1. Phys.: Condens. Mater 7 (1995) 9075-9080. Printed in the UK (001)-oriented PbTiO3 ferroelectric <a href="/keyword/thin-films/" ><a href="/keyword/thin-film/" >thin film</a> s</a> grown on by metal-organic chemical vapour deposition Wen-Hui Ma, Ming-Sheng Zhang. Li Shun, Yang-Feng Chen and Nai-ben Ming National Laboratory of Solid State Microstructures and Center for Advanced Studies in Science and Technology of Microstructures, Nanjing University, Nanjing 210093, People's Republic of China Received 27 lune 1995 Abstract. PbTiOs ferroelectric <a href="/keyword/thin-films/" ><a href="/keyword/thin-film/" >thin film</a> s</a> have been prepared on Si (001) by metal-organic chemical vapour deposition. The ai-grown films Were characterized by scanning electmn microscopy, x-ray diffraction and Raman spectroscopy. It is shown that the films were highly (001) oriented and had essentially the same lanice constants as the bulk single crystal. However, im the %-grown f l s were subject to internal stress a shown by a downshift in the Raman modes when compared with a bulk single crystal. 1 Introduction . Perovskite-type. oxides, such as PbTiO3. BaTiO, and PbZrxTil-,03, are ferroelectric materials which have remarkable ferroelectric, pyroelectric and electro-optic properties. Potential applications of these oxides in non-volatile memories, infrared sensors, space optical modulators, etc, have stimulated the development of thin-film growth [l]. Epitaxial films would be advantageous for device applications. Recently, much effort has been given to the epitaxial growth of <a href="/keyword/thin-films/" ><a href="/keyword/thin-film/" >thin film</a> s</a> of perovskite-type oxides; for example. epitaxial films have been achieved on singlecrystal substrates such as MgO, S r T i q and LaAIO3 t2-41, while films grown on fused quartz were polycrystalline [5]. However, in order to integrate ferroelectric <a href="/keyword/thin-films/" ><a href="/keyword/thin-film/" >thin film</a> s</a> into CMOS technology, it is desirable to prepare high-quality <a href="/keyword/thin-films/" ><a href="/keyword/thin-film/" >thin film</a> s</a> on a Si substrate. In the past few years, a series of techniques for the preparation of <a href="/keyword/thin-films/" ><a href="/keyword/thin-film/" >thin film</a> s</a> has been developed, including sputtering, sol-gel and metal-organic chemical vapour deposition (MOCVD) [2-101. Of these techniques, MOCVD shows some advantages over the others, such as good uniformity o composition, high probability of achieving epitaxial growth, f high deposition rate, adaptability to multicomponent deposition and large-scale commercial production. By using MOCVD, many epitaxial ferroelectric <a href="/keyword/thin-films/" ><a href="/keyword/thin-film/" >thin film</a> s</a> have been achieved, including PbTi03 [24], Bali03 [8] and PZT [9]. However, it is still difficult to obtain highquality PbTiO3 <a href="/keyword/thin-films/" ><a href="/keyword/thin-film/" >thin film</a> s</a> on Si substrates. Kim etal [lo] have grown polycrystallinePbTiO3 <a href="/keyword/thin-films/" ><a href="/keyword/thin-film/" >thin film</a> s</a> with random orientation, which was attributed to the formation of an amorphous interfacial layer, and Okada et al [2] obtained polycrystalline films with some [OOl] texture. Although epitaxial or single-crystal films are preferred, the c-axis-oriented <a href="/keyword/thin-films/" ><a href="/keyword/thin-film/" >thin film</a> s</a> are sufficient and suitable for piezoelectric, pyroelectric and other applications. A oaxisoriented PbTiO3 film may have a small dielectric constant and a large pyroelectric coefficient 0953-8984/95/479075+06S19.50 1995 IOP Publishing Ltd @ 9075 9076 Wen-ffui et al Ma because the c axis is the polarization axis. In this paper, we report the preparation of highly (001)-oriented PbTiOs <a href="/keyword/thin-films/" ><a href="/keyword/thin-film/" >thin film</a> s</a> on Si (001) single-crystal substrates and characterization of the as-grown films by scanning electron microscopy (SEM), x-ray diffraction (XRD) and Raman spectroscopy. Substrate o* N , J- -i TIP Thermocouple Hnnifold SUSCeptor and Heater Exhaust Figure 1. Schematic diagram of fhc MOCVD appmlus used in chis work. 2. Experimental methods The PbTiOa films were grown on Si: (001) by MOCVD. The MOCVD apparatus is shown schematically in figure 1. It is mainly composed of three parts: a gas supply system, a horizontal quartz reactor and an exhaust gas-handling system. In the present work, two purified metal-organic precursors, namely titanium isopropoxide (TIP) and tetraethyllead (TEL) were used. The temperatures of the two precursors TIP and TEL were maintained at 65 C and 3 5 T , respectively. The substrate temperature was elevated using a resistive heater and kept at 600 C. The flow rates of TIP, TEL and 0 2 were 200 sccm, 250 sccm and 200 sccm, respectively, with the total pressure of the reactor chamber set at 10 Torr. The surface