Ferroelectrics Dielectric, pyroelectric and structural ...

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Dielectric, pyroelectric and structural properties of LiTaO3 thin films grown on silicon by a modified molecular beam epitaxy a

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Ye Tao , Frank Gitmans , Zlatko Sitar a

a

a b

, Harald a

Pierhöfer , Armin Kündig , Iris Gamboni & Peter Günter a a

Swiss Federal Institute of Technology, Institute of Quantum Electronics, Nonlinear Optics Laboratory , CH-8093, Zurich, Switzerland b

North Carolina State University, Department of Materials Science and Engineering , Raleigh, NC, 27695, USA Published online: 07 Mar 2011.

To cite this article: Ye Tao , Frank Gitmans , Zlatko Sitar , Harald Pierhöfer , Armin Kündig , Iris Gamboni & Peter Günter (1997) Dielectric, pyroelectric and structural properties of LiTaO3 thin films grown on silicon by a modified molecular beam epitaxy, Ferroelectrics, 201:1, 245-253, DOI: 10.1080/00150199708228374 To link to this article: http://dx.doi.org/10.1080/00150199708228374

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0 1997 OPA (Overseas Publishers Association) Amsterdam B.V. Published under license in The Netherlands under the Gordon and Breach Science Publishers imprint Printed in India

!v,roe/i,ctrii~. 1997, Vol. 201,pp. 245-253 Reprints avdilahle directly from the publisher l’hotocopying permitted by license only


DIELECTRIC, PYROELECTRIC AND STRUCTURAL, PROPERTIES OF LiTa03 THIN FILMS GROWN ON SILICON BY A MODIFIED MOLECULAR BEAM EPITAXY YE TAO, FRANK GITMANS, ZLATKO SITAR*, HARALD PIERHOFER, ARMIN KUNDIG, IRIS GAMBONI and PETER GUNTER Swiss Federal Institute of Technology, Institute of Quantum Electronics, Nonlinear Optics Laboratory, CH-8093 Zurich, Switzerland. *North Carolina State University, Department of Materials Science and Engineering, Raleigh, NC 27695, USA (Rrtelued28 August 1996, In final form 15 January 1997)

Abstract Polycrystalline lithium tantalate (LiTa03) thin films have been prepared using a modified molecular beam epitaxy process on silicon (1 11) substrates which have an epitaxial platinum silicide (PtSi) buffer layer serving as a growth template and oxygen barrier. X-ray photoelectron spectroscopy on the films showed a stoichiometric composition. X-ray diffraction revealed a preferentially oriented polycrystalline structure with LiTaO3 (012) parallel to the surface. Atomic force microscope measurements on a 200 nm film showed growth islands with typical diameters of 0.4 pm and a root mean square roughness of 50 nm. For the first time, the ferroelectric properties of these very thin LiTa03 films (< 0.5 pm) grown on silicon substrates have been investigated. The samples show a phase transition temperature between 580 and 650°C a coercive field of 15-22 kV/cm and an apparent spontaneous polarization up to 1 pC/cm2. The pyroelectric current response showed a value of 100-200 pNW. INTRODUCTION Due to its excellent piezoelectric, pyroelectric and optical properties, lithium tantalate iLiTa03) is a well-known and extensively studied ferroelectric crystal.’ The large pyroelectric coefficient, high Curie temperature, low dielectric constant and stable physical and chemical properties of LiTaO3 crystals make them very attractive for pyroelectric applications. The techniques for growing large LiTa03 single crystals have been developed during the ~ O ’ S ,but the current trend toward miniaturization of electronic and optic devices has put high demands on thin films of LiTaO3. Moreover, in order to be compatible with the main stream of microelectronics and to fabricate monolithic infrared detectors using the state of the art silicon technology, LiTaO3 thin films grown on silicon substrates are highly desirable. However, the growth of epitaxial single crystalline LiTa03 thin films on silicon substrates Gas been hampered due to the lattice mismatch between the two materials, and the easy oxidation of silicon surfaces in an oxygen-containing atmosphere. To grow instead preferentially oriented polycrystalline thin films on silicon substrates is a possible 245


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Y. TAO et al.

solution and is also a challenging topic in materials research. As long as the polar axis of LiTaOB polycrystalline films has a large enough component normal to the surface, a polycrystalline thin film is suitable for infrared detector applications. There have been some attempts to grow LiTaO3 thin films on silicon substrates using different growth techniques, such as RF sputtering,2.3 pulsed laser deposition (PLD),4 and sol-gel technique.5 The problem of growing preferentially oriented LiTa03 thin films on silicon substrate using molecular beam epitaxy (MBE) technology has been addressed in our previous papeP by introducing an epitaxially grown PtSi buffer layer on Si (1 11) to serves as a growth template for LiTa03 and an oxygen barrier to prevent the underlying Si from being oxidized. In this work, we report the dielectric, pyroelectric and structural properties of these LiTa03 thin films grown on PtSi/Si substrates.

