x)LiNbOs+XMoOs
+ (Lil_,~,)(Nbl_,5+Mo,e+)Os,
where + denotes a vacancy in a lithium site. Under a series of very limited growth conditions, the multidomain appearance seen on polishing and etching(l) disappears completely. The single domain nature of such crystals has been confirmed by pyroelectric,(s) acoustic,@) phase matched harmonic generation,(sJQ difference frequency generation,(T) and tuneable optical parametric oscillation@) experiments. The following listing presents a particular set of conditions, which appears satisfactory, but not always sufficient, for the growth of single domain LiNbOs :
ADDITION
When molybdenum in the form of Moos is added at low concentrations to LiNbOs, about one twentieth of this amount appears in a crystal pulled from the melt as described previously.(l) It is assumed that the molybdenum replaces niobium, forming a lithium vacancy for charge 989
(1) 0.5 at. % Moos added to the melt, (2) growth direction 20-40” from the c-axis, e.g. perpendicular to (11*12), (3) negative dipole end of crystal(l) facing the melt, (4) flat melt-crystal interface, (5) seed free from twin boundaries, and very stable temperature so that twins will not form, (6) steady, slow pulling rate, e.g. l/2-3/4 in./hr.
990
K. NASSAU,
H. J. LEVI~STE~N
In addition to these necessary requirements there are several optional conditions which reduce cracking of the. crystals: (7) composition ranging from nominally stoichiometric LiNbOs to 2 at. 0/Oin either direction have been used; it is sometimes helpful to purify by first pulling single crystal material and then using this as feed material, and (8) use of platinum only in contact with the melt; both iridium and rhodium make it difficult or impossible to obtain colorless crystals. The crystals are annealed in oxygen at about 1100°C overnight and cooled slowly to room temperature. This removes the pale brown color which probably originates from oxygen vacancies and niobinm in a lower valence state.(Q) The flat interface of (4) has been achieved by growing at a low level in the crucible or by the use of an after heater as in the second technique. In crystals grown from a rhodium crucible O-030.1 y0 rhodium is found spectroscopically. Platinum and iridium are not detectable spectroscopically in crystals grown from these crucibles. A variety of additives have been tried, but tungsten is the only other ion that has shown any activity. The mechanism is not understood at present. One possibility is that the low distribution coefficient of about O-05 in the case of MO, and the non-turbulent stirring present, result in a layer of rejected MO** ions close to the growth interface, thus producing a small electric field and simulating the second technique. 3. TECHNIQUE 2: GROWTH IN AN ELECTRIC FIELI)
The application of a large electric field at the melt interface was at first considered to be impracticable due to the high resistance of the cooler seed-end of the crystal and the low resistance of the hot region near the growth interface. However, high voltages were found not to be necessary for single domain production. Resistance measurements made on single crystals at ternperat~~ near the melting point indicated resistivity values of a few hundred !&cm. The actual resistance (that of the seed, crystal, crucible, etc.) was found to range from 1000 to 5000 B at the start of crystaf growth. This depends on the length and diameter of the seed and depth in the crucible. As little ss l/Z v applied to a l/2 in. seed, producing a current of
and G. M. LXOACONO
l/2 II& has resulted in the complete removal of one type of antiparallel domain, yielding a single domain crystal. The pulling apparatus used for these experiments was a modified one as shown in Fig. 1. A rotating friction contact is used to apply the voltage to the insulated crystal holder. The circuit shown in Fig. 1 is designed to hold the current steady at approximately 24 or 5 mA even though the resistance of the crystal changes. In exploratory work a variable voltage supply capable of 1000 V and 100 mA was used. The electrical circuit is not grounded except through the crucible so as not to interfere with the temperature controlling system. The direction of the field can be changed by a reversing switch not shown; the voltage drop across the crystal plus melt waQ measured by a vacuum tube voltmeter. Table 1
Growth time without after-heater (hr) 0 112 :.
6,500 25,000 145,000 -
with after-heater 3,200 10,600 17,500 66,000
As the crystal grows its resistance increases rapidly. Table 1 shows this effect during the growth of about 1 cm dia. crystals at a pulling rate of 314 in,lhr. The table also shows that the use of the after-heater shown in Fig. 1, held at an uncorrected optical pyrometer temperature of 105O”C, considerably reduced this increase in resistance. Since the electric field can only be active in the vicinity of the growth interface, it is convenient to control this at a constant value by maintaining a constant current; ideally the magnitude of the current should be adjusted to the cross-sectional area of the crystal, and the orientation of the c-axis with respect to the pulling direction. The results of a typical experiment with C-Z&J growing LiNbOs is illustrated in Figs. 2 and 3 ; the field designation indicates the electrical palarity connected to the crystal. The multidomain
FERROELECTRIC
LITHIUM
NIOBATE.
