Industries such as the automotive, aerospace or military, as well as environmental and biological research have promoted the development of ultraviolet (UV) photodetectors capable of operating at high temperatures and in hostile environments. UV-enhanced Si photodiodes are hence giving way to a new generation of UV detectors fabricated from wide-bandgap semiconductors, such as SiC, diamond, III-nitrides, ZnS, ZnO, or ZnSe. This paper provides a general review of latest progresses in wide-bandgap semiconductor photodetectors.

https://iopscience.iop.org/article/10.1088/0268-1242/18/4/201/pdf

Wide-bandgap semiconductor ultraviolet photodetectors

A “visible-blind” solution-processed UV photodetector is realized on the basis of colloidal ZnO nanoparticles. The devices exhibit low dark currents with a resistance >1 TΩ and high UV photocurrent efficiencies with a responsivity of 61 A/W at an average intensity of 1.06 mW/cm2 illumination at 370 nm. The characteristic times for the rise and fall of the photocurrent are <0.1 s and about 1 s, respectively. The photocurrent of the device is associated with a light-induced desorption of oxygen from the nanoparticle surfaces, thus removing electron traps and increasing the free carrier density which in turn reduces the Schottky barrier between contacts and ZnO nanoparticles for electron injection. The devices are promising for use in large-area UV photodetector applications.

Solution-processed ultraviolet photodetectors based on colloidal ZnO nanoparticles.

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

The photoconductive characteristics of CdS single nanoribbons were investigated. The device characteristics, including spectral response, light intensity response, and time response, were studied systematically. It is found that CdS nanoribbon has the response speed substantively faster than those ever reported for conventional film and bulk CdS materials and the size of nanoribbons has a significant influence on the response speed with smaller CdS nanoribbons showing higher response speed. The high photosensitivity and high photoresponse speed are attributable to the large surface-to-volume ratio and high single-crystal quality of CdS nanoribbons and the reduction of recombination barrier in nanostructures. Measurements in a different atmosphere demonstrate that the absorption of ambient gas (mainly oxygen) can significantly change the photosensitivity of CdS nanoribbons through trapping electrons from the nanoribbons.

Photoconductive characteristics of single-crystal CdS nanoribbons.

We report on the fabrication of n-ZnO/p-AlGaN heterojunction light-emitting diodes on 6H-SiC substrates. Hydride vapor phase epitaxy was used to grow p-type AlGaN, while chemical vapor deposition was used to produce the n-type ZnO layers. Diode-like, rectifying I-V characteristics, with threshold voltage ~3.2V and low reverse leakage current ~10(-7)A, are observed at room temperature. Intense ultraviolet emission with a peak wavelength near 389 mn is observed when the diode is forward biased; this emission is found to be stable at temperatures up to 500K and shown to originate from recombination within the ZnO.

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Fabrication and characterization of n-ZnO/p-AlGaN heterojunction light-emitting diodes on 6H-SiC substrates

Published data on the properties of metal-semiconductor ohmic contacts and mechanisms of current flow in these contacts (thermionic emission, field emission, thermal-field emission, and also current flow through metal shunts) are reviewed. Theoretical dependences of the resistance of an ohmic contact on temperature and the charge-carrier concentration in a semiconductor were compared with experimental data on ohmic contacts to II–VI semiconductors (ZnSe, ZnO), III–V semiconductors (GaN, AlN, InN, GaAs, GaP, InP), Group IV semiconductors (SiC, diamond), and alloys of these semiconductors. In ohmic contacts based on lightly doped semiconductors, the main mechanism of current flow is thermionic emission with the metal-semiconductor potential barrier height equal to 0.1–0.2 eV. In ohmic contacts based on heavily doped semiconductors, the current flow is effected owing to the field emission, while the metal-semiconductor potential barrier height is equal to 0.3–0.5 eV. In alloyed In contacts to GaP and GaN, a mechanism of current flow that is not characteristic of Schottky diodes (current flow through metal shunts formed by deposition of metal atoms onto dislocations or other imperfections in semiconductors) is observed.

