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Appl Phys A DOI 10.1007/s00339-010-5961-0

Phase transition of TiO2 thin films detected by the pulsed laser photoacoustic technique
A. Pérez-Pacheco · R. Castañeda-Guzmán · C. Oliva Montes de Oca · A. Esparza-García · S.J. Pérez Ruiz

Received: 5 January 2010 / Accepted: 1 July 2010 © Springer-Verlag 2010

Abstract In this work, we present characterization of titanium oxide thin films byphotoacoustic measurements to determine the ablation threshold and phase transitions from amorphous to crystalline states. The important advantages of this method are that it does not require amplification at the detection stage and that it is a non-destructive technique. The correlation analysis of the photoacoustic signals allows us to visualize the ablation threshold and the phase transitions withenhanced sensitivity. This correlation analysis clearly exhibits the changes in the thin-film morphology due to controlled variations of the fluence (energy/area) and the temperature of the surrounding medium. This is particularly important for those cases where the crystalline changes caused by temperature variations need to be monitored. The thin-film samples were prepared by the sputteringtechnique at room temperature in the amorphous state. The phase transformations were induced by controlled temperature scanning and then corroborated with Raman spectroscopy measurements.

1 Introduction In recent years, particular interest has been shown in the study of metal oxide thin films. This has been due to the

A. Pérez-Pacheco ( ) · R. Castañeda-Guzmán · C. Oliva Montes de Oca · A.Esparza-García Laboratorio de Fotofísica y Películas Delgadas, Universidad Nacional Autónoma de México, CCADET-UNAM, Cd. Universitaria, A.P. 70-186, C.P. 04510, México D.F., Mexico e-mail: ekargy@hotmail.com Fax: +52-55-56228651 S.J. Pérez Ruiz Acústica y Vibraciones, CCADET-UNAM, México D.F., Mexico

many possible technological applications of these thin films in information storage devices, solar cells,gas sensors, photocatalysis, coatings, and so on [1–3]. The structural transformation of various materials has been widely investigated by means of radiation–matter interaction, using pulsed or continuous lasers [4, 5]. Special attention has been devoted to amorphous–crystalline transformation or vice versa by means of laser radiation. This is because it has been the basis for optical storagedevices and the advantages it has over the more conventional methods of hole burning, such as the requirement of low power radiation [6]. Therefore, it is very important that the material has a high absorption coefficient at the desired wavelength [3]. For storage devices, such as the CD-RW (compact disc rewritable), it was observed that system operation is based on mechanisms of phase transitions ofa given material (from the amorphous to crystalline state or vice versa). In the present work, we apply the photoacoustic (PA) technique to amorphous thin films to induce the transition to a crystalline state with pulsed laser irradiation as a function of the temperature. Titanium oxide (TiO2 ) is one of the most extensively studied transition–metal oxides; it has three different crystallinephases: brookita, anatase, and rutile. The transformation from amorphous to anatase and anatase to rutile in powder or bulk materials usually takes place at temperatures between 300–600°C and 700–1000◦ C, respectively [7]. These temperatures depend on the initial particle size, the impurity content, the initial phase atmospheric reactions, temperature and pressure, among other factors [8, 9]. In athin-film, the temperatures at which the transitions occur are determined by the method of synthesis and by the conditions of deposition. The PA technique has been used in the characterization of a wide variety of materials, regardless of their nature. Par-

A. Pérez-Pacheco et al.

ticularly, this technique has proven to be a powerful tool for studying phase transitions in condensed matter...