-PULLING
MECHANISM
.-CERAMIC
INSULATOR
2
991
8
R
I
---PLATINUM __..e....
MOLTEN
FIG. 1.
e
LiNbO,
Apparatus for the Czochralski growth of LiNbOa in an electric field.
regions have the same appearance previously described,(r) single domain regions showing no structure. Changing the magnitude of the current has no effect once the necessary minimum has been exceeded. Reversing the direction of the field, however, produces a sharp 180” domain
wall; the position and shape apparently corresponds to the growth interface. In crystals grown along the c-axis l/2 mA, or about 2 mA/cms, is the
FIG. 2. Appearance of a LiNbOa crystal grown under varying polarities and currents.
minimum current necessary. With the crystal electrically negative (with respect to the melt) up to 25 mA has been used, but above this value the current becomes unstable and difficult to control, perhaps due to localized resistive heating in the crystal. With the crystal positive the limit is approximately 10 mA. More current is required as the orientation departs from c-axis, as then only that part of the field resolved along the c-axis is active. Nevertheless, crystals up to 80 degrees from the c-axis have been grown single domain at 10 mA. The actual voltage drop required in the vicinity of the growth interface is apparently very low. With the crystal electrically negative with respect to the melt, the upper surface of the crystal when cut transversely is the positive dipole surface and is found to etch slowly(r) as expected. This is the surface which becomes positive in a pyroelectric experiment on cooling, and has been defined as the +c end of the crystal.(s)
992
K.
NASSAU,
H.
J.
LEVINSTEIN
In view of the results of the third technique (poling after growth) it is expected that the effect of the field would be significant only at the 1210°C isotherm in the crystal; it is neverthless possible that due to electrolytic action some structure is being imposed at the growth interface and survives through the paraelectric region. Impurities such as vanadium or molybdenum up to the 1 y0 level do not interfere with this process, but tungsten
and
G.
M.
LOIACONO
4. TECHNIQUE
3: POLING
Poling, i.e. the electric reversal of the polarization direction of a ferroelectric, has usually been carried out on thin slices of material with high fields applied at temperatures near room temperature.(ls) These conditions do not produce any poling on LiNbOs. The reverse conditions, namely low fields at very high temperatures, were tried and did convert multidomain lithium niobate
ION
D ENT
PLATINUM
tb)
FIG. 4. Arrangements for poling LiNbOs crystals; crystals as in (a), short crystals as in (b).
does seem to prevent single domain growth with an electric field applied. The addition of impurities may be desirable to adjust the refractive indexes so that phase matched harmonic generation without double refraction(s) can occur at a convenient temperature. Interaction is often observed at the platinumcrystal contact when the crystal is held electrically positive. X-ray diffraction examination of the tan product shows this to consist of NbaOs, apparently produced by electrolytic action. Cracking of the seed is often a result of this attack, and it seems preferable to grow with the crystal negative, since this also permits stability at higher currents.
long
to single domain crystals. Fields of O-2-5 V/cm have been used for this purpose at temperatures from 1090 to 1210°C. At such high temperatures it is important to avoid alternate parallel paths through the various furnace components. The two arrangements shown in Fig. 4 have been used successfully. Long thin crystals are hung from platinum wires anchored in grooves cut into the crystal. Platinum legs may also be used at one end of the crystal only. The wires which also carry the current are kept under tension to support the crystal in a horizontal tube furnace; contact with any part of the furnace is permitted at only one point. Short wide crystals
FIG. 3. Polished LiNbOs crystal showing transition to single domain growth when the field is turned on ( x 100).
FERROELECTRIC
LITHIUM
NIOBATE.
