Design and Fabrication of Vertically-Integrated CMOS Image Sensors

Technologies to fabricate integrated circuits (IC) with 3D structures are an emerging trend in IC design. They are based on vertical stacking of active components to form heterogeneous microsystems. Electronic image sensors will benefit from these technologies because they allow increased pixel-level data processing and device optimization. This paper covers general principles in the design of vertically-integrated (VI) CMOS image sensors that are fabricated by flip-chip bonding. These sensors are composed of a CMOS die and a photodetector die. As a specific example, the paper presents a VI-CMOS image sensor that was designed at the University of Alberta, and fabricated with the help of CMC Microsystems and Micralyne Inc. To realize prototypes, CMOS dies with logarithmic active pixels were prepared in a commercial process, and photodetector dies with metal-semiconductor-metal devices were prepared in a custom process using hydrogenated amorphous silicon. The paper also describes a digital camera that was developed to test the prototype. In this camera, scenes captured by the image sensor are read using an FPGA board, and sent in real time to a PC over USB for data processing and display. Experimental results show that the VI-CMOS prototype has a higher dynamic range and a lower dark limit than conventional electronic image sensors

Multi material rectifying device fibers

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2009.Cataloged from PDF version of thesis.Includes bibliographical references.Electronic and optoelectronic device processing is commonly thought to be incompatible with much simpler thermal drawing techniques used in optical fiber production. The incorporation of metals, polymer insulators, and chalcogenide semiconductors into structured fibers has reversed this paradigm and made it possible to realize optoelectronic device functionalities at fiber optic length scales and cost. In spite of the surprising robustness of this processing technique, the electronic performance and complexity of these optoelectronic fiber devices has been constrained by the small set of materials compatible with the fabrication method and the disordered nature of the semiconductor. Specifically, the high density of defects inherent to the amorphous chalcogenide semiconductors precludes the ability to create spatially extended internal electric fields necessary to create more sophisticated devices such as diodes and transistors. In this work, the design, fabrication, and characterization of the first fiber-integrated diode is described. The relevant optical, thermal, and electronic properties of candidate materials compatible with the thermal fiber drawing process are described and measured. Phase changing semiconductors are incorporated into the fiber having both amorphous properties amenable to thermal drawing and crystalline properties ideal for electronic devices. Combinations of metals and semiconductors that form both blocking and non-blocking contacts are identified and combined to form the first diode device that is compatible with the thermal drawing process. Techniques are developed to reduce the dimensions of the resulting devices by an order-of- magnitude compared to all previous multimaterial device fibers.(cont.) A series of measurements of both compositional and potential spatial variation are used to determine that compound formation at specific metal semiconductor interfaces control the rectifying behavior of the fiber integrated rectifying junction. This work demonstrates the ability to synthesize compounds during fiber drawing to create complex electronic structures and combine them to form basic building blocks of circuits into arbitrary long fiber, paving the way to increasingly complex electronic structures and truly intelligent fibers and fabrics.by Nicholas D. Orf.Ph.D

Electrical excitation of colloidally synthesized quantum dots in metal oxide structures

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 163-172).This thesis develops methods for integrating colloidally synthesized quantum dots (QDs) and metal oxides in optoelectronic devices, presents three distinct light emitting devices (LEDs) with metal oxides surrounding a QD active layer, and uses these novel metal oxide based QD-LEDs to study mechanisms for electrical excitation of QDs. QD-LEDs have generated considerable interest for applications such as thin film displays with improved color saturation and white lighting with high color rendering index. This work demonstrates that air-stable metal oxides can be used to achieve QD-LEDs that have long shelf lives and operate at constant luminance in ambient conditions, unpackaged. Because metal oxides range from conductors to dielectrics, they can be used to develop a variety of different device architectures to explore mechanisms for electrical excitation of QDs. We report the first all-inorganic QD-LEDs with n- and p-type metal oxide charge transport layers and present design rules to enable systematic improvement of device efficiency. To shift away from direct charge injection as a means for electroluminescence (EL) in inorganic-based QD-LED structures, we develop a unipolar device architecture that presents the first evidence of field driven EL in QDs. To further explore this field driven excitation mechanism, we develop a structure that situates QDs between two insulating metal oxide layers. By eliminating the need for energy band alignment, these devices enable EL from QDs with emission peaks from 450 nm-1500 nm as well as from novel nanoparticles, such as phosphor doped-core/shell nanocrystals.by Vanessa Claire Wood.Ph.D

