Ultraviolet (UV) detectors have attracted considerable attention in the past decade due to their extensive applications in the civil and military fields. Wide bandgap semiconductor-based UV detectors can detect UV light effectively, and nanowire structures can greatly improve the sensitivity of sensors with many quantum effects. This review summarizes recent developments in the classification and principles of UV detectors, i.e., photoconductive type, Schottky barrier type, metal-semiconductor-metal (MSM) type, p-n junction type and p-i-n junction type. The current state of the art in wide bandgap semiconductor materials suitable for producing nanowires for use in UV detectors, i.e., metallic oxide, III-nitride and SiC, during the last five years is also summarized. Finally, novel types of UV detectors such as hybrid nanostructure detectors, self-powered detectors and flexible detectors are introduced.

Ultraviolet Detectors Based on Wide Bandgap Semiconductor Nanowire: A Review.

Homogenous crystallization in solution, in the absence of external influences, is expected to lead to growth that is symmetric at least in two opposite facets. Such was not the case when we attempted to synthesize ZnO nanostructures by employing a solvothermal technique. The reaction product, instead, consisted of bullet-shaped tiny single crystals with an abrupt hexagonal base and a sharp tip. A careful analysis of the product and the intermediate states of the synthesis reveals that one of the reaction intermediates with sheet-like morphology acts as a self-sacrificing template and induces such unexpected and novel growth. The synthesis was further extended to dope the nanobullets with nitrogen as previous studies showed this can induce p-type behavior in ZnO, which is technologically complementary to the naturally occurring n-type ZnO. Herein, a soft-chemical approach is used for the first time for this purpose, which is otherwise accomplished with high-temperature techniques. Cathodoluminesce (CL) investigations reveal stable optical behavior within a pure nanobullet. On the other hand, the CL spectra derived from the surfaces and the cores of the doped samples are different, pointing at a N-rich core. Finally, even though N-doped ZnO is known to have high electrical conductivity, the study now demonstrates that the field-emission properties of ZnO can also be greatly enhanced by means of N doping.

Solvothermal synthesis, cathodoluminescence, and field-emission properties of pure and N-doped ZnO nanobullets

We build sensors, capable of detecting and measuring ultraviolet (UV) light, by depositing zinc oxide (ZnO) powder from a solvent suspension over common white paper. Although these sensors are easy to fabricate and require inexpensive materials, they feature characteristics similar to those of UV sensors made with complex and expensive procedures. The good performance in terms of conductivity change of our simple devices can be attributed to the conductivity and porosity properties of paper, which effectively binds the ZnO crystals. We perform analyses using quantum chemistry methods to describe possible mechanisms that explain the conductivity changes observed on the ZnO surface due to doping interactions with interstitial hydrogen and doping depletion caused by oxygen adsorption.

ZnO-Paper Based Photoconductive UV Sensor

An inductor is fabricated on a substrate having a top surface and a bottom surface. The inductor includes a plurality of holes extending through the substrate, wherein the plurality of holes interconnect the top surface and the bottom surface of the substrate. The inductor also includes a plurality of conductive posts formed in the plurality of holes and a plurality of conductive segments formed on the top surface and on the bottom surface that interconnect the conductive posts such that a continuous conductive coil is formed. The inductor also includes a magnetic core that occupies substantially the entire volume enclosed by the conductive coil.

Integrated circuit inductor with a magnetic core

An ultraviolet photosensor has been successfully constructed by the in situ lift-out method in a focused ion beam system. The prototype consists of individual naturally self-assembled crossed ZnO nanorods grown by an aqueous solution process. The current–voltage (I–V) characteristics show linear behavior. The photosensor exhibits a response of ∼15 mA/W for UV light (361 nm) under 1 V bias. Response measurements showed that such a photosensor is suitable for low levels of ultraviolet detection. The method is simple, rapid and applicable to research prototypes for further studies of crossed ZnO nanorods for nano-device applications.

https://physics.ucf.edu/~lc/X_ZnO_Sensor_09.pdf

Crossed zinc oxide nanorods for ultraviolet radiation detection

Photoconductivity of ZnO single crystals grown by a hydrothermal method for ultraviolet light was measured. The ZnO single crystals have photo response in a range of wavelengths shorter than 380 nm. O-faces of ZnO single crystals of the c -plane showed higher photocurrents and wider spectral response than did Zn-faces. It is thought that band bending at the surface regions of ZnO crystals is large at the O-face but small at the Zn-face, because the surface state density for electron capture at the O-face was higher than that of the Zn-face and, therefore, carriers generated at the surface region by UV light were effectively separated at the O-face region before their recombination. The difference between photoresponces of the O-face and the Zn-face is also useful for determining the polarity of a ZnO crystal. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Applicability of ZnO single crystals for ultraviolet sensors

PROBLEM TO BE SOLVED: To provide an apparatus for counting the number of microorganisms, capable of accurately counting the number of the microorganisms. SOLUTION: The structure of the apparatus for counting the number of the microorganisms has a container 10 capable of storing a liquid containing the microorganisms and having a pair of flat electrodes 11a and 11b facing to each other face in the interior, an electric source circuit 12 for applying a voltage between the electrodes 11a and 11b, a circuit 14 for measuring the quantity of electricity, capable of measuring the quantity of the electricity between the electrodes 11a and 11b, and a calculating circuit 15 for calculating the microorganisms based on the quantity of the electricity measured by the circuit 12 for measuring the quantity of the electricity. As a result, the quantity of the electricity between the electrodes 11a and 11b can be detected, and the microorganisms can be counted by calculating the result. The facing area of the electrodes is enlarged and the microorganisms can be accurately counted because the electrodes 11a and 11b are obtained by using the flat electrodes and arranging the flat electrodes so as to face to each other face. COPYRIGHT: (C)2003,JPO

Apparatus for counting number of microorganisms

Photoluminescence properties of nitrogen-doped ZnO films deposited on ZnO single crystal substrates by the plasma-assisted reactive evaporation method

Non-contact magnetic temperature sensor for biochemical applications

A polymer magnetic composite (PMC) is a flexible thick film and is composed of a ferrite powder with variable possible Curie temperatures, polymer and solvent. It is found that high and low Curie temperature PMCs can be used for pressure and photosensors, respectively. The set up combined with the PMC and a pressure element acts as a pressure sensor. The device, painted with a radiant flux absorbent on the surface of the PMC, responds to the visible rays. The pressure and light signals are read out by a Hall element in noncontact with the PMC. >

Shigeki Chiba

Papers

Applicability of ZnO single crystals for ultraviolet sensors

Apparatus for counting number of microorganisms

Photoluminescence properties of nitrogen-doped ZnO films deposited on ZnO single crystal substrates by the plasma-assisted reactive evaporation method

Non-contact magnetic temperature sensor for biochemical applications

Pressure/photo sensor utilizing polymer magnetic composite