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Microstructural, electrical, and transmittance properties of PbTiO3 films grown on p-InP (100) substrates at low temperature
Journal of Physics and Chemistry of Solids 60 (1999) 935–942
Microstructural, electrical, and transmittance properties of PbTiO3 films grown on p-InP (100) substrates at low temperature T.W. Kim a,*, S.S. Yom b b
a Department of Physics, Kwangwoon University, 447-1 Wolgye-dong, Nowon-ku, Seoul, 139-701, South Korea Applied Physics Laboratory, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, South Korea
Received 16 October 1998; accepted 21 December 1998
Abstract PbTiO3 thin films were grown on p-InP (100) substrates by using metalorganic chemical vapor deposition via thermal pyrolysis at a relatively low temperature ( , 3208C) using Pb(tmhd)2, Ti(OC3H7)4, and N2O. X-ray diffraction measurements showed that the PbTiO3 film layer grown on the InP substrate was polycrystalline, and Auger electron spectroscopy measurements indicated that the as-grown films consisted of lead, titanium, and oxygen. Transmission electron microscopy measurements showed that the grown PbTiO3 was a polycrystalline layer with small domains and that the PbTiO3 films had interdiffusion and local epitaxial formations near the PbTiO3/p-InP (100) heterointerface. Room-temperature current–voltage and capacitance–voltage (C–V) measurements clearly revealed a metal–insulator–semiconductor behavior for the PbTiO3 insulator gates, and the interface state densities at the PbTiO3/p-InP interfaces, as determined from the C–V measurements, were approximately low 10 11 eV 21 cm 22 at an energy of about 0.7 eV below the conduction-band edge. The dielectric constant of the PbTiO3 thin film, as determined from the C–V measurements, was as large as 60.4. The temperature-dependent Fourier transform infrared spectroscopy measurements showed several sharp absorption edges in the transmission spectra. These results can help improve understanding of the physical properties of the PbTiO3 layers grown on p-InP (100) substrates at low temperatures for potential high-density dynamic-memory and high-speed applications based on the InP substrates. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Thin films; Metal-insulator-semiconductor
1. Introduction Recently, InP compound semiconductors have attracted much attention because of their applications as substrates in optoelectronic and high-speed electronic devices [1–5]. In contrast to GaAs-metal–insulator–semiconductor (MIS) capacitors, since the Fermi level in the InP–MIS diodes can be moved nearly across the whole energy gap, both inversion and accumulation layers can be achieved [6]. Many groups have investigated the growth of PbTiO3 films on Si substrates as possible deposited insulator layers for Si-MIS applications [7–10]. However, the temperature for deposition of the PbTiO3 insulator layer on the InP substrate must be kept below the InP decomposition * Corresponding author. Tel.: 1 82-02-940-5234; fax: 1 82 02 942 0108.
temperature in order to obtain a high-quality PbTiO3/InP heterointerface. Even though Al2O3 insulator layers were grown on p-InP (100) at 2808C, the interface state density of Al/Al2O3/p-InP, which was approximately high 10 11 eV 21 cm 22 at the middle of the energy gap, was too high for MIS-diode applications [11]. For these reasons, we chose to investigate low-temperature deposition of ferroelectric PbTiO3 on p-InP substrates in order to look for evidence a new kind of Au/PbTiO3/p-InP diode with a high-quality heterointerface. This article reports the microstructural, electrical, and transmittance properties of PbTiO3 thin films grown on p-InP (100) substrates by using metalorganic chemicalvapor deposition (MOCVD) at 3208C. X-ray diffraction (XRD) measurements were carried out to investigate the crystallization of the PbTiO3 layer. Scanning electron microscopy (SEM) measurements were performed in order
0022-3697/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0022-369 7(98)00351-5
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Fig. 1. An XRD curve of the PbTiO3/p-InP heterostructure grown at 3208C.
to investigate the PbTiO3 surface morphology, and Auger electron spectroscopy (AES) was carried out in order to characterize the stoichiometry of the grown films. Transmission electron microscopy (TEM) was performed to investigate the atomic structure of the PbTiO3/p-InP (100), and current–voltage (I–V) and capacitance–voltage (C–V) measurements were carried out to investigate the possibility of MIS behavior for the Au/PbTiO3/p-InP diode and to determine both the interface state density at the PbTiO3/pInP (100) interface and the dielectric constant of the PbTiO3 film. The temperature-dependent Fourier transform infrared (FTIR) spectroscopy measurements were performed to investigate the optical transmittance of the PbTiO3 film.
