Rozprawy doktorskie na temat „Light Emitting Diode - Semiconductor Nanocrystals”

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2008.
Vita.
Includes bibliographical references.
This thesis employs colloidal semiconductor nanocrystals (NCs) as nanoscale emissive probes to investigate the physics of light emitting diodes (LEDs), as well as to unveil properties of cells that conventional imaging techniques cannot reveal. On the LED side, in particular, Chapter 2 utilizes individual NCs to alter layered organic LED structures at nanometer scale, resulting in spectrally resolved electroluminescence from single colloidal CdSe/ZnS (core/shell) NCs at room temperature. Chapter 3 takes NCs as emissive probes in layered organic LEDs, and shows that the photoluminescence of single NCs is bias dependent which helps elucidate the interactions between NCs and organic semiconductors, knowledge useful for designing efficient NC organic optoelectronics. Instead of using a planar LED geometry, Chapter 4 presents a technique for making nanoscale gap LEDs which allow the spectrally coincidental photoluminescence and electroluminescence from NCs. The work investigates the interactions between NCs and different metal gaps, and suggests electromigrating leads made of different metals as a promising route to fabricating nanoscale gaps with workfunction offsets for optoelectronic devices. On the cell biology side, we develop a three-dimensional sub-diffraction limited single fluorophore imaging method for proteins labeled with NCs. Chapter 5 applies the method to measure the endothelial glycocalyx thickness in vitro for the first time, by labeling different proteins with NCs of different emission wavelengths. Taking a step further, Chapter 6 utilizes the NC based imaging method to investigate the flow induced dynamics of endothelial glycocalyx, and measures the shear modulus of glycocalyx.
by Hao Huang.
Ph.D.

InGaN heterostructures are at the core of blue light emitting diodes (LEDs) which are the basic building blocks for energy efficient and environment friendly modern white light generating sources. Through quantum confinement and electronic band structure tuning on the opposite end of the spectrum, Ge1−xSnx alloys have recently attracted significant interest due to its potential role as a silicon compatible infra-red (IR) optical material for photodetectors and LEDs owing to transition to direct bandgap with increasing Sn. This thesis is dedicated to establishing an understanding of the optical processes and carrier dynamics in InGaN heterostructures for achieving more efficient visible light emitters and terahertz generating nanocavities and in colloidal Ge1−xSnx quantum dots (QDs) for developing efficient silicon compatible optoelectronics. To alleviate the electron overflow, which through strong experimental evidence is revealed to be the dominating mechanism responsible for efficiency degradation at high injection in InGaN based blue LEDs, different strategies involving electron injectors and optimized active regions have been developed. Effectiveness of optimum electron injector (EI) layers in reducing electron overflow and increasing quantum efficiency of InGaN based LEDs was demonstrated by photoluminescence (PL) and electroluminescence spectroscopy along with numerical simulations. Increasing the two-layer EI thickness in double heterostructure LEDs substantially reduced the electron overflow and increased external quantum efficiency (EQE) by three fold. By incorporating δ p-doped InGaN barriers in multiple quantum well (MQW) LEDs, 20% enhancement in EQE was achieved due to improved hole injection without degrading the layer quality. Carrier diffusion length, an important physical parameter that directly affects the performance of optoelectronic devices, was measured in epitaxial GaN using PL spectroscopy. The obtained diffusion lengths at room temperature in p- and n-type GaN were 93±7 nm and 432±30 nm, respectively. Moreover, near field scanning optical microscopy was employed to investigate the spatial variations of extended defects and their effects on the optical quality of semipolar and InGaN heterostructures, which are promoted for higher efficiency light emitters owing to reduced internal polarization fields. The near-field PL from the c+ wings in heterostructures was found to be relatively strong and uniform across the sample but the emission from the c- wings was substantially weaker due to the presence of high density of threading dislocations and basal plane stacking faults. In case of heterostructures, striated regions had weaker PL intensities compared to other regions and the meeting fronts of different facets were characterized by higher Indium content due to the varying internal field. Apart from being the part and parcel of blue LEDs, InGaN heterostructures can be utilized in generation of coherent lattice vibrations at terahertz frequencies. In analogy to LASERs based on photon cavities where light intensity is amplified, acoustic nanocavity devices can be realized for sustaining terahertz phonon oscillations which could potentially be used in acoustic imaging at the nanoscale and ultrafast acousto-optic modulation. Using In0.03Ga0.97N/InxGa1-xN MQWs with varying x, coherent phonon oscillations at frequencies of 0.69-0.80 THz were generated, where changing the MQW period (11.5 nm -10 nm) provided frequency tuning. The magnitude of phonon oscillations was found to increase with indium content in quantum wells, as demonstrated by time resolved differential transmission spectroscopy. Design of an acoustic nanocavity structure was proposed based on the abovementioned experimental findings and also supported by full cavity simulations. Optical gap engineering and carrier dynamics in colloidal Ge1−xSnx QDs were investigated in order to explore their potential in optoelectronics. By changing the Sn content from 5% to 23% in 2 nm-QDs, band-gap tunability from 1.88 eV to 1.61 eV, respectively, was demonstrated at 15 K, consistent with theoretical calculations. At 15 K, time resolved PL spectroscopy revealed slow decay (3 − 27 μs) of luminescence, due to recombination of spin-forbidden dark excitons and effect of surface states. Increase in temperature to 295 K led to three orders of magnitude faster decay (9 − 28 ns) owing to the effects of thermal activation of bright excitons and carrier detrapping from surface states. These findings on the effect of Sn incorporation on optical properties and carrier relaxation and recombination processes are important for future design of efficient Ge1−xSnx QDs based optoelectronic devices. This thesis work represents a comprehensive optical study of InGaN heterostructures and colloidal Ge1−xSnx QDs which would pave the way for more efficient InGaN based LEDs, realization of terahertz generating nanocavities, and efficient Ge1−xSnx based silicon compatible optoelectronic devices.

In this study, silicon nanocrystals (NC) were synthesized in silicon dioxide matrix by ion implantation followed by high temperature annealing. Annealing temperature and duration were varied to study their effect on the nanocrystal formation and optical properties. Implantation of silicon ions was performed with different energy and dose depending on the oxide thickness on the silicon substrate. Before device fabrication, photoluminescence (PL) measurement was performed for each sample. From PL measurement it was observed that, PL emission depends on nanocrystal size determined by the parameters of implantation and annealing process. The peak position of PL emission was found to shifts toward higher wavelength when the dose of implanted Si increased. Two PL emission bands were observed in most cases. PL emission around 800 nm originated from Si NC in oxide matrix. Other emissions can be attributed to the luminescent defects in oxide or oxide/NC interface. In order to see electroluminescence properties Light Emitting Devices (LED) were fabricated by using metal oxide semiconductor structure, current-voltage (I-V) and electroluminescence (EL) measurements were conducted. I-V results revealed that, current passing through device depends on both implanted Si dose and annealing parameters. Current increases with increasing dose as one might expect due to the increased amount of defects in the matrix. The current however decreases with increasing annealing temperature and duration, which imply that, NC in oxide behave like a well controlled trap level for charge transport. From EL measurements, few differences were observed between EL and PL results. These differences can be attributed to the different excitation and emission mechanisms in PL and EL process. Upon comparision, EL emission was found to be inefficient due to the asymmetric charge injection from substrate and top contact. Peak position of EL emission was blue shifted with respect to PL one, and approached towards PL peak position as applied voltage increased. From the results of the EL measurements, EL emission mechanisms was attributed to tunneling of electron hole pairs from top contact and substrate to NC via oxide barrier.

This thesis is about new ways to experimentally realise materials with desired nano-structures for solution-processable optoelectronic devices such as solar cells and light-emitting diodes (LEDs), and examine structure-performance relationships in these devices. Short exciton diffusion length limits the efficiency of most exciton-based solar cells. By introducing nano-structured architectures to solar cells, excitons can be separated more effectively, leading to an enhancement of the cell’s power conversion efficiency. We use diblock copolymer lithography combined with solvent-vapour-assisted imprinting to fabricate nano-structures with 20-80 nm feature sizes. We demonstrate nanostructured solar cell incorporating the high-performance polymer PBDTTT-CT. Furthermore, we demonstrated the patterning of singlet fission materials, including a TIPS-pentacene solar cell based on ZnO nanopillars. Recently perovskites have emerged as a promising semiconductor for optoelectronic applications. We demonstrate a perovskite light-emitting diode that employs perovskite nanoparticles embedded in a dielectric polymer matrix as the emissive layer. The emissive layer is spin-coated from perovskite precursor/polymer blend solution. The resultant polymer-perovskite composites effectively block shunt pathways within the LED, thus leading to an external quantum efficiency of 1.2%, one order of magnitude higher than previous reports. We demonstrate formations of stably emissive perovskite nanoparticles in an alumina nanoparticle matrix. These nanoparticles have much higher photoluminescence quantum efficiency (25%) than bulk perovskite and the emission is found to be stable over several months. Finally, we demonstrate a new vapour-phase crosslinking method to construct full-colour perovskite nanocrystal LEDs. With detailed structural and compositional analysis we are able to pinpoint the aluminium-based crosslinker that resides between the nanocrystals, which enables remarkably high EQE of 5.7% in CsPbI3 LEDs.

