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.

Controlling anisotropic thermal properties of graphene aerogel by compressive strain.

Materials with adjustable wide-ranging thermal conductivity are desired to tackle the problem of thermal management for electronic devices operating in an extended range of temperature. In this study, graphene aerogels (GAs) are fabricated and transformed from thermal insulators to thermal conductors by high-temperature annealing. The highest through-plane and in-plane thermal conductivity of annealed GA reaches 3.3 and 96 W/m·K, respectively, under 95% compressive strain. Using the annealed GA as thermal interface material leads to superior performance than commercially available products that have higher through-plane thermal conductivity in dissipating heat for high-power electronic devices (e.g., LED lamp). Furthermore, due to excellent elasticity, the thermal resistance of annealed GAs can be reversibly tuned about six-fold by compressive strain. This paves a novel venue in designing thermal management system for devices, which not only need excellent heat dissipation but also good thermal insulation at various operating environments.

Gradient heterostructure perovskite single crystals enable the improvement of radiative recombination for scintillator application.

Recently, organic-inorganic hybrid perovskites (OIHPs) are rising as promising candidates for light-emitting applications, due to their superior optical properties. High performance light-emitting applications such as scintillators require minimum non-radiative recombination and high fractions of radiative recombination. Here, we report a simple solution-processing strategy for the synthesis of funnel-type CH3NH3(MA)PbCl3/CH3NH3(MA)PbBrxCl3-x heterostructure perovskite materials that improve the light emission performances. The single crystal X-ray diffraction pattern indicates that the lattice mismatch is only ∼3.24% in the heterointerface. The halide gradient is helpful for driving the photoexcited carriers from the internal high bandgap material to the low bandgap light-emitter layer. The steady-state photoluminescence (PL) and radioluminescence (RL) spectra show that the luminescence intensity has been significantly improved by this heterostructure perovskite. Time-resolved photoluminescence (TRPL) exhibits carrier transport along the halide gradient. Our research suggests that the gradient halide perovskite heterostructure with specific optical properties could be a prospect for commercial scintillator applications.

Bulk Organic-Inorganic Methylammonium Lead Halide Perovskite Single Crystals for Indirect Gamma Ray Detection.

Scintillators that convert ionization radiation photons to UV-visible photons have attracted extraordinary attention. Traditional scintillators are associated with a vacuum photomultiplier tube that faces strict constraints of fragility, magnetic fields, and operated voltage, or coupled to a silicon photomultiplier (SiPM) with optical silicone grease. Here, we report a high-performance radiation detector with an indirect photon-to-photon conversion radiation detection model based on perovskite single crystals (SCs), where perovskite SCs have been directly integrated into the window of SiPM by using the solution growth method at low temperature. Tunable X (γ)-ray excited light emission in the range of 414 to 600 nm is obtained with different concentrations of Br doping, which greatly matches the response wavelength of SiPM. Small Br-doped CH3NH3PbBr0.05Cl2.95 SCs exhibit high transmittance and weak self-absorption, resulting in improved scintillation light emissions. Moreover, we have successfully collected a 137Cs source gamma-ray pulse height spectrum with the SiPM readout. The MAPbBr0.05Cl2.95 scintillator exhibits a decay time of 0.14 ± 0.02 ns and a light yield of 18 000 photons/MeV with an energy resolution of 10.5 ± 0.4% at 662 keV. The results indicate that the CH3NH3PbBrxCl3-x perovskite SCs could enable the next generation of low-cost, fast, and fine-energy resolution scintillators.

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.