G. de Cesare

Solar-blind UV photodetectors for large area applications

In this paper, we extensively investigate a family of solar-blind thin-film photodetectors optimized for the ultraviolet spectrum (UV). The devices are p-i-n structures made of hydrogenated amorphous silicon (a-Si:H) and silicon carbide (a-SiC:H) on a glass substrate. At room temperature the photodetectors exhibit values of quantum efficiency of 20% at /spl lambda/=187 nm, and are transparent to visible radiation. The excellent sensitivity of the device at short wavelengths is explained within the framework of a diffusive model of transport, taking into account the effects of hot carrier relaxation. The rejection of visible light is obtained with an appropriate design of the energy gap and intrinsic layer thickness. The great advantage of this technology lies in the possibility to produce low-cost, large-area arrays of photodetectors.

Amorphous Si/SiC three‐color detector with adjustable threshold

An adjustable threshold color detector (ATCD) is demonstrated, based on a hydrogenated amorphous silicon (a‐Si:H) and silicon–carbide (a‐SiC:H) p+‐i‐n+‐i‐n+‐i‐p+ multilayer. The ATCD is able to discriminate between blue (λ=450 nm), green (550 nm), and red (≳650 nm) illumination by varying the externally applied voltage within a few volts. The operation of the detector can be explained regarding the ATCD as three independent devices connected in series: a p+‐i‐n+, a n+‐i‐n+, and a n+‐i‐p+. The novel feature is the n+‐i‐n+ cell. It acts as a short circuit under strong illumination, whereas in dark it is equivalent to two low quality back‐to‐back diodes which introduce a shift in the threshold in the photocurrent detection. Thanks to the large number of physical parameters such as layer thickness and band gap the ATCD appears extremely versatile.

Hydrogenated amorphous silicon ultraviolet sensor for deoxyribonucleic acid analysis

In this letter, we show the results achieved using a hydrogenated amorphous silicon photosensor for a “label-free” deoxyribonucleic acid (DNA) analysis based on the measurements of the absorbance in the ultraviolet range. The optimization of the sensor structure allowed us to bring the detection limit for a 30-mer DNA sample down to 1nM×cm, limited by the experimental setup. Taking advantage of the hypochromic effect, we also demonstrated the detection of single- and double-stranded DNA molecules in a melting experiment. From the noise characterization of our setup, we estimated the minimum DNA absorbance required to detect the occurrence of a hybridization/separation process to be below 10−3.

Tunable photodetectors based on amorphous Si/SiC heterostructures

We describe a novel family of two-color photodetectors based on p-i-n-i-p heterostructures in hydrogenated amorphous Si/SiC. The devices operate as bias-controlled light detectors with enhanced absorption in either the blue or the red regions. Compared to other two-color sensors based on amorphous Si/SiC n-i-p-i-n structures, they exhibit sharper wavelength selection and higher rejection ratios with excellent quantum efficiencies: full width at half maximum (FWHM) less than 130 mm, suppression up to 27 dB, and peak responsivity of about 150 mA/W are reported for both the blue and the red window. Simulation of the photoresponse of the device under steady-state and time-dependent voltage bias and under continuous illumination is also presented. >

Amorphous silicon/silicon carbide photodiodes with excellent sensitivity and selectivity in the vacuum ultraviolet spectrum

An innovative family of thin‐film photodetectors optimized for the ultraviolet (UV) spectrum is presented here. The devices are made of hydrogenated amorphous silicon (a‐Si:H) and silicon carbide (a‐SiC:H) on glass substrates. At room temperature, the photodetectors exhibit values of quantum efficiency of 21% in the vacuum UV and 0.08% at 750 nm, without external voltage. The great advantage of this technology lies in the possibility to produce low‐cost, large‐area arrays of photodetectors on glass or flexible substrates. All these features candidate the a‐Si/SiC:H photodetectors as possible, concurrent to specialized commercial devices.