Since the advent of precise semiconductor engineering techniques in the 1960s, considerable effort has been devoted both in academia and private industry to the fabrication and testing of complex structures. In addition to other techniques, molecular beam epitaxy (MBE) has made it possible to create devices with single mono-layer accuracy. This facilitates the design of precise band structures and the selection of specific spectroscopic properties for light source materials. The applications of such engineered structures have made solid state devices common commercial quantities. These applications include solid state lasers, light emitting diodes and light sensors. Band gap engineering has been used to design emitters for many wavelength bands, including the short wavelength (SWIR) infrared region which ranges from 1.5 to 2.5 μm [1]. Practical devices include sensors operating in the 2-2.5 μm range. When designing such a device, necessary concerns include the required bias voltage, operating current, input impedance and especially for emitters, the wall-plug efficiency. Three types of engineered structures are considered in this thesis. These include GaInAsSb quaternary alloy bulk active regions, GaInAsSb multiple quantum well devices (MQW) and GaInAsSb cascaded light emitting diodes. The three structures are evaluated according to specific standards applied to emitters of infrared light. The spectral profiles are obtained with photo or electro-luminescence, for the purpose of locating the peak emission wavelength. The peak wavelength for these specimens is in the 2.2-2.5μm window. The emission efficiency is determined by employing three empirical techniques: current/voltage (IV), radiance/current (LI), and carrier lifetime measurements. The first verifies that the structure has the correct electrical properties, by measuring among other parameters the activation voltage. The second is used to determine the energy efficiency of the device, including the wall-plug and quantum efficiencies. The last provides estimates of the relative magnitude of the Shockley Read Hall, radiative and Auger coefficients. These constants illustrate the overall radiative efficiency of the material, by noting comparisons between radiative and non-radiative recombination rates.

Deep ultraviolet (UV) light sources are useful in a number of applications that include sterilization, medical diagnostics, as well as chemical and biological identification. However, state-of-the-art deep UV light-emitting diodes and lasers made from semiconductors still suffer from low external quantum efficiency and low output powers. These limitations make them costly and ineffective in a wide range of applications. Deep UV sources such as lasers that currently exist are prohibitively bulky, complicated, and expensive. This is typically because they are constituted of an assemblage of two to three other lasers in tandem to facilitate sequential harmonic generation that ultimately results in the desired deep UV wavelength. For semiconductor-based deep UV sources, the most challenging difficulty has been finding ways to optimally dope the (Al,Ga)N/GaN heterostructures essential for UV-C light sources. It has proven to be very difficult to achieve high free carrier concentrations and low resistivities in high-aluminum-containing III-nitrides. As a result, p-type doped aluminum-free III-nitrides are employed as the p-type contact layers in UV light-emitting diode structures. However, because of impedance-mismatch issues, light extraction from the device and consequently the overall external quantum efficiency is drastically reduced. This problem is compounded with high losses and low gain when one tries to make UV nitride lasers. In this thesis, we provide a robust and reproducible approach to resolving most of these challenges. By using a liquid-metal-enabled growth mode in a plasma-assisted molecular beam epitaxy process, we show that highly-doped aluminum containing III-nitride films can be achieved. This growth mode is driven by kinetics. Using this approach, we have been able to achieve extremely high p-type and n-type doping in (Al,Ga)N films with high aluminum content. By incorporating a very high density of Mg atoms in (Al,Ga)N films, we have been able to show, by temperature-dependent photoluminescence, that the activation energy of the acceptors is substantially lower, thus allowing a higher hole concentration than usual to be available for conduction. It is believed that the lower activation energy is a result of an impurity band tail induced by the high Mg concentration. The successful p-type doping of high aluminum-content (Al,Ga)N has allowed us to demonstrate operation of deep ultraviolet LEDs emitting at 274 nm. This achievement paves the way for making lasers that emit in the UV-C region of the spectrum. In this thesis, we performed preliminary work on using our structures to make UV-C lasers based on photonic crystal nanocavity structures. The nanocavity laser structures show that the threshold optical pumping power necessary to reach lasing is much lower than in conventional edge-emitting lasers. Furthermore, the photonic crystal nanocavity structure has a small mode volume and does not need mirrors for optical feedback. These advantages significantly reduce material loss and eliminate mirror loss. This structure therefore potentially opens the door to achieving efficient and compact lasers in the UV-C region of the spectrum.

In this thesis we examine the feasibility of developing a white light source capable of producing colors between 2500 and 7500 Kelvin on the black-body radiator spectrum by simply adjusting amperage to a blue and ultraviolet (UV) light emitting diode (LED). The purpose of a lighting source of this nature is to better replicate daylight inside a building at a given time of day. This study analyzes the proposed light source using a 385 nm UV LED, a 457 nm blue LED, a 479 nm blue LED, a 562 nm peak cerium doped yttrium aluminum garnet (YAG:Ce) phosphor, and a 647 nm peak selenium doped zinc sulfide (ZnS:Se) phosphor. Our approach to this study initially examined optical performance of yellow-emitting phosphor (YAG:Ce) positioned at specific distances above a blue LED using polydimethylsiloxane (PDMS) as a substrate. An understanding of how phosphor concentration within the PDMS, the thickness of the PDMS, and how substrate distance from the LED die affected light intensity and color values (determined quantitatively by utilizing the 1931 CIE 2° Standard Observer) enabled equations to be developed for various lens designs to efficiently produce white light using a 457 nm peak wavelength LED. The combination of two luminescent sources (457 nm LED and YAG:Ce) provided a linear trend on the 1931 CIE diagram which required a red illumination source to obtain Kelvin values from 2500 to 7500. Red-emitting phosphor (ZnS:Se), selected to compliment the system, was dispersed with YAG:Ce throughout PDMS where they were stimulated with a blue LED thereby enabling all desired Kelvin values with differing concentration lenses. Stimulating ZnS:Se with the addition of a UV LED did not provide the ability to change the color value of the set up to the degree required. Many other factors resulted in the decision to remove the UV LED contribution from the multi-Kelvin light source design. The final design incorporated a combination of ZnS:Se and YAG:Ce stimulated with a blue LED to obtain a 2500 Kelvin value. A separate blue LED provides the means to obtain 7500 Kelvin light and the other color values in between, with a linear approximation, by adjusting the amperages of both LEDs. In addition to investigating the feasibility of obtaining the Kelvin values from 2500 to 7500, this thesis also examined the problem of ZnS:Se’s inability to cure in PDMS and a method to create a lens shape to provide equal color values at all points above a phosphor converted LED source. ZnS:Se was found to be curable in PDMS if first coated with a low viscosity silicon oil prior to dispersion within PDMS. The lens configuration consists of phosphors equally distributed in PDMS and cured in the shape of a Gaussian distribution unique to multiple factors in LED-based white light design.

The purpose of this thesis work is to design and fabricates organic-inorganic hetero junction White Light Emitting Diode (WLED). In this WLED, inorganic material is n- type ZnO and organic material is p-type conjugated polymer. The first task was to synthesise vertically aligned ZnO nano-rods on glass as well as on plastic substrates using aqueous chemical growth method at a low temperature. The second task was to find out the proper p- type organic material that gives cheap and high efficient WLED operation. The proposed polymer shouldn’t create a high barrier potential across the interface and also it should block electrons entering into the polymer. To optimize the efficiency of WLED; charge injection, charge transport and charge recombination must be considered. The hetero junction organic-inorganic structures have to be engineered very carefully in order to obtain the desired light emission. The layered structure is composed of p-polymer/n-ZnO and the recombination has been desired to occur at the ZnO layer in order to obtain white light emission. Electrical characterization of the devices was carried out to test the rectifying properties of the hetero junction diodes.

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This project focused on reducing the production cost of organic light emitting diodes and organic semiconductors through intuitive engineering design. Included in the thesis is a comprehensive, bottom-up method of design, synthesis, fabrication and analysis of both a set of luminescent materials and semiconductor device structure for use in organic light emitting diodes. The results demonstrate the viability of the method and reveal novel findings about the "thermally activated delayed fluorescence" phenomenon and its application in organic light emitting diodes.

Gallium nitride (GaN) light emitting diodes (LED) embody a large field of research that aims to replace inefficient, conventional light sources with LEDs that have lower power, higher luminosity, and longer lifetime. This thesis presents an international collaboration effort between the State Key Laboratory for Mesoscopic Physics in Peking University (PKU) of Beijing, China and the Electrical Engineering Department of California Polytechnic State University, San Luis Obispo. Over the course of 2 years, Cal Poly’s side has simulated GaN LEDs within the pure blue wavelength spectrum (460nm), focusing specifically on the effects of reflection gratings, transmission gratings, top and bottom gratings, error gratings, 3-fold symmetric photonic crystal, and 2-fold symmetric nano-imprinted gratings. PKU used our simulation results to fabricate GaN high brightness LEDs from the results of our simulation models. We employed the use of the finite difference time domain (FDTD) method, a computational electromagnetic solution to Maxwell’s equations, to measure light extraction efficiency improvements of the various grating structures. Since the FDTD method was based on the differential form of Maxwell’s equations, it arbitrarily simulated complex grating structures of varying shapes and sizes, as well as the reflection, diffraction, and dispersion of propagating light throughout the device. We presented the optimized case, as well as the optimization trend for each of the single grating structures within a range of simulation parameters on the micron scale and find that single grating structures, on average, doubled the light extraction efficiency of GaN LEDs. Photonic crystal grating research in the micron scale suggested that transmission gratings benefit most when grating cells tightly pack together, while reflection gratings benefit when grating cells space further apart. The total number of grating cells fabricated on a reflection grating layer still affects light extraction efficiency. For the top and bottom grating structures, we performed a partial optimization of the grating sets formed from the optimized single grating cases and found that the direct pairing of optimized single grating structures decreases overall light extraction efficiency. However, through a partial optimization procedure, top and bottom grating designs could improve light extraction efficiency by 118% for that particular case, outperforming either of the single top or bottom grating cases alone. Our research then explored the effects of periodic, positional perturbation in grating designs and found that at a 10-15% randomization factor, light extraction efficiency could improve up to 230% from the original top and bottom grating case. Next, in an experiment with PKU, we mounted a 2-fold symmetric photonic crystal onto a PDMS hemi-cylinder by nano-imprinting to measure the transmission of light at angles from near tangential to normal. Overall transmission of light compared with the non-grating design increases overall light extraction efficiency when integrated over the range of angles. Finally, our research focused on the 3-fold symmetric photonic crystal grating structure and employed the use of 3-D FDTD methods and incoherent light sources to better study the effects of higher-ordered symmetry in grating design. Grating cells were discovered as the source of escaping light from the GaN LED model. The model revealed that light extraction efficiency and the far-field diffraction pattern could be estimated by the position of grating cells in the grating design.

