Development of Broadband Operation and Low Power-Consumption of Photo-Electronic-Thermo Detection and Conversion Devices

Photo-electronic-thermo conversion devices have been applied in a variety of applications, such as image sensing, optical communication, environmental monitoring, astronomical studies, and solar energy harvesting. However, insufficient detection capability, limited detection bandwidth, and requiring large bias voltage restricted the practical applications of devices. Therefore, the goal of the researches in this thesis is to develop photo-electronic-thermo conversion devices featuring high conversion efficiency, detection over broad bandwidth, and low power consumptions. Direct improving the quality of materials can intrinsically enhance the conversion efficiencies of devices. In the first part of thesis, we describe a low-cost cadmium sulfide (CdS) photoconductor that behaves as a highly sensitive and rapidly responding detector toward low-intensity light. Through the observation of transmission electron microscopy images and analysis of photoluminescence and Raman spectra, the degree of crystallization of CdS increased and defects were removed effectively in the region of a few tens nanometers beneath the surface of CdS after treatment with several shots/distinct intensities from a krypton fluoride (KrF) excimer laser. Moreover, we found the improvements of detection capability of CdS devices in the ultraviolet (UV) regime are much larger than that in the visible regime. At a low bias voltage of 1 mV, the treated CdS device provided a record high responsivity of 74.7/7200 A W-1 and a detectivity of 1014/1015 Jones in UV/visible regime. The measured response time of the treated CdS device from the dark to illumination at 10-2 fW µm-2 was only 40 ms—much faster than the shutter speed or exposure time required for a professional digital camera for such low-light image detection. This strategy appears to hold great potential for ultralow-light image detection with ultralow power consumption. In the second part of thesis, we developed a compact surface plasmonics-based color-image sensor comprised only a multifunctional electrode based on a single-layer structured aluminum (Al) film and an underlying silicon (Si) substrate. This approach significantly simplifies the device structure and fabrication processes. Moreover, such Schottky-based plasmonic electrodes perform multiple functions, including color splitting, optical-to-electrical signal conversion, and photogenerated carrier collection for color-image detection. The device took advantage of the near-field surface plasmonic effect around the Al–Si junction to enhance the optical absorption of Si, resulting in a significant photoelectric current output even under zero bias voltage. These plasmonic Schottky-based color-image devices could convert a photocurrent directly into a photovoltage and provided sufficient voltage output for color-image detection even under a light intensity of only several femtowatts per square micrometer. Therefore, this strategy has great potential for direct integration with complementary metal–oxide–semiconductor (CMOS)-compatible circuit design, increasing the pixel density of imaging sensors developed using mature Si-based technology. In the third part of thesis, we propose the concept of deep-trench/thin-metal (DTTM) active antenna that take advantage of surface plasmon resonance (SPR) phenomena, three-dimensional (3D) cavity effects, and large-area metal/semiconductor junctions to effectively generate and collect hot electrons arising from plasmon decay and, thereby, increase photocurrent for photodetection well below the semiconductor band edge. The DTTM-based devices exhibited superior photoelectron conversion ability and high degrees of detection linearity under infrared light of both low and high intensity. Therefore, these DTTM-based devices have the attractive properties of high responsivity, extremely low power consumption, and polarization-insensitive detection over a broad bandwidth, suggesting great potential for use in photodetection and on-chip Si photonics in many applications of telecommunication fields. In the fourth part of thesis, we describe a broadband perfect absorber based on loading effect–induced single-layer/trench-like thin metallic (LISTTM) structures. These LISTTM structures take advantage of both SPR and 3D cavity effects to provide efficient, tunable, and polarization-insensitive absorption from the UV to the infrared (IR) regime. The optimized hole-width of the LISTTM arrays was approximately one half of the designed wavelength. Therefore, even when the designed absorption band was in the visible regime, the feature sizes of the LISTTM structure could remain on the order of several hundred nanometers—dimensions much larger, and structures much simpler, than those of metamaterial-based absorbers. Moreover, because the absorbance of the specific LISTTM structure was similar to the air mass 1.5 solar spectrum, much solar energy was absorbed by the LISTTM arrays. Besides, these LISTTM structures exhibited superior photothermal performance; they also displayed very low emissivity, thereby decreasing heat dispersion through thermal radiation. Therefore, the LISTTM arrays could efficiently absorb light of higher photon energy in the UV, visible, and near-IR regimes, effectively conduct the generated heat through the continuous metal films, and barely disperse any heat through thermal radiation. Accordingly, these attractive properties suggest that such LISTTM absorbers might have promising applications in many fields related to energy harvesting. In the last, we developed a filter-free, junctionless color image sensor that exhibited superior photo–thermo–electrical response under a low bias voltage and a short response time. Although our compact sensor had a simple single-layer trenchlike aluminum (Al) structure, it could perform multiple functions, including light harvesting, color-selective absorption, photo–thermo–electrical transformation, and the ability to collect of photoinduced differences in electrical signals. This device exploited near-field surface plasmon resonance and cavity effects to enhance the intensity of the electric field and the color-selective absorption, ultimately resulting in significant current signals in its structured Al film. With its ability to provide functional filter-free, junctionless structures, this strategy has great potential for the production of devices that operate on different kinds of substrates, thereby bridging various applications of color imaging technologies.