2. Experimental details The b -diketonate complex Pb(tmhd)2 (tmhd 2,2,6,6tetramethyl-3,5-heptanedionate), Ti(OC3H7)4, and N2O were used as sources, and argon as the carrier gas. The temperatures of the oil baths for the Pb(tmhd)2 and the Ti(OC3H7)4 were maintained at 1258C and 158C, respectively. The flow rates of the argon carrier gas for the Pb(tmhd)2 and the Ti(OC3H7)4 were 15 and 10 sccm, respectively. Heating tape was wrapped around the metalorganic-source
vapor-transport line, which was at 2508C, to prevent condensation from the source bath to the growth chamber. N2O gas was supplied through separate lines into a resistively heated, vertical, cold-wall reaction chamber at a system pressure of 1 Torr. The carrier concentration of the Zn-doped p-InP substrates with a (100) orientation used in this experiment was 1 × 10 16 cm 23. The InP substrates obtained from Sumitomo Co. were alternately degreased in warm acetone and trichloroethylene (TCE) thrice, etched in a Br–methanol solution mechanochemically, rinsed in de-ionized water thoroughly, etched in a mixture of H2SO4, H2O2, and H2O (4 : 1 : 1) at 408C for 10 min, and rinsed in TCE again. As soon as the chemical processing of the InP wafers was finished, the substrates were mounted onto a molybdenum susceptor. A typical deposition was carried out for 60 min and was followed by a slow cooling to room temperature at a rate of 1008C/h in an oxygen atmosphere in order to prevent strain-induced microcracks. The source was changed for every experimental run in order to reduce the problem of vapor–pressure reduction caused by surface melting of the barium precursor [12]. The XRD measurements were performed using a Rigaku D/Max-B diffractometer with CuKa radiation. The AES measurements were performed on the as-grown films by using a Perkin-Elmer phi 400 scanning Auger microprobe. The TEM observations were performed in a JEOL 200CX transmission electron microscope operating at 400 kV. The samples for the TEM measurements were prepared by cutting and polishing to an approximately 30 mm thickness by using diamond paper, and then argon-ion milling at liquid-nitrogen temperature to electron transparency. FTIR measurements were performed with a Bomem DA-3 far infrared spectrometer in conjunction with a liquid helium dewar, and the sample substrates were wedged 58 to avoid multiple-reflection interference. The detector used in these transmission measurements was a gallium-doped germanium photoconductive detector operating at 4.2 K. The sample temperature was controlled between 4.2 and 350 K by using a liquid He system and was determined by means of a chromel–constantan thermocouple.
3. Results and discussion The PbTiO3 films as-grown by MOCVD had mirror-like surfaces without any indications of pinholes, which was confirmed by using Normarski optical microscopy and SEM measurements. Although the PbTiO3 films were grown in the temperature range between 3008C and 5008C, only the physical properties of the film grown at 3208C are reported because the films grown below 3208C had stoichiometry problems and those grown above 3508C had significant interdiffusion problems. The typical thickness of the PbTiO3 films, as determined from the TEM ˚ , and this value measurements, was approximately 2000 A
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Fig. 2. AES curves obtained from a PbTiO3/p-InP heterostructure grown at 3208C. The upper curve was obtained at the surface of the as-grown PbTiO3/p-InP heterostructure, and the lower one was obtained at a 0.1 mm depth.
was in reasonable agreement with that obtained from ellipsometric measurements. The XRD results for the PbTiO3 film grown on a p-InP (100) substrate at 3208C are shown in Fig. 1. Even though the possibility of forming an amorphous or other phase cannot be neglected totally, there is no clear evidence of any phase other than the perovskite PbTiO3, as shown in Fig. 1. The (001), (100), (101), (002), (200), and (201) Ka 1 diffraction peaks corresponding to the PbTiO3 film together with the (200) and (400) Ka 1 diffraction peaks corresponding to the InP (100) substrate are clearly observed. These results indicate that the PbTiO3 films grown on InP (100) substrates by using MOCVD at 3208C
are polycrystalline. Since the c-axis direction of PbTiO3 displays a higher electromechanical coupling coefficient [13], the growth of a ferroelectric PbTiO3 thin film with a c-axis orientation is particularly interesting. The PbTiO3 film grown on the InP substrate at 3208C shows a slight (001) or c-axis-preferred orientation, and the intensity of the (001) reflection is almost same as that of the (100) reflection. The compositions of the PbTiO3 thin films grown on the InP substrates were investigated by AES measurements, and the results are presented in Fig. 2. These results show that the grown film consisted of lead, titanium, oxygen, and carbon at the surface and of lead, titanium, and oxygen at
Fig. 3. A bright-field TEM image of the PbTiO3/p-InP heterostructure grown at 3208C.
