Sandro Rao

Electro-optically induced absorption in α-Si:H/α-SiCN waveguiding multistacks

Electro optical absorption in hydrogenated amorphous silicon (α-Si:H) - amorphous silicon carbonitride (α-SiCxNy) multilayers have been studied in two different planar multistacks waveguides. The waveguides were realized by plasma enhanced chemical vapour deposition (PECVD), a technology compatible with the standard microelectronic processes. Light absorption is induced at λ=1.55 µm through the application of an electric field which induces free carrier accumulation across the multiple insulator/ semiconductor device structure. The experimental performances have been compared to those obtained through calculations using combined two-dimensional (2-D) optical and electrical simulations.

Use of Amorphous Silicon for Active Photonic Devices

Silicon photonics is a new emerging and disruptive technology aimed at using cost-effective silicon-based materials for the generation, control, and detection of modulated light signals for optical communication. Hydrogenated amorphous silicon (a-Si:H) is a particularly promising platform for enabling the desired matching between electronics and on-chip photonics. Thin a-Si:H layers can be in fact deposited using the CMOS-compatible low-temperature plasma-enhanced chemical vapor deposition technique, with no impact at all on the microelectronic layers. This paper provides an overview of the progress and the state of the art of a-Si:H-based active photonic devices, focusing, in particular, on the low technological complexity required for an easy integration within a single photonic microchip. This paper consists of three main sections, in each of which the exploitable optoelectronic effects present in a-Si:H are presented. A comparison between some experimental a-Si:H and crystalline-Si photonic components available in the literature is presented.

Electro-optical modulation at 1550 nm in an as-deposited hydrogenated amorphous silicon p-i-n waveguiding device.

Hydrogenated amorphous silicon (a-Si:H) has been already considered for the objective of passive optical elements, like waveguides and ring resonators, within photonic integrated circuits at λ = 1.55 μm. However the study of its electro-optical properties is still at an early stage, therefore this semiconductor in practice is not considered for light modulation as yet. We demonstrated, for the first time, effective electro-optical modulation in a reverse biased a-Si:H p-i-n waveguiding structure. In particular, phase modulation was studied in a waveguide integrated Fabry-Perot resonator in which the V(π)⋅L(π) product was determined to be 63 V⋅cm. Characteristic switch-on and switch-off times of 14 ns were measured. The device employed a wider gap amorphous silicon carbide 
(a-SiC:H) film for the lower cladding layer instead of silicon oxide. In this way the highest temperature involved in the fabrication process was 170°C, which ensured the desired technological compatibility with CMOS processes.

A 2.5 ns switching time Mach­Zehnder modulator in as-deposited a-Si:H

A very simple and fast Mach-Zehnder electro-optic modulator based on a p-i-n configuration, operating at λ = 1.55 μm, has been fabricated at 170 °C using the low cost technology of hydrogenated amorphous silicon (a-Si:H). In spite of the device simplicity, refractive index modulation was achieved through the free carrier dispersion effect resulting in characteristic rise and fall times of ~2.5 ns. By reverse biasing the p-i-n device, the voltage-length product was estimated to be V(π)∙L(π) = 40 V∙cm both from static and dynamic measurements. Such bandwidth performance in as-deposited a-Si:H demonstrates the potential of this material for the fabrication of fast active photonic devices integrated on standard microelectronic substrates.

4H-SiC p-i-n diode as Highly Linear Temperature Sensor

The linear dependence on temperature of the voltage drop VD across a forward-biased 4H-SiC p-i-n diode is investigated experimentally. The results show that the fabricated temperature sensor has a high degree of linearity in the range from room temperature up to 573 K corresponding to a root-mean-square error lower than 0.5%. A maximum sensitivity of 2.66 mV/K was calculated. The low saturation current of the p-i-n diode, well below the forward biasing current also at high temperatures, reduces the nonlinear effects in the VD-T characteristic allowing the design and fabrication of highly linear sensors operating in a wider temperature range.

High-Performance Temperature Sensor Based on 4H-SiC Schottky Diodes

A high-performance temperature sensor based on coupled 4H-SiC Schottky diodes is presented. The linear dependence on temperature of the difference between the forward voltages appearing on two diodes biased at different constant currents, in a range from 30 °C up to 300 °C, was used for temperature sensing. A high sensitivity of 5.11 mV/°C was measured. This is, to the best of our knowledge, the first experimental result about a proportional-to-absolute-temperature sensor made with SiC diodes, showing both a good degree of linearity and long-term stability performance.

Highly Linear Temperature Sensor Based on 4H-Silicon Carbide p-i-n Diodes

The linear dependence on temperature of the voltage drop difference measured on two diodes biased at different constant currents has been characterized in a range from room temperature up to 573 K. The realized proportional to absolute temperature sensor shows a good level of linearity and the corresponding rms error lower than 0.3%. Moreover, a maximum sensitivity of 610 μV/K has been obtained, with an extrapolated output converging to 0 V at T = 0 K, in agreement with theory and allowing a single-point temperature calibration.

Electrooptical Modulating Device Based on a CMOS-Compatible

In this paper, we report results on a field-effect-induced light modulation at λ = 1.55 μm in a high-index-contrast waveguide based on a multisilicon-on-insulator platform. The device is realized with the hydrogenated amorphous silicon (α-Si:H) technology, and it is suitable for monolithic integration in a CMOS IC. The device exploits the free-carrier optical absorption electrically induced in the semiconductor core waveguide. The amorphous silicon waveguiding layer contains several thin dielectric films of amorphous silicon carbonitride (α-SiCN) embedded along its thickness, thus highly enhancing the absorbing action of the modulator held in the on state.

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Abstract. A p-i-p configuration of an electro-optical modulator based on hydrogenated amorphous silicon (a-Si:H) is characterized and compared with an a-Si:H based p-i-n modulator. In particular, we estimate the performances in terms of optical losses, voltage-length product, and bandwidth at λ=1550  nm for waveguide-integrated p-i-p versus p-i-n configurations. Both devices are fabricated on a silicon substrate by plasma enhanced chemical vapor deposition at low temperature ensuring the back-end integration with a CMOS microchip. We demonstrate a factor of merit for the p-i-p waveguide integrated Fabry-Perot resonator of Vπ×Lπ=19  V×cm allowing the design of shorter devices with respect to p-i-n structure.

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Hydrogenated amorphous silicon (a-Si:H) shows interesting optoelectronic and technological properties that make it suitable for the fabrication of passive and active micro-photonic devices, compatible moreover with standard microelectronic devices on a microchip. A temperature sensor based on a hydrogenated amorphous silicon p-i-n diode integrated in an optical waveguide for silicon photonics applications is presented here. The linear dependence of the voltage drop across the forward-biased diode on temperature, in a range from 30 °C up to 170 °C, has been used for thermal sensing. A high sensitivity of 11.9 mV/°C in the bias current range of 34–40 nA has been measured. The proposed device is particularly suitable for the continuous temperature monitoring of CMOS-compatible photonic integrated circuits, where the behavior of the on-chip active and passive devices are strongly dependent on their operating temperature.