2
993
are painted on both ends with platinum pasteol) and placed on a platinum foil electrode in a vertical furnace. Contact with the upper face is made by a plunger consisting of platinum foil at the end of a platinum wire passing through a ceramic tube. This slides freely in a quartz guide tube, neither touching any part of the furnace. Some platinum diffuses into the crystal, particularly at the positive electrode, but sanding away a few mils from each electrode area appears to remove this. Even at 1200°C poling takes about 10 min, and longer periods are necessary at lower temperatures. If the process is interrupted before completion, it can be seen that one type of polarity domains are growing at the expense of the other type; accordingly the process occurring appears to be one of sidewise domain wall motion.(ls) Alternatively, the crystal may be poled by heating above 121O”C, and then cooling to below this temperature with the electric field turned on. If such a single domain crystal is reheated above 1210°C without an electric field, a multidomain crystal results. Below 1210°C however even extended heating does not affect the domain structure in the absence of afield, indicating 1210°C to be the Curie temperature, with an estimated uncertainty of + 10°C. Experiments on the variation of the resistivity with temperature, differential thermal analysis, and the thermal expansion and high temperature X-ray workos) all show no evidence of a phase change between room temperature and the Curie temperature. Preliminary pyroelectric measurements indicate that the spontaneous polarization decreases monotonically with increasing temperature,(s) substantiating the assumption previously made.(5)
The thermal expansion coefficient a in the a-axis direction is 16.7 x 10-s per “C over the range room temperature up to 800°C; the c-axis coefficient is 2 x 10-s per “C over the range room temperature to 6OO”C.(ls) The density is 4.64 g/cm3 at 25”C, and the Moh’s hardness about 5. The material is brittle and a reproducible Knoop diamond penetration hardness at loads as low as 5 g could not be obtained due to cracking. Lithium niobate is optically uniaxial negative, with ns = 2.2967 and ne = 2.2082 at 6000 A, giving a birefringence of 0*0885.(s) The optical nonlinear coefficients and the indices of refraction from 0.42 to 4.00~ have been reported,(s) as have been the temperature variation of the birefringence and the dispersion.@) The optical transmission curve of LiNbOs is given in Fig. 5. When allowance is made for reflection losses at the crystal surfaces, the transmission becomes lOOo/o within experimental accuracy in the central region.
5. MISCELLANEOUS PROPERTIES A number of additional crystalline properties have been determined in this and related work on LiNbOs, and are summarized below. The resistivity p of a c-axis slice was measured and found to vary from 5 x 10s n cm at 400°C to 140 fi cm at 1200°C. Expressing the temperature in degrees Kelvin, the approximate resistivity in fi cm over this temperature range is given by the expression
The melting point has been given as 1253’%,(s) 125O”C,(ls) and 117O”C,(14)the value of 1260 + 10°C was obtained in the present work. Piezoelectric parameters have been reported by WARNER;~~) coupling constants as high as 57 o/o have been obtained. The velocity of sound in LiNbOs is 7.5 x 105 cm/set longitudinal and 3.7 x 105 cm/set transverse; the elastic Q is about 105.(4) The dielectric constant (K) and loss tangent (dissipation factor) variation with crystal orientation, temperature, and frequency was determined. For l-100 MC/S a Model 33A-Sll admittance
7150 logp = 7 -2.823
FIG. 5. Optical transmission curve of LiNbOs.
994
K. NASSAU,
H.
J. LEVINSTEIN
bridge, and for 100 kc/s a Model 74C-58 capacitante bridge, both from the Boonton Electronics Corp. were used. In Fig. 6 is shown the dielectric constant variation with temperature at 100 kc/set for a c-axis slice [cut parallel to (00-l)] and an a-axis slice [cut parallel to (10-O)]. The c-axis curve shows the expected high temperature increase as the Curie temperature is approached.
100
zoo
300
400
and G. M. LOIACONO possibility of oxygenvacancymovements. Investigation is continuing on this matter. It has always been assumed that lithium niobate and lithium tantalate have the same structure and have similar properties. Several properties are so different however, that in the absence of detailed work this near identity must remain an open question; some of these differences are summarized
500
600
700
TEMPERATURE
PC)
FIG. 6. Dielectric constant variation of LiNbOs temperature at lo6 c/s.
A plot of the reciprocal of the susceptibility (x = K- 1) vs. temp.@) for the c-axis data gives a straight line from 300 to 95O”C, with an extrapolated Curie-Weiss intercept of about 1080°C. If this extrapolation, admitted a long one, were valid, it would imply a first order phase change at the Curie temperature of 1210°C. Dielectric loss tangent data for Fig. 6 were as follows: tan6
400°C
SOO’C 600°C 7OO’C 8OO’C 900°C
c-axis a-axis
0.001 0.0006
0.016 0.01
0.12 0.1
1.0 0.8
5 8
11 25
The variation ofthe dielectric constant at various temperatures and the loss tangent at 25°C with frequency is presented in Figs. 7 and 8. At least one strong dispersion and associated loss appear at about 107 c/s; this does not correspond to an acoustic mode for the size samples used (O-3 x O-3 x O-1 in. approximately). Loss peaks such as this have been attributed to the movement of charged particles or to second phase effects.(s2) There appears to be no obvious mechanism available in LiNBOs to account for the loss except for the
with
in Table 2. The electro-optic experiments on LiTaOs of Ref. 18 were performed with multidomain crystals; recent observation@) on single domain crystals of LiTaOs do show the presence of a significant electro-optic effect. Some of the electro-optic constants rif of LiTaOs are similar in magnitude, others are smaller by orders of magnitude than those of LiNbOs. Table 2 Property Curie temp. Melting temp. Birefringence at 0.6 p
LiNbOs
LiTaOs
Reference
1210°c 126O’C
665°C 1650°C
16, this work this work, 13
+O.OW
5, 17
-0.0885
Recent work by SHAPIRO et aZ,(23) on LiTaOsLiNbOs system reports a continuous isomorphous solid solution series, identified as being ilmenite structure on the basis of powder diffraction patterns only. Curie temperatures were determined from ceramic dielectric constant measurements in the O-8Oo/o LiNbOs range; extrapolation
FERROELECTRIC
LITHIUM
NIOBATE.
