Ultraviolet (UV) photodetectors have drawn extensive attention owing to their applications in industrial, environmental and even biological fields. Compared to UV-enhanced Si photodetectors, a new generation of wide bandgap semiconductors, such as (Al, In) GaN, diamond, and SiC, have the advantages of high responsivity, high thermal stability, robust radiation hardness and high response speed. On the other hand, one-dimensional (1D) nanostructure semiconductors with a wide bandgap, such as β-Ga2O3, GaN, ZnO, or other metal-oxide nanostructures, also show their potential for high-efficiency UV photodetection. In some cases such as flame detection, high-temperature thermally stable detectors with high performance are required. This article provides a comprehensive review on the state-of-the-art research activities in the UV photodetection field, including not only semiconductor thin films, but also 1D nanostructured materials, which are attracting more and more attention in the detection field. A special focus is given on the thermal stability of the developed devices, which is one of the key characteristics for the real applications.

A Comprehensive Review of Semiconductor Ultraviolet Photodetectors: From Thin Film to One-Dimensional Nanostructures

β-Ga2O3 epitaxial thin films were deposited using laser molecular beam epitaxy technique and oxygen atmosphere in situ annealed in order to reduce the oxygen vacancy. Metal/semiconductor/metal structured photodetectors were fabricated using as-grown film and annealed film separately. Au/Ti electrodes were Ohmic contact with the as-grown films and Schottky contact with the annealed films. In compare with the Ohmic-type photodetector, the Schottky-type photodetector takes on lower dark current, higher photoresponse, and shorter switching time, which benefit from Schottky barrier controlling electron transport and the quantity of photogenerated carriers trapped by oxygen vacancy significant decreasing.

Oxygen vacancy tuned Ohmic-Schottky conversion for enhanced performance in β-Ga2O3 solar-blind ultraviolet photodetectors

Low-dimensional (LD) nanostructures are ideal systems for constructing high-performance photodetectors due to their tailored geometries, high surface-area-to-volume ratios and rationally designed surfaces. This article provides a brief summary about recent progress on LD nanostructures based visible-light-blind ultraviolet photodetectors. The current challenges and an outlook on the future developments of this research field are summarized and highlighted.

Low-dimensional nanostructure ultraviolet photodetectors.

Recent advances in ultraviolet photodetectors

Ultrahigh external quantum efficiency from thin SnO2 nanowire ultraviolet photodetectors.

SnO 2 nanowires can be synthesized on alumina substrates and formed into an ultraviolet (UV) photodetector. The photoelectric current of the SnO 2 nanowires exhibited a rapid photo-response as a UV lamp was switched on and off. The ratio of UV-exposed current to dark current has been investigated. The SnO 2 nanowires were synthesized by a vapor–liquid–solid process at a temperature of 900 °C. It was found that the nanowires were around 70–100 nm in diameter and several hundred microns in length. High-resolution transmission electron microscopy (HRTEM) image indicated that the nanowires grew along the [200] axis as a single crystallinity. Cathodoluminescence (CL), thin-film X-ray diffractometry, and X-ray photoelectron spectroscopy (XPS) were used to characterize the as-synthesized nanowires.

Ultraviolet photodetectors made from SnO2 nanowires

Single-crystal indium antimony (InSb) nanowire was fabricated into middle-infrared photodetectors based on a metal–semiconductor-metal (M-S-M) structure. The InSb nanowires were synthesized using an electrochemical method at room temperature. The characteristics of the FET reveal an electron concentration of 3.6 × 1017 cm−3 and an electron mobility of 215.25 cm2 V−1 s−1. The photodetectors exhibit good photoconductive performance, excellent stability, reproducibility, superior responsivity (8.4 × 104 A W−1), and quantum efficiency (1.96 × 106%). These superior properties are attributed to the high surface-to-volume ratio and single-crystal 1D nanostructure of photodetectors that significantly reduce the scattering, trapping, and the transit time between the electrodes during the transport process. Furthermore, the M-S-M structure can effectively enhance space charge effect by the formation of the Schottky contacts, which significantly assists with the electron injection and photocurrent gain.

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High sensitivity of middle-wavelength infrared photodetectors based on an individual InSb nanowire.

Vertically aligned single-crystal InSb nanowires were synthesized via the electrochemical method at room temperature. The characteristics of Fourier transform infrared spectrum revealed that in the syntheses of InSb nanowires, energy bandgap shifts towards the short wavelength with the occurrence of an electron accumulation layer. The current–voltage curve, based on the metal–semiconductor–metal model, showed a high electron carrier concentration of 2.0 × 1017 cm−3 and a high electron mobility of 446.42 cm2 V−1 s−1. Additionally, the high carrier concentration of the InSb semiconductor with the surface accumulation layer induced a downward band bending effect that reduces the electron tunneling barrier. Consequently, the InSb nanowires exhibit significant field emission properties with an extremely low turn-on field of 1.84 V μm−1 and an estimative threshold field of 3.36 V μm−1.

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Room temperature-synthesized vertically aligned InSb nanowires: electrical transport and field emission characteristics.

Selective area growth of single crystalline Sn-doped In2O3 (ITO) nanowires synthesized via vapor–liquid–solid (VLS) method at 600°C was applied to improve the field emission behavior owing to the reduction of screen effect. The enhanced field emission performance reveals the reduction of turn-on fields from 9.3 to 6.6 V μm−1 with increase of field enhancement factors (β) from 1,621 to 1,857 after the selective area growth at 3 h. Moreover, we find that the screen effect also highly depends on the length of nanowires on the field emission performance. Consequently, the turn-on fields increase from 6.6 to 13.6 V μm−1 with decreasing β values from 1,857 to 699 after the 10-h growth. The detailed screen effect in terms of electrical potential and NW density are investigated in details. The findings provide an effective way of improving the field emission properties for nanodevice application.

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Sn-doped In2O3 nanowires: enhancement of electrical field emission by a selective area growth.

Single crystalline Sn doped In(2)O(3) (ITO) NWs (nanowires) were synthesized via an Au-catalyzed VLS (vapor-liquid-solid) method at 600 °C. The different sizes (~20, ~40, ~80 nm) of the Au NPs (nanoparticles) provided the controllable diameters for ITO NWs during growth. Phase and microstructures confirmed by high-resolution transmission electron microscope images (HRTEM) and X-ray diffraction (XRD) spectra indicated that the phase of In(2)O(3) NWs had a growth direction of [100]. X-ray photoelectron spectroscopy (XPS) was employed to obtain the chemical compositions of the ITO NWs as well as the ratio of Sn/In and oxygen concentrations. The findings indicated that low resistivity was found for ITO NWs with smaller diameters due to higher concentrations of oxygen vacancies and less incorporation of Sn atoms inside the NWs. The resistivity of NWs increases with increasing diameter due to more Sn atoms being incorporated into the NW and their reduction of the amount of oxygen vacancies. Low resistivity NWs could be achieved again due to excess Sn atoms doped into the large diameter NWs. Therefore, by optimizing the well-controlled growth of the NW diameter and interface states, we are able to tune the electrical properties of Sn-doped ITO NWs.

Cheng-Hsiang Kuo

Papers

Ultraviolet photodetectors made from SnO2 nanowires

High sensitivity of middle-wavelength infrared photodetectors based on an individual InSb nanowire.

Room temperature-synthesized vertically aligned InSb nanowires: electrical transport and field emission characteristics.

Sn-doped In2O3 nanowires: enhancement of electrical field emission by a selective area growth.

Synthesis of single crystal Sn-doped In2O3 nanowires: size-dependent conductive characteristics