EXPERJMENTAL The MBE system used for the deposition has been described in detail el~ewhere.~ Briefly, it is a multi-chamber MBE system with growth, transfer and surface analysis facilities. The MBE chamber itself consists of two electron-beam evaporators and two Knudsen cells. An electron cyclotron resonance (ECR) plasma source was attached to supply reactive oxygen during the growth. All Si (1 11) substrates used for the deposition were treated with an HF-last RCA cleaning procedure prior to loading into the ultra high vacuum (UHV) system. PtSi was prepared by evaporating metallic Pt from an electron beam evaporator onto the Si (1 1 1) substrate at a low deposition rate of 0.05 rids. The substrate temperature was held at 800°C during the deposition. The PtSi was formed upon Pt reacting with the silicon surface layer. The samples were normally given a post-growth anneal at the same temperature for 15 min. in UHV to improve the film quality. LiTa03 thin films were deposited thereafter. Metallic lithium and tantalum pentooxide were used as source materials. The material flux can be actively controlled better than 1% via a quadrupole mass spectrometer. Reactive oxygen was supplied by the ECR source. The substrate temperature was kept at 900°C during the deposition. LiTaO3 thin films with thickness up to 500 nm were prepared. Details about the growth procedure and parameters can be found in ref. 6. After the deposition, the samples were transferred to the surface analysis chamber via a UHV tunnel. The chemical composition was measured using a dual-anode x-ray photoelectron spectrometer (XPS). X-ray diffraction (XRD) and atomic force microscopy (AFM) were performed ex-sifu.Gold(200 nm)/tantalum ( 10 nm) interdigital electrodes for electrical measurements were prepared on the samples by using electron beam evaporation and lift-off photolithographic techniques. Fig. 1 illustrates the electrodes used in our experi-

PROPERTIES OF LiTaOj THIN FILMS

241

4.0 mm

FIGURE 1 Interdigital electrodes for dielectric measurements. The right drawing shows a cross-section of the prepared sample.


ments. Dielectric permittivity curves were measured using a HP 4192A low frequency impedance analyzer. During the measurement, the samples were heated in a hot tube furnace with a temperature ramp of 2S°C/min., and they were mounted on an Inconela holder which provides a uniform temperature distribution across the film. Pyroelectric measurements were performed by heating the sample with a pulsed laser diode (633 nm, average power 1.7mW). A 100 nm-thick evaporated gold film was used as absorption layer. The gold layer also served as top electrode, the bottom electrode was made of platinum paste on the backside of the silicon wafer. The laser diode was used in a pulsed mode at different frequencies with a lock-in amplifier used to gate the diode by TTL signals and to measure the resulting current or voltage. RESULTS AND DISCUSSION Epitaxial Relationship Between PtSi and Si( 111) Our previous XRD and transmission electron spectroscopy measurements have shown that the PtSi buffer layer prepared on Si (1 1 1) substrate has a (010) surface and an abrupt interface with the substrate.6 Since PtSi has an orthorhombic structure and the Si ( 1 11) surface has a six-fold symmetry, a single-crystallineepitaxial growth is difficult to realize, while a columnar growth of strained epitaxial PtSi domains is possible.*. 9 Fig. 2 shows a low energy electron diffraction (LEED) patter from a 30 nm-thick PtSi buffer layer and illistrates the epitaxial relationship between the PtSi and silicon (1 1 1). The hexagonal appearance of the LEED pattern arises from three identical but 120" rotated domains of a strained PtSi surface, with the PtSi (010) parallel to the Si (1 11) as confirmed by XRD measurements, and the PtSi [OOI] parallel to a Si direction. The same epitaxial relationship was found on the PtSi/Si( 1 11) samples prepared by co-evaporation of platinum and silicon. lo


Y .TAO et al.