Experimental Techniques in Nuclear and Particle Physics

I have been teaching courses on experimental techniques in nuclear and particle physics to master students in physics and in engineering for many years. This book grew out of the lecture notes I made for these students. The physics and engineering students have rather different expectations of what such a course should be like. I hope that I have nevertheless managed to write a book that can satisfy the needs of these different target audiences. The lectures themselves, of course, need to be adapted to the needs of each group of students. An engineering student will not qu- tion a statement like “the velocity of the electrons in atoms is ?1% of the velocity of light”, a physics student will. Regarding units, I have written factors h and c explicitly in all equations throughout the book. For physics students it would be preferable to use the convention that is common in physics and omit these constants in the equations, but that would probably be confusing for the engineering students. Physics students tend to be more interested in theoretical physics courses. However, physics is an experimental science and physics students should und- stand how experiments work, and be able to make experiments work. This is an open access book. ; I have been teaching courses on experimental techniques in nuclear and particle physics to master students in physics and in engineering for many years. This book grew out of the lecture notes I made for these students. The physics and engineering students have rather different expectations of what such a course should be like. I hope that I have nevertheless managed to write a book that can satisfy the needs of these different target audiences. The lectures themselves, of course, need to be adapted to the needs of each group of students. An engineering student will not qu- tion a statement like “the velocity of the electrons in atoms is ?1% of the velocity of light”, a physics student will. Regarding units, I have written factors h and c explicitly in all equations throughout the book. For physics students it would be preferable to use the convention that is common in physics and omit these constants in the equations, but that would probably be confusing for the engineering students. Physics students tend to be more interested in theoretical physics courses. However, physics is an experimental science and physics students should und- stand how experiments work, and be able to make experiments work

NASA patent abstracts bibliography: A continuing bibliography. Section 1: Abstracts (supplement 23)

Abstracts are cited for 129 patents and patent applications introduced into the NASA scientific and technical information system during the period January 1983 through June 1983. Each entry consists of a citation, an abstract, and in most cases, a key illustration selected from the patent or patent application

Mechanochemical Synthesis of Functional Layered Materials

As society continues to create new digital content, the telecommunications industry is seeking new technologies to enable higher bandwidth and lower costs to keep pace with the growing demand. Two-dimensional black phosphorus is proposed as a replacement for III-V compound semiconductors as the optically active material in next-generation silicon photonics as it can enable device scaling with lower power consumption. Therefore, the primary motivation of this dissertation was to investigate BP synthesis and chemical doping using an industrially scalable process, high energy ball milling. Initially, the work focused on understanding the ball mill conversion kinetics of red to black phosphorus, hitherto unknown, and is detailed in Chapter 2. The process follows a nucleation and growth dominated mechanism whose rate is controlled by the collision energy and milling intensity. Photoluminescence on mechanochemically synthesized BP showed visible and infrared emissions at the few-layer limit, indicating this process route provides optically viable BP suitable for silicon photonics. To address feasibility of doping, arsenic alloys with phosphorus were subsequently produced by ball milling in order to better understand how the crystal structure changes with substitutional doping; this work is described in Chapter 3. A similar conversion kinetics study was also performed showing a two-step mechanism. First, within a few minutes of milling, the trigonal PAs structure forms followed by a much slower phase transformation to the orthorhombic structure. This work provided a solid benchmark for how substitutional atoms affects the crystal structure, vibrational modes, binding energies, and photoluminescence. Candidate dopants for BP beyond arsenic included germanium, sulfur, selenium, and tellurium. Milling results for germanium phosphides are presented in Chapter 4 and results for phosphorus with sulfur, selenium, and tellurium are presented in Chapter 5. Germanium appears to dope BP (\u3c 1 \u3e at% Ge) as do sulfur (\u3c 10 \u3eat%) and selenium (\u3c 10 \u3e at%). Tellurium does not appear to form a stable dopant with black phosphorus via ball milling. Higher concentrations produced layered trigonal and monoclinic Ge-P crystals, while several crystalline and amorphous phosphorus sulfides and selenides are synthesized by this novel route. Together, Chapters 3-5 indicated that mechanochemical doping of BP with arsenic, germanium, sulfur and selenium is feasible with future work to explore electrical measurements. Finally, within the appendix, a discussion is presented challenges for ball mill doping of BP including milling material, red phosphorus purity, and candidate dopants; limited structural characterization of BP doped with germanium and selenium are included. Less comprehensive work on ball mill reactions of phosphorus with boron, tin, antimony, and bismuth are also reported in the appendix. These results confirmed inability to form phosphorus antimonides while several of the known tin phosphides were successfully synthesized. Independent of the black phosphorus work, a separate study on the synthesis of several intermetallic half-Heuslers for thermoelectric applications is also included in the Appendix. The half-Heusler work shows the versatility of ball milling to synthesize a wide range of intermetallic compounds and revealed nuances regarding challenges of milling together high temperature refractory metals, transition metals, and soft metalloids, in terms of particle size reduction, single phase synthesis, and milling media contamination