2
995
C-AXIS
a-AXIS
rd
(CYCLES,&DND)
FIG. 7. Dielectric constant variation of LiNbOs frequency, temperature, and orientation.
to pure LiNbOa is said to give a value above their melting point of 1170°C; from the diagram given, a value of about 1210°C may be estimated in good agreement with this work. 0'25,
0 10’
I
106 FREQUENCY
10’ (CYCLES/SECOND)
,
7
ti FREQUENCY
100
FIG. 8. Loss tangent variation of LiNbOs with frequency at 25°C.
with
A detailed X-ray diffraction determination crystal structure of LiNbOs is described following paper.
of the in the
authors wish to thank G. D. Bon, and G. E. PETERSON,whose enthusiasm provided the initial stimulus for much of this investigation. Helpful discussion with R. C. MILLER and R. A. LAUDISE were also appreciated. The assistance of C. D. CAPIO in many of the measurements is gratefully acknowledged, as is also the permission given to use unpublished data by the various workers cited as well as Miss D. M. DODD and R. L. BARNS.
Acknowledgements--The
REFERENCES 1. NASSAUK., LEVINSTEINH. J. and LOIACONOG. M., J. Phys. Chem. Solids 27, 983 (1966). 2. NASSAU K., LEVINSTEINH. J. and LOIACONOG. M., Appl. Phys. Lett. 6, 69 (1965). 3. SAVAGEA. and MILLER R. C. (unpublished observations). 4. SPENCER E. G., LENZO P. V. and NASSAUK., Appl.
Phys. Lett. 6, 67 (19651. 5. BOG G. D., ~&LLA R. d., NASSAUK., BOND W. L. and SAVAGEA., Appl. Phys. Lett. 5,234 (1964).
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K. NASSAU,
H. J. LEVINSTEIN
6. MILLER R. C., Bovn G. D. and SAVAGE A., Appl. Phys. Lett. 6, 77 (1965). 7. A~HKIN A. and Bon, G. D. (unpublished observations) . 8. GIORDMAINEJ. A. and MILLER R. C., Phys. Rev. Lett. 14, 973 (1965). 9 REISMAN A. and HOLTZBERC F., J. Am. Chem. Sot. Sot. 80, 6503 (1958). 10. JONA J. and SHIRANEG., Ferroelectric crystals, p. 171. MacMillan, New York (1962). 11. Platinum Paste #6082 Hanovia Liquid Gold Cornpany, East Newark, New Jersey. 12. AEXAHAMSS. C., LEVINSTEINH. J. and REIIDY J. M., J. Phys. Chem. Solids 27, 1019 (1966). 13. BALLMANA. A., J. Am. &ram. Sot. 48,112 (1965). 14. FEDULOV S. A., SHAPIRO 2. I. and LAIJYZHENSKII P. B., Kristallografya 10, 268 (1965). 15. WARNER A. W., Proc. of the 19th Annual Symposium on Frequency Control, Atlantic City, New Jersey, 19 April 1965 (to be published). Yu N. 16. SHAPIROZ. I., FEDULOVS. A. and VENEVTSEV Fiz. Tverd. Tela 6, 316 (1964).
and
G.
M.
LOIACONO
17. BOND W. L., J. Appl. Phys. 36, 1674 (1965). 18. PETERSONG. E.. BALLMANA. A.. LENZO P. V. and BRIDENHAUCHP. M., Appl. Ph&. Lett. 5,62 (1964) J. M. and BERNSTEINJ. L., 19. ABRAHAMSS. C., REDDY J. Phys. Chem. Solids 27, 997 (1966). 20. LENU) P. V., SPENCERE. G. and BALLMAN A. A. (to be published). to Solid State Physics; 21. KITTEL C., Introduction p. 199, 2nd edition. John Wiley, New York (1956). Sot. 47, 539 22. See e.g. FLOYD J. R., J. Am. Gram. (1965). 23. SHAPIRO Z. I., FEDULOV S. A., VENEVTSEVYu. N. and RIGERMANL. G., Izv. Akad. Nauk. SSSR. Ser. Fiz. 29, 1047 (1965).
Note added in proof-Note 1 added in proof to Paper 5 of this series(rs) should also be considered part of this paper.