FIGURE 2 Epitaxial relationship between the Si (111) and the three possible PtSi (010) domains. An ideal unit cell of a PtSi (010) plane is also shown. The mismatch between this cell and the lattice parameters of the silicon equals 6 % at the PtSi [OOl] direction and 11% at the PtSi [loo] direction. A LEED pattern (64eV) from an annealed 30 nm-thick PtSi film on a Si (1 11) substrate is also shown. Chemical and Structural Prouerties of LiTaOs Grown on PtSi(OlOYSi(l11) XPS was used to determine the chemical composition of the films. Due to the small photoionization cross-section of lithium atoms, the signal-to-noiseratio of the lithium peak was relatively low as compare to the others, the measured Li concentration is estimated to be accurate within 20 percent, the XPS results show that LiTa03 thin films prepared with optimized deposition parameters6 have a composition of Li l.o*o.2Ta03. The amount of lithium in the film depends strongly on the deposition parameters such as substrate temperature, lithium flux and overall growth rate. Very thin LiTa03 films (1-2 nm) deposited on PtSi/Si substrates showed V-shape streaks in RHEED patterns (Fig. 3), indicating the existence of twinning in the films. A typical XRD spectrum from a 500 nmthick LiTaO3 film grown at 9OOOC on a PtSi(OlO)/Si (1 11) substrate is shown in Fig. 4. Besides the peaks of the substrate (Si and PtSi), all peaks belong to LiTa03. There exists a strong preferential orientation of LiTa03(012)/PtSi(OlO)/Si(111). i. e., the spontaneous polarization vector, Ps,of the films is about 57" off the surface normal.


PROPERTIES OF LiTaOj THIN FILMS

249

FIGURE 3 RHEED pattern of a thin LiTa03 film (=ISnm) on a PtSilSi (111) substrate, indicating the existence of twinning in the film.

k

Si

.

2 0 (deg)

FIGURE 4 XRD spectrum of a 500 nm LiTaO3 film deposited on a PtSi/Si (1 11) substrate at 900°C, indicating a strong preferential orientation of LiTa03(012)/ PtSi(O1 O)/Si( 11 1).

The crystallinity of the films has a strong dependence on the substrate temperature used during the deposition, as shown in Fig. 5. These five spectra were taken from the samples deposited at different substrate temperatures. All spectra were normalized to the silicon peak, and calibrated to the same thickness. At low substrate temperatures (< 750°C) no XRD peak for LiTa03 could be observed. Starting from 750"C, a LiTa03 (012) peak develops and it reaches a maximum at about 900°C. From AFM measurements on a 200 nm-thick LiTaO3 film deposited on PtSi/Si substrate at 900"C, we found that the film has growth islands with typical diameters of 0.4 p m and a root mean square roughness of

Y. TAO et al.

250

50 nm. Some voids were also found in the film. These features might resulted from the columnar growth which is related with the multi-domain PtSi buffer layer.

900°C


800°C 750°C 700°C

540°C 22

24

26

28

)

2 8 (deg) FIGURE 5 XRD spectra of the li;hiu% tantalate (012) peak for films grown at different temperatures. The crystallinity of the films improves with increasing substrate

temperature.

Dielectric Permittivitv and Pvroelectric Response The dielectric permittivity was measured by using the interdigital electrodes illustrated in Fig. 1. The samples were heated with a temperature ramp of 2.5Wmin. Fig. 6 shows a dielectric curve of a 500 nm-thick LiTa03 thin film deposited on PtSi(OlO)/Si(11I). It has a dielectric permittivity peak at a temperature around 615-640°C. This temperature is very close to the Curie temperature of 610°C for LiTa03 bulk crystals. The broad feature of the dielectric peak is probably caused by different Curie temperatures across the film due to the fluctuation in lithium content, which has been proved to affect the phase transition temperature. 1 1 Ferroelectric hysteresis loops measured from a 100 nm-thick film, which have been corrected for the leakage current12 are shown in Fig. 7. The spontaneous polarization component perpendicular to the sample surface showed a strong temperature dependence. At 520°C an averaged spontaneous polarization of 0.05 pC/cm2 at a coercive field of 95 kV/cm has been observed. For temperatures close to the Curie point the apparent spontaneous polarization increased by a factor of ten to about 0.8 pC/cm2 and the coercive field reduced to 20 kV/cm, which is in very good agreement with the values of 18 kV/cm measured from bulk crystals13 and 17 kVlcm measured from a sputtered LiTa03

25 1

PROPERTIES OF LiTa03 THIN FILMS

I

2000,

LiTa03film


-

0 100

300

500

700

Temperature (CO) FIGURE 6 Dielectric permittivity measured at 1 kHz (oscillator voltage: 100 mv) from a 500 nm

thick LiTaO3 film shows a broad maximum at around 630OC. The increase in the curve at around 700OC is due to the conductivity at high temperatures.

film.14 On thicker samples (-400 nm), apparent spontaneous polarization up to 1 pC/cm2 have been obtained. This value is already quite promising by considering, as mentioned earlier, that the P, of the polycrystalline LiTa03 films is about 57" off the surface normal, the P, and the direction of the applied electrical field had an angle of 57", i. e. we just switched one component of the P,, which reduced the value of the measured spontaneous polarization by half, and the voids in the film as indicated by would also reduce the apparent spontaneous polarization in the measurements. 1.o