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Fig. 4. A bright-field TEM image of a PbTiO3 thin layer grown at 3208C.
Fig. 5. An electron diffraction pattern from TEM of the PbTiO3/p-InP heterostructure grown at 3208C.
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Fig. 6. A high-resolution TEM image of the PbTiO3/p-InP heterostructure grown at 3208C.
a 0.06 mm depth. The AES signals for Pb, Ti, O, and C are located at 83, 378, 488, and 261 eV, respectively. The appearance of the carbon impurities at the PbTiO3 surface might originate from contamination as a result of the metal source materials or to the growth chamber; the existence of the carbon was also confirmed by X-ray photoelectron spectroscopy measurements. The ratios of the peak-to-peak intensities among the lead, titanium, and oxygen peaks of the PbTiO3 films grown on the
InP substrates were similar to those of the PbTiO3 grown on p-Si by using MOCVD [8,14]. In addition to the XRD and the AES measurements, a bright-field TEM image showing the top PbTiO3 layer and the bottom InP substrate is presented in Fig. 3. Even though our main purpose was the growth of a PbTiO3/p-InP heterostructure with a high-quality interface at low temperature, hemispherical pores caused by interdiffusion or outdiffusion at the PbTiO3/p-InP interface were observed occasionally.
Fig. 7. The I–V characteristic for an Au/PbTiO3/p-InP (100) diode.
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Fig. 8. One megaHertz C–V curve for an Au/PbTiO3/p-InP (100) diode.
The line patterns perpendicular to the PbTiO3/p-InP heterointerface are caused by defects. A bright-field TEM image of the PbTiO3 thin layer is shown in Fig. 4. The grain size in the PbTiO3 film is very small, and the average grain size is approximately 70 nm. These microstructures, such as the small grain sizes and the large number of grain boundaries, could affect the electrical and the optical properties of the PbTiO3 films. A selected-area electron-diffraction pattern from the TEM measurements at the PbTiO3/p-InP heterointerface is shown in Fig. 5. The regular strong spots originate from the InP substrate, and the irregular weak spots and the weak diffused ring are related to the PbTiO3 polycrystalline film and the amorphous interfacial layer, respectively. A high-resolution TEM image of the PbTiO3/p-InP structure is shown in Fig. 6. The results indicate that an interfacial amorphous layer and a hemispherical pore formed at the PbTiO3/p-InP heterointerface and that almost all of the PbTiO3 thin film was polycrystalline. The formation of a PbTiO3 epitaxial film was observed randomly, as shown in Fig. 6, and local epitaxy existed in the PbTiO3 film near the PbTiO3/p-InP heterointerface. Even though the residual oxide layer during the initial growth stage can not be neglected totally as a possible cause of the formation of the amorphous interfacial layer at the PbTiO3/p-InP heterointerface, the interfacial layer may originate from the strain effect resulting from the difference between the lattice mismatch between the PbTiO3 thin film and the InP substrate. However, the detailed origins of the hemispherical pores at the PbTiO3/p-InP heterointerface and the local epitaxy at the PbTiO3 thin film are still under investigation. The I–V and the C–V characteristics were measured on adjacent pieces of the same samples which were investigated by XRD, AES, and TEM. Ohmic contacts
were fabricated by evaporating gold on the front sides and by soldering indium on the backsides of the samples. The diameter of the top electrode used for the I–V and the C–V measurements was 0.5 mm. The I–V characteristics for Au/ PbTiO3/p-InP at room temperature are shown in Fig. 7. The dc measurements were performed using a HP 4140B picoammeter with a ramp rate of 0.05 V/s. The I–V behavior is similar to those of an Al/Al2O3/InP diode [11] and an Al/ Ta2O5/SiO2/Si capacitor [15]. The leakage current for the Au/PbTiO3/p-InP was smaller than that for the Au/PbTiO3/ p-Si [16], and the decrease in the leakage current for Au/ PbTiO3/p-InP might originate from a reduction of the interfacial layer thickness between the PbTiO3 and the p-InP in comparison with that between the PbTiO3 and the p-Si. A 1 MHz C–V profile for the Au/PbTiO3/p-InP structure is shown in Fig. 8. The behavior of the C–V curve is similar to those from C–V measurements on an ordinarily prepared Al/SiO2/Si diode [17]. The value of the maximum capacitance was 524.7 pF, and the dielectric constant obtained from the maximum capacitance