Abstract
Ultrathin Si films have a reduced thermal conductivity in comparison to Si bulk due to phonon scattering at the surfaces. Furthermore, the small thickness guarantees a reduced thermal mass (in the µJ/K range), which opens up the possibility of developing thermal sensors with a high sensitivity. Based on these premises, a thermoelectric (TE) microsensor based on ultrathin suspended Si films was developed and used as a thermal photosensor. The photoresponse of the device was evaluated with an argon laser (λ = 457 nm) with a variable power ranging from 0 to 10 mW in air at atmospheric pressure, with laser diodes at 406 nm, 520 nm and 638 nm wavelengths, and fixed powers in high vacuum conditions. The responsivity per unit area, response time (τ) and detectivity (D*) of the device were determined in air at ambient pressure, being 2.6 × 107 V/Wm2, ~4.3 ms and , respectively. Temperature differences up to 30 K between the central hot region and the Si frame were achieved during open-circuit voltage measurements, with and without laser diodes. During illumination, the photogeneration of carriers caused a slight reduction of the Seebeck coefficient, which did not significantly change the sensitivity of the device. Moreover, the measurements performed with light beam chopped at different frequencies evidenced the quick response of the device. The temperature gradients applied to the thermoelectric Si legs were corrected using finite element modeling (FEM) due to the non-flat temperature profile generated during the experiments.
Highlights
Photosensors are sensors that can detect light
Previous work has shown that this strategy can be applied satisfactorily to fabricate Si-based compatible thermoelectric microsensors complementary metal-oxide-semiconductor (CMOS)-compatible microgenerators as energy harvesters for low-power applications [9,10,11,12]
This study presents a thermoelectric microsensor that relies on an array of ultrathin single-crystalline n- and p-type thermoelectric legs to generate the output voltage
Summary
Photosensors are sensors that can detect light (electromagnetic waves). Depending on the desired spectral sensitivity, different physical principles should be considered. Thermal detectors, either bolometers or thermoelectric-based (thermopiles), have a broad spectral range of detection and rely on the temperature increase produced by the absorbed radiation. Among the wide family of thermal absorption photosensors, those based on the thermoelectric (TE) principle do not require an external current or voltage source, and may offer extremely low energy detection limits depending on the design. Appropriate designs and materials can be used to boost absorption at the sensing/absorbing regions The simplicity of these types of devices enable their miniaturization. Previous work has shown that this strategy can be applied satisfactorily to fabricate Si-based compatible thermoelectric microsensors complementary metal-oxide-semiconductor (CMOS)-compatible microgenerators as energy harvesters for low-power applications [9,10,11,12]. The device was microfabricated from a silicon-on-insulator (SOI) wafer where a central silicon free-standing membrane (0.25 mm area) was connected and supported to the bulk silicon frame with 40, 100 nm thick thermoelectric silicon legs (20 doped with B and 20 doped with P) [11]
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