0.10

605 "C

520°C

-

0.05 TO -T=90°C n

%

5 W

PIv:

'

-0.10 ' ' -400 -200

- -0.5 I

0

200

E [kV/cm]

400

-400

,

-200

'

I

0

200

-1.0

400

E (kV/cm)

FIGURE 7 Hysteresis loops for a 100 nm-thick LiTa03 film. Note the different scaling of the Ps. Transition temperature for LiTa03 TO = 610 "C. Pyroelectric measurements were performed at room temperature. For a 200 nm-thick LiTaOs film both the voltage response (Fig. 8 left) and the current response (Fig. 8 right) have been measured. The sample was heated up by a laser diode with chopper frequencies from 0.1 Hz to 100 lcHz and the resulting pyroelectric current was measured by a lock-in amplifier. The voltage response showed an almost ideal behavior with a low-frequency

Y. TAO et ul

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I +

To* loo

10'

lo2 lo3 lo4

[

,

,,

los


Frequency [Hz]

FIGURE 8 Left: pyroelectric voltage response of a 200 nm-thick LiTaO:, film on PtSi/Si(1 1 1). Right: Current response of the same sample.

increase, which is due to the thermal time constant, to a constant plateau. A decrease in the signal at high frequencies was due to the electrical time constant, which depends on the dielectric constant and the conductivity of the film. The current signal shows a similar behavior. Across the whole film, uniform pyroelectric current response of more than 100 pA/W over a large frequency range has been obtained. For thicker films (360 nm) this value increased to about 170 pLA/w yielding a pyroelectric coefficient o f p = 15 pC/m2K. Considering the fact that there is no thermal isolation between the active layer and substrate, and the porosity of the LiTa03 films, a pyroelectric coefficient of 15 pC/m*K is quite promising for polycrystalline thin films as compare to the value of single crystalline LiTa03 (p = 230 pC/m2K, ref. 15 ). It can be expected that the pyroelectric coefficient will further increase once the thermal isolation is introduced. CONCLUSIONS Preferentially oriented polycrystalline lithium tantalate (LiTa03) thin films have been prepared by the MBE technique on Si (111) substrate using an epitaxial platinum silicide buffer layer as a growth template and oxygen barrier. The films showed typical ferroelectric characteristics. Apparent spontaneous polarization (the component perpendicular to the surface) up to 1 pC/cm2 and a coercive field of 15-22 kV/cm have been measured for films of 100-400 nm-thick. The pyroelectric current response showed a large value of about 100-200 pAlW. The results show that the polycrystalline films of LiTa03 prepared on silicon substrates by MBE are promising candidates for infrared detector applications.

253

PROPERTIES OF LiTa03 THIN FILMS

ACKNOWLEDGMENTS This work was supported by the Swiss priority research program “Power Electronics, System and Information Technology (LESIT)”. REFERENCES 1.


2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Y. Xu, Ferroelectric materials and their applications (North-Holland,Amsterdam, 1991). T.A. Rost, L. He, T.A. Rabson, R.C. Baumann, and D.L. Callahan, J. Appl. Phvs. 72,4336 (1992). 2989 (1990). R.C. Baumann, T.A. Rost, and T.A. Rabson, J. Appl. Phvs. 8, Y. Shibata, K. Kaya, K. Akashi, M. Kanai, T. Kawai, and S. Kawai, &DL Phvs. Lett. 62,3046 (1993). S . Hirano and K. Kato, Mat. Res. SOC. Svmu.Proc. 155. 181 (1989). F. Gitmans, Z. Sitar, and P. Gunter, Microelectronic Engineerin% 29,289 (1995). F. Gitmans, Z. Sitar, and P. Gunter, Vacuum, 46,939 (1995). J.-R. Chen, T.-S. Heh, and M.-P. Lin, Surface Science, 162,657 (1985). Phvs. Lett. 57, R.W. Fathauer, Q.F. Xiao, H. Shin, and C.W. Nieh, &I. 686 (1990). Y. Kumagai, F. Hasegawa, and K. Park, J. Appl. Phys. 321 1( 1994). T. Yamada, N. Niizeki, and H. Toyoda, Jap. J. Appl. Phvs. Z, 298 (1968). H. Pierhofer, F. Gitmans, J. Fousek, and P. Gunter, To be published ,(1996). S.S. Ping and L.H. Lin, Ferroelectrics, 387,821( 1981). A. Kanduser, B. Zigon, D. Mandrino, M. Kosec. P. Panian. and B.B. Lavrencic. Intep. Ferroelectrics, 4,13 (1994). H.P. Beerman, Infrared Physics, 15,225 (1975).

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