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. Contents 1. Introduction 33 2. Photodetector parameters 34 3. Semiconductors for UV photodetection 34 4. SiC photodetectors 36 4.1. Schottky photodiodes 36 4.2. The p-n junction 37 4.3. Charge-coupled devices 37 4.4. Particle detectors 38 5. Diamond photodetectors 38 5.1. Photoconductors 38 5.2. Schottky photodiodes 39 5.3. Metal-semiconductor-metal photodiodes 39 5.4. Phototransistors 39 5.5. X-ray and particle detectors 40 5.6. Photocathodes 40 6. III-nitride photodetectors 40 6.1. Photoconductors 41 6.2. Schottky photodiodes 42 6.3. Metal-semiconductor-metal photodiodes 44 6.4. The p-i-n photodiodes 45 6.5. Avalanche photodiodes 46 6.6. Phototransistors 46 6.7. Photocathodes 46 7. II-VI semiconductors 47 7.1. ZnS, ZnSe and related compounds 47 7.2. ZnO 48 8. Conclusion and perspectives 48

Wurtzite GaN nanocolumns are reproducibly grown by plasma-assisted molecular beam epitaxy on Si͑111͒ and c-sapphire substrates. The nanocolumns density and diameter ͑600-1500 Å͒ are effectively controlled by means of the III/V ratio. The nanocolumns are fully relaxed from lattice and thermal strain, having a very good crystal quality characterized by strong and narrow ͑2 meV͒ low-temperature photoluminescence excitonic lines at 3.472-3.478 eV. In addition, the spectra reveal a doublet at 3.452-3.458 eV and a broad line centered at 3.41 eV. This broad emission shows a sample-dependent spectral energy dispersion, from 3.40 to 3.42 eV, explained as due to the effect of strain and/or electric fields associated with extended structural defects located at the nanocolumns bottom interface. From cathodoluminescence data, it is concluded that the doublet emission lines originate at the nanocolumns volume, most probably related to Ga I defects, given the column growth mode ͑Ga balling͒.

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The PTOLEMY project aims to develop a scalable design for a Cosmic Neutrino Background (CNB) detector, the first of its kind and the only one conceived that can look directly at the image of the Universe encoded in neutrino background produced in the first second after the Big Bang. The scope of the work for the next three years is to complete the conceptual design of this detector and to validate with direct measurements that the nonneutrino backgrounds are below the expected cosmological signal. In this paper we discuss in details the theoretical aspects of the experiment and its physics goals. In particular, we mainly address three issues. First we discuss the sensitivity of PTOLEMY to the standard neutrino mass scale. We then study the perspectives of the experiment to detect the CNB via neutrino capture on tritium as a function of the neutrino mass scale and the energy resolution of the apparatus. Finally, we consider an extra sterile neutrino with mass in the eV range, coupled to the active states via oscillations, which has been advocated in view of neutrino oscillation anomalies. This extra state would contribute to the tritium decay spectrum, and its properties, mass and mixing angle, could be studied by analyzing the features in the beta decay electron spectrum.

III nitrides have become the most exciting challenge in optoelectronic materials in the last decade. Their intrinsic properties and an intense technological effort have made possible the fabrication of reliable and versatile detectors for short wavelengths. In this work, materials and devices issues are considered to provide a full picture of the advances in nitride UV photodetection. First, basic structures like photoconductors, Schottky, p-i-n and metal-semiconductor-metal photodiodes and phototransistors are compared, with emphasis on their specific properties and performance limitations. The efforts in the design and fabrication of more advanced detectors, in the search for higher quantum efficiency, contrast, signal-to-noise or speed operation, are reviewed afterwards. Metal-insulator-semiconductor diodes, avalanche photodetectors and GaN array detectors for UV imaging are also described. Further device optimization is linked with present materials issues, mainly due to the nitride quality, which is a direct result of the substrate used. The influence of substrates and dislocations on detector behaviour is discussed in detail. As an example of AlGaN photodetector applications, monitoring of the solar UV-B radiation to prevent erythema and skin cancer is presented.