Low Power Resistive Photon- and Gas- Sensors
Manohar, Gollakota Venkata Sai
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Sensors, be it for temperature, pressure, acoustic, light or gas to name a few have become an indispensable part of human life. Light sensors or photodetectors have widespread usage in modern portable electronics as camera sensors, fibre optic communication systems, surveillance, firefighting and medical imaging amongst others. Commercial light sensors such as charge coupled device (CCD) and complementary metal-oxide-semiconductor (CMOS) detectors, used extensively for ultraviolet (UV) and visible detection, have a low responsivity of ~ 0.7 A/W. While high responsivity of ~ 50 A/W can be achieved using avalanche photodiodes (APDs), they operate under very high bias voltages (>150 V). In infrared (IR) region, thermal and photodetectors are used. Thermal detectors are slow and have low photoresponse, while requiring the use of special filters for IR-only detection. On the other hand, photon detectors always require cooling for operation often up to cryogenic temperatures, increasing their power consumption. Hence, there is a clear need for the development of high response photodetectors with low power consumption across the UV, visible and IR regions. Gas sensors are widely used for the detection of combustible, flammable and toxic gases, or oxygen depletion. Also, with air pollution reaching alarming levels across the world, there is a demand for constant air quality monitoring and hence, the development of portable and low-cost gas sensors with low power consumption are of utmost importance. In this work, we have explored UV, visible/NIR and IR (>1040 nm) photodetection using ZnO, Si and IrP2 nanoparticles (NPs) embedded few-layer graphene (FLG), respectively. ZnO with a bandgap of ~3.2 eV is suitable for UV photodetection. However, the exciton binding energy of ~60 meV leads to a high recombination and hence, low photoresponse. Therefore, nitrogen-doped carbon nanotubes (NCNTs) and PEDOT:PSS/p-Si were used to separate the electrons and holes, respectively, increasing the photoresponse by ~6 x 104 and ~ 1.5 x 103 over a bare ZnO film in linear (Ag/ZnO/Ag) and non-linear (p-Si/ZnO) configurations. Responsivity of ~0.153 A/W (for linear device) and ~10.5 A/W (for non-linear device) were obtained with power consumptions of ~0.6 and ~2.5 mW, respectively. For, visible and NIR detection, we considered Si photodiodes in Ag/p-Si (111)/Ag configuration. The device shows a fast and broadband photoresponse with excellent responsivity ~65 A/W under 840 nm illumination at a reverse bias of 21 V, 2 orders of magnitude greater than the commercial CCD and CMOS detectors. The results are comparable to a commercial APD, while operating at a reverse bias at least an order of magnitude lower than the commercial one. For IR detection, IrP2 NPs embedded FLG was used, where the NPs act like trap states and lead to photo-gating effect. Responsivity of ~1.81 A/W and ~0.54 A/W at 1550 nm were obtained under 80 and 2200 mW/cm2 of power density, with a low power consumption of ~75 μW. The device also showed photoresponse in the MWIR and LWIR regions while being UV and visible blind. The same material was used for fabricating a flexible NO2 gas sensor. The device shows fast and selective response towards NO2 with a LOD of ~ 57 ppb and a low power consumption of ~40 μW. The device also shows stable response towards NO2 even when bent to an angle of 60o and can also be used as a breath analyser. Finally, we fabricated sensors using NCNTs and studied their properties towards pressure sensing. The sensors showed a linear variation in resistance with chamber pressure in the range of 1 - 0.092 atm at sensor powers as low as ~1.8 μW. In summary, the thesis reports development of high response photo- and gas- sensors with low power consumption.