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4H-silicon carbide ͑SiC͒ metal-semiconductor-metal ͑MSM͒ ultraviolet ͑UV͒ photodetectors with Al 2 O 3 / SiO 2 ͑A/S͒ films employed as antireflection/passivation layers have been demonstrated. The devices showed a peak responsivity of 0.12 A / W at 290 nm and maximum external quantum efficiency of 50% at 280 nm under 20 V electrical bias, which were much larger than conventional MSM detectors. The redshift of peak responsivity and response restriction effect were found and analyzed. The A/S/4H-SiC MSM photodetectors were also shown to possess outstanding features including high UV to visible rejection ratio, large photocurrent, etc. These results demonstrate A/S/4H-SiC photodetectors as a promising candidate for OEIC applications. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2949318͔Silicon carbide ͑SiC͒ is a promising material for both optoelectronic and power devices due to its properties such as wide band gap, high break down electric field, and high thermal stability. SiC based photodetectors [1][2][3][4][5][6] have been fabricated for ultraviolet ͑UV͒ detection applications in chemical sensing, ozone-hole sensing, flame detection, etc. Planar metal-semiconductor-metal ͑MSM͒ photodetectors were considered to be more suitable for optoelectronic integrated circuits ͑OEIC͒ than p-i-n and avalanche detectors.7 Therefore, it is important to fabricate high-performance 4H-SiC based MSM photodetectors for OEIC application.However, 4H-SiC MSM photodetectors suffered from low responsivity and quantum efficiency 6 ͑QE͒ due to the optical losses including reflection and effects of surface states. Therefore, Al 2 O 3 / SiO 2 ͑A/S͒ films were selected as antireflection coatings and passivation layers on the detectors in order to reduce these losses.8 Al 2 O 3 and SiO 2 are the most important oxide film materials with the least absorption in the UV spectral region.9 Furthermore, the refractive index of Al 2 O 3 ͑n 1 ͒ films is most suitable for 4H-SiC substrates 10 in the UV region. The SiO 2 film was prepared to enhance the wavelength selectivity and adhesion between the Al 2 O 3 film and 4H-SiC substrate as well as to passivate the interface. Thus the A/S films were considered to be the most promising antireflection/passivation coatings to improve the performance of 4H-SiC MSM photodetectors. In this letter, we demonstrated A/S/4H-SiC MSM photodetectors with much better responsivity, higher QE, and larger photocurrent than conventional 4H-SiC MSM devices. The MSM photodetectors with A/S films and thermally grown layers were fabricated, respectively, for comparison on two identical n-type 4H-SiC wafers with a 3.4 m n − epilayer ͑N D = 3.1ϫ 10 15 cm −3 ͒ grown on a n + substrate ͑N D =10 20 cm −3 ͒. The orientations of the substrate and epilayer were both 8°off the ͑0001͒ Si face. The wafer for SiO 2 / 4H -SiC ͑S/4H-SiC͒ was oxidized at 1150°C for 1 h, and then etched by hydrofluoric acid in order to form a fresh surface before oxidation. Immediately, the wafer was oxidized at 1150°C for 4 h. The S/4H-SiC structur...

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Doping Ti into WO x is widely used to improve the performance of various WO x -based devices. However, Ti-doped WO x (Ti:WO x ) based memristors have not been investigated in depth, especially regarding their synaptic properties. In this report, Ti:WO x films are deposited by sputtering WTi alloy target in Ar + O2 atmosphere, in which the substrates are not heated, avoiding the precipitation of TiO2 phase. As-grown Ti:WO x films exhibit polymorphous crystallized structures, originating from the topological deformation of WO6 octahedron by replacing W with Ti. Furthermore, the memristor with the structure of W/Ti:WO x / indium-doped tin oxide demonstrates typical synaptic properties, including the controllable synapse weight update by adjusting the input pulse amplitude, interval, as well as the transformation from the paired-pulse facilitation to the paired-pulse depression. Moreover, the higher Ti–O band energy leads to smaller hysteresis loop areas of its current–voltage (I–V) curves, which may mean better synaptic performances, such as higher resolution. Additionally, contrary to other metal-oxide based memristors, our device conductance increases by applying positive voltage scans or a series of positive pulses on the metal electrode. Otherwise, it will decrease. With I–V fitting and the energy-band diagrams, we analyzed deep carrier transport processes, revealing a unique synaptic mechanism.

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Natural Science Foundation of Fujian Province [2009J05151]The titanium dioxide (TiO2) films prepared by sol-gel processing were used to fabricate metal-semiconductor-metal ultraviolet photodetectors. A very low dark current of 5.38 pA (current density of 3.84 nA/cm(2)) at 5 V bias is obtained, which is ascribed to the high effective Schottky barrier between Au and TiO2 films. The x-ray photoelectron spectroscopy analysis demonstrates that the concentration of oxygen vacancies is very low in the surface of the TiO2 films, which is responsible for the high effective Schottky barrier. The devices exhibit a cutoff wavelength at about 380 nm and a large UV-to-visible rejection ratio (340 versus 400 nm) of three orders of magnitude. The peak responsivity of the devices is 17.5 A/W at 5 V bias, indicating the presence of internal photoconductive gain induced by desorption of oxygen on the TiO2 surface. (C) 2011 American Institute of Physics. [doi:10.1063/1.3537918

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