D. Caputo

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.

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.

On-chip detection of multiple serum antibodies against epitopes of celiac disease by an array of amorphous silicon sensors

In this paper, we present the preliminary results of an ELISA-on-chip device, intended as a technological demonstrator of a novel analytical system suitable for the diagnosis and follow-up of celiac disease. The idea of the work is to combine an array of amorphous silicon photosensors with a pattern of a poly(2-hydroxyethyl methacrylate) polymer brush film, which acts as anchor for the immobilization of gliadin peptides containing the celiac disease epitopes. Recognition relies on a sandwich immunoassay between antibodies against the peptides and secondary antibodies marked with horseradish peroxidase to obtain a chemiluminescent signal. Detection is based on the measurement of photocurrent induced in the array of amorphous silicon photosensors by the chemiluminescent signal. An ad-hoc procedure has been developed in order to enable the fabrication of the photodiode array and the polymer brush pattern on the two sides of the same glass substrate ensuring the compatibility of the different technological steps. The sensitivity and the selectivity of the chip for multiplex immunoassays were demonstrated using two gliadin peptides (VEA and DEC). In particular, we found that the average amount of the bound HRP revealed by our analytical protocol is 3.5(±0.3) × 10−6 pg μm−2 and 0.85(±0.3) × 10−6 pg μm−2 for specific and non-specific interactions, respectively.

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.

Amorphous Silicon Photosensors for Detection of Ochratoxin a in Wine

The presence of ochratoxin A (OTA) in different food commodities deserves great attention because of its toxic and carcinogenic effects on humans and animals. In this paper, we report on the detection of OTA both in standard solutions and in contaminated samples of red wine by using amorphous silicon photosensors. The method relies on the excitation by ultraviolet radiation of the toxin molecules and on the absorption of the toxin reemitted light by the photosensor. The device is a p-i-n stacked structure, whose electro-optical characteristics have been optimized in order to maximize the photosensor responsivity and the limit of detection. For standard solutions, we found minimum detected OTA amount to be equal to 0.1 ng, while for contaminated red wine samples the technique coupled with very simplified and rapid extraction procedures has allowed the detection of OTA at the 1-ppb level.

Lab-on-glass system for DNA analysis using thin and thick film technologies

In this paper, we present a compact lab-on-chip system suited for label-free DNA analysis. The system can be fabricated on a conventional microscope glass slide using thin-film and thick-film technologies. It integrates a heating chamber, an electrowetting-based droplet handling system and a hydrogenated amorphous silicon (a-Si:H) photosensor array for DNA detection. At this stage of research we have designed and tested the individual functional units. The heating chamber incorporates a thin metal film heater optimized for uniform temperature distribution on a 1cm 2 area. A forward-biased a-Si:H p-i-n junction is used for temperature monitoring, achieving a linear temperature dependence with -3.3 mV/K sensitivity. The droplet-handling unit, relying on the electrowetting method, is designed to move the sample from the heating chamber to the sensor array. The unit includes a set of metal pads beneath a layer of PDMS that provides both the electric insulation of the electrodes and the hydrophobic surface needed by the electrowetting technique. The UV sensor array allows measuring the DNA absorbance variation at 254nm related to the hybridization between probe-molecules contained in the sample and reference target molecules immobilized on the sensor surface. A preliminary test to detect the hybridization between a 25-mer single-stranded oligonucleotides and denaturated pBR 322 4162-mer single-stranded oligonucleotides has been carried out successfully.