Mechanisms of current flow in metal-semiconductor ohmic contacts

After a brief review of classification, application and sources of near-ultraviolet (UV) radiation the methods for fabricating UV photodetectors and characteristics of the photoconductive cells, p-n junction structure and Schottky barrier photodiodes are discussed. Characteristics of some light filters used in photodetectors and measuring devices are also reported. Now Si p-n structures are commonly used but Schottky diodes based on wide-gap (GaAsP, GaP, GaN, AlGaN, SiC) semiconductors are very attractive. They are insensitive to the infrared radiation and if necessary simple glass filters can be used for correcting the spectrum in such way that it covers just the near-UV region.

http://iopscience.iop.org/article/10.1088/0268-1242/14/7/201/pdf

Semiconductor near-ultraviolet photoelectronics

Recently, ultraviolet photoelectronics emerged in response to the needs of medicine, biology, military equipment, and the problem of the hole in the ozone layer. A specific feature of this field of photoelectronics is the need to detect weak (albeit appreciably affecting vital human functions) signals against a background of intense radiation in the visible and infrared regions of the spectrum. Ultraviolet electronics relies on Si-based p-n structures and GaP-based Schottky barriers, p-n structures and Schottky barriers based on GaN and AlGaN (“solar-blind,” i.e., solar-radiation-insensitive devices), SiC structures with potential barriers (high-temperature devices), and ZnO-and ZnS-based photoresistors and Schottky diodes. In this review, the parameters of starting wide-gap semiconductors are given, physical foundations for photoelectric conversion and the principles of formation of ohmic contacts are described, characteristics of corresponding devices are given, and the envisaged lines of further studies are outlined.

Semiconductor photoelectric converters for the ultraviolet region of the spectrum

Ultraviolet Schottky photodetectors based on n-4H–SiC (Nd − Na = 4 × 1015 cm−3) epitaxial layers of high purity have been fabricated. Their spectral sensitivity range is 3.2–5.3 eV peaking at 4.9 eV (quantum efficiency is about ~0.3 electron/photon), which is close to the bactericidal ultraviolet radiation spectrum. The temperature dependence of the quantum efficiency of 4H–SiC Schottky structure has been investigated to determine the temperature stability and the mechanism of the photoelectric conversion process. At low temperatures (78–175 K) the quantum efficiency increases with increasing temperature for all photon energy values and then tends to saturate. We suppose that some imperfections in the space-charge region act as traps that capture both photoelectrons and photoholes. After some time the trapped electron–hole pairs recombine due to the tunnelling effect. At high temperatures (more than 300 K), the second enhancement region of the quantum efficiency is observed in the photon energy range of 3.2–4.5 eV. It is connected with a phonon contribution to indirect optical transitions between the valence band and the M-point of the conduction band. When the photon energy is close to a direct optical transition threshold this enhancement region disappears. This threshold is estimated to be 4.9 eV. At photon energies more than 5 eV a drastic fall of the quantum efficiency has been observed throughout the temperature interval. We propose that in this case the photoelectrons and photoholes are bound to form hot excitons in the space-charge region due to the Brillouin zone singularity, and do not contribute to the following photoelectroconversion process.

http://iopscience.iop.org/article/10.1088/0268-1242/20/8/010/pdf

Temperature dependence of the photoelectric conversion quantum efficiency of 4H?SiC Schottky UV photodetectors

The temperature dependence of the electric resistance of the In-GaP ohmic contact has been studied in the range from 77 to 420 K. The resistance was measured in GaP plates of various thickness with two In ohmic contacts. The measured ohmic contact resistance increases with temperature in the interval from 230–420 K. It is suggested that the In-GaP ohmic contact is formed by metallic shunts appearing upon deposition of In atoms on dislocations and other imperfections present (with a density evaluated at (4.5–8)×107 cm−2) in the subsurface region of the semiconductor.

Yu. A. Goldberg

Papers

Mechanisms of current flow in metal-semiconductor ohmic contacts

Semiconductor near-ultraviolet photoelectronics

Semiconductor photoelectric converters for the ultraviolet region of the spectrum

Temperature dependence of the photoelectric conversion quantum efficiency of 4H?SiC Schottky UV photodetectors

Peculiarities in the mechanism of current flow through an ohmic contact to gallium phosphide