mophologies of the as-grown PbTiO, films were investigated by SEM. Before examination, a thin layer of gold was deposited on the surface. The crystallinity and orientation of the films were examined using XRD. XRD measurements were performed on a Ragaku x-ray diffractometer with nickel-filtered Cu Kor radiation. Raman spectroscopy measurements were performed on a Spex 1403 Raman spectrometer using the backscattering geometry. The 488 nm line of an Art laser with 100 mW output power was used. The widths of both the entrance and the exits slits were set at 150 pm. 3. Results and discussion A scanning electron micrograph is shown in figure 2. A few particulate structures with a size of about 1 fim can be observed. Considering that the typical film thickness is 1.5 pm, such surface fluctuation or roughness is understandable. The surface roughness is expected to reduce as the film thickness is decreased. In fact, for the intergrated ferroelectrics applications, films of only a few hundred nanometres thickness are generally needed. From figure 2, some equilateral triangular faceted grains of about 0.5 fim in size can be obseryed. These are related to the (111)-oriented texture of the film, consistent with the following XRD measurements. PbTiOs ferroelectric ttiinjilms grown on S by MOCVD i 9077 Figure 2. A scanning electron micrngrilph showing the surface morphology of the PbTiO3 films on Si(llll1). Figure 3. XRD panem of highly (001)-oriented PbTiO) films on Si by low-pressure MOCVD (a.&quot;., arbitray units). Figure 3 shows the XRD pattern from the as-grown PbTiO3 films on an Si substrate by MOCVD. The XRD data are also clearly listed in table 1. It is shown that the film is polycrystalline but highly (001) textured and has no secondary phases. Careful checks by focusing the x-rays on different regions were performed and similar patterns were obtained, indicating good uniformity of the films. The c-axis orientation ratio a is calculated to be 0.71, which is, to our knowledge, the highest value ever reported on Si. Tetragonality of ~~ 9078 Wen-HuiMa et ai Table 1 XRD data for MOCVD-grownPbTiO, films on Si . 001 100 42 50 ~ 101 110 111 002 200 102 201 210 112 5 20 211 Film JCPDS No. 6-452 ~ 100 25 46 9 55 10 ~40 28 16 , 20 30 6 14 3 . 12 7 40 100 -20 the film was calculated to be 1.063 which is comparable to that reported for bulk single crystals. Raman spectroscopy is used conventionally to characterize the frequencies of optical phonons or long-wavelength lattice vibrations. Raman scattering from bulk PbTiO3 single crystals has been fully studied [11,121. Recently Taguchi et al [13] reported a Raman scattering study of PbTiOj <a href="/keyword/thin-films/" ><a href="/keyword/thin-film/" >thin film</a> s</a> prepared by RF sputtering and post-annealing treatment. It was found that phonon frequencies are generally more sensitive than XRD peaks to microstructural changes in the films. So Raman spectroscopy can give more subtle information on the microstructure of the as-grown films and should be a good supplement to the XRD measurements. Here, Raman spectroscopy measurements were performed to examine further the crystalline quality of the as-grown films. 50 450 850 RAMAN SHIFT (cm-') Figure 4. Raman spectrum of the as-grownPbTiOs film on Si obtained at room tempemure (ao.,arbiaary mirs). Figure 4 displays the Raman spectrum from the PbTiO3 films at room temperature. It closely resembles the Raman spectra from bulk single crystals. The 520 cm-I mode is a first-order mode from the Si substrate. All the other phonon modes were from PbTi03 and clearly resolved. No extra Raman modes indicative of impurities or interfacial phases were detected. We suppose that the 650 C growth temperature used here, which was higher than that used by Kim etal [lo], had restrained the formation of interfacial phases. The 83 cm-' mode is a well known soft mode [ 1I]. Near 634 and 734 cm-' there are two broader lines PbliOj ferroelectric thinfilm grown on S by MOCVD i 9079 . with relatively lower intensities. It is reported that the linewidths of these two modes are sensitive to microstructure 11I]; their broadening was expected and consistent with highly textured polycrystalline films with some randomly oriented grains. Table 2. Mode frequencies of optical phonons in Pb'TiOs single crystal and PbTiO3 film at room temperature (cm-'). Single crystal Mode 0 Gpaz 0 GPab 1 GP3C 83 120 Presenl work E(2TO) BitE Aj(2LO) 89 127 128 215 221 290 364 445 508 89 148 130 83 I09 149 214 288 354 454 503. 630 713 770 220 290 362 440 508 218 289 349 515 634 718 65 1 717 797 1 650 720 - [Ill - [I21 - 1141 b All the phonon modes are listed in table 2 for comparison with the Raman data on bulk single crystals. From table 2 it is found that the phonon frequencies for the films are somewhat lower than those for single crystals. This downshift of Raman lines or modesoftening behaviour can be attributed to pressure effects [I41 induced by internal stress in the <a href="/key...