of the Au/PbTiO3/p-InP structure under accumulation conditions was about 60.4. The high magnitude of the dielectric constant for the PbTiO3 thin layer might be caused by the local epitaxial PbTiO3 and the relatively smaller interfacial layer. The hysteresis of the C–V results might have originated from the mobile carriers in the PbTiO3 thin layer and the interfacial layer. The interface state density distribution at the PbTiO3/p-InP interface, as determined by the Terman method [18], was asymmetrically U shaped over energy from the conduction-band edge to the midgap and was approximately low 10 11 eV 21 cm 22 at an energy about 0.7 eV below the conduction-band edge. The interface state densities measured on the Au/PbTiO3/p-InP diodes
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4. Summary and conclusions XRD, AES, and TEM measurements showed that PbTiO3 polycrystalline films could be grown on p-InP substrates by using low-pressure MOCVD at a relatively low temperature. The results of the I–V and the C–V measurements at room temperature clearly demonstrated MIS behaviors for the Au/ PbTiO3/p-InP structures. The dielectric constant of the PbTiO3 thin film, as determined from the C–V measurements, was as large as 60.4, and the interface state density, as determined by the Terman method, was approximately low 10 11 eV 21 cm 22 at an energy of approximately 0.7 eV below the conduction-band edge. The results of the temperature-dependent FTIR spectroscopy measurements showed several sharp absorption edges, which were related to Pb– Ti, Pb–O, Ti–O, and Si–O bonds and to deep traps. These results indicate that PbTiO3/p-InP grown by MOCVD has promising applications as insulator films for new kinds of MIS diodes based on InP substrate. Acknowledgements
Fig. 9. Transmission measurements on the PbTiO3/p-InP heterostructure grown at 3208C for several different sample temperatures.
This work was supported by the Basic Science Research Institute Program, Ministry of Education, in 1998, Project No. BSRI-98-2423. One of the authors (T.W. K.) would like to thank Professor B.D. McCombe for providing the FTIR facilities and for helpful discussion during his stay in SUNY at Buffalo. References
were lower than those for comparable structures on Al/ Al2O3/p-InP diodes [11]. As the minimum value of the surface state density was located at an energy about 0.7 eV below the conduction-band edge, a missing phosphorus surface defect seemed to have developed at the InP substrate [19]. In addition to the microstructural and the electrical measurements, temperature-dependent FTIR measurements were performed to investigate the atomic bonding structure of the PbTiO3 film. The transmittance measurements of the PbTiO3/p-InP heterostructure for several different sample temperatures are shown in Fig. 9. Several sharp absorption edges are observed between 400 and 2000 cm 21. These absorption edges are related to Pb–Ti, Pb–O, Ti–O, and Si–O bonding, as well as to deep traps. As the sample temperature changes from 250 to 370 K, the peak position of the absorption edge changes slightly. Even though detailed studies of the origins of the absorption edges have not been done yet, this behavior indicates that the atomic structure of the PbTiO3 film does not change in the temperature range between 250 and 370 K.
[1] C.W. Wilmsen, Physics and Chemistry of III–V Compound Semiconductor Interfaces, Plenum Press, New York, 1985. [2] F. Capasso, Physics of Quantum Electron Devices, Springer, Heidelberg, 1990. [3] C.S. Sundararaman, P. Milhelich, R.A. Masut, J.F. Currie, Appl. Phys. Lett. 64 (1994) 2279. [4] J. Faist, F. Capasso, D.L. Sivco, A.L. Hutchinson, S.N.G. Chu, A.Y. Cho, Appl. Phys. Lett. 72 (1998) 680. [5] T.W. Kim, M. Jung, D.U. Lee, Y.S. Lim, J.Y. Lee, Appl. Phys. Lett. 73 (1998) 61. [6] P.V. Staa, H. Rombach, R. Kassing, J. Appl. Phys. 54 (1983) 4014. [7] W.G. Lee, S.I. Who, J.C. Kim, S.H. Choi, K.H. Oh, Appl. Phys. Lett. 63 (1993) 2511. [8] T.W. Kim, Y.S. Yoon, S.S. Yom, J.Y. Lee, Appl. Phys. Lett. 64 (1994) 2676. [9] C.J. Lu, S.B. Ren, H.M. Shen, J.S. Liu, Y.N. Wang, J. Vac. Sci. Technol. A 15 (1997) 2167. [10] V.R. Palkar, S.C. Purandare, P.R. Apte, R. Pinto, M.S. Multani, Appl. Phys. Lett. 71 (1997) 3637. [11] T.W. Kim, W.N. Kang, Y.S. Yoon, S.S. Yom, J.Y. Lee, C.Y. Kim, H. Lim, H.L. Park, J. Appl. Phys. 74 (1993) 760. [12] T. Nakamori, H. Abe, T. Kanamori, S. Shibata, Jpn. J. Appl. Phys. 27 (1988) L1265. [13] K. Kushida H. Takeuchi, IEEE Trans. Ultrason. Ferroelectr. Freq. Ctrl. 38 (1991) 656.
