Monolithically Integrable Si-Compatible Light Sources

On the road to integrated optical circuits, the light emitting device is considered the bottleneck preventing us from arriving to the fully monolithic photonic system. While the development of silicon photonics keeps building momentum, the indirect bandgap nature of silicon represents a major problem for obtaining an integrated light source. Novel nanostructured materials based on silicon, such as silicon-rich oxide (SRO) containing silicon nanoparticles, present intense luminescence due to quantum phenomena. Using this material, electroluminescent devices have already been fabricated and even integrated in monolithic photonic circuits by fully complementary metal oxide semiconductor (CMOS) compatible techniques, opening the door to seamless electronic and photonic integration. The present work discusses some of the strategies used to improve the performance of SRO-based electroluminescent devices fully compatible with CMOS technology. Results from the characterization of devices obtained using different approaches are presented and compared

Luminescent Devices Based on Silicon-Rich Dielectric Materials

Luminescent silicon‐rich dielectric materials have been under intensive research due to their potential applications in optoelectronic devices. Silicon‐rich nitride (SRN) and silicon‐rich oxide (SRO) films have been mostly studied because of their high luminescence and compatibility with the silicon-based technology. In this chapter, the luminescent characteristics of SRN and SRO films deposited by low‐pressure chemical vapor deposition are reviewed and discussed. SRN and SRO films, which exhibit the strongest photoluminescence (PL), were chosen to analyze their electrical and electroluminescent (EL) properties, including SRN/SRO bilayers. Light emitting capacitors (LECs) were fabricated with the SRN, SRO, and SRN/SRO films as the dielectric layer. SRN‐LECs emit broad EL spectra where the maximum emission peak blueshifts when the polarity is changed. On the other hand, SRO‐LECs with low silicon content (~39 at.%) exhibit a resistive switching (RS) behavior from a high conduction state to a low conduction state, which produce a long spectrum blueshift (~227 nm) between the EL and PL emission. When the silicon content increases, red emission is observed at both EL and PL spectra. The RS behavior is also observed in all SRN/SRO‐LECs enhancing an intense ultraviolet EL. The carrier transport in all LECs is analyzed to understand their EL mechanism

Synthesis and Luminescent Properties of Silicon Nanocrystals

Nowadays, study of silicon-based visible light-emitting devices has increased due to large-scale microelectronic integration. Since then different physical and chemical processes have been performed to convert bulk silicon (Si) into a light-emitting material. From discovery of Photoluminescence (PL) in porous Silicon by Canham, a new field of research was opened in optical properties of the Si nanocrystals (Si-NCs) embedded in a dielectric matrix, such as SRO (silicon-rich oxide) and SRN (silicon-rich nitride). In this respect, SRO films obtained by sputtering technique have proved to be an option for light-emitting capacitors (LECs). For the synthesis of SRO films, growth parameters should be considered; Si-excess, growth temperature and annealing temperature. Such parameters affect generation of radiative defects, distribution of Si-NCs and luminescent properties. In this chapter, we report synthesis, structural and luminescent properties of SRO monolayers and SRO/SiO2 multilayers (MLs) obtained by sputtering technique modifying Si-excess, thickness and thermal treatments

Fabrication of electroluminescent silicon diodes by plasma ion implantation

This thesis describes the fabrication and testing of electroluminescent diodes made from silicon subjected to plasma ion implantation. A silicon-compatible, electrically driven light source is desired to increase the speed and efficiency of short-range data transfer in the communications and computing industries. As it is an indirect band gap material, ordinary silicon is too inefficient a light source to be useful for these applications. Past experiments have demonstrated that modifying the structural properties of the crystal can enhance its luminescence properties, and that light ion implantation is capable of achieving this effect. This research investigates the relationship between the ion implantation processing parameters, the post-implantation annealing temperature, and the observable electroluminescence from the resulting silicon diodes. Prior to the creation of electroluminescent devices, much work was done to improve the efficiency and reliability of the fabrication procedure. A numerical algorithm was devised to analyze Langmuir probe data in order to improve estimates of implanted ion fluence. A new sweeping power supply to drive current to the probe was designed, built, and tested. A custom software package was developed to improve the speed and reliability of plasma ion implantation experiments, and another piece of software was made to facilitate the viewing and analysis of spectra measured from the finished silicon LEDs. Several dozen silicon diodes were produced from wafers implanted with hydrogen, helium, and deuterium, using a variety of implanted ion doses and post-implantation annealing conditions. One additional device was fabricated out of unimplanted, unannealed silicon. Most devices, including the unimplanted device, were electroluminescent at visible wavelengths to some degree. The intensity and spectrum of light emission from each device were measured. The results suggest that the observed luminescence originated from the native oxide layer on the surface of the ion-implanted silicon, but that the intensity of luminescence could be enhanced with a carefully chosen ion implantation and annealing procedure

