S. Salvatori

Photoelectrical characteristics of diamond UV detectors: Dependence on device design and film quality

Abstract The application of CVD diamond films in UV photodetectors is analysed according to device configuration. In particular photoconductive devices with sandwich and coplanar contacts and photodiodes with single and double Schottky barriers are considered. Ideal dark and photo-electrical characteristics are compared with experimental properties and the role of defects and impurities in collection efficiency, UV-visible discrimination and time response is discussed.

Optimized spectral collection efficiency obtained in diamond-based ultraviolet detectors using a three-electrode structure

A diamond detector structure for uniform and efficient collection of photogenerated carriers in the ultraviolet spectral range is presented. The device operating principle is based on the contemporary collection of both photogenerated carriers in the bulk, and photoelectrons emitted from the diamond surface. A three-electrode device structure is used, having dual side contacts on the diamond surfaces, and a separated third electrode in the vacuum. A large improvement of the detector responsivity, as well as a compensation of the efficiency losses usually occurring for highly absorbed radiation is achieved, while keeping comparable wavelength selectivity.

Secondary electron emission from diamond: Physical modeling and application to scanning electron microscopy

Secondary electron emission from diamond films is studied as a function of the primary electron beam energy and bulk materialproperties. A formulation of a simple model of the secondary electron emission coefficient, as a function of the primary electron beam energy, has been found to be helpful in defining physical criteria able to guide the optimization of the diamond film electron emission performance. The secondary electron mean escape depth deduced from the model is indeed related to the density of defects in the material and represents the main factor in determining the low energy secondary electron yield. These results are supported by Raman spectroscopy measurements, indicating a lower graphitic content and a higher crystalline quality of the diamond phase in films showing better secondary electron and photoemission yields. We demonstrate that a diamond film, acting as a stable and proportional electron multiplier, can be used as a converter of backscattered electrons into secondary electrons in scanning electron microscopy. It will be shown that the use of a diamond film converter is suitable to improve the signal to noise ratio of images providing an enhanced compositional contrast.

Diamond Detectors for UV and X-Ray Source Imaging

This work reports on the realization and test of a compact beam-profiling system for UV and X-ray sources, based on polycrystalline CVD diamond detectors. Multistrip and pixel structures have been used for 1-D and 2-D photodetectors, respectively. A dedicated read-out electronic circuitry has been designed and used to independently sample the signal produced by each strip (or pixel), enabling a real-time beam profile reconstruction.

Diamond photoluminescence spectra: Dependence on excitation energy and microstructure

Abstract Photoluminescence data are presented for diamond films grown by different techniques (hot-filament CVD, microwave PECVD, DC arc-jet) and excited in the range 450–650 nm by different laser sources. In large-grain textured samples, sharp vibronic, structures, related to nitrogen and silicon impurity centers, are clearly observed, whereas in small-grain, randomly oriented films spectra are dominated by broad luminescence bands, whose maxima move with excitation energy. In the former case a careful deconvolution of emission spectra allows us to obtain vibronic parameters of the centers and their fine structure details, while excitation spectra give the energy location of higher electronic excited states. On the other hand, the line-shape and the excitation-dependent shift of broad luminescence bands are related to a continuous distribution of gap states, able to trap photogenerated electron-hole pairs.

Performance of diamond-based photoconductive devices in the UV range

Abstract A detailed investigation of diamond-based UV planar photoconductors with interdigitated contacts is presented, analyzing in particular the influence of film morphology and texturing on device performance. Photoluminescence and photocurrent spectroscopies reveal a strong correlation between structural defects and detector selectivity, whereas impurities affect photocurrent yield mainly in highly oriented materials. Responsivity measurements show that diamond-based devices give the highest UV-visible discrimination in comparison to UV-enhanced Si- and GaN-based sensors. For λ ≤225 nm, an almost linear photocurrent-light intensity relationship is observed, although, in a few cases, a superlinear dependence on illumination intensity is found. A theoretical model, capable of describing the observed field dependendence of photocurrent gain is presented, giving minority carriers lifetime and mobility in the range 0.1–1 ns and 10–300 cm2V−1s−1, respectively.

Optimised contact-structures for metal–diamond–metal UV-detectors

Abstract UV detection performances of planar metal–diamond–metal devices have been analysed as a function of the distance between the two metal contacts. In particular, underlying the importance of the penetration depth of electric field lines, it is shown that good characteristics are obtained by means of a contact distance comparable to the absorption length of UV photons to be detected. In addition, the importance of chemical treatment in a sulphochromic mixture, able to reduce leakage current in the planar contact configuration, is also discussed.

Laser-induced nanocrystalline silicon formation in a-SiO matrices

Abstract Nanocrystalline silicon formation by CW laser-irradiation of amorphous silicon–oxygen alloys of variable composition has been investigated as a function of both laser power density and alloy composition. Structural changes during the annealing treatment were monitored ‘in situ’ by micro Raman spectroscopy. Careful Raman lineshape analysis in the 450–550 cm −1 range allowed the evaluation of Si nanocrystal size distribution. It is shown that depending on the size of the nanocrystallites, irradiation can yield to a strong photoluminescence (PL) enhancement or quenching.

Solar-blind UV-photodetector based on polycrystalline diamond films: basic design principle and comparison with experimental results

Abstract A detailed investigation of metal-semiconductor-metal UV photodetectors based on polycrystalline diamond film is presented, The roles of material quality, device configuration (planar or sandwich) and contact properties (blocking or ohmic) in determining both darkand photo-electrical behaviours are particularly discussed.

Effect of nanostructure and back contact material on the field emission properties of carbon films

The emission properties of carbon films grown by pulsed laser ablation are investigated in relation to their nanostructure, which changes from amorphous to nanostructured carbon, according to the substrate temperature. In addition to an increasing number and size of six-member carbon rings, Raman scattering measurements reveal light polarisation sensitivity, reflecting a temperature-induced orientation of graphene domains. Such characteristics largely affect the electron emission properties, resulting in a close relation among threshold field values, graphene domain size and probably their average orientation. These results are interpreted suggesting that hot electron generation and transport through graphene domains is one of the main mechanisms enhancing the electron emission probability. A lowering of the threshold field strength is also observed when carbon films are deposited on titanium substrate in respect to the silicon one.