Detailed Study of Amorphous Silicon Ultraviolet Sensor With Chromium Silicide Window Layer

In this paper, we present a detailed investigation of an amorphous silicon sensor for the detection of ultraviolet (UV) radiation. The device is an n-i-p stacked structure with a grid-patterned top metal contact through which the incident radiation reaches the active layers. The performances of the sensor have been enhanced by using a very thin chromium silicide (CrSi) film formed on top of the p-doped layer. In particular, this film enhances the surface conductivity, reducing the effect of the self-forward bias that occurs in the device due to the high resistivity of the p-doped layer. As a result, the sensitivity and the linearity of the response increase, reaching a responsivity above 60 mAAV at 254.3 nm. Furthermore, the CrSi layer leads to a stable device because it hides the effect of the p-doped layer resistivity variation under UV radiation. The comparison between two sets of devices with different grid geometries, one with and one without the CrSi film, demonstrates the effectiveness of the alloy film.

Microfluidic Chip With Integrated a-Si:H Photodiodes for Chemiluminescence-Based Bioassays

On-chip optical detection of chemiluminescent reactions is presented. The device is based on the integration of thin film hydrogenated amorphous silicon photosensors on a functionalized glass substrate ensuring both a good optical coupling and an optimal separation between biological or chemical reagents and the sensing elements. The sensor has been characterized and optimized using the chemiluminescent system composed by the enzyme horseradish peroxidase (HRP) and luminol/peroxide/enhancer cocktail. The detectability of HRP is at the attomole level with a sensitivity of 1.46 fA/fg. Experiments, involving the detection of immobilized bio-specific probes on the functionalized surface have been performed both in bulk and microfluidics regime, proving the ability of the system to effectively detect chemiluminescent reactions and their kinetics. In particular, results achieved using conventional polydimethylsiloxane microfluidics for samples and reagents handling confirmed the good detection capabilities of the proposed system.

Electrical Properties of ITO/Crystalline-Silicon Contact at Different Deposition Temperatures

In this letter, the effect of deposition temperature on the barrier height between indium tin oxide (ITO) and crystalline silicon (c-Si) is presented. ITO films have been deposited by RF magnetron sputtering in the range between room temperature and 200 °C on both p- and n-type doped c-Si substrates. From current-voltage and capacitance-voltage characteristics of the ITO/c-Si junctions, we found that ITO deposited on 1-Ω · cm n-type doped silicon forms a rectifying junction with barrier heights varying from 0.9 to 0.3 eV, while at room temperature, an ohmic behavior on 1-Ω · cm p-type c-Si is obtained.

Monitoring of Temperature Distribution in a Thin Film Heater by an Array of a-Si:H Temperature Sensors

In this paper, we propose the use of an array of amorphous silicon (a-Si:H) p-i-n diodes to monitor the spatial temperature distribution over a thin film heater used for thermal treatments in lab-on-chip systems. The effects of heater geometry and operating conditions on the spatial temperature distribution have been preliminarily investigated by using COMSOL Multiphysics, coupling the electrostatic problem with the thermal problem via the Joule effect. Depending on the analyzed system, nonuniform temperature profiles can be induced over the heater surface revealing the need for a temperature point-monitoring. An example of whole device, constituted by a serpentine shaped TiW/Al/TiW thin film heater and five a-Si:H diodes deposited between the resistor meanders, has been fabricated on a microscope glass slide and characterized. Voltage-temperature characteristics of the a-Si:H sensors, measured at constant forward current, show a sensitivity around . The spatial temperature distribution along the heater has been derived measuring the voltage across each a-Si:H diode. A good agreement between modeled and measured data is obtained, demonstrating the suitability of the a-Si:H array as temperature distribution sensors in lab-on-chip application.

Amorphous Silicon Sensors for Single and Multicolor Detection of Biomolecules

In this paper, we report on a system for single and multicolor detection of biomolecules based on amorphous silicon photosensors. The system promises to be compact, portable, and low cost. It allows the quantitative detection without using optics for focusing both the excitation and the emitted radiation. The revealed biomolecules can be chemi- or naturally luminescent or can be labeled with fluorochromes. Here, we focus on the detection of DNA molecules labeled with a single or with two fluorochromes by using a p-i-n and a p-i-n-i-p amorphous silicon stacked structure, respectively. The device design has been optimized in order to maximize the signal-to-noise ratio and to match the sensor spectral response with the emission spectra of the fluorochromes. This optimization process has been carried out by means of a numerical device simulator, which takes into account the optical and electrical properties of the amorphous silicon. Detection limit in the order of a few nmol/l have been achieved for both the single and the two-color photosensors. Comparison with commercial measurement equipment shows the suitability of our system for practical applications.