This thesis describes the development of III-Nitride materials for light emitting applications. The goals of this research were to create and optimize a green light emitting diode (LED) and laser diode (LD). Metalorganic chemical vapor deposition (MOCVD) was the technique used to grow the epitaxial structures for these devices. The active regions of III-Nitride based LEDs are composed of InₓGa₁₋ₓN, the bandgap of which can be tuned to attain the desired wavelength depending on the percent composition of Indium. An issue with this design is that the optimal growth temperature of InGaN is lower than that of GaN, making the growth temperature of the top p-layers critical to the device performance. Thus, an InGaN:Mg layer was used as the hole injection and p-contact layers for a green led, which can be grown at a lower temperature than GaN:Mg in order to maintain the integrity of the active region. However, the use of InGaN comes with its own set of drawbacks, specifically the formation of V-defects. Several methods were investigated to suppress these defects such as graded p-layers, short period supper lattices, and native GaN substrates. As a result, LEDs emitting at ~532 nm were realized. The epitaxial structure for a III-Nitride LD is more complicated than that of an LED, and so it faces many of the same technical challenges and then some. Strain engineering and defect reduction were the primary focuses of optimization in this study. Superlattice based cladding layers, native GaN substrates, InGaN waveguides, and doping optimization were all utilized to lower the probability of defect formation. This thesis reports on the realization of a 454 nm LD, with higher wavelength devices to follow the same developmental path.

Previous research in InGaN/GaN light emitting diodes (LEDs) employing semi-classical drift-diffusion models has used reduced polarization constants without much physical explanantion. This paper investigates possible physical explanations for this effective polarization reduction in InGaN LEDs through the use of the simulation software SiLENSe. One major problem of current LED simulations is the assumption of perfectly discrete transitions between the quantum well (QW) and blocking layers when experiments have shown this to not be the case. The In concentration profile within InGaN multiple quantum well (MQW) devices shows much smoother and delayed transitions indicative of indium diffusion and drift during common atomic deposition techniques (e.g. molecular beam epitaxy, chemical vapor deposition). In this case the InGaN square QW approximation may not be valid in modeling the devices' true electronic behavior. A simulation of a 3QW InGaN/GaN LED heterostructure with an AlGaN electron blocking layer is discussed in this paper. Polarization coefficients were reduced to 70% and 40% empirical values to simulate polarization shielding effects. QW shapes of square (3 nm), trapezoidal, and triangular profiles were used to simulate realistic QW shapes. The J-V characteristic and electron-hole wavefunctions of each device were monitored. Polarization reduction decreased the onset voltage from 4.0 V to 3.0 V while QW size reduction decreased the onset voltage from 4.0 V to 3.5 V. The increased current density in both cases can be attributed to increased wavefunction overlap in the QWs.

GaN-based light emitting diodes (LEDs) face several challenges if the technology is to make a significant impact on the solid state lighting market. The two most pressing of these challenges are cost and efficiency. The development of alternative substrate technologies shows promise toward addressing both of these challenges, as both GaN-based device technology and the associated metalorganic chemical vapor deposition (MOCVD) technology are already relatively mature. Zinc oxide (ZnO) and silicon (Si) are among the most promising alternative substrates for GaN epitaxy. This work focuses on the development of MOCVD growth processes to yield high quality GaN-based materials and devices on ZnO and Si. ZnO, because of its similar lattice constant and thermal expansion coefficient, is a promising substrate for growth of low defect-density GaN. The major hurdles for GaN growth on ZnO are the instability of ZnO in a hydrogen atmosphere and out-diffusion of zinc and oxygen from the substrate. A process was developed for the MOCVD growth of wurtzite GaN and InxGa1-xN on ZnO, and the structural and optical properties of these films were studied. High zinc and oxygen concentrations remained an issue, however, and the diffusion of zinc and oxygen into the subsequent GaN layer was studied more closely. Silicon is the most promising material for the development of an inexpensive, large-area substrate technology. The challenge in GaN growth on Si is the tensile strain induced by the lattice and thermal mismatch between GaN and Si. A thin atomic layer deposition (ALD)-grown Al2O3 interlayer was employed to relieve strain while also simplifying the growth process. While some strain was still observed, the oxide interlayer leads to an improvement in thin film quality and a reduction in both crack density and screw dislocation density in the GaN films. A comparison of GaN-based LEDs grown on sapphire and Al2O3/Si shows similar performance characteristics for both devices. IQE of the devices on silicon is ~32%, compared to ~37% on sapphire. These results show great promise toward an inexpensive, large-area, silicon-based substrate technology for MOCVD growth of GaN-based optoelectronic devices.

Large area OLEDs show pronounced Joule self-heating at high brightness. This heating induces brightness inhomogeneities, drastically increasing beyond a certain current level. We discuss this behavior considering 'S'-shaped negative differential resistance upon self-heating, even allowing for 'switched-back' regions where the luminance finally decreases (Fischer et al., Adv. Funct. Mater. 2014, 24, 3367). By using a multi-physics simulation the device characteristics can be modeled, resulting in a comprehensive understanding of the problem. Here, we present results for an OLED lighting panel considered for commercial application. It turns out that the strong electrothermal feedback in OLEDs prevents high luminance combined with a high degree of homogeneity unless new optimization strategies are considered.

One of the major challenges in the field of organic semiconductors is to develop molecular design rules and processing routes which optimise the charge carrier mobility, whilst independently controlling the radiative and non-radiative processes. To date there has existed a seeming trade-off between charge carrier mobility and photoluminescence efficiency, which limits the development of some devices such as electrically pumped laser diodes. This thesis investigates fluorescence enhancement strategies for high-mobility polymer semiconductor systems and the mechanisms by which they currently display poor emission properties. Four independent approaches were taken and are detailed as follows. 1. Solubilising chain engineering It is shown that for the high mobility polymer poly(indacenodithiophene-co-benzothiadiazole), the addition of a phenyl- initiated side chain can enhance the solid-state fluorescence quantum yield, exciton lifetime and exciton diffusion length significantly in comparison to that without phenyl-addition. 2. Energy transfer to a highly fluorescent chromophore It is shown that for the high mobility polymer poly(indacenodithiophene-co-benzothiadiazole) efficient energy transfer to a more emissive squaraine dye molecule is possible despite fast non-radiative decay short exciton diffusion lengths. This results in a significant fluorescence enhancement, which in turn facilitates an order of magnitude increase of the efficiency of polymer light emitting diodes made from this material combination. 3. Energy gap engineering The well known Energy Gap Law predicts an increase in the non-radiative rate as the optical bandgap of an organic chromophore decreases in energy. In combination with this, almost all polymer semiconductors reported to date with high charge carrier mobility have low optical bandgaps. Therefore, molecular design principles which act to increase the optical bandgap of polymer semiconductors whilst retaining a high mobility were sought out. One specific system was successfully identified and showed a significant fluorescence enhancement compared to is predecessor poly(indacenodithiophene-co-benzothiadiazole) in both the solution and the solid state. It is found that the Frenkel exciton lifetime in this new system is a factor of four larger which also results in a significantly increased exciton diffusion length. An inter-chain electronic state is also identified and discussed. 4. Hydrogen substitution For some low-bandgap material systems such as erbium chromophores, high energy vibrational modes such as the C-H stretching mode can act as non-radiative pathways. The effect of hydrogen substitution with deuterium and fluorine was therefore investigated in a series of polythiophene derivative families. It was found that in the solid state, fluorescence and exciton lifetime enhancement occurred when the backbone hydrogen atoms were replaced with fluorine. However, evidence is given that this was not owing to the initial hypothesis, and is more likely owing to structural differences which occur in these substituted material systems.

To fully exploit the potential of gallium nitride (GaN) devices for optoelectronics and power electronic applications, the structures of device need to be investigated and optimized. In particular carrier densities, conductivities and localised charges can have a significant impact to device performances. Electrical scanning probe microscopy techniques, including scanning capacitance microscopy (SCM), conductive atomic force microscopy (C-AFM) and kelvin probe force microscopy (KPFM), were utilized to study the structures of nitride devices such as high electron mobility transistors (HEMTs), light emitting diodes (LEDs) and junction diodes. These results combine with other characterisation techniques to give an enhanced understanding about the nitride structures. Leakage currents are one of the major challenges in HEMTs, especially leakages in buffer layers which deteriorate the breakdown voltage of the devices. To achieve an insulating buffer layer, carbon doping is usually used to compensate the unintentional n-type doping of nitride materials. Here, I show that vertical leakage can originate from the formation of inverted hexagonal pyramidal defects during the low temperature growth of an AlGaN:C strain relief layer. The semi-polar facets of the defects enhanced the oxygen incorporation and led to the formation of leakage pathways which were observed using SCM. Leakage occurring at HEMT surfaces will lead to current collapses of devices. In this work, I discovered nano-cracks on a HEMT surface. C-AFM showed enhanced conductivity along these nano-cracks. A model based on stress relaxation analysis was proposed to explain the drop of surface potential along the nano-cracks. Advances in the quality of epitaxial GaN grown by MOVPE have been facilitated by understanding the formation of defects within the materials and structures. However, hillocks as a specific type of defects have not been intensively studied yet. In this work, three types of hillocks were discovered on GaN p-i-n diodes and a GaN film grown on patterned sapphire substrates. It was found that pits were always present around the centres of hillocks. Multi-microscopy results showed these pits were developed from either an inversion domain or a nano-pipe or a void under the sample surface. Formation of hillocks was usually associated with a change of growth condition, such as an increase in Mg doping or a decrease in growth temperature and gas flows, despite the formation mechanism is still unclear. GaN$_{1-x}$As$_x$ is a highly mismatched alloy semiconductor whose band-gap can be engineered across the whole visible spectrum. For this reason and the potential to achieve high p-type doping, GaN$_{1-x}$As$_x$ is a promising material for optoelectronic applications. However, the growth of GaN$_{1-x}$As$_{x}$ at intermediate As fraction while maintaining a high conductivity and uniformity of the material is still challenging. Two n-GaN/p-GaN$_{1-x}$As$_x$ diodes with different Ga flows were investigated. Both samples demonstrated that highly Mg-doped GaN$_{1-x}$As$_x$ with high As fraction is achievable. However, the samples contained both amorphous and polycrystalline regions. The electrical scanning probe microscopy results suggested the amorphous structure has a lower hole concentration and hence conductivity than the polycrystalline structure. Nevertheless, there is still a lack of understanding about the electrical properties and conduction mechanisms of the GaN$_{1-x}$As$_x$ alloy.

