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

Based on electronic structure and atomic size considerations, nitrogen has been regarded as the most suitable impurity for p-type doping in ZnO. However, numerous experimental efforts by many different groups have not resulted in stable and reproducible p-type material, casting doubt on the efficacy of nitrogen as a shallow acceptor. Based on advanced first-principles calculations we find that nitrogen is actually a deep acceptor, with an exceedingly high ionization energy of 1.3 eV, and hence cannot lead to hole conductivity in ZnO. In light of this result, we reexamine prior experiments on nitrogen doping of ZnO.

http://absimage.aps.org/image/MAR10/MWS_MAR10-2009-001476.pdf

Why nitrogen cannot lead to p-type conductivity in ZnO

The group-III sesquioxides possess material properties that render them interesting for applications such as high-power rectifiers and transistors, solar-blind UV detectors and inter-sub-band infrared detectors. Technology for growing large, single-crystalline bulk material and for wafer fabrication exists, enabling homoepitaxial growth of thin films with high crystalline quality. The bandgap can be tuned in an energy range from about 4 to 8 eV for the ternary alloys and allows growth of heterostructures with large band offset. Here, past results and recent investigations on the growth, the material properties, contact fabrication and the alloying of group-III sesquioxides are reviewed, and an overview on demonstrator devices is provided.

Group‐III Sesquioxides: Growth, Physical Properties and Devices

The creation of large magnetic fields is a necessary component in many technologies, ranging from magnetic resonance imaging, electric motors and generators, and magnetic hard disk drives in information storage. This is typically done by inserting a ferromagnetic pole piece with a large magnetisation density MS in a solenoid. In addition to large MS, it is usually required or desired that the ferromagnet is magnetically soft and has a Curie temperature well above the operating temperature of the device. A variety of ferromagnetic materials are currently in use, ranging from FeCo alloys in, for example, hard disk drives, to rare earth metals operating at cryogenic temperatures in superconducting solenoids. These latter can exceed the limit on MS for transition metal alloys given by the Slater-Pauling curve. This article reviews different materials and concepts in use or proposed for technological applications that require a large MS, with an emphasis on nanoscale material systems, such as thin and ultra-thin films. Attention is also paid to other requirements or properties, such as the Curie temperature and magnetic softness. In a final summary, we evaluate the actual applicability of the discussed materials for use as pole tips in electromagnets, in particular, in nanoscale magnetic hard disk drive read-write heads; the technological advancement of the latter has been a very strong driving force in the development of the field of nanomagnetism.

/pdf/a-review-of-high-magnetic-moment-thin-films-for-microscale-mhfn2l7x2f.pdf

A review of high magnetic moment thin films for microscale and nanotechnology applications

With a wide band gap, high critical breakdown voltage and commercially available substrates, Ga2O3 is a promising material for next-generation power electronics. Like most wide-band-gap semiconductors, obtaining better control over its electrical conductivity is critically important, but has proven difficult to achieve. Although efficient p-type doping in Ga2O3 is not expected, since theory and experiment indicate the self-trapping of holes, the full development of this material will require a better understanding of acceptor dopants. Here the properties of group 2, group 5 and group 12 acceptor impurities in β-Ga2O3 are explored using hybrid density functional calculations. All impurities are found to exhibit acceptor transition levels above 1.3 eV. After examining formation energies as a function of chemical potential, Mg (followed closely by Be) is determined to be the most stable acceptor species.

https://iopscience.iop.org/article/10.1088/1361-6641/aaba98/pdf

A survey of acceptor dopants for β-Ga2O3

It is significant to design an electrode with high carrier separation and transport efficiency in a photoelectrochemical water splitting system. In this study, we utilized a facile H2O2 etching method for modulating S-vacancies in a TiO2/MoS2 nanostructure to enhance photoelectrochemical performance. The as-prepared TiO2/MoS2 with S-vacancies demonstrates an increased photocurrent density of 1.56 mA cm−2 at 1.23 V vs. RHE, which is 2.14 times higher than that of pristine TiO2 (0.73 mA cm−2 at 1.23 V vs. RHE). An appropriate energy level alignment between TiO2 and MoS2 and defect modulation are demonstrated in this study. The photo-generated electrons and holes are effectively separated and transported due to the built-in electric field in TiO2/MoS2. Moreover, the surface oxygen production reaction is enhanced due to its plenty exposed reaction active sites. This study demonstrates the synergistic effects of S-vacancies and built-in electric field in enhancing the carrier separation and transport properties. Furthermore, this study provides a universal strategy in designing high quality photoelectrodes in the photoelectrochemical system.

The synergistic effect with S-vacancies and built-in electric field on a TiO2/MoS2 photoanode for enhanced photoelectrochemical performance

Using plane-wave pseudopotential (PWPP) method, the magnetism and spin-resolved electronic properties of Cu-doped ZnS system are studied. Our calculations indicate that ferromagnetic (FM) state is ground state in Cu-doped ZnS. The FM coupling strength in ZnS doping with Cu fluctuates with the variation of distance between two dopants and the fluctuation gets larger with increase in distance. Room temperature ferromagnetism can be observed in Cu-doped ZnS with high dopant concentration. Formation energy calculation implies that the clustering effect is not obvious in Cu-doped ZnS. Thus, Cu-doped ZnS can be a promising dilute magnetic semiconductor (DMS), which promises to be free of magnetic precipitates.

Ferromagnetic properties of Cu-doped ZnS: A density functional theory study

The electronic and optical properties of the Cu-doped and intrinsic β-Ga2O3 are studied by using the first-principles calculation method. Results show that Cu-doped β-Ga2O3 can be fabricated in experiments. Two acceptor impurity levels are introduced near the top of the valence band by Cu dopant, indicating that Cu-doped gallium oxide is a promising p-type semiconductor. Cu-doped β-Ga2O3 can be used as intermediate band semiconductor in solar cell. Cu dopant induced 100% spin polarization near the Fermi level. The analysis results of optical properties reveal that Cu-doped β-Ga2O3 is a promising potential candidate for p-type ultraviolet (UV) transparent semiconductor.

Huiyu Yan

Papers

The synergistic effect with S-vacancies and built-in electric field on a TiO2/MoS2 photoanode for enhanced photoelectrochemical performance

Ferromagnetic properties of Cu-doped ZnS: A density functional theory study

First-principles study on electronic structure and optical properties of Cu-doped β-Ga2O3

Zinc ferrite-based p–n homojunction with multi-effect for efficient photoelectrochemical water splitting

Ferromagnetism in Cu-doped MgO: Density-functional calculations