The role of oxygen-related defects and hydrogen impurities in HfO 2 and ZrO 2 J.L. Lyons ⇑ , A. Janotti, C.G. Van de Walle Materials Department, University of California, Santa Barbara, CA 93106-5050, USA article info Article history: Available online 30 March 2011 Keywords: High-k dielectrics First-principles calculations Defects in semiconductors Oxygen vacancy Oxygen interstitial Hydrogen impurities ZrO 2 HfO 2 Sources of fixed charge abstract We investigate the properties of oxygen-related defects and hydrogen impurities in monoclinic HfO 2 and ZrO 2 using first-principles calculations based on a hybrid functional. We examine how the formation energy of these defects depend on the Fermi level and the chemical potential. By aligning the valence- and conduction-band edges of these oxides to those of Si, we evaluate the possibility of these defects act- ing as carrier traps or sources of fixed charge in Si/HfO 2 and Si/ZrO 2 . While the oxygen vacancy (V O ) appears more likely to act as a carrier trap, we find oxygen and hydrogen interstitials (O i and H i ) as well as substitutional hydrogen (H O ) to have much lower formation energies for relevant Fermi levels and chemical potentials. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction HfO 2 and ZrO 2 are two high-k oxides that offer a promising alternative to the traditional gate dielectric SiO 2 [1]. With the introduction of HfO 2 and ZrO 2 into the gate stack, new issues have arisen. In particular, the high-k oxides appear to suffer from the presence of fixed charge and carrier traps [2]. These issues are likely related to defects and impurities in the bulk of the gate oxide or at interfaces. First-principles calculations are an excellent tool for studying defects and impurities in these systems, since they of- fer the ability to isolate individual defects and examine their struc- tural, optical, and electronic properties. Previous work has suggested that carrier trapping or the pres- ence of fixed charge in these gate oxides may be due to oxygen- related native defects [3–6]. Most studies have focused on the oxygen vacancy as the dominant defect responsible for these phe- nomena [3–5]. A spectroscopic ellipsometry study observed a strong absorption signal about 1 eV below the conduction-band minimum of oxidized hafnium [3]. This signal was thought to be caused by V O , specifically since the samples may have been un- der-oxidized and the signal strength decreased with increasing oxidation temperature. Another study has shown that annealing HfO 2 gate stacks with increasing oxygen partial pressure ðp O 2 Þ caused a decrease in positive fixed charge, which the authors sug- gested was due to the filling of V O in the bulk of the gate dielectric [4]. Theoretical studies have also focused on V O as the dominant native defect in HfO 2 [5,7]. However, it is not assured that V O is the only relevant defect in monoclinic HfO 2 . Other authors have suggested that the oxygen interstitial may also be present in HfO 2 when in it is grown as a gate dielectric [6]. An electron spin resonance (ESR) study has ob- served signals that suggest that negatively charged defects, possi- bly negatively charged oxygen ions or undercoordinated hafnium atoms, play an important role in HfO 2 by acting as electron traps [8]. Similar results have been reported in ESR studies on ZrO 2 [9]. By examining the high temperature conductivity in thin films with varying p O 2 , other researchers have proposed that defects other than V O were dominant in undoped monoclinic HfO 2 [10]. Hydrogen may also play a critical role in determining the prop- erties of HfO 2 and ZrO 2 . Water is a precursor in atomic layer depo- sition (ALD), a common method for depositing these high-k oxides, and the resulting films are often subjected to forming gas anneals [11]. Thus, hydrogen may be incorporated unintentionally into these oxides during growth or processing. Recent work has sug- gested that degradation mechanisms in HfO 2 gate stacks and the presence of positive fixed charge are related to hydrogen impuri- ties [12]. First-principles calculations based on density functional theory (DFT) have previously been utilized to investigate native defects and impurities in HfO 2 and ZrO 2 [13–20]. However, most of these studies employed traditional approaches based on either the local density approximation (LDA) or the generalized gradient approxi- mation (GGA) of the density functional theory [21], in which the band gaps of semiconductors and insulators are severely underes- timated. This leads to large errors in the defect levels and forma- tion energies, especially in wide-band gap materials. Studies which employed more advanced techniques either only considered 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.03.099 ⇑ Corresponding author. Tel.: +1 6084494653. E-mail address: jllyons@engineering.ucsb.edu (J.L. Lyons). Microelectronic Engineering 88 (2011) 1452–1456 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee