A directionally solidified cobalt-base alloy, DZ40M, was solidified with a columnar grained austenitic matrix with a ͗001͘ preferential orientation and primary carbides of chromium-rich M 7 C 3 and MC at grain boundaries and interdendritically. Secondary carbides in DZ40M alloy are chromium-rich M 23 C 6 and tungsten-rich M 6 C. The M 23 C 6 carbide has a cube-cube orientation relationship with the austenitic matrix. Initial precipitation of secondary carbide, M 23 C 6 , occurred on dislocations in the austenitic matrix of the as-cast DZ40M alloy during cooling. Aging treatment (100 to 1000 hours at 850 ЊC) produced a profusive precipitation of the M 23 C 6 carbide mainly around the primary carbides. In the interior of grains, M 23 C 6 precipitated preferentially on dislocations and stacking faults. Subsequently, M 23 C 6 grew into laths near the primary carbides and coalesced into chains. The precipitation behavior of M 23 C 6 can be explained by the following reaction: 23M ϩ 6C → M 23 C 6 . The primary carbides are a carbon reservoir for the precipitation of M 23 C 6 . The M 6 C carbide was found only on the surface of the primary M 7 C 3 carbide adjacent to tungsten-rich MC in the aged condition. The precipitation of the tungsten-rich M 6 C is attributed to the tungsten segregation, which resulted from decomposition of the tungsten-rich MC and good lattice match between M 6 C and M 7 C 3 . The inhomogeneity of secondary precipitation is due to the uneven distribution of alloying elements.

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Large-area metal -semiconductor -metal (MSM) solar-blind photodetectors with a device area of 5 × 5 mm 2 have been fabricated on Al 0.4 Ga 0.6 N/AlN/sapphire epistructure. The photodetector exhibits ultra-low dark current density of 3.2 × 10 212 A/cm 2 under 20 V bias and a corresponding breakdown voltage of up to 385 V. The solar-blind/ultraviolet rejection ratio of the photodetector is more than four orders of magnitude with a maximum quantum efficiency of 28% at 275 nm.Introduction: Wide bandgap AlGaN-based materials are especially suitable for detecting ultraviolet (UV) radiation owing to their tunable bandgap (3.4 -6.2 eV), good thermal conductivity, excellent radiation hardness and high breakdown electric field. Solar-blind AlGaN-based UV photodetectors (PDs) with cutoff wavelength smaller than 280 nm have important potential applications, including missile plume sensing, chemical/biological agent detection, solar astronomy and covert space-to-space communications [1]. For many of these applications, capability of detecting very weak UV signals is often required. Thus, the PDs are desired to have both large detector area and low dark current, which are nevertheless hard to be realised simultaneously. In past studies, small-area AlGaN-based solar-blind PDs with low dark current density and high signal-to-noise ratio have been reported [2][3][4][5]. Although there are limited reports on AlGaN-based solar-blind Schottky PDs with active area of more than 1 mm 2 , those devices suffer from high leakage current owing to material quality or uniformity-related issues [6]. In this work, by growing the AlGaN active layer on a high-temperature (HT) AlN buffer layer on sapphire, we demonstrate a high-performance AlGaN-based metal -semiconductor -metal (MSM) PD with large device area of up to 25 mm 2 , which exhibits record low dark current density and high solar-blind/ultraviolet rejection ratio.

In this work, a well-operated broadband UV-visible photodetector based on a Ga 2 O 3 /BiFeO 3 (Ga 2 O 3 /BFO) heterojunction is fabricated and characterized. Under the ultraviolet (UV) light illumination with an intensity of 100 μW•cm −2 at 254 nm in UVC waveband, at a biasing voltage of 2 V, an ultra-low dark current (I dark ) of 0.12 pA, a high photo-to-dark current ratio (PDCR) of 1.0×10 5 , a responsivity (R) of 12.0 mA•W −1 , a specific detectivity (D * ) of 6.1×10 12 Jones, an external quantum efficiency (EQE) of 5.9%, and a UVC/visible rejection ratio (R 235 /R 600 ) of 4.47×10 3 are achieved. In addition, the rise time and decay time are 0.25 s and 0.04 s, respectively. Meanwhile, the potential of the device as a self-powered photodetector is proved, and the principle of energy-band diagram is analysed for the fabricated heterojunction device. The results show that the Ga 2 O 3 /BFO heterojunction is a potential candidate for preforming desired broadband UV-visible photodetection.

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