Raman-spectroscopy study of PbTiO 3 thin films grown on Si substrates by metalorganic chemical vapor deposition

Raman spectra have been investigated in PbTiO 3 thin films grown on Si by metalorganic chemical vapor deposition. A large grazing-angle scattering technique was taken to measure the temperature dependence of Raman spectra below room temperature. All Raman modes in the thin films are assigned and compared with those in the bulk single crystal, a new A 1 (TO) soft mode at 104 cm~1 was recorded which satisfies the Curie-Weiss relation u2"A(1 # !1 ). Intensities of the A 1 (1TO) and E(1TO) modes were anomalously strengthened with increasing temperature. Raman modes for the thin films exhibit remarkable frequency downshift and upshift which is related to the effect of internal stress. PACS: 78.30.Hv; 63.20.!e; 81.15.Gh In recent years, ferroelectric thin films have attracted great attention due to their potential applications in microelectronic and optoelectronic devices [1]. There have been several techniques of thin-film growth including sputtering, sol-gel and MetalOrganic Chemical Vapour Deposition (MOCVD). Among them, MOCVD shows advantages over the others and can be used to prepare devicequality ferroelectric thin films including PbTiO 3 , Pb(Zr x Ti 1~x )O 3 , and BaTiO 3 [2—4]. Perovskite lead titanate PbTiO 3 is an important ferroelectric material with remarkable physical properties such as a large spontaneous polarization and a high Curie temperature. Studies of PbTiO 3 are of considerable interest from not only practical applications but also fundamental subjects, such as the origin of ferroelectricity and the mechanism of phase transition [5]. Raman spectroscopy had been employed in studies of the soft modes and phase transition in bulk PbTiO 3 , where Raman selection rules were rigorously obeyed in both the ferroelectric and paraelectric states [6, 7]. Recently, Raman scattering was also used to investigate the microstructures and phase transition in ferroelectric thin films. Taguchi et al. studied the Raman spectra of PbTiO 3 thin films prepared by rf sputtering and thermal treatment [8], where the frequency downshift of Raman modes was attributed to the pressure effect. Scott et al. studied KNO 3 thin films using Raman spectroscopy and found the effect of surface electric field on Raman modes of the thin films [9]. In the case of thin films, theoretical studies suggested that there may be some new phenomena such as the critical-thickness effect of the polar state and the change of the order of phase transitions [10]. In PbTiO 3 , there has also been a controversy on the phase-transition behaviour at low temperatures. Kobayashi et al. [11] reported an additional phase transition at !90 °C by X-ray and optical studies although it has long been believed to undergo only a phase transition at 490 °C. In this paper, we have successfully prepared highly (001)-oriented PbTiO 3 thin films on Si (001) substrates by MOCVD. We have investigated Raman spectra from the thin films and discuss the causes leading to the frequency shift of the Raman modes. We have also studied the temperature dependence of the Raman spectra and examined the behaviour of the PbTiO 3 thin films at low temperature. The low-pressure MOCVD technique was described in detail elsewhere [2]. We have chosen the purified Titanium IsoPropoxide (TIP) and TetraEthylLead (TEL) as the metalorganic precursors; N 2 as the carrier gas and oxygen gas as the oxidant. The temperatures of the precursors TIP and TEL were 65 and 35 °C. The substrate temperature was kept at 650 °C. The flow rates of TIP, TEL and O 2 are 200, 250, and 250 sccm, respectively, with a total pressure of the reactor chamber at 15 Torr. Crystal structure of the thin films was checked by a Rigaku X-ray diffractometer with nickel-filtered a radiation. The results showed that the films were highly [001]-oriented with a pure PbTiO 3 single phase. Ramanspectroscopy measurements were performed using a SPEX 1403 Raman spectrometer. The 488 nm line of an Ar` laser with 100 mW output power was used. The widths of both entrance and exit slits were set at 150 lm. A large grazing-angle scattering technique was taken to collect the weak Raman signals from the thin films. The geometrical arrangement is shown in Fig. 1, where the incident angle / was about 70° to the normal of the film Fig. 2. X-ray diffraction pattern of the MOCVD-grown PbTiO 3 thin films Fig. 1. Schematic diagram of the near-right-angle Raman-scattering geometry in PbTiO 3 thin films and the Si substrate system (1 incident light; 2 reflected light; 3 scattered light; 4 quartz window; 5 lens; 6 cryostat; 7 copper rod; 8 thermo couple; 9 Si substrate; 10