Conducting metal oxide materials for printed electronics

Printed electronics as a manufacturing process has many advantages, mainly, it allows for the high throughput rapid fabrication of thin, flexible electronic components with minimal waste. There are many printing processes that can be utilised for printing electronics and although each process can differ vastly, the materials currently used in these processes are generally the same, silver and carbon. However, to develop printing as a more mainstream manufacturing method for electronics, a wider variety of materials are required which can provide better stability and longevity of components, new functionality for printed applications and allow for in-situ processing and tuning of components. Conducting metal oxides are a good candidate for integrating into printed electronics processes, these materials are typically semiconductors, they have bandgaps, and properties can be altered via altering the band gap. They are also oxides, so they cannot oxidise further and therefore atmospheric damage is reduced compared to pure metals. They can also be fabricated into a wide range of particle morphologies, all with advantages in different fields and electronic applications. Therefore, the ability to print these materials is valuable to the field. In this thesis, the integration of conducting metal oxide electro-ceramic materials into the field of printed electronics has been explored. This was performed through the completion of five research objectives including, the selection of appropriate materials for the research, the formulation of conductive inks with the materials, the investigation of post-processing techniques for printed films and further research into passive component fabrication and sensor applications. Firstly, following an extensive literature review, four materials were selected including three doped zinc oxide materials synthesised via different methods. The fourth material is commercially sourced indium tin oxide (ITO). A nitrocellulose vehicle was determined to be the most compatible with the oxides and selected for ink formulation. Inks were then formulated with all four materials, with optical and electrical properties analysed. Gallium doped Zinc Oxide (GZO) and ITO were selected for further investigation based on the excellent conductivity of the indium tin oxide (57.77Ω□-1) and the highly transparent optical properties of the gallium doped zinc oxide (>84% transmittance). Laser processing was selected as a post processing method. It was found that the laser processing dramatically increased conductivity. The GZO improving from a non-conductive film to 10.21% of bulk conductivity. The ITO improved from 3.46% to 40.47% of the bulk conductivity. It was also found that the laser processing invoked a carbothermal reduction process allowing for a rapid manufacturing process for converting spherical particles into useful nanoparticle morphologies (nanorods, nanowires etc). Following this, resistive and capacitive applications involving laser processing and conventionally heat-treated conductive oxide inks were developed. Combining the new materials and manufacturing processes, tuneable printed resistors with a tuning range of 50 to 20M could be fabricated. All metal oxide, ITO based capacitors were also fabricated and characterised. These were then developed into humidity sensors which provided excellent humidity sensing properties, showing linearity between 5 and 95% relative humidity (RH) and sensitivities of up to 7.76pF/RH%, demonstrating higher performance than commercial equivalents (0.2 – 0.5pF/RH%). In conclusion, this work provides a breakthrough for conductive metal oxide materials research and its place in Printed Electronics research by providing insight into the processes required to make these materials conduct and by developing useful manufacturing methods, post processing techniques and applications.</div

Novel Processing and Electrical Characterization of Nanowires

This thesis investigates novel electrical nanowire characterization tools and devices. Conventional characterization methods, long available to bulk semiconductor samples, have been adapted and transferred to the nanowire geometry. The first part of the thesis describes the development of Hall effect measurements, an entirely new characterization tool for nanowires. It is shown that Hall effect measurements can be performed on InP core-shell nanowires using a self-aligned lifting layer. By combining experimental data with results from simulations of band diagrams and current distribution under the influence of a magnetic field, the carrier density in the n-type nanowire shell can be determined. We found that the nanowire shell exhibits a doping gradient along the length of the nanowire. This doping inhomogeneity is important to account for and engineer when making devices using InP nanowires. The second part of the thesis demonstrates how capacitance-voltage measurements can be performed on arrays of InAs nanowires. Using a novel device structure, the capacitance signal from the nanowires can be distinguished from parasitic capacitances. A model was developed to simulate the capacitance-voltage behavior of the nanowires and was fitted to the experimental data to extract the doping concentration. Furthermore, the hysteresis observed in the capacitance-voltage sweeps was used to calculate the trap density close to the InAs-HfO2 dielectric interface. Finally, studies of gate effects in nanowires are presented. We demonstrate how the gating efficiency is improved by means of wrapped or semiwrapped gates on nanowires. Field-effect transistors made from InP nanowires are described that exhibit both ambipolar behavior and gating efficiency only 13% lower than the theoretical limit. These properties were used to fabricate a field-effect diode, a device in which a p-n junction is formed without any doping incorporation in the active region. We also demonstrate how nanowires positioned laterally on a substrate can be equipped with a fully wrapped gate electrode. This device was developed as part of a platform to perform basic research in which a uniform gating effect is desired