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[14] W.G. Lee, S.I. Who, J.C. Kim, S.H. Choi, K.H. Oh, Appl. Phys. Lett. 63 (1993) 2511. [15] G.Q. Lo, D.L. Kwang, S. Lee, Appl. Phys. Lett. 60 (1992) 3286. [16] Y.S. Yoon, W.N. Kang, S.S. Yom, T.W. Kim, M. Jung, H.J. Kim, T.H. Park, H.K. Na, Appl. Phys. Lett. 63 (1993) 1104.
[17] S.M. Sze, Physics of Semiconductor Devices, 2, Wiley, New York, 1981. [18] L.M. Terman, Solid State Electron. 5 (1962) 285. [19] C.J. Huang, Y.K. Su, J. Electron. Mater. 19 (1990) 753.
Microstructural, electrical, and transmittance properties of PbTiO3 films grown on p-InP (100) substrates at low temperature T.W. Kim a,*, S.S. Yom b b
a Department of Physics, Kwangwoon University, 447-1 Wolgye-dong, Nowon-ku, Seoul, 139-701, South Korea Applied Physics Laboratory, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, South Korea
Received 16 October 1998; accepted 21 December 1998
Abstract PbTiO3 thin films were grown on p-InP (100) substrates by using metalorganic chemical vapor deposition via thermal pyrolysis at a relatively low temperature ( , 3208C) using Pb(tmhd)2, Ti(OC3H7)4, and N2O. X-ray diffraction measurements showed that the PbTiO3 film layer grown on the InP substrate was polycrystalline, and Auger electron spectroscopy measurements indicated that the as-grown films consisted of lead, titanium, and oxygen. Transmission electron microscopy measurements showed that the grown PbTiO3 was a polycrystalline layer with small domains and that the PbTiO3 films had interdiffusion and local epitaxial formations near the PbTiO3/p-InP (100) heterointerface. Room-temperature current–voltage and capacitance–voltage (C–V) measurements clearly revealed a metal–insulator–semiconductor behavior for the PbTiO3 insulator gates, and the interface state densities at the PbTiO3/p-InP interfaces, as determined from the C–V measurements, were approximately low 10 11 eV 21 cm 22 at an energy of about 0.7 eV below the conduction-band edge. The dielectric constant of the PbTiO3 thin film, as determined from the C–V measurements, was as large as 60.4. The temperature-dependent Fourier transform infrared spectroscopy measurements showed several sharp absorption edges in the transmission spectra. These results can help improve understanding of the physical properties of the PbTiO3 layers grown on p-InP (100) substrates at low temperatures for potential high-density dynamic-memory and high-speed applications based on the InP substrates. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Thin films; Metal-insulator-semiconductor
1. Introduction Recently, InP compound semiconductors have attracted much attention because of their applications as substrates in optoelectronic and high-speed electronic devices [1–5]. In contrast to GaAs-metal–insulator–semiconductor (MIS) capacitors, since the Fermi level in the InP–MIS diodes can be moved nearly across the whole energy gap, both inversion and accumulation layers can be achieved [6]. Many groups have investigated the growth of PbTiO3 films on Si substrates as possible deposited insulator layers for Si-MIS applications [7–10]. However, the temperature for deposition of the PbTiO3 insulator layer on the InP substrate must be kept below the InP decomposition * Corresponding author. Tel.: 1 82-02-940-5234; fax: 1 82 02 942 0108.
