Coexistence of different charge states in Ta-doped monoclinicHfO2: Theoretical and experimental approaches

Abstract

A combination of experiments and ab initio quantum-mechanical calculations has been applied to examine hyperfine interactions in Ta-doped hafnium dioxide. Although the properties of monoclinic HfO 2 have been the subject of several earlier studies, some aspects remain open. In particular, time differential perturbed angular correlation spectroscopy studies using 181 Ta as probe atom revealed the coexistence of two hyperfine interactions in this material but an explanation was only given for the more populated one. Until now, no models have been proposed that explain the second interaction, and it has not yet been associated with a specific crystallographic site. In this work, a detailed study of the different charge states for the impurity-probe atom ͑Ta͒ was performed in order to understand the second interaction observed in Ta-doped monoclinic HfO 2 . The combination of experiments and theory suggests that two different charge states coexist in this compound. Further, ab initio calculations predict that, depending on the impurity charge state, a sizeable magnetic moment can be induced at the probe site. This is confirmed by a new analysis of experimental data.

Structural, electronic, and hyperfine properties of pure and Ta-dopedm-ZrO2

“…32 A similar result was obtained in other systems such as Cd complexes in Si and Ge. 31 Based on these results spin-polarized calculations were performed.…”

“…[28][29][30][31] Very recently, a combination of hyperfine experiments [using time-differential perturbed angular correlations spectroscopy (TDPAC) with 181 Ta as a probe] and ab initio calculations has been applied to study the electronic structure of Ta-doped hafnium dioxide. 32 The study revealed that Ta introduces impurity levels in the band gap of semiconducting monoclinic HfO 2 (m-HfO 2 ), and induces significant structural distortions in the host lattice. Also, the structural relaxations as well as the hyperfine parameters strongly depend on the charge state of the impurity.…”

“…This trapping effect could be related to the magnetism observed in thin films of pure m-HfO 2 . 32 ZrO 2 and HfO 2 are similar systems regarding their chemical and physical properties. They have the same crystallographic structures with very similar cell parameters and atomic positions, and both can be stabilized in their more symmetric structures by means of the addition of low percentages of dopants.…”

Experimental and First-Principles Theoretical Study of Structural and Electronic Properties in Tantalum-Doped In2O3 Semiconductor: Finding a Definitive Hyperfine Interaction Assignment

“…Each Ta atom has 12 In atoms as next nearest neighbors, whose positions slightly change when we allow the full relaxation (their distances to the impurity increase in less than 0.5% of those corresponding to the pure In 2 O 3 system). This result shows that the Ta atom tends to locally reconstruct the environment that it has in its own oxide TaO 2 (in which the Ta atom is 6-fold O coordinated and d NN = 2.02 Å), as it was observed for Ta in other host oxides. ,,, …”

“…This result shows that the Ta atom tends to locally reconstruct the environment that it has in its own oxide TaO 2 (in which the Ta atom is 6-fold O coordinated and d NN = 2.02 Å), 58 as it was observed for Ta in other host oxides. 27,35,40,59 In parts b and c of Figure 2, we show the DOS for In 2 O 3 :Ta(D) and In 2 O 3 :Ta(C) systems, respectively, after full structural relaxation. Comparing these DOS with that of the pure In 2 O 3 system (Figure 2a), we see that unfilled Ta-5d impurity states appear about 2 eV over the CB minimum (see The integration of the total DOS of Figure 2, parts b and 2c, in the occupied CB states energy region (from 7.0 eV to E F ) gives a value of 2.0 electrons, in agreement with the nominal double donor character of the substitutional Ta impurity in In 2 O 3 .…”