Electrical, forming and switching characteristics of metal-semiconductor-metal (MSM) memory switches of ion bombarded hydrogenated amorphous silicon (a-Si:H) and its alloys are presented. MSM devices for memory switching applications are known to be characterised by instabilities as well as non-uniformity and irreproducibility of the forming and switching characteristics. It is believed that the presence of defect states in the semiconductor layer plays a significant role in the observation of memory switching in these MSM devices. Gas-phase doping and current stressing of the semiconductor are some of the techniques that have been used in the past to introduce mid-gap defect states. In this work, we use for the first time, ion bombardment as a novel tool for defect introduction into the semiconductor material of the MSM devices and we compare the electrical, forming and switching characteristics of these devices to those fabricated using the previous techniques mentioned above. A significant observation is that as the density of defects is increased in the semiconductor film with increasing implantation dose, conduction in the devices changes from barrier-controlled thermionic emission to bulk controlled where carriers hop through the defect states in a Poole-Frenkel manner. This transformation eliminates problems associated with Schottky barriers such as quality of contacts, oxidation, etc. In the forming characteristics, not only do we report enhanced uniformity of the forming voltages (VF) but also the magnitude of VF is observed to vary systematically with the implantation dose used. The ON states and switching characteristics in the bombarded devices are also observed to be much more stable presumably as a result of the uniformity of the defects introduced by ion bombardment. We report also an enhanced switching ratio in the ion bombarded devices, especially after partial annealing of the bombardment induced Si dangling bond defects.

Metal-semiconductor-metal (MSM) photodetectors are becoming attractive devices for optoelectronic integrated circuits due to their high speed and simplicity. Optoelectronic matrix switches based on MSM detector arrays offer many advantages such as zero-bias off-state, low bias voltage, high speed and large bandwidth. While in many applications the optical input is coupled in through the top surface of the device, optical signals can also be distributed through transparent waveguides that are located below the absorbing detector layer. Such waveguide-coupled detectors will act as optical taps when the coupling between the waveguide and detector layers is well under control. In this thesis, a 4x4 MSM waveguide-coupled optoelectronic matrix switch was demonstrated and analyzed. The strength of the coupling between the waveguide and detector layers was predicted theoretically and confirmed experimentally. Franz-Keldysh effect in this device was also demonstrated. / Thesis / Master of Engineering (ME)

The design, simulation, and initial fabrication of a novel ultra-compact 2x2 silicon multimode-interference device evanescently coupled to a dual germanium metal-semiconductor-metal (MSM) photodetector is presented. For operation at the standard telecom wavelength of 1.5 µm, the simulations demonstrate high-speed operation at 30 GHz, low dark current in the nanoamp range, and external quantum efficiency of 80%. Error analysis was performed for possible tilt error introduced by hybrid integration of the MSM layer on top of the MMI waveguides by use of surface mount technology (SMT) and direct wafer bonding.

In this paper, the performance simulations of a novel ultra-compact balanced coherent photodetector for operation at a wavelength of 1.5 mu m are presented and design proposals for future fabrication processes are provided. It consists of a compact 2x2 MMI that is evanescently coupled into a germanium MSM photodetection layer. The simulations demonstrate dark current less than 10 nA, capacitance less than 20 fF, and optical bandwidth in the 10-30 GHz range. We propose utilizing the simplicity of direct wafer bonding to bond the detection layer to the output waveguides to avoid complicated epitaxial growth issues. This ultra-compact device shows promise as a high-speed, low-cost integrated silicon photonics solution for the telecommunications infrastructure.

The innovative design of flat panel volume computed tomography (CT) systems has recently led to the emergence of a wide spectrum of new applications for both diagnostic and interventional purposes, such as ultra-high resolution bone imaging, image guided interventions, dynamic CT angiography, and interventional neuroradiology. Most of these applications require low X-ray dose to limit potential harm to the patient. One of the main challenges of low dose imaging is to maintain a quantum noise limited system to achieve the highest possible signal to noise ratio (SNR) at a given dose. One potential method to achieve a quantum noise limited system is to employ a high gain detector. Current flat panel CT technology is based on indirect conversion detectors that contain a scintillator and hydrogenated amorphous silicon (a-Si:H) p-i-n photodetectors which have a gain below unity and require a specialized p-layer. In this thesis, an alternative detector to the p-i-n photodetector, which can achieve gain above unity and thus aid in achieving quantum noise limited systems is investigated for large area flat panel imaging. The proposed detector is based on amorphous selenium (a-Se). Amorphous selenium is the most highly developed photoconductor for large area direct conversion X-ray imaging and is still the only commercially available large area direct conversion flat panel X-ray detector. However, the use of a-Se for indirect conversion imaging has not been significantly explored. Amorphous selenium has field dependent mobility and conversion efficiency, which increase with increasing electric field. It is also the only large area compatible avalanche-capable material; a property that was discovered more than 30 years ago. This unique property could be leveraged to provide the gain necessary for low dose medical imaging applications. The only current commercial avalanche capable a-Se optical detector uses electron beam readout in vacuum, which is not large area compatible and makes integration with pixelated readout electronics challenging. The detector structure proposed in this research seeks to address the challenges associated with integration of an avalanche capable a-Se detector with large area X-ray imager. One important aspect in the development of a-Se avalanche detectors is reducing the dark current and preventing a-Se breakdown as the electric field across the device is increased. A high dark current reduces the dynamic range of the detector, it increases the noise level, and it can lead to crystallization of the detector due to joule heating. To overcome the dark current problem, different blocking layers that allow for integration with large area flat panel imagers were investigated. Experimental results from fabricated devices provided the basis for the choice of the most suitable blocking layer. Two device structures are proposed using the selected blocking layer, a vertical structure and a lateral structure, each having associated benefits and drawbacks. It was shown that introducing a polyimide blocking layer brought down the dark current more than four orders of magnitude at high electric fields and does not deteriorate the charge transport properties of the detectors. The polyimide blocking layer also greatly minimizes physical stress related crystallization in a-Se improving reliability. Gain above unity was observed in the vertical structure and the initiation of impact ionization was verified by performing time-of-flight experiments. Although impact ionization was not verified in the lateral structure, this device structure was found to be highly sensitive to ultraviolet light due to the absence of a top contact layer. Devices were fabricated on several different substrates, including a CMOS substrate, to demonstrate their integration compatibility with large area readout electronics. The exhibited performance of the vertical device structure demonstrates that it is a suitable alternative to the p-i-n photodetector for low dose imaging applications.