temperature in order to obtain a high-quality PbTiO3/InP heterointerface. Even though Al2O3 insulator layers were grown on p-InP (100) at 2808C, the interface state density of Al/Al2O3/p-InP, which was approximately high 10 11 eV 21 cm 22 at the middle of the energy gap, was too high for MIS-diode applications [11]. For these reasons, we chose to investigate low-temperature deposition of ferroelectric PbTiO3 on p-InP substrates in order to look for evidence a new kind of Au/PbTiO3/p-InP diode with a high-quality heterointerface. This article reports the microstructural, electrical, and transmittance properties of PbTiO3 thin films grown on p-InP (100) substrates by using metalorganic chemicalvapor deposition (MOCVD) at 3208C. X-ray diffraction (XRD) measurements were carried out to investigate the crystallization of the PbTiO3 layer. Scanning electron microscopy (SEM) measurements were performed in order
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Fig. 1. An XRD curve of the PbTiO3/p-InP heterostructure grown at 3208C.
to investigate the PbTiO3 surface morphology, and Auger electron spectroscopy (AES) was carried out in order to characterize the stoichiometry of the grown films. Transmission electron microscopy (TEM) was performed to investigate the atomic structure of the PbTiO3/p-InP (100), and current–voltage (I–V) and capacitance–voltage (C–V) measurements were carried out to investigate the possibility of MIS behavior for the Au/PbTiO3/p-InP diode and to determine both the interface state density at the PbTiO3/pInP (100) interface and the dielectric constant of the PbTiO3 film. The temperature-dependent Fourier transform infrared (FTIR) spectroscopy measurements were performed to investigate the optical transmittance of the PbTiO3 film.
2. Experimental details The b -diketonate complex Pb(tmhd)2 (tmhd 2,2,6,6tetramethyl-3,5-heptanedionate), Ti(OC3H7)4, and N2O were used as sources, and argon as the carrier gas. The temperatures of the oil baths for the Pb(tmhd)2 and the Ti(OC3H7)4 were maintained at 1258C and 158C, respectively. The flow rates of the argon carrier gas for the Pb(tmhd)2 and the Ti(OC3H7)4 were 15 and 10 sccm, respectively. Heating tape was wrapped around the metalorganic-source
vapor-transport line, which was at 2508C, to prevent condensation from the source bath to the growth chamber. N2O gas was supplied through separate lines into a resistively heated, vertical, cold-wall reaction chamber at a system pressure of 1 Torr. The carrier concentration of the Zn-doped p-InP substrates with a (100) orientation used in this experiment was 1 × 10 16 cm 23. The InP substrates obtained from Sumitomo Co. were alternately degreased in warm acetone and trichloroethylene (TCE) thrice, etched in a Br–methanol solution mechanochemically, rinsed in de-ionized water thoroughly, etched in a mixture of H2SO4, H2O2, and H2O (4 : 1 : 1) at 408C for 10 min, and rinsed in TCE again. As soon as the chemical processing of the InP wafers was finished, the substrates were mounted onto a molybdenum susceptor. A typical deposition was carried out for 60 min and was followed by a slow cooling to room temperature at a rate of 1008C/h in an oxygen atmosphere in order to prevent strain-induced microcracks. The source was changed for every experimental run in order to reduce the problem of vapor–pressure reduction caused by surface melting of the barium precursor [12]. The XRD measurements were performed using a Rigaku D/Max-B diffractometer with CuKa radiation. The AES measurements were performed on the as-grown films by using a Perkin-Elmer phi 400 scanning Auger microprobe. The TEM observations were performed in a JEOL 200CX transmission electron microscope operating at 400 kV. The samples for the TEM measurements were prepared by cutting and polishing to an approximately 30 mm thickness by using diamond paper, and then argon-ion milling at liquid-nitrogen temperature to electron transparency. FTIR measurements were performed with a Bomem DA-3 far infrared spectrometer in conjunction with a liquid helium dewar, and the sample substrates were wedged 58 to avoid multiple-reflection interference. The detector used in these transmission measurements was a gallium-doped germanium photoconductive detector operating at 4.2 K. The sample temperature was controlled between 4.2 and 350 K by using a liquid He system and was determined by means of a chromel–constantan thermocouple.
