| dc.description.abstract | The electronics industry is ruled by silicon which is realized as the solution to meet the requirements of higher bandwidth and low cost high density monolithic integration. However, silicon has its limitations when it comes to high frequency or optoelectronic applications. Fusion of several functionalities on one chip can be obtained by heterogeneous integration of materials that have optimized performance for a specific application. The ability to tailor heterogeneous materials to be able to integrate them with each other, in rigid or flexible form, can lead to novel and superior functionalities, with impact in multitude of areas like electronics, optoelectronics, spintronics, biosensing and photovoltaics. Imaging systems benefit highly from integrating materials with different bandgaps, which in turn leads to detection in several wavelength ranges. Multi-band and multispectral imaging has various applications in remote sensing, industrial surveillance systems, bio photonics, automotive cameras, fluorescent imaging, spectrometer on a chip, LADAR and free space optical communications. The aim of this thesis is to explore various materials, configurations and integration schemes for efficient photodetection in several wavelength bands, and to achieve this with low cost, less bulky substrates, and easy fabrication techniques. The predictions of performance and efficiency for the structure and designs employed for various configurations are validated by measurement results and performance analysis after fabrication.
To realize a photodetector in visible wavelength band, silicon is the most widely used material. With its bandgap of 1.1 eV, silicon can cover a wide range of wavelengths from 400nm to 1.1 µm. An 8x8 array of silicon photodetectors is first demonstrated, with characterization results. The structure and thickness of each layer in the silicon active region is optimized to obtain maximum absorption and hence high photo response in the blue, green and red wavelengths. The rejection ratio or the ability of each P-N junction to have maximum responsivity to a specific wavelength compared to the others is measured. By the process of membrane transfer, silicon active layer is transferred to a kapton substrate, enabling flexibility and bending. The 8x8 Si PD array is fabricated on the kapton substrate, and the performance is analyzed and compared with that of a Si PD array on rigid silicon substrate.
InGaAs is one of the materials that can be used to detect the Near IR wavelengths due to its bandgap of 0.75 eV. InGaAs PDs have high responsivity, low power consumption and lower dark currents. InGaAs detector fabrication is explored to cover detection in the near IR wavelength range. Initially, the fabrication of a single detector is described, with measurement results of responsivity to IR wavelengths. This paves way for InGaAs 8x8 photodetector array, which is first fabricated on rigid substrate. Photoresponse to 980 nm and 1550 nm lasers are investigated, and responsivities are measured. InGaAs photodetector array on flexible substrate is investigated next. The InGaAs photodetector array is fabricated on InP substrate and then transferred to kapton by black wax transfer method. Performance comparison is made before and after transfer of the PD array to kapton.
Different heterointegration techniques and architectures are investigated to combine Si and InGaAs to form a detector capable of detection in visible and Near IR regime. Vertical integration of Si on InGaAs reduces the real estate but has more fabrication complications as compared to adjacent integration. Both the layouts are explored in detail. A new method of transferring unconnected silicon mesas from the SOI substrate to the InGaAs substrate is proposed and investigated in detail. This technique of transferring Si mesas allows for an aligned transfer and can be extended to any material. | |
