Electronic structure and crystallinity of the HfO 2 –TiO 2 thin films Yew Von Lim, Ten It Wong, Shijie Wang ⁎ Institute of Materials Research and Engineering, 117602 Singapore abstract article info Available online 8 April 2010 Keywords: Controllable synthesis HfO 2 Thin films Band structure The effects of TiO 2 on the band structure and the crystallinity of the HfO 2 were studied. The bandgap energy decreases by increasing TiO 2 concentration. The shift in the valence band edge and the conduction band edge are due to the changes in electronic band structures and in microstructure. First principle calculations were carried out on the electronic band structures. Our experimental results confirmed the theoretical studies. © 2010 Elsevier B.V. All rights reserved. 1. Introduction One of the technological advancement of electronic devices is the vigorous downscaling of device dimensions. Issues were raised in terms of the replacement of SiO 2 gate dielectrics in the nanometer regime [1]. Among many of the suggested and future technologies studied [2], replacing SiO 2 with an insulator substitute with a higher dielectric constant is a more direct solution to the problem. HfO 2 is very promising to be utilized as a replacement due to its thermal stability in contact with silicon, high dielectric constant (∼ 30) and large band offset (Δ ∼ 1.5 eV) [3–6] compared to other high-κ materials. However, to obtain HfO 2 with equivalent oxide thickness (EOT) at nano-scale regime requires both high permittivity and large band offsets [7]. TiO 2 has a high permittivity of nearly 60 and highly possible of forming solid solutions with HfO 2 [11–13], in which a thin film system serving the purpose can be formed. Methods of obtaining such systems are possible in the form of composite thin films involving direct incorporation of TiO 2 and HfO 2 [7–10]. In this letter, we present the band structure studies of the HfO 2 –TiO 2 thin films grown on silicon (100) and α-Al 2 O 3 substrates. The focus of our study is on the effects of TiO 2 on the bandgap energies of the thin films. 2. Experimental details The HfO 2 –TiO 2 thin films were synthesized by high vacuum (10 -7 ) DC co-sputtering in a combined process air pressures of oxygen and argon at 10 -2 Torr. High purity hafnium and titanium metallic targets (99.99%) were used. The DC power of the hafnium and titanium targets were varied to fabricate samples with different TiO 2 concentration (at.%) in HfO 2 . The samples were post-annealed by furnace in air at target temperatures of 400 °C and 750 °C for 60 min. The crystalline structures of the thin films were analyzed by X-ray diffraction (XRD) with Cu Kα radiation. The UV–VIS absorption spectroscopy was performed on a UV–VIS–NIR Scanning Spectropho- tometer. X-ray photoelectron spectroscopy measurements were performed using a monochromatic AL K αl source (1486.6 eV) to determine the concentrations of TiO 2 in the samples, results are shown in Table 1. 3. Results and discussion The XRD results of the thin films are shown in Fig. 1. The XRD results show that all HfO 2 –TiO 2 samples are polycrystalline except for T0, which is pure anatase. For the samples annealed at 400 °C, the increase in TiO 2 concentration interrupts crystallization of HfO 2 . This is indicated by the weaker monoclinic XRD peak and the amorphous broad maxima in the XRD spectra of HT samples. For samples annealed at 750 °C, the stronger intensity of the monoclinic HfO 2 indicates that thermal annealing enhanced the crystallinity. The intensity of the monoclinic (-111) phase pattern were also found decreasing with increasing TiO 2 concentration, in conjunction with increasing intensity of (002) peak. This represents that there is an evolution in the majority monoclinic phase orientation, from (-111) and (111) to (002), with annealing. The monoclinic peaks (-111) and (111) are also shown shifted significantly with TiO 2 concentration, attributed to the change in lattice constant due to the incorporation of Ti in HfO 2 . The peak position of (-111) and (111) peak of different samples are shown in Table 1. As shown in Fig. 2, the absorption data were represented in terms of (αhυ) 2 versus the photon energy (hυ) derived from the UVS data measured. α represents the absorption coefficient and related to the transmittance, T = exp(-αd), where d is the thickness of the film thickness. The absorption refers to the valence band to conduction band transition from which the bandgap energy (E g ) can be estimated by assuming a direct transition between these bands. The absorption Thin Solid Films 518 (2010) e107–e110 ⁎ Corresponding author. E-mail address: sj-wang@imre.a-star.edu.sg (S. Wang). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.03.098 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf