The amorphous/crystalline silicon technology has demonstrated its potentiality leading to high efficiency solar cells. We propose the use of surface photovoltage technique as a contact-less tool for the evaluation of the energetic distribution of the state density at amorphous/crystalline silicon interface. We investigate the effect hydrogen plasma treatments performed on thin amorphous silicon buffer layer deposited over crystalline silicon surface and we compare its effect with that of thermal annealing on the interface. The surface photovoltage technique results to be very sensitive to the different experimental treatments, and therefore it can be considered a precious tool to monitor and improve the interface electronic quality.
In this paper, we present the first integration of an amorphous silicon balanced photosensor with a microfluidic network to perform on-chip detection for biomedical applications, where rejection of large background light intensity is needed. This solution allows to achieve high resolution readout without the need of high dynamic range electronics. The balanced photodiode is constituted by two series-connected a-Si:H/a-SiC:H n-i-p stacked junctions, deposited on a glass substrate. The structure is a three terminal device where two electrodes bias the two diodes in reverse conditions while the third electrode (i.e. the connection point of the two diodes) provides the output signal given by the differential current. The microfluidic network is composed of two channels made in PolyDimetilSiloxane (PDMS) positioned over the glass substrate on the photodiode-side aligning each channel with a diode. This configuration guarantees an optimal optical coupling between luminescence events occurring in the channels and the photosensors. The experiments have been carried out measuring the differential current in identical and different conditions for the two channels. We have found that: the measurement dynamic range can be increased by at least an order of magnitude with respect to conventional photodiodes; the balanced photodiode is able to detect the presence or absence of water in the channel; the presence of fluorescent molecules in the channel can be successful detected by our device without any need of optical filter for the excitation light. These preliminary results demonstrate the successful integration of a microfluidic network with a-Si:H photosensor for on-chip detection in biomedical applications.
A detailed characterization of the performances of amorphous silicon photodiodes in the detection of chemiluminescent signal is carried out. Comparison with commercial CCD acquisition system has been done as benchmark. The underlying idea is the development of stand-alone and compact micro-total-analysys-systems (μ-TAS) that do not need bulky and expensive equipment for their operation as external focusing optics and excitation sources. The photosensor is p-i-n structures deposited by Plasma Enhanced Chemical Vapour Deposition on a glass substrate covered with a transparent conductive oxide that acts as bottom electrode and window layer for the light impinging through the glass. A PDMS layer with wells has been fabricated using an aluminum mold and bonded on the glass substrate with a well aligned with a photosensor. The experiments have been performed by filling a well with solutions containing different quantities of horseradish peroxidase. A good linearity of the photosensor response is observed across the entire measurement range that spans over three orders of magnitude. The system detection limit is 70 fg/μL. A very good agreement between results achieved with conventional off-chip CCD detection and the on-chip photodiode has been observed. Experiments with target molecules immobilized on a functionalized glass surface have been also performed in microfluidic regime, confirming the validity of the proposed integrated approach based on a-Si:H technology.
In this work we present a system for the detection of labeled DNA by means of a two-color amorphous silicon photosensor. The device is a p-i-n-i-p structure, whose spectral response is controlled by tuning the voltage applied to its electrodes. The thicknesses of the different layers has been optimized to match the emission spectra of the two utilized fluorochromes. Minima detectable concentrations range in the order of few nmol/l. Very good linearity in the photosensor responses, comparable with those of commercial equipment, has been achieved.
In this work we investigate, for the first time, the performances of a system based on hydrogenated amorphous silicon
photosensors for the detection of Ochratoxin A. The sensor is a n-type/intrinsic/p-type amorphous silicon stacked
structure deposited on a glass substrate. The mycotoxin is deposited on a thin layer chromatographic plate and aligned
with the sensor. An ultraviolet radiation excites the ochratoxin A, whose fluorescence produces a photocurrent in the
sensor. The photocurrent value is proportional to the deposited mycotoxin quantity. An excellent linearity of the detector
response over more than two orders of magnitude of ochratoxin A amount is observed. The minimum detected
mycotoxin quantity is equal to 0.1ng, suggesting that the presented detection system could be a good candidate to
perform rapid and analytical ochratoxin A analysis in different kind of samples.
A Bismuth Germanate (BGO) 'veto' shield surrounds on five faces the detector planes of the IBIS instrument on-board the satellite INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory). The Veto System provides anti-coincidence signals to the two imager layers covering the energy range from 20 keV to 10 MeV. The area to be shielded is about 8000 cm2, and with a shield thickness of 20 mm, this leads to a total BGO crystal weight of about 115 kg. This paper describes the shield design, and how some scientific and engineering requirements are implemented. Also results from tests with the Engineering Model are presented. Particular emphasis is given to the electronic signal chain, and its response to overload particles, mainly high energy protons, expected in the INTEGRAL orbit (Elliptic Earth Orbit with 72 h period). The overload response has been studied in detail both with a built-in Light Emitting Diode (LED) in the laboratory, and at a proton beam facility. Based on the lab measurements the expected blinding of the shield in-orbit is around 1%. This is obtained with a simple, but optimized chain, consisting of a front-end amplifier and a bi-polar shaper, that provides input to the trigger generator. Results from beam tests with proton energies from 60 to 300 MeV are reported, and it is demonstrated that the proton pulses in terms of amplitude, shape and duration are very similar to the simulated ones, and thus confirm the expected system response.
We present the first semiconductor p-i-n photodiode with excellent sensitivity in the VUV range and high rejection of visible radiation. The device is based on the thin-film technology of amorphous silicon and silicon carbide and can be integrated in large area arrays on glass or flexible substrate. Its internal quantum efficiency is over 50 percent in the VUV and decreases with wavelength. In the visible range the sensitivity can be tuned by variations to the technology parameters. Solar blind photodiodes have been fabricated, with 1 percent quantum efficiency at 400 nm and 0.1 at 650 nm. Working bias voltage is very low since its best sensitivity is achieved when reversely biased with 0.3V. Linearity of the photocurrent was verified with incident UV light in the range 5nW to 4mW. Response times under UV illumination was tested with a N2 laser: 500 ns rise times and 6microsecond(s) FWHM were measured. The excellent behavior of the photodetector in the UV range was explained in the UV range was explained within the hypotheses that generation of hot carriers in the p-doped layer occurs and that a pure diffusion mechanism rules transport, being the thickness of the p-doped layer comparable with the effective diffusion length of electrons.
A new family of photodetectors based on hydrogenated amorphous silicon (a-Si:H) and silicon carbide (a-SiC:H) is described. They are p-i-n photodiodes whose thin layers are grown by glow discharge on cheap substrates as glass or flexible materials. Modulating the absorption profile in the semiconductor and the thickness of the layers, it is possible to select, during the growing process, the wavelength range where the photodetector is more sensitive. A first generation prototype of photodetectors optimized for UV detection was tested at room temperature and with no external bias voltage, illuminating it with visible and vacuum-UV radiation. The results show that the measured quantum efficiency is above 15% in the 58.4 - 250 nm spectral range and about 300 times lower at longer wavelengths (0.05% at 700 nm). An improved second generation has been also tested in the same experimental conditions and the preliminary data exhibit a better noise level (less than 1 pA), a higher response stability and an enhanced efficiency. A linear dependence on the radiation intensity has been verified over three orders of magnitude at 400 nm. Noise figure evaluation and response times will be also presented.
An optical hysteresis and nonlinear absorption of the microsecond and nanosecond laser pulse at interband excitation of different a-Si:H and a-SiC:H films are reported. To explain the experimental results a new physical model is suggested, taking into account the light interaction with nonequilibrium localized lattice vibrations anharmonically coupled with the extended phonons.