Electroluminescent mis structures incorporating langmuir-blodgett films

The Langmuir-Blodgett {LB) technique provides an excellent method of depositing thin, uniform, insulating films of accurately defined thickness. They can therefore be used effectively in the investigation of metal/thin insulator/ semiconductor electroluminescent structures where the insulator requirements are good dielectric properties allied to uniformity and an accurately defined thickness. The insertion of a fatty acid LB film into the gold/n-type GaP system produces an increase in the effective barrier height of the device and also enables electroluminescence to be observed. The electroluminescent efficiency is shown to depend on the thickness of the organic film whereas the increase in the effective barrier height is relatively independent of this parameter. The optimum efficiency is obtained for a film thickness of approximately 27 mm, well above that predicted on the basis of direct quantum mechanical tunnelling, The increase in the effective barrier height is shown not to be due to an increased band bending in the semiconductor and a simple energy band model, which accounts for most of the experimental observations, is proposed. Measurements made on the diodes under illumination both support the proposed model and provide information about the mechanism of minority carrier injection. Diodes fabricated using phthalocyanine as the LB film exhibit very different characteristics, in particular the optimum thickness for EL efficiency is approximately 5.6 mm and the diodes appear to conform to the conventional tunnel injection theory. The prospects for commercial exploitation are quite promising in the case of the phthalocyanine-based diodes, particularly if the system can be extended to incorporate an efficient II-VI phosphor as the luminescent material. The results of preliminary experiments using ZnSe as the semiconductor are encouraging in this respect. Preliminary results for two other potential applications of LB films in metal/insulator/semiconductor devices are also described. The first of these concerns an attempt to invert the surface of p-type GaAs, with a view to producing an n-channel inversion mode field effect transistor, and the second describes the high field injection of charge into silicon dioxide, which has potential applications in the field of semiconductor memory devices

Novel Concepts For Alternating Current Operated Organic Light-Emitting Devices

Inorganic alternating current electroluminescent devices (AC-ELs) are known for their ruggedness and extreme long-term reliability, which is why they can often been found in industrial and medical equipment as well as in applications in the military sector. In contrast to the inorganic phosphors used in AC-ELs, organic materials offer a number of advantages, in particular a significantly higher efficiency, easier processibility, and a wide selection of emitter materials spanning the entire visible spectrum. Several efforts towards alternating current driven organic light-emitting devices have recently been made, however, important operating mechanism are still not well understood. In the first part of this theses, alternating current driven, capacitively coupled, pin-based organic light-emitting devices are investigated with respect to the influence of the thickness of the insulating layer and the intrinsic organic layer on the driving voltage. A three-capacitor model is employed to predict the basic behavior of the devices and good agreement with the experimental values is found. The proposed charge regeneration mechanism based on Zener tunneling is studied in terms of field strength across the intrinsic organic layers. A remarkable consistency between the measured field strength at the onset point of light emission (3–3.1 MV/cm) and the theoretically predicted breakdown field strength of around 3 MV/cm is obtained. The latter value represents the field required for Zener tunneling in wide band gap organic materials according to Fowler-Nordheim theory. In a second step, asymmetric driving of capacitively coupled OLEDs is investigated. It is found that different voltages and/or pulse lengths for positive and negative half-cycle lead to significant improvements in terms of brightness and device efficiency. Part two of this work demonstrates a device concept for highly efficient organic light-emitting devices whose emission color can be easily adjusted from, e.g., deep-blue through cold-white and warm-white to saturated yellow. The presented approach exploits the different polarities of the positive and negative half-cycles of an alternating current driving signal to independently address a fluorescent blue emission unit and a phosphorescent yellow emission unit vertically stacked on top of each other. The electrode design is optimized for simple fabrication and driving and allows for two-terminal operation by a single source. The presented approach for color-tunable OLEDs is versatile in terms of emitter combinations and meets application requirements by providing a high device efficiency of 36.2 lm/W, a color rendering index of 82 at application relevant brightness levels of 1000 cd/m², and warm-white emission color coordinates. The final part demonstrates an approach for full-color OLED pixels that are fabricated by vertical stacking of a red-, green-, and blue-emitting unit. Each unit can be addressed separately which allows to efficiently generate every color that is a superposition of spectra of the individual emission units. The device is built in a top-emission geometrywhich is highly desirable for display fabrication as the pixel can be directly deposited onto the back-plane electronics. Furthermore, the presented device design requires only three independently addressable electrodes which simplifies fabrication and electrical driving. The electrical performance of each individual unit is on par with standard pin single emission unit OLEDs, showing very low leakage currents and achieving high brightness levels at moderate voltages of around 3–4 V

