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

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

https://iopscience.iop.org/article/10.1149/2.0341907jss/pdf

Editors' Choice—Review—Theory and Characterization of Doping and Defects in β-Ga2O3

Heterojunction p-Cu2O/n-β-Ga2O3 diodes were fabricated on an epitaxially grown β-Ga2O3(001) layer. The reverse breakdown voltage of these p-n diodes reached 1.49 kV with a specific on-resistance of 8.2 mΩ cm2. The leakage current of the p-n diodes was lower than that of the Schottky barrier diode due to the higher barrier height against the electron. The ideality factor of the p-n diode was 1.31. It indicated that some portion of the recombination current at the interface contributed to the forward current, but the diffusion current was the dominant. The forward current more than 100 A/cm2 indicated the lower conduction band offset at the hetero-interface between Cu2O and Ga2O3 layers than that predicted from the bulk properties, resulting in such a high forward current without limitation. These results open the possibility of advanced device structures for wide bandgap Ga2O3 to achieve higher breakdown voltage and lower on-resistance.

Heterojunction p-Cu2O/n-Ga2O3 diode with high breakdown voltage

Cr-doped ZnS nanoparticles with Cr concentrations of 0.5, 1, 2 and 3 atm.% were successfully synthesized by the chemical co-precipitation method using ethylenediaminetetraacetic acid (EDTA) as the capping agent. The structural, optical and magnetic properties of the prepared samples were studied. Energy dispersive spectroscopy (EDS) measurements showed the presence of Cr in the Cr-doped ZnS. No mixed phase was observed from X-ray diffraction (XRD) studies and all the peaks were indexed to the cubic phase of ZnS. The average diameter of the particles was in the range of 6–10 nm, and it was confirmed by TEM studies. The magnetic behavior of the nanoparticles for different chromium concentrations was investigated by magnetism measurements using a vibrating sample magnetometer (VSM). The nanoparticles with lower Cr concentration exhibited strong ferromagnetism, where as in samples of higher Cr concentrations the ferromagnetism suppressed. The electron paramagnetic resonance (EPR) spectra of the nanocrystals showed the resonance of electron centers with a g-value of 1.989. The signal intensity and linewidth of the EPR signal increased with increasing Cr content. FTIR studies indicated that the nanoparticles were sterically stabilized by EDTA.

Room-temperature ferromagnetism in EDTA capped Cr-doped ZnS nanoparticles

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.

Yanrui Guo

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

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