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Identification of Defects in Metal Oxides

January 20, 2015.

 

         The synchrotron x-ray absorption near edge structures (XANES) technique was used in conjunction with first-principles calculations to characterize Al-doped ZnO films. Standard characterizations revealed that the amount of carrier concentration and mobility depend on the growth conditions, i.e. H2 (or O2)/Ar gas ratio and Al concentration. First-principles calculations showed that Al energetically prefers to substitute on the Zn site, forming a donor AlZn, over being an interstitial (Ali). The measured Al K-edge XANES spectra are in good agreement with the simulated spectra of AlZn (Fig. 1), indicating that the majority of Al atoms are substituting for Zn. The reduction in carrier concentration or mobility in some samples can be attributed to the AlZn-VZn and 2AlZn-VZn complex formations that have similar XANES features. In addition, XANES of some samples showed additional features that are the indication of some α-Al2O3 or nAlZn-Oi formation, explaining their poorer conductivity.

 

         For over three decades, the infrared spectroscopy peaks of around 3500 cm-1 observed in hydrogen-doped SrTiO3 samples have been assigned to an interstitial hydrogen (Hi) attached to a lattice oxygen with two possible configuration models: the octahedral edge (OE) and the cubic face (CF) models. Based on our first-principles calculations of Hi around O, both OE and CF configurations are not energetically stable. Starting from either configuration, the Hi would spontaneously relax into an off axis (OA) site; lowering the energy by 0.25 eV or more (Fig. 2). The calculated vibrational frequency of 2745 cm-1 for OA invalidates the assignment of Hi to the observed 3500 cm-1 peak. In addition, the calculated diffusion barrier is low, suggesting that Hi can be easily annealed out. We propose that the observed peaks around 3500 cm-1 are associated with defect complexes. A Sr vacancy (VSr) can trap Hi and form a H-VSr complex which is both stable and has the frequency in agreement with the observed main peak. The complex can also trap another Hi and form 2H-VSr; consistent with the observed additional peaks at slightly higher frequencies (3510–3530 cm−1).

 

        In a recent Letter [1],Scanlon and Watson (SW) reportedtheir first -principles results on hydrogen in Cu2O. They find that

 

(1) an interstitial H in Cu2O prefers to occupy the tetrahedral site (Htet), which is coordinated with four Cu cations, in all three charge states (+1, neutral, and -1), and

 

(2) H will bind with a Cu vacancy and form an electrically active H-VCu defect complex, which is amphoteric with (+/0) and (0/-) transition levels at Ev + 0.1 and Ev + 1.1 eV, respectively.

 

        However, these two findings contradict two generally observed behaviors of H in oxides, i.e.,

 

 (i) cationic H usually binds with an O atom, forming a single O-H bond, while the anionic H usually binds withcations with multicoordination (Fig. 3), and

 

(ii) H usually passivatescation vacancies in oxides. In this Comment, we explicitly show that with charge state +1, H prefers to bind with a single O anion rather than with four Cu cations and that H-VCu does not induce any defect levels inside the band gap. Our results were obtained by using hybrid density functional method, similar to Ref. 1, but absent the error noted in their calculations (Fig. 4).

 

 

 

Figure 1. Al K-edge XANES spectra of Al-doped ZnO thin films grown (a) under 0.3% H2/Ar gas with Al concentration ranging from 0.1 to 2.0% and (b) under different H2/Ar and O2/Ar gas ratios with Al concentration at 1.0%.  (c) The simulated Al K-edge XANES spectra obtained from different models.

 

 

 

Figure 2. Illustration of hydrogen location in SrTiO3 crystal, which has been reported for a long time that CF and OE were the most stable sites [5-6].  In this work, we reported that OA is the most stable site for hydrogen interstitial with lower energy than previous works.

 

 

 

Figure 3. Calculated migration barrier forthe hopping of proton from different path ways.  It is quite clear that hydrogen can easily move throughout the crystal due to a very low migration barrier.

 

 

 

Figure 4. Illustration of bonding between oxygen and hydrogen at (a) CF site, (b) OA site, (c) OA site with Sr vacancy (VSr).  (d) and (e) show two oxygen and hydrogen bonds with VSr resulting in increasing the vibrational frequencies close to the experimental observation of 3,500 cm-1.

 

 

 

Figure 5. X-ray absorption spectroscopy station at beamline 8 of the Synchrotron Light Research Institute, Ministry of Science and Technology, Nakornrachasima, Thailand.

 

References:

1. D. O. Scanlon and G. W. Watson, Erratum :Uncovering the Complex Behavior of Hydrogen in Cu2O, Phys. Rev. Lett. 108, 129901(2012).

2. J. T-Thienprasert, S. Rujirawat, W. Klysubun, J. N. Duenow, T. J. Coutts, S. B. Zhang, D. C. Look, and S. Limpijumnong. Compensation in Al-doped ZnO by Al-related acceptor complexes: Synchrotron x-ray absorption spectroscopy and theory. Physical Review Letters 110, 055502 (2013).

3. J. T-Thienprasert, I. Fongkaew, D. J. Singh, M.-H.Du, and S. Limpijumnong. Identification of hydrogen defects in SrTiO3 by first-principles local vibration mode calculations. Physical Review B 85, 125205 (2012)

4. K. Biswas, M.-H.Du, J. T-Thienprasert, S. Limpijumnong, and D. J. Singh. Comment on “Uncovering the complex behavior of hydrogen in Cu2O”. Physical Review Letters 108, 219703 (2012).

5. G. Weber, S. Kapphan, and M. Wöhlecke. Spectroscopy of the O-H and O-D stretching vibrations in SrTiO3 under applied electric field and uniaxial stress. Physical Review B 34, 8406 (1986).

6. S. Klauer and M. Wöhlecke. Local symmetry of hydrogen in cubic and tetragonal SrTiO3 and KTaO3: Li determined by polarized Raman scattering. Physical Review Letters 68, 3212 (1992).

 

Reported by :

Assistant Professor Dr. Jiraroj T-Thienprasert, Condensed Matter Physics Laboratory, Thailand Center of Excellence in Physics (ThEP) and Department of Physics, Faculty of Science, Kasetsart University, Bangkok

Telephone / Fax :+662-562-5555  ext. 3534

E-mail :  fscicwt@ku.ac.th