3. Results and discussion The PbTiO3 films as-grown by MOCVD had mirror-like surfaces without any indications of pinholes, which was confirmed by using Normarski optical microscopy and SEM measurements. Although the PbTiO3 films were grown in the temperature range between 3008C and 5008C, only the physical properties of the film grown at 3208C are reported because the films grown below 3208C had stoichiometry problems and those grown above 3508C had significant interdiffusion problems. The typical thickness of the PbTiO3 films, as determined from the TEM ˚ , and this value measurements, was approximately 2000 A
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Fig. 2. AES curves obtained from a PbTiO3/p-InP heterostructure grown at 3208C. The upper curve was obtained at the surface of the as-grown PbTiO3/p-InP heterostructure, and the lower one was obtained at a 0.1 mm depth.
was in reasonable agreement with that obtained from ellipsometric measurements. The XRD results for the PbTiO3 film grown on a p-InP (100) substrate at 3208C are shown in Fig. 1. Even though the possibility of forming an amorphous or other phase cannot be neglected totally, there is no clear evidence of any phase other than the perovskite PbTiO3, as shown in Fig. 1. The (001), (100), (101), (002), (200), and (201) Ka 1 diffraction peaks corresponding to the PbTiO3 film together with the (200) and (400) Ka 1 diffraction peaks corresponding to the InP (100) substrate are clearly observed. These results indicate that the PbTiO3 films grown on InP (100) substrates by using MOCVD at 3208C
are polycrystalline. Since the c-axis direction of PbTiO3 displays a higher electromechanical coupling coefficient [13], the growth of a ferroelectric PbTiO3 thin film with a c-axis orientation is particularly interesting. The PbTiO3 film grown on the InP substrate at 3208C shows a slight (001) or c-axis-preferred orientation, and the intensity of the (001) reflection is almost same as that of the (100) reflection. The compositions of the PbTiO3 thin films grown on the InP substrates were investigated by AES measurements, and the results are presented in Fig. 2. These results show that the grown film consisted of lead, titanium, oxygen, and carbon at the surface and of lead, titanium, and oxygen at
Fig. 3. A bright-field TEM image of the PbTiO3/p-InP heterostructure grown at 3208C.
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Fig. 4. A bright-field TEM image of a PbTiO3 thin layer grown at 3208C.
Fig. 5. An electron diffraction pattern from TEM of the PbTiO3/p-InP heterostructure grown at 3208C.
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Fig. 6. A high-resolution TEM image of the PbTiO3/p-InP heterostructure grown at 3208C.
a 0.06 mm depth. The AES signals for Pb, Ti, O, and C are located at 83, 378, 488, and 261 eV, respectively. The appearance of the carbon impurities at the PbTiO3 surface might originate from contamination as a result of the metal source materials or to the growth chamber; the existence of the carbon was also confirmed by X-ray photoelectron spectroscopy measurements. The ratios of the peak-to-peak intensities among the lead, titanium, and oxygen peaks of the PbTiO3 films grown on the
InP substrates were similar to those of the PbTiO3 grown on p-Si by using MOCVD [8,14]. In addition to the XRD and the AES measurements, a bright-field TEM image showing the top PbTiO3 layer and the bottom InP substrate is presented in Fig. 3. Even though our main purpose was the growth of a PbTiO3/p-InP heterostructure with a high-quality interface at low temperature, hemispherical pores caused by interdiffusion or outdiffusion at the PbTiO3/p-InP interface were observed occasionally.
Fig. 7. The I–V characteristic for an Au/PbTiO3/p-InP (100) diode.
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Fig. 8. One megaHertz C–V curve for an Au/PbTiO3/p-InP (100) diode.
The line patterns perpendicular to the PbTiO3/p-InP heterointerface are caused by defects. A bright-field TEM image of the PbTiO3 thin layer is shown in Fig. 4. The grain size in the PbTiO3 film is very small, and the average grain size is approximately 70 nm. These microstructures, such as the small grain sizes and the large number of grain boundaries, could affect the electrical and the optical properties of the PbTiO3 films. A selected-area electron-diffraction pattern from the TEM measurements at the PbTiO3/p-InP heterointerface is shown in Fig. 5. The regular strong spots originate from the InP substrate, and the irregular weak spots and the weak diffused ring are related to the PbTiO3 polycrystalline film and the amorphous interfacial layer, respectively. A high-resolution TEM image of the PbTiO3/p-InP structure is shown in Fig. 6. The results indicate that an interfacial amorphous layer and a hemispherical pore formed at the PbTiO3/p-InP heterointerface and that almost all of the PbTiO3 thin film was polycrystalline. The formation of a PbTiO3 epitaxial film was observed randomly, as shown in Fig. 6, and local epitaxy existed in the PbTiO3 film near the PbTiO3/p-InP heterointerface. Even though the residual oxide layer during the initial growth stage can not be neglected totally as a possible cause of the formation of the amorphous interfacial layer at the PbTiO3/p-InP heterointerface, the interfacial layer may originate from the strain effect resulting from the difference between the lattice mismatch between the PbTiO3 thin film and the InP substrate. However, the detailed origins of the hemispherical pores at the PbTiO3/p-InP heterointerface and the local epitaxy at the PbTiO3 thin film are still under investigation. The I–V and the C–V characteristics were measured on adjacent pieces of the same samples which were investigated by XRD, AES, and TEM. Ohmic contacts
were fabricated by evaporating gold on the front sides and by soldering indium on the backsides of the samples. The diameter of the top electrode used for the I–V and the C–V measurements was 0.5 mm. The I–V characteristics for Au/ PbTiO3/p-InP at room temperature are shown in Fig. 7. The dc measurements were performed using a HP 4140B picoammeter with a ramp rate of 0.05 V/s. The I–V behavior is similar to those of an Al/Al2O3/InP diode [11] and an Al/ Ta2O5/SiO2/Si capacitor [15]. The leakage current for the Au/PbTiO3/p-InP was smaller than that for the Au/PbTiO3/ p-Si [16], and the decrease in the leakage current for Au/ PbTiO3/p-InP might originate from a reduction of the interfacial layer thickness between the PbTiO3 and the p-InP in comparison with that between the PbTiO3 and the p-Si. A 1 MHz C–V profile for the Au/PbTiO3/p-InP structure is shown in Fig. 8. The behavior of the C–V curve is similar to those from C–V measurements on an ordinarily prepared Al/SiO2/Si diode [17]. The value of the maximum capacitance was 524.7 pF, and the dielectric constant obtained from the maximum capacitance of the Au/PbTiO3/p-InP structure under accumulation conditions was about 60.4. The high magnitude of the dielectric constant for the PbTiO3 thin layer might be caused by the local epitaxial PbTiO3 and the relatively smaller interfacial layer. The hysteresis of the C–V results might have originated from the mobile carriers in the PbTiO3 thin layer and the interfacial layer. The interface state density distribution at the PbTiO3/p-InP interface, as determined by the Terman method [18], was asymmetrically U shaped over energy from the conduction-band edge to the midgap and was approximately low 10 11 eV 21 cm 22 at an energy about 0.7 eV below the conduction-band edge. The interface state densities measured on the Au/PbTiO3/p-InP diodes
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4. Summary and conclusions XRD, AES, and TEM measurements showed that PbTiO3 polycrystalline films could be grown on p-InP substrates by using low-pressure MOCVD at a relatively low temperature. The results of the I–V and the C–V measurements at room temperature clearly demonstrated MIS behaviors for the Au/ PbTiO3/p-InP structures. The dielectric constant of the PbTiO3 thin film, as determined from the C–V measurements, was as large as 60.4, and the interface state density, as determined by the Terman method, was approximately low 10 11 eV 21 cm 22 at an energy of approximately 0.7 eV below the conduction-band edge. The results of the temperature-dependent FTIR spectroscopy measurements showed several sharp absorption edges, which were related to Pb– Ti, Pb–O, Ti–O, and Si–O bonds and to deep traps. These results indicate that PbTiO3/p-InP grown by MOCVD has promising applications as insulator films for new kinds of MIS diodes based on InP substrate. Acknowledgements
Fig. 9. Transmission measurements on the PbTiO3/p-InP heterostructure grown at 3208C for several different sample temperatures.
This work was supported by the Basic Science Research Institute Program, Ministry of Education, in 1998, Project No. BSRI-98-2423. One of the authors (T.W. K.) would like to thank Professor B.D. McCombe for providing the FTIR facilities and for helpful discussion during his stay in SUNY at Buffalo. References
were lower than those for comparable structures on Al/ Al2O3/p-InP diodes [11]. As the minimum value of the surface state density was located at an energy about 0.7 eV below the conduction-band edge, a missing phosphorus surface defect seemed to have developed at the InP substrate [19]. In addition to the microstructural and the electrical measurements, temperature-dependent FTIR measurements were performed to investigate the atomic bonding structure of the PbTiO3 film. The transmittance measurements of the PbTiO3/p-InP heterostructure for several different sample temperatures are shown in Fig. 9. Several sharp absorption edges are observed between 400 and 2000 cm 21. These absorption edges are related to Pb–Ti, Pb–O, Ti–O, and Si–O bonding, as well as to deep traps. As the sample temperature changes from 250 to 370 K, the peak position of the absorption edge changes slightly. Even though detailed studies of the origins of the absorption edges have not been done yet, this behavior indicates that the atomic structure of the PbTiO3 film does not change in the temperature range between 250 and 370 K.
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