Top-Gate Nanocrystalline Silicon Thin Film Transistors

Thin film transistors (TFTs), the heart of highly functional and ultra-compact active-matrix (AM) backplanes, have driven explosive growth in both the variety and utility of large-area electronics over the past few decades. Nanocrystalline silicon (nc-Si:H) TFTs have recently attracted attention as a high-performance and low-cost alternative to existing amorphous silicon (a-Si:H) and polycrystalline silicon (poly-Si) TFTs, in that they have the strong potentials which a-Si:H (low carrier mobility and poor device stability) and poly-Si (poor device uniformity and high manufacturing cost) counterparts do not have. However, the current nc-Si:H TFTs expose several challenging material and devices issues, on which the dissertation focuses. In our material study, the growth of gate-quality SiO2 films and highly conductive nc-Si:H contacts based on conventional plasma-enhanced chemical vapor deposition (PECVD) is systematically investigated, which can lead to high performance, reproducibility, predictability, and stability in the nc-Si:H TFTs. Particularly to overcome a low field effect mobility in the p-channel transistors, the possibility of B(CH3)3 as an alternative dopant source to current B2H6 is examined. The resultant p-doped nc-Si:H contacts demonstrate comparable performance to the state of the art with the maximum dark conductivity of 1.11 S/cm over 70% film crystallinity. Based on the highest-quality SiO2 and nc-Si:H contacts developed, complementary (n- and p-channel) top-gate nc-Si:H TFTs with a staggered source/drain geometry are designed, fabricated, and characterized. The n-channel TFTs demonstrate a threshold voltage VTn of 6.4 V, a field effect mobility of electrons μn of 15.54 cm2/Vs, a subthreshold slope S of 0.67 V/decade, and an on/off current ratio Ion/Ioff of 10^5, while the corresponding p-channel TFTs exhibit VTp of -26.2 V, μp of 0.24 cm2/Vs, S of 4.72 V/ decade, and Ion/Ioff of 10^4. However, the TFTs show significant non-ideal behaviors that considerably limit device performance: high leakage current in the off-state, transconductance degradation under high gate bias, and threshold voltage instability in time. Quantitative insight into each non-ideality is provided in this research. Our study on the off-state conduction in the nc-Si:H TFTs reveals that the responsible mechanism for high leakage current, particularly at a high bias regime, is largely due to Poole-Frenkel emission of trapped carriers in the reverse-biased drain depletion region. This could be effectively suppressed by proposed offset-gated structure without compromising the on-state performance. A numerical analysis of the transconductance degradation shows that the parasitic resistance components that are present in the nc-Si:H TFTs strongly degrade transconductance and thus a field effect mobility. Correspondingly, strategies for reduction in parasitic resistance of the TFT are presented. Lastly, the threshold voltage shift in the nc-Si:H TFT is attributed to the flatband voltage shift, which is mainly due to charge trapping in the PECVD SiO2 gate dielectric. Material and device study, and physical insight into non-ideal behaviors in the top-gate nc-Si:H TFTs reported in the dissertation constitute an arguably important step towards monolithic integration of pixels and peripheral driving circuits on a versatile active-matrix TFT backplane for high-performance and low-cost large-area electronics. However, the gate dielectric and the highly doped nc-Si:H contacts, still imposing considerable challenges, may require entirely new approaches