This thesis is a step towards exploiting singlet and triplet excitons in organic semiconductors for multi-functional diodes. It details the device design and fabrication processes for realisation of reversible organic optoelectronic diodes that can sense as well as emit light on demand. It explores new avenues for multi-exciton harvesting and triplet energy transfer in organic semiconductors in conjunction with physical mechanisms of singlet fission and triplet-triplet annihilation. It details optoelectronic characteristics of multi-chromophore, organic cascades that operate as photodetectors, light-emitting diodes and photovoltaic device with spectral response extending from visible to NIR.

In this work, well-defined and stable thin films based on polythiophene and its derivative, are employed as the hole-injection contact of organic light-emitting diodes (OLED). The polymer films are obtained by the electropolymerization or the electrochemical doping/dedoping of a spin-coated layer. Their electrical properties and energetics are tailored by electrochemical adjustment of their doping levels in order to improve the hole-injection from the anode as well as the performance of small molecular OLEDs. By using dimeric thiophene and optimizing the electrodeposition parameters, a thin polybithiophene (PbT) layer is fabricated with well-defined morphology and a high degree of smoothness by electro-polymerization. The introduction of the semiconducting PbT contact layer improves remarkably the hole injection between ITO anode and the hole- transport layer (NPB) due to its favourable energetic feature (HOMO level of 5.1 eV). The vapor-deposited NPB/Alq3 bilayer OLEDs with a thin PbT interlayer, show a remarkable reduction of the operating voltage as well as enhanced luminous efficiency compared to the devices without PbT. Investigations have also been made on the influence of PbT thickness on the efficiency and I-V feature as well as device stability of the OLED. It is demonstrated that the use of an electropolymerization step into the production of vapor deposited molecular OLED is a viable approach to obtain high performance OLEDs. The study on the PbT has been extended to poly(3,4-ethylenedioxythiophene) (PEDT) and the highly homogenous poly(styrenesulfonate) (PSS) doped PEDT layer from a spin-coating process has been applied. The doping level of PEDT:PSS was adjusted quantitatively by an electrochemical doping/dedoping process using a p-tuoluenesulfonic acid containing solution, and the redox mechanism was elucidated. The higher oxidation state can remain stable in the dry state. The work function of PEDT:PSS increases with the doping level after adjusting at an electrode potential higher than the value of the electrochemical equilibrium potential (Eeq) of an untreated film. This leads to a further reduction of the hole-injection barrier at the contact of the polymeric anode/hole transport layer and an ideal ohmic behavoir is almost achieved at the anode/NPB interface for a PEDT:PSS anode with very high doping level. Molecular Alq3-based OLEDs were fabricated using the electrochemically treated PEDT:PSS/ITO anode, and the device performance is shown to depend on the doping level of polymeric anode. The devices on the polymer anode with a higher Eeq than that for the unmodified anode, show a reduction of operating voltage as well as a remarkable enhancement of the luminance. Furthermore, it is found that the operating stability of such devices is also improved remarkably. This originates from the removal of mobile ions such as sodium ions inside the PEDT:PSS by electrochemical treatment as well as the planarization of the ITO surface by the polymer film. By utilizing an Al/LiF cathode with an enhanced electron injection and together with a high Eeq- anode, a balanced injection and recombination of hole and electron is achieved. It leads to a further reduction of the operating voltage and to a drastic improvement of EL efficiency of the device as high as 5.0 cd/A. The results demonstrate unambiguously that the electrochemical treatment of a cast polymer anode is an effective method to improve and optimize the performance of OLEDs. The method can be extended to other polythiophene systems and other conjugated polymers in the fabrication of the OLEDs as well as organic transistors and solar cells.

In this work, well-defined and stable thin films based on polythiophene and its derivative, are employed as the hole-injection contact of organic light-emitting diodes (OLED). The polymer films are obtained by the electropolymerization or the electrochemical doping/dedoping of a spin-coated layer. Their electrical properties and energetics are tailored by electrochemical adjustment of their doping levels in order to improve the hole-injection from the anode as well as the performance of small molecular OLEDs. By using dimeric thiophene and optimizing the electrodeposition parameters, a thin polybithiophene (PbT) layer is fabricated with well-defined morphology and a high degree of smoothness by electro-polymerization. The introduction of the semiconducting PbT contact layer improves remarkably the hole injection between ITO anode and the hole- transport layer (NPB) due to its favourable energetic feature (HOMO level of 5.1 eV). The vapor-deposited NPB/Alq3 bilayer OLEDs with a thin PbT interlayer, show a remarkable reduction of the operating voltage as well as enhanced luminous efficiency compared to the devices without PbT. Investigations have also been made on the influence of PbT thickness on the efficiency and I-V feature as well as device stability of the OLED. It is demonstrated that the use of an electropolymerization step into the production of vapor deposited molecular OLED is a viable approach to obtain high performance OLEDs. The study on the PbT has been extended to poly(3,4-ethylenedioxythiophene) (PEDT) and the highly homogenous poly(styrenesulfonate) (PSS) doped PEDT layer from a spin-coating process has been applied. The doping level of PEDT:PSS was adjusted quantitatively by an electrochemical doping/dedoping process using a p-tuoluenesulfonic acid containing solution, and the redox mechanism was elucidated. The higher oxidation state can remain stable in the dry state. The work function of PEDT:PSS increases with the doping level after adjusting at an electrode potential higher than the value of the electrochemical equilibrium potential (Eeq) of an untreated film. This leads to a further reduction of the hole-injection barrier at the contact of the polymeric anode/hole transport layer and an ideal ohmic behavoir is almost achieved at the anode/NPB interface for a PEDT:PSS anode with very high doping level. Molecular Alq3-based OLEDs were fabricated using the electrochemically treated PEDT:PSS/ITO anode, and the device performance is shown to depend on the doping level of polymeric anode. The devices on the polymer anode with a higher Eeq than that for the unmodified anode, show a reduction of operating voltage as well as a remarkable enhancement of the luminance. Furthermore, it is found that the operating stability of such devices is also improved remarkably. This originates from the removal of mobile ions such as sodium ions inside the PEDT:PSS by electrochemical treatment as well as the planarization of the ITO surface by the polymer film. By utilizing an Al/LiF cathode with an enhanced electron injection and together with a high Eeq- anode, a balanced injection and recombination of hole and electron is achieved. It leads to a further reduction of the operating voltage and to a drastic improvement of EL efficiency of the device as high as 5.0 cd/A. The results demonstrate unambiguously that the electrochemical treatment of a cast polymer anode is an effective method to improve and optimize the performance of OLEDs. The method can be extended to other polythiophene systems and other conjugated polymers in the fabrication of the OLEDs as well as organic transistors and solar cells.

京都大学
0048
新制・課程博士
博士(工学)
甲第22449号
工博第4710号
新制||工||1736(附属図書館)
京都大学大学院工学研究科電子工学専攻
(主査)教授 川上 養一, 教授 野田 進, 教授 山田 啓文
学位規則第4条第1項該当
Doctor of Philosophy (Engineering)
Kyoto University
DFAM

