Effects of gamma-ray irradiation on material and electrical properties of AlN gate dielectric on 4H-SiC.

This article investigates the radiation effects on as-deposited and annealed AlN films on 4H-SiC substrates under gamma-rays. The AlN films are prepared using plasma-enhanced-atomic-layer-deposition on an n-type 4H-SiC substrate. The AlN/4H-SiC MIS structure is subjected to gamma-ray irradiation with total doses of 0, 300, and 600 krad(Si). Physical, chemical, and electrical methods were employed to study the variations in surface morphology, charge transport, and interfacial trapping characteristics induced by irradiation. After 300 krad(Si) irradiation, the as-deposited and annealed samples exhibit their highest root mean square values of 0.917 nm and 1.190 nm, respectively, which is attributed to N vacancy defects induced by irradiation. Under irradiation, the flatband voltage (Vfb) of the as-deposited sample shifts from 2.24 to 0.78 V, while the annealed sample shifts from 1.18 to 2.16 V. X-ray photoelectron spectrum analysis reveals the decomposition of O-related defects in the as-deposited AlN and the formation of Al(NOx)ycompounds in the annealed sample. Furthermore, the space-charge-limits-conduction (SCLC) in the as-deposited sample is enhanced after radiation, while the barrier height of the annealed sample decreases from 1.12 to 0.84 eV, accompanied by the occurrence of the SCLC. The physical mechanism of the degradation of electrical performance in irradiated devices is the introduction of defects like N vacancies and O-related defects like Al(NOx)y. These findings provide valuable insights for SiC power devices in space applications.

Polycrystalline Few-Layer Graphene as a Durable Anticorrosion Film for Copper.

Corrosion of metals in atmospheric environments is a worldwide problem in industry and daily life. Traditional anticorrosion methods including sacrificial anodes or protective coatings have performance limitations. Here, we report atomically thin, polycrystalline few-layer graphene (FLG) grown by chemical vapor deposition as a long-term protective coating film for copper (Cu). A six-year old, FLG-protected Cu is visually shiny and detailed material characterizations capture no sign of oxidation. The success of the durable anticorrosion film depends on the misalignment of grain boundaries between adjacent graphene layers. Theoretical calculations further found that corrosive molecules always encounter extremely high energy barrier when diffusing through the FLG layers. Therefore, the FLG is able to prevent the corrosive molecules from reaching the underlying Cu surface. This work highlights the interesting structures of polycrystalline FLG and sheds insight into the atomically thin coatings for various applications.

Nanoscale avalanche photodiode with self-quenching and ultrahigh ultraviolet/visible rejection ratio.

A 4H-SiC based separate-absorption-multiplication (SAM) avalanche photodiode with a nanoscale multiplication region and a bulk absorption region is proposed and its optoelectronic performance is modeled. The results show that the avalanche breakdown voltage of the device is found to be dependent on the illumination condition. This is attributed to the existence of an illumination-dependent hole potential well in the upper center of the absorption region. Based on the illumination-dependence of avalanche breakdown voltage, a self-quenching and an ultrahigh UV/visible rejection ratio have been realized in this structure.

Band alignment of ZnO-based nanorod arrays for enhanced visible light photocatalytic performance.

A ternary semiconductor ZnO/MoS2/Ag2S nanorod array in an intimate core-shell structure was synthesized on glass substrates. The physicochemical properties and photocatalytical performance of the specimen were characterized and compared with single ZnO and binary ZnO/Ag2S and ZnO/MoS2 nanorod arrays. It is found that the coating layers depressed the band edge emission of the ZnO core, improved light absorption in the visible range, reduced charge transfer resistance, and increased photocatalytic activity. The ternary heterojunction nanorod array possessed full solar absorption with an efficiency of 52.88% for the degradation of methylene blue under visible light in 30 min. The efficiency was higher than other arrays and was 7.6 times that of the ZnO array. Theory analysis revealed that the coating layer brought different band alignment in the heterojunctions for efficient charge separation and conduction, which was beneficial for the photocatalytic performance.

Effect of interfacial dipole on heterogeneous ice nucleation.

In this work, we performed molecular dynamics simulations of ice nucleation on a rigid surface model of cubic zinc blende structure with different surface dipole strength and orientation. Our results show that, although substrates are excellently lattice-matched to cubic ice, ice nucleation merely occurred as the interfacial water molecules (IWs) show identical or similar orientations to that of water molecules in cubic ice. Free energy landscapes revealed that, as substrates have non-suitable dipole strength/orientation, there exist large free energy barriers for rotating dipole IWs to the right orientation to trigger ice formation. This study stresses that, beyond the traditional view of lattice match and the similarity of lattice length between the substrate and new-formed crystal, the similarity between molecular orientations of interfacial component and component in the specific new-formed crystalline face is also critical for promoting ice nucleation.

Raman Spectroscopy of Multi-Layer Graphene epitaxially Grown on 4H-SiC by Joule Heat Decomposition.

We developed a Joule heating decomposition (JHD) method, which applied direct current on the SiC for the epitaxial growth of multi-layer graphene (MLG) films on Si-terminated (0001) face of the high doping 4H-SiC substrate. By this JHD method, the growth time for preparing MLG was only several minutes. Raman spectroscopy was employed to study the influence of the temperature caused by the Joule heating on the quality and the uniformity of the sample. Then, other properties, such as the strain, the layer's number, and the electric characteristics, of the MLG were studied in details. It was found that the quality of the MLG was substantially dependent on the growth temperature (operation current) and the growth time, while the layer's number was only dependent on the growth temperature but not the growth time. Finally, less-defect and homogeneous MLG (~ 45 layers) with an area of ~ 12 × 5 mm2 could be obtained at a heating temperature of ~ 1470 °C with duration time of 5 min. By using the linear transmission line method, the specific contact resistance of Au and MLG was 5.03 × 10-5 Ω cm2, and the sheet resistance was 52.36 Ω/sq, respectively.