Extensive calculations based on density functional theory have been carried out to understand the origin of magnetism in undoped ZnO thin films as found in recent experiments. The observed magnetism is confirmed to be due to Zn, instead of O, vacancy. The main source of the magnetic moment, however, arises from the unpaired 2p electrons at O sites surrounding the Zn vacancy with each nearest-neighbor O atom carrying a magnetic moment ranging from 0.490 to 0.740 B. Moreover, the study of vacancy-vacancy interactions indicates that in the ground state, the magnetic moments induced by Zn vacancies prefer to ferromagnetically couple with the antiferromagnetic state lying 44 meV higher in energy. Since this is larger than the thermal energy at room temperature, the ferromagnetic state can be stable against thermal fluctuations. Calculations and discussions are also extended to ZnO nanowires that have larger surface to volume ratio. Here, the Zn vacancies are found to lead to the ferromagnetic state too. The present theoretical study not only demonstrates that ZnO samples can be magnetic even without transition-metal doping but also suggests that introducing Zn vacancy is a natural and an effective way to fabricate magnetic ZnO nanostructures. In addition, vacancy mediated magnetic ZnO nanostructures may have certain advantages over transition-metal doped systems in biomedical applications.

https://absimage.aps.org/image/MAR08/MWS_MAR08-2007-004197.pdf

Zinc Vacancy induced magnetism in ZnO thin films and nanowires

Defect engineering is considered as one of the most efficient strategies to regulate the electronic structure of materials and involves the manipulation of the types, concentrations, and spatial distributions of defects, resulting in unprecedented properties. It is shown that a single-layered MnO2 nanosheet with vacancies is a robust half-metal, which was confirmed by theoretical calculations, whereas vacancy-free single-layered MnO2 is a typical semiconductor. The half-metallicity of the single-layered MnO2 nanosheet can be observed for a wide range of vacancy concentrations and even in the co-presence of Mn and O vacancies. This work enables the development of half-metals by defect engineering of well-established low-dimensional materials, which may be used for the design of next-generation paper-like spintronics.

Half‐Metallicity in Single‐Layered Manganese Dioxide Nanosheets by Defect Engineering

We report on the fabrication and characterization of solar blind photodetectors (SBPs) based on undoped β-Ga2O3 and Zn doped (∼5 × 1020 cm−3) β-Ga2O3 (ZnGaO) epitaxial films with cutoff wavelength of ∼260 nm. The epilayers were grown on c-sapphire by the metal organic chemical vapor deposition technique and their structural, electrical and optical properties were characterized using various methods. As grown films have a large number of defects, resulting in detectors with enhanced internal gain, hence, high spectral responsivity >103 A/W. Post growth annealing in oxygen improved the quality of the epilayers, leading to detectors with reduced dark current (∼nA to ∼pA) and increased out of band rejection ratio. At 20 V bias, a ZnGaO detector showed a peak responsivity of 210 A/W (at 232 nm) and an out of band rejection ratio (i.e., R232 nm/R320 nm) of 5 × 104. Alternatively, for a β-Ga2O3 detector these parameters were found to be five times and three times lower, respectively, suggesting that ZnGaO detectors have superior performance characteristics. These results provide a roadmap toward achieving high responsivity SBPs based on epitaxial ZnGaO films, laying a solid foundation for future applications.

Solar blind photodetector based on epitaxial zinc doped Ga2O3 thin film

The difficulty in fabricating p-type Ga2O3 is a crucial issue which restricts its applications in practical devices. In the present work, we have performed first principles studies on formation energies, electronic structures and optical properties for a series of metal doped β-Ga2O3, involving a long list of main group and transition metals, even some lanthanides, to search for potential p-type dopants. Calculations have shown that Li and Be, both with small atomic radius, prefer interstitial doping rather than substitutional doping of Ga, resulting eventually in an n-type character to the doped system. In addition, an O-rich atmosphere is more favorable for p-type substitutional dopings by comparison with the Ga-rich condition. A number of metal dopants show potential in achieving p-type β-Ga2O3, for example, Na, Mg, Ca, Cu, Ag, Zn, Cd, which are all worth a further emphasis study in experiment, although a satisfying holes concentration may only be possible to achieve by simultaneously committing to both the elimination of n-type background carriers and the activation mechanism of dopants. Absorption spectra have shown that all the above-mentioned potential p-type dopants are suitable for deep UV applications. The major peaks of the absorption spectra are red-shifted in most cases, due to the introduction of new states to the forbidden gap by dopants, which have been discussed in detail by inspecting into the partial density of states. Ga2O3 doped by some transition metals show potential as magnetic devices.

Electronic structure and optical property of metal-doped Ga2O3: a first principles study

Carbon doped hexagonal boron nitride nanoribbon as efficient metal-free electrochemical nitrogen reduction catalyst

Electronic structure and magnetic interactions in Zn-doped β-Ga2O3 from first-principles calculations

Nitrogen-vacancy induced intrinsic ferromagnetism and half-metallicity in BN

The electronic properties and ferromagnetic coupling interactions of Mg-doped cubic boron nitride (BN) have been investigated by using first-principles calculation method. The calculated results show that Mg-doped BN takes ferromagnetic ground state with magnetic moment of 0.565 μ
 B which mainly comes from the unpaired 2p electron of N atoms around the Mg dopant. The magnetic coupling between Mg dopants induced moments is long-ranged. It also was found that the Mg-doped BN system has high spin polarization near the Fermi level.

Songqing Guo

Papers

Electronic structure and magnetic interactions in Zn-doped β-Ga2O3 from first-principles calculations

Nitrogen-vacancy induced intrinsic ferromagnetism and half-metallicity in BN

Electronic Structure and Ferromagnetic Properties of MgDoped Cubic BN from First-Principles Calculation