Phases organiques luminescentes qui sont incorporés dans une matrice inorganique conductrice est proposé dans cette étude pour la couche active d'une diode émettant de la lumière hybride. Dans ce composite, le colorant organique joue le rôle de site de recombinaison radiative de porteurs de charge qui sont injectées dans la matrice de transport ambipolaire inorganique. Comme l'un des combinaisons de matériaux de candidat, bicouche et des films minces composites de ZnSe et un complexe d'iridium rouge (Ir(BPA)) émetteur de lumière organique ont été préparé in situ par UHV technique d'évaporation thermique. Les alignements de bande d'énergie mesurée par spectroscopie de photoélectrons (PES) pour le ZnSe/Ir(BPA)et deux couches de ZnSe+Ir(BPA) révèlent que le composite HOMO et LUMO du colorant organique sont positionnées dans la largeur de bande interdite de ZnSe. Cette gamme offre les forces motrices énergiques nécessaires pour les transferts d'électrons et de trous de ZnSe à Ir(BPA). Par l'interprétation des données du PES,la composition chimique des interfaces ont également été déterminés. Le ZnSe/Ir(BPA) interface est réactive, même si elle est d'une pureté de matériaux de haute.Pendant ce temps, l'Ir (BPA)/ZnSe interface ne présente pas la pureté matériel. Ceci est représenté à la nature de ZnSe évaporation comme Zn particuliers et des fluxSE2, associée à des interactions chimiques avec le Ir(BPA) substrat. L'interface est,de ce fait, composé d'une multitude de phases, les phases de Se0, ZnSe rares, réduit Se et oxydé molécules de colorant, et de Zn qui sont intercalées atomes dans leIr(BPA) substrat. PES des composites ZnSe+Ir(BPA) révèle des tendances similaires à l'Ir(BPA)/ZnSe interface. A des émissions de lumière rouge surfaciques et intermittents fanées ont été observés à partir de dispositifs qui incorporent couches alternées séquences de ZnSe et Ir(BPA) pour la couche active
Luminescent organic phases that are embedded in a conductive inorganicmatrix is proposed in this study for the active layer of a hybrid light-emitting diode. Inthis composite, the organic dye acts as the radiative recombination site for chargecarriers that are injected into the inorganic ambipolar transporting matrix. As one ofthe candidate material combinations, bilayer and composite thin films of ZnSe and ared iridium complex (Ir(BPA)) organic light emitter were prepared in situ via UHVthermal evaporation technique. The energy band alignments measured byphotoelectron spectroscopy (PES) for the ZnSe/Ir(BPA) bilayer and ZnSe+Ir(BPA)composite reveal that the HOMO and LUMO of the organic dye are positioned in theZnSe bandgap. This lineup provides the required energetic driving forces for electronand hole transfers from ZnSe to Ir(BPA). By interpreting PES data, the chemicalcomposition of the interfaces were also determined. The ZnSe/Ir(BPA) interface isreactive even though it is of high material purity. Meanwhile, the Ir(BPA)/ZnSeinterface does not exhibit material purity. This is accounted to the nature of ZnSeevaporation as individual Zn and Se2 fluxes, coupled with chemical interactions withthe Ir(BPA) substrate. The interface is, thereby, composed of an abundance of Se0phases, sparse ZnSe phases, reduced Se and oxidized dye molecules, and Znatoms that are intercalated into the Ir(BPA) substrate. PES of the ZnSe+Ir(BPA)composites reveals similar trends to the Ir(BPA)/ZnSe interface. A faded areal andintermittent red light emissions were observed from devices that incorporatedalternating layer sequences of ZnSe and Ir(BPA) for the active layer

L'étude structurale des polymères conjugués montre que leur désordre moléculaire et structural limite la qualité de leurs propriétés électroniques, et restreint donc leur éventuelle application à des composants électroniques. L'utilisation de systèmes moléculaires structuralement bien définis devrait par contre permettre de s'affranchir de ces défauts chimiques et physiques, et d'améliorer ainsi l'efficacité du transport de charges dans ces matériaux semi-conducteurs. Cette démarche est illustrée ici par l'étude du sexithiophène (6T) et de ses dérivés substitués. La première partie est consacrée à la description des techniques expérimentales de préparation des films minces, puis des composants électroniques réalisés à partir de ces semi-conducteurs organiques, tels que transistors à effet de champ en couche mince et diodes électroluminescentes. L'étude des propriétés structurales et optiques a mis en évidence que ces films sont polycristallins, et que l'orientation des chaînes moléculaires sur le substrat est contrôlée par la nature du substrat, par la température de dépôt et par la position de substitution de groupes alkyles sur cette molécule (6T). Les mesures de conductivité et de mobilité d'effet de champ du sexithiophène et de ses dérivés montrent que pour le , -dihexyl-sexithiophène, la conductivité présente une anisotropie importante, alors que b,b'-dihexyl-sexithiophène se comporte comme un isolant. La température du substrat utilisée lors du dépôt du film influe sur la conductivité et sur la mobilité du sexithiophène. La mobilité plus faible de 6T à température ambiante est attribuée à une plus grande concentration de défauts tels que joints de grains. Les caractéristiques statiques des diodes électroluminescentes, dont les couches actives sont constituées de sexithiophène ou de l'un de ses dérivés, montrent un effet redresseur (semi-conducteur de type-p). Les caractéristiques transitoires montrent un courant capacitif bref, suivi d'un courant permanent donnant lieu à émission lumineuse. Dans une structure bicouche, on remarque une augmentation de l'émission lumineuse et une augmentation du rendement d'électroluminescence par rapport à la structure monocouche. L'intensité du spectre d'électroluminescence enregistré pour la structure bicouche dans la gamme 500-700 nm montre qu'en baissant la température l'émission augmente, ceci est attribué à une diminution du rendement des transitions non radiatives

碩士
國立臺灣大學
光電工程學研究所
96
In this thesis, we study optoelectrical characteristics and material analysis of silicon-rich silicon nitride film (SRSN) with silicon nanocrystals (Si-ncs). The SRSN films are deposited by plasma enhanced chemical vapor deposition (PECVD) using SiH4 and N2 or NH3. Si-ncs embedded in Si3N4　would form after high temperature annealing. We conclude that NH3 is the better reactant gas instead of N2. The SRSN films with different composition are deposited by detuning NH3 fluence. From results by means of RBS, we gain the ratio of N/Si raise with NH3 increasing, and SRSN changes from Si-rich SiNx to pure Si3N4. This phenomenon is proved by means of FTIR, the absorption peak corresponds Si-H stretching mode shift toward long wavenumber. From images of HRTEM, we observe that the size of Si-ncs decrease with NH3 increasing. The photoluminescence (PL) ranges from 675 nm to 385 nm by quantum confinement effect (QCE). The strongest PL reveals from SiN1.16. In addition, we discuss the electroluminescence of SRSN LED. The low turn-on voltage is 3 V because of low barrier between metal and dielectric layer. However, the optical power just reaches 45 nW. As the result, we study the charge storage effect in SRSN LED. In capacitance-voltage and retention time measurement, we conclude electron and hole are hardly trapped in Si-ncs so that the efficiency of e-h recombination is low, compared to Si-rich silicon oxide (SRSO) LED.

碩士
臺灣大學
光電工程學研究所
96
In this thesis, correlation between N2O/SiH4 fluence ratio and O/Si composition ratio for optimizing Si nanocrystal precipitation in Si-rich SiOx grown by low-plasma PECVD is demonstrated. The O/Si composition ratio of SiOx can be adjustable from 1.38 to 0.88 by detuning N2O fluence and N2O/SiH4 ratio to obtain a nonlinearly Gaussian-like dependency with near-infrared photoluminescence (PL). By reducing N2O/SiH4 ratio, abundant Si-H bonds with absorption at 870 and 2250 cm-1 assist small-size Si nanocrystal precipitation and prevent outer surface re-oxidation. Maximum PL at 760 nm at O/Si=1.24 with corresponding Si concentration of 44.64 atom.% is obtained at N2O/SiH4 ratio of 5.5. In particular, the N2O fluence remains as small as 25 sccm to restrict oxygen desorption and to complete SiH4 decomposition, thus minizing the hydrogen passivation on dangling bonds at Si nanocrystal surface. The N2O:SiH4 fluence is decreased to 5:1 and the optimized annealing are achieved as short as 15 min at 1100oC in comparison with typical 1-hr process. HRTEM analysis reveals such tiny Si nanocrystals exhibit diameter of only 1.5±0.2 nm. From FTIR results, we conclude that the ultra-low fluence PECVD can completely decompose the Si from SiH4 with minimum hydrogen passivation, which facilitates the precise control of Si nanocrystal size and greatly enhances the blue PL intensity. The blue-light EL pattern is observed at 290 V for the MOSLED made on SiOx grown at N2O fluence as low as 25 sccm. The maximum emitting power is about 333~500 nW for the blue-light MOSLED as compared to that of 270 nW for red-light MOSLED associated with a PI slope of 0.37 mW/A. Higher output power of MOSLED on low-N2O-fluence grown SiOx is attributed to the smaller Si nanocrystals with larger density.

碩士
國立臺灣師範大學
化學研究所
89
We have synthesized CdSe and CdSe(CdS) core/shell nanorcrystals in tri-n-octylphosphine oxide (TOPO) micellar solution using dimethylcadmium (Cd(CH3)2), selenium (Se) powder and bis-trimethylsilane sulfide ((TMS)2S) as the reactants. The sizes of the nanocrystals were controlled by varying the experimental conditions such as the concentration of Cd(CH3)2, reaction temperature, and reaction time. The nanocrystals were characterized using UV-Vis absorption and fluorescence spectra. The absorption and fluorescence spectra suggested that the band edges of the resulting nanocrystals shift to higher energy than that of the bulk CdSe crystals. The transmission electron microscopy images indicated that the CdSe nanocrystals are about 3 nm. We have demonstrated the electrical and optical characteristics of the organic light emitting diode (OLED) devices using nanocrystals as the emitting layer, Poly (9-vinylcarbazole) (PVK) as the hole-transport layer, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as the electron-transport layer, and poly(3,4-ethylenedioxythiophene) -poly(4-styrenesulphonate) (PEDT-PSS) as the hole-injection layer. We have investigated the characteristics of CdSe for ITO/PEDT-PSS/PVK/CdSe/BCP/Mg:Ag structure and CdSe(CdS) for ITO/PEDT-PSS/PVK/CdS(CdS)/BCP/Mg:Ag structure. Then, we change the thickness of CdSe for ITO/PEDT-PSS/PVK/CdSe/BCP/Mg:Ag structure. We found that emission wavelength of these heterostructure devices was affected by the thickness of the light emitting layer of LED. The intensity of the electroluminescence (EL) at the position of 600 nm from nanocrystals and at 400 nm from PVK change with different voltages applied to the devices. We also found that ITO/PEDT-PSS/PVK/CdSe/BCP/Mg:Ag structure using CdSe nanocrystals in their emitting layer can provide emission tunable in the visible spectrum, because of the size-dependent luminescence of the quantum dots.

碩士
國立臺灣大學
光電工程學研究所
96
By changing the RF plasma power during the PECVD system, photoluminescence (PL) wavelength control of Si-rich SiOx film was proposed. The O/Si composition ratio increased as the RF power increased, which was confirmed by TEM XEDS. After high temperature annealing, the average sizes of the nc-Si embedded in the SiOx film decreased when the O/Si composition ratio in the SiOx film increased. We could control the O/Si composition ratio in the SiOx film by detuning RF plasma power in the PECVD system, which leads to different sizes of Si nanocrystals after high-temperature annealing. Hence, we could obtain nc-Si size-related and wavelength-tunable PL spectrum from 390 to 780 nm. Subsequently, the EL properties of PECVD–grown Si-rich SiOx based MOSLED was investigated. The turn-on voltage of the Si-rich SiOx film increased as the RF plasma power increased from 50 to 70 W, resulting in the same operational electric field strength at 6.6 × 10^6 V/cm for such nc-Si based devices. The EL microscopy images of device under RF plasma power of 50, 60 and 70 W revealed the red, green, and blue emission under forward bias current to the Si substrate. A significant size-dependent blueshift was clearly shown.

碩士
國立臺灣大學
光電工程學研究所
92
In this thesis, the novel metal-oxide-semiconductor (MOS) tunneling diodes with high leakage current were utilized as photodetectors. The leakage of inversion carrier through ultrathin oxide makes the device to operate in the deep depletion region. The dark current is limited by the thermal generation process and can be reduced by the high growth temperature of oxide. In order to increase the speed of the MOS tunneling photodetectors, the novel fully-depleted silicon-on-insulator (SOI) MOS photodetector is proposed. For devices with 1020 cm-3 buffer layer doping, the device can reach high bandwidth (22 GHz) and are fully compatible with ultra-large scale integration (ULSI) technology. For thin devices, the transit time can be determined by the drift mechanism. For thick devices, however, the diffusion mechanism is needed to describe the device behavior. DBR (distributed Bragg reflector) model is used to design the device for better responsivity. The metal-insulator-semiconductor light emission diode (MIS LED) using high k insulators is successfully demonstrated. The enhancement of quantum external efficiency of MIS LED is accomplished well due to more quantum confinement holes created by larger electric field on Si. From the simulations, it is confirmed that the electric field on Si is increased when HfO2 replaced SiO2. The long wavelength EL spectrum is observed for the high k LED with many interface states. The normalized EL spectrum of MOS LED and high k LED are similar. The quantum efficiency of high k LED is 2 * 10-6, which is about ten times larger than oxide LED. Surface plasmon is applied on MOS LED for better light intensity. By controlling the size of hole array, we can have enhanced transmission for silicon emitted light through Aluminum film. These simple and high performance Si-based photodetectors together with other devices can be used as building blocks for the future optical signal process and the optoelectronic applications on Si chips.

博士
國立成功大學
化學工程學系
103
Silver nanowire (Ag NW)/inorganic semiconductor composite films and gallium nitride (GaN) films were fabricated for the application of organic light emitting diode (OLED) devices. The Ag NW ink was prepared using a simplified polyol method. After synthesis, a reusable porous membrane was utilized to purify the Ag NW solution. Nearly 90% of silver nanoparticles (Ag NPs) could be removed. In order to increase the conductivity of Ag NW films, antimony tin oxide (ATO) nanoparticles were added to form composite films. By emitting infrared (IR) light for 30 sec, the sheet resistance of the composite film could be decreased to 34 ohm/sq with a light transmittance of 91%. For the OLED devices using composite films as anodes, the maximum luminance and efficiency could reach 7020 cd/m2 and 2.7 cd/A, respectively, which was better than that of the device with indium tin oxide (ITO) anode. Next, the growth of GaN (0002) films deposited on sapphire substrates by inductively coupled-plasma (ICP)-enhanced reactive magnetron sputtering was investigated. X-ray diffraction (XRD) measurements confirmed that the high quality GaN crystallites could be obtained at a temperature as low as 500°C. The N:Ga ratio of the film grown at 500°C was almost 1:1. Afterwards, the crystalline GaN film was applied to the OLED device as a carrier transporting layer. The hybrid OLED that could be operated at high voltage showed the improved device durability. The maximum luminance of the hybrid OLED was 3451 cd/m2, higher than that of the conventional device.

碩士
國立臺灣大學
光電工程學研究所
101
In this thesis, we present the fabrication and characterization of ZnO/GaN and ZnO/Si light-emitting diode (LED) using self-assembled nanosphere lithography and atomic layer deposition (ALD). First, we discuss the theory and process of self-assemble nanosphere lithography, and present the theory of ALD system followed by material analysis. Second, we fabricate and measure the ZnO/GaN LED devices. Third part, we fabricate and measure the ZnO/Si LED devices. From the PL analysis pumped by a 266nm Nd:YAG solid-state laser, we observed a peak emission wavelength at 383nm with a full width at half maximum (FWHM) of 25nm. Data from the XRD analysis suggest the ZnO film grown by the ALD system to be poly crystalline. For fabricated the ZnO/GaN LED devices from the current-voltage and the electroluminescence (EL) data, these devices exhibit non-ideal electrical characteristic. The devices emit ultraviolet and visible light under both forwarded and reversed bias, respectively. For the ZnO/Si LED devices, they exhibit current rectification characterization and have a turn-on voltage of 6V and the devices emit continuous visible light under forward bias. In addition, we fabricated planar type for ZnO LED on (001) and (111) P-Si substrates. We detect five-fold increase in the emission intensity for a 6nm-thick ZnO/Si LED compared with a 3nm-thick ZnO/Si LED.

碩士
國立臺灣大學
化學研究所
107
In recent years, the zero-dimensional Cs4PbBr6 crystals are respected as the new generation of functional materials for their good thermal stability and optical performance. Not only do the zero-dimensional perovskite Cs4PbBr6 nanocrystals retain the optical properties of traditional perovskite nanocrystals with a narrow full width at half maximum (FWHM) of ∼20 nm, high absorbance and high quantum efficiency, but they also solve a long-lasting stability problem in perovskite nanocrystals. However, the mechanism of the green emission from the Cs4PbBr6 crystals is still under debate. In order to solve this controversy, we attempt to clarify the controversy by using fluorescence spectrometer and confirm that the green light source of Cs4PbBr6. In this study, the Cs4PbBr6 nanocrystals were successfully synthesized by low-temperature microemulsion method. The temperature tolerance of the structure was determined by temperature-dependent fluorescence spectroscopy. It was found that the 96% emission intensity was maintained after the high-temperature heating at 150oC. About the optical properties, the fluorescence behaviour of Cs4PbBr6 and CsPbBr3 were similar at low temperature. However, the previous literature pointed out that the band gap of Cs4PbBr6 is about 3.9 eV, so it should not emit the green light. Furthermore, the synchrotron XRD didn’t show the existence of CsPbBr3 phase. We proposed that it generated CsPbBr3 clusters which cause the strong green light in Cs4PbBr6 crystals. At the same time, we found that Cs4PbBr6 has strong absorption at 310 nm, but it could not be stimulated. However, when the temperature was lower than 200 K, Cs4PbBr6 emitted the light in 375 nm and 518 nm. This phenomenon proved the energy transfer, and it also showed the thermal quenching effect at room temperature. In addition, we hypothesized that the Cs+ vacancies in the Cs4PbBr6 nanocrystals induced the formation of CsPbBr3 clusters and we proved it by adjusting the ratio of different Cs/Pb precursors. While the Cs/Pb ratio decreased to 2.5, the quantum efficiency kept increasing and the product maintained the pure Cs4PbBr6 phase. It can be confirmed that the CsPbBr3 clusters were the reason for high quantum efficiency. When the Cs/Pb ratio was less than 2.5, the quantum efficiency quickly decreased and the CsPbBr3 impurity phase appeared. It indicated that the green light was not caused by the CsPbBr3 impurity. The optical properties of Cs4PbBr6 nanocrystals under different pressures were also determined by pressure-dependent fluorescence spectroscopy. Compared with the traditional CsPbBr3 nanocrystals, the Cs4PbBr6 nanocrystals possess better stability and tolerance in environmental factors. These advantages make Cs4PbBr6 have better performance in the backlighting used light emitting diode. About the mechanism of the green light from the Cs4PbBr6 crystals, we also successfully clarified the debate. Finally, the Cs4PbBr6 crystals were succeeded fabricating as traditional WLED and Mini-LED which achieved high color gamut of 129% and 126% and proved Cs4PbBr6 nanocrystals are the potential material for backlight application.

碩士
國立臺灣大學
光電工程學研究所
99
In this thesis, the synthesis of a-SixC1-x films embedded with Si-ncs and SiC-ncs by fluence-ratio detuned PECVD at high-temperature growth is investigated to modify its luminescent property and to enrich the crystallinity after thermal annealing at 1100oC. With changing the deposition temperature from 450oC to 650oC, the Si concentration increases from 64.7% to 71.6%. However, the carbon and oxygen contents decrease from 27.7% to 22.8% and 7.6% to 5.5% and the O/Si ratio is reduced from 0.12 to 0.07 from the XPS analysis due to growth of better crystallinity to prevent the oxygen invasion in a-SixC1-x films. A significant signal at 510 cm-1 is shown to confirm the existence of Si-ncs after post-annealing. The other two intensive peaks at 744 and 933 cm-1 are red-shifted than bulk 3C-SiC Raman peaks at 796 (TO) and 972 cm-1 (LO) and ascribed to reduced nanograin size of SiC-ncs, respectively. From the results of XRD spectra, the average crystallite sizes of Si and 3C-SiC nanocrystals are around 4.2±0.5 nm and 2.4±0.3 nm, respectively. On the basis of FTIR analysis, the Si-H3 stretching mode is transformed into Si-H stretching mode after annealing since the hydrogen bond is broken up to diffuse out. Accordingly, Si-ncs can be easily aggregated by dehydrogenation in Si-H3 radical. A distinct band at 792-806 cm-1 is ascribed to Si-C stretching mode and the blue-shifted peak from 792 to 802 cm-1 is due to enhanced strength of bonds between Si and C atoms. The intense visible PL centered at 485 nm is found in annealed sample of g=60% and it attributed to the luminescence of SiC-ncs due to the self-trapped excitons at the surface states between SiC-ncs and surrounding. In addition, the PL peak at 580 nm is also observed from the contribution of Si-ncs in view of quantum confinement effect. Composition ratio x in SixC1-x is detuned from 0.74 to 0.62 with increasing fluence ratio from 40 to 70% by XPS spectra. The resistivity of P type a-SixC1-x network at g of 50% reduces to 2.2×101 Ω-cm when B2H6 doping mole fraction increases to 2% since the appropriate amounts of boron atoms occupy the position in tetrahedral SiC network to release enough holes to form the electrically active dopant after thermal process of 650oC. The dopant density is also increased to 1.35×1016 cm-3 when doping mole fraction is at 2% corresponding to activation energy of 0.17 eV. On the other hand, the resistivity is reduced from 22 to 0.72 Ω-cm and dopant density is increased from 1.35×1016 to 4.35×1017 cm-3 due to the minimization of the overdoping phenomenon to reduce the influence on excess impurity atoms scattering and collision between released carriers when enlarged triply doping gaseous fluence and it is equivalent to gaseous dilution owing to various dissociation energy for different process gas. The resistivity of N-SiC is decreased abruptly to 11.3 Ω-cm when RF power changes to 80 W corresponding PH3 dopant density of 1.46×1015 cm-3. The PIN thin film light emitting diode with intrinsic layer embedded with Si-ncs and SiC-ncs is fabricated to enlarge optical power, reduce turn on voltage and enhance carrier injection efficiency. Carrier injection and transport properties can be improved with the higher doping concentration P-SiC layer. The thicker intrinsic layer caused the larger series resistance thus whether turn on voltage or injection current is larger than the thinner I-layer and the optical power emitted from PIN TFLED with I layer thickness of 50 nm is triple than I layer thickness of 25 nm at g of 60%. With increasing thickness of intrinsic layer from 25 nm to 100 nm at g of 50% since the carrier tunneling probability is decreased with enlarging the intrinsic layer thickness since insufficient electric field across the I-SiC film is to reduce the carrier injection and the luminescent centers are more plentiful owing to the Si-rich SiC matrix at g of 50% with embedded more quantity of Si-ncs. The optical power from PIN LED with g of 50% at I layer thickness of 50 nm is three point five times than I layer at 25 nm. The PCR increasing trend is due to the production of more luminescent centers at thickness of 50 nm but subsequently decreased is owing to too thicker I-layer caused the carrier tunneling probability reduction. The EQE is larger twice than others with increasing intrinsic layer thickness from 25 to 50 nm at g of 60% and the P-I slope is four times than others since the trade off relation between carrier transport and tunneling into active layer and the more luminescent centers in active layer are observed thus the optimized thickness of intrinsic layer is 50 nm. The EQE is four times than others with increasing g of 50% intrinsic layer thickness from 25 to 50 nm and the P-I slope is six times than others since the appropriate intrinsic thickness at 50 nm is needed to enhance light emission and preserve the carrier injection ability. Carrier transport via band to band tunneling is confirmed owing to high electric field and then radiative transition is also occurred due to the carrier tunneling into intrinsic region and its neighborhood. The principal EL peak at 495 nm with narrower shape is assigned to self-trapped excitons at the surface states between SiC-ncs and surrounding at g of 60% corresponding to blue-white EL emission pattern. Moreover, the main EL wavelength centered at 570 nm with broader shape is attributed to nearly direct band to band transition by Si-ncs at g of 50% and consistent with orange-yellow EL emission pattern. The injection efficiency is six times than N-SiC with 1015 cm-3 when dopant density increased to 1016 cm-3. The EQE is enhanced nearly six times when injection efficiency is increased from 7.84% to 46 %.

碩士
國立成功大學
化學工程學系
104
This research is involving of five parts. They are sequentially the growth of GaN nanorods without doping, the growth of n-GaN nanorods with doping SiH4, Cl2-assisted InGaN growth, Cl2-assisted p-GaN growth with doping Mg3N2 and Cl2-assisted AlGaN growth. For GaN and n-GaN, high crystal quality as well as good uniformity of quality and nanorods’ density could be seen in PL and SEM. Then p-GaN epitaxial film was grown at 505℃ with appearance of characteristic peak around 425 nm in PL. On the other hand, InGaN epitaxial film was grown at 505℃ with 16% atomic indium estimated by XRD. Furthermore, AlGaN epitaxial film was grown at 600℃ with 68% atomic aluminum estimated by XRD.

博士
國立成功大學
微電子工程研究所碩博士班
92
In this dissertation, the growth and characterization of SiN/GaN double buffer and InGaN/AlGaN MQW ultraviolet LED layer have been studied. In addition, we also investigate the influence of O-containing annealing environments on the activation of Mg-doped GaN and Mg-doped AlGaN/GaN strained-layer superlattices. The primary result obtained in this dissertation are summarized as follows: It was found that the GaN grown on SiN buffer showed low dislocation density in TEM images and narrow peak in DCXRD. We could achieve a low-resistive p-type GaN by pure O2 annealing at a temperature as low as 400oC. The electrical properties of Al0.15Ga0.85N/GaN SLs were measured and compared to conventional Mg-doped GaN films. Ni/Au contacts to p-type Al0.15Ga0.85N/GaN SLs with a low specific contact resistance as low as 4.0×10-6Ω-cm2 has been successfully achieved. We could reduce the 20 mA LED forward voltage from 3.78 V to 2.94 V and also reduce the series resistance of the LED from 41 W to 10 W by introducing such an n+-InGaN/GaN SPS top contact. It was also found that we could improve the LED output power and lifetime by employing such a SPS structure. (d) For UV LED, it was found that the 20 mA EL intensity of InGaN/Al0.1Ga0.9N MQW LED was two times larger than that of InGaN/GaN MQW LED. The larger maximum output intensity and the fact that maximum output intensity occurred at larger injection current suggest that Al0.1Ga0.9N barrier layers can provide a better carrier confinement and effectively reduce leakage current. For UV led with transparent ITO layer, , it was found that we could achieve a 36% larger output intensity by using such an ITO on n+-SPS upper contact. Phosphor converted LED lamps were fabricated by precoating blue/green/red phosphors onto n-UV LED chips prior to packaging. It was also found that no changes in color temperature, Tc, or color rendering index, Ra, could be observed when we increased the injection from 20 mA to 60 mA. These results indicate that such “n-UV+blue/green/red” white LED lamps are much more optically stable than conventional “blue+yellow” white LED lamps.

A novel, non-volatile, organic capacitive memory device called p-i-n-metal-oxide-semiconductor (pinMOS) memory is demonstrated with multiple-bit storage that can be programmed and read out electrically and optically. The diode-based architecture simplifies the fabrication process, and makes further optimizations easy, and might even inspire new derived capacitive memory devices. Furthermore, this innovative pinMOS memory device features local charge up of an integrated capacitance rather than of an extra floating gate. Before the device can perform as desired, the leakage current due to the lateral charge up of the doped layers outside the active area needs to be suppressed. Therefore, in this thesis, lateral charging effects in organic light-emitting diodes (OLEDs) are studied first. By comparing the results from differently structured devices, the presence of centimeter-scale lateral current flows in the n-doped and p-doped layers is shown, which results in undesirable capacitance increases and thus extra leakage currents. Such lateral charging can be controlled via structuring the doped layers, leading to extremely low steady-state leakage currents in the OLED (here 10-7 mA/cm2 at -1 V). It is shown that these lateral currents can be utilized to extract the conductivity as well as the activation energy of each doped layer when modeled with an RC circuit model. Secondly, pinMOS memory devices that are based on the diode with structured doped layers are investigated. The memory behavior, which is demonstrated as capacitance switching for electrical signals, and light emission for optical signals, can be tuned either by the applied voltage or ultraviolet light illumination, respectively. The working mechanism is explained by the existence of quasi steady-states as well as the width variation of space charge zones. The pinMOS memory shows excellent repeatability, an endurance of more than 104 write-read-erase-read cycles, and currently already over 24 h retention time. Furthermore, an early-stage investigation on emulating synaptic plasticity reveals the potential of pinMOS memory for applications in neuromorphic computing. Overall, the results indicate that pinMOS memory in principle is promising for a variety of future applications in both electronic and photonic circuits. A detailed understanding of this new concept of memory device, for which this thesis lays an important foundation, is necessary to proceed with further enhancements.:1 Introduction 1 2 Fundamentals of organic semiconductors 5 2.1 Electronic states of a molecule 5 2.1.1 Atomic orbitals and molecular orbitals 5 2.1.2 Solid states 9 2.1.3 Singlet and triplet states 12 2.2 Charge transport 13 2.2.1 Charge carrier mobility 13 2.2.2 Charge carrier transport 14 2.3 Charge injection 17 2.3.1 Current limitation 17 2.3.2 Charge injection mechanisms 20 2.4 Doping 22 3 Organic junctions and devices 25 3.1 Metal-semiconductor junction 25 3.1.1 Schottky junction 25 3.1.2 Surface states 27 3.2 Metal-oxide-semiconductor capacitor 29 3.3 Junctions and diodes 31 3.3.1 PN junction and diode 31 3.3.2 PIN junction and diode 32 4 Organic non-volatile memory devices 35 4.1 Basic concepts 35 4.2 Organic resistive memory devices 37 4.2.1 Device architecture and switching behavior 38 4.2.2 Working mechanisms 38 4.3 Organic transistor-based memory devices 41 4.3.1 Organic field-effect transistor and memory devices based thereon 41 4.3.2 Floating gate memory 43 4.3.3 Charge trapping memory 45 4.4 Organic ferroelectric memory devices 46 4.4.1 Ferroelectric capacitor memory 47 4.4.2 Ferroelectric transistor memory 48 4.4.3 Ferroelectric diode memory 49 5 Experimental methods 53 5.1 Device fabrication 53 5.2 Device characterization 55 5.3 Materials 57 6 Lateral current flow in semiconductor devices having crossbar electrodes 61 6.1 Introduction 61 6.2 Device architecture 62 6.3 Characteristics comparison between unstructured and structured devices 63 6.3.1 Charging measurement 63 6.3.2 Current-voltage characteristics 64 6.3.3 Capacitance-frequency characteristics 67 6.4 Influence of conductivity of doped layers 69 6.4.1 Dependence on doped layers thickness 69 6.4.2 Dependence on temperature 73 6.5 Lateral charging simulation 74 6.5.1 Analytical description 74 6.5.2 RC circuit simulation 76 6.5.3 Parameters for doped layers gained by simulation 79 6.6 Pseudo trap analysis 81 6.6.1 The pseudo trap density of states determination 81 6.6.2 The pseudo trap analysis under simulated identical conditions 84 6.7 Summary 85 7 The pinMOS memory: novel diode-capacitor memory with multiple-bit storage 87 7.1 Introduction 87 7.2 Device architecture 88 7.2.1 Dependence on layout and pixel 89 7.2.2 Fundamental memory behavior characterization 93 7.3 Working mechanism 96 7.3.1 Working mechanism of quasi-steady states 97 7.3.2 Working mechanism of dynamic states 101 7.4 Tunability of the memory effect 105 7.4.1 Operation parameters 106 7.4.2 Photoinduced tunability 108 7.4.3 Intrinsic layer thickness 110 7.5 Potential in neuromorphic computing application 111 7.5.1 Extracting capacitance at 0 V sequentially 112 7.5.2 Mimicking the long-term plasticity (LTP) behavior 113 7.6 Summary 114 8 Optoelectronic properties of pinMOS memory 117 8.1 Introduction 117 8.2 Measurement setup 117 8.3 pinMOS memory emission intensity 118 8.4 Pulse characteristics and device brightness 119 8.5 Conclusion 124 9 Conclusion 125 Bibliography 129 List of Figures 145 List of Tables 151 List of Abbreviations 153 Publications and Conference 157 Acknowledgment 159
Es wird ein neuartiges, organisches kapazitives Speicherelement demonstriert, das p-i-n-Metalloxid-Halbleiter (pinMOS) Speicher genannt wird und eine Mehrfachbitspeicherung besitzt, die elektrisch und optisch programmiert und ausgelesen werden kann. Die auf einer Diode basierende Architektur vereinfacht den Herstellungsprozess sowie die weitere Optimierung und könnte sogar Inspiration für neue kapazitive Speichermedien sein. Darüber hinaus basiert dieses innovative pinMOS Speicherelement auf der lokalen Aufladung einer integrierten Kapazität und nicht auf einem zusätzlichem “Floating Gate”. Bevor das Speicherelement wie gewünscht funktioniert, muss der Leckstrom, der durch die laterale Aufladung der dotierten Schichten außerhalb des aktiven Bereichs verursacht wird, unterdrückt werden. Deshalb werden in dieser Arbeit zuerst die lateralen Aufladungseffekte in organischen Leuchtdioden (OLEDs) untersucht. Beim Vergleich verschiedener Device-Strukturen wird die Existenz von lateralen Stromflüssen im Zentimeterbereich in den n- und p-dotierten Schichten gezeigt, was zu einer unerwünschten erhöhten Kapazität und folglich einem höheren Leckstrom führt. Diese laterale Aufladung kann durch die Strukturierung der dotierten Schichten kontrolliert werden, was zu extrem geringen Gleichgewichtsleckströmen in den OLEDs (10-7 mA/cm2 bei -1 V) resultiert. Es wird auch gezeigt, dass die lateralen Ströme genutzt werden können um die spezifische Leitfähigkeit sowie die Aktivierungsenergie der einzelnen dotierten Schichten zu extrahieren, wenn diese mit einem RC-Modell modelliert werden. Im zweiten Teil werden pinMOS Speicherelemente, die auf der Diode mit strukturierten dotierten Schichten basieren, untersucht. Das Speicherverhalten, dass durch Kapazitätsschaltung für elektrische Signale und als Lichtemission für optische Signale gezeigt wird, kann entweder durch die angelegte Spannung, beziehungsweise durch die Belichtung mit ultraviolettem Licht eingestellt werden. Die Wirkungsweise wird durch die Existenz quasistatischer Gleichgewichte sowie durch die Größenänderung der Raumladungszonen erklärt. Der pinMOS Speicher zeigt eine hervorragende Wiederholbarkeit, eine Beständigkeit über mehr als 104 Schreiben-Lesen-Löschen-Lesen Zyklen und aktuell schon eine Retentionszeit von über 24 h. Weiterhin offenbaren erste Versuche in der Nachahmung von Neuronaler Plastizität das Potenzial von pinMOS Speichern für Anwendungen im “Neuromorphic Computing”. Insgesamt deuten die Ergebnisse an, dass pinMOS Speicher prinzipiell vielversprechend für eine Vielzahl von zukünftigen Anwendungen in elektronischen und photonischen Schaltkreisen ist. Ein tiefgreifendes Verständnis von diesem Konzept neuartiger Speicherelemente, für das diese Arbeit eine wichtige Grundlage bildet, ist notwendig, um weitere Verbesserungen zu entwickeln.:1 Introduction 1 2 Fundamentals of organic semiconductors 5 2.1 Electronic states of a molecule 5 2.1.1 Atomic orbitals and molecular orbitals 5 2.1.2 Solid states 9 2.1.3 Singlet and triplet states 12 2.2 Charge transport 13 2.2.1 Charge carrier mobility 13 2.2.2 Charge carrier transport 14 2.3 Charge injection 17 2.3.1 Current limitation 17 2.3.2 Charge injection mechanisms 20 2.4 Doping 22 3 Organic junctions and devices 25 3.1 Metal-semiconductor junction 25 3.1.1 Schottky junction 25 3.1.2 Surface states 27 3.2 Metal-oxide-semiconductor capacitor 29 3.3 Junctions and diodes 31 3.3.1 PN junction and diode 31 3.3.2 PIN junction and diode 32 4 Organic non-volatile memory devices 35 4.1 Basic concepts 35 4.2 Organic resistive memory devices 37 4.2.1 Device architecture and switching behavior 38 4.2.2 Working mechanisms 38 4.3 Organic transistor-based memory devices 41 4.3.1 Organic field-effect transistor and memory devices based thereon 41 4.3.2 Floating gate memory 43 4.3.3 Charge trapping memory 45 4.4 Organic ferroelectric memory devices 46 4.4.1 Ferroelectric capacitor memory 47 4.4.2 Ferroelectric transistor memory 48 4.4.3 Ferroelectric diode memory 49 5 Experimental methods 53 5.1 Device fabrication 53 5.2 Device characterization 55 5.3 Materials 57 6 Lateral current flow in semiconductor devices having crossbar electrodes 61 6.1 Introduction 61 6.2 Device architecture 62 6.3 Characteristics comparison between unstructured and structured devices 63 6.3.1 Charging measurement 63 6.3.2 Current-voltage characteristics 64 6.3.3 Capacitance-frequency characteristics 67 6.4 Influence of conductivity of doped layers 69 6.4.1 Dependence on doped layers thickness 69 6.4.2 Dependence on temperature 73 6.5 Lateral charging simulation 74 6.5.1 Analytical description 74 6.5.2 RC circuit simulation 76 6.5.3 Parameters for doped layers gained by simulation 79 6.6 Pseudo trap analysis 81 6.6.1 The pseudo trap density of states determination 81 6.6.2 The pseudo trap analysis under simulated identical conditions 84 6.7 Summary 85 7 The pinMOS memory: novel diode-capacitor memory with multiple-bit storage 87 7.1 Introduction 87 7.2 Device architecture 88 7.2.1 Dependence on layout and pixel 89 7.2.2 Fundamental memory behavior characterization 93 7.3 Working mechanism 96 7.3.1 Working mechanism of quasi-steady states 97 7.3.2 Working mechanism of dynamic states 101 7.4 Tunability of the memory effect 105 7.4.1 Operation parameters 106 7.4.2 Photoinduced tunability 108 7.4.3 Intrinsic layer thickness 110 7.5 Potential in neuromorphic computing application 111 7.5.1 Extracting capacitance at 0 V sequentially 112 7.5.2 Mimicking the long-term plasticity (LTP) behavior 113 7.6 Summary 114 8 Optoelectronic properties of pinMOS memory 117 8.1 Introduction 117 8.2 Measurement setup 117 8.3 pinMOS memory emission intensity 118 8.4 Pulse characteristics and device brightness 119 8.5 Conclusion 124 9 Conclusion 125 Bibliography 129 List of Figures 145 List of Tables 151 List of Abbreviations 153 Publications and Conference 157 Acknowledgment 159