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Title: Origins of the Doping Asymmetry in Oxides: Hole Doping in NiO versus Electron Doping in ZnO

Abstract

The doping response of the prototypical transparent oxides NiO (p-type), ZnO (n-type), and MgO (insulating) is caused by spontaneous formation of compensating centers, leading to Fermi-level pinning at critical Fermi energies. We study the doping principles in these oxides by first-principles calculations of carrier-producing or -compensating defects and of the natural band offsets, and identify the dopability trends with the ionization potentials and electron affinities of the oxides. We find that the room-temperature free-hole density of cation-deficient NiO is limited by a too large ionization energy of the Ni vacancy, but it can be strongly increased by extrinsic dopants with shallower acceptor levels.

Authors:
; ;
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
939293
DOE Contract Number:
AC36-99-GO10337
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review. B, Condensed Matter and Materials Physics; Journal Volume: 75; Journal Issue: 24, 2007; Related Information: Article No. 241203R
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; ASYMMETRY; DEFECTS; ELECTRONS; FERMI LEVEL; IONIZATION; IONIZATION POTENTIAL; OXIDES; Basic Sciences; Solid State Theory

Citation Formats

Lany, S., Osorio-Guillen, J., and Zunger, A. Origins of the Doping Asymmetry in Oxides: Hole Doping in NiO versus Electron Doping in ZnO. United States: N. p., 2007. Web. doi:10.1103/PhysRevB.75.241203.
Lany, S., Osorio-Guillen, J., & Zunger, A. Origins of the Doping Asymmetry in Oxides: Hole Doping in NiO versus Electron Doping in ZnO. United States. doi:10.1103/PhysRevB.75.241203.
Lany, S., Osorio-Guillen, J., and Zunger, A. Mon . "Origins of the Doping Asymmetry in Oxides: Hole Doping in NiO versus Electron Doping in ZnO". United States. doi:10.1103/PhysRevB.75.241203.
@article{osti_939293,
title = {Origins of the Doping Asymmetry in Oxides: Hole Doping in NiO versus Electron Doping in ZnO},
author = {Lany, S. and Osorio-Guillen, J. and Zunger, A.},
abstractNote = {The doping response of the prototypical transparent oxides NiO (p-type), ZnO (n-type), and MgO (insulating) is caused by spontaneous formation of compensating centers, leading to Fermi-level pinning at critical Fermi energies. We study the doping principles in these oxides by first-principles calculations of carrier-producing or -compensating defects and of the natural band offsets, and identify the dopability trends with the ionization potentials and electron affinities of the oxides. We find that the room-temperature free-hole density of cation-deficient NiO is limited by a too large ionization energy of the Ni vacancy, but it can be strongly increased by extrinsic dopants with shallower acceptor levels.},
doi = {10.1103/PhysRevB.75.241203},
journal = {Physical Review. B, Condensed Matter and Materials Physics},
number = 24, 2007,
volume = 75,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • To elucidate the hole doping mechanism, we have systematically studied the Ni and Cu [ital K]-edge x-ray-absorption spectra in doped Nd[sub 2]NiO[sub 4], La[sub 2]NiO[sub 4], and La[sub 2]CuO[sub 4]. We find a significant difference in the character of the doped hole between the nickelates and cuprate. We observe that both strontium substitution and introduction of excess oxygen lead to a shift of the Ni absorption edge to higher energies, by nearly 3 eV in NaSrNiO[sub 4]. This result demonstrates that the doped holes, in addition to the O 2[ital p] character, have a substantial amount of Ni 3[ital d]more » character (30--40 %). The intensity of the 1[ital s]-3[ital d] transition increases upon doping, consistent with increasing number of [ital d] holes. In contrast, the Cu absorption edge in the doped cuprate is shifted by much smaller amounts, indicative of a lower amount of Cu 3[ital d] character for the doped hole. The strongly mixed Ni 3[ital d] and O 2[ital p] character of the doped hole distinguishes the electronic structure of the nickelate from that of the cuprate.« less
  • Dilute magnetic semiconductors (DMS) which exhibit ferromagnetism (FM) at and above room temperature are a highly desirable class of materials for future spin- tronics devices. Zn 1-xCo xO (Co:ZnO) is a heavily studied DMS material in this context. Although controversially discussed in the literature, there is a growing con- sensus, that phase-pure Co:ZnO is paramagnetic (PM)[1–3]. Altering the preparation conditions can easily lead to phase separation and consequently superparam- agnetism (SPM) [3]. Nonetheless there are recent experimental data claiming that FM can be switched on inCo:ZnO by controlling the carrier concentration [4]. On the other hand, no FM was foundmore » in structurally excellent Al-codoped Co:ZnO [5]. However, in the latter work the magnetic characterization was restricted to room temperature measurements. In parallel, theory has also revealed that defect-free, insulating Co:ZnO is not ferromagnetic [6, 7] whereas the role of n-type carriers remains under debate, ranging from ferromagnetic coupling [8], or oscil- latory behavior with Co-Co distance [9] to antiferromagnetic coupling [10]. It is rather common to manipulate the n-type carrier concentration of ZnO by Al-doping to yield high conductivity [4, 11]. On the other hand, it had been shown that Al-codoping of Co:ZnO may promote the onset of phase separation [11]. It is extremely difficult to detect such secondary Co-containing phases even with the most careful x-ray diffraction (XRD) analysis [11, 12] or depth-profiling photoelectron spectroscopy (DP-XPS) [13]. Such careful materials characterization is lacking in Ref. [4]. An alternative to extensive XRD or DP-XPS to look for potential phase separation in Co:ZnO is the combination of x-ray absorption near edge spectra (XANES), x-ray linear dichroism (XLD), and x-ray magnetic circular dichroism (XMCD). This suite of atom-specific x-ray spectroscopies nicely complements integral superconducting quantum interference device (SQUID) magnetometry. For example, combined XLD simulations and experiments at the Co K-edge have been used to verify the phase purity of Co:ZnO [2] and characteristic spectroscopic signatures with appropriate quality thresholds for PM and SPM have been identified recently in the XANES and XMCD at the Co K-edge of Co:ZnO [3]. Along the same line, a careful combination of XANES and extended x-ray absorption fine structure (EXAFS) was employed to study Co:ZnO films similar to those in [4] which found evidence for Co(0) secondary phases [14].« less