www.spm.com.cn Structural, morphological, optical and photoluminescence properties of HfO 2 thin films C.Y. Ma ⁎, W.J. Wang, J. Wang, C.Y. Miao, S.L. Li, Q.Y. Zhang Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024, China abstract article info Article history: Received 18 July 2012 Received in revised form 13 August 2013 Accepted 15 August 2013 Available online 26 August 2013 Keywords: Hafnium dioxide Sputtering Optical Properties Luminescence Nanocrystalline monoclinic HfO 2 films with an average crystal size of 4.2–14.8 nm were sputter deposited under controlled temperatures and their structural characteristics and optical and photoluminescence properties have been evaluated. Structural investigations indicate that monoclinic HfO 2 films grown at higher temperatures above 400 °C are highly oriented along the (-111) direction. The lattice expansion increases with diminishing HfO 2 crystalline size below 6.8 nm while maximum lattice expansion occurs with highly oriented monoclinic HfO 2 of crystalline size about 14.8 nm. The analysis of atomic force microscopy shows that the film growth at 600 °C can be attributed to the surface-diffusion-dominated growth. The intensity of the shoulderlike band that initiates at ~5.7 eV and saturates at 5.94 eV shows continued increase with increasing crystalline size, which is intrinsic to nanocrystalline monoclinic HfO 2 films. Optical band gap varies in the range 5.40 ± 0.03– 5.60 ± 0.03 eV and is slightly decreased with the increase in crystalline size. The luminescence band at 4.0 eV of HfO 2 films grown at room temperature can be ascribed to the vibronic transition of excited OH • radical while the emission at 3.2–3.3 eV for the films grown at all temperatures was attributed to the radiative recom- bination at impurity and/or defect centers. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Hafnium dioxide (HfO 2 ) is a wide band gap, high dielectric constant, and high refractive index insulator with good thermal stability. On the basis of these properties, thin film HfO 2 has been investigated for use in the field of electronics, magnetoelectronics, structural ceramics, and optoelectronics [1,2]. HfO 2 has been identified as one of the most prom- ising materials for high-k replacement of SiO 2 in the next generation of complementary metal-oxide-semiconductor devices [3]. These interest- ing applications have led to numerous efforts to synthesize HfO 2 films by various techniques, including chemical vapor deposition [4], sol–gel process [5], pulsed laser deposition [6], electron beam evaporation and sputtering [7,8], among them, reactive sputtering being one of the most widely used. In the sputtering technique, controlled growth and manipulation of specific crystal structures at the nanoscale dimensions have important implications for the applications of HfO 2 . However, it is well known that the film's optical and electronic properties, which are essentially af- fected by the structural defects or trap centers in hafnia [9,10], are highly dependent on the structural characteristics (i.e., structure, crystallite size, crystallographic texture and morphology) and chemistry, which is in turn controlled by the fabrication technique, growth conditions and post-deposition processes etc. From the viewpoint described above, the ability to tailor the properties so as to optimize performance requires a detailed understanding of the structural characteristics and optical and electronic properties of nanocrystalline HfO 2 films. One approach to in- vestigate defect energy levels in HfO 2 films is using photoluminescence (PL) spectroscopy. Several emission bands of HfO 2 in the photo energy range of 2–4.5 eV have been noticed. Ito et al. [11] used PL to study HfO 2 films synthesized with various methods. They found that HfO 2 films have similar PL characteristics regardless of preparation methods. Ni et al. [12] showed two broad bands in the visible range of the PL emis- sion spectra of HfO 2 films which were due to oxygen vacancies involved during deposition. The intensity of the PL absorptions can be either en- hanced by annealing in argon or diminished by annealing in oxygen. Kiisk et al. further investigated the intensity of PL emission of HfO 2 in- creased due to its defects and self-trapped excitions [13]. Interestingly, markedly different results have been obtained depending on growth conditions indicating the complexity of the defect state in HfO 2 films. In this paper, we report our experimental results on the structural char- acteristics, optical properties and PL features of nanocrystalline HfO 2 films grown at various conditions by RF reactive sputtering. The correla- tion between structural characteristics and optical and PL properties was systematically studied with x-ray diffraction (XRD), atomic force micros- copy (AFM), UV/VIS spectrophotometry, and PL measurements. Thin Solid Films 545 (2013) 279–284 ⁎ Corresponding author. E-mail address: chunyuma@dlut.edu.cn (C.Y. Ma). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.08.068 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf