Giampiero de Cesare

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.

New a-Si:H two-terminal switching device for active display

Abstract This work presents a new two-terminal switching amorphous silicon (a-Si:H) device for active flat panel display. The device is a multilayer stacked structure n-i-δp-i-n. The electrical characteristics of the diode are strongly determined by its geometry and by the doping level of the δp layer. The current-voltage data show a symmetrical behaviour with an OFF current of 10 −11 A and an ON/OFF current ratio of six orders of magnitude calculated at applied voltage of 3 and 1 V. An active matrix based on this device has been realized by using only two photolithographic masks.

Variable spectral response photodetector based on crystalline/amorphous silicon heterostructure

A novel photodetector based on a p-i-n amorphous silicon/n-p crystalline silicon stacked heterostructure, which exhibits either infrared or visible response, depending on the polarity of the applied bias is described. The energy gap and the thickness of the layers inside amorphous diode have been optimized to obtain a wavelength selection (centered at 480 nm and 780 nm) with high rejection ratio and good quantum efficiencies. Absolute values of the quantum yield as high as 80% in both the two spectral bands have been obtained thanks to an Al-doped ZnO conductivity transparent film deposited on the top of the device.

Amorphous Silicon p-i-n Structure Acting as Light and Temperature Sensor

In this work, we propose a multi-parametric sensor able to measure both temperature and radiation intensity, suitable to increase the level of integration and miniaturization in Lab-on-Chip applications. The device is based on amorphous silicon p-doped/intrinsic/n-doped thin film junction. The device is first characterized as radiation and temperature sensor independently. We found a maximum value of responsivity equal to 350 mA/W at 510 nm and temperature sensitivity equal to 3.2 mV/K. We then investigated the effects of the temperature variation on light intensity measurement and of the light intensity variation on the accuracy of the temperature measurement. We found that the temperature variation induces an error lower than 0.55 pW/K in the light intensity measurement at 550 nm when the diode is biased in short circuit condition, while an error below 1 K/µW results in the temperature measurement when a forward bias current higher than 25 µA/cm2 is applied.

Electrowetting-on-dielectric system based on polydimethylsiloxane

In this paper we present a detailed characterization of an electro-wetting on dielectric (EWOD) system able to move drop of liquid and to detect its position over an array of electrodes covered with a 1μm thick polydimethylsiloxane (PDMS) layer. In the presented system, the PDMS layer acts as both insulation and hydrophobic material. An electronic board controls all the signals needed for the actuation and sensing functionalities of the EWOD system. Sessile drop experiments show the saturation of the contact angle at negative bias voltage applied to the droplet. This behavior is ascribed to trapped carrier in the PDMS layer and explains the movement of the droplet toward the grounded electrode found in EWOD experiment. The procedure chosen for the drop movement achieves speed around 5cm/s with applied voltages around 200V. Detection of drop position is successfully achieved implementing the time-constant method, which evaluates the variation of electrode capacitance induced by the droplet presence on the PDMS surface corresponding to the metal electrode.

Adjustable Threshold a-Si/SiC:H Color Detectors

We present the Adjustable Threshold Color Detectors (ATCDs) based on stacked heterojunctions of amorphous silicon (a-Si:H) and silicon carbide (a-SiC:H). These devices work as bias-controlled photocurrent sources with different values of the voltage threshold depending on the wavelength of the incident radiation. The ATCDs detect the three fundamental chromatic components (blue, green, red) by varying the external voltage within a few Volts.

Built-in Enhancement in a-Si:H Solar Cell by Chromium Silicide Layer

To increase the performances of amorphous silicon n-i-p solar cell, we investigate the possibility to enhance the built-in potential inserting thin high-conductivity chromium silicide layers at interfaces between metal electrodes and doped regions. We found that chromium silicide, formed on top of amorphous doped layer during chromium film evaporation, allows a reduction of activation energy of about 0.225 eV for n- and p-doped amorphous films. The activation energy reduction contributes to both built-in and open-circuit voltage enhancements, as demonstrated by comparing simulated and measured photovoltaic performances of n-i-p amorphous silicon solar cell with and without the chromium silicide layers.

Contact Formation on a-Si:H/c-Si Heterostructure Solar Cells

In this chapter a description of the contact formation in a-Si:H/c-Si heterojunction solar cell is detailed. Firstly the doping of amorphous films is reported together with the possibility to enhance the amorphous film conductivity by using Chromium Silicide formation on top of the doped films. Then a finite difference numerical model is used to describe the a-Si:H/c-Si heterojunction solar cell in which both contacts are made by amorphous films. In particular to evaluate the effect of the bandgap mismatch between amorphous and crystalline silicon at the base contact a detailed investigation is presented comparing experimental current voltage characteristics of heterojunction contacts with the results of a simulation based on numerical model. Subsequently, details about formation and properties of a transparent conductive oxide and a screen printing procedure to form metallic grids are presented as a common way to form the heterojunction solar cell electrodes. Finally three examples of heterojunction solar cells are proposed using different approaches to form the contacts. In particular a double side heterojunction cell fabricated on multicrystalline silicon is presented, a laser fired local contact for the rear side of the cell is shown and finally an interdigitated back contact is described. All the investigations are based on our experience on heterostructure solar cells developed in the past years.

Thermal control system based on thin film heaters and amorphous silicon diodes

In this paper we present a system able to perform thermal treatments on lab-on-chip devices fabricated on glass substrates. The system includes a thin film resistor acting as heater and thin film hydrogenated amorphous silicon diodes acting as temperature sensors. An electronic system controls the lab-on-chip temperature through a Proportional-IntegralDerivative algorithm. In particular, an electronic board infers the system temperature measuring the voltage across the amorphous silicon diodes and drives the heater to achieve the set-point temperature. Taking into account the 16-bit ADC resolution and the sensors sensitivity, which is around 3.6 mV/oC, we estimate that our system is able to detect temperature variation as low as 3.5·10-3oC. Furthermore, the experimental results show that the system is able to stabilize the system temperature with a precision better than 0.1 oC.

On the relation between defect density and dopant concentration in amorphous silicon films

Abstract In this work we present a study of the relation between the concentration of dopant gas used in the deposition of amorphous silicon films and the active dopant concentration in the material. Fitting of experimental dark and light conductivity data have been performed by a numerical model to evaluate the ratio between dangling bond density and phosphine concentration. Simulation results show that this ratio is constant for dopant concentration up to 8×10 18 , and increases to 1 for larger dopant concentration.