Amorphous Selenium (a-Se) and its Compounds: Photo-induced Metastability and Application in a Novel Gamma Camera

Despite the large number of successful commercial applications of chalcogenide glasses (ChGs) ranging from memory devices to photonics and medical imaging detectors, the understanding of a fundamental property – photo-induced structural metastability is not yet complete. The inherent trend of amorphous chalcogenides to convert to its crystalline counterpart, can on the one hand, be directly utilized in phase-change memories, while on the other hand can degrade glass properties for applications in sensing. Furthermore, because the structure of amorphous semiconductors is not fixed by thermodynamic equilibrium conditions, its transformation can be triggered by optical/X-ray excitation and influenced by thermal heating or applied electric field. Each case has its own peculiarities of transformation. Thus, to ensure effective application of a given ChG, a solid understanding of the causes for the structural transformation on the microscopic level has to be developed. This thesis is devoted to the study of photo-induced metastability of the most widely used ChG – amorphous selenium (a-Se), and archetypal a-GexSe100-x glass compounds relevant for applications in sensing. We focus on the research of defect creation and defect relaxation processes in these materials since it is hypothesized to be the root cause for structural transformations affecting stability. The detailed experimental and theoretical study is carried out to reveal the pathways from a micro-level, i.e. defect creation/relaxation, to the macro-level photo-induced effects formation, namely photodarkening (PD), photobleaching (PB) and photo-induced crystallization (PC) and their kinetics. The effects of temperature and energy of excitation on PD kinetics are studied in a- Se. Distinctly different PD effects are observed in the case of sub-bandgap and above- bandgap excitation. It is found that above-bandgap excitation causes only transient photodarkening with a temperature independent relaxation time and no significant structural rearrangements. In contrast, sub-bandgap excitation causes both transient and metastable PD effects. Whereas the mechanism responsible for transient PD for sub- bandgap excitation is analogous to that in the case of above-bandgap excitation, metastable PD is controlled by the formation of self-trapped excitons resulting in structural transformations into configuration defects. Subsequent relaxation in the latter case is shown to be a thermally activated process at elevated temperatures; or configuration tunneling below the room temperature. It is confirmed that if left unrestored, metastable PD acts as a precursor for photo-induced crystallization. Detailed investigation of the effects of different substrates and temperature on photo- induced crystallization in a-Se demonstrates that the onset of PC is suppressed by softening of the film-substrate interface and/or by operating at temperatures near the glass transition. Further, photodarkening and photobleaching effects are investigated in a-GexSe100-x as a function of composition across the glass forming region. The Ge:Se ratio is found to play a decisive role in the observed effects. The critical concentration of Ge ≈30% is highlighted to correspond to the crossover from transient PB to a mixture of transient PD and metastable PB. The underlying microscopic mechanism is governed by the availability of lone pair (LP) states at Se atoms. For low-Ge content films (x<20%) LPs are primarily involved in photoexcitation, causing a photodarkening effect (like in a-Se). As Ge concentration reaches 30%, Ge-Se bond breakage becomes prevalent under light exposure, which results in the generation of dangling bonds and the saturation of the available LP states. This is found to be responsible for the transient PB effect. With further increase of Ge %, the amount of homopolar Ge-Ge bonds starts to increase along with the obvious deficiency of lone pairs. This leads to Ge-Ge bond breakage upon excitation, which causes transient photodarkening. In the post-excitation period, however, it initiates transition to a more ordered state with the main feature of the formation of 3D nanostructures. This is reflected in a bandgap increase and an experimentally observed metastable photobleaching effect. Finally, our understanding of a-Se properties allowed us to extend its application in low-energy nuclear medicine devices. A novel fluence integrating gamma camera is proposed, which if commercialized, will be used to guide advanced breast cancer treatment that involves the placement of low-energy radioactive seeds. The feasibility of the proposed approach is confirmed in realistic breast phantom studies. Our results demonstrate the potential of the gamma camera to fulfill the clinical requirements of both spatial resolution and sensitivity

Synthesis and characterization of hybrid multiwalled carbon nanotube-polycaprolactone-selenium nanoparticles nanofibres and its antibacterial properties

Antibacterial materials are particularly important nowadays in many applications such as disinfecting surfaces and maintaining a healthy, clean, and safe environment. This is to prevent any bacterial infection and kill potentially harmful microbes that can cause morbidity and mortality. Hybrid poly-(ε-caprolactone) (PCL) nanofibres are biodegradable antibacterial biomaterials that are essential for preventing and combating dangerous bacterial infections. Hybrid PCL nanofibres were synthesised by incorporating multi-walled carbon nanotubes (MWCNTs) or/and selenium nanoparticles (SeNPs) with PCL nanofibres. Firstly, MWCNTs were purified and functionalised using mild acid, followed by the synthesis of SeNPs by oxidising selenious acid (H2SeO3) with ascorbic acid. The carboxyl group was attached to the MWCNTs surface while trigonal SeNPs were successfully synthesised with a purity of 97.15%. The synthesis of PCL nanofibres with nanoparticles at different concentrations by electrospinning was optimised, having concentrations of 0.08 wt.% and 0.6 wt.% of MWCNTs and SeNPs, respectively. FESEM images showed the formation of aligned fibres with a size of less than 530 nm. The FESEM images confirmed that PCL-MWCNTs-SeNPs nanofibres degraded faster followed by PCL-SeNPs, PCL-MWCNTs, and PCL nanofibres. The presence of nanoparticles enhanced the biodegradation process by the agglomeration of nanofibres before holes appeared, degrading the nanofibres. The inhibition zone for PCL-MWCNTs, PCL-SeNPs, and PCL-MWCNTs-SeNPs nanofibres against Escherichia coli was around 9–13 mm and 10–16 mm for Staphylococcus aureus. The synergistic effects of MWCNTs and SeNPs in PCL-MWCNTs-SeNP nanofibres began degradation in the fourth month and are more effective in inhibiting E. coli and S. aureus. The characteristics of PCL nanofibres are still maintained and capable of decreasing the hydrophobicity and enhancing the biodegradation rate, as well as antibacterial properties of the hybrid PCL nanofibres. From this study, hybrid PCL nanofibres have the potential to be used in broad and various applications, from daily products to specific applications, especially in healthcare and medical applications

Electrical properties of amorphous selenium based photoconductive devices for application in x-ray image detectors

In the last 10-15 years there has been a renewed interest in amorphous Se (a-Se) and its alloys due to their application as photoconductor materials in the new fully digital direct conversion flat panel x-ray medical image detectors. For a number of reasons, the a-Se photoconductor layer in such x-ray detectors has to be operated at very high electric fields (up to 10 Volts per micron) and one of the most difficult problems related to such applications of a Se is the problem of the dark current (the current in the absence of any radiation) minimization in the photoconductor layer. This PhD work has been devoted to researching the possibilities for dark current minimization in a-Se x-ray photoconductors devices through a systematic study of the charge transport (carrier mobility and carrier lifetimes) and dark currents in single and multilayered a-Se devices as a function of alloying, doping, deposition condition and other fabrication factors. The results of the studies are extensively discussed in the thesis. We have proposed a new technological method for dark current reduction in single and multilayered a-Se based photoconductor for x-ray detector applications. The new technology is based on original experimental findings which demonstrate that both hole transport and the dark currents in a-Se films are a very strong function of the substrate temperature (Tsubstrate) during the film deposition process. We have shown that the new technique reduces the dark currents to approximately the same levels as achievable with the previously existing methods for dark current reduction. However, the new method is simpler to implement, and offers some potential advantages, especially in cases when a very high image resolution (20 cycles/mm) and/or fast pixel readout (more than 30 times per second) are needed. Using the new technology we have fabricated simple single and double (ni-like) photoconductor layers on prototype x-ray image detectors with CCD (Charge Coupled Device) readout circuits. Dark currents in the a-Se photoconductor layer were not a problem for detector operation at all tested electric fields. Compared to the currently available commercial systems for mammography, the prototype detectors have demonstrated an excellent imaging performance, in particular superior spatial resolution (20 cycles/mm). Thus, the newly proposed technology for dark current reduction has shown a potential for commercialization