A number of Bangladeshi wheat genotypes (varieties and advanced lines) have been tested for boron efficiency through sand culture experiments over two years (2007-08 & 2008-09) against two Thai check varieties ‘Fang 60’ (boron efficient) and ‘SW41’ (boron inefficient). Performances of the genotypes were evaluated with respect to grain set, boron uptake and grain yield. The genotypes differed considerably in their response to added boron. Some commonly grown varieties such as ‘Sourav’, ‘Prodip’ and ‘Sufi’ showed little response to boron application, while ‘Shatabdi’ had a high response to added boron, thus the former varieties can be regarded as boron efficient and the later as boron inefficient. Some advanced lines, ‘BAW 1051’, ‘BAW 1086’ and ‘KRL-1-4’, also demonstrated their performances as boron efficient genotypes. Wheat genotype response to added boron showed considerable variation between two years, so it is unclear that under what conditions the boron translocation from the vegetative part to the reproductive part is affected and at what stage of plant development does this take place. Our field data indicate that high temperature and water stress at specific stages of plant development might have an interacting effect on boron translocation to the reproductive part.

Over the last five decades, there has been continuous development in the field of optical communication and sensing applications. Timely development of cost-effective, power and absorption efficient, low-noise and ultra-fast optical interconnects/sensors is crucial to meet the high demand for data transfer in the era of the Internet of Things (IoT), augmented reality (AR), virtual reality (VR), light detection and ranging (LIDAR), quantum communication, biomedical imaging and emerging applications that are expected to connect billions of devices/sensors with different functionalities. Datacenters are envisioned to scale up to meet the high connectivity demand as big data and cloud computing continues to grow exponentially. Intra- and inter-datacenter communications require optical links for reach gap (500 m–2 km), long-reach (∼10 km), and extended-reach communications (up to 40 km), which requires optical transceivers/PDs that work in a wide range of the optical spectrum. In a surface-illuminated PD, high speed and high efficiency are often a tradeoff since a high-speed device needs a thin absorption layer to reduce the carrier transit time. In contrast, a high-efficiency device needs a thick absorption layer to compensate for the low absorption coefficient of some semiconductors such as Si Germanium (Ge), GaAs, and InGaAs at wavelengths near the bandgap. This thesis presents the recent development in enhancing the photon–material interactions by utilizing photon-trapping (PT) nanostructures that can control light for more interaction with the photoabsorbing materials, slow down the propagation group velocity and reduce surface reflection. Since ultra-fast PDs suffer from low optical absorption, photon-trapping nanostructures can be utilized to enhance their efficiency. We demonstrated that a perpendicular light beam could be bent to allow guiding parallel to the surface of the PDs, greatly enhancing the interaction of light with the absorption material, which allows for improving broadband absorption by photon manipulation. Consequently, the speed of carrier collection can be increased by designing a thin absorbing layer with a reduced transient time without losing the sensitivity of the PD. Another advantage of developing PT nano-designs is to reduce the junction capacitance by decreasing the junction area. That helps reduce RC time, which is one factor limiting a photodetector's speed. The capacitance reduction in designed PT PDs results in faster response compared to its counterpart without PT PDs. Additionally, thinner absorbing material with integrated PT nano-designs could also help to reduce the bulk dark current, which is one of the noise components in the PDs. Different passivation methods were applied to improve the surface damages/traps to achieve low leakage of less than 1 nA. Moreover, photon-trapping designs add another parameter to guide the light to a specific preferential depth to maximize the gain bandwidth and absorption efficiency in PDs.

This thesis presents the modeling, fabrication, and characterization of various photon-trapping designs in Si, Ge, III-V, and quantum-well PDs. Si photon-trapping PDs enable the development of efficient ultra-fast PDs suitable for monolithic integration with CMOS electronics for the short-reach (850 nm) multimode optical data links used in datacom and computer networks. Such an all-Si optical receiver offers great potential to reduce the cost of short reach, <300 m optical data links in data centers. Additionally, Ge-on-Si PT PDs have the potential to be monolithically integrated with CMOS/ BiCMOS ASICs. Si and Ge-on-Si photon-trapping per pixel designs are presented, which show high absorption efficiency and enable high-performance CMOS image sensors. The unique response of the Si photontrapping PDs paves the way for computational imaging development and spectroscopy on chips utilized for biomedical applications. In addition, highly sensitive photon-trapping Avalanche Photodetectors (APDs) and Single Photon Avalanche Photodetectors (SPADs) are designed with low noise, high gain, and ultra-fast characteristics. The monolithic integration of Si and Ge-on-Si offers low-cost packaging solutions and allows low parasitics, resulting in high-performance on-chip detection. Ge-on-Si, InGaAs, III-V quantum-well PT PDs can be utilized for short- and long-reach communication at intra- and inter-datacenters, passive optical networks, LIDAR, and quantum communication systems, as well as enhancing the capacity of long-haul DWDM systems beyond the L band. III-V PT PD modeling is presented to enhance their bandwidth to meet future THz optical detection and communication demand in the C and L bands and other emerging applications.

Extremely sensitive optical receivers operating at high bandwidth are critical for optical communications technologies and other emerging applications that require the detection of an extremely low number of photons. Light imaging, detection and ranging (LIDAR), quantum communications, biophotonics, medical imaging systems, and other emerging applications will require a new generation of photodetectors that can detect extremely low power levels and be mass manufacturable. Today, Avalanche Photodetectors (APDs) and Single Photon Avalanche Photodetectors (SPADs) are the only detectors that can meet those requirements, but their bandwidth, sensitivity, and noise limitation must be overcome.

This thesis presents the development of new silicon and germanium-on-silicon-based APDs and SPADs with high speed and absorption efficiency at visible and near-infrared wavelengths. Through the modeling, fabrication, and characterization we show that the implementation of photonic nanostructures in photodetectors enhances their responsivity and gain by guiding the light parallel to its surface, greatly enhancing the interaction with the semiconductor material. These nanostructures also allow enhancing their time response by two methods: (i) use of thinner absorption layers to reduce the transit time of the photogenerated carriers, without sacrificing their absorption capabilities and (ii) reduction of junction capacitance by reducing the effective area of the device through the removal of material. This thesis also develops the concept of penetration depth engineering. We show that it is possible to guide photons to a critical depth in semiconductors and maximize the gain-bandwidth performance and absorption efficiency in avalanche-based photodetectors by integrating photon-trapping nanoholes. Our new Si APDs have shown superior amplification gain and speed compared to their conventional counterparts.

The advantages of photon-trapping are exploited in Germanium (Ge) on Si photodetectors. Our Ge-on-Si PDs show enhanced absorption capabilities at the O (original band: 1260-1360 nm) and C (conventional band: 1530 nm to 1565 nm) optical wavelengths of bands and enable their use at longer wavelengths in the L band (long wavelength: 1565 nm to 1625 nm). These PDs with photon-trapping holes have the potential to be monolithically integrated with CMOS/BiCMOS ASICs and offer a promising solution for waveguide PDs required for Photonic Integrated Circuits (PICs).

A new generation of APDS and SPADs are now designed with the implementation of appropriate doping profiles for high amplification, thin semiconductor layers for high speed, and the accurate design of micro/nanoholes